NUREG/CR-6666 

CNWRA 98-005

Survey of Waste 

Solidification Process 

Technologies 

Center for Nuclear Waste Regulatory Analyses

U.S. Nuclear Regulatory Commission 

Office of Nuclear Material Safety and Safeguards 

Washington, DC 20555-0001

0f&( 

*40 0
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I
NUREG/CR-6666 

CNWRA 98-005

Survey of Waste 

Solidification Process 

Technologies

Manuscript Completed: January 2001 

Date Published: February 2001 

Prepared by 

V. Jain 

Center for Nuclear Waste Regulatory Analyses 

Southwest Research Institute 

6220 Culebra Road 

San Antonio, Texas 78238-5166

R E. Shewmaker, NRC Project Manager 

Division of Fuel Cycle Safety and Safeguards 

Office of Nuclear Material Safety and Safeguards 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001 

NRC Job Code J5164
ABSTRACT

This report provides a current status of the 

high-temperature solidification technologies that 

have been used or proposed in the United States 

and abroad. The technologies presented in this 

report can be broadly classified into the 

following categories: Inconel-based, joule

heated melters; high-temperature, joule-heated

melters; induction melters; cold-crucible 

induction melters; plasma melters; combustion 

melters; microwave melters; molten metal 

technology; and Synroc technology. The 

operating experience and safety issues 

associated with major solidification technologies 

are summarized.

Ill NUREGICR-6666
CONTENTS 

Page 

A bstract ........................................................................... iii 

Executive Summary ................................................................. xix 

Acknowledgments .................................................................. xxi 

A cronym s ........................................................................ xxiii 

Registered Trademarks ............................................................. xxvii 

1 Introduction .................................................................... 1 

2 Relevant Aspects of Commercial Technology ......................................... 3 

3 Joule-Heated, High-level Radioactive Waste Processing Plants ............................ 7 

3.1 West Valley Demonstration Project Vitrification Facility at West Valley ............... 7 

3.1.1 Waste Pretreatment and Feed Preparation .............................. 8 

3.1.2 Vitrification .................................................... 12 

3.1.3 Offgas Treatment System .......................................... 14 

3.1.4 Process Issues and Upsets ......................................... 16 

3.1.4.1 Melter Nozzle Failure ...................................... 16 

3.1.4.2 Melter Pour Spout Failure ................................... 16 

3.1.4.3 Formation of Glass Fibers ................................... 17 

3.1.4.4 Spread of Contamination .................................... 17 

3.1.4.5 M elter Level Probes ....................................... 17 

3.1.4.6 Slurry Backflow .......................................... 17 

3.2 Defense Waste Processing Facility at Savannah River Site ......................... 17 

3.2.1 Waste Pretreatment and Feed Preparation ............................. 18 

v NUREG/CR--6666
CONTENTS (cont'd)

3.2.2 

3.2.3 

3.2.4 

3.3 Pamela 

3.3.1 

3.3.2 

3.3.3 

3.3.4

Page 

V itrification .................................................... 23 

Offgas Treatment System .......................................... 25 

Process Issues and Upsets ......................................... 25 

3.2.4.1 Hydrogen Gas Generation ................................... 25 

3.2.4.2 Corrosion of Tanks and Transfer Lines ......................... 26 

3.2.4.3 Benzene Deflagration ...................................... 26 

3.2.4.4 Moisture and Electrical Problems with Canister Manipulating 

M echanism .............................................. 26 

3.2.4.5 Glass Pour Problems ....................................... 27 

3.2.4.6 Foaming in the Melter ...................................... 27 

3.2.4.7 Canister Turntable ......................................... 27 

3.2.4.8 Process Component ........................................ 27 

Vitrification Plant, Mol, Belgium ...................................... 28 

Waste Pretreatment and Feed Preparation ............................. 28 

V itrification .................................................... 31 

Offgas Treatment System .......................................... 33 

Process Issues and Upsets ......................................... 34 

3.3.4.1 Noble Metal Issues in the Melter .............................. 34 

3.3.4.2 Electrode Damage ......................................... 35 

3.3.4.3 Failure of the Bottom Draining System ......................... 35 

3.3.4.4 Blockage of the Outlet of the Overflow Channel ................. 35

NUREG/CR-6666 vi
CONTENTS (cont'd) 

Page 

3.4 Tokai Vitrification Plant, Japan .............................................. 35 

3.4.1 Waste Pretreatment and Feed Preparation ............................. 37 

3.4.2 Vitrification .................................................... 38 

3.4.3 Offgas Treatment System .......................................... 39 

3.4.4 Process Issues and Upsets ......................................... 39 

3.4.4.1 Remote Operations ........................................ 39 

3.4.4.2 Glass Accumulation in the Melter ............................. 39 

3.4.4.3 Offgas Plugging ........................................... 40 

3.5 High-Level Waste Vitrification Plant, Mayak, Russia ............................. 40 

3.5.1 Waste Pretreatment and Feed Preparation ............................. 40 

3.5.2 Vitrification .................................................... 40 

3.5.3 Offgas Treatment System .......................................... 41 

3.5.4 Process Issues and Upsets ......................................... 41 

4 Induction Melting High-level Waste Processing Plants ................................. 43 

4.1 Marcoule and La Hague R7/T7 Vitrification Plants, France ........................ 43 

4.1.1 Waste Pretreatment and Feed Preparation ............................. 43 

4.1.2 Vitrification .................................................... 46 

4.1.3 Offgas Treatment System .......................................... 48 

4.1.4 Process Issues and Upsets ......................................... 48 

4.2 Waste Vitrification Plant, Sellafield, United Kingdom ............................ 48 

4.2.1 Waste Pretreatment and Feed Preparation ............................. 49 

vii NUREG/CR-6666
CONTENTS (cont'd) 

Page 

4.2.2 Vitrification .................................................... 49 

4.2.3 Offgas Treatment System .......................................... 50 

4.2.4 Process Issues and Upsets .......................................... 50 

4.2.4.1 M elter Failures ........................................... 50 

4.2.4.2 Failures of Cranes and Manipulators ........................... 51 

4.2.4.3 Shield Door Interlock Failure ................................ 51 

4.3 Waste Immobilization Plant, Tarapur India ..................................... 51 

5 Joule-Heated, Mixed Low-level Radioactive and Hazardous Waste Plant ................... 53 

5.1 M-Area Vitrification Plant at Savannah River Site, South Carolina .................. 53 

5.1.1 Waste Pretreatment and Feed Preparation ............................. 53 

5.1.2 Vitrification .................................................... 53 

5.1.3 Offgas Treatment System .......................................... 55 

5.1.4 Process Issues and Upsets ......................................... 56 

6 High-Temperature, Joule-Heated Non-Radioactive Mixed Waste Plants .................... 57 

6.1 Fernald Silo Waste Vitrification System, Fernald, Ohio ........................... 57 

6.1.1 Waste Pretreatment and Feed Preparation ............................. 57 

6.1.2 V itrification .................................................... 57 

6.1.3 Offgas Treatment System .......................................... 58 

6.1.4 Process Issues and Upsets ......................................... 58 

6.1.4.1 Separation Wall Between the Chambers ........................ 58 

6.1.4.2 Auto Discharging of Glass from the Melter ..................... 61

NUREG/CR-6666 viii
CONTENTS (cont'd) 

Page 

6.1.4.3 Corrosion of MoSi2 Components ............................. 61 

6.1.4.4 Foaming Events in the Melter ................................ 61 

6.2 Transportable Vitrification System ........................................... 61 

6.2.1 Feed Preparation Module .......................................... 62 

6.2.2 Vitrification Module .............................................. 62 

6.2.3 Offgas Treatment System .......................................... 62 

6.2.4 Process Issues and Upsets ......................................... 64 

7 Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience ...... 65 

7.1 Envitco, Inc . ............................................................ 65 

7.1.1 Vitrification Process .............................................. 65 

7.1.2 Test Results .................................................... 67 

7.1.3 Technical Issues ................................................. 67 

7.2 Vectra Technologies, Inc .................................................... 68 

7.2.1 Vitrification Process .............................................. 68 

7.2.2 Test Results .................................................... 68 

7.2.3 Technical Issues ................................................. 70 

7.3 Penberthy Electromelt International, Inc ........................................ 70 

7.3.1 Vitrification Process .............................................. 70 

7.3.2 Test Results .................................................... 72 

7.3.3 Technical Issues ................................................. 72 

ix NUREG/CR-6666
CONTENTS (cont'd) 

Page 

7.4 GTS Duratek, Inc . ........................................................ 72 

7.4.1 Vitrification Process .............................................. 72 

7.4.2 Test Results .................................................... 73 

7.4.3 Technical Issues ................................................. 74 

7.5 U.S. Bureau of Mines, Albany Research Center ................................. 74 

7.5.1 Vitrification Process .............................................. 74 

7.5.2 Test Results .................................................... 74 

7.5.3 Technical Issues ................................................. 75 

7.6 Westinghouse Science and Technology Center .................................. 76 

7.6.1 Vitrification Process .............................................. 76 

7.6.2 Test Results .................................................... 76 

7.6.3 Technical Issues ................................................. 78 

7.7 Babcock & Wilcox, Alliance Research Center .................................. 79 

7.7.1 Vitrification Process .............................................. 79 

7.7.2 Test Results .................................................... 79 

7.7.3 Technical Issues ................................................. 79 

7.8 Recommendations of the Evaluation Board ..................................... 81 

8 Mixed Waste Solidification Technology Experience ................................... 83 

8.1 Plasma Hearth Process ..................................................... 83 

8.2 Plasma Arc Centrifugal Treatment System ..................................... 85 

8.3 High-temperature, Joule-Heated Melter ........................................ 86 

NUREG/CR-6666 x
CONTENTS (cont'd) 

Page 

8.4 Direct Current Graphite Arc Melter ........................................... 86 

8.5 Alternating Current Arc Melter .............................................. 87 

8.6 Hybrid Plasma-Induction, Cold-Crucible Melter ................................. 87 

8.7 M icrowave M elter ........................................................ 87 

8.8 Quantum-Catalytic Extraction Process ......................................... 88 

8.9 Vortec Combustion and Melting System .......... ....................... 88 

8.10. In Situ Vitrification ........................................................ 89 

8.11 Russian High-Frequency Induction Melter ..................................... 91 

8.12 Transportable Vitrification System ........................................... 91 

8.13 Vitreous Ceramics ........................................................ 91 

9 Research and Test Melter Experience ............................................... 93 

9.1 Clemson University Vitrification Research Laboratory ............................ 93 

9.1.1 Envitco EV-16 Melter ............................................ 94 

9.1.2 Stir-M elter W V-0.25 ............................................. 94 

9.1.3 Electro-Pyrolysis, Inc., DC Arc Melter ............................... 96 

9.2 Vitreous State Laboratory at the Catholic University of America .................... 96 

9.2.1 High-Temperature, Joule-Heated Minimelter System .................... 96 

9.2.2 Duramelter' 100 Melter ........................................... 97 

9.2.3 Duramelter® 1000 (Asbestos Melter) ................................. 99 

9.3 Pacific Northwest National Laboratories ...................................... 101 

9.3.1 Radioactive Liquid-Fed Ceramic Melter ............................. 102 

xi NUREG/CR-6666
CONTENTS (cont'd) 

Page 

9.3.2 Liquid-Fed Ceramic Melter ....................................... 106 

9.3.3 Pilot-Scale Ceramic Melter ....................................... 106 

10 Pilot-scale Vitrification Systems for High-Level Waste Plants ........................... 107 

10.1 Scale Vitrification System at the West Valley Demonstration Project ............... 107 

10.1.1 Feed Preparation System ......................................... 109 

10.1.2 Vitrification System ............................................. 110 

10.1.3 Offgas Treatment System ................................. 111 

10.1.4 Process Issues and Upsets ................................. 111 

10.2 Integrated Defense Waste Processing Facility Melter System at the 

Savannah River Site ...................................................... 112 

10.2.1 Feed Preparation System ......................................... 112 

10.2.2 Vitrification System ..................................... 112 

10.2.3 Offgas Treatment System ................................. 114 

10.2.4 Process Issues and Upsets ................................. 114 

11 Emerging Solidification Technologies .............................................. 117 

11.1 Cold-Crucible Melting Technology .......................................... 117 

11.2 Hybrid, Plasma-Induction, Cold-Crucible Melter ............................... 119 

11.3 Submerged Combustion Melter ............................................. 120 

12 Non-Vitrification Solidification Process Experience ................................... 121 

12.1 Synroc Demonstration Plant, Australia ....................................... 121 

12.2 Quantum-Catalytic Extraction Process ........................................ 124 

13 Technological Assessments ............................................ 127 

NUREG/CR-6666 xii
CONTENTS (cont'd)

13.1 Joule-Heated Inconel° Melters ............. 

13.2 High-Temperature, Joule-Heated Melters ...... 

13.3 Induction Melting Technology ............. 

13.4 Cold-Crucible Induction Melting Technology ... 

13.5 Plasma Melting Technology ............... 

13.6 Combustion Melting Technology ........... 

13.7 Microwave Melting Technology ............ 

13.8 Molten Metal Technology ................ 

13.9 Synroc Technology .................... 

14 Process System Design Considerations ............ 

14.1 Tank Farms and Feed Preparation ........... 

14.1.1 Waste Transfer Lines ............ 

14.1.2 Waste Transfer and Mixing ........ 

14.1.3 Waste Homogeneity ............. 

14.1.4 Waste Pretreatment ............. 

14.1.5 Feed Preparation for Solidification ... 

14.1.6 Selection of Reducing Agents ...... 

14.2 Waste Solidification and Storage ............ 

14.2.1 Chemical Reactions ............. 

14.2.2 Glass Redox .................. 

14.2.3 Conductive Sludges and Noble Metals

NUREG/CR-6666

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. 134 

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* 134 

* 135 

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. 136 

. 139 

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Xlll
CONTENTS (cont'd) 

Page 

14.2.4 Glass Properties ....................................... 141 

14.2.4.1 Viscosity ..................................... 141 

14.2.4.2 Electrical Resistivity ............................. 141 

14.2.4.3 Liquidous Temperature ............................ 142 

14.2.4.4 Phase Separation ................................ 142 

14.3 Offgas Treatment and Release ..................................... 142 

14.4 Remote Operations and Human Factors ............................... 143 

15 Summary and Recommendations ........................................ 145 

16 References ....................................................... 151 

Appendices 

A Primer on Glass Production ..................................................... A-I 

B G lossary ..................................................................... B-1 

Figures 

3.1 The West Valley Demonstration Project Vitrification Process Flowsheet 

(Jain and Barnes, 1997) ........................................................ 9 

3.2 Sectional View of the West Valley Demonstration Project Slurry-Fed 

Ceramic M elter (Carl et al., 1990) ............................................... 13 

3.3 The West Valley Demonstration Project Integrated Offgas Treatment System 

(V ance, 1991) ............................................................... 15 

3.4 High-Level Waste Evaporation Process Flowsheet (Savannah River Operations Office, 1997) 19 

3.5 Sludge and Salt Pretreatment Process Flowsheet (Savannah River Operations Office, 1997).. 21 

3.6 Defense Waste Processing Facility Vitrification Process Flowsheet (Pearson, 199 1) ........ 22

NUREG/CR-6666 xiv
Figures (cont'd) 

3.7 Sectional View of the Defense Waste Processing Facility Vitrification Melter 

(W eisman et al., 1988) ........................................................ 23 

3.8 Defense Waste Processing Facility Melter Offgas Treatment System 

(W eisman et al., 1988) ........................................................ 25 

3.9 Pamela Vitrification Process Flowsheet (Hohlein et al., 1986) ......................... 30 

3.10 Sectional View of the Joule-Heated Ceramic Melter at the Pamela Facility 

(Hohlein et al., 1986) ......................................................... 32 

3.11 Offgas Treatment System at the Pamela Facility (Hohlein et al., 1986) ................... 34 

3.12 Tokai Vitrification Plant Process Flowsheet (Uematsu, 1986) .......................... 37 

3.13 Three-Dimensional View of the Tokai Vitrification Plant Joule-heated Melter 

(Uem atsu, 1986) ............................................................. 38 

3.14 Tokai Vitrification Plant Offgas Treatment System (Uematsu, 1986) .................... 39 

4.1 La Hague/marcoule Vitrification Process Flowsheet (Liberge et al., 1998) ................ 45 

4.2 La Hague/marcoule Vitrification Induction Melter (Liberge et al., 1998) ................. 47 

4.3 Process Diagram for the Sellafield Waste Vitrification Plant in the United Kingdom 

(Elsden and W oodall, 1988) .................................................... 49 

5.1 Mixing Strategy for the M-Area Mixed Waste at the Savannah River Site 

(Bowan and Brandys, 1995) .................... ............................... 54 

5.2 The Savannah River Site M-Area Mixed Waste Process Flowsheet 

(Bowan and Brandys, 1995) .................................................... 56 

6.1 Top View of the Duramelter I1000-HT Melter at the Fernald Site (FERMCO, 1997) ....... 58 

6.2 Back View of the Duramelter® 1000-HT Melter Showing Bottom Drain Tubes 

(FERM CO, 1997) ............................................................ 59 

6.3 Side View of the Duramelter® 1000-HT Melter Showing Bottom Bubblers 

(FERM CO, 1997) ............................................................ 60 

6.4 Process Flow Diagram for the Transportable Vitrification System (Zamecnik et al., 1998)... 63 

7.1 Envitco Cold-Cap, EV-16 Melter for Hanford Low-Level Waste (Wilson, 1996a) ......... 66

XV NUREG/CR-6666
Figures (cont'd) 

7.2 Vectra Vitrification Process Flowsheet for Hanford Low-Level Waste (Wilson, 1996a) ..... 69 

7.3 Penberthy Electromelt International, Inc. Vitrification Process Flowsheet for 

Hanford Low-Level Waste (Wilson, 1996a) ....................................... 71 

7.4 Duratek® Diagram of the Vitrification System for Hanford Low-Level Waste 

(W ilson, 1996a) ............................................................. 73 

7.5 Feed Preparation and Melter Schematic for Carbon Electrode Melter System 

(W ilson, 1996a) ............................................................. 75 

7.6 Westinghouse Science and Technology Center Vitrification Process Flowsheet for 

Hanford Low-Level Waste (Wilson, 1996a) ....................................... 77 

7.7 Westinghouse Science and Technology Center Marc' 11 Plasma Torch and Melter 

System (W ilson, 1996a) ....................................................... 78 

7.8 Babcock & Wilcox Vitrification System for Hanford Low-Level Waste (Wilson, 1996a) .... 80 

8.1 Plasma Hearth Process System Flowsheet (Science Application 

International Corporation, 1996) ................................................ 84 

8.2 Plasma Arc Centrifugal Treatment System Flowsheet (Whitworth and Filius, 1995) ........ 85 

8.3 Vortec Combustion and Melting System Flowsheet (Hnat et al., 1994) .................. 89 

9.1 Stir-Melter WV-0.25 at Clemson University (Marra et al., 1996a) ...................... 95 

9.2 Duramelter• 100 Melter Schematic (Bowan et al., 1994a) ............................. 98 

9.3 Duramelter® 100 Process Flowsheet (Matlack et al., 1995) ............................ 99 

9.4 Duramelter® 1000 Melter (Bowan et al., 1994b) ................................... 100 

9.5 Duramelter® 1000 Process Flowsheet for Asbestos Processing (Bowan et al., 1994b) ...... 101 

9.6 Process Flowsheet for the Radioactive Liquid-Fed Ceramic Melter at 

Pacific Northwest National Laboratories (Dierks et al., 1987) ......................... 103 

9.7 Sectional View of the Radioactive Liquid-Fed Ceramic Melter System 

(D ierks et al., 1987) ......................................................... 104 

10.1 Scale Vitrification System III Vitrification Process Schematic (Jain et al., 1997) .......... 108 

10.2 Integrated Defense Waste Processing Facility Melter System and Feed Preparation 

Flowsheet (Ritter et al., 1991) .................................................. 113 

NUREG/CR-6666 xvi
Figures (cont'd) 

10.3 Integrated Defense Waste Processing Facility Melter System and Offgas Flowsheet 

(Ritter et al., 1991) .......................................................... 113 

10.4 Integrated Defense Waste Processing Facility Melter System (Hutson, 1994) ............ 114 

11.1 Cold-Crucible, High-Frequency Induction Melter Schematic (Ladirat et al., 1995) ........ 118 

11.2 Typical Temperature Distribution in a Cold-Crucible Induction Melting 

(Jouan et al., 1996a) ......................................................... 118 

11.3 Hybrid Plasma/induction Cold-Crucible Melting System 

(Schumacher and Bickford, 1995) .............................................. 119 

11.4 Submerged Combustion Melter Schematic (Olabin et al., 1996) ....................... 120 

12.1 Synroc Process Flowsheet (Ringwood et al., 1988) ................................. 122 

12.2 Powder Mixing and Filling Process During Synroc Manufacture (Ringwood et al., 1988)... 123 

12.3 400-mm Diameter Synroc Bellows Before and after Hot Pressing (Ringwood et al., 1988).. 124 

Tables 

3.1 West Valley Demonstration Project Target Glass Composition (Jain and Barnes, 1997) ..... 11 

3.2 History of Vitrification Melter Development in Germany/Belgium (Elliott, 1994) .......... 29 

3.3 Composition of the Glass Frit and the Low-Enriched Waste Concentrate Glass Produced 

in the Pamela Plant (Hohlein et al., 1986) ......................................... 30 

3.4 History of Vitrification Melter Development in Japan (Elliott, 1994) .................... 36 

4.1 Operational Summary as of October 1997 for High-Level Liquid Waste Vitrification 

Plants in France (Liberge et al., 1998) ............................................ 44 

5.1 Composition of the Savannah River Site M-Area Blended Waste Batches 

(Bowan and Brandys, 1995) .................................................... 55 

7.1 Technologies for Phase 1 Testing of Hanford Low-Level Waste (Wilson, 1996a) .......... 66 

9.1 Surrogate Studies Performed at Clemson University (Overcamp, 1995) .................. 93 

12.1 Composition of Synroc-C (Jostons, 1994) ........................................ 122 

14.1 Summary of Melter-Related Incidents ........................................... 137 

xvii NUREG/CR-6666
EXECUTIVE SUMMARY

The objective of this report is to provide the 

Nuclear Regulatory Commission (NRC) a 

current status of high-temperature solidification 

technologies that have been used or proposed 

throughout the world. The report includes a 

survey of technologies used or proposed in the 

waste management (chemical and nuclear 

materials) industries, and relevant information 

on commercial technologies used in the ceramic 

and glass industries. This report has been 

prepared to assist the NRC in: 

" determining whether sufficient information 

exists to assess safety considerations 

regarding the Hanford Tank Waste 

Remediation System (TWRS) 

" determining whether current regulatory 

guidelines are adequate for controlling 

implementation of the technology to solidify 

Hanford waste 

• identifying the existence of technical 

uncertainties 

"* assessing where future guidance may be 

warranted 

"* evaluating a "first-of-a-kind" process that is 

being proposed or utilized 

The survey of waste solidification processes 

used in the United States and abroad indicates 

that, although major accomplishments have 

occurred in the area of vitrification in the last 

20 yr, developments are continuing in other 

solidification technologies such as 

glass-ceramics, Synroc (synthetic rock), and 

more recently, molten metal processes. The 

technologies reviewed in this survey can be 

broadly classified into the following categories: 

Inconel®-based, joule-heated melters; 

high-temperature, joule-heated melters; 

induction melters; cold-crucible induction 

melters; plasma melters; combustion melters;

microwave melters; molten metal technology; 

and Synroc technology. The development of a 

successful solidification process requires a 

reasonable level of characterization of the 

wastes that need to be solidified before a 

technology can be selected. This selection 

should be based on the nature of the waste and 

the final waste form that will provide effective 

encapsulation. 

In chapters 3 and 4, information from high-level 

waste (HLW) vitrification experience at existing 

facilities, such as those at the West Valley 

Demonstration Project (WVDP) West Valley, 

New York; Defense Waste Processing Facility 

(DWPF), Savannah River Site (SRS), Aiken, 

South Carolina; Tokai Vitrification Facility 

(TVF) Japan; Waste Vitrification Plant (WVP) 

Sellafield (United Kingdom); Mayak 

Vitrification Plant (Russia); Waste 

Immobilization Plant (WIP), India; and the 

Marcoule and La Hague vitrification facilities in 

France, is provided for use in the development 

of the regulatory framework for the privatized 

Hanford facility. In addition, information from 

other sources such as experience from 

commercial glass and ceramic industries 

(chapter 2), mixed low-level radioactive, 

hazardous, and non-radioactive waste 

vitrification plants (chapters 5 and 6), Hanford 

low-level waste vitrification program 

(chapter 7), mixed waste solidification 

technology development program (chapter 8), 

research-scale vitrification plants (chapter 9), 

pilot-scale vitrification plants (chapter 10), 

emerging solidification technologies 

(chapter 11), and non-vitrification technologies 

(chapter 12) is provided for use. The key 

elements of the solidification processes, 

including a description of feed preparation, 

melting, and offgas treatment systems for each 

technology/operating plant, and process upsets 

and events leading to incidents and accidents, 

are reviewed in each chapter.

xix NUREG/CR-6666
Executive Summary

In chapter 13, technological assessments, in 

terms of their strengths and weaknesses, are 

provided. The joule-heated Inconel® melter 

technology and induction melting technology 

are the two most widely accepted and mature 

technologies for vitrifying HLW. Both 

technologies provide extremely low volatility 

for alkalis, boron, and radioactive components 

such as technetium and cesium, and 

homogeneous products due to long residence 

time in the melter. While the size of the 

joule-heated melter can be increased 

significantly (Mayak Vitrification Plant, Russia) 

to meet production rate requirements, the size of 

the induction melter cannot be increased due to 

the attenuation of induction frequencies in 

large-diameter crucibles. In addition, life of a 

joule-heated melter ranges from 3 to 5 yr, while 

the melting crucible for the induction melting is 

considered a disposable item with replacement 

time ranging from 1,500 to 4,000 hr. Due to the 

design of the induction melting system, only 

calcined feeds can be processed in the melter 

while a joule-heated melter can easily adopt to 

wet or dry feeding. Other technologies that are 

discussed in the report are either in the 

development stage or have never demonstrated 

solidification of radioactive wastes. 

In chapter 14, a review of potential safety issues 

that could be encountered during processing of 

Hanford wastes is presented. Safety issues 

associated with the key elements of the 

solidification processes such as feed preparation, 

solidification, and offgas treatment systems are 

reviewed. Based on the review of the 

solidification technologies, technological

assessments, and evaluation of the process and 

operational safety issues, a number of areas 

associated with the solidification options for 

TWRS activities that will require evaluation 

have been identified. These include: 

"• Explosion hazard due to flammable gas 

(hydrogen and ammonia) generation 

"• Criticality hazard from plugging and 

accumulation of radionuclides in transfer 

and offgas pipes 

"• Uncontrolled chemical reactions, leading to 

radionuclide release, due to mixing of 

incompatible wastes and chemicals 

"• Corrosion of melter components, leading to 

melter failure, due to processing of wastes 

containing corrosive chemicals 

"* Gap formation in the melter, due to thermal 

stresses and thermal shock during heatup 

and cool-down cycles 

" Precipitation of metals, leading to electrical 

shorting of the melter, due to improper 

redox control methodology or settling of 

noble metals 

" Failure of the process components, leading 

to radionuclide release, due to 

incompatibility of the 

materials-of-construction with waste and 

feed chemistry

NUREG/CR-6666 XX
ACKNOWLEDGMENTS

This report was prepared to document work 

performed by the Center for Nuclear Waste 

Regulatory Analyses (CNWRA) for the Nuclear 

Regulatory Commission (NRC) under Contract 

No. NRC-02-97-009. The activities reported 

here were performed on behalf of the NRC 

Office of Nuclear Material Safety and 

Safeguards (NMSS), Division of Fuel Cycle 

Safety and Safeguards (FCSS). The report is an 

independent product of the CNWRA and does

not necessarily reflect the views or regulatory 

position of the NRC. 

The author wishes to acknowledge the technical 

review performed by D. Daruwalla, 

programmatic review by W. Patrick, and the 

editorial review by C. Gray and B. Long. The 

assistance from J. Gonzalez in the preparation of 

the document is also acknowledged.

xxi NUREGICR-6666
% TS 

AVM 

AZS 

B&W 

CCM 

CEP 

CFMT 

CMM 

CMS 

CNWRA 

CRV 

CUA 

DC 

DNFSB 

DOE 

DST 

DWPF 

EPA 

EPI

ACRONYMS 

Percent of Total Solids FACTS 

Atelier de Vitrification de Marcoule 

(Marcoule Vitrification Facility), FHT 

Marcoule, France 

FRG 

Alumina-Zirconia-Silicate 

GTAW 

Babcock & Wilcox 

HEME 

Cold-Crucible Melter 

HEPA 

Catalytic Extraction Process 

HEWC 

Concentrator Feed Makeup Tank 

HLLW 

Canister Manipulating Mechanism 

HLW 

Combustion and Melting System 

ICC 

Center for Nuclear Waste 

Regulatory Analyses IDMS 

Counter-Rotating Vortex IFO 

Catholic University of America ILDS 

Direct Current INEEL 

Defense Nuclear Facilities Safety 

Board ISPV 

U.S. Department of Energy ISV 

Double-Shell Tank ITP 

Defense Waste Processing Facility IWDV 

U.S. Environmental Protection 

Agency LEWC 

Electro-Pyrolysis, Inc. LFCM 

LFL

NUREG/CR-6666

Functional and Checkout Testing of 

Systems 

Feed Hold Tank 

Federal Republic of Germany 

Gas Tungsten Arc Welder 

High-Efficiency Mist Eliminator 

High-Efficiency Particulate Air 

High-Enriched Waste Concentrate 

High-Level Liquid Waste 

High-Level Waste 

Inner Canister Closure 

Integrated DWPF Melter System 

Index of Feed Oxidation 

Infrared Level Detection System 

Idaho National Engineering 

Laboratory 

In Situ Plasma Vitrification 

In Situ Vitrification 

In-tank Precipitation Process 

In Situ Waste Destruction and 

Vitrification 

Low-Enriched Waste Concentrate 

Liquid-Fed Ceramic Melter 

Lower flammability limit

°°.i
Acronyms

LLW Low-Level Radioactive Waste 

LSFM Large Slurry-Fed Melter 

MAWS Minimum Additive Waste 

Stabilization 

MFHT Melter Feed Hold Tank 

MFT Melter Feed Tank 

MIT Massachusetts Institute of 

Technology 

MMT Molten Metal Technology, Inc. 

MSM Master Slave Manipulators 

MTRU Mixed Transuranic 

MTU Metric Ton Uranium 

MWFA Mixed Waste Focus Area 

NRC Nuclear Regulatory Commission 

OGCT Offgas Condensate Tank 

OU-4 Operable Unit 4 

ORNL Oak Ridge National Laboratory 

PACT Plasma Arc Centrifugal Treatment 

System 

PCCS Product Composition Control 

System 

PCT Product Consistency Test 

PEI Penberthy Electromelt 

International, Inc. 

PHA Precipitate Hydrolysis Aqueous

SAS 

SBS 

SCR 

SDP 

SFCM 

SME 

SMT

Steam Atomizing Scrubber 

Submerged Bed Scrubber 

Selective Catalytic Reduction 

Synroc Demonstration Plant 

Slurry-Fed Ceramic Melter 

Slurry Mix Evaporator 

Slurry Mix Tank

NUREG/CR-6666

PHP Plasma Hearth Process 

PICCM Plasma-Induction Cold-Crucible 

Melter 

PNC Power Reactor and Nuclear Fuel 

Development Corporation 

PNNL Pacific Northwest National 

Laboratories [originally called 

Pacific Northwest Laboratories 

(PNL)] 

PSCM Pilot-Scale Ceramic Melter 

PUREX Plutonium-Uranium 

Recovery Extraction 

Q-CEP Quantum-Catalytic Extraction 

Process 

RASP Reactive Additive Stabilization 

Process 

RFP Rocky Flats Plant 

RLFCM Radioactive Liquid-Fed Ceramic 

Melter 

SAIC Science Applications International 

Corporation

xxiv
SRAT Sludge Receipt and Adjustment 

Tank 

SRS Savannah River Site 

SST Single-Shell Tank 

STAR Science and Technology 

Applications Research 

SVS-III Scale Vitrification System III 

Synroc Synthetic Rock 

TECO Toledo Engineering Company 

THOREX Thorium Recovery Extraction 

TOC Total Organic Carbon 

TPB Tetraphenyl Borate 

TRM Telerobotic Manipulator 

TRP Tokai Reprocessing Plant 

TRU Transuranic Waste 

TS Fraction of Total Solids 

TVP Tokai Vitrification Plant

TVS 

TWRS 

USBM 

VF 

VPPS 

VPS 

VRL 

VSL 

WHC 

WIP 

WQR 

WSTC 

WVDP 

WVP

Acronyms 

Transportable Vitrification System 

Tank Waste Remediation System 

U.S. Bureau of Mines 

Vitrification Facility 

Fernald Vitrification Pilot Plant 

System 

Vitrified Product Store 

Vitrification Research Laboratory 

Vitreous State Laboratory 

Westinghouse Hanford Company 

Waste Immobilization Plant 

Waste Form Qualification Report 

Westinghouse Science and 

Technology Center 

West Valley Demonstration Project 

Waste Vitrification Plant

XXV NUREG/CR-6666
REGISTERED TRADEMARKS 

Bento Grout is a registered trademark of Inframetrics® 760 is a registered trademark of 

AMCOL International Corporation, Arlington, Inframetrics, Inc., Billerica, Massachusetts. 

Illinois. 

Marc is a registered trademark of 

Chemineer9 is a registered trademark of Westinghouse, Inc., Pittsburgh, Pennsylvania. 

Robbins & Myers, Inc., Dayton, Ohio. 

Monofrax® is a registered trademark of the 

Duratek® is a registered trademark of GTS Carborundum Company, Falconer, New York. 

Duratek, Inc., Columbia, Maryland 

NC-300® is a registered trademark of the Norton 

Duramelter is a registered trademark of GTS Company, Worchester, Massachusetts. 

Duratek, Inc., Columbia, Maryland. 

Vac-U-Max® is a registered trademark of 

Hastelloy® is a registered trademark of Haynes Vac-U-Max, Belleville, New Jersey. 

International, Inc., Kokomo, Indiana. 

Zirmul® is a registered trademark of North 

Inconel® is a registered trademark of Inco American Refractories Co., Cleveland, Ohio. 

Alloys International, Inc., Huntington, 

West Virginia.

xxvii NUREG/CR-6666
1 INTRODUCTION

The U.S. Department of Energy (DOE) 

established the Tank Waste Remediation System 

(TWRS) program at the Hanford site in 1991 to 

manage the maintenance and cleanup of 

high-level radioactive waste contained in 177 

aging underground storage tanks. The DOE has 

agreed to commitments and milestones for 

remediation of the waste tanks under the 

Hanford Federal Facilities Agreement and 

Consent Order of 1989 (Ecology, 1994), also 

known as the Tri-Party Agreement. To 

accomplish the TWRS requirements, the DOE 

plans to privatize the waste treatment and 

immobilization operations. The TWRS 

privatization is divided into two phases, a 

proof-of-concept or demonstration phase 

(Phase I) and a full-scale-operations phase 

(Phase II). A Memorandum of Understanding 

has been signed between the DOE and the 

Nuclear Regulatory Commission (NRC)' for 

Phase I activities, which provides for the NRC

to acquire sufficient knowledge of the physical 

and operational situation at the Hanford waste 

tanks and processes involved in Phase I 

activities to (i) assist the DOE in performing 

reviews in a manner consistent with the NRC 

regulatory approach, and (ii) be prepared to 

develop an effective regulatory program for the 

possible licensing of DOE contractor-owned and 

contractor-operated facilities during Phase II. 

The objective of this report is to provide NRC 

staff an overview and current status of 

high-temperature solidification technologies that 

have been used, tested, or proposed throughout 

the world. The report includes a survey of 

technologies used or proposed in the waste 

management (chemical and nuclear materials) 

industries, and includes a survey of commercial 

technologies in the ceramic and glass industries 

that are relevant to high-level radioactive waste 

(HLW). Information presented in this report is 

intended to assist the NRC in conducting safety 

assessments and developing potential regulatory 

requirements for the Hanford TWRS and 

subsequent solidification operations.

IMemorandum of Understanding Between the Nuclear 

Regulatory Commission and the U.S. Department of 

Energy, January 29, 1997, Federal Register, V. 62, No. 52, 

12861. March 18, 1997.

I NUREG/CR-6666
2 RELEVANT ASPECTS OF COMMERCIAL TECHNOLOGY

The glass, ceramic, and metals industries have 

been in existence for a long time. Consequently, 

the technologies used in these industries for 

manufacturing glass are well developed for 

manufacturing traditional products. There are 

thousands of operating plants around the world, 

and a huge body of information is available in 

the literature. In fact, the development of the 

vitrification technology for HLW depended 

largely on input from the commercial glass 

industry. The total glass produced in a float 

glass plant in one day is equivalent to processing 

the entire HLW inventory at West Valley in 30 

to 36 mo. While the ultimate purpose for the two 

products are dramatically different, there is a 

significant similarity in their conceptual design 

and processing behavior. In both traditional and 

nuclear waste applications, the process is 

designed around the melter/furnace/firing kilns, 

and in most cases, the performance of the melter 

or firing kilns determines the life of the process. 

Designing the melting systems to meet the 

process requirements and determining the 

operating limits for the process are the keys to a 

successful plant. The melter design and 

performance depend upon the melter operating 

temperatures, the selection of refractories to 

meet thermal expansion and thermal 

conductivity requirements, the compatibility of 

refractories with melter components and the 

melt, and glass properties such as corrosivity, 

electrical conductivity, and viscosity. In 

addition, supporting systems such as raw 

material (feed) preparation systems or offgas 

treatment systems also require compatibility 

evaluation for materials and processing 

requirements. 

A survey of the traditional industries is beyond 

the scope of this report, but a brief overview of 

parameters that influence the process designs are 

discussed in the following text. The melting 

chambers for glass or metals are designed to 

meet processing constraints. The contents of the

melting chambers could either be heated by 

thermal energy such as burning fossil fuels, or 

by electrical energy that could be dissipated by 

joule heating, microwave heating, plasma 

heating, or induction heating. Commercial glass 

melters are typically air/fuel fired rectangular 

melters lined with a thick layer of corrosion 

resistant refractories. Electric glass melters are 

used in the production of several specialty 

glasses such as lead glasses. The blast furnaces 

for the iron and steel industry have barrel 

shapes. The barrel shape provides for spalling 

resistance and improves integrity for the 

refractories by helping cracked refractories 

retain their position. Cement kilns are also 

cylindrical in design. 

A rectangular base for glass furnaces provides 

better conditioning of the glass. The depth of the 

glass furnaces depends upon the emissivity and 

thermal conductivity of the melt. If all 

parameters are fixed, dark glasses have a shorter 

depth compared to clear glasses, because of high 

absorption in dark glasses. Therefore, 

understanding product characteristics is 

important even before a process can be 

designed. The glass melting temperatures could 

range from 1,000 'C for certain glasses to 

2,200 °C for fused silica. A key element of 

furnace design is the selection of refractories to 

withstand required operating temperatures. 

Additional parameters that are a part of 

refractory selection criteria are operating 

conditions and the nature of the glass melt. 

The operating conditions include issues such as 

thermal cycling during production and thermal 

expansion of the melter during furnace start-up 

and cool-down. Generally, the refractories that 

exhibit excellent corrosion resistance are also 

excellent in terms of heat transfer, but poor in 

thermal shock resistance. The glass contact 

refractory is always the highest corrosion

resistant-material, but insulating refractories are

3 NUREG/CR-6666
Relevant Aspects of Commercial Technology 

used as a backup to minimize heat losses. 

Industry prefers to use a graded approach in 

their selection of refractories. The glass contact 

refractory, the most corrosion resistant and the 

least adequate in thermal insulation, is followed 

by a series of refractories with decreasing 

corrosion resistance and increasing thermal 

insulation. The last layer is composed of 

fiberboard, which does not have any corrosion 

resistance to glass melts, but its high porosity 

provides the best thermal insulation. There are 

two types of refractories that are widely used in 

the glass industry; alumina-zirconia-silicate 

(AZS) and high-chrome refractories. While 

high-chrome refractories have much better 

corrosion resistance than AZS refractories, glass 

manufacturers prefer to use AZS because 

corrosion of Cr20 3 from high-chrome 

refractories at times leaves a streak of coloration 

in products. Most of the melters designed for 

nuclear waste vitrification use high-chrome as a 

glass contact refractory. 

The thickness of the refractory provided in a 

melter usually depends on the design life of the 

melter, which is a function of operating 

conditions such as temperature and the corrosive 

nature of glass melts. The thicker the refractory, 

the longer the life of the melter. But the melter 

has components other than the refractories, such 

as electrodes and penetrations for bubblers and 

drains, that could fail before the refractories. An 

optimum design focuses on selecting 

components so that all parts corrode uniformly. 

The refractories in the melter are designed to 

provide uniform wear in the entire melter. In the 

area of higher wear, refractory blocks are made 

thicker, if possible, or made of better performing 

refractory. The selection of an inappropriate 

refractory will reduce the life of the melter. For 

example, the use of basic refractories such as 

MgO and dolomite for acidic melts will result in 

extensive degradation of the refractories. While 

MgO refractories are suitable for cement 

calciners, acidic refractories such as fused cast

AZS are preferred for manufacturing float and 

container glass. The selection of roof 

refractories requires special consideration, 

because it depends upon the nature of offgases 

generated from the decomposition reactions and 

the burning of fuel/air or fuel/C2 mixtures. Most 

of the melter roof also undergoes extensive 

temperature cycling. Therefore, the roof needs 

to be corrosion resistant to the offgas 

components as well as have a good thermal 

shock resistance. In all refractories, a better 

thermal shock resistance always compromises 

corrosion resistance or chemical attack, making 

selection of roof refractories a challenging task. 

The selection of refractory materials by itself is 

not sufficient for designing the melter. A 

compatibility evaluation with other components 

present in the melter is required. If the melter is 

electrically heated, electrode materials and their 

compatibility with the refractory need to be 

evaluated. If the electrode material wears faster 

than the refractory or if the refractory materials 

wear faster than the electrodes, the melter will 

have to be shut down prematurely. Other 

components that require further evaluation are 

the bubblers, thermocouples, and drains. 

After the selection of refractories and a heating 

source such as an air/fuel mixture or electricity 

is made, the most important design criterion that 

determines the furnace longevity is the thermal 

expansion of different components during 

heat-up, cool-down, or normal operating thermal 

cycles. The presence of different types and 

shapes of refractories makes this a challenging 

task. The failure of pour-spout and alumina 

inserts in the nozzles of the West Valley 

Demonstration Project (WVDP) melter and 

failure of the roof or formation of gaps between 

the refractory blocks in some of the old melters 

are a result of thermal stresses that were not 

relieved during heat-up. The failure of the 

separation block between melting chambers in 

the Fernald melter was attributed to the use of a

NUREG/CR-6666 4
poor thermal shock resistant refractory in an 

area of extensive thermal cycling. While the 

blocks required a high corrosion resistance, the 

material selection failed to account for poor 

thermal shock characteristics. These safety 

problems will be reviewed in greater detail in 

chapter 4. 

Finally, the melt properties determine whether 

the glass will process (i.e., melt and flow) as 

desired. In electric melters, the electrical 

conductivity of the melt is of prime concern. If 

the electrical conductivity is low, it will result in 

a high-voltage drop between the electrodes 

causing conduction within the refractory 

material, which will heat the refractories rather 

than the melt. If the electrical conductivity is 

high, it may cause the current required to heat 

the glass to exceed the recommended maximum 

current density for the melter electrodes, which 

will eventually damage the electrodes. 

Therefore, the selection of electrode size and 

type depends on the electrical conductivity of 

the glass melt. For certain waste types 

containing excessive amounts of metals or small 

amounts of ionic conductors, electric melters 

may not be suitable. 

The operating temperatures and corrosivity of 

the melt greatly influences the selection of 

electrodes. In the crystal glass industry 

(high-lead glass), molybdenum electrodes 

cannot be used for melting lead glass because 

lead oxide reacts with molybdenum to form lead 

metal and molybdenum oxide, which is volatile. 

This reaction would result in the destruction of 

the electrodes as well as precipitation of lead 

metal on the melter floor, resulting in premature 

melter failure. In the crystal glass industry, tin 

oxide electrodes are preferred. The failure of the 

melter at Fernald, to be discussed in detail in

Relevant Aspects of Commercial Technology 

section 6.1, was in fact a result of molydisilicide 

(MoSi 2) bubbler tubes reacting with the lead 

oxide in the waste glass. In most of the HLW 

plants, Inconel® Alloy 690 is used as an 

electrode material because the melting 

temperatures required for HLW vitrification are 

within the operating range for Inconel® 

Alloy 690. Lower operating temperatures allow 

minimizing volatilization of radioactive species. 

Melt properties such as viscosity and liquidous 

temperature are important for processing. The 

liquidous temperature in glasses is defined as the 

lowest temperature below which crystal 

formation occurs in glass. Liquidous 

temperature is an important property of the melt. 

If the temperature of the melt in the melter is 

below the liquidous temperature, crystal 

formation occurs in the melt. If the crystalline 

phase is conductive, it may result in formation 

of an alternate path for the current in the melter, 

making joule heating ineffective. Also, it may 

clog bottom drains and pour spouts making glass 

pouring impossible. The viscosity of glass is 

another important processing property. In glass 

manufacturing, if viscosity of the melt is off by 

a few percent, the entire process is shut down 

due to the problems in the forming processes. In 

addition, high viscosity will result not only in 

glass pouring problems but also in poor 

convective currents in the melter. Convective 

currents are essential to homogenize and 

completely dissolve the raw materials into glass. 

If viscosity is high, unmelted raw materials 

could be discharged into the forming process 

line. In nuclear waste vitrification, this may 

result in an inhomogeneous waste form and 

problems in glass pouring.

5 NUREG/CR-6666
3 JOULE-HEATED, HIGH-LEVEL RADIOACTIVE WASTE 

PROCESSING PLANTS

Initial research on the development of the 

vitrification process for HLW started in the early 

1960s. The first vitrification plant that was 

operational in the United States was 

commissioned for radioactive operations in 

December 1985 to produce 32 heat and radiation 

source canisters for the Federal Republic of 

Germany (FRG). This facility was shut down in 

March 1987. The melter was developed at the 

Pacific Northwest National Laboratories 

(PNNL) as part of a bilateral agreement between 

the Bundesminiterium fur Forschung und 

Technologie and the DOE. The first joule-heated 

melter plant, the Pamela vitrification facility in 

Mol, Belgium, was commissioned in 1985. In 

the United States, vitrification plants at the 

WVDP, West Valley, New York, and the 

Defense Waste Processing Facility (DWPF) at 

the Savannah River Site (SRS), Aiken, South 

Carolina, were commissioned in 1996. The 

Tokai Vitrification Plant (TVP) in Japan was 

commissioned in 1995 and radioactive 

operations at the Mayak vitrification facility 

(S. Ural) in Russia were started in 1986. Except 

for the Pamela and Mayak vitrification plants, 

all plants are currently operational. The Pamela 

plant concluded its operation, after 6 yr of 

radioactive operations, in September 1991 and 

the Mayak plant was shut down in 1997. The 

process flowsheet, including process issues for 

each of the above plants, is discussed in the 

following sections. 

3.1 West Valley Demonstration 

Project Vitrification Facility at 

West Valley 

In 1980, the United States Congress authorized 

establishment of the WVDP as a DOE HLW 

management project. Immobilization of the 

HLW into a stable and non-dispersible form 

using vitrification was successfully initiated at

the WVDP in July 1996. However, before the 

start of radioactive operations, one full-scale 

non-radioactive vitrification system and three 

scaled vitrification systems were operated to 

support the development of the radioactive 

vitrification process (Barnes and Jain, 1996). 

The full-scale vitrification system was operated 

for 5 yr, from December 1984 through 

December 1989, under the Functional and 

Checkout Testing of Systems (FACTS) 

campaign. In parallel, the scaled vitrification 

system was operated to develop redox control 

methodology. The second scale vitrification 

system was operated at the end of the full-scale 

non-radioactive testing in 1989. The third scale 

vitrification system (SVS-III) was operated in 

1995 to optimize and develop techniques to 

control radioactive operations. The SVS-III 

testing and operations are discussed in 

section 10.1. 

The WVDP HLW vitrification facility is a 

remotely operated facility designed using 

canyon remote technology. All major 

components are designed for fully remote 

replacement by using only the impact wrench 

and crane hook. Master slave manipulators 

(MSM) are provided to support activities such 

as sampling, canister closure, welding, and 

decontamination. As of August 15, 1997, 320 m3 

(84,500 gal.) of HLW has been vitrified into 130 

stable canistered waste forms (Hamel et al., 

1998). The HLW transferred from Tank 8D-2 is 

mixed with the glass-forming chemicals and fed 

to a joule-heated melter at 1,150 'C. The glass is 

then poured into stainless steel canisters. After 

the canisters have cooled, they are capped, 

welded, decontaminated, and stored on-site for 

eventual disposal. The description of the HLW 

vitrification process provided in the following 

sections has been extracted from Ploetz and 

May (1996), Firstenberg et al. (1995), West 

Valley Nuclear Services (1996), Barnes and

7 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

Jain (1996), Chapman and Drosjack (1988), Carl 

et al. (1990), Jain (1993), Jain and Barnes (1993, 

1997), and Hamel et al. (1998). 

3.1.1 Waste Pretreatment and Feed 

Preparation 

The HLW at the WVDP was generated by the 

commercial reprocessing of 640 MT of spent 

reactor fuel. At the inception of the WVDP, two 

types of wastes were stored in underground 

tanks. The majority, 2,270 m3 (600,000 gal.), 

was neutralized plutonium-uranium recovery 

extraction (PUREX) waste stored in the 

carbon-steel Tank 8D-2. Another 40 m3 

(10,560 gal.) of partially processed thorium 

recovery extraction (THOREX) waste was 

stored in stainless steel Tank 8D-4. The PUREX 

waste was washed to minimize the sodium and 

sulfate salts to be vitrified. The original 

supernatant and wash solutions were 

decontaminated using a zeolite ion-exchange 

process to remove the radioactive cesium 

(Cs-137). The process was conducted in Tank 

8D-1. The effluent was encapsulated in cement. 

Prior to the start of vitrification operations, the 

THOREX was transferred to Tank 8D-2, mixed 

with washed PUREX waste and the combined 

PUREX and THOREX waste was washed one 

or more times. The washed PUREX/THOREX 

waste was then blended with Cs-137 loaded 

zeolite. The volume of the waste at the start of 

vitrification operations was approximately 1,000 m3 (264,000 gal.). The vitrification 

process can be separated into four phases: 

(i) tailoring the waste slurry by adding 

glass-forming chemicals to achieve the desired 

glass composition, (ii) melting the waste slurry 

in a melter and casting the glass into canisters, 

(iii) closing the canistered waste-forms, and 

(iv) treating the offgas before releasing it to the 

stack. The overall vitrification process flowsheet

is shown in figure 3.1, and each phase is 

discussed in the following text. 

The initial step in the vitrification process is the 

pumping of a batch of HLW slurry from 

Tank 8D-2 to the concentrator feed makeup 

tank (CFMT) in the Vitrification Facility (VF). 

The CFMT is a 3-m-diameter (10-ft) and 

4.3-m-high (14.1-ft) HastelloyO Alloy C-22 tank 

with a nominal capacity of 20.8 mn3 (5,500 gal.). 

Half-pipe heat exchange jackets are included 

outside the tank on the sides and the bottom to 

provide both heating (steam) and cooling 

(water) to the tank. Since the CFMT undergoes 

much more thermal cycling and higher 

temperature excursions compared to other tanks, 

Hastelloy® Alloy C-22 was selected to provide 

better corrosion resistance from the waste 

components. To homogenize slurry, the CFMT 

has an agitator equipped with dual high

efficiency impellers. The HLW is transferred 

from Tank 8D-2 to the CFMT through an 

underground shielded trench. Approximately 

100 batch transfers may be required to process 

the HLW inventory in Tank 8D-2. The volume 

of waste required for each transfer is calculated 

based on the heel present in the CFMT, the 

volume of recycled solutions, and the maximum 

glass that could be made without exceeding tank 

volumes. Nominal HLW transfers have been 

from 7.6 m3 (2,000 gal.) to 10 mn3 (2,620 gal.) of 

waste from Tank 8D-2. After the transfer is 

complete, the contents of the CFMT are sampled 

and analyzed for composition, density, total 

solids, nitrates, and nitrites. The samples are 

obtained at a slurry sample station. An air 

displacement slurry pump transfers the slurry 

from the CFMT to the sample station. At the 

sample station, samples are collected in 15-cm3 

(0.9-in.3 ) glass vials and sent to the analytical 

laboratory by a pneumatic sample transfer 

system. While the samples are being analyzed, 

the contents of the CFMT are concentrated by 

evaporating water from the slurry. The amount

NUREG/CR-6666 8
--------------------------

Cesium Waste 

Supernatant 

E 813-2 

8D-4 PUREX WasteI 

THOREX 

Waste 

Waste Tank Form

MelterI 

FeedI 

HoldI 

TankI 

(MF HT) Feed Delivery 

Syte I Submer-gedT II

C a n is t e l i 

I I 

I II 

WasM TI eltnrter 

SStorage 

i I I Fa c t 

CansteIterm TunCniserl 

StorageCosr 

Vitrifiaionspctiont

.I.q 

0•

Figure 3.1. West Valley Demonstration Project Vitrification Process Flowsheet (Jain and Barnes, 1997) (Permission to use this 

copyrighted material is granted by WM Symposia, Inc., Tucson, Arizona)

0 

0

0

"O
Joule-Heated, High-Level Radioactive Waste Processing Plants

of HLW that could be transferred or the 

maximum glass that could be produced from a 

waste transfer depends on the concentration 

potential of the wastes in the CFMT. The 

concentration potential is enhanced in the CFMT 

by the addition of sodium metasilicate as a 

deflocculating agent (Firstenberg et al., 1995). 

Based on the chemical analysis of the CFMT 

contents, the total amount of glass-forming 

chemicals required to produce the WVDP target 

glass composition is calculated. The glass 

formers are prepared in a non-radioactive 

facility and transferred to the CFMT. The target 

borosilicate glass composition and its acceptable 

process limits for composition are shown in 

table 3.1. The waste-feed slurry is the final mix 

of the HLW and the glass-former slurries. The 

mix contains all chemical species needed to 

produce the target waste form. The chemical 

analysis of this slurry is the key hold-point for 

approval to transform the final slurry mix into 

glass. The Waste Form Qualification Report 

(WQR) (West Valley Nuclear Services, 1996) 

requires that the feed batch be of acceptable 

composition for making the waste form, should

In addition, WQR requires that, before the final 

mix is approved for making glass, the mean of 

the normalized release rates for B, Na, and Li in 

the feed (as estimated by the prediction 

equations based on chemical analysis) (West 

Valley Nuclear Services, 1996) is at least two 

standard deviations below the mean normalized 

release rates for B, Na, and Li in the 

Environmental Assessment glass using the 

product consistency test (PCT) (Jantzen et al., 

1992). The product consistency in a sample is 

measured by subjecting a known amount of a 

sample (particle size between 100 and 200 

mesh), with 10 times its mass of deionized water 

at 90 'C for 7 days in a closed stainless steel 

vessel. After 7 days, the leachate is filtered and 

analyzed for B, Na, and Li. 

The final step in feed preparation is to estimate 

the amount of reductant, usually sucrose, to be 

added to the feed. The reductant addition is 

necessary to provide proper redox conditions in 

the melter. The amount of sucrose needed is 

calculated from Total Organic Carbon (TOC) 

using the empirical relationship

TOC = NO3 (1 - TS)IFO

fall within the processing region shown in 

table 3.1. After the glass formers are transferred 

to the CFMT, the contents of the CFMT are 

sampled again and analyzed. If the composition 

of the feed falls outside the acceptance region, a 

new set of samples is taken and analyzed, and 

then the analyses are grouped with the previous 

set to redetermine the chemical composition of 

the slurry. Furthermore, if the batch is still not 

acceptable, the required amount of shim 

chemicals is added and the contents resampled 

and reanalyzed. All the batches processed to 

date were within the processing region of the 

target glass composition (Jain and Barnes, 

1997).

where Index of Feed Oxidation (IFO) is the 

index of feed oxidation, NO3 is the total nitrate 

concentration, TS is the fraction of total solids in 

the feed, and TOC is the total organic carbon 

concentration. A target IFO of 3.0 is used to 

estimate the amount of sucrose required to 

process the feed and ensure that the Fe2 ÷/Fe 3

1 

ratio remains between 0.01 and 0.20. 

Uncontrolled redox excursions could lead to 

major process upsets. If the melt becomes too 

oxidized (Fe2

l/Fe3÷ < 0.01), a stable foam layer 

develops at the glass surface, diminishing the 

glass production rate. Alternatively, if the glass 

melt becomes overly reduced (Fe2

+/Fe3 > 1), 

electrically conductive phases precipitate from 

the melt and interfere with the electrical current 

flow within the glass. The impacts of such

NUREG/CR-6666

(3-1)

10
Joule-Heated, High-Level Radioactive Waste Processing Plants

Table 3.1. West Valley Demonstration Project Target Glass Composition (Jain and Barnes, 1997) 

(Permission to use this copyrighted material is granted by WM Symposia, Inc., Tucson, Arizona) 

Lower Process Limit Target Composition Upper Process Limit 

Oxide (weight %) (weight %) (weight %) 

A120 3 5.43 6.00 6.57 

B 2 0 3 11.00 12.90 14.80 

CaO 0.36 0.48 0.55 

Fe2O3 10.20 12.00 13.80 

K20 4.37 5.00 5.63 

Li2O 3.25 3.71 4.17 

MgO 0.76 0.89 1.02 

MnO 0.70 0.82 0.94 

Na2O 7.00 8.00 9.00 

P20 5 1.02 1.20 1.38 

SiO2 38.70 41.00 43.20 

Th0 2 2.67 3.56 4.09 

TiO2 0.68 0.80 0.92 

U0 3 0.47 0.63 0.72 

ZrO2 1.12 1.32 1.52 

Others* na 1.70 na

process upsets have been further discussed in 

detail Barnes and Jain, 1996 and 

Jain et al.,1997). 

The waste-feed slurry containing sucrose is 

transferred to the melter feed hold tank (MFHT) 

by a steamjet and mixed with the heel of the 

previous batch. The MFHT is a 3-m (10-ft)

diameter and 3-m-high (10-ft) stainless steel 

tank with a 22-M3 (5,800-gal.) capacity, and the 

agitator is equipped with a dual high-efficiency 

impeller. The combined slurry is then fed to the 

melter. The feed rate depends on the melting 

characteristics of the feed. Higher total solids 

and sucrose concentrations allow higher feed 

rates, while higher water and nitrate

11 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

concentrations decrease feed rates in the melter. 

An optimum balance is maintained between total 

solids, nitrates, and sucrose to maximize the 

feed rate while maintaining the Fe2

+/Fe3+ ratio 

within the specified range. 

3.1.2 Vitrification 

The second phase is initiated by pumping the 

slurry from the MFHT to the melter. In the 

melter, the remaining water is evaporated from 

the slurry, forming a solid crusty layer on the 

surface (cold-cap), and the waste fuses with the 

glass-forming chemicals thereby producing the 

target waste-form composition. Figure 3.2 

shows a sectional view of the slurry-fed ceramic 

melter (SFCM). The SFCM is encased inside a 

water-cooled jacketed box. The internal surface 

of the box is constructed from Inconel® Alloy 

690 to provide corrosion resistance to molten 

glass or condensed salts from the melting 

process. All penetrations for measurements and 

service are made from the top of the melter. The 

interior of the melter assembly is a composite of 

various refractory materials. Like all 

joule-heated melters, high-chrome fused cast is 

used as a glass contact refractory to minimize 

corrosion from the molten glass. Between the 

glass contact refractory and the metal box, a 

series of refractories with increasing thermal 

insulation and decreasing corrosion resistance 

are used. The high-chrome refractory is backed 

up by Zirmul®, which is an AZS refractory. 

This, in turn, is backed up by castable bubble 

alumina and, finally, thermally insulated 

fiberboard is used. The fiberboard 

accommodates expansion from interior 

refractories on melter heat-up. The interior of 

the melter box is composed of three chambers: 

one melting and two discharge chambers. The 

two discharge chambers are provided for 

redundancy. The inner cavity of the melter is an 

irregular shaped prismoid similar to an inverted 

prism and contains three electrodes: two 

electrodes on the side and one at the bottom.

The electrodes are made of Inconel® Alloy 690 

and are air-cooled to keep the electrode 

temperature below 1,100 °C, to minimize 

corrosion. The floor and the side walls slope 

toward the bottom electrode, as illustrated in 

figure 3.2. The melter has 2.15 in2 (23.1 ft2) of 

glass pool surface area and contains about 860 L 

(227 gal.) of glass. The molten glass is 

maintained at operating temperatures by 

adjusting the current between the three 

electrodes. The sloped side wall design for this 

melter was selected primarily because of 

concerns over noble metals or conductive 

sludges precipitating from the molten glass. 

Temperature and glass level control in the 

melter is accomplished by two thermowells (an 

Inconel® Alloy 690 tube closed at one end) and a 

set of three Inconel® tubes acting as a bubbler 

set. Each thermowell contains nine 

thermocouples of various lengths, which are 

introduced in the melt chamber from the melter 

lid. These thermowells are replaced every 6 mo. 

The temperature profiles and the glass level 

obtained from these thermowells and bubbler 

tubes, along with power consumption and mass 

balances, are used to optimize the process and 

determine the need for adjusting feed rates and 

glass pour rates into canisters. 

When the glass reaches a predetermined level in 

the melter, it is poured into the canister by 

activating an airlift mechanism. The glass is 

pushed out of the melter by introducing air into 

the melter discharge passage. Air is introduced 

via a platinum air-lift lance, called a riser tube, 

which is inserted from the melter lid into the 

discharge passage. Air bubbles reduce the 

density of glass in the riser tube, causing the 

glass to flow from the melter into the discharge 

trough. The glass flows from the trough into the 

canister. The glass flow is controlled by the 

airflow rate in the riser. The discharge or 

overflow chamber is heated by silicon carbide 

radiant heaters to maintain the temperature of 

the melt and prevent it from thickening before

NUREG/CR-6666 12
Floo Electrode 

las- Floor 

z LOverflow CDischarge Chamber Passage 

((IQ 

011 

N Figure 3.2. Sectional View of the West Valley Demonstration Project Slurry-Fed Ceramic Melter (Carl et al., 1990) ON
Joule-Heated, High-Level Radioactive Waste Processing Plants

exiting the melter. Molten glass is periodically 

poured from the melter into a 3-in (10-ft) tall by 

0.6-m (2-ft) diameter stainless steel canister 

located under the discharge chamber. During 

each airlift, 150 to 200kg (330 to 440 lb) of 

glass is poured into the canister. Typically it 

takes about 12 to 15 airlifts to fill one canister. 

A full canister contains between 1,800 to 

2,000 kg (3,960 to 4,400 lb) of glass. The fill 

height is monitored using an infrared level 

detection system (ILDS). The ILDS consists of 

an Inframetrics® 760 imaging radiometer 

operating in the wavelength of 8-12 ym. The 

signal is displayed in the control room and 

tracks the fill level in the canister. In addition, 

load cells are used to monitor the weight of the 

canisters as the glass is poured. After a canister 

is full, the turntable on which the canister sits is 

rotated to bring a new empty canister under the 

discharge section and the turntable is sealed to 

the canister. The turntable structure is made of 

mostly 304L stainless steel. It consists of a 

stationary frame and a rotating frame. The 

rotating frame has positions for four canisters to 

move in a carousel fashion; the upper region of 

the turntable is sealed to the melter discharge 

section. 

The full canister is allowed to cool under natural 

convection. Once the canister is cool, it is 

removed from the turntable and moved to a 

welding station. At the welding station, the 

canisters are closed by welding on a lid. The 

welding technique is an automated, autogenous, 

pulsed-gas tungsten-arc welding process 

(Kocialski and Dinsmore, 1994). Following a 

visual quality inspection of the closure weld, the 

canisters are decontaminated using an acidic, 

cerium-based, chemical oxidation procedure. 

After decontamination, a smear test is performed 

on the canisters to assure contamination levels 

are below the specification level (West Valley 

Nuclear Services, 1996). Finally, the canistered 

waste-forms are stored on-site until they can be

shipped off-site to a federal repository for 

storage or disposal. 

3.1.3 Offgas Treatment System 

The WVDP offgas treatment system schematic 

is shown in figure 3.3. Basic components of the 

offgas treatment system located in-cell are a 

submerged bed scrubber (SBS), high-efficiency 

mist eliminator (HEME), and high-efficiency 

particulate air (HEPA) filter. The ex-cell 

components include another HEPA filter, a 

blower, and a NOQ abatement system. 

The offgas first passes through a film-cooler, 

which is included as a component of the melter 

itself, and is located at the offgas nozzle on the 

melter lid. This device consists of a louvered 

insert that supplies cool airflow along the inner 

surface of the offgas line, thus minimizing 

buildup of carry-over glass and condensed salts 

on the pipe walls. The device also includes an 

offgas reamer, which consists of a brush that 

rotates as well as travels down the film cooler, 

discharging condensed salt deposits back into 

the melter. The offgas from the film cooler goes 

to an SBS. The SBS is designed for cooling and 

condensing melter vapor emissions. The SBS is 

also used as an interim storage for condensed 

fluids and spent canister decontamination 

solutions. The SBS has a double-tank 

configuration; the inner tank is a scrubber 

packed bed filled with ceramic spheres and 

water. The offgases are drawn from the bottom 

of the tank by vacuum and percolated through 

the packed bed. This results in condensation of 

water vapor and removal of particulates from the 

gas. As the water is condensed, the inner tank 

overflows into an outer receiver tank. The offgas 

is cooled from 300-400 'C to 30-40 °C. Both 

the sludge collected in the inner tank and 

condensate in the outer tank are recycled into 

the feed preparation process. The offgas from 

the SBS goes to a mist eliminator that removes 

liquid entrained in the gas during scrubbing.

NUREG/CR--6666 14
Stack

In Cell Off-MCI 

VF Cell

Tank (MFMT) Empty HEPA ' 

: ~Housing 1 

HEME Filter 

supernatant Preheter Peheater 

Sliny-Fed Submerged Bed Mb HEPA #2 

Sludge CerarZo FSrer Mistlo (HEMit 

8D-2 Purex Waste (SFCM) (I BI) r 

(D 

Pos HeterHEPA#1I 

G las s; Produced I' .

Trench 0 

EmptV Canister CPO Interim Eiitralmnmefit F t 

Canister Turntable Closure & Storage Seperetor 

Storage Decontamination 

z 

Out of Coll co Off-Gas 

01-14 Bldg. -. 

ON Figure 3.3. The West Valley Demonstration Project Integrated Offgas Treatment System (Vance, 1991)
Joule-Heated, High-Level Radioactive Waste Processing Plants

After the mist eliminator, the offgas passes 

through the HEME, which removes more than 

95 percent of the particulates. The gases are then 

heated and prefiltered through the HEPA, which 

captures small radioactive particulates. The 

HEPA prefilter contains two filter elements, 

each of which removes greater than 99 percent 

of 0.3 gim particulates. Upon exiting the HEPA 

prefilter, the offgas, primarily air, water vapor, 

NO,, SO2, and a minimal amount of particulates 

is sent to the ex-cell portion of the offgas 

system. The ex-cell provides further filtration of 

any radioactive particulates that may have 

eluded capture by the in-cell system and 

destruction of NOx. 

Offgas exiting from the in-cell portion of the 

system is sent to another building, the 0 1-14 

building, through a pipe contained within an 

underground tunnel. Pipe insulation and an 

entrainment separator within the pipe are used to 

minimize or eliminate offgas condensation 

before it reaches the 0 1-14 building. At the 

01-14 building, the offgas is preheated to ensure 

it reaches the HEPA filters at a temperature 

above the dew point. HEPA filters remove more 

than 99.999 percent (decontamination factor of 

l x 10') of all the remaining particles. The offgas 

is then sent to the NO. abatement system. Three 

blowers are installed in parallel. One is used, 

and two are for backup. The blower maintains 

the vitrification system in negative pressure. The 

NO, abatement system consists of a heater, a 

catalytic reactor, and an ammonia injection 

system. The NO, is destroyed by heating the 

offgas and causing it to react with ammonia in 

the presence of a catalyst. The catalyst is 

NC-300®, a zeolite-based material proprietary to 

Norton Company. The reaction produces water 

vapor, nitrogen, and oxygen. The amount of NO 

and NOx in the offgas is continuously monitored 

by separate analyzers before and after it is sent 

to the catalytic reactor. Following the NO. 

destruction, the offgas is directed to the main 

plant stack for discharge.

3.1.4 Process Issues and Upsets 

3.1.4.1 Melter Nozzle Failure 

During the non-radioactive operations from 

December 1984 to December 1989, it was 

observed that several cylindrical penetrations, 

lined with Inconel® Alloy 690 tubes, in the lid 

were corroded due to a high-temperature 

sulfur/halide attack. These penetrations are used 

for inserting instrumentation to measure 

temperature, density, pressure, and glass levels 

in the melter. The penetrations are also used for 

feeding and exhausting the offgas from the 

melter. After the conclusion of the 

non-radioactive testing, several materials were 

evaluated for improved corrosion resistance. 

Based on the evaluation, the cylindrical 

penetrations in the lid were replaced by alumina 

tubes. During the initial cold startup of the 

melter, several alumina pieces were found 

floating on the melt surface. This observation 

led to the shutdown of the melter. The alumina 

nozzles were removed and replaced by a single 

composite uniflange with replaceable Inconel® 

Alloy 690 nozzle liners. Nozzle failure was 

attributed to insufficient thermal expansion 

allowance provided for the alumina nozzles 

during heatup of the melter. 

3.1.4.2 Melter Pour Spout Failure 

After nozzle repairs were completed, melter 

heatup was resumed. During September and 

October 1995, irregularities were noted with 

glass pours, including erratic flow and glass 

buildup at the bottom of the discharge cavity. 

Problems were also experienced with melter 

pressure fluctuation. The melter has two 

discharge sections: the west chamber is used for 

glass pouring, the east chamber is a backup. In 

November 1995, a large glass buildup was 

noticed in the west discharge chamber bottom as 

well as under the pour spout. Melter operations

NUREG/CR-6666 16
Joule-Heated, High-Level Radioactive Waste Processing Plants

were suspended and the melter was allowed to 

cool. Initial examination revealed that the trough 

(pour spout) was displaced from its original 

location. A detailed examination revealed that 

the seal welds that held the pour spout to the 

dam (vertical Inconel® Alloy 690 wall that 

separates-the melting chamber and discharge 

chamber) had failed. The movement of the 

trough from its original location was attributed 

to the transposition of a weld symbol in the 

engineering drawing that resulted in a missing 

weld at the dam and trough interface. This 

provided insufficient support to the seal welds to 

withstand the weight of the pour spout in 

addition to the stresses generated by the thermal 

expansion of the dam constrained between the 

walls of the melter. The corrective action plan 

included welding, reinforcement, and refractory 

modification in the discharge area. Weld repairs 

included three changes: increasing the weld size 

to provide additional reinforcement, adding a 

weld to the bottom of each trough, and using the 

weld procedure specifically developed to reduce 

distortion and stress during welding. 

Reinforcements were accomplished by 

providing additional gussets around the trough. 

Refractory modifications included the use of six 

keystone shaped bricks placed on top of 

rectangular bricks to provide greater structural 

support to the trough. The repairs were 

completed and melter heatup was resumed in 

February 1996. 

3.1.4.3 Formation of Glass Fibers 

During the early stages of the radioactive 

operations, formation of glass fibers in the 

discharge orifice led to the blockage of the 

discharge orifice. The incident was attributed to 

high airflow through the discharge orifice to the 

melter. The corrective action included 

installation of a flow-reducing orifice at the 

discharge orifice.

3.1A.4 Spread of Contamination 

In January 1997, during radioactive operations, 

contamination was observed in the operating 

aisle through an electrical jumper connecting the 

discharge heaters in the melter. At the time, this 

spread of contamination was attributed to a 

slightly positive pressure existing in the 

discharge chamber as a result of pressure 

fluctuations in the vitrification system. 

3.1.4.5 Melter Level Probes 

Melter level probes are considered consumable 

components and are designed for a 6-mo 

operating life. Continuous erosion and corrosion 

of the level probe tips by feed particles and glass 

melt, with time, provides a biased glass level 

reading in the melter. A weekly calibration of 

probes and adjustments were necessary to 

maintain proper glass level in the melter and to 

discharge required amounts of glass to the 

canister. 

3.1.4.6 Slurry Backflow 

In November 1996, during a backflushing 

operation of the slurry sample station, diluted 

HLW slurry was forced into the water line 

outside the shielded cell and activated the area 

radiation alarm. The incident was attributed to 

the lack of an approved procedure for the 

backflush operation. Corrective actions included 

implementation of vigorous review 

of procedures. 

3.2 Defense Waste Processing 

Facility at Savannah River Site 

The SRS has been storing radioactive residues 

from over 30 yr of reprocessing irradiated 

nuclear fuels for national defense. The inventory 

is approximately 130 million L (34.3 million 

gal.) stored in 51 underground carbon-steel

17 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

tanks (Marra et al., 1996b). The DWPF was the 

first high-level radioactive waste plant in the 

United States to start radioactive operations. 

However, before the start of radioactive 

operations, four scaled vitrification systems 

were operated to support the development of the 

vitrification process (Iverson, 1993). 

The first vitrification melter, S-1 941, was 

operated from August 1980 to September 1981. 

The S-1941 melter was a refractory-lined, 

water-cooled, cylindrical melter with four 

15.2 cm (6 in.) diameter Inconel® Alloy 690, 

water-cooled electrodes installed through the top 

head. Glass was poured into a canister by tilting 

the melter (similar to a teakettle). The campaign 

produced. 74,000 kg (162,800 lb) of glass from 

54 dry runs and 20 slurry runs. Analysis of the 

Inconel® Alloy 690 components concluded that 

the observed wear supported the premise of a 

2-yr operating life. The next melter was a large 

slurry-fed melter (LSFM). The LSFM ran 

campaigns over a period of 749 days during 

which it was operated with slurry for 193 days 

producing a total of 234,000 kg (514,800 Ib) of 

glass. The LSFM melter was octagonal in shape 

with four plate electrodes, 90 degrees apart. The 

design of the electrodes was changed to provide 

more uniform temperature distribution in the 

melt. Glass was poured, similar to the S-1941 

melter, by tilting the melter. The third scale 

melter, S-4234, was a modification to the 

original S-1941 melter. The modifications 

included side penetrations for the plate 

electrodes and incorporation of most of the 

subsequent DWPF melter equipment design 

features for electrodes, plenum heaters, 

riser/pour spout heater, pour spout bellows, 

thermowell, feed tube, level dip tube, TV 

cameras, bottom refractory and insulation, 

vacuum pouring, recirculating loop slurry-feed 

system, and offgas system equipment. The 

S-4234 was used to verify operation of DWPF 

equipment design. The fourth melter, integrated 

DWPF melter system (IDMS), had a fully

integrated one-fifth scale DWPF feed 

preparation system and a one-ninth scale DWPF 

melter and offgas system, and was used to 

develop process control strategy for radioactive 

vitrification operations. The IDMS testing and 

operations are discussed in section 10.2. 

The DWPF was commissioned in March 1996 to 

vitrify 130-million L (34.3-million gal.) of 

HLW. The DWPF HLW vitrification facility is a 

remotely-operated facility designed using 

canyon remote technology. All major 

components are designed for fully remote 

replacement. The facility has two long canyons. 

Remote cranes are used to remove failed 

components. The description provided in the 

following sections has been extracted from 

Pearson (1991); Weisman et al. (1988); 

Marra et al. (1996b); Schumacher and Ramsey 

(1994); Little et al. (1997); Carter et al. (1997, 

1998); Kerley et al. (1997). 

3.2.1 Waste Pretreatment and Feed 

Preparation 

The radioactive wastes transferred to the tank 

farms at the SRS, initially consisted of a mixture 

of insoluble metal oxide sludge and soluble salt 

solutions. The wastes, after reprocessing, were 

aged for 1 yr to allow decay of short-lived 

radioisotopes. After the initial cool-down period, 

the soluble salt solution, was decanted from the 

insoluble sludge, concentrated by evaporation to 

concentrated salt solution and transferred to 

storage tanks as a salt cake, as shown in 

figure 3.4. The current inventory of the salt cake 

is approximately 53.4 million L (14.1 million 

gal.), constituting 173 million Ci, in 22 tanks. 

The sludge, consisting primarily of precipitates 

of the hydroxides of iron, aluminum, and 

manganese, contained most of the radioactivity 

in the waste, except Cs-l137. The current 

inventory of sludge is approximately

NUREG/CR-6666 18
Waste Receipt 

Tanks

0- Overheads 

Evaporator 

• Salt Receipt 

Tanks

Figure 3.4. High-Level Waste Evaporation Process Flowsheet (Savannah River Operations Office, 1997)

C

0 

CD 

K. 

Z4 

•0
Joule-Heated, High-Level Radioactive Waste Processing Plants

15.-1 million L (4 million gal.), including 

residual soluble salts and aluminum, constituting 

323 million Ci, in 23 tanks. This salt supernatant 

constitutes 60.2 million L (15.9 million gal.). 

The total radionuclide inventory is 500 million 

Ci. Both sludge and salt cake undergo 

pretreatment before they are transferred to the 

vitrification facility. Pretreatment steps are 

shown in figure 3.5. 

The sludge is washed and undergoes aluminum 

dissolution operation at the Extended Sludge 

Processing Facility. The main objective of this 

process step is to remove aluminum and soluble 

salts to reduce HLW glass volume and improve 

glass quality. The process dissolves and extracts 

soluble non-radioactive components such as 

aluminum and soluble salts, which are further 

processed with the salt solution into low-level 

radioactive waste (LLW). The sludges that 

contain high aluminum concentration undergo 

two processing steps. First, sludge is treated 

with 3-molar caustic at 90 'C to dissolve 

alumina and supernatant containing dissolved 

aluminum is decanted before sludge washing. 

The sludge washing is a water dilution process 

where water is added to the waste tank and 

mixed. After mixing, the waste is allowed to 

settle and supernatant is decanted. The washed 

sludge is transferred to the vitrification facility. 

The salt cake is subjected to an in-tank 

precipitation process (ITP) to remove Cs-137 

along with residual Sr and Pu contained in the 

salt cake via sodium tetraphenyl borate (TPB) 

addition. The main objective is to remove 

soluble sodium salts from the salt cake so that it 

can meet compositional requirements for the 

vitrified glass product. The salt cake solution is 

pumped to a separate waste tank where sodium 

TPB, along with minor additions of sodium 

titanate, is added to precipitate insoluble salts of 

potassium, cesium, and NH'. Sodium titanate 

helps adsorption of residual strontium and

plutonium. The treatment calls for addition of 

sodium TPB, quantity in excess of 

stoichiometric quantity required to precipitate 

cesium as Cs TPB due to a significant amount of 

potassium present in the salt solution that is also 

precipitated as potassium TPB. The excess 

sodium TPB also ensures complete precipitation 

of Cs-137. Next, the precipitate is concentrated 

by continuous filtration to 10 wt. % solids, 

washed to remove soluble salts, and transferred 

to the salt process cell. The processed soluble 

salt solution is mixed with cement, slag, and 

flyash, and immobilized as an LLW at the 

Saltstone Facility. 

The washed 10 wt % precipitated slurry is 

transferred to the DWPF salt process cell to 

remove most of the organic materials. The 

DWPF process flowsheet is shown in figure 3.6. 

About 90 percent of the phenyl groups are 

converted to an immiscible benzene phase by a 

formic acid hydrolysis process. The benzene is 

steam distilled, further decontaminated if 

necessary, and incinerated as LLW. The TPB 

compounds in the precipitate react with formic 

acid in the presence of copper (II) formate 

catalyst, producing aromatic organic 

compounds, (benzene, phenol, and minor 

amounts of higher boiling aromatics) and an 

aqueous phase known as precipitate hydrolysis 

aqueous (PHA). The PHA contains most of the 

radioactive Cs, soluble formate salts, boric acid, 

and excess formic acid. 

The absorbed radiation doses from the Cs-137 

in the precipitate required additional levels of 

copper formate catalyst than initially anticipated 

for the PHA formation. This increase in copper 

concentration in the melter feed increases the 

potential for precipitation of metallic copper 

phases in the melter. These metallic phases 

could accumulate over a period of time and 

interfere with the joule heating (Schumacher and 

Ramsey, 1994). Anticipated levels of elemental

NUREG/CR-6666 20
Sludge I1

Waste Removal

In-Tank I 

Waste Processing 

for DWPF 

DWPF

'2I

ranks Salt Tanks 

I In-Tank 

& Washing Precipitation & Aluminum 

Dissolution 

Salt Tanks 

ý_w ý1%1 Salt Solution 

SaEttone

Figure 3.5. Sludge and Salt Pretreatment Process Flowsheet (Savannah River Operations Office, 1997)

i-..

0 

.0 

('b 

0 

0•
Joule-Heated, High-Level Radioactive Waste Processing Plants

D MeWter I - C

-DEFENSE WASTE PROCESSING FACIUTYFigure 3.6. Defense Waste Processing Facility Vitrification Process Flowsheet (Pearson, 1991) (Permission to use this copyrighted material is granted by American Ceramic Society, Westerville, 

Ohio)

copper in glass are 0.4 to 0.5 wt. %. The DWPF 

redox control studies have indicated that, in 

order to avoid precipitation of metallic copper 

phases in the melter, and maintain elemental 

copper levels in glass at 0.5 wt. %, the 

Fe2

+/Fe(total) ratio should be below 0.1. Since 

the redox ratio measurement at 0.1 or below is 

not accurate, an empirical model based on 

formate, nitrate, and percent total solids was 

developed to determine anticipated 

Fe2+/Fe(total) response in glass. In a feed 

containing nominal total solids of 45 wt. %, the 

molar difference between the formate and nitrate 

concentration should be maintained below 0.5. 

This provides an upper bound for operating the 

required redox control in the melter. 

At the DWPF, the feed preparation begins with 

the transfer of the washed sludge directly to the 

sludge receipt and adjustment tank (SRAT). The 

SRAT is a 41-in3 (10,800-gal.) alloy N10276 

tank equipped with an agitation system, 

cooling/heating coils, and vertical cantilever

centrifugal pumps for sampling and transfer. 

The washed sludge is first neutralized with nitric 

acid and then the PHA is added to the sludge (at 

boiling). The excess formic acid in the PHA 

reduces the mercuric oxide in the slurry mix to 

elemental mercury. The elemental mercury is 

then steam stripped from the SRAT into a 

holding tank from which it is later pumped and 

decontaminated. After the PHA and sludge are 

blended and processed in the SRAT, the mix is 

transferred to the slurry mix evaporator (SME). 

Note that due to benzene generation (discussed 

in detail in section 3.2.4.3) during the ITP 

process, Defense Nuclear Facilities Safety 

Board (DNFSB) has recommended that the ITP 

process should not be commenced until 

mechanisms involved in benzene generation are 

well understood. Therefore, the DWPF is only 

processing sludge along with simulated PHA. 

The SME is a 41-in 3 (10,800-gal.) alloy N10276 

tank equipped with an agitation system, 

heating/cooling coils, and a vertical cantilever 

centrifugal pump for sampling and transfer. The

NUREG/CR-6666 22
Joule-Heated, High-Level Radioactive Waste Processing Plants

wastes are then mixed with the borosilicate glass 

frit and concentrated to produce melter feed. 

The ratios of PHA, sludge, and frit are 

determined by the product composition control 

system (PCCS). The PCCS is a computer 

program that uses glass property models and 

statistical algorithms to develop blending 

strategies and to determine the acceptability of 

the feed before it is transferred to the melter. 

PCCS input includes chemical analyses of the 

wastes, tank volumes, and tank heels. The 

sample analyses of the SME are used by the 

PCCS to determine acceptability of the melter 

feed. 

3.2.2 Vitrification 

Once the feed is approved for vitrification, the 

feed from the SME is transferred to the melter 

feed tank (MFT) and is fed to the melter. In the

melter, the remaining water is evaporated from 

the slurry, forming a solid crusty layer on the 

surface (cold-cap) of the glass pool. The 

cold-cap prevents volatilization of radioactive 

components like Cs-137. The formates and the 

nitrates in the crusted slurry thermally 

decompose, forming a calcined oxide product. 

The calcined oxides fused to produce 

borosilicate glass melt. 

The DWPF melter is the largest HLW 

vitrification melter operating in the world. The 

melter has a 2.6-m2 (28-ft2) glass pool surface 

area and contains about 2,200 L (581 gal.) of 

glass. It is designed to vitrify a continuous slurry 

stream of glass flit mixed with treated waste. 

The cross-section of the DWPF melter is shown 

in figure 3.7. The melter is cylindrical in shape; 

this geometry prevents the cave-in of the 

refractories. The glass contact refractory is 

high-chrome material to minimize corrosion by 

the waste glass. Two pairs of diametrically

C

VALVE 

Figure 3.7. Sectional View of the Defense Waste Processing Facility Vitrification Melter 

(Weisman et al., 1988) (Permission to use this copyrighted material is granted by WM Symposia, Inc., 

Tucson, Arizona)

NUREG/CR-6666 23
Joule-Heated, High-Level Radioactive Waste Processing Plants

opposed Inconel® Alloy 690 electrodes provide 

about 150 kW of power to the melt. In addition, 

plenum heaters provide additional heat to 

maintain a plenum temperature high enough to 

combust organics. Plenum heaters are resistance 

heated Inconel® Alloy 690 tubes. The DWPF 

melter operates between 1,050 and 1,150 *C. A 

temperature higher than 1,150 'C could result in 

creeping and eventual damage to the Inconel® 

Alloy 690 electrodes. In addition to meeting 

waste form, disposal requirements, the glass 

composition is optimized to meet the viscosity 

and electrical resistivity requirements that would 

provide a maximum benefit ofjoule heating and 

glass mixing by convective flows. 

As the waste slurry is transformed into glass, the 

glass level in the melter rises. When the glass 

level reaches a predetermined level in the 

melter, glass is poured into a stainless steel 

canister. To pour glass, a differential pressure is 

introduced between the melting cavity and the 

pour spout. This is done by introducing a slight 

vacuum into the canister end. As the glass level 

increases in the riser, the glass overflows into 

the pour spout and travels vertically down into 

the canister. The melter is also provided with a 

bottom drain for complete draining. 

The glass is intermittently poured into a 3-m 

(10-ft) tall and 61-cm (2-ft) diameter stainless 

steel canister. A full canister contains about 

1,800 to 2,000 kg (3,960 to 4,400 lb) of glass. 

The fill height is monitored using an ILDS. In 

addition, load cells are used to monitor the 

weight of the canisters as they are filled. After a 

canister is full, the turntable on which the 

canister sits is rotated to bring a new empty 

canister under the discharge section. 

After the canister is full, it is sealed using an 

inner canister closure (ICC) plug that provides a 

temporary water-tight seal to prevent ingress of 

water during the decontamination process. The 

ICC plug is tapered to facilitate installation into

the canister throat while the canister is still hot 

and on the turntable. If the canister is not hot 

enough to form a shrink fit between the canister 

throat and the ICC plug, the canister is removed 

from the turntable and reheated. Next, the 

canister is moved to the ICC station where the 

ICC seal is leak tested. All canisters have to pass 

less than 2.0 x 10-1 atm-cc/sec He. 

The ICC plugged canister is then moved to the 

canister decontamination cell and 

decontaminated prior to welding to remove the 

oxide layer from the outside surface of the 

canister. The canister is placed in a 

decontamination cell and the outside surface of 

the canister is blasted with an air-injected frit 

slurry. This technique uses an 8-percent slurry 

of glass frit, driven by a 100-psig air stream that 

is sprayed in a helical pattern, covering the 

entire surface. Each canister decontamination 

generates about 5,700 L (1,500 gal.) of frit 

slurry, which is recycled to the SME. After 

blasting, the canister is water rinsed, air dried, 

and moved to a smear station. Canisters that fail 

the smear test are sent back for additional 

decontamination. Each canister is required to 

pass the smear testing acceptance criteria that is 

less than 220 dpm/100 cm2 for alpha radiation 

and less than 2,200 dpm/cm2 for beta/gamma 

radiation. 

The canisters are welded before they are 

transferred for interim storage. Each canister is 

moved to the weld test cell and placed in the 

canister trolley. The trolley moves the canister to 

the ICC press where the ICC plug is pressed 

down by an 890,000 N (200,000 lbf) hydraulic 

press into the canister neck to allow space for 

the weld plug. The weld plug is then installed 

into the canister throat and the trolley transfers 

the canister to the weld station. At the weld 

station, a combination of a 400,000 A DC 

capacity solid state resistance welder system and 

an ICC hydraulic press is used to pass current 

through the weld plug-canister flange interface.

NUREG/CR-6666 24
Joule-Heated, High-Level Radioactive Waste Processing Plants

This generates enough heat to cause plastic flow 

of metal, and simultaneously allows the plug to 

be pushed into the canister throat. The canisters 

are smear tested again and transferred to an 

interim storage facility. 

3.2.3 Offgas Treatment System 

The offgas system is shown schematically in 

figure 3.8. The reaction products from the 

melter, along with the steam, pass through a film 

cooler, a quencher, an offgas condensate tank 

(OGCT), a two-stage steam atomizing scrubber 

(SAS), a HEME, and a HEPA filter. After 

required decontamination factors are obtained, 

the treated offgas is released to the stack. The 

offgas film cooler reduces the deposits at the 

entrance of the offgas system. A film cooler

brush is used to clean any deposits that are 

formed. The quencher (venturi scrubber) cools 

the offgas and removes large particulates. The 

offgas condensate tank separates liquids and 

gases. The two SASs remove submicron 

particulates. The HEME removes the fine mist 

and particulates and the HEPA filter removes 

the trace particulates. 

3.2.4 Process Issues and Upsets 

3.2.4.1 Hydrogen Gas Generation 

At the SRS, hydrogen is generated by wastes in 

all the tanks and constitutes a significant hazard. 

The hydrogen generation is mostly radiolytic. 

High nitrate concentration inhibits hydrogen 

generation. To prevent hydrogen accumulation

S 50 

STEAM ATOMIZED 

SCRUBBER

40 20o 

MIST HEPA 

ELIMINATOR

200 8 x 10 

SAND 

FILTER

Figure 3.8. Defense Waste Processing Facility Melter Offgas Treatment System 

(Weisman et al., 1988) (Permission to use this copyrighted material is granted by WM Symposia, Inc., 

Tucson, Arizona)

NUREG/CR-6666

DF (Cs-137) I 1 

QUENCHER

25

I
Joule-Heated, High-Level Radioactive Waste Processing Plants

in the tanks, considerable redundancy and 

independence is provided. This includes 

ventilation systems served by diesel backup 

power with alarms, upon its failure. The 

ventilation systems are checked by operators 

every 12-hr shift. Hydrogen monitors, equipped 

with failure- and low-flow alarms, are provided 

for high-hydrogen generation tanks. The alarms 

are set at 10 percent of the lower flammability 

limit. In addition, tanks are continuously purged 

with nitrogen. Time to reach lower flammability 

limit (LFL) for each tank is estimated once 

every month, which is approximately 9 days. 

3.2.4.2 Corrosion of Tanks and Transfer 

Lines 

In tanks and transfer lines, leaks due to 

corrosion are a significant hazard at the SRS 

site. Five of twelve tanks constructed in 1954 

and all four tanks constructed in 1956 have 

leaked into their secondary containment vessels 

while two out of the eight tanks constructed in 

1959 have penetration. All tanks have a 

secondary containment with leak-detection 

devices. The waste chemistry is adjusted 

periodically to inhibit corrosion. All transfer 

lines are made of 304 L stainless steel and have 

a secondary containment. The secondary 

containment carries two transfer lines and drains 

to a leak detection box that is fitted with 

conductivity probes to detect leaks. All leaks go 

to a sump. In addition, secondary and primary 

containment integrity is verified biannually 

using a pneumatic pressure test, if possible. The 

test involves maintaining a 15-psig air pressure 

in the annulus for 20 min. Pressure drop of 

1 psig indicates a leak. 

3.2.4.3 Benzene Deflagration 

The SRS uses sodium TPB and sodium titanate 

to separate Cs, Sr, and Pu from the salt cake 

solution in an ITP system. During batch 1 (first 

radioactive batch) processing, the benzene

release was much greater than expected from 

historical tests conducted a decade ago, which 

postulated that the benzene is generated due to 

radiolytic destruction of TPB. In addition, when 

the ITP system pumps were restarted after 

several days or weeks of shutdown, the benzene 

release rate increased 2-3 orders of magnitude, 

indicating some sort of benzene retention 

phenomenon. The current opinion of the SRS 

staff, based on laboratory tests conducted since 

the batch 1 test, is that benzene is produced 

principally from the catalytic decomposition of 

TPB ions in solution. Catalytic decomposition is 

caused by palladium, which is present 

principally in the sludge and is carried along 

with the supematent as an entrained solid during 

transfer. In 1996, the DNFSB issued 

recommendation 96-1, which stated that the ITP 

process cannot be resumed until a scientific 

understanding of the reactions leading to the 

generation of benzene, the mechanisms involved 

with the retention of benzene in the IT? system, 

and the mechanisms involved with the release of 

benzene in the ITP system are understood 

sufficiently well to ensure that the defense-in

depth measures taken to prevent deflagration are 

adequate. In addition, better understanding of 

chemistry issues related to ITP should be 

developed to determine the combination of 

controls and engineered systems necessary to 

prevent and mitigate benzene deflagration in 

process vessels. Engineered controls that are in 

place to avoid benzene deflagration in the 

sodium TPB tank and the filtrate hold tank 

include blanketing the tank plenum space with 

nitrogen, redundant nitrogen supply, and 

installation of instruments for monitoring tank 

pressure. 

3.2.4.4 Moisture and Electrical Problems 

with Canister Manipulating 

Mechanism 

The canister manipulating mechanism (CMM) is 

used for lifting and rotating the canister past the

NUREG/CR-6666 26
Joule-Heated, High-Level Radioactive Waste Processing Plants

spray nozzles during decontamination. In 1996, 

standing water was found in CMM electrical 

jumpers and caused electrical faults between the 

lift and rotate motors. The source was 

determined to be water from in-cell washing that 

leaked into remote electrical connectors. Other 

problems with the CMM included pin-to-pin 

shorts on CMM electrical connector plates, a 

failed electrical connector on a transfer motor, a 

failed remote electrical connector for the 

recirculating pump electrical jumper, failed limit 

switches for the CMM grapple and motors, and 

a CMM rotational indicator that failed to operate 

during blasting operations. 

3.2.4.5 Glass Pour Problems 

Glass pouring problems have impacted the 

DWPF overall production performance. Glass 

pouring is accomplished by creating a slight 

vacuum at the pour spout end relative to the 

melter plenum space. The differential pressure 

forces glass to overflow into the pour spout and 

travel vertically 38 cm (15 in.) down the side 

wall of the pour spout where the wall is cut back 

to form a sharp knife edge to disengage glass. 

The glass then falls free about 360 cm (12 ft) 

through the bellows, into the canister. Any 

wavering of the glass stream, resulting from the 

pressure transients and airflow fluctuations, 

through the discharge section during this free 

fall could result in the glass stream contacting 

the unheated bellows and adhering to the 

bellows surface, gradually leading to a buildup 

of glass sufficient to clog the flow of glass. 

After the initial plugging of the discharge 

orifice, the DWPF designed a telerobotic 

manipulator (TRM) to clear the blockage. While 

the TRM was effective in removing the clogged 

glass from the discharge orifice, it did not 

resolve pouring problems. Pouring problems 

were significantly reduced after a removable 

insert was designed and installed at the end of 

the pour spout that lowered the knife edge from 

its existing location. Since the installation of the

insert, pouring problems have been significantly 

reduced. 

3.2.4.6 Foaming in the Melter 

Foaming in the melter is related to the glass 

composition and redox conditions in the melt. 

Generation of foam in the DWPF melter is 

attributed to the presence of manganese dioxide 

and oxidizing conditions. If foaming is left 

uncontrolled, it can cause process upsets such as 

plugging of offgas lines, pressure transients, and 

unstable temperatures in the melter. The 

combination of these events can even lead to 

shortening of melter life. To date, generation of 

foam in the DWPF melter has not caused any 

significant process problems. The process is 

continuously monitored for foam. 

3.2.4.7 Canister Turntable 

A drive failure was observed in the canister 

turntable. The turntable is positioned under the 

pour spout and rotates to provide empty 

canisters under the discharge section. When the 

bellows or glass plug needs to be cleaned, the 

turntable is driven out from under the pour spout 

by an electric motor/gear box drive. The drive 

failure was attributed to overload in the 

drivetrain. 

3.2.4.8 Process Component 

The performance of all process equipment 

components was evaluated before the start of 

radioactive operations. The evaluation indicated 

that most of the equipment components except 

for the SME components met the design criteria. 

The examination of the SME agitator blades 

indicated severe erosion on the back of the 

lower blades due to continuous impact of glass 

frit slurry particles. Similarly, severe localized 

erosion was observed in the SME coil assembly. 

The blades were overlayed with W73006 alloy 

coating and the damaged areas of the SME coils

27 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

were repaired before the start of radioactive 

operations (Gee et al., 1996). Despite the 

coating, SME blade erosion recently caused 

temporary suspension of radioactive operations 

to repair the SME agitator blades again. 

3.3 Pamela Vitrification Plant, 

Mol, Belgium 

The Pamela vitrification plant in Mol, Belgium, 

was the first vitrification plant to process HLW 

using joule-heated melter technology. The plant 

was constructed between 1981 and 1984. The 

Pamela vitrification plant is a remotely operated 

facility constructed from reinforced concrete 

with multiple shielded areas in which process 

and handling equipment are installed. After 1 yr 

of cold operations, hot operations were started 

on October 1, 1985. Radioactive processing was 

successfully completed in September 1991, 

vitrifying almost 900 m3 (238,000 gal.) of 

high-level liquid waste (HLLW) into 500 tons of 

borosilicate glass in 2,201 canisters (National 

Research Council, 1996). During the process, 

two German-designed flat-bottom, joule-heated, 

liquid-fed ceramic melters were used 

(Scott et al., 1988). The first melter was used for 

3 yr and processed approximately 275 m3 

(72,650 gal.) of HLLW. This melter experienced 

noble metals buildup on the melter floor 

resulting in shorting of the melter electrical 

system and eventual failure. A second identical 

melter was installed and the campaign was 

successfully completed. Before and during 

radioactive operations, several vitrification 

systems (as shown in table 3.2) were developed 

and operated at the Kernforschungszentrum 

Karlsruhe site in Germany to support the 

development of vitrification processes at the 

Pamela vitrification facility in Mol, Belgium 

(Elliott et al., 1994). The HLW vitrification 

processing description provided in the following 

sections has been extracted from National

Research Council (1996), Hohlein et al. (1986), 

Ewest and Wiese (1987), and Wiese et al. (1988). 

3.3.1 Waste Pretreatment and Feed 

Preparation 

There are two types of HLLW stored at the 

Pamela site. There is approximately 50 m3 

(13,120 gal.) of low-enriched waste concentrate 

(LEWC) from the reprocessing of spent fuel 

from commercial nuclear power stations with a 

low enrichment ofU-235, and 800 m3 

(211,360 gal.) of high-enriched waste 

concentrate (HEWC) from the reprocessing of 

spent Materials Test Reactor fuel, with a high 

U-235 enrichment. LEWC contains significant 

amounts of process chemicals such as sodium, 

in addition to actinides and fission products. The 

HEWC is mostly aluminum nitrate solution and 

has about 10 percent radioactivity of LEWC. 

LEWC also contains a significant amount of 

sulfur. The presence of sulfur limits the waste 

loading in glass to 11 percent (Wiese et al., 

1988), attributed to low sulfur solubility in 

borosilicate glass. 

The basic flowsheet of the vitrification process 

is shown in figure 3.9. The LEWC is transferred 

from the storage tanks at the EUROCHEMIC 

reprocessing plant via steam jets through a 

shielded overhead pipeline to the receiving tank 

at the Pamela vitrification facility. Each transfer 

is approximately 1.5 m3 (400 gal.). After each 

transfer, the receiver tank contents are sampled 

for solids and activity. Next, the waste is batch 

transferred in 10-L (2.64-gal.) increments to the 

feed tank using a two-stage airlift transfer 

system. This waste is then continuously fed to 

the melter. Simultaneously, 5-kg (11-1b) bags of 

1.5 mm (0.06 in.) glass flit beads are loaded in a 

solids handling system and are fed to the melter. 

The amount of flit added to the melter depends 

on the desired waste loading. Table 3.3 shows

NUREG/CR-6666 28
Joule-Heated, High-Level Radioactive Waste Processing Plants

Table 3.2. History of Vitrification Melter Development in Germany/Belgium (EIllott et al., 1994) 

Melter Characteristics I Campaign/Run Dates 

K-iNA-I 0.36-m2 area, bottom drain, flat 18 m3 glass 1977-1979 

bottom 

K-2NA-2 0.64-m2 area, bottom drain, flat 25 tons glass 1980

bottom 

K-3/VA-3 0.72-m2 area, bottom and 22 campaigns, 37 tons 1983-1985 

overflow drain, flat bottom glass 

K-4/Pamela* 0.72-m2 area, bottom and Low-enriched waste 1/85-12/86 

overflow drain, 4' sloped concentrate campaign, 

bottom 77.8 tons glass 

K-4 Pamela* 0.72-m2 area, bottom and High-enriched waste 1/86-1/89 

overflow drain, 4° sloped concentrate campaign, 

bottom 248.3 tons glass 

K-5/Pamela same as K-4 1987

K-WiN-Wi 1.4-n 2 area, bottom drain, 1986

overflow, vacuum suction, flat 

bottom 

K-W2 1.4-m2 area, bottom drain, Campaign W4, 13.2 tons 4/88-5/88 

overflow, vacuum suction, 450 glass 

sloped floor 

K-W2. 1.4-m2 area, bottom drain, Campaign W5, 12.8 tons 1/89-3/89 

overflow, vacuum suction, 45° glass 

sloped floor, air bubblers 

K-6 0.88-m2 area, bottom drain, 8.5 tons glass 6/90-present 75'/60' sloped floor 

K-6 0.88-rn2 area, overflow system, 5.5 tons glass 6/90-present 

75 0/600 sloped floor, air 

sparging 

*Radioactive melters

29 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

Figure 3.9. Pamela Vitrification Process Flowsheet (Hohlein et al., 1986) (Permission to use this 

copyrighted material is granted by WM Symposia, Inc., Tucson, Arizona) 

Table 3.3. Composition of the Glass Frit and the Low-Enriched Waste Concentrate Glass 

Produced in the Pamela Plant (Hohlein et al., 1986) (Permission to use this copyrighted material 

is granted by WM Symposia, Inc., Tucson, Arizona) 

Constituent Glass Frit, wt% I Waste Glass, wt% 

SiO 2 58.6 52.2 

B20 3 14.7 13.1 

A1203 3.0 2.7 

Li20 4.7 4.2 

Na.O 6.5 5.8 

MgO 2.3 2.0 

CaO 5.1 4.5 

TiO2 5.1 4.5 

Low-enriched waste concentrate-oxides -- _11.0

NUREG/CR-6666 30
the composition of the glass frit and the LEWC 

waste glass. 

3.3.2 Vitrification 

The joule-heated ceramic melter is a major 

component of the Pamela process. Figure 3.10 

shows major parts of the melter. Like all 

joule-heated melters, the high-chrome ceramic 

refractory is used as a glass contact refractory to 

minimize corrosion from the molten borosilicate 

glass. The melter has 0.77 m2 (8.29 ft') of glass 

pool surface area and a normal glass pool 

operating depth of 0.4 m (1.3 ft) and contains 

approximately 0.31 m3 (82 gal.) of glass. 

The joule heating for melting is provided by 

four Inconel® Alloy 690 plate electrodes. These 

electrodes are air cooled to keep the electrode 

temperatures below 1,120 °C to minimize 

electrode corrosion. The electrical input to the 

melter is controlled by fixing the current to the 

power supplied to the electrodes. The power 

consumption of the melter is about 55 kW 

(187,710 BTU/hr) for the upper electrodes and 

20 kW (68,258 BTU/hr) for the lower pair of 

electrodes. 

The slurry feeding to the melter results in water 

and free nitric acid evaporation. This leaves 

behind a crusty layer of salts (cold-cap) on the 

surface of the glass melt. The cold-cap 

formation reduces the volatilization of 

radioactive species like Cs-137. The solids 

between the crusty layer and the glass melt 

surface are calcined to their oxides and 

incorporated into the glass melt at about 

1,150 °C. The melt chamber temperatures at 

various points in the melter are measured by 

thermocouples. Four thermocouples, placed in 

Inconel® Alloy 690 tubes, are installed through 

the melter lid to various depths to measure the 

plenum, calcination, and molten glass zones 

temperatures. These temperature profiles, along 

with the power consumption and mass balances,

Joule-Heated, High-Level Radioactive Waste Processing Plants

are used to determine the need for changing feed 

rates or pouring glass into a canister. 

In the production of glass blocks, glass is 

drained to a canister batch-wise by using a 

bottom drain. When the glass in the melter 

(based on temperature readings) reaches a 

predetermined level, approximately 150 kg 

(330 lb) or about 60 L (15.8 gal.) of glass is 

removed from the melter. The glass flow is 

accomplished by providing induction heat to the 

bottom drain valve from a water-cooled, 

medium-frequency induction heating coil. 

Additional heat is provided by passing current 

from the bottom drain tube to the bottom 

electrodes. The initial fill rate is about 150 kg/hr 

(330 lb/hr) and averages about 120 kg/hr 

(204 lb/hr). The total time from start of heating 

to filling the canister to the required level is 

about 2 hr. The weight of glass in the canister is 

determined by a strain gauge balance located on 

the canister lifting carriage, and these readings 

are integrated periodically to determine the fill 

rate. Two gamma detectors are installed to 

measure the glass level in the canister to confirm 

the present weighing system. The upper detector 

is set for the maximum fill level and the lower 

detector is 10 cm (4 in.) below the maximum 

fill. 

The bead production unit consists of an Inconel® 

Alloy 690 pot, heated by a resistance furnace to 

1,100 'C, that breaks the continuous glass 

stream from the melter into droplets. These 

droplets of glass fall onto a rotating turntable 

where they solidify. A scraper transports the 

beads to a hole in the center of the turntable 

where they fall into a container. During the time 

when the container is changed, the continuous 

stream of glass beads is temporarily collected in 

a buffer. After cooling down, the void volume 

between beads is filled with lead by induction 

heating the container to about 400 'C. These 

filled containers are called VITROMET.

31 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

Feeding 

Tube 

i *i Glass-Aidift

r-- • Bottom 

S~Drain 

Figure 3.10. Sectional View of the Joule-Heated Ceramic Melter at the Pamela Facility 

(Hohlein et al., 1986) (Permission to use this copyrighted material is granted by WM Symposia, inc., 

Tucson, Arizona)

NUREG/CR-6666 32
Joule-Heated, High-Level Radioactive Waste Processing Plants

Pamela canisters are 1.2 m (4 ft) in height and 

0.3 m (1 ft) in diameter. The canister can hold 

150 kg (330 lb) of glass [about 60 L (15.8 gal.)]. 

In-cell, these canisters are put in a heat 

insulating overpack and moved to a canister 

lifting carriage. The carriage is driven along a 

rail to position the canister directly underneath 

the bottom of the fill valve. It then lifts the 

canister up to the bottom valve bellows for 

filling. After filling, the canister and the 

overpack are returned to the handling cell for 

cooling. After the canister is cooled for 60 hr to 

about 60 °C, it is removed from the overpack 

and moved to the welding station. A cover is 

welded using a gas tungsten arc welder 

(GTAW). 

The welded canister is decontaminated in an 

ultrasonic bath containing 1 molar nitric acid. It 

is then moved to a separate chamber where it is 

rinsed with water. A smear test is performed on 

the canisters to assure contamination levels are 

below 10-7 Ci/m2. Finally, the canisters are 

moved to the engineered storage building. 

3.3.3 Offgas Treatment System 

The offgas treatment system schematic is shown 

in figure 3.11. First, the offgas passes through a 

dust collector where large particulates are 

removed. The dust scrubber consists of a pot 

with a 204-mm-diameter column containing 

nine plates at 175 mm (6.9 in.) spacing. The 

scrubber solution is held at 80-85 'C by an 

electric heater and is circulated counter-current 

to the offgas flow. The temperature is 

maintained to prevent steam condensation. The 

solids are periodically removed and recycled 

back to the process. A mechanical system is 

used to push back the solids that are deposited 

between the dust scrubber and the melter. 

The offgas from the dust scrubber goes to a 

condenser where cooling water at 30 'C is used

as a condensing media. The upper part of the 

condenser is a set of tubes and a lower section 

has a vapor disengaging and collection vessel. 

The condenser contact surface is enough to 

produce an offgas exit temperature of 40 0C. 

The condensate, which is usually a nitric acid 

solution, is periodically transferred to the 

secondary waste evaporation system. 

The offgas leaving the condenser is mostly NO1, 

small particulates, and aerosols, and it is 

combined with the offgas from vents of the 

other radioactive tanks. The mixture is fed to a 

jet scrubber where it is contacted by a 

recirculating fluid stream at a pressure of 

500 kPa (5 bar). The recirculating scrub-solution 

energy is provided by a pump. Routine analyses 

of the scrub solution are made and, when the 

activity reaches 0.5 Ci/L, used scrub solution is 

sent to the secondary waste evaporation system. 

The scrubbed offgas is next sent to the NO, 

absorber. In this case, the scrubbing medium is 

5 to 6 molar nitric acid. The scrubber solution 

also contains 0.2 molar hydrogen peroxide to 

oxidize NO to NO2. The solution is maintained 

at 15 °C by a cooling water coil to cool the 

offgas and remove the heat of solution. 

The offgas exiting from the NO, scrubber is 

essentially air, nitric acid aerosols, and fine 

particulates. The offgas is next treated in a 

fiberglass filter where the nitric acid aerosols are 

removed and then heated to 70 °C to ensure that 

it is above the dew point before passing through 

two HEPA filters in series to minimize 

radioactive discharge. The gas is cooled to 

30 'C, sent through an impingement, and 

discharged to the stack by an offgas blower.

33 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

NOx - Absorber

Figure 3-11. Offgas Treatment System at the Pamela Facility (Hohlein et al., 1986) (Permission to 

use this copyrighted material is granted by WM Symposia, Inc., Tucson, Arizona)

3.3.4 Process Issues and Upsets 

3.3.4.1 Noble Metal Issues in the Melter 

During the initial processing of 47 m3 

(12,400 gal.) LEWC, approximately 90 kg 

(198 lb) of noble metals (Pd, Rh, Ru) were fed 

into the melter;, Ru accounted for 64.4 kg 

(141.7 lb). In addition, during idling (time 

during which the melter is not fed), the melter 

temperature was lowered to around 800 °C by 

decreasing the voltage between the electrodes. 

On restart of the feeding to the melter, the 

system showed a glass production rate decrease 

from 30 kg/hr (66 lb/hr) to 20 kg/hr (44 lb/hr), 

and a loss in energy consumption. A glass 

sample taken from the melter bottom indicated 

an 80 times higher Ru concentration in the glass.

This implies 27-61 kg (59.4-134.2 lb) of noble 

metals accumulation in a layer approximately 

5 cm (2 in.) thick on the melter floor. 

Investigations with simulated bottom glass 

indicated that the electrical resistance was three 

times lower and the viscosity about five times 

higher than those of the normal LEWC glass at 

1,150 'C. The decrease in the glass production 

rate from 30 kg/hr (66 lb/hr) to 20 kg/hr 

(44 lb/hr) or a loss of energy consumption in the 

melt could be explained because a significant 

portion of the current was passing through the 

highly electrically conductive bottom glass 

containing noble metals. Noble metals 

accumulated on the melter floor, because of 

their limited solubility in glass melt and their 

decrease in solubility with decreasing 

temperature. To eliminate this bottom glass, the 

melter was successfully rinsed with inactive

NUREG/CR-6666 34
Joule-Heated, High-Level Radioactive Waste Processing Plants

glass and then sparged with nitrogen. 

Fortunately, the HEWC solution that was 

vitrified after this incident in the plant had 

10 percent lower noble metal than LEWC. The 

rinse by inactive glass was only marginally 

successful in reducing noble metal concentration 

in the melter. 

Premature failure of the Pamela plant melter 

occurred in 1987, caused by rapid accumulation 

of noble-metal-enriched conductive sludges. The 

conductive sludge caused bottom drain plugging 

and resulted in electrode failure caused by 

shorting through the sludge (Scott et al., 1988). 

3.3.4.2 Electrode Damage 

At the beginning of the HEWC campaigns in 

October 1986, a hole in an Inconel® Alloy 690 

electrode next to the bottom of the melter was 

discovered when electrode cooling air was 

leaking into the melt. It was attributed to the 

layer of noble metals that contacted the lower 

side of the electrodes, resulting in a spot attack 

from the current. Since the bottom electrodes are 

not used for normal operation, the vitrification 

process continued. 

3.3.4.3 Failure of the Bottom Draining 

System 

In November 1986, cooling air from the bottom 

draining flange, which contacts the refractory, 

was leaking into the glass melt. Because of the 

limited capacity of the induction coil, the glass 

was kept running with direct heating. A much 

higher heat dissipation was maintained in the 

circuit outlet channel of the bottom draining 

system, causing the region to overheat. This 

overheating resulted in glass flow beside the 

metallic outlet channel of this system. Because 

of the loss of the cooling circuit at this outlet, it 

was decided to fill the containers with the 

overflow system. Stopping and starting the glass 

stream was managed by the nitrogen fed glass

airlift inserted in the riser of the overflow 

channel. Within three days, the lifting device for 

the containers was remotely adapted for the 

overflow operation in order to be able to 

produce glass blocks. 

3.3.4.4 Blockage of the Outlet of the 

Overflow Channel 

In January/February 1987, the outlet of the 

overflow channel was blocked several times 

with solidified glass. The blockage was remotely 

removed by inserting Kanthal heaters into the 

outlet to melt the solidified glass. The viscosity 

of the glass was too high for the glass to flow 

through the outlet nozzle, particularly during 

starts and stops of the glass stream. Blockage 

problems were resolved by changing the 

viscosity (at 1,150 'C) of the glass by reducing 

the waste oxide content from 22.5 to 21 wt % 

and by increasing the temperature in the outlet 

region by 40 'C. In addition, it was observed 

during these incidents that the glass was partly 

crystalized in the outlet channel as a result of the 

long residence time involved. The crystals were 

identified as being predominantly rutile needles 

(TiO2). As a consequence, a glass flit without 

TiO2 was developed for HEWC. 

3.4 Tokai Vitrification Plant, 

Japan 

The Power Reactor and Nuclear Fuel 

Development Corporation (PNC) of Japan 

started development of vitrification technology 

in 1975 in cooperation with universities, 

national research institutes, and private 

companies. The TVP was designed to vitrify 

HLLW from the Tokai Reprocessing Plant 

(TRP). The radioactive operations were started 

in December 1995, employing ajoule-heating 

melter system similar to the Pamela, WVDP, 

and DWPF vitrification systems. TVP is a 

remotely operated facility with two main cells.

35 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

The vitrification cell has most of the vitrification 

process equipment such as the receiving vessel, 

evaporator, melter, welding system, and offgas 

treatment equipment. For maintenance, 

overhead systems such as two-armed 

servo-manipulators, in-cell cranes, and a "rack 

system" that mounts the process equipment on 

modular frames, are disposed in one cell. The 

other cell is a transfer cell with storage pits 

where the canisters are inspected and stored. 

The glass production rate is 9 kg/hr (19.8 lb/hr), 

and the glass is poured from the bottom of the 

melter into a 304 L stainless steel canister. The 

waste form is produced as borosilicate glass

with approximately 25 percent waste loading. 

However, before the start of radioactive 

operations, several vitrification systems, as 

shown in table 3.4, were developed and operated 

to support the development of the vitrification 

process. The HLLW vitrification processing 

description provided in the following sections 

has been extracted from Senba et al. (1997), 

Tomikawa and Yoshioka (1996), National 

Research Council (1996), Uematsu (1986), and 

Tsuboya and Tsunoda (1988).

Table 3.4. History of Vitrification Melter Development in Japan (Elliott et al., 1994) 

Melter Characteristics Campaigns Dates 

ETP A 0.3-m2 area, overflow drain, flat bottom - 1978-1984 

ETP B 0.59-r 2 area, overflow drain, flat 1980-1983 

bottom 

ETP C 0.3-m2 area, bottom drain, flat bottom - 1978-1984 

MTP Mock-up 160-L volume, overflow and bottom 5 campaigns 1982-1983 

drain, flat bottom 

Advanced-B 450 floor, 0.53-m2 area, bottom drain 3 campaigns 11/85-8/86 

CPF bottom drain 11 runs 12182-1987 

Small Scale 300 floor, 0.054-m2 area bottom drain 8 batches estimated 1983-1984 

Small Scale 450 floor, 0.054-r 2 area, bottom drain 13 batches estimated 1983-1984 

Pilot Scale IHI 0.25-1 2 area, 450/530 floor, bottom 2.3 tons glass 1987 

drain 

Mock-up II data not available - 1984-1986 

Mock-up III 0.66-m2 area, 450 floor, bottom drain 14.5 tons glass 2/88-1/89 

Large Scale 2.2-m2 area, 49.60/53.30 sloped floor, 30.3 tons glass 1991 

bottom drain

NUREG/CR-6666 36
Joule-Heated, High-Level Radioactive Waste Processing Plants

3.4.1 Waste Pretreatment and Feed 

Preparation 

The TRP was the first reprocessing facility in 

Japan, and it is capable of reprocessing 

700 kg/day (1,540 lb/day) of fuel. The plant has 

been operational since 1977. As of 1988, the 

TRP has processed 350 MT (770,000 lb) of 

light-water reactor fuels and has generated, as a 

result, 300 m3 (79,260 gal) of HLLW. The 

reprocessed fuels also include 5 tons from the 

prototype "Fugen" Advanced Test Reactor 

(Tsuboya and Tsunoda, 1988). The process is 

accomplished through four major systems: the 

waste retrieval and feed preparation system, 

vitrification, the offgas treatment system, and 

the canister closure and storage system. The 

schematic of the vitrification process is shown in 

figure 3.12.

The HLLW is transferred once a week from the 

reprocessing plant to a receiving tank in the 

TVP through an underground trench system. 

The composition of the HLLW is analyzed and 

adjusted by the addition of chemicals such as 

sodium. The waste is concentrated using an 

evaporator to a target concentration 

500 L/metric ton uranium (132-galJMTU). The 

pretreated HLLW is transferred to the feed tank 

by a steam jet. Before HLLW is fed to the 

melter, it is soaked into cylindrical glass fibers. 

The size of the cylinder is approximately 70 mm 

(2.75 in.) in diameter and 70 mm (2.75 in.) in 

length. The diameter of the glass fiber is about 

10 pm. The soaking capacity is 4 ml HLLW 

glass fiber. This keeps the glass melting stable, 

without rapid evaporation.

Figure 3.12. Tokai Vitrification Plant Process Flowsheet (Uematsu, 1986) (Permission to use this 

copyrighted material is granted by WM Symposia, Inc., Tucson, Arizona)

37 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

3.4.2 Vitrification 

Figure 3.13 shows a schematic of the TVP 

melter. The PNC melter is square in 

cross-section with a 45-degree-sloped floor; 

sloped walls facilitate discharge of noble metals 

with glass. The melt surface area is 

approximately 0.53 m2 (5.7 ft2) and the volume 

capacity is 300 L (79.3 gal.). The melter uses a 

composite refractory design. Four layers of 

refractory are used in the melter tank wall and 

three in the lid; this design is similar to the West 

Valley concept. The glass contact refractory is 

fused-cast high-chrome-alumina, backed up by 

successive layers of mullite, alumina castable, 

and a ceramic fiberboard. The containment 

vessel is cooled by natural air convection. The 

joule heating is provided by two main plate 

electrodes and another set of smaller plates that 

act as auxiliary electrodes below the main 

electrodes. The electrodes are made of Inconel® 

Alloy 690. The melt temperature is maintained

between 1,100 and 1,250 *C. The electrodes are 

cooled by forced air through the channels 

incorporated inside the electrodes. Current 

density is restricted to avoid excessive electrode 

corrosion. 

The molten glass is discharged periodically, 

through a metallic nozzle located at the bottom 

of the melter, into a canister. A bottom freeze 

valve is heated by two-zone induction coils. This 

provides a higher degree of control over the 

starting and terminating of the glass draining 

into the canister. 

Electroconductive deposits such as ruthenium 

oxide that have the potential to cause electrical 

shorting in the melter are also discharged along 

with the glass. This eliminates their 

accumulation on the floor. An estimated design 

life of the melter is 5 yr. 

After the canisters are cooled, the cap is welded 

using a GTAW, and the canister surfaces are

Figure 3.13. Three-Dimensional View of the Tokai Vitrification Plant Joule-Heated Melter (Uematsu, 

1986) (Permission to use this copyrighted material is granted by WM Symposia, Inc., Tucson, Arizona)

NUREG/CR-6666 38
Joule-Heated, High-Level Radioactive Waste Processing Plants

decontaminated by high-pressure water jet spray 

with wire brushing. Decontamination is verified 

using smears. The canisters are further inspected 

for appearance, length, and diameter before they 

are stored. 

3.4.3 Offgas Treatment System 

Figure 3.14 shows the melter offgas treatment 

system. After release from the melter, the offgas 

goes through the air-film cooler, dust scrubber, 

venturi scrubber, perforated plate water 

scrubber, HEME, a ruthenium adsorber (silica 

gel), an iodine adsorber, and HEPA filters. 

3.4.4 Process Issues and Upsets 

3.4.4.1 Remote Operations 

Two years before the start of the radioactive 

operations, maintenance tests were conducted on 

all remote-type process equipment. During these 

tests, several problems were identified related to 

hardware and software (Senba et al., 1997). The 

majority of the problems were related to 

interference in giving remote handling

equipment access to process equipment. Most of 

the software problems were solved by changing 

operational procedures; hardware problems were 

solved by demonstration tests conducted using 

simulated objects. For example, equipment with 

electrical cables was improved by shortening 

and rearranging cable routes; cameras were 

moved to achieve better viewing; easy access to 

objects was enhanced by improving the electric 

drive system; and higher clearance was provided 

to impact wrenches, to prevent fracture of 

impact wrench sockets and bolt heads. 

3.4.4.2 Glass Accumulation in the Melter 

During the radioactive operations, radioactive 

glass accumulated in the coupling device 

between the melter and canister (Tomikawa and 

Yoshioka, 1996). This was attributed to 

extremely low temperatures at the discharge 

orifice region due to the lack of proper 

temperature control in the region. The discharge 

orifice was remotely modified and the location 

of the thermocouples was changed to provide 

better temperature control near the discharge 

section.

Figure 3.14. Tokai Vitrification Plant Offgas Treatment System (Uematsu, 1986) (Permission to 

use this copyrighted material is granted by WM Symposia, Inc., Tucson, Arizona)

39 NUREG/CR-6666
Joule-Heated, High-Level Radioactive Waste Processing Plants

3.4.4.3 Offgas Plugging 

A high-pressure differential in the melter offgas 

line leading to the SBS was encountered during 

cold testing, and was determined to be caused by 

the formation of deposits in this line. The 

problem was rectified by the repositioning and 

modification of the exhaust gas coupling device 

in this line assembly. 

3.5 High-Level Waste Vitrification 

Plant, Mayak, Russia 

3.5.1 Waste Pretreatment and Feed 

Preparation 

No information available. 

3.5.2 Vitrification 

Research on HLW vitrification was initiated in 

Russia in the 1950s. Several pilot-scale 

vitrification plants were designed and operated 

using non-radioactive simulated wastes, as well 

as radioactive wastes between 1967 and 1987. 

The first large-scale, liquid-fed melter at 

Chelyabinsk-65 in Kyshtym started operation in 

1986 and was commissioned in February 1987. 

The melter is lined with a high-alumina zirconia 

refractory and is fired using top-entering 

molybdenum electrodes. The melting surface 

area is 10.7 m2 (115.2 ft2) with a nominal glass 

depth of 0.4 m (1.3 ft). The outside dimensions 

of the melter are 9.48 m (31.1 ft) x 4.2 m 

(13.8 ft) x 3.2 m (10.5 ft) and it uses 56 

electrodes (4 groups of 14). The melter suffered 

an electrode failure after 12 mo in operation due 

to a very high current load (2,000 A) that caused 

accelerated corrosion and eventual failure. 

Between 1987 and 1988, the HLW vitrification 

facility produced 160 MT (352,000 lb) of 

phosphate glass containing 3.9 million Ci of 

radioactivity (Bradley and Schneider, 1990).

A second melter, similar in design, started 

testing operations in December 1990 and was 

commissioned for radioactive operations on 

June 25, 1991. This melter has produced more 

than 2,010 MT of glass containing 235 million 

Ci of radioactivity. The glass production rate 

was between 400 and 450 L/hr (105.7 and 

118.9 gal./hr). The facility was shut down in 

January 1997, 2.5 yr past its operational design 

life of 3 yr (Nuclear Waste News, 1997). 

Each mild steel pour container was filled with 

200 L (52.8 gal.) of phosphate glass, in a batch 

process, with a pour time of 1-2 min. A 

mechanical valve was used to open and close the 

melter side-drain. After lids were welded, three 

containers were placed in a stainless steel 

overpack. The pour canisters were not 

decontaminated prior to placement in the 

overpack. The overpacks were welded using a 

tungsten inert gas weld fixture attached to the 

cell floor, and rotating the canister by the weld 

head to obtain the fusion seal. No filler material 

was used during welding. Except for the visual 

inspection, there were no leak checks or other 

quality checks of the welds. The overpacks were 

placed in a storage building that is designed to 

cool canister heat loads of up to 5 kW 

(17,065 BTU/hr) by a forced-air circulation. 

The liquid HLW used as a feed was based on 

reprocessing fuel of both the BM-type and from 

VVER-440 reactor fuel. Unlike the rest of the 

world, the Soviets use phosphate glass as their 

high-level wasteform. 

Phosphate glasses, unlike borosilicate glasses, 

offer lower melting temperatures (approximately 

900 °C) coupled with higher sulphate solubility 

but have poor chemical durability and thermal 

stability. Furthermore, the melts are highly 

corrosive (Donald et al., 1997). Even though 

phosphate glasses are melted at a lower 

temperature, the corrosive nature of the melt 

limits the operating life of the melter. While

NUREG/CR-6666 40
Joule-Heated, High-Level Radioactive Waste Processing Plants

significant improvements have been made in 

improving chemical durability and thermal 

stability of phosphate glass wasteform, their 

performance is still well below borosilicate glass 

wasteforms, and they still remain highly 

corrosive. Significant process characteristics 

are: 

Molasses is used in the feed to trap the 

volatile radionuclides. 

Frit is not used, but the feed is spiked with 

additives to get the proper phosphate glass 

composition. 

The melter does not have any problems with 

noble metals because they alternate HLW 

solutions with solutions that contain no 

noble metals (such as intermediate-level 

waste streams). In addition, the large melter 

volume, 12 m3 (3,170 gal.), helps 

dilution/dispersion of noble metals.

• Electrode and refractory corrosion is a 

problem, but they expect that the electrodes 

will last as long as the refractories. 

3.5.3 Offgas Treatment System 

No information is available. 

3.5.4 Process Issues and Upsets 

The first HLW melter suffered an electrode 

failure due to excessive current load that caused 

accelerated corrosion and eventual failure of the 

molybdenum electrodes (Bradley, 1992). The 

electrode failure was attributed to failure of the 

cooling system that resulted in overheating of 

the electrodes and, hence, increased corrosion of 

electrode material. The facility was shut down 

and the damaged melter was replaced by a 

similar melter with an improved electrode 

cooling system.

41 NUREG/CR-6666
4 INDUCTION MELTING HIGH-LEVEL WASTE PROCESSING 

PLANTS

Induction melting technology using calcined 

wastes for encapsulating HLWs was first 

developed in France. The Atelier de Vitrification 

de Marcoule (AVM), also known as the 

Marcoule Vitrification Facility, was 

commissioned in 1978 to vitrify fission product 

solutions from the UPI reprocessing plant in 

France. The success of the AVM led to the 

startup of two similar facilities, R7 and T7. The 

R7 and T7 facilities were commissioned in 1989 

and 1992, respectively, at La Hague 

reprocessing plant. R7 and T7 are identical 

plants, each having three vitrification lines. Each 

vitrification line is designed for a production 

rate of 30 kg/hr (66 lb/hr). R7 and T7 are being 

used for vitrifying wastes generated from 

reprocessing 800 MTU at the UP2 and UP3 

reprocessing facilities, respectively. Each plant 

has an interim glass storage facility of 4,500 

canisters. Table 4.1 shows the operating data as 

of October 1997 for the three vitrification 

facilities in France. Furthermore, in 198 1, the 

United Kingdom adopted this technology at 

Sellafield. The Waste Vitrification Plant (WVP) 

at the Sellafield site was commissioned in 1991. 

In addition, similar technology was adopted in 

India to process HLW. 

4.1 Marcoule And La Hague 

R7/T7 Vitrification Plants, 

France 

The HLW vitrification processing description 

provided in the following sections has been 

extracted from the National Research Council 

(1996), Merlin (1986), Maillet and Sombert 

(1988), Desvaux and Delahaye (1994), 

Elliot et al. (1994), Jouan et al. (1997),

Pierson (1997),' Jain (1997), and Liberge et al. 

(1998)9. Since all French facilities are similar in 

operations, a combined description is provided 

for Marcoule and La Hague facilities. 

The remotely operated vitrification facility 

layout consists of several cells, organized as 

workshops at different levels, which are 

separated based on process functions. This was 

done to reduce cross-contamination risks, 

especially for the glass containers. The cells are 

stainless-steel lined, with viewing windows and 

manipulators along one side. Equipment in the 

plant is modular, to facilitate remote 

replacement. The facility design takes into 

consideration potential accidents such as failure 

of high activity liquid waste cooling system, loss 

of air for the dilution of radiolytic hydrogen, 

loss of cooling for the vitrified waste storage, 

fall of a heavy load (container) in the vitrified 

waste storage, flooding of the building from an 

external origin, extreme weather conditions, an 

aircraft crash, a fire, or an earthquake. 

4.1.1 Waste Pretreatment and Feed 

Preparation 

The characteristics of the waste solutions that 

constitute the blend feed for the melter are the 

following. 

• Fission product solutions-These are the 

concentrates from the fission product 

lPierson, R.C. 1997. Visit to Regulators and Facilities in 

France and England. Trip Report December 1, 1997. 

Washington, DC: Nuclear Regulatory Commission. 

2

Jain, V. 1997. Technical Exchange with the Regulators 

and Site Operators of the High-Level Waste (HLW) 

Vitrification Facilities at La Hague, France and Sellafield, 

UK. Trip Report. San Antonio, TX: Center for Nuclear Waste 

Regulatory Analyses.

43 NUREG/CR-6666
Induction Melting High-Level Waste Processing Plants

Table 4.1. Operational Summary as of October 1997 for High-level Liquid Waste Vitrification 

Plants in France (Liberge et al., 1998) (Permission to use this copyrighted material is granted 

by WM Symposia, Inc., Tucson, Arizona) 

Marcoule La Hague R7 La HagueT7 

Commissioned 1978 1989 1992 

Waste treated (mn) 2,140 3,622 1,904 

Curies Processed (million) 435 1,254 832 

Glass Produced (tons) 957 1,313 817 

Canisters 2,680 3,330 2,060

evaporators, including raffinates from the 

first extraction cycle and from other 

extraction cycles after preconcentration in 

acid recovery units. 

" Clarification fines-The fines are made of 

zircaloy and undissolved fission products, 

such as platinoids and molybdenum, and are 

separated by centrifugation. 

" Alkaline waste-These solutions come from 

the solvent regeneration of the first 

extraction cycle and the Pu purification 

cycles and contain mostly sodium nitrate 

and sodium carbonate. They also come from 

alkaline rinsing of fission-product 

concentration evaporators; in this case, they 

contain mostly sodium hydroxides. 

A vitrification process flowsheet is shown in 

figure 4.1. Fission product solutions, alkaline 

waste, and clarification fines are transferred 

from the reprocessing plant to the vitrification 

plant and stored in separate stainless steel tanks. 

The tanks are water-cooled and equipped with 

mechanical stirrers. Alkaline solutions are 

neutralized using nitric acid. Both fission 

product solutions and neutralized alkaline

wastes, in required proportions, are blended in 

an adjustment feed tank. The batch is analyzed 

and additives are included. The additives, nitric 

acid, and aluminum nitrate are intentionally 

combined with the waste to produce the desired 

glass composition. These solutions are 

separately metered, in required proportions, in a 

tank and blended with adjustment additives. 

Separate feeding lines are provided for the blend 

of fission products, alkaline waste and additives, 

and for the clarification fines. The two lines feed 

directly to the calciner through a metering 

wheel. The metering wheel outlet is connected 

to a hydraulic gauge that collects the solution 

from the recycle stream of the offgas treatment 

system. The hydraulic gauge is then connected 

to a small pot where the solution is mixed with 

sugar before flowing into the calciner. 

In this process, the liquid waste, along with 

scrubbing solutions and sugar, is converted to 

the oxide form by heating above the 

decomposition temperature of the waste 

components. The calciner is an inclined 

(3-degree) tube heated by electrical resistance 

that rotates at a speed of 20 rpm, and is operated 

at about 400 'C. Sugar is added upstream of the 

calciner to reduce ruthenium oxide to ruthenium

NUREG/CR-6666 44
Induction Melting High-Level Waste Processing Plants

Glass Frit

Scrubber

Calciner

gInterim Storagme ]" ---

Figure 4.1. La Hague/Marcoule Vitrification Process Flowsheet (Liberge et al., 1998) (Permission 

to use this copyrighted material is granted by WM Symposia, Inc., Tucson, Arizona)

45 NUREG/CR-6666
Induction Melting High-Level Waste Processing Plants 

metal. The heat decomposes nitrates to oxides. 

The offgas generated during calcination is sent 

to the offgas treatment system while the calcined 

waste is delivered directly to the melter crucible. 

4.1.2 Vitrification 

The waste exiting the calciner is mixed with the 

required amount of glass frit and fed to the 

inductively-heated, oval-shaped, melting 

crucible, shown in figure 4.2. The melting 

crucible sits in an induction furnace that has four 

induction coils around the crucible and operates 

at 4,000 Hz frequency. The melting crucible is 

1 in (3.3 ft) long, 0.25 in (0.82 ft) wide and is 

1.38 m (4.5 ft) tall and is made of 10 mm 

(0.4 in.) thick Inconel® Alloy 601. The air space 

between the melting pot and furnace is under a 

slight vacuum, and exhausts separately from the 

melter offgas. Unlike joule-heated melters in the 

United States and elsewhere that have an 

average operating life span of 3 or more yr, the 

melting crucible in induction melting technology 

is considered a disposable component. It takes 

about 2 wk to replace the failed melting pot. The 

failed components are sent to a breakdown cell 

where they are cut into pieces and disposed. A 

major achievement has been made in the 

operating life of the melting pot. The operating 

life has increased from 500 to 7,000 hr. The 

crucible material is still Inconel® Alloy 601, but 

better heat control and distribution to reduce 

thermal, chemical, and electrical stresses has 

resulted in improved life. Other factors 

contributing to improved pot life include 

changes in pot fabricating technology, and 

preheating of the pot to 900 'C before the first 

glass pour. A typical failure mode of the melting 

pot is a leak in the melting pot due to vapor 

phase corrosion. Leaks occur above the glass 

melt line. The operators run the melter to its 

failure. The failure is predicted by the changes 

reflected in pressure drop orincrease in the 

offgas flow rates.

Recently, the T7 vitrification plant added 

stirring by mechanical stirrers and argon 

bubbling to homogenize the melt. The melting 

pot has two mechanical stirrers introduced from 

the top. Additional stirring is accomplished by 

introducing argon under the melt. Since the 

introduction of the mechanical stirrers, the T7 

facility has seen a 10-percent increase in 

production rate. The glass is poured every 8 hr, 

with pouring triggered by heating the bottom 

nozzle. Heating is stopped as soon as pouring 

begins, and the pouring stops by itself when the 

glass level reaches the siphon level and a glass 

plug forms in the nozzle. The melter crucible 

has another glass plug at the bottom, which is 

used to completely drain the melter crucible. 

Each canister is 1.335 m (4 ft) tall and 0.43 m 

(1.3 ft) in diameter, can hold about 0.1 5-m 3 

(5.3 ft) of glass, and requires two pours to fill. 

Before the first pour, the canister is pre-heated 

and, after filling, is transferred to the cooling 

and welding cell with a crane. After 24 hr of 

cooling, the canisters are transferred to a 

welding machine. Tacking and welding are 

performed without any filler metal by rotation of 

the plasma torch around the canister lid. Next, 

the canister is transferred to the contamination 

monitoring unit. If the canisters are 

contaminated, they are transferred back to the 

decontamination cell and decontaminated using 

a high-pressure 25,000-kPa (250-bar) water jet. 

After decontamination, the canisters are first 

transferred to the contamination monitoring unit 

and then to the vitrified product store (VPS)3. 

The VPS is an array of 500 stainless tubes that 

can hold nine containers in each tube for a total 

of 4,500 containers. The VPS is forced-air

cooled to keep the temperature of the walls 

below 90 'C and the glass temperature below 

510 'C. In case of power failure, the containers 

3

The vitrified product resulting from reprocessing fuel 

for foreign clients is sent back to the clients.

NUREG/CR-6666 46
Induction Melting High-Level Waste Processing Plants

Calciner + Glass Frit

"""- AInduction Heaters 

wring ozzleDrainage Nozzle 

Figure 4.2. La HaguelMarcoule Vitrification Induction Melter (Liberge et al., 1998) (Permission 

to use this copyrighted material is granted by WAM Symposia, Inc., Tucson, Arizona)

NUREG/CR-6666 47
Induction Melting High-Level Waste Processing Plants 

are cooled by natural convection. The change 

from normal ventilation to natural draft is 

automatic. The average heat generation rate is 

about 2 kW (6,826 BTU/hr) per canister. 

4.1.3 Offgas Treatment System 

The function of the offgas treatment system is to 

scrub offgas from the calciner, recycle scrubber 

solution from the calciner, condense the steam at 

the scrubber outlet, absorb NO. vapors, transfer 

liquid effluents to collecting tanks, ensure 

proper negative pressure in the equipment, wash 

offgas, and release offgas to the atmosphere. 

Dusts and offgases from the calciner are first 

passed through a dust scrubber. The dust 

scrubber is a cylindrical vessel, with the upper 

portion equipped with inclined plates, and the 

lower portion containing boiling nitric acid. The 

inclined plates trap the dust and are washed 

down by nitric acid sprays. The used nitric acid 

feeds back to the calciner with the recycling 

solution. The offgas at the scrubber outlet is 

passed through a condenser. Condensate is 

collected in a buffer tank located below the 

condenser, and the offgas goes to the NO, 

absorption column. In the washing column, the 

offgas is mixed with other offgases coming from 

various units of the plant. The offgas then passes 

through a cold trap, demister, heater to limit 

droplet carryover, two shielded HEPA filters, 

and one non-shielded HEPA filter. Two pumps 

ensure negative pressure in the washing column 

and all the other equipment. The pumps also 

ensure that the offgas is exhausted from the 

plant stack.

4.2 Waste Vitrification Plant, 

Sellafield, United Kingdom 

The WVP at Sellafield, United Kingdom, is an 

adaptation of the French AVM process. The 

construction of the WVP at Sellafield began in 

1981 to vitrify HLLW from the Magnox 

reprocessing plant and the Thermal Oxide 

reprocessing plant, in addition to processing the 

backlog of the stored HLLW. The WVP consists 

of two vitrification lines, vit line 1 and vit line 2, 

and has a higher throughput than the AVM. The 

active operation of the plant started in 1991. Vit 

line 3 is currently being constructed and 

incorporates lessons learned from vit lines 1 and 

2 operations. These changes include improved 

cell layout, an improved dust scrubbing system, 

a dual melter on the turntable to reduce 

downtime from 2 wks to a few hours, improved 

decontamination systems, and improved saws in 

the breakdown cell. The description provided in 

the following sections has been extracted from 

Elsden and Woodall (1988), National Research 

Council (1996), Pierson (1997)4, and Jain 

(1997)'. The main steps of the process are 

similar to the Marcoule and La Hague facilities, 

as discussed in detail in the earlier section. 

Specific aspects of the WVP process are 

described in the following sections.

4.1.4 Process Issues and Upsets 

No information is available.

4Pierson, R.C. 1997. Visit to Regulators and Facilities in 

France and England. Trip Report December 1, 1997. 

Washington, DC: Nuclear Regulatory Commission. 

5

Jain, V. 1997. Technical Exchange with the Regulators 

and Site Operators of the High-Level Waste (HLW) 

Vitrification Facilities at La Hague, France and Sellafield, 

UK. Trip Report. San Antonio, TX: Center for Nuclear Waste 

Regulatory Analyses.

NUREG/CR-6666 48
4.2.1 Waste Pretreatment and Feed 

Preparation 

Reprocessing at the British Nuclear Fuels 

Limited started in 1952 but no waste was treated 

until 1990. Currently, there are 21 storage tanks; 

tanks 1-6 contain old waste and tanks 7-21 

contain active solutions. There are about 180 

different waste streams. Characteristics of waste 

include well-defined composition and high heat 

load, mostly fission products. Criticality issues 

are minimal. 

Figure 4.3 shows a schematic diagram of the 

WVP process. The feed preparation takes place 

in two tanks. HLLW is transferred from the 

reprocessing facility to the tank in WVP. 

Lithium nitrate solution is added to the HLLW 

to ensure adequate reactivity of the calcined 

HLLW. The prepared feed is characterized by 

sampling.and analysis before being fed forward 

for vitrification. A constant-flow meter is used 

Highly Ative Calcination 

Liquid Waste Additives

Induction Melting High-Level Waste Processing Plants 

to control the flow of feed to the calciner. In 

addition to HLLW, other feeds that go to the 

calciner include effluent recycle from the dust 

scrubber, sugar solution, additional lithium 

nitrate, and dilution waste. The calciner consists 

of an inclined stainless steel tube rotated inside 

of an electrical inductance furnace. As the feeds 

flow down the calciner, they are evaporated, 

dried, and heated to decompose nitrates. The 

dried calcine is directed to the melter. 

4.2.2 Vitrification 

The melter furnace is inductively heated and 

controlled at 1,100 *C. Glass frit is fed along 

with calcined HLLW in controlled batches. As 

the level of the vitrified waste reaches a 

predetermined height in the melter, a freeze 

valve at the base of the melter is activated, and 

molten glass is cast into the product container 

under the melter. The pouring takes place every 

8 hr, and each container takes about two pours

Storag 

Surface Lid Container 

Decontamination Welding for Glass

Figure 4.3. Process Diagram for the Sellafield Waste Vitrification Plant in the United Kingdom 

(Elsden and Woodall, 1988) (Permission to use this copyrighted material is granted by WM 

Symposia, Inc., Tucson, Arizona)

49 NUREG/CR-6666
Induction Melting High-Level Waste Processing Plants 

to fill. The glass container is 1.3 in (4.26 ft) tall 

and 0.42 m (1.38 ft) in diameter and holds about 

150 L of vitrified HLLW glass. The glass 

container is held in a preheated furnace at 

500 °C before and during pouring. This 

minimizes thermal shock on the glass. After 

filling, the container is lowered from the preheat 

furnace and allowed to cool. A lid is then 

loosely fitted, and the container and lid are 

completely sealed by an automatic fusion 

welding technique. The container is then placed 

in a decontamination vessel where high-pressure 

water or nitric acid is used to remove loose 

contamination from the external surfaces. After 

decontamination, the canister is checked and, if 

acceptable, it is transferred to the VPS. The VPS 

has a capacity to store 8,000 containers. The 

safety rule requires the air temperature not to 

exceed 110 'C. In addition, the centerline 

temperature of the glass cannot exceed 510 °C, 

to ensure that the glass is stored below the glass 

transition temperature. At temperatures higher 

than the glass transition temperature, the glass 

behaves like a viscous fluid. The VPS is cooled 

by natural convection. The typical heat 

generation rate is 1.2 kW (4,096 BTU/hr) per 

container, which is well below the maximum 

allowable heat generation rate of 2.4 kW 

(8,192 BTU/hr). 

4.2.3 Offgas Treatment System 

The offgas treatment system consists of a dust 

scrubber, a tube condenser, a NO. scrubber, an 

electrostatic precipitator, a scrubber column, and 

high-efficiency filters. 

4.2.4 Process Issues and Upsets 

In the 1980s, the induction melting technology 

was purchased from the French. While the 

technology was adopted from the French, the 

waste processed at Sellafield was significantly 

different compared to French waste. Major

process problems resulted during the initial 

operations. Even though fundamentals of the 

technology were good, one of the lessons 

learned very early in the process was that 

success is embedded in knowing the fine levels 

of detail. Some specific problems that have a 

severe impact on vitrification operations are 

discussed in the following section. 

4.2.4.1 Melter Failures 

Melting pot life is of significant concern. The 

best performance of the ovoidal 1 cm (0.40 in.) 

thick Inconel® Alloy 601 melting pot has been 

2,000 hr. Typically, melting pot life has ranged 

from 900 to 2,000 hr (this is in contrast to the La 

Hague facility where melting pot life of more 

than 4,000 hr is normally achieved). Routinely 

observed failures in the melting pot are: 

Leaks in the melting pot due to vapor phase 

corrosion followed by a vertical crack 

through the melting pot within 10 min. Due 

to the catastrophic failure of the melting 

pots, BNFL Inc. never runs melting pots to 

failure. The axial power profile and 

temperature gradients are monitored to 

estimate thinning of the Inconel® Alloy 601 

melting pot, which have been quite 

successful for replacing melting pots prior to 

failure. The improved life of the melting 

pots has been attributed to reducing thermal 

stresses by controlled heating and providing 

an even balance of temperature around the 

melting pots. In addition, an insulating

fiber-wool blanket is placed around the 

melting pot to ensure uniform distribution of 

heat. 

Failure of pour nozzles was the most 

frequent mode of melting pot failure. With 

melting pots, glass pouring is accomplished 

by inductively heating the pour nozzle to 

melt the solidified glass plug in the nozzle. 

When the glass has been poured, the

NUREGICR-6666 50
induction heater is switched off and the 

glass is allowed to solidify in the nozzle to 

stop further glass pouring. Wavering of the 

glass pour stream due to pressure 

fluctuations in the melting pot results in an 

imperfect seal, allowing the glass to drip and 

form an external coating around the pour 

nozzle. The external corrosion of the pour 

nozzle by the glass is responsible for its 

failure. This type of failure has been 

minimized by controlling offgas flow 

characteristics and, in fact, in the last 18 mo, 

pour nozzle failure has not occurred. 

Early failure is attributed to pour nozzle failure; 

midlife failure is attributed to the splitting of 

external and internal reinforcing ribs at the ends; 

and general failure occurs around 1,300 to 

1,400 hr due to vapor phase corrosion. 

4.2.4.2 Failures of Cranes and 

Manipulators 

The WVP requires significant usage of MSMs 

and in-cell cranes during normal operating 

conditions. There are five in-cell cranes and 80 

positions for MSMs, of which 56 are regarded 

as key MSMs (Millington, 1995). Low 

reliability of the in-cell cranes and the MSMs 

was a major process issue. The main problem 

encountered with the in-cell cranes was the 

degradation of electrical cables, connectors, and 

electronic components at a rate greater than 

anticipated during design, and high failure rates 

of MSMs were coupled with inadequacies in the 

workshop and backup facilities needed for 

repairs. A secondary issue was the slow rate at 

which used contaminated crucibles were 

sectioned and disposed from the breakdown cell 

to allow room for a new crucible. In one 

instance, a portable grinder in the breakdown 

cell caught fire. Since the start of the operation 

in the cell, cranes, MSMs, and cutting 

equipment in the breakdown cell have been

Induction Melting High-Level Waste Processing Plants 

upgraded. These changes have resulted in a 

significant reduction in process downtime. 

4.2.4.3 Shield Door Interlock Failure 

A potentially fatal incident occurred during 

1991. Two shield doors are located between the 

area where maintenance workers enter the hot 

cells, and the control cell where the swabbing 

robot is located. An interlock prevents a canister 

access gate on the floor of the control cell from 

opening while the inner shield door is open, and 

a gamma monitor is located between the two 

shield doors. During the early morning hours of 

Friday, September 13, 1991, workers opened 

both doors and entered to perform maintenance 

on the swabbing robot. Upon exiting, they left 

the doors open. Early Sunday, the 

September 15, 1991, an operator opened the 

gate on the floor of the control cell and lifted a 

canister of vitrified glass from the 

decontamination cell into the control cell. The 

gamma alarm sounded, the operator observed on 

a television monitor that the shield doors were 

open, and the canister was immediately lowered. 

The interlock, which was supposed to prevent 

the gate on the floor from opening, had not 

functioned as intended. The radiation level at the 

gamma monitor was measurable in tens of 

thousands of Rem per hour. Had maintenance 

workers been in the cell, they could have 

received fatal doses within a few minutes.

4.3 Waste Immobilization Plant, 

Tarapur India

Solidification research on HLW was initiated at 

the Bhabha Atomic Research Center in India 

during the early 1960s. A remotely operating 

HLW Waste Immobilization Plant (WIP) was 

constructed in 1981 and commissioning tests 

were completed in 1985-86 at Tarapur to 

convert HLW to borosilicate glass. Information

51 NUREG/CR-6666
Induction Melting High-Level Waste Processing Plants 

provided in this section has been extracted from 

the U.S. Department of Energy (1994). 

Unlike the French induction melting process, the 

WIP is designed around a "pot-glass" process 

where HLW is first calcined at 600 °C in an 

Inconel® Alloy 690 melter vessel. The HLW mix 

is transferred until the vessel is about 75 percent 

full; it is then calcined at 600 °C, and finally 

melted in the same vessel at 950-1,000 °C using 

a three-zone induction furnace. The melting 

vessel is 0.325 m (1 ft.) outside diameter and 

1.8 m (5.9 ft) long. The contents of the melter 

are poured through a drain pipe into an insulated 

304 L stainless steel canister, which is 0.325 m 

(1 ft) outside diameter and 0.775 m (2.5 fi) tall

with a 10-mm (0.4 in.) wall thickness, that can 

hold up to 45 L (11.9 gal.) of glass. A fully 

loaded canister has a heat generation rate of up 

to 1.75 kW (5,973 BTU/hr) and contains 

approximately 94,000 Ci. The HLW processing 

rate is 25 L/hr (6.6 gal./hr), and by using two 

vitrification lines, one canister/day is produced. 

The filled canisters are sealed by welding a lid. 

Two welded canisters are inserted into a carbon

steel overpack, which is sealed by welding a lid 

and placed in an interim storage facility 

designed with stack-induced natural draft air 

cooling. The current plans call for interim 

storage of vitrified wastes for 20-25 yr to cool 

and ensure integrity of the waste package before 

disposal in a repository.

NUREG/CR-6666 52
5 JOULE-HEATED, MIXED LOW-LEVEL RADIOACTIVE AND 

HAZARDOUS WASTE PLANT

5.1 M-Area Vitrification Plant at 

Savannah River Site, South 

Carolina 

The SRS stores approximately 2,500 m3 

(660,000 gal.) of mixed low-level radioactive 

and hazardous waste at the M-Area. In 

November 1993, GTS Duratek was awarded the 

contract to construct a facility to vitrify the 

M-Area low-level radioactive mixed waste. The 

construction of the vitrification plant was 

completed in January 1996, and it was 

commissioned to process low-level radioactive 

mixed wastes in October 1996. The Duratek 

melter, Duramelter" 5000, is currently the 

largest low-level radioactive melter system 

operating in the world, and is also a one-third 

size prototype of the melter system to be built 

and operated by BNFL Inc. under the Hanford 

TWRS Program (Duratek, 1996). 

The process description provided in the 

following sections has been extracted from 

Bowan and Brandys (1994, 1995). Since all of 

the published information on this melter 

precedes the construction of the plant at the 

SRS, it is. likely that the current design and 

operating process may be different. 

5.1.1 Waste Pretreatment and Feed 

Preparation 

The M-Area waste at SRS is stored in three 

partially filled 2,900-m3 (766,000-gal.) tanks, 

6 132-m 3(35,000-gal.) tanks and about 

125 0.21-m 3 (55-gal.) drums. The 0.21-m 3 

(55-gal.) waste drums contain mixed TRU 

wastes such as Watts Plating Solution and Mark 

15 Filtercake. Prior to vitrification, the wastes 

are blended to provide two large batches of 

similar compositions. Because of the limited 

free volume in the tanks, one-step mixing is not

possible. The mixing strategy is shown in 

figure 5.1. It is planned to transfer the entire 

contents of the 132-m3 (35,000-gal.) tanks (tanks 

100-1 to 100-6) to Tank 100-7. The contents of 

Tank 100-10 will be evenly split between Tanks 

100-7 and 100-8. Since Mark 15 drums contain 

a substantial quantity of uranium, they are added 

to the Tank 100-7 mixture, which has a deficit in 

uranium concentration. The Watts Plating 

solution is added to the Tank 100-8 wastes. The 

estimated composition of the blended wastes is 

shown in table 5.1. 

The vitrification process flowsheet is shown in 

figure 5.2. The waste mixed with glass-forming 

chemicals is transferred to the M-Area plant for 

vitrification. The waste slurry mixed with 

glass-forming chemicals is pumped to a 

single-stage vitrification melter. The slurry 

enters the melter through water-cooled entrance 

ports and is deposited on the surface of a 

molten-glass bath. In the melter, the remaining 

water is evaporated from the slurry, and nitrate 

salts are dried into a solid crusty layer (cold-cap) 

on the surface. The salts under the cold-cap 

calcine and fuse with the glass-forming 

chemicals, thereby producing glass melt of the 

target composition. 

5.1.2 Vitrification 

The Duramelter® 5000 is a joule-heated electric 

melter with a glass holding capacity of 1,950 kg 

(4,280 lb) and a glass pool surface area of 5 m2 

(54 ft2). The glass pool is powered by three 

parallel submerged electrodes. The power 

supply to the electrodes controls the melt 

temperature at 1,150 *C. The melter has two 

discharge ports for the molten glass. One pour 

spout is dedicated to filling 0.27 m3 (71 gal.) 

square drums while the second pour spout is 

coupled to a gem-machine to produce glass 

gems. Glass gems are flattened marbles about

53 NUREG/CR-6666
Joule-Heated, Mixed Low-Level Radioactive and Hazardous Waste Plant

1S Transferred Tanks 100-1 Thru 100-6 to 100-7 

* 50% Tank 100-10 

Transferred to 100-7 

* 50% Tank 100-10 

Transferred to 100-8 

Watts Step 2: 

Plating Bath * Tanks 100-7 Transferred to 

Tanks 100-1/100-2 with Mark 

100-715, Chemicals Added, Transferred to Vitrification 

"• Tanks 100-8 Transferred to 

Tanks 100-1/100-2 with Watts Plating, Chemicals Added, Transferred to Vitrification 

Mark 15 Fitercake 

Glass Cemicals Vitrification 

Figure 5.1. Mixing Strategy for the M-Area Mixed Waste at the Savannah River Site (Bowan and Brandys, 1995) (Permission to use this copyrighted material is granted by American Ceramic Society, 

Westerville, Ohio)

NUREG/CR-6666 54
Joule-Heated, Mixed Low-Level Radioactive and Hazardous Waste Plllt

Table 5.1. Composition of the Savannah River Site M-Area Blended Waste Batches (Bowan and 

Brandys, 1995) (Permission to use this copyrighted material is granted by American Ceramic 

Society, Westerville, Ohio) 

Tank 100-7 Tank 100-8 Tank 100-7 Tank 100-8 

Blend Dry Blend Dry Blend Dry Blend Dry 

Species Wt (kg) Wt (kg) Species Wt (kg) Wt (kg) 

At 26,468 39,986 Zr 24 29 

Ca 1,404 1,235 Ti 90 109 

Fe 2,212 2,030 K 3,881 2,963 

Mg 446 473 P 6,185 6,828 

Mn 504 1,252 Ba 13 152 

Na 44,170 30,859 Pb 454 431 

Li 4 7 Mo 18 30 

Ni 4,528 3,262 Zn 1,374 2,459 

Si 70,470 69,946 Cu 195 66 

Cr 20 54 F 198 118 

B 99 33 CI 1,939 515 

U 11,596 14,834 SO 4 1,232 1,721 

Sr 2 16 NO3 66,874 64,405

1.5 cm (0.6 in.) in diameter. Flattened marbles 

offer flexibility of repackaging, if needed. A 

major disadvantage is&lower volume reduction 

compared to glass monolith and an additional 

operational step in the process. Each pour spout 

is activated by an airlift for controlled glass 

discharge into the canister or gem-making 

machine. Additional lid heaters are provided in 

the melter plenum for startup and operations. In 

addition, the melter is equipped with a patented 

air-bubbling stirring system. This system 

accelerates melter production and oxidizes 

organics and metallic inclusions in the waste

stream. The melter is designed to produce a 

minimum of 5,000 kg (11,000 lb) of glass per 

day and a maximum of 15,000 kg (33,000 lb) of 

glass per day. 

5.1.3 Offgas Treatment System 

The offgas treatment system maintains the 

melter at a constant, but slightly negative, 

pressure, traps particulate radionuclides, and 

treats the offgas emissions for release to the 

atmosphere. The offgas from the melter goes to 

a water-spray quencher followed by two

55 NUREG/CR-6666
Joule-Heated, Mixed Low-Level Radioactive and Hazardous Waste Plant

Figure 5-2. The Savannah River Site M-Area Mixed Waste Process Flowsheet (Bowan and Brandys, 

1995) (Permission to use this copyrighted material is granted by American Ceramic Society, 

Westerville, Ohio)

aqueous-based, packed-bed scrubbers connected 

in series, charged with NaOH and H202 to 

provide 90-95 percent removal of NO. 

Following the packed-bed scrubbers, a dry 

filtration process retains most of the particulates. 

These filtration units are baghouses and HEPA 

filters. The final exhaust contains steam, carbon 

dioxide, and a small fraction (less than 

10 percent) of untreated NO,. 

5.1.4 Process Issues and Upsets 

Since the startup of the plant, there have been 

numerous problems associated with the 

performance of the melter and other 

components. Except for the recent shutdown of 

the plant to evaluate melter components and the

decision to replace the melter, the author is not 

aware of any published documents that detail 

these issues. 

On March 31, 1997, the waste processing was 

temporarily suspended after observing possible 

signs of wear on melter components. Visual 

inspections indicated that hot spots formed in 

the melter were a result of refractory corrosion 

by molten glass, especially near the refractory 

joints, that were attributed to chemicals used in 

the melter startup. On April 16, 1997, a decision 

was made by GTS Duratek to replace the 

existing melter with a new one. On 

December 11, 1997, a new melter was installed, 

and the operations were restarted.

NUREG/CR-6666 56
6 HIGH-TEMPERATURE, JOULE-HEATED LOW-LEVEL 

RADIOACTIVE MIXED WASTE PLANTS

Several high-temperature, joule-heated melters 

designed to vitrify low-level mixed waste are 

operational around the world. In this section, 

discussion will be focused on the Fernald 

Vitrification Pilot Plant System (VPPS) and the 

Transportable Vitrification System (TVS). Even 

though both systems were shut down for major 

repairs because of glass leaks from the melter, 

they provide insight into the issues relevant to 

the safety of the vitrification operations. The 

TVS melter was redesigned and demonstrated 

vitrification of low-level radioactive mixed 

wastes; the VPPS is still shut down. 

6.1 Fernald Silo Waste 

Vitrification System, Fernald, 

Ohio 

The Fernald Vitrification plant was designed to 

vitrify K-65 (Silos 1 and 2) and Silo 3 residues 

stored onsite in Fernald, Ohio. The Fernald 

vitrification melter, a Duramelter® 1000-HT, 

was designed, constructed, and delivered by 

GTS Duratek for vitrification operations. The 

Duramelter* 1 000-HT is a high-temperature 

melter with a production capacity of 1,000 kg of 

glass/day (2,200 lb/day). 

6.1.1 Waste Pretreatment and Feed 

Preparation 

The three silos in Operable Unit 4 (OU-4) at the 

Fernald Environmental Restoration Management 

Project in Fernald, Ohio, contain residues from 

the processing of pitchblende 

(mineral-containing uraninite) ores (Merrill and 

Janke, 1994). Silos 1 and 2, designated jointly as 

K-65, contain the depleted ore with Bento 

Groute cap over the materials. Bento Grout® is a 

synthetic mineral used to reduce radon 

emanation. Silo 3 contains calcined residue from 

processing solutions. The residues from the

three silos contain radium, uranium, uranium 

daughters, and heavy metals (primarily lead). 

6.1.2 Vitrification 

The Duramelter' 1000-HT melter is unique in 

its design. The melter is designed to operate at 

1,400 'C. Unlike traditional waste glass melters, 

it has three melting chambers. A top view of the 

melter is shown in figure 6.1. The two side 

chambers contain benign borosilicate glass 

while the central chamber contains waste glass 

melt. Since the waste glass melt contains almost 

10 percent lead oxide, the presence of benign 

glass in the side chambers was designed to 

protect the molybdenum electrodes from 

corrosion and by lead oxide in molten waste 

glass. The three chambers are separated by 

high-chrome, high-electrical conductivity 

refractory Monofrax-E blocks. The two side 

chambers contain five molybdenum tapered rod 

electrodes. The glass contact refractory, as for 

all waste melters, is high-chrome Monofrax® 

K-3. The Monofrax® K-3 is backed up by 

bonded Zirmul® and castable aluminum oxide. 

The glass containment refractory is housed in an 

Inconel® Alloy 690 inner shell, whereas the 

entire melter is housed in a stainless steel outer 

shell. The space between the inner shell and the 

outer shell is filled with insulating fiberboard. 

The outer shell of this melter is not water 

cooled. Other unique features include the 

presence of several bottom and side penetrations 

in the melter. There are five bottom penetrations 

for the bubblers, one for the airlift lance, and 

three for the bottom drains. These are shown in 

figures 6.2 and 6.3. The bubbler tubes are placed 

uniformly through the floor of the melter down 

the middle of the center chamber. These tubes 

are made of molydisilicide and are used to 

provide agitation by blowing air into the center 

chamber glass melt. Additionally, air would 

provide an oxidizing environment to promote

57 NUREG/CR-6666
High-Temperature, Joule-Heated Low-Level Radioactive Mixed Waste Plants

Bottom Drain

Surface. 

Drain

A _j

TOP VIEW (SECTION C-C) 

Figure 6.1. Top View of the Duramelter 1000-HT Melter at the Fernald Site (FERMCO, 1997)

oxidation of metals in the feed. The airlift lance 

is also made of molydisilicide and is used to 

discharge glass down the pour spout and out into 

the canister. Four bottom drains are provided to 

empty the melter. There are two drains for the 

center chamber and one each for the side 

chambers. The bottom penetrations are provided 

to reduce corrosion of the tubes from sulfur and 

halide emissions at temperatures greater than 

1,150 'C when exposed from the top of the 

melter. The melter heatup was started on 

May 16, 1996, and was shut down on 

December 26, 1996 when 6,000 kg (13,200 lb) 

glass drained out of the melter in 15 min. from 

one of the corroded air bubbler tubes. 

6.1.3 Offgas Treatment System 

No information is available.

6.1.4 Process Issues and Upsets 

The information provided in this section has 

been extracted from a single report (FERMCO, 

1997). The process issues related to feed 

preparation and offgas system operations are not 

published anywhere. The melter incidents 

leading to its failure are presented in detail in the 

following section. Other minor incidents 

included coolant leaks in bottom drains and 

electrical shorts to ground. 

6.1.4.1 Separation Wall Between the 

Chambers 

The separation wall between the chambers was 

designed to avoid any mixing between the waste 

glass and the inert glass. Soon after the melter 

was heatedup, cracks were observed in the 

separation walls. These cracks allowed 

migration of glass from one chamber to the

NUREG/CR-6666 58
High-Temperature, Joule-Heated Low-Level Radioactive Mixed Waste Plants

'-

View Port 

K-3 

AZS 

Castable 

Inner Shell 

Insulation 

Outer Shell

Air Bubbler--

Discharge Hole-

•L "--ýSide Chamber Drain 

Bottom Drain

BACK VIEW (SECTION B-B) 

Figure 6.2. Back View of the Duramelter 1000-HT Melter Showing Bottom Drain Tubes (FERMCO, 

1997)

59 NUREG/CR-6666
High-Temperature, Joule-Heated Low-Level Radioactive Mixed Waste Plants

Emergency Exhaust- [-- nPB /-Feeding Port

Off Gas FSpare Port 

SMonofrax H Backup Air Lift Lance (Option) 

Discharge Heaters 

Start-Up Heater Discharge 

View Port 

Side Chamber 

Feeder 

CC 

Discharge Port 

Thermowell Discharge Raiser 

SE-Monofrax K-3 

Surface Drain -Mo Electrodes /''1 -Air Lift Lance 

Air Bubblers -"B 

SIDE VIEW (SECTION A-A) 

Figure 6.3. Side View of Durameltere 1000-HT Melter Showing Bottom Bubblers (FERMCO, 1997)

NUREG/CR-6666 60
High-Temperature, Joule-Heated Low-Level Radioactive Mixed Waste Plants

other. The refractory was also used as a 

secondary electrode that allowed current to flow 

from molybdenum electrodes to the center 

chamber. However, the presence of electrical 

currents in the separation wall and 

inhomogeneous distribution of crystalline phase 

resulted in the formation of hot spots on the 

wall. These hot spots, when combined with the 

poor thermal shock resistance of the Monofrax® 

E refractory, caused the wall to crack, which 

allowed easy migration of lead into the side 

chambers. 

6.1.4.2 Auto Discharging of Glass from the 

Melter 

Auto discharging was a frequent event. On 

July 12, 1996, a large chunk of glass was 

removed from the discharge section. 

Investigations indicated that the discharge 

orifice brick had a large crack. During campaign 

1, a torch'was used to melt the glass from the 

discharge chamber. A cooling jacket was placed 

around the pour spout to solidify the glass and 

stop the migration into the discharge section. 

The failure of the brick could be due to thermal 

stresses in the melter generated during heatup. 

6.1.4.3 Corrosion of MoSi2 Components 

The MoSi2 tubes were used to determine the 

glass level in the side chambers. Inspection of 

the level probes during campaign 1 indicated 

corrosion of the tips. Corrosion of the MoSi2 

bubbler tubes was a major contributing factor to 

the failure of the melter. The MoSi2 tubes in the 

presence of PbO in the waste glass caused 

formation of lead metal in the melter. As MoSi2 

tubes corroded in the presence of PbO in the 

melt, low-corrosion-resistant bonded Zirmul® 

bricks were exposed to the lead attack. Metal 

drilling of bonded Zirmul® bricks by metallic 

lead resulted in dissolution of more than 

50 percent of the Zirmul brick layer under the 

glass contact refractory Monofrax® K-3. The

post-incident evaluation of the melter indicated 

that the K-3 bricks were still in good condition, 

while the Zirmul® brick layer was completely 

destroyed. Corrosion was further enhanced by 

abrasion, oxidation, and molten glass circulation 

via compressed air from the bubblers. The lead 

formation, in addition to corroding the MoSi2, 

led to the formation of hot spots that disrupted 

electrical current paths and short circuited 

normal current paths in the melt. This 

accelerated Zirmul® dissolution and corrosion. 

Finally, the integrity of the melter was breached 

causing glass to flow out from the melt cavity. 

6.1.4.4 Foaming Events in the Melter 

Foaming was frequently observed in the melter. 

Foaming is the generation of stable gas bubbles 

in the glass that forms a stable froth on the 

surface. This froth is usually very insulating and 

disrupts the temperature profile in the glass. The 

disruption of the temperature profiles results in 

erratic power supply to the melter. Another 

event similar to foaming is the formation of an 

immiscible sulfate layer on the melt surface. In 

the Fernald melter, urea was used as a reducing 

agent to control redox and enhance destruction 

of sulfates in the melt. While reducing the melt, 

foaming excursions are minimized, but 

uncontrolled addition of urea could result in 

reduction of transition metal oxides to their 

metallic states. In one instance, the sulfate foam 

boiled over into the side chambers. No redox 

control strategy was implemented in Fernald. 

These incidents can be attributed to the lack of 

understanding of control of foaming, urea 

reactions, redox conditions, and sulfate 

chemistry. 

6.2 Transportable Vitrification 

System 

The TVS is designed to treat low-level 

radioactive and mixed wastes. The unit is a

61 NUREG/CR-6666
High-Temperature, Joule-Heated Low-Level Radioactive Mixed Waste Plants

transportable large-scale, fully integrated 

vitrification system. The equipment is housed in 

modules that can be disassembled and sealed for 

shipment on standard trailers. Major modules 

include feed preparation, melter, offgas, process 

control and services, and a process laboratory. 

Each of these modules is discussed in the 

following (Jantzen et al., 1995; Zamecnik et al., 

1998). TVS started its operations at the Oak 

Ridge National Laboratory (ORNL). A glass 

leak was observed in the TVS melter in 

November 1996 and the melter was shutdown. 

The leak was attributed to the movement of the 

refractories during repeated heatup and cool

down cycles. Figure 6.4 shows the process flow 

diagram for the TVS. The design of the TVS 

melter was modified, was rebuilt and on October 

17, 1997, the demonstration of the TVS with 

mixed waste was completed. A total of 7,300 kg 

(16,220 Ib) of sludge was processed, producing 

7,600 kg (16,890 lb) of glass (Cozzi et al., 

1999). The system is currently shut down due to 

a lack of funding. 

6.2.1 Feed Preparation Module 

The waste is introduced in the feed module 

where it is characterized and blended with the 

glass-forming chemicals. Pumps are used to 

feed the waste plus glass-former slurry to the 

melter. Both dry or liquid waste could be used in 

the module to make the slurry. 

6.2.2 Vitrification Module 

As the slurry falls on the glass-melt surface, 

water evaporates leaving behind a crusty layer 

of salts (cold-cap) on the surface of the glass 

melt. The cold-cap formation reduces the 

volatilization. The solids between the crusty 

layer and the glass-melt surface are calcined to 

their oxides and incorporated into the glass melt.

The melter module consists of ajoule-heated 

melter and auxiliary equipment. The melter, 

which was manufactured by Envitco, Inc., 

Toledo, Ohio, is capable of operating between 

1,100 and 1,400 *C. Since the melter operates at 

temperatures greater than 1,150 °C, 

molybdenum electrodes are used for glass 

melting. Anticipated glass production rates for 

this melter are between 50 kg/hr (110 lb/hr) and 

150 kg/hr (330 lb/hr). The melter has three 

chambers, a center chamber and two side 

chambers. The center chamber is a melting 

chamber where waste and glass formers are 

vitrified; one side discharge chamber is used to 

pour glass into 610-mm (24-in.) stainless steel 

cubes, and the other side chamber is used to 

remove molten salts such as chlorides and 

sulfates from the molten-glass surface. Separate 

heaters are provided in side chambers to keep 

the glass molten while pouring or removing salts 

from the surface. A gas burner is placed at the 

top to start the melter system from cold 

conditions. 

6.2.3 Offgas Treatment System 

The offgas from the melter is sent through a 

refractory-lined pipe with an inside diameter of 

200 mm (7.8 in.). Offgas is drawn from all three 

melter chambers, and the entire melter is 

maintained at a pressure of 0.25 to 1.25 kPa 

(0.036 to 0.18 psi) below atmospheric pressure. 

The blowers maintain a proper flow through the 

pipes. The other components of the offgas 

module are the quencher, packed bed cooler, 

variable throat venturi, mist eliminator, reheater, 

HEPA filter, and stack. 

The quencher cools the offgas and removes 

some coarse particles. The packed bed cooler 

further cools the gas and removes some 

moisture. The variable throat venturi removes 

most of the particulates. Downstream of the 

venturi is a mist eliminator to remove most of 

the moisture from the venturi. Offgas is then

NUREG/CR-6666 62
Venturi.. 

S~~Melter Scrubber '] 

Wat Dr Chamber Main L )p To Fan 

IxGlass LE 

TakDrainR 

O• [• • i Wqsteds • Packed Coe Bed "'Water Entrainment I 

Cllass Separator (ES) 

I t ooe 

[__ o tainer Wastewater 

HEPA Tanks • 2-Stage Filters Induced I Dr Adiivs nduced .ft.•an 

DryAditiesDraft. F Heater 

From .••Stack 

ES Stack 

Figure 6.4. Process Flow Diagram for the Transportable Vitrification System (Zamecnik et al., 1998) (Permission to use this 

O copyrighted material is granted by American Ceramic Society, Westerville, Ohio)
High-Temperature, Joule-Heated Low-Level Radioactive Mixed Waste Plants

sent to a reheater to ensure that the temperature 

of the offgas is above the dewpoint before it 

enters a HEPA filter. The offgas exiting from 

the HEPA filter goes to the stack. 

6.2.4 Process Issues and Upsets 

During the shutdown of the TVS at the Oak 

Ridge K-25 site on November 5, 1996, the 

melter was flushed by adding bottle glass 

composition. During this addition, 

approximately one ton of glass leaked from the 

melter at a vertical joint between ceramic 

refractory blocks and flowed between the steel 

floor and subfloor of the melter module, burning 

cooling hoses, power cables, and instrument 

wiring routed between the floor and subfloor 

(Lockheed Martin Energy Systems, 1996).

Visual observations during disassembly of the 

melter verified that the leak was caused by 

failure to maintain adequate compression of 

joints between refractory blocks in the melter 

chamber, which allowed vertical joints between 

refractory blocks to open on cooling. This may 

have occurred progressively over the four heat

up and cool-down cycles that the melter had 

experienced during its operating life. Once the 

glass entered and froze in the refractory joints, it 

would be extremely difficult to close it when 

heating, due to thermal expansion or external 

compression. In order to mitigate the risk of 

future glass losses, jack bolts were added to 

maintain refractory compression during cooling, 

and coolers were installed along the vertical 

joints to solidify any glass that may penetrate 

through the melting chamber.

NUREG/CR-6666 64
7 HANFORD LOW-LEVEL WASTE VITRIFICATION TEST 

PROGRAM (TRI-PARTY AGREEMENT) EXPERIENCE

The 1994 revision of the Tri-party Agreement 

(Wilson, 1996a) between the DOE, the 

U.S. Environmental Protection Agency (EPA), 

and the Washington State Department of 

Ecology, specified vitrification as the treatment 

method for LLW streams derived from retrieval 

and pretreatment of single-shell tank (SST) and 

double-shell tank wastes at Hanford. The main 

objective of Phase 1 of the program was to test 

and evaluate promising commercial vitrification 

technologies to support selection of reference 

technologies for the Hanford site LLW 

vitrification. The technical objectives included 

evaluation of feed preparation systems, melter 

operations, melter offgas characterization, and 

treatment of secondary process streams. 

Volatilization and entrainment of feed 

components were important features of the 

offgas characterization objective. 

Based on the Request for Proposal submitted by 

the Westinghouse Hanford Company (WHC), 

seven vendors were selected for demonstration 

of their vitrification technologies. Table 7.1 

shows a list of vendors that were selected and 

technologies that were evaluated. Note that the 

high-temperature, joule-heated melter means 

melters that are capable of operating at 

temperatures above 1,300 *C. Melters that run 

on Inconel® Alloy 690 electrodes at 1,150 °C or 

less are referred to as low-temperature, 

joule-heated melters. 

WHC supplied pre-formulated, non-radioactive 

LLW simulant. The simulant composition and 

its characterization data are provided by Wilson 

(1 996a). Each vendor was allowed to select their 

own feed preparation and offgas treatment 

systems. Vendors also had a choice of either 

using one of the five glass compositions 

proposed by WHC or using 1996a): (i) the waste 

loading should be 25 percent, and (ii) the

normalized Na release rate measured by PCT 

will be less than 1 g/m2/day. 

The following sections describe each melter 

technology with some discussion on feed 

preparation and offgas systems. Each process 

description is followed by testing results, 

technical issues, and technology strengths. The 

performance of all seven technologies was 

assessed by an Evaluation Board consisting of 

experts from the nuclear waste industry and the 

commercial glass industry. Their 

recommendations are presented as a summary. 

The information presented in the following 

sections has been extracted from Wilson 

(1996a,b) and Whyatt et al. (1996). 

7.1 Envitco, Inc. 

Envitco, Inc. (Envitco) is affiliated with the 

Toledo Engineering Company (TECO). TECO 

provides high-temperature, cold-top, 

commercial melting systems to the commercial 

glass industry. Envitco used the Clemson 

University Center for Vitrification Research 

(Marra et al., 1996a) for melter demonstration 

testing. 

7.1.1 Vitrification Process 

The Envitco EV-16 is a high-temperature, 

joule-heated melter that uses four molybdenum 

side-electrodes. A simplified melter schematic is 

shown in figure 7.1. This melter has a square 

cross section with a melt surface area of 0.21 m2 

(22.6 ft2) and a working depth of 0.4-m (1.3 ft). 

The melter is heated by four horizontal 

electrodes that enter through the center of each 

sidewall. The glass production capacity is 500 to 

1,000 kg/day (1,100 to 2,200 lb/day). The melter 

is lined with a thin layer of refractory and

65 NUREG/CR-6666
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience 

Table 7.1. Technologies for Phase 1 Testing of Hanford Low-Level Waste (Wilson, 1996a) 

Vendor I Technology 

Envitco, Inc., Toledo, Ohio Wetted, pelletized feed, high-temperature, -joule-heated melter, molybdenum electrodes 

Vectra Technologies, Inc., Richland, Slurry-fed, high-temperature, joule-heated 

Washington melter, molybdenum electrodes 

Penberthy Electromelt International, Inc., Moist granular feed, high-temperature, Seattle, Washington joule-heated melter, molybdenum electrodes 

GTS Duratek, Inc., Columbia, Maryland Slurry-fed, low-temperature, joule-heated 

melter, Inconel Alloy 690 electrodes 

U.S. Bureau of Mines, Albany Research Pre-reacted dry feed, carbon-electrode melter, 

Center, Oregon radiant heat by arcing or joule-heating 

Westinghouse Science and Technology, Slurry-fed, plasma-torch-fired cupola furnace 

Pittsburgh, Pennsylvania 

Babcock and Wilcox, Alliance Research Slurry-fed, gas-fired, cyclone-combustion 

Center, Alliance, Ohio melter

Refractory 

Lined --,

Water 

Cooled 

Shell

Figure 7.1. Envitco Cold-Cap, EV-16 Melter for Hanford Low-Level Waste (Wilson, 1996a)

NUREG/CR-6666 66
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

depends largely on a water-cooled jacket around 

the melter shell to form a protective layer of 

solid glass at the refractory-glass interface to 

reduce refractory corrosion. The melter also has 

a proprietary, mechanically-controlled drain 

assembly for glass pouring. This drain assembly 

penetrates through the center of the melter 

bottom. A batch hopper with a screw charger on 

the side of the top of the melter uniformly 

delivers dry batch materials on the surface of the 

glass melt pool. 

Envitco selected both dry and liquid slurry feed 

materials for demonstration purposes and used 

the glass formulation specified by WHC. The 

simulant supplied by WHC was mixed with 

glass forming chemicals to attain the specified 

glass composition. A finely powdered activated 

carbon was added as a reductant additive to the 

slurry to break down nitrate and nitrites. 

Preparation of dry feeds required spray drying 

followed by compaction. 

The offgas exiting the melter passes through a 

film cooler and a quench chamber, where its 

temperature is reduced and heavy particulates 

are removed and sent back to the main tank for 

recycling. The saturated offgas then goes to a 

SAS where gases and scrub solutions are 

aggressively mixed. The scrub solution and 

contaminants are then removed through a 

cyclone separator and flushed back to the main 

tank for recycling. Finally, offgas passes 

through a packed column where residual soluble 

acidic gases are removed prior to demisting and 

exhausting. 

7.1.2 Test Results 

The melter operations proceeded smoothly, with 

the exception of a melt reboil (foaming) at the 

end of the dry feed run and a cooling system 

interlock trip during the slurry run. The glass 

production rates with slurry were essentially 

identical to the spray dried feed. The observed

melt rates ranged from 14 kg/hr (30.8 lb/hr) at 

1,350 -C to 26.4 kg/hr (58.1 lb/hr) at 1,520 -C. 

Offgas measurements indicated low volatility 

and limited feed entrainment losses. 

Before the start of the demonstration testing, the 

melter was re-bricked and new electrodes were 

placed. After testing, refractory blocks and 

electrodes were visually examined and sectioned 

to determine the amount of dissolution. 

Examination indicated cracks on the refractory 

walls with little refractory loss. The electrodes 

showed larger-diameter endcaps that were 

beveled on the end that faced the center of the 

melter. There was some minor pitting on the 

endcaps. 

7.1.3 Technical Issues 

Foaming is an issue with all melting systems 

that contain large pools of molten glass with 

transition metal oxides such as Mn, Fe, and Ni, 

and variable feed chemistry (e.g., reductants 

such as sugar and activated carbon, and 

oxidizers such as nitrates and nitrites). The 

Envitco process control strategy needs further 

testing and evaluation. A proper understanding 

of the underlying reactions, use of reductant, 

and its control methodology could minimize 

foaming in the melt. 

When the melter is overfed such that the entire 

melt pool is covered with cold-cap, the cold-cap 

could bridge with melter walls and form an 

impermeable layer. This impermeable layer, in 

turn, could trap feed reaction product gases and 

pressurize the glass melt under the cold-cap. 

This could either result in sudden release of 

reaction gases when the pressure inside exceeds 

the cold-cap strength or it may automatically 

syphon the glass into the container if the cold

cap is not breached. In both cases, it will upset 

the process. Bridging could be easily avoided by 

proper feed rates and monitoring of the melt 

surface.

67 NUREG/CR-6666
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

Molybdenum electrodes are easily corroded with 

LLW feeds containing corrosive salts such as 

halides, sulfates, phosphates, or metals such as 

Pb, Sn, Cu, and Ni. The demonstration testing 

was not long enough to determine the long-term 

performance of the electrodes and needs to be 

verified before the electrodes could be installed 

for LLW vitrification. 

Melter drains require periodic maintenance or 

replacement. Due to the limited duration (2 wk) 

of the test, the long-term performance of the 

bottom drain and its ability to remove metals 

and conductive sludges from the floor are yet to 

be demonstrated. Alternate designs should be 

explored.

7.2 Vectra Technologies, Inc.

Vectra Technologies, Inc. (Vectra) is a 

diversified provider of services to commercial 

nuclear utilities and the federal government. 

Their services include design and operation of 

nuclear waste management processes, 

equipment, transportation casks, and processing 

systems. Vectra demonstrated the performance 

of its Ve-Skull melter system. 

7.2.1 Vitrification Process 

The Vectra Ve-Skull is a joule-heated melter 

with nearly vertical molybdenum electrodes. 

The melter is a double-shelled cylindrical 

pressure vessel with cooling-water circulation 

between the inner and outer shell. The inner 

shell is I m (3.3 ft) in diameter and has a thin 

refractory lining. The melter depends largely on 

a water-cooled jacket between the inner and 

outer shell to form a protective layer of a frozen 

glass skull at the refractory-glass interface to 

reduce refractory corrosion. The effective melt 

surface area is 0.57 m2 (9 ft2

). The melter has 

three molybdenum electrodes that enter from the 

top of the melter and a single port for feeding

and venting offgas. For slurry feeding, the feed 

tube extends down through the offgas port, 

discharging near the center of the melt pool 

above the glass level. For dry feeding, an auger 

is used to transfer powdered feed from a hopper. 

The melter has a single-drain port located at the 

center of the bottom, for glass pouring. The 

drain ring is a thick metal disc that is heated by 

passing electric current through it to initiate 

glass pour. The glass flow is stopped by 

inserting a water-cooled plug into the drain hole. 

The Vectra process flow sheet is shown in 

figure 7.2. Vectra selected two types of dry 

feeds and one liquid slurry for the 

demonstration. The simulant supplied by WHC 

was mixed with dry glass-forming chemicals to 

attain the specified glass composition. Powdered 

sugar was added as a reductant additive to the 

simulant to reduce nitrates and nitrites. Smaller 

batches of the slurry mix were transferred using 

a pump to an agitated day-tank located near the 

top of the melter. Feed was drawn from the tank 

through a peristaltic pump and delivered to the 

melter. 

The offgas exiting the stack is sent to a spray 

scrubber where offgas and scrub solutions 

(sodium hydroxide and water) are rapidly mixed 

to remove soluble acidic gases. 

7.2.2 Test Results 

The melter system functioned well during 

testing when it was operated with direct slurry 

feeding and dry feeding. The Vectra test melter 

ran for 32 days producing 10,000 kg (22,000 lb) 

of glass. Approximately 500 kg (1,100 Ib) of 

calcined feed was prepared and melted. 

Attempts to make pre-reacted, dry-pressed feed 

were unsuccessful. The glass production rates 

achieved with slurry feed were greater than or 

equal to glass production rates estimated for 

calcined or dry feeds. The melt rate was 

estimated at about 34 kg/hr (74.8 lb/hr).

NUREG/CR-6666 68
C-4 Z~~~ i LOe~ C...-a Filt dGF Tank 

Glass Quench 1.CF 

CWater Pump 

C% Figure 7.2. Vectra Vitrification Process Flowsheet for Hanford Low-Level Waste (Wilson, 1996a)

0 

EL 

CO
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

During the demonstration testing, severe 

corrosion of the electrodes was experienced. 

Electrode material was corroded at the melt

electrode-air interface to a point where ends of 

the two electrodes broke off. The ends of the 

electrodes that were submerged in the glass pool 

were not as severely corroded. The water-cooled 

protective jackets designed to prevent exposure 

of the electrodes to the atmosphere were also 

corroded. Excessive corrosion is attributed to the 

fact that, during testing, the glass level was 

allowed to drop below the protective sleeves, 

thus exposing the molybdenum electrodes to air. 

Molybdenum oxidizes (bums) very rapidly in air 

at high temperatures. 

Before the start of the test program, the melter 

was re-bricked and new electrodes were placed. 

After the test program was over, refractory 

blocks and electrodes were visually examined 

and sectioned to determine the amount of 

dissolution. Examination indicated a significant 

loss of refractory mortar used between the 

bricks. 

7.2.3 Technical Issues 

Foaming and cold-cap bridging are common 

issues for all melters and have already been 

discussed in section 7.1.3. The bottom drain is 

an important aspect of the Vectra Ve-Skull 

melter. Even though it allowed draining of non

vitrified sludges and metal precipitates during 

normal glass pouring, the use of bottom drains is 

considered a high risk. Because of the lack of 

experience in long-term continuous use of 

bottom drains and a history of problems with 

bottom drains in the commercial glass industry, 

reliable performance of the bottom drain design 

need to be verified before being installed for 

LLW vitrification. 

The use of vertical electrodes with a bottom 

drain melter design raises special issues in 

regard to electrode performance. As discussed,

severe corrosion of electrodes was experienced 

during testing. Other electrode materials or a 

better electrode configuration needs to be 

evaluated before the melter should be used for 

LLW vitrification. 

The melter is lined with a thin layer [10 cm 

(4 in.) thick] of refractory and depends largely 

on a water-cooled jacket between the inner and 

outer shell to form a protective layer of a frozen 

glass skull at the refractory-glass interface to 

reduce refractory corrosion. The long-term 

performance of the refractory and skull layer for 

the Vectra Ve-Skull melter is not known. 

7.3 Penberthy Electromelt 

International, Inc. 

Penberthy Electromelt International, Inc. (PEI) 

is a small company with expertise in designing 

electrical, joule-heating melters for waste 

vitrification. PEI used a high-temperature, 

joule-heated melter with molybdenum electrodes 

for demonstration testing. 

7.3.1 Vitrification Process 

The PEI melter is a high-temperature, 

joule-heated melter that uses molybdenum 

sidewall rod electrodes. The melter has thick 

ceramic refractories [20 cm (8 in.) thick] and 

does not rely on the formation of a protective 

frozen skull layer to reduce refractory corrosion. 

The melter has a melt area of 0.5 m2 (5.4 ft2) and 

a glass depth of 0.5 m (1.6 ft). Two 

molybdenum rod electrodes extend more than 

halfway across the melter interior. It has three 

electrically-heated drains in the lower side wall 

for glass discharge. 

Figure 7.3 shows the process flow schematic for 

the PEI vitrification process. The feed system 

mixes the liquid LLW with absorbent glass 

former additives in screw chargers. This system

NUREG/CR-6666 70
j D atmiz 7 FuZI( e

Weighing & Mixing 

Z irconr uI 

Flour Meal Sugar

Melter 

Control

Figure 7.3. Penberthy Electromelt International, Inc. Vitrification Process Flowsheet for Hanford Low-Level Waste (Wilson, 

1996a)

',4

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ON

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CD 

CD 

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0 

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0 

CD 

CD 

CD 

0 CD
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

is called mix-in-the-charger. The reductant, 

sugar, is split equally between LLW and glass 

formers before they are mixed together. The 

screw chargers deliver moist granular feed 

directly to the melter. Even though, in this feed 

system, the composition of the final feed cannot 

be confirmed before sending it to the melter, it 

requires minimal radioactive material processing 

equipment. A cold-cap is maintained over the 

glass melt and provides low volatility and minor 

feed entrainment losses. 

The offgas exiting the melter passes through an 

offgas filter alcove filled with ceramic fibers. 

When the filter becomes loaded with 

particulates, it is pushed into the melter where it 

melts away with the LLW. The offgas is 

quenched and scrubbed before it is demisted and 

exhausted. 

7.3.2 Test Results 

Melter drain failures caused termination of the 

testing before all the testing objectives were met. 

7.3.3 Technical Issues 

Two feed streams, LLW and dry glass former 

mix, are independently metered to control the 

glass composition. In this system, feed 

composition before it enters the melter cannot be 

confirmed. An accurate characterization of the 

LLW and accurate metering of the LLW and 

glass former mix are critical to this type of 

system. 

Foaming and cold-cap bridging, as discussed in 

section 7.1.3 for the Envitco melter, is a 

potential concern. 

As in the Envitco and Vectra melters, remote 

maintenance and replacement of the 

molybdenum electrodes is a potential concern.

Extended testing is needed to ensure long-term 

electrode performance. 

Drains, as indicated in both the Envitco and 

Vectra melters, are an important part of the 

melter. The failure of three lower sidewall 

drains, followed by a loss of control on the 

bottom drain, was the immediate cause of 

termination of the demonstration testing for the 

PEI melter. Reliable performance of the bottom 

drain design needs to be verified before the 

melter should be installed for LLW vitrification. 

7.4 GTS Duratek, Inc. 

GTS Duratek, Inc. (Duratek) provides 

vitrification technology to treat radioactive and 

hazardous waste materials. For the 

demonstration testing, Duratek used two low

temperature, joule-heated melter systems. 

7.4.1 Vitrification Process 

The Duratek® melter is a low-temperature, joule

heated melter system with Inconel® 

Alloy 690 plate electrodes. The melter has a 

thick-walled refractory and does not depend on 

melter-shell cooling to form a skull layer at the 

melt-refractory interface. The melter uses 

bubblers to increase mixing within the melt. In 

addition, the glass melt exits via an 

airlift-assisted overflow drain. Duratek® used 

two melters, the Duramelter® 100 and the 

Duramelter' 1000, for testing. The Duramelter® 

100 is rated for 100 kg/day (220 lb/day) and the 

Duramelter® 1000 is rated for 1,000 kg/day 

(2,200 lb/day) of glass production. 

A schematic diagram of this vitrification system 

is shown in figure 7.4. The feed system for the 

Duramelter® 100 consisted of two 208-L 

(55-gal.) drums, equipped with agitators. The 

first drum is a mixing tank with a 

recirculating/transfer pump and the other is a

NUREG/CR-6666 72
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

Fed Pump

Simulant Drum Mbdng Tank

Figure 7.4. Duratek Diagram of the Vitrification 

1996a) 

feed tank with a peristaltic pump and a single 

water-cooled feed nozzle in the center of the top 

of the melter. Feed is prepared by transferring a 

required amount of LLW simulant to the mixing 

tank and a preweighed batch of glass former 

additives. Additives are added via a hopper into 

the mixing drum. After the feed is mixed, it is 

transferred to the feed tank. Urea is added as a 

reductant to the feed to reduce NO, production 

in the melter. 

The glass is poured from the melter through a 

copper or a graphite tube extending through a 

water-cooled melter shell just below the desired 

glass level. 

The feed system for the Duramelter® 1000 is 

functionally the same as the Duramelter® 100 

feed system. The Duramelter® 1000 used two 

0.9-m3 (238-gal.) tanks. In addition, the

Overflow AMilt 

Drain 

4" 

Disharoe 

Chamber

Recarculation 

Pumo

System for Hanford Low-Level Waste (Wilson, 

Duramelter' 1000 has two feed nozzles to feed 

the slurry on both sides of the bubblers. 

7.4.2 Test Results 

The testing for both melters, the Duramelter® 

100 and the Duramelter' 1000, went smoothly 

and was uneventful, with both melters exceeding 

the rated 100 kg/day (220 lb/day) and 

1000 kg/day (2,200 lb/day) production rate. The 

Duramelter® 100 test melted 600 kg (1,320 lb) 

of glass with an average processing rate of 

7.7 kg/hr (17 lb/hr). The Duramelter® 1000 test 

melted 10,700 kg (23,100 lb) of glass with an 

average processing rate of 75 kg/hr (165 Ib/hr). 

Results from the small melter indicated a 

significant reduction in NO, after the addition of 

urea. In addition, urea improved rheological 

properties of the feed.

73 NUREG/CR-6666
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

The feed entrainment losses were not greater 

than 0.6 percent. The volatility of components 

was greater than for a true cold-top melter in 

which 100 percent of the melt surface is 

blanketed but was less than that of hot-top or 

exposed-surface melters. 

7.4.3 Technical Issues 

There were no significant technical issues that 

need to be addressed. Foaming and cold-cap 

bridging, as with every melter discussed in 

section 7.1.3, is a potential concern. 

The technology is similar to HLW technology 

adopted at WVDP and DWPF. The impact of 

bubblers on the erosion of the melter refractory 

lining has to be evaluated for long-term 

performance. In addition, bubbling gives rise to 

more severe feed plugging in the offgas line and 

should be evaluated. The testing of the M-Area 

melter at SRS should provide more data to 

support these issues (see section 5). 

7.5 U.S. Bureau of Mines, Albany 

Research Center 

The U.S. Bureau of Mines (USBM) 

demonstrated a carbon-electrode melter at the 

USBM Albany Research Center in Oregon. The 

center is supported by several contributors 

including Idaho National Engineering 

Laboratory (INEEL). 

7.5.1 Vitrification Process 

USBM technology is based on a 3-phase electric 

arc furnace. This technology is widely used in 

the metals industry and is referred to by several 

names such as electric arc furnace, electric arc 

melter, carbon arc furnace, and carbon electrode 

melter. A schematic diagram of the feed 

preparation and melter is shown in figure 7.5.

The melter uses top-entering vertical carbon 

electrodes of 10.2 cm (4 in.) in diameter. The 

electrodes are arranged in an equilateral triangle 

with about a 28.5 cm (17.2 in.) separation 

between the electrode centers. The depth of the 

electrodes can be adjusted during melter 

operations. The heat could either be supplied as 

radiant heat by arcing above the melt surface or 

as joule-heat with the electrode submerged in 

glass melt. The melter consists of a steel shell 

lined with a thin layer of alumina refractory. 

Water cascades down over the outer shell of the 

melter, cooling the refractory and forming a 

protective frozen skull layer at the 

refractory-glass interface to reduce refractory 

corrosion. The bottom of the melter is lined with 

a thicker refractory layer and is cooled only by 

ambient air. The melter operates using dry 

materials to avoid reaction between water and 

the carbon electrodes. The melt surface has a 

circular cross-section with about 1.12 m2 (12 ft2) 

surface area. 

The USBM developed two dry feed preparation 

processes in which nitrites and nitrates were 

mostly reduced by reaction with sugar and 

activated powdered carbon during feed drying. 

Type A feed is prepared by pelletizing high

surface-area, glass-forming chemicals and 

combining with liquid waste, which is absorbed 

in the pellets. The Type B feed is prepared by 

mixing the glass-forming chemicals with the 

LLW before pelletizing. In both cases, the feed 

is dried and reductant additives reacted with 

NO-3/NO- 2 before being fed to the melter. The 

feed tubes enter from the top of the melter and 

are within 46 cm (18.1 in.) of the melt surface. 

The glass melt is poured from the melter 

through a water-cooled copper tapping fixture. 

7.5.2 Test Results 

During the demonstration, the USBM melter 

produced 4,700 kg (10,340 lb) of glass in a 

24-hr period. However, excessive melting

NUREG/CR-6666 74
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

RatianM Hoaters

Reacdon Furnace Carbon Secho•e Molter 

Figure 7.5. Feed Preparation and Melter Schematic for Carbon Electrode Melter System (Wilson, 

1996a)

temperatures resulted in high-volatile 

component losses. In the other two 

demonstration runs, USBM reduced the volatile 

losses by using large-diameter electrodes and 

lower voltage. The volatile losses were lower 

but were still greater than cold-top, joule-heated 

melters. Volatility was high for boron, alkali 

metals, and halides. 

A total of 1,784 kg (3,925 lb) of Type A or 

11,889 kg (26,156 lb) of Type B feed were 

produced. The sugar, plus activated carbon feed, 

effectively destroyed an average of 78 percent 

of the NO-2 and NO- 3 in the Type B feed while 

with only activated carbon present in Type B 

feeds, the destruction varied from 

15 to >90 percent. The degree of reaction for 

Type A feed was not determined. 

During the initial test run, the electrode 

consumption per ton of glass was 12.6 kg

(27.7 lb). After replacement with large-diameter 

electrodes, the electrode consumption (per ton of 

glass) was 10.5 kg (23.1 lb), indicating lower 

electrode consumption with increasing electrode 

diameter. Most of the glass product was 

homogeneous, except for some cases where 

inclusions were observed, but the glass was 

mostly in the reduced state. The measured 

Fe2"/Fe (total) was 1.0. The visual examination 

indicated refractory corrosion, which was also 

reflected as an increase in Cr content in the glass 

composition. 

7.5.3 Technical Issues 

There were several technical issues that need to 

be addressed before this technology could be 

adopted for LLW vitrification. Preparation of 

dry feed requires extensive equipment and 

mechanical complexity that may not be suitable 

for radioactive operations. The wear and

75 NUREG/CR-6666
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

consumption of electrodes are matters of 

concern. It is necessary to change and feed the 

electrode into the melter as it is consumed. The 

presence of additional carbon arising from the 

electrodes causes significant reduction in glass. 

Volatility and entrainment of feed were 

significant; even though the use of a larger 

electrode reduced the volatility, design changes 

need to be made to operate this melter like a 

cold-top melter. The glass pour system is not 

suitable for continuous pouring of viscous glass, 

and excessively high temperatures are required 

to be maintained for continuous glass flow. The 

situation becomes worse when the alkali metals 

and boron volatilize, resulting in a high

viscosity glass. A glass tapping temperature of 

1,600 'C was required for pouring, which will 

cause volatilization of Cs and Tc during 

pouring. 

Carbon electrode arc melters have been widely 

used in melting specialty metals, steel, and 

ceramic refractories. The melt rates are higher 

than all the joule-heated melters, and the melter 

can melt wastes with a variety of characteristics. 

7.6 Westinghouse Science and 

Technology Center 

The Westinghouse Science and Technology 

Center (WSTC) is a division of Westinghouse, 

Inc. and the demonstration test was performed at 

the Westinghouse Plasma Center at the Waltz 

Mill Site, Madison, Pennsylvania. 

7.6.1 Vitrification Process 

The vitrification technology is based on a 

plasma torch-fired furnace. A cupola furnace is 

equipped with a single Marc' 11 plasma torch 

with as output power of 700-1,400 kW. A 

schematic diagram of the overall vitrification 

system is shown in figure 7.6. The plasma

melter is 6.7 m high (22 ft) with an outer shell 

diameter of 1.2 m (3.9 ft) and an inner shell 

diameter of 0.76 m (2.5 ft). The melter is fired 

by a single Marce 11 plasma torch mounted on a 

tuy~re that directs the plasma plume to the melt 

crucible. A tap hole on the bottom side of the 

crucible connects to an overflow side pour spout 

for glass pouring. The cupola shaft is lined with 

A120 3/SiO 2 refractory, the crucible is lined with 

high-chrome refractory, and the tuy~re is lined 

with AI203/CaO insert sleeves. Figure 7.7 shows 

a cross section of the cupola. The tuy~re and 

crucible are water-cooled by copper tube 

cooling coils. 

WSTC uses glass flit as a raw material to be 

mixed with LLW simulant. The flit is 

volumetrically metered through an auger while 

LLW simulant is metered through a mass 

flowmeter. Both simulant and the glass flit are 

fed as separate streams to a slurry pump where 

they are mixed and fed as a slurry to the furnace 

tuy~re. In the tuy~re, feed is rapidly heated and 

carried along with the hot plume from the 

plasma torch to the melt crucible. The glass 

product is continuously tapped from an overflow 

pour spout and poured into carbon steel boxes. 

The gas/solid streams exit the tuy~re and 

entrained solids impinge on the glass plume and 

dissolve in the melt. Offgases proceed up the 

melter shaft. The offgases travel up the shaft, are 

partially air quenched, and flow out the top of 

the melter shaft via a horizontal duct, to a 

vertical duct where they are water quenched, 

and scrubbed in a venturi scrubber. The 

scrubbed offgas passes through a demister and is 

sent out to the stack through an exhaust fan. 

7.6.2 Test Results 

WSTC produced approximately 7,000 kg 

(15,400 lb) of glass during a 24-hr run. The 

glass composition was close to target 

composition. The glass was well reacted and

NUREG/CR-6666 76
Horizontal Duct 

Charge Door 

Glassformer Hopper 

-Prequench Stack 

Feed Auger -Radlation Spray 

Baffle 

SDemister/Separator 

Simulant Tank 

lurryPumpScrubber 

Tank 

Plasma.,,i Exhankst 

Torch Spout Venturi Exhaust 

Tuybre Crucible Product Scrubber Bor 

. Collector -. 

Heat 

Exchanger 

z 

S Figure 7.6. Westinghouse Science and Technology Center Vitrification Process Flowsheet for Hanford Low-Level Waste 

• (Wilson, 1996a)
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

Product MW3IL TaHl olco 

Level Crucible Tap Hole Tap Hole I 

(End View) (Side View) 

Figure 7.7. Westinghouse Science and Technology Center Marc 11 Plasma Torch and Melter System

transparent but had numerous chords resulting 

from limited convection and mixing due to low 

melt level in the crucible. The glass production 

rate was about 300 kg/hr (660 lb/hr) 

at 1,550 kW. 

High volatility and entrainment losses were 

observed. Entrainment losses were estimated to 

be 2.7 percent, while volatilization of B20 3 and 

Na2O was 22 and 15 percent, respectively. 

Significant offgas deposits were observed in the 

upper horizontal duct. A 7- to 100-cm (2.7- to 

39.4-in.) width in a 30-cm (12-in.) diameter duct 

was completely filled with deposits. The 

analysis indicated sodium and borate salts as 

major constituents. The offgas deposits blocked 

the offgas scrubber solution flow resulting in a 

temporary shutdown of the system. 

Refractory wear was highest in the alumina

calcia lining of the tuy~re while the melt 

crucible, pour spout, and shaft plate surface 

refractories performed well. The vertical shaft

showed some attack by the glass in the lower 

levels of the shaft. Some spallation of refractory 

was also observed in the upper section of the 

shaft. 

7.6.3 Technical Issues 

The high-feed entrainment and volatility losses 

were a major issue with this technology. Since 

this melter cannot be operated in a cold-top 

mode, minimization of losses may be difficult. 

Erosion was significant in the refractory used 

for tuy~re lining. A redesigning of the tuyire 

should reduce wear of its lining. 

This technology can melt feeds with a variety of 

compositions and characteristics. Its advantage 

consists of significantly higher production rates 

than joule-heated melters.

NUREG/CR-6666

SwtlvsO~l J.YYoa)

78
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

7.7 Babcock & Wilcox, Alliance 

Research Center 

The Babcock & Wilcox (B&W) cyclone 

vitrification technology is based on 50 yr of 

experience with slagging combustion units for 

the electric utility industry. Slag is a vitreous 

byproduct of impurities in coal from coal 

combustion. The B&W cyclone vitrification 

technology has been demonstrated for soil 

vitrification for the EPA Superfund Innovative 

Technologies Evaluation Program. B&W 

demonstrated a slurry-fed cyclone melter system 

for the Hanford LLW at the Alliance Research 

Center in Alliance, Ohio. 

7.7.1 Vitrification Process 

The cyclone furnace is a water-cooled horizontal 

cylinder attached to the wall of the main furnace 

cavity. The cyclone wall consists of a network 

of water-cooled metal tubes fitted with metal 

studs pointing into the center of the cylinder. A 

19- to 51-mm (0.75- to 2-in.) layer of refractory 

cement is imbedded between and over the studs 

to protect the metal structure. During operation, 

formation of a skull layer on the walls of the 

cyclone melter minimizes corrosion. Figure 7.8 

shows the schematic of the B&W LLW 

vitrification system. The cyclone furnace is fired 

by natural gas with a small amount of excess air. 

LLW and dry glass formers are mixed outside 

the melter and injected into the melter through 

an atomizing nozzle. Slurry is injected onto the 

cyclone wall where it melts and flows down the 

cylindrical wall and is collected in the bottom of 

the cyclone. Glass drains from the cyclone 

section through a notch in the back baffle of the 

cyclone to a sump in the main furnace cavity. 

The sump is drained to a quench tank.

7.7.2 Test Results 

The glass production rate was approximately 

600-kg/day (13,200-lb/day). The volatile and 

feed entrainment losses were the highest among 

all the melter systems. Estimates indicate that 

23.4 percent of the glass forming components 

were lost because of volatilization. In addition, 

dry feed accumulated as stalagtites hanging off 

the injector tube and the glass buildup on the 

combustion air inlet resulted in globs of molten 

glass falling and interfering with the feed 

atomizer. 

The post-test inspection of the melter indicated 

significant erosion of the cyclone chamber 

refractory during the demonstration. The glass 

product indicated the presence of unmelted 

refractory grains and significant glass 

inhomogeneity, suggesting a need for longer 

refining times. 

7.7.3 Technical Issues 

Entrainment losses during testing were 

7-10 percent of the feed and were the highest 

observed among the various technologies. A 

modification to the feed delivery system that 

will allow feed to be delivered to the cyclone 

system before entrainment is needed. The high 

volatilization losses of alkali metals, halides, and 

B are attributed to temperatures as high as 

1,600 °C at the combustion section. The 

volatilization losses have to be minimized before 

this technology can be used for LLW 

vitrification. 

The glass composition was significantly 

depleted of B and Na due to entrainment and 

volatilization losses, and the glass was visually 

inhomogeneous. Some of the contamination was 

from refractory erosion. A longer residence time 

is needed for glass to completely homogenize 

and produce an acceptable product. Extensive

79 NUREG/CR-6666
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

Stack

S~P = -2 

S-.o T - 9540C Economizer :c\ Reheater 

T 10= --------.. "Of'" ID 

ol .- 0I \Secondary .- .- superheater 

0 ... .. "•" " I•Soothlower 

4.6 mi Superheater deposition probe 

S•• Furnace arch 

SSecondary air T = Gas temperature (Ceiss) 

S n399eC air P = Operating pressure (inches water column) 

Natural gas P 0 

Atomizing t D 

air & feed P=9 

Cyclone furnace h T = 1538 to 1649PC 

(0.46 m ID) Glass quench tank 

Glass sampling port 

Figure 7.8. Babcock & Wilcox Vitrification System for Hanford Low-Level Waste (Wilson, 1996a)

NUREG/CR-6666 80
Hanford Low-Level Waste Vitrification Test Program (Tri-Party Agreement) Experience

erosion of refractory indicates that the cyclone 

chamber needs to be redesigned and cooled 

enough to form a thick skull layer. 

Cyclone vitrification technology could provide a 

modular melter design that is smaller than 

conventional joule-heated melters. This 

technology is capable of producing glass 

throughputs in the range of 100,000 to 

200,000 kg/day. This technology produced a 

self-draining glass and, therefore, potential for 

drain valve failure does not exist. The 

electrically conductive glass is not a limitation. 

7.8 Recommendations of the 

Evaluation Board 

An evaluation board consisting of experts was 

chartered by the WHC to review the 

performance of melter technologies. This 

evaluation rated cold-top, joule-heated melters 

as the best available technology. These 

technologies were considered mature and 

exhibited superior mass balance. However, the 

evaluation board was concerned about the use of 

a bottom drain on a large-capacity production 

melter and the control of the glass level so as not

to expose the top-entering molybdenum 

electrodes to oxidation. The board preferred the 

Envitco overflow side drain and Duratek® airlift 

drain design over the Vectra bottom drain. 

The board rated slurry feeding and mix-in-the

charger damp feeding, as used by PEI, superior 

to dry feeding. The size and complexity of 

equipment required for preparing dry feed was a 

concern. In addition, complexity of the 

equipment would result in difficult remote 

maintenance and operations. The slurry feed 

was further supported by the fact that there is no 

significant difference in production rate between 

liquid and dry feeds. 

The board recommended that further testing of 

cyclone combustion and plasma technology 

should not be continued. In their opinion, these 

technologies require significant additional 

modifications and data at this stage. The carbon 

electrode technology is worthy of additional 

consideration. The board believed that the 

incorporation of an improved drain design will 

allow the melter to operate continuously in a 

cold-top mode, reducing entrainment and 

volatilization.

NUREG/CR-6666 81
8 MIXED WASTE SOLIDIFICATION TECHNOLOGY 

EXPERIENCE

In 1992, Congress passed the Federal Facilities 

Compliance Act mandating the treatment and 

disposal of the DOE mixed waste inventory 

(Office of Science and Technology, 1996). The 

DOE complex has approximately 167,000 m3 

(44.1 million gal.) of mixed waste in storage 

including 51,600 m3 (13.6 million gal.) of mixed 

transuranic (MTRU) and 115,400 m3 

(30.5 million gal.) of mixed low-level waste. It 

is anticipated that this inventory will increase 

due to ongoing processes, environmental 

restoration, facility decontamination, and facility 

transition activities within the DOE complex. 

The Mixed Waste Focus Area (MWFA) was 

established by the Office of Science and 

Technology of the DOE to develop and facilitate 

implementation of technologies required to meet 

DOE commitments for characterization, 

treatment, and disposal of mixed wastes. Under 

MWFA sponsorship, a number of thermal 

treatment technologies to stabilize mixed wastes 

have evolved. In this survey, technologies that 

are relevant to the scope of this report will be 

discussed. Several technologies that were 

evaluated under this program are not 

solidification technologies but were thermal 

treatment technologies such as steam reforming 

or system integration concepts like minimum 

additive waste stabilization (MAWS) and 

reactive additive stabilization process (RASP). 

The MAWS (Pegg, 1994) is not a solidification 

technology but a systems approach to integrate 

multiple technologies into one and is not 

discussed in this review. Similarly, RASP 

(Jantzen et al., 1994) is not a technology but a 

systems approach to optimize glass composition 

by using various waste streams and reactive 

chemicals as glass formers. The information 

reported in this section has been extracted from 

a series of reports prepared by the Office of 

Technology Development (1995ab,c).

8.1 Plasma Hearth Process 

Plasma arc technology is widely used in various 

industry applications (Gillins and Poling, 1994). 

Some of the applications are 

"• Metal and ceramic powder production 

"* Composites (metal on metal, metal on 

ceramic, etc.) production 

"* Titanium dioxide pigment production 

"• Chemical production (acetylene, ethylene, 

etc.) 

* Metal ore smelting 

0 Metal and refractory recycling 

0 Metal refining 

* Metal cutting and welding 

Plasma hearth process (PHP) technology for 

stabilizing mixed wastes is being developed by 

the Science Applications International 

Corporation (SAIC) for the MWFA (Science 

Applications International Corporation, 1996). 

This technology is targeted for mixed wastes 

such as sludges and heterogeneous debris that 

cannot be treated by conventional technologies. 

Nonradioactive testing was done on a 

bench-scale plasma system at the SAIC Science 

and Technology Applications Research (STAR) 

Center. The PHP is a high-temperature thermal 

treatment process using a plasma direct current 

(DC) arc torch in a stationary, refractory lined 

melter. The plasma arc destroys organics and 

stabilizes residuals in a vitrified slag. The major 

advantage of the process is that whole drums 

could be fed into the process chamber, thus 

reducing waste handling and potential exposure 

to toxic and radioactive contaminants. 

Major components of the STAR Center plasma 

system are the plasma chamber, secondary 

chamber, quench air, dry absorbent air, bag 

house, HEPA filter, stack, and instrumentation 

and controls. A PHP system process schematic

83 NUREG/CR-6666
Mixed Waste Solidification Technology Experience 

is shown in figure 8.1. The plasma chamber is 

79 cm (31 in.) diameter by 76 cm (30 in.) long 

horizontal cylinder with interior walls lined with 

the combination of MgO, A120 3 and Cr20 3 

refractories. A water-cooled plasma torch is 

mounted from the top of the plasma chamber. 

The torch uses nitrogen as its primary gas to 

stabilize electric discharge between two 

electrodes. The second electrode is the material 

that is being processed. The plasma chamber 

also contains additional ports for viewing, 

material feeding, and offgas exit. The feed port 

is used to feed either 0.95 L (1 quart) or 3.7 L 

(1 gal.) steel containers to simulate 208 L 

(55 gal.) drums. The materials are drip melted 

into the plasma chamber. 

The reaction gases from the plasma chamber go 

to the secondary chamber. The secondary 

chamber ensures full destruction of the residual 

organics left in the offgas. The secondary 

chamber is a vertical cylinder and operates at a 

temperature of 980 *C by a natural gas burner at 

the side of the chamber. The offgas enters the 

chamber from the bottom and flows in the

chamber in a cyclonic fashion. The mean 

residence time in the chamber is about 2 sec. 

The offgas leaving the chamber is quenched by 

injecting water/air at a temperature of 540 °C 

and is sent for acid removal by a dry sorbent 

injection with sodium bicarbonate. The sodium 

bicarbonate is injected into the offgas. At 

540 °C, sodium bicarbonate is converted to 

sodium carbonate, and sodium carbonate reacts 

with acidic gases to form sodium salts. The 

offgas and alkali reagents are then passed 

through a dry reactor, which provides enough 

residence time for reaction of acidic gases. The 

offgas then passes through a baghouse that 

collects unused sodium bicarbonate, sodium 

salts, and fly ash. Collected solids are recycled 

back into the offgas. As needed, fresh sodium 

bicarbonate is added to the system. Results 

indicate 95-percent removal of HC1 and 

98-percent or more for metal removal except for 

Hg, which is 50 percent. The HEPA filter then 

removes fine particulates from the offgas with a 

removal efficiency of 99.97 percent. The offgas 

is then exhausted to the stack.

Stack 

t

Figure 8.1. Plasma Hearth Process System Flowsheet (Science Applications International 

Corporation, 1996)

NUREG/ CR-6666 84
This technology has the ability to process large 

amounts of metal-bearing wastes. The 

technology is robust enough to accept and 

process papers, plastics, metals, soils, liquids, 

and sludges. The resulting waste forms do not 

require additional stabilization. Because the 

wastes can be processed at high temperatures 

(1,500 °C), a wide range of feed compositions 

can be accommodated. 

A major disadvantage is the increased 

volatilization of metals such as Pb, Hg, and Cs 

compared to the joule-heated systems. Other 

disadvantages are the short life of plasma 

torches, remote material handling, and plugging 

of slag drains. These characteristics make 

radioactive operations risky and difficult, 

requiring frequent maintenance.

Mixed Waste Solidification Technology Experience 

8.2 Plasma Arc Centrifugal 

Treatment System 

The plasma arc centrifugal treatment (PACT) 

system was installed at the DOE Western 

Environmental Technology Office and was 

operated by MSE, Inc., Butte, Montana. MSE, 

Inc. has been involved in the development of 

this system since 1989. Some of the waste 

streams that have been tested and evaluated 

include copper mill tailings from a Superfund 

site in Montana, small caliber and handheld 

pyrotechnic devices, smoke and spotting dyes 

from the Department of Defense Ordnance 

demilitarization stockpile, and several simulated 

DOE mixed waste streams. 

The system uses a transferred electric arc, 

similar to PHP, as a heat source. Figure 8.2 

shows a general process schematic of the PACT 

system. A screw feeder is used to feed waste 

uniformly and continuously into a primary

SLAG 

COLLECTION 

CHAMBER

Figure 8.2. Plasma Arc Centrifugal Treatment System Flowsheet (Whitworth and Filius, 1995) 

(Permission to use this copyrighted material is granted by American Ceramic Society, Westerville, 

Ohio)

NUREG/CR-6666 85
Mixed Waste Solidification Technology Experience 

chamber through a chute connecting the feeder 

and the primary chamber. The primary chamber 

consists of a rotating tub with a central orifice or 

throat lined with copper. 

The system uses a transferred electric arc as a 

heat source. The copper-lined throat is used to 

strike the arc of the plasma torch. As waste is 

fed to the primary chamber, the torch is moved 

away from the throat. The molten slag remains 

in the primary chamber by the centrifugal force 

generated by the rotating tub. The torch is 

slowly moved around to provide uniform 

distribution of heat. After about 450 kg 

(1,000 lb) of material is fed to the chamber and 

has attained molten state, the rotation speed of 

the primary chamber is reduced, and the 

glass/slag is allowed to pour into a 208-L 

(55-gal.) drum under the primary chamber. The 

temperature of the melt is approximately 

1,600 'C. After pouring, the process is started 

again. 

The offgas from oxidation of the organic 

materials consists of combustion gases, mostly 

CO2 and H20, and NO, in case of nitrated waste. 

The temperature of the offgas exiting the 

primary chamber is around 1,100 to 1,300 °C, 

and it is sent to a secondary combustion 

chamber to ensure complete destruction of any 

residual organic species. The secondary 

chamber is a natural gas fired system. The 

exhaust gases are then passed through a 

quencher/absorber, particulate scrubber, 

high-energy scrubber, super heater, baghouse, 

HEPA, carbon-absorption bed, induced draft 

blower, gas preheater, NO, removal system, and 

released to the stack. 

Advantages and disadvantages of this 

technology are similar to the PHP technology 

and are discussed in section 8-1. The entire 

system is sealed from the atmosphere and is 

operated under a slightly negative atmosphere. 

Even though the PACT system is operated under

slightly negative pressure, the failure of the 

rotating tub could result in premature pouring of 

the waste into the canister and possible release 

of offgas from the primary chamber into the 

facility. 

Currently, there are several PACT systems that 

are either in their construction phase or have 

been installed to vitrify heterogeneous mixed 

wastes (Haun et al., 1996). Three PACT-8 units, 

with a 2.4-in (8-ft) diameter centrifuge, are 

being constructed, and one unit is being built for 

remediating Pit 9 waste at the INEEL. One unit 

is at the Retech Plant in Ukiah, California, for 

further development, and another unit will be 

located at Munster, Germany. In addition, two 

PACT-2 units, with a 0.6-m (2-ft) diameter 

centrifuge, have been installed at the Retech 

plant. 

8.3 High-Temperature, 

Joule-Heated Melter 

High-temperature, joule-heated melters have 

been discussed in detail in chapter 6. 

8.4 Direct Current Graphite Arc 

Melter 

DC arc furnaces with graphite electrodes have 

been used in the steel industry since the 

beginning of this century. Drawn from the steel 

industry experience, the DC graphite arc melter 

system for mixed wastes was developed and 

tested by a collaborative National 

Laboratory-University-Industry program 

between PNNL, Massachusetts Institute of 

Technology (MIT), and T&R Associates. The 

program was initiated in 1992. Currently, there 

are three graphite electrode DC arc furnaces 

available for testing. A pilot-scale furnace, 

Mark II with processing rates of 450 kg/hr 

(1,000 lb/hr), is installed at the Plasma Fusion 

Center Laboratory at MIT. An engineering-scale

NUREG/CR-6666 86
system, capable of processing 45 kg/hr 

(100 lb/hr), and a bench-scale sygtem are being 

installed at PNNL to perform radioactive, 

mixed, and hazardous waste processing. In this 

section, discussion will be focused on the 

Mark II Pilot-Scale Furnace system installed at 

MIT. 

The Mark II melter is a carbon steel vessel lined 

with refractory and is approximately 

7.0 m (23 ft) high and 2.1 m (7 ft) in diameter. 

The feed system and the glass discharge system 

are attached diametrically opposite to each 

other. The electrode is a unique coaxial 

arrangement with a 40.6 cm (16 in.) outside 

diameter and 25.4 cm (10 in.) inside diameter 

outer electrode, and a 15.2 cm (6 in.) diameter 

solid inner electrode. This electrode system 

provides the capability of operating either in the 

transfer mode (arcing from the electrode to the 

material) or non-transferred mode (arcing 

between the inner and outer electrode). 

A significant effort is being made to develop a 

furnace diagnostic system. The efforts include 

development of analytical equipment for 

spatially resolved measurements of furnace and 

glass temperatures as well as continuous in situ 

monitoring of offgas emissions. 

Like all other plasma systems, this technology is 

suitable for heterogeneous wastes difficult to 

vitrify when volatilization is not a major issue in 

stabilization. This technology is still in its 

development stage. 

8.5 'Alternating Current Arc 

Melter 

A pilot-scale alternating current arc plasma 

melting technology for mixed wastes and 

contaminated soils is being developed at the 

Albany Research Center of the USBM, Albany, 

Oregon. The technology is similar to the

Mixed Waste Solidification Technology Experience 

three-phase carbon arc technology discussed in 

section 7.5. 

8.6 Hybrid Plasma-Induction, 

Cold-Crucible Melter 

The hybrid plasma-induction cold-crucible 

Melter (PICCM) technology is discussed in 

chapter 11, Emerging Solidification 

Technologies. 

8.7 Microwave Melter 

Microwave technology is not new to United 

States industry or to the public. For the last two 

decades, microwave technology has been used 

daily by millions of people for cooking. Other 

extensive users include the chemical and food 

industries. Since 1985, the Rocky Flats Plant 

(RFP) in Colorado has been developing this 

technology for solidification of radioactive 

wastes. Several studies have been conducted in 

bench-, pilot-, and full-scale demonstration 

systems. The bench-scale studies have included 

both radioactive as well as nonradioactive 

processing using microwave systems. The 

full-scale demonstration plant has shown that the 

technology is scalable from bench to production 

level, but, due to the lack of funding, integrated 

testing of ancillary units like the offgas 

treatment systems has stopped. 

The microwave melting system developed by 

INEEL is called the In-Drum Melting System. In 

this process, a 113-L (30-gal.) drum is placed in 

an insulation cage and connected to a 

microwave source. The microwave energy is 

transmitted via a 60-kW (204,780-BTU/hr), 

915-MHz generator. A small amount of waste 

material is first transferred from a hopper to the 

drum through a screw conveyor. The microwave 

energy raises the temperature of the waste 

materials to 1,000 'C. After an initial charge is 

melted, a continuous charge of waste materials

87 NUREG/CR-6666
Mixed Waste Solidification Technology Experience 

is fed to the drum. After a required amount of 

waste material is fed, the microwave system is 

stopped, and the drum is removed from the 

insulation cage and allowed to cool. Offgas is 

removed from the processing chamber by a 

down-stream blower system. The offgas is 

passed through a bed of granulated, 

activated-carbon filter to remove oxides of 

sulfur and nitrogen and through HEPA filters to 

remove particulates before being released to the 

stack. 

Microwave technology for waste treatment and 

solidification is not fully developed, and its 

performance as an integrated system has not 

been established. The technology is itself robust 

and can be applied to a large number of waste 

streams. In addition, a single container can act 

as a melting crucible and waste-form canister, 

thus reducing costs and maintenance. However, 

due to low throughput and inability of the 

microwaves to penetrate large-diameter drums, 

the technology is only useful for processing 

small quantities of waste. 

8.8 Quantum-Catalytic Extraction 

Process 

The Quantum-Catalytic Extraction Process 

(Q-CEP) technology is discussed as part of 

Non-vitrification Solidification Processes 

Experience in chapter 12.

8.9 Vortec Combustion and 

Melting System

The Vortec-patented Combustion and Melting 

System (CMS) is a high-temperature 

thermal/chemical solid waste treatment and 

recycling process for remediating soils, 

sediments, sludges, and mill tailings that have 

organic, inorganic, and heavy metals 

contaminations (Hnat et al., 1994). The process

oxidizes organic contaminants and immobilizes 

inorganic contaminants in a vitrified matrix. 

Figure 8.3 shows the Vortec process schematic. 

The basic elements of the system are CMS; 

material handling, storage, and feeding system; 

vitrified product separation and reservoir 

assembly; waste heat recovery unit; flue gas 

(offgas) cleanup system; and vitrified product 

handling system. The CMS consists of two 

major assemblies: a counter-rotating vortex 

(CRV) in-flight suspension pre-heater and a 

cyclone melter. First, feed (wet or dry) is 

introduced into the CRV combustor. The CRV 

combustion burner preheats the suspended waste 

and oxidizes organics. The average temperature 

of the materials leaving the CRV is between 

1,200 and 1,540 0C. The preheated solids exit 

the CRV and enter the cyclone melter, where 

they are dispersed onto the chamber walls to 

form a molten glass product. The vitrified 

molten glass product exits the cyclone melter 

through a tangential exit channel into a glass/gas 

separation chamber. The exhaust gases then 

enter a pre-heater for waste heat recovery and 

are sent to the offgas treatment system. The 

glass exits through a tap to a water quench 

assembly for subsequent disposal. 

Since 1988, Vortec has been processing 

non-hazardous industrial waste materials at its 

20,000 kg/day (44,000 lb/day) operational 

pilot-scale facility. Even though CMS 

technology offers high production rates, the 

issues relating to the safety of fuel/air mixtures 

in the presence of organics and other 

contaminants is unknown. In addition, the lack 

of a cold-cap and temperatures in the range of 

1,600 °C could result in extremely high 

volatilization losses.

NUREG/CR-6666 88
Mixed Waste Solidification Technology Experience

PRE-COUBUSTOR 

CRY C(

cO&ESUS'rONi AF

ELECTROSTATIC 

PRECIPITATOR

MELTER ID FAN SEPARATOR

RESERVOIR

GLASS 

Figure 8.3. Vortec Combustion and Melting System Flowsheet (Hnat et al., 1994) (Permission to use 

this copyrighted material is granted by American Ceramic Society, Westerville, Ohio)

8.10 In Situ Vitrification 

The In Situ Vitrification (ISV) process 

(Luey, 1995) was invented in 1980 and patented 

in 1983 by the PNNL. The technology is 

commercially available through Geosafe 

Corporation. In recent years, the ISV has been 

extended to In Situ Plasma Vitrification (ISPV) 

(Kielpinski et al., 1995) and In Situ Waste 

Destruction and Vitrification (IWDV) (Ferrar, 

1995). 

ISV has been evaluated and implemented for a 

number of waste streams/sites within the DOE, 

Department of Defense, Superfund sites in the 

United States, and internationally in Australia 

and Japan. Waste types that have been vitrified 

include contaminated soil, contaminated soil 

with debris, liquid seepage cribs, buried waste, 

and underground storage tanks. The waste

streams include organics, heavy metals, 

radionuclides, and construction debris. 

The ISV process involves forming a molten pool 

of soil in the treatment zone. The treatment zone 

is determined by the placement of electrodes in 

the soil. As the electrical energy is applied 

between the electrodes, the joule heating starts 

melting the soil. Typical melting temperatures 

are 1,600 to 2,000 -C. The process continues 

until the desired depth and width is reached. The 

power is shutdown, electrodes are pulled out (if 

required) and the molten mass is allowed to 

cool. A typical single, full-scale ISV setting is 

9.1 m (30 ft) in diameter and 4.6 m (15 ft) deep 

and takes about 10 to 15 days per setting to 

vitrify. 

The vitrified product is analogous to natural 

obsidian and is estimated to retain its integrity 

for geologic periods. The vitrified product has

89 NUREG/CR-6666
Mixed Waste Solidification Technology Experience 

outstanding physical, weathering, and chemical 

properties and is typically 5 to 10 times stronger 

than reinforced concrete. In a typical ISV 

product, unlike vitrification, which produces a 

single homogeneous product, there are generally 

five distinct regions: (i) the vitreous frothy 

surface layer; (ii) the partially reacted ring layer, 

which marks the vitrification surface of the 

monolith; (iii) the vitrified material; (iv) the 

devitrified material near the center of the 

monolith; and (v) the potential pools of metals at 

the electrodes or at the bottom of the monolith. 

To characterize such a monolith, a detailed 

characterization of megascopic regions is 

necessary. If major regions can be identified, a 

stratified random sampling can provide 

sufficient data to determine chemical, 

morphological, and durability properties of each 

region. 

ISV can be used to treat the waste at its origin 

and organics, inorganics, and radioactive 

contaminants can be processed simultaneously. 

In addition, the process has high tolerance for 

debris. Some of the limitations of this 

technology are that the media must be 

acceptable for joule heating. The total organic 

loading must be less than 10 wt %. and the 

treatment depth less than 6.1 m (20 ft). The 

presence of high groundwater recharge rates or 

very large voids makes the process difficult. The 

presence of sealed containers also poses a 

processing risk. 

In the ISPV, a plasma torch, in a nontransferred 

mode, is inserted to a target depth and, as the 

soil is melted, the torch is slowly withdrawn 

upwards. The technology does not require that 

the soils be conducive to joule heating and 

avoids safety concerns of explosion due to vapor 

pressure buildups in the top-down vitrification. 

In addition, ISPV uses the existing systems 

technology for ISV such as site 

preparation/characterization, in-field power 

systems, and offgas capture and treatment.

Ex situ test trials were conducted on SRS soils at 

Georgia Institute of Technology and Mississippi 

State University, and it was found that fluxing 

agents were necessary to achieve a good 

vitrified product. Actual field testing of this 

technology has not been performed. 

The IWDV is another extension of ISV 

technology. In this process, a steel-lined 

borehole is installed through a waste pit. Then, a 

high-temperature graphite arc-melter system is 

lowered through the bore hole to the bottom of 

the buried waste and arc melting is initiated. The 

melter consumes the steel liner, volatilizes 

organics, and melts the inorganic wastes and 

soils at the bottom. As the waste and 

contaminated soils are vitrified, the torch is 

withdrawn slowly until a required depth and 

width is reached. This technology has only gone 

past the proof-of-concept stage and needs major 

resources for pilot-scale demonstration of waste 

remediation. 

One of the major concerns related to the ISV 

technology is the unknown and probably 

nonuniform melting characteristics of the 

underground materials including offgas (steam 

from soil moisture and reaction products) 

characteristics. This issue is evident from the 

recent melt expulsion in the ISV plant at the 

ORNL in Oak Ridge, Tennessee. A 20,000-kg 

(44,000-1b) melt expulsion of radioactively 

contaminated soil occurred at the plant on 

April 21, 1996 (Oak Ridge National Laboratory, 

1996). The expulsion was caused by pressurized 

steam venting rapidly through the molten glass 

body. The ISV was going on for the previous 

17 days. The vitrification pit was covered by a 

stainless steel hood and maintained under 

negative pressure to collect, filter, scrub, and 

monitor offgas. During expulsion, the offgas 

was rapidly heated from 250 to 1,000 'C, and 

the pressure surge was sufficient to lift the 

6,750 kg (15,000 lb) hood approximately 30 cm 

(12 in.) off the ground. A pool of glass was able

NUREG/CR-6666 90
to flow up to 0.9 m (3 ft) outside the hood while 

the hood was still raised. The escaping offgas 

and molten glass ignited several small fires in 

the combustible components near the hood. 

The air sample analysis indicated that there was 

no airborne contamination. However, gas 

samples directly from the ISV hood, coupled 

with an estimated volume of offgas release, 

indicated a maximum release of 0.13 IiCi of 

Cs-137. The analyses of offgas processing filters 

indicated that 99.7 percent of all offgas 

radioactivity was retained in the filters. The 

residual molten glass was found to contain 

99.99 percent Cs- 137. Small fragments of the 

glass pieces were found as far as 91 m (300 ft) 

away from the site. The overall environmental 

impact of the incident relative to the spread of 

contamination was considered to be 

insignificant. The root cause of the melt 

expulsion incident was inadequate vent design. 

Although similar expulsion events have been 

reported in 3 out of 70 large-scale ISV melts, 

previous incident investigations implicated 

man-made subterranean structures like tanks, 

concrete vaults, and drums, which were thought 

to constrict the normal flow and dissipation of 

offgas around the growing molten mass. The 

offgas hood was modified to handle future 

expulsion events in a controlled manner. 

Due to operating temperatures of greater than 

1,600 °C, and the absence of cold-cap to prevent 

volatilization of radionuclides such as Tc and 

Cs, ISV technology in its present form cannot be 

applied to vitrification of HLW in the tanks. In 

contrast, the risk of having melt expulsion in the

Mixed Waste Solidification Technology Experience 

joule-heated slurry-fed melter systems is 

extremely low, even though the slurry may 

contain 40-50 percent waste, which will flush as 

steam in the melter. This is mainly due to the 

fact that the slurry is fed on top of the hot melt 

and does not percolate through the melt, as is the 

case with the Q-CEP process for example. This 

risk of expulsion is consequently much higher in 

the Q-CEP process. 

8.11 Russian High-Frequency 

Induction Melter 

The Russian high-frequency induction melter is 

combined with the cold-crucible melter and is 

discussed in section 11.1. 

8.12 Transportable Vitrification 

System 

The TVS has been discussed in section 6.2. 

8.13 Vitreous Ceramics 

The vitreous ceramics concept is still in the 

laboratory-testing phase. Laboratory testing has 

shown that a vitreous ceramic waste form offers 

high waste loadings and much higher volume 

reduction. Since this concept is still 

experimental, and an adequate technology has 

not been developed, further discussion on the 

topic at this time is beyond the scope of this 

report.

91 NUREG/CR-6666
9 RESEARCH AND TEST MELTER EXPERIENCE

The PNNL, the Vitreous State Laboratory (VSL) 

at the Catholic University of America (CUA), 

and the Clemson University Vitrification 

Research Laboratory have established a set of 

pilot-scale melter systems to provide industries 

preliminary process evaluation for their waste 

types. In fact, the PNNL ran the first radioactive 

melter that provided information for the WVDP 

melter at West Valley and DWPF melter at the 

SRS. In addition, WVDP and DWPF both used 

pilot-scale melters to develop process flow 

sheets. In this section, an overall description of 

the technologies is presented and discussed.

9.1 Clemson University 

Vitrification Research 

Laboratory 

In 1992, the DOE/Industrial Laboratory for 

Vitrification Research was established in 

cooperation with the Westinghouse Savannah 

River Company; Envitco, Inc.; and Stir Melter, 

Inc. The Vitrification Research Laboratory 

(VRL) currently houses three melters. Of these, 

two are joule-heated melters supplied by 

Envitco, Inc., and Stir Melter, Inc., and the DC 

arc graphite electrode melter supplied by 

Electro-Pyrolysis, Inc. (EPI). Each melter 

system has its own offgas system. Since the 

inception of the program, VRL has evaluated 

several waste streams, using surrogate waste 

compositions. Table 9.1 lists the type of waste

Table 9.1. Surrogate Studies Performed at Clemson University (Overcamp, 1995) 

Electro

Envitco Stir-Melter Pyrolysis, Inc. 

Surrogate Waste EV-16 WV-0.25 Mark I 

Ion exchange resins & high-level waste slurry - 1 

Savannah River Site M-Area sludge (dry feed) 4 1 -

Savannah River Site M-Area sludge (slurry) 1 1 

Oak Ridge Reservation Y-12 West End 4 2 

Treatment Facility sludge 

Rocky Flats Plant precipitate sludge 1 

Los Alamos National Laboratory TA-50 1 

precipitate sludge 

Hanford high-sodium waste 1 

Oak Ridge K-25 Plant B & C Pond sludge 1 -

Soil and debris - - 4

93 NUREG/CR-6666
Research and Test Melter Experience

stream processed, the number of campaigns 

associated with each waste stream, and the 

melting system used. 

The discussion of results for each waste stream 

shown in table 9.1 is beyond the scope of this 

review, but in the sections that follow, a brief 

description and discussion of each melter 

technology is provided, along with the operating 

experiences. The information presented here has 

been extracted from Erich and Overcamp 

(1996), Overcamp (1995), Overcamp et al. 

(1996), and Marra et al. (1996a). 

9.1.1 Envitco EV-16 Melter 

The Envitco EV-16 is a high-temperature, joule

heated melter that uses four molybdenum 

side-electrodes. This is the same melter and 

offgas system that was used by Envitco in the 

Hanford LLW vitrification test program 

discussed in section 7.1. In addition to the 

Hanford program, vitrification tests were 

conducted for other waste management 

programs as shown in table 9.1. 

A significant amount of devitrification was 

observed in the Oak Ridge West End Treatment 

Facility and the RFP precipitate sludge 

campaigns. This was attributed to relatively high 

concentrations of calcia, which resulted in the 

formation of wollastonite (CaSiO 3) phase having 

a melting temperature of 1,800 'C. The 

devitrification caused extreme difficulties in 

discharging glass from the melter. It is 

recommended that, for calcium aluminosilicate 

glasses, the calcia concentration should not 

exceed 18 percent. A complete characterization 

of the glass is warranted before processing 

waste through the melter. 

Severe corrosion of the molybdenum electrodes 

was observed after the SRS M-Area and RFP 

campaigns. The electrodes lost about 10 cm 

(4 in.) of their length during testing. Both high

temperatures and reducing conditions resulted in 

alloying of electrodes with reduced metals. 

Nickel alloying was clearly evident. This 

warrants careful control of redox conditions in 

the melter and nickel contents in glass. 

During one of the campaigns, four different 

refractory materials were used for the melter 

walls. These materials included an AZS sintered 

refractory, an AZS fused cast refractory, a 

high-chrome pressed refractory, and a 

chromium-aluminum-zirconium-silicate fused 

cast refractory. After the campaign, the AZS 

sintered refractory was heavily damaged near 

the electrode opening losing about 5 cm (2 in.) 

of 7.56 cm (3 in.) thickness. The AZS fused 

cast, high-chrome sintered, and Cr/Al/Zr silicate 

showed about 2.2 cm (0.875 in.), 1.25 cm 

(0.5 in.), and negligible degradation, 

respectively. 

Based on the above results, the authors 

recommend that either AZS fused cast or 

high-chrome refractories should be used in 

actual radioactive melter operations. 

The melt rates were severely affected by 

(i) reactive offgassing and foaming and 

(ii) sulfate salt formation in the high sulfate 

feeds. Dry feeding in the M-Area tests caused 

significant offgas line plugging. The plugging of 

offgas lines was attributed to the entrainment of 

relatively fine particles of the feed materials in 

the offgas line. 

9.1.2 Stir-Melter WV-0.25 

The stir-melter WV-0.25 is a joule-heated melter 

that uses an Inconel Alloy 690 chamber. A 

simplified melter schematic is shown in 

figure 9.1. This chamber has a square cross 

section with a melt surface area of 0.025 m2 

(0.27 ft) and a working depth of 0.3 m (1 ft). 

The melter has an Inconel® Alloy 690 stirrer. 

The stirrer is driven by a 325-W (1,1 10-BTU/hr)

NUREG/CR-6666 94
Research and Test Melter Experience

Liquid Feed

Dry Feed

Auxilliary 

Heaters 

Exhaust 

Port 

Drain Normal 

Valve Flow 

Figure 9.1. Stir-Melter WV-0.25 at Clemson University (Marra et al., 1996a) (Copyright 1996 by the 

American Nuclear Society, LaGrange Park, Illinois)

variable-speed motor at speeds up to 1,000 rpm. 

The impeller can be raised or lowered to 

optimize mixing. The stirrer and the chamber act 

as electrodes. A 10-kW (34,130-BTU/hr) power 

source is available for joule heating through the 

stirrer/chamber circuit. In addition, there are 

external resistance heaters with 7 kW 

(23,890-BTU/hr) power for startup and 

maintaining temperature during melting. The 

maximum operating temperature of the melter is 

1,070 'C because of the melter construction 

material. The melt chamber has an internal 

"teapot" spout with an external drain tube. The 

melter has separate ports for dry feeding, slurry 

feeding, and other additives. The offgas system 

consists of a spray/ejector venturi, jet sparging 

scrubber, and a counter-flow packed tower for 

acid gas removal and neutralization. The 

vitrification campaigns are shown in table 9.1.

The operation of the stir-melter was limited due 

to the lower operating temperatures of the 

melter. Inconel® Alloy 690 has a melting point 

of about 1,350 'C, but the creep rate increases 

with a decline in the elastic modulus above 

1,100 °C. Therefore, a maximum use 

temperature of 1,070 °C was placed in the 

melter to avoid damage to its impeller and the 

melt vessel. Despite operation at 1,070 °C, the 

impeller was severely damaged. The bottom ring 

of the impeller was missing, and severe 

corrosion of the blades was evident. Large 

metallic drops were found on the bottom of the 

melter. Analysis of the melter after the failure 

indicated that, in addition to the reducing 

conditions, localized extreme temperatures 

(resulting from excessive power dissipation 

caused by a faulty controller) in the impeller 

caused severe degradation. 

During the RFP campaign, waste containing 

high sulfur and activated carbon were processed.

95 NUREG/CR-6666
Inconel® Alloy 690 under reducing conditions is 

susceptible to severe corrosion and failure from 

aggressive chemical attack, especially 

sulfidation; the operation of the melter in the 

presence of activated carbon increases the 

potential for extremely reducing conditions. Due 

to catastrophic impeller failure with the RFP 

feed, the campaign never extended beyond 

operational parameter studies. 

Foaming and sulfate salt formation were much 

more severe than in the Envitco melter. At 

times, the glass drained from the melter was 

foamy, indicating the inability of the melter to 

remove foam from the melt. At times, the 

molten salt layer volatilized or became entrained 

with particulates in the offgas line. Significant 

offgas plugging problems similar to those of the 

Envitco system were observed in this system. 

This plugging severely impaired the melter 

performance. 

As in the case of the Envitco melter, the high 

calcia feeds caused plugging of melter drains, 

foaming in the melter, and devitrification of 

glass. 

9.1.3 Electro-Pyrolysis, Inc., DC Arc 

Melter 

The EPI DC arc melter consists of a graphite 

crucible contained inside a sealed, water-cooled, 

atmosphere-controlled vessel. This melter has a 

circular cross section with a melt surface area of 

0.085 m2 (0.91 ft) and a working depth of 

0.53 m (1.7 ft). The melter is capable of 

processing about 50 kg (110 lb) of feed per 

charge. The melter can be operated either in a 

cold-top or hot-top mode. Power at 100 kW 

(341,300 BTU/hr) is supplied through 

consumable electrodes. The melter has three 

ports, located 120 degrees apart, for installation 

of extra cameras, feed systems, or 

thermocouples.

Several test runs were made (Overcamp et al., 

1996) on INEEL soils and South Carolina 

high-silica soil with fluxing agents, but 

operational data or process upsets and issues 

were not discussed. 

9.2 Vitreous State Laboratory at 

the Catholic University of 

America 

The VSL of the CUA has been operating a 

variety of joule-heated melters since 1985 and a 

radioactive system since 1987. The joule-heated 

melter systems installed range from production 

rates of 1-kg/hr to more than 1,000-kg/hr 

(2,200-lb/hr) and are innovative derivatives of 

the joule-heated systems for HLW vitrification. 

CUA-VSL is extending the I-ILW vitrification 

systems that operate at 1,150 0C to 

high-temperature, joule-heated systems that 

operate up to 1,500 °C by the use of new 

materials. Several melters installed and operated 

by CUA-VSL have been designed by 

GTS Duratek. 

Some major accomplishments (Pegg, 1995) 

include (i) the glass development, as well as the 

melter and offgas test runs, to support GTS 

Duratek M-Area mixed waste facility at SRS; 

(ii) glass development and melter and offgas test 

runs to support the MAWS demonstration 

conducted at the Fernald site; (iii) process 

development and melter and offgas runs for 

DuraChem commercial nuclear power spent 

organic ion exchange resins; and 

(iv) development, characterization, and melter 

testing of glasses for the WVDP at West Valley. 

9.2.1 High-Temperature, 

Joule-Heated Minimelter 

System 

This high-temperature, joule-heated system is a 

scaled version of the melter installed at Femald 

(section 6.1). Before the full-scale melter was 

installed at Fernald, this scaled version provided

NUREG/CR-6666 96
Research and Test Melter Experience

data on the performance of innovative designs 

incorporated in the melter chamber. Even 

though this minimelter has been successfully 

operated at the CUA-VSL for the last 18 mo for 

a variety of waste streams, the full-scale version 

at Fernald suffered a catastrophic failure and 

was shut down within 6 mo of operations as 

noted in section 6.1. 

Similar to the Fernald melter, this melter 

consists of a center chamber separated by 

conductive ceramic bricks. The center chamber 

is used for melting waste glasses. The side 

chambers contain molybdenum rod electrodes 

immersed in benign glass to avoid oxidation. 

The surface area of the melting chamber is 

194 cm 2 (30 in2) with a volume of 2.95 L 

(0.78 gal.) for waste glass. The side chambers 

have a volume of 2 L (0.64 gal.). The melter can 

be operated between 1,150 to 1,350 °C. The 

offgas leaving the melter is successively 

introduced into a film-cooler, a water-spray 

quencher, an NaOH scrubber, and a mist 

eliminator. After the mist eliminator, gases are 

heated to reduce the relative humidity before 

entering the baghouse where residual 

particulates are removed on bags precoated with 

diatomaceous earth. Finally, gases pass through 

a HEPA filter before being exhausted to stack. 

The melter was operated to show the feasibility 

of melting feeds high in lead, barium, 

phosphorous, and sulfur (Fu et al., 1996). The 

results indicated that the feeds containing up to 

9 wt % PbO, 5 wt % BaO, 8 wt % P20 5, and 

12 wt % S03 can be processed in the 

temperature range of 1,150 and 1,350 'C. The 

results also indicated that feeds with more than 

5 wt % S0 3 can produce excessive foaming 

events, but with the addition of urea (a reducing 

agent), the number of foaming incidents was 

reduced. Urea was found to reduce sulfates to 

SO 2, decrease the concentration of NO., and 

produce a homogeneous product.

It should be noted that the use of urea in 

excessive amounts can promote significant 

reducing of the melt and result in the formation 

of conductive sludges that could cause 

premature failure of the melter. In addition, 

urea, when not present in stoichiometric 

amounts, can result in formation of ammonia in 

the offgas. The presence of ammonia and NO,, 

can be a major safety issue. Even though the 

minimelter runs did not indicate severe 

molybdenum, molydisilicide, or refractory 

corrosion, the full-scale melter failed due to 

excessive corrosion and erosion. 

9.2.2 DuramelterO 100 Melter 

The Duramelter 100 is a joule-heated melter 

that uses Inconel® Alloy 690 side electrodes 

(Bowan et al., 1994a). This same melter and 

offgas system was used by GTS Duratek in the 

Hanford LLW vitrification test program 

discussed in section 7.4. A simplified melter 

schematic is shown in figure 9.2. This melter 

has a square cross section with a melt surface 

area of 0.13 m2 (1.38 ft) and melt volume of 

49 L (13 gal.). The melter is lined with 

high-chrome refractory in the glass contact area. 

The contact refractory is backed up by AZS 

refractory and 95 percent alumina castable. The 

entire glass containment is housed in an interior 

Inconel® Alloy 690 liner. A thick, ceramic 

fiberboard is placed between the Inconel 

Alloy 690 liner and external melter shell. The 

melter has three drains a bottom drain, a surface 

sulfate layer drain, and a side discharge riser 

with a pour spout for glass pouring. The glass 

pouring is initiated by an airlift mechanism. 

Multiple air bubblers are installed near the floor 

to enhance convection in the melt pool and 

increase production rate. The melter operates 

between 1,100 and 1,200 'C. The glass 

production capacity is 100 kg/day (220 lb/day).

97 NUREG/CR-6666
0~ 

ON 

ON• 

(2%

Discharge-,. 

Heaters A

Trough--- 00

RiserFiber-•--

Insulation

Castable 

Bottom Drain

Gas Port 

Emergency Vent 

Lid Heater 

AZS (Zirmul) 

Outer Shell 

Inner Shell 

Electrodes 

Monofrax K-3

L Sulfate Drain

Figure 9.2. Duramelter® 100 Melter Schematic (Bowan et al., 1994a) (Permission to use this copyrighted material is granted by 

American Ceramic Society, Westerville, Ohio)

(D 

H 

'4 

'-I 

hi 

x 

0
Research and Test Melter Experience

Duramelter' 100 has been used for testing a 

variety of waste streams. A discussion on 

processing of the simulated Hanford 

high-sodium waste was provided is section 7-4. 

This melter was also used for processing 

radioactive Fernald Pit 5 sludge (containing as 

much as 25 percent fluorine as fluorides) as a 

part of the MAWS program. The offgas system 

for this testing was specifically designed to 

handle high-fluoride content. A process 

schematic is shown in figure 9.3. The major 

offgas components are the quencher, which 

operates as a spray-drying wet scrubber, and a 

conventional wet scrubber. The hot offgases 

flow from the melter into the quencher. In the 

quencher, 2-3 M sodium hydroxide solution is 

sprayed and evaporated to aid in decreasing 

offgas temperature. Fluoride in the offgas reacts 

with sodium hydroxide to form sodium fluoride 

solid. The solids are collected at the bottom of 

the quencher and recycled back into the melter 

through the feed system. The wet scrubber is a 

secondary cleaner that uses 0.1 M sodium 

hydroxide. From the scrubber, offgases pass

through the mist eliminator, heater, baghouse, 

pre-filter, and a HEPA before they are released 

to the stack. 

The performance data presented by 

Matlack et al. (1995) do not indicate any 

problems or issues related to the processing of 

the high-fluoride radioactive wastes. A 

Duramelter" 300, of similar design, was 

installed at the Fernald site under the MAWS 

program to vitrify Pit 5 radioactive sludges. No 

information is available on the performance of 

this melter. 

9.2.3 Durameltere 1000 (Asbestos 

Melter) 

Another major pilot-scale melter system 

installed at CUA-VSL is a demonstration plant 

for the vitrification of the asbestos-containing 

wastes. This was the second of the two melter 

and offgas systems used by GTS Duratek in the 

Hanford LLW vitrification test program 

discussed in section 7.4.

Figure 9.3. Duramelter 100 Process Flowsheet (Matlack et al., 1995) (Permission to use this 

copyrighted material is granted by American Ceramic Society, Westerville, Ohio)

99 NUREG/CR-6666
Research and Test Melter Experience

The pilot-scale asbestos melter is a joule-heated 

melter that uses Inconel® Alloy 690 side 

electrodes (Bowan et al., 1994b). A simplified 

melter schematic is shown in figure 9.4. The 

melter is lined with a 12.7-cm-thick (5-in.-thick) 

layer of high-chrome refractory in the glass 

contact area. The contact refractory is backed up 

by a 15.2-cm-thick (6-in.-thick) layer of AZS 

refractory and a 6.4-cm-thick (2.5-in.-thick) 

layer of 95 percent alumina castable refractory. 

The entire glass containment is housed in a 

0.64-cm-thick (0.25-in.-thick) interior liner 

constructed from Inconel® Alloy 690. A 

10.2-cm-thick (4-in.-thick) ceramic fiberboard is 

placed between the Inconel® Alloy 690 liner and 

external melter shell. The melter electrodes are 

two pairs of 2.5-cm-thick (1-in.-thick) Inconel® 

Alloy 690 plates on the opposing walls of the 

melter. The melter has a bottom drain and a side 

discharge riser with a pour spout for glass 

pouring. Glass pouring is initiated by an airlift 

mechanism. Multiple air bubblers, installed near 

the floor to enhance convection in the melt pool, 

increase the production rate. The melter operates

on AZS (Canmn Bon Drain

between 1,100 and 1,200 -C. The glass 

production capacity is 900 kg/day 

(1,980 lb/day). 

Asbestos wastes are typically contained in 

approved, multiple polyethylene bags in 

amounts not exceeding 22.5 kg. (50 lb). The 

wastes can contain wire, gypsum, cements, 

sections of steel pipes, water, and other debris 

collected during remediation. Large steel pipes 

are usually not fed to the melter. Figure 9.5 

shows a process flow diagram for the asbestos 

waste vitrification. The melter receives these 

bags and feeds them directly along with some 

glass-forming chemicals. Organic components 

of the waste are oxidized, and inorganic 

components are vitrified in the melter. Offgas, 

steam, and particulates from the oxidation and 

reaction processes are sent to an offgas 

treatment system. The offgas system consists of 

a water spray quencher in series with a packed 

bed scrubber. Following the packed bed 

scrubber is a mist eliminator and a multistage 

dry filtration system including HEPA. The

MO.,r Food 

Room Room

Figure 9.4. Duramelter 1000 Melter (Bowan et al., 1994b) (Permission to use this copyrighted material is granted by American Ceramic Society, Westerville, Ohio)

NUREG/CR-6666 100
Research and Test Melter Experience

CLASS..... 

CHEMIICAL ADDITIVIES -IPRE FILTIER HIEPA F ILTER 

'WASTE 

CLASS RESIDUALS 

DISC1ARGE 

Figure 9-5. Duramelter 1000 Process Flowsheet for Asbestos Processing (Bowan et al., 1994b) 

(Permission to use this copyrighted material is granted by American Ceramic Society, Westerville, 

Ohio)

residuals accumulated by the scrubber are 

recycled with the feed. Glass is discharged from 

the melter through either a bottom drain or a 

side exit to a riser-pour discharge system. 

The information presented in Bowan et al. 

(1994b) does not indicate any problems or 

safety issues related to the processing of 

asbestos wastes. It is interesting to note that the 

Duramelterl 1000 was also used to convert 

about 2,000 kg of hazardous asbestos waste 

from the White House into glass. 

9.3 Pacific Northwest National 

Laboratories 

PNNL has been developing joule-heated 

ceramic lined glass melters since 1973. The first

experimental, full-scale electric melter began 

operation in February 1977. The vitrification 

systems that have been operated at the PNNL 

include (i) Liquid-Fed Ceramic Melter (LFCM), 

(ii) Pilot-Scale Ceramic Melter (PSCM), 

(iii) Radioactive Liquid-Fed Ceramic Melter 

(RLFCM), (iv) Engineering-Scale Melter, 

(v) Research-Scale Melter, (vi) Small-Scale 

High-Temperature Melter, (vii) Pilot-Scale 

High-Temperature Melter, and (viii) DC Arc 

Melter. 

A major thrust of research at PNNL has been on 

joule-heated melter systems. A summary of all 

the tests and melter research conducted at PNNL 

is beyond the scope of this report. However, the 

information relevant to this report, presented in 

the following sections, has been extracted from 

publications by Barnes and Larson (1981),

101 NUREG/CR-6666
Research and Test Melter Experience

Larson (1996), Holton et al. (1988), and Koegler (1987). Most of the discussion is focused on the 

RLFCM. 

9.3.1 Radioactive Liquid-Fed 

Ceramic Melter 

The fundamental design of the RLFCM is based 

on the successful designs of the PSCM and the 

LFCM. The pilot-scale RLFCM was designed, 

fabricated, and remotely operated in shielded 

cells from 1983 to 1987. Radioactive operations 

were initiated in October 1984. The operations 

were concentrated on testing vitrification 

flowsheets simulating wastes stored at the West 

Valley site and preparation of special isotopic heat and radiation dose canisters for the FRG. 

Radioactive operations were terminated in 

March 1987. The RLFCM consists of a melter 

system and supporting equipment for slurry preparation and an offgas treatment system. 

Figure 9.6 shows a schematic diagram of the 

vitrification process. 

The feed preparation system consists of two 

tanks, each equipped with its own agitator 

system. Both the feed make-up tank and the 

melter feed tank have a volume capacity of 

1,250 m3 (330,000 gal.). Radioactive slurries are 

prepared in the feed make-up tank where 

radioactive waste is blended with glass-forming chemicals. The contents are allowed to fully mix 

before transferring to the feed tank. An air 

displacement pump is used to move the slurry through the feed line to a water-cooled feed 

nozzle located on the top of the melter. 

The RLFCM is a joule-heated, ceramic-lined 

melter. A schematic of the RLFCM is shown in 

figure 9.7. The molten glass is contained in a 

high-chrome refractory. The melt cavity 

measures 0.61 m (2 ft) by 0.91 m (3 ft) with a 

nominal glass depth of 0.36 m (1.2 ft) and the 

nominal melt surface area is 0.6 M2 (6.45 ft).

The melter is designed to operate at a feed rate 

of 40 L/hr (10.6 gal.) and glass production rate 

of 20 kg/hr (44 lb/hr). The melt cavity contains 

two air-cooled Inconel® Alloy 690 plate 

electrodes. The nozzles penetrating the melter 

lid provide access to the interior of the melter 

tank. Five 5.1-cm. (2-in.) nozzles are used for 

glass tank thermocouples, electrode surface 

thermocouples, refractory wall thermocouples, 

an airlift lance, and cold-cap thermocouples. 

Two 7.6-cm (3-in.) nozzles provide access for 

the water-cooled slurry feed nozzle and the melt 

level detection sensor. Two 10.2-cm (4-in.) 

nozzles provide an offgas port and plenum 

sampling package. One 12.7-cm (5-in.) nozzle is 

provided for a television viewing system. The 

melter is also equipped with various sensors for 

process control and interpretation of off-normal 

events. These include sensors for the glass 

weight factor and specific gravity, the melter 

plenum pressure, and seven sensors for 

monitoring the temperature of the molten glass 

at different levels. The temperature sensors were 

used to control the power to the melter, 

determine feeding conditions, and identify 

off-normal conditions. The melter plenum 

pressure sensor was used to indicate stability of 

the feed pump. The dip tubes were used for 

measuring melt level and specific gravity of the 

melt in the melter. The dip tubes needed 

replacement every 6 mo due to corrosion. 

The canisters are stationed on a turntable. The 

turntable consists of a three-position 

motor-driven carousel located inside a 

containment vessel. Three canisters can be 

loaded on the turntable at a time. The glass level 

in the canister being filled is monitored by a 

gamma detection system. Once the canister is 

full, it is rotated from the filling position while 

an empty canister is rotated into the filling 

position. The glass level detection system was 

an indispensable tool for monitoring the glass 

level in the canisters. The system encountered 

several design problems that were eventually

NUREG/CR-6666 102
Glass

pump Uoleor 

Food Make-Up MUllor Food 

Tank Tank 

Turntable 

Figure 9.6. Process Flowsheet for the Radioactive Liquid-Fed Ceramic Melter at Pacific Northwest National Laboratories 

S0(Dierks et al., 1987)
Figure 9.7. Sectional View of the Radioactive Liquid-Fed Ceramic Melter System (Dierks et al., 1987)

(7%

CD 

CD 

CD 

CD 1 

x 

CD 

CD
Research and Test Melter Experience

corrected by out-of-cell modifications to the 

system electronics. These problems 

compromised its performance during initial 

operation. The filled canister is allowed to cool, 

is removed from the turntable, covered, and 

transferred to another part of the process cell for 

decontamination by electropolishing and 

non-destructive characterization. 

The offgas from the melter consists mostly of 

in-leakage air, water vapors, chemical 

decomposition products, and entrained 

particulates. The offgas is treated through an 

ejector venturi scrubber, a packed-bed 

scrubber/condenser, an evaporator/fractionator 

system, offgas heaters, HEPA filters, and 

process offgas blowers. The decontaminated 

offgas is then discharged to the stack. 

During initial radioactive operations, the 

reliability of the delivery system to feed the 

melter adequately was one of the major 

concerns. Too-high or too-low feed rates could 

result in major off-normal conditions in the 

melter. A series of modifications were made to 

the feed make-up tank, the feed tank, the feed 

transfer pump, the feed transfer line, the feed 

flow rate -indicator, and the melter feed nozzle to 

improve the reliability of the system. In 

addition, both the feed makeup tank and the feed 

tank exhibited erroneous weight and specific 

gravity readings from the conventional dip tube 

assembly. The agitating system compromised 

both the weight and the specific gravity 

indications. In addition, the dip tubes plugged 

frequently as slurry evaporated at the tips. 

Clearing the dip tubes required prolonged 

soaking followed by water flushing through the 

purge air inlet. The design and installation of 

replaceable flange units resolved the problem. 

Also, the sensitivity of the slurry flow sensor in 

the cell was compromised due to high-radiation 

fields. Special radiation-hardened components 

were required to operate the sensors accurately.

During radioactive operations, typical 

off-normal conditions included bridging of the 

cold-cap and foaming in the melter. The 

foaming incidents were detected mainly by the 

melter-specific gravity sensor and glass 

temperatures near the melt surface. The cold-cap 

bridging was detected by the glass weight factor 

sensor. The weight factor, which is measured as 

a pressure, increases rapidly and becomes very 

unstable with the formation of a cold-cap bridge. 

The weight-factor increase is a direct result of 

reaction gases pressurizing the melt. The molten 

glass exits the melting cavity through a hole 

(throat) in one of the side walls near the floor of 

the melter. The glass pouring is activated by 

bubbling air via the air-lift lance into the riser 

tube. The riser tube connects the throat to the 

discharge trough. As the bubbles are introduced, 

a decrease in the pressure head in the riser tube 

allows the glass from the melt cavity to push up 

the riser tube. After the glass leaves the riser, it 

flows down the Inconel® Alloy 690 discharge 

trough into the receiving canister. Glass flow to 

the canister is controlled by the airflow rate in 

the riser. 

During one of the runs, bridging of the cold-cap 

caused pressurization under the cold-cap, 

resulting in an uncontrolled glass discharge that 

not only completely filled the canister in one 

pour but caused some glass overflow onto the 

discharge floor. This eventually acted as an 

anchor for forming a glass plug in the discharge 

cavity on subsequent pours. After the glass had 

cooled in the discharge cavity, a rotary masonry 

drill with extended shaft was used to penetrate 

the glass plug 45.7 cm (18 in.) deep with 

successively larger drill bits. This was followed 

by heating the discharge cavity by a silicon 

carbide heater in a mild steel shroud. The heater 

was lowered into the hole to completely melt the 

glass plug in the discharge tube. The discharge 

section is heated with six silicon carbide 

electrical-resistance heaters, arranged in a 

circular pattern around the discharge trough, to

105 NUREG/CR-6666
Research and Test Melter Experience

maintain glass in a molten state while it is 

discharged into a stainless steel canister. The 

discharge heaters failed on a regular basis. The 

failure was attributed to electric arcing caused 

by condensation of conductive volatile salts on 

the nonheated zone of the discharge heaters. The 

failures were minimized by increasing the 

nominal spacing between heaters. 

Offgas plugging was frequent during the initial 

operations. The plugs occurred in the offgas line 

between the melter and the inlet to the venturi 

scrubber. Monitoring the plug was difficult 

because a pressure drop in the offgas line does 

not occur until the line is more than 80 percent 

plugged. Also, the pressure spikes caused by the 

cold-cap bridging and foaming interfered with 

the monitoring of offgas plugging. Plugging was 

virtually eliminated by designing an offgas 

reamer, a cleaning device that rotates a shaft 

connected with four stainless steel chains. The 

centrifugal force generated from the chains is 

enough to clear salt deposits in the offgas line. 

The lessons learned from the RLFCM 

operations proved to be very useful for other 

HLW sites in optimizing their designs, as well 

as process flow sheet. 

9.3.2 Liquid-Fed Ceramic Melter 

The LFCM was designed to vitrify simulated 

high-level waste (Barnes and Larson, 1981). The 

melter operated from February 1977 to February 

1980 and processed both slurry and dry feeds. 

The LFCM produced 80,000 kg (176,000 lbs) of 

glass from 54,000 kg (118,800 lb) of calcine 

plus glass formers and 19.0 m3 (671 ft3

) of slurry 

feed.

During the 3-yr service period, the LFCM was 

used to develop ceramic-lined melter 

technology, establish melter operating 

parameters, and study processing characteristics 

of glass formulations. 

In February 1980, the melter was shut down and 

a comprehensive investigation was conducted to 

determine the performance of melter materials. 

The investigation indicated that the service life 

of the melter can be increased beyond 3 yr. The 

most important factor for extended life is 

adequate accommodation of thermal expansion 

properties of the construction materials. 

9.3.3 Pilot-Scale Ceramic Melter 

The PSCM operated for 5-1/2 yr between 1978 

and 1985, producing 57,000 kg (125,400 Ib) of 

glass. The melter was shut down due to the 

corrosion of tank refractories and Inconel® Alloy 

690 electrodes (Koegler, 1987) in 1985 and was 

rebuilt with an improved lid, electrode, and 

overflow drain designs. A portion of the lid had 

fallen into the tank due to inadequate vertical 

support at the walls and failure of anchors 

welded to the lid. The anchors were replaced by 

more corrosion-resistant ceramic anchors and 

Inconel® Alloy 690 clips. The electrodes failed 

at the upper bus connection and showed severe 

corrosion at the glass melt interface. Corrosion 

may have been due to a combination of 

oxidizing material buildup and mechanical stress 

at the welds. The overflow trough was modified 

because of corrosion at the pour tip. The 

ceramic components were replaced with an 

Inconel® Alloy 690 trough and dam to increase 

corrosion resistance and prevent glass seepage.

NUREG/CR-6666 106
10 PILOT-SCALE VITRIFICATION SYSTEMS FOR HIGH-LEVEL 

WASTE PLANTS

10.1 Scale Vitrification System at 

the West Valley 

Demonstration Project 

At the WVDP, the SVS-III verification tests 

were conducted, at only one-seventh scale, 

between February and August 1995 as a 

supplemental means to support the vitrification 

process flowsheet (Jain et al., 1997). The 

principal reason for having minimelter tests was 

to optimize and develop techniques to control 

the vitrification process during radioactive 

operation. The need to operate the SVS-III prior 

to radioactive vitrification operations was a 

result of the changes in the vitrification process 

flowsheet that had occurred since the conclusion 

of the full-scale non-radioactive campaign. 

The three primary areas of interest during the 

SVS-III testing were the redox state of the 

molten and solid glass, feed make-up strategy, 

and establishment of a statistical approach to the 

remote glass composition analysis. The amount 

of oxygen available to the glass melt, as 

reflected in the ferrous/ferric (Fe2+/Fe3+) or 

redox ratio, affects the melting process. Too 

much oxygen causes the melt to foam, at best 

slowing production, at worst plugging the offgas 

system. Too little oxygen causes oxides such as 

Fe20 3, NiO, and MnO to convert to metallic 

form and'become suspended in the molten glass. 

The metal precipitate can eventually buildup to 

short out the electrodes, thus permanently 

damaging the melter. During non-radioactive 

operations and prior Scale Vitrification System 

runs, an empirical relationship was developed 

between the redox ratio and the amount of sugar, 

nitrates, and total solids in the feed. The SVS-III 

runs were designed to test that relationship and 

provide assurance that a similar relationship did 

exist for the melter during radioactive 

operations. If it could be demonstrated that the

redox ratio could be predicted from an analysis 

of the feed (i.e., sugar, nitrates, and total solids), 

then it would not be necessary to determine the 

redox ratio by converting feed into glass and 

analyzing for the Fel2/Fe+3 ratio prior to feeding 

the melter. This approach would significantly 

reduce feed makeup cycle time. 

Various aspects of the feed preparation strategy 

were investigated during the SVS-III. These 

included the waste concentration cycle, the use 

of antifoaming agents in glass-formers, the 

addition of nitric acid either during glass-former 

preparation or directly to the waste, the addition 

of sodium metasilicate, and laboratory analysis 

of the feed materials. 

Sampling for feed analyses was a critical 

operation. Because the chemical analysis of the 

feed slurry provides assurance in producing the 

target glass composition, the sampling process 

must provide a representative sample of the 

material being fed to the melter. It was known 

that foaming of the slurry and the design of the 

sampling system both affect the ability to obtain 

a representative sample. Resolving the problems 

in acquiring and analyzing representative 

samples was one of the goals of the SVS-III. 

The SVS-II1, the third Scale Vitrification 

System at the WVDP, was a pilot-scale, 

joule-heated, slurry-fed, non-radioactive 

vitrification system that was independent of the 

full-scale, radioactive vitrification system. It 

incorporated several operational features of the 

full-scale system. The SVS-III included its own 

slurry preparation, offgas treatment, and 

distributed control systems, similar to those 

installed on the full-scale vitrification system 

(section 3.1). 

The SVS-III vitrification process schematic is 

shown in figure 10.1. The process started by

107 NUREG/CR-6666
z •To SVS-02 

T-12 K-12 Atmos. 

TO SVS-01 

I Atmos. o• T-16 K-16 

ON -I' Va UMax , 

- v; 

SVS-04 

M To 

SScal GAtmos. 

DrumStc 

Outside V0F 

Inside VTF - -. Cat. 

181"-11 

Waste ANH3 

Slmulant G-01 Analyzer 

Grinder Sug l Air In-Bleed SVS-05 

TT 

Atmos. Redx Probe 

Vent SVS-06 Melter 

-06TO eat Ex. 

SVS Emission FedHldAms. yln Point F HoSeparator 

Pump G G-04 Drum Cond or Conveyor Hold G-08 

Blower K Melter C yl 

S Quick Feed G06 To Disposal • 

Connect I To Slurry Mix g

Figure 10.1. Scale Vitrification System III Vitrification Process Schematic (Jain et al., 1997)

I
Pilot-Scale Vitrification Systems for High-Level Waste Plants

mixing and preparing waste simulant using the 

slurry mix tank (SMT). The chemicals were 

added to the SMT using either the Vac-U-Max® 

system or a liquid pump. The Vac-U-Max® was 

a pneumatic device for transfer of dry materials. 

After the simulant was prepared, it was sampled 

and analyzed. While the sample analysis was in 

progress, waste simulant was routed through a 

grinder to the feed hold tank (FHT) where it was 

concentrated to maximize the percent of total 

solids (% TS) in the waste. The addition of 

sodium metasilicate to the waste simulant helps 

to maximize the % TS. The contents were 

concentrated at approximately 100 °C using 

steam coils. After concentration, the simulant 

was allowed to cool to room temperature. 

Based on the waste simulant analysis, the 

amounts of glass-forming chemicals needed to 

produce a target glass composition were 

calculated. The glass formers were added to the 

SMT using either the Vac-U-Max® system for 

dry chemicals or a liquid pump for solutions or 

slurries. The completed glass-former batch was 

sampled and analyzed. After the waste and 

glass-former analyses were combined, and the 

results indicated a mix having the composition 

within the target region, the glass formers were 

transferred to the FHT using the grinder. The 

combined slurry was sampled in the FHT and 

analyzed for elements, NO- , TOC, density, and 

% TS. Based on the TOC, NO3-, and % TS, 

sugar was added to the FHT and mixed. The 

mixture was again sampled to ensure the proper 

NO3/TOC ratio. If the ratio was within the test 

parameters, the feed was transferred to the MFT. 

Slurry from the MFT was metered into the 

melter and processed into glass. Both glass and 

vapors exited the melter as a result of the 

processing. Glass was poured into a canister 

beneath the melter, and vapors went through the 

SVS-III offgas treatment system.

10.1.1 Feed Preparation System 

All free-flowing powders were transferred to the 

SMT using the Vac-U-Max®. Powders were 

typically delivered in 0.21-m3 (55-gal.) drums. 

These drums were positioned on a calibrated 

scale that controlled the amount of feed to the 

SMT based on loss in weight. A pneumatic 

conveyor pickup wand placed in the drums 

provided the means of transport. The powder 

was picked up by the vacuum in the wand and 

discharged by a solid separator stationed 

immediately above the SMT. The collection 

hopper, transfer lines, and filter were made of 

stainless steel to ensure chemical purity and to 

make cleaning easier. The flexible portion of the 

wand was made of polyethylene. 

Liquid chemicals were handled using a similar 

drum and wand system set on calibrated 

weighing scales. Chemical-resistant pumps were 

employed to transport the liquids from the 

drums to the SMT. To prevent overflow, the 

SMT and the FHT had high-level alarms that 

were interlocked to the pumps feeding them. 

The SMT was a 1.89-m3 (500-gal.), 304-L 

jacketed stainless steel tank equipped with a 

Chemineer® agitator and internal heating and 

cooling coils. The tank was vented by a separate 

offgas system that filtered the offgas first before 

discharging it to the atmosphere via a 5-hp fan 

capable of pushing 200 SCFM of air. The SMT 

ventilation system was skid-mounted and 

included a filter and a slidegate damper to 

control airflow. The SMT fan maintained a 

slight vacuum on the SMT to aid in filling and in 

keeping dust and nuisance vapors in the 

immediate area to a minimum. 

The feed preparation cycle was designed so that 

the only chemical reaction that took place in the 

SMT was acid/base neutralization. Chilled water 

was run through the jacket to absorb the heat 

produced in the neutralization process. Chemical 

additions to the SMT were made through liquid

109 NUREG/CR-6666
Pilot-Scale Vitrification Systems for High-Level Waste Plants

transfer, dry powder transfer, or portable nitric 

acid 0.21 -m3 (55-gal.) drum transfer lines. 

Chemical transfers out of the SMT to the FHT 

were routed through a grinder to ensure a 

maximum particle diameter of just under 50 gim 

in the slurry. The FHT was a 2.8-m3 (740-gal.), 

304-L semi-jacketed, stainless steel tank 

equipped with a Chemineer' agitator and 

internal heating and cooling coils. The FHT was 

maintained under negative pressure controlled 

by the offgas system. This tank was utilized for 

boiling the waste simulant, mixing the waste 

simulant with the glass formers, and adding 

sugar to obtain the desired carbon-to-nitrate 

ratio. After verification and approval by sample 

analysis, the melter-fed slurry was transferred 

from the FHT to the MFT. 

10.1.2 Vitrification System 

The MFT was a 0.28-m3 (74-gal.), 304-L 

stainless steel tank equipped with an agitator and 

a peristaltic pump. The peristaltic pump 

provided the feed to the melter at a constant 

flow rate through a water-cooled feed nozzle. 

The melter feed pump had a variable frequency 

drive that gave it the ability to manually control 

the feed rate to the melter. The SVS-III melter is 

capable of simulating the operations of the 

VF melter. It was built with one-seventh the 

surface area [0.325 m2 (3.5 ft), 0.46 in (1.5 ft) 

by 0.70 m (2.3 ft), with a maximum pool depth 

of 0.18 in (0.6 ft)] and one-fourteenth the 

volume [62 L (16.4 gal.) maximum capacity] of 

the VF melter. This allowed the SVS-III melter 

to reach steady state much faster and have a 

quicker volume changeout. 

The SVSIII melter was a simple rectangular 

geometry tank, straddled by two paddle-shaped 

Inconel' Alloy 690 electrodes. Above the glass 

pool was a set of four plenum or lid heaters. 

These heaters were silicon carbide resistance 

heaters centrally located within Inconel® Alloy

690 sheaths. The plenum heaters were used 

during startup to raise the temperature of the 

glass surface until it became conductive enough 

for the electrodes to become productive. Once 

the glass was hot enough (- 700 °C), an electric 

current was passed through the electrodes. The 

resistance to the electric flow by the contents of 

the melter creates the heat required to melt the 

glass. Glass was discharged from the melter 

through a single riser activated by an airlift 

lance. The glass flowed down an Inconel® Alloy 

690 trough and into a 1.1-in 3 (290-gal.), stainless 

steel drum. The discharge chamber around the 

trough was heated by six silicon carbide 

elements. The molten pool of glass was 

contained by a Monofrax® K-3 (Cr/Al) 

refractory, backed by a Zirmul® (AI/Zr/Si type 

refractory), and an A120 3 refractory. A single 

discharge orifice exited the melter cavity into 

the Inconel® Alloy 690 riser. Temperature in the 

melter was monitored in four primary areas: the 

discharge chamber, main pool, plenum area, and 

lid heater sheaths. 

A 1.1-im 3 (290-gal.) drum was mounted on a 

chain-driven conveyor and moved under the 

melter discharge port. With a Fiberfrax gasket 

around the perimeter of the drum lip, the sealing 

bellows would be extended to press firmly 

around the drum, sealing it to the discharge 

chamber. The seal maintained the vacuum 

integrity of the melter. Glass was discharged 

into the drum in increments of approximately 7 

to 8 L (1.8 to 2.1 gal.) each. The level of glass in 

the drum was monitored from the drum exterior 

using surface temperature indicators and an 

infrared pyrometer for backup. When full, the 

bellows were lifted from the drum, and the drum 

was moved out. Once the drum was out of the 

way of the bellows, a plate cover could be slid 

under the bellows to curb heat loss and air in

leakage.

NUREG/CR-6666 110
Pilot-Scale Vitrification Systems for High-Level Waste Plants

10.1.3 Offgas Treatment System 

The offgas treatment system provided a 

controlled negative gauge pressure for the FHT, 

MFT, and melter. It was operated in two distinct 

modes depending on whether feed was being 

prepared or the melter was processing slurry. 

The equipment was sized so that it would not 

support simultaneous operation. The volume of 

airflow was constant for both modes. An air 

in-bleed in the melter offgas line was used to 

control the pressure in the melter and the FHT 

while the offgas treatment system was operating. 

Offgas treatment was required to mitigate the 

effects of temperature, NO. formation, and 

particulate generation. A venturi scrubber with 

cyclonic separator quenched the offgas and 

removed large particulate matter. The offgas 

was then sent through the HEME to remove 

unwanted mist and particulates greater than 

submicron size. The water from the scrubber 

was collected in the condensate hold tank, and 

was cooled and filtered before being reused in 

the scrubber. Excess accumulations of scrubber 

water were stored in the scrubber water 

collection tank. This water was either used in the 

SMT during feed preparation activities or 

disposed of off-site. A skid-mounted, offgas 

blower provided the vacuum necessary to pull 

the vapors from their origin to the NO, reactor. 

The blower was a 7.46-kW (10-hp), rotary 

positive displacement type made with stainless 

steel wetted parts. The selective catalytic 

reduction (SCR) process reduced NO, in the 

offgas to nitrogen and water. The offgas was 

first heated to 300 'C using natural gas in a 

combustion chamber. NH3 was then added to the 

offgas before it was passed through a fluidized 

bed of zeolite that had been treated with a 

catalyst. The effluent came out the top of the 

SCR and entered the atmosphere from the stack. 

The offgas was sampled for NO, concentration 

before and after the reactor in order to determine 

reactor efficiency and control NH3 usage.

10.1.4 Process Issues and Upsets 

Based on the results of this program, it was 

determined that there is a lower limit that 

separates a region of predictable oxidizing 

behavior in the glass melt from a region where 

small changes in the sugar/nitrates 

concentrations in the slurry have large effects on 

the redox state of the glass product. The SVS-III 

test series successfully demonstrated that a glass 

product with acceptable composition could be 

made consistently from a slurry recipe 

incorporating waste simulant and glass-former 

components. In addition, melter operation 

parameters such as feed rate, cold-cap coverage, 

glass melt and plenum temperatures, and melter 

power were controlled successfully to provide 

steady state operating conditions during the 

melter test runs. 

The SVS-III melter exhibited irregularities in 

glass pours similar to the WVDP production 

melter, including erratic flow and glass buildup 

at the bottom of the discharge cavity. Unlike the 

WVDP production melter, SVS-III had only one 

discharge section. In the SVS-IlI melter, the 

glass leaked from the main melt chamber to the 

discharge section during the test campaign and 

was shut down several times to clear the glass 

blockage in the discharge section. Detailed 

examination revealed that the seal welds that 

held the pour spout to the dam (i.e., the vertical 

Inconel® Alloy 690 wall that separates melting 

chamber and discharge chamber) had failed. The 

failure was attributed to the insufficient support 

provided by the seal welds to withstand stresses 

generated from the thermal expansion of the 

dam during melter heatup.

ill NUREG/CR-6666
Pilot-Scale Vitrification Systems for High-Level Waste Plants

10.2 Integrated Defense Waste 

Processing Facility Melter 

System at the Savannah River 

Site 

The IDMS was a pilot-scale, non-radioactive VF 

designed to support the startup and operation of 

the HLW radioactive processing at the DWPF. 

The IDMS consisted of a one-fifth-scale feed 

preparation system and a one-ninth-scale melter 

with its associated feeding and offgas treatment 

systems (Ritter et al., 1991). The IDMS 

operations were started in June 1990. 

Part of the IDMS operating plan was to develop 

a process control strategy for the DWPF 

(Jantzen et al., 1993), study the behavior of 

noble metals in the melter (Hutson, 1994), and 

perform materials evaluation under processing 

conditions (Imrich and Jenkins, 1996). The 

IDMS consisted of two major systems: the feed 

preparation system, and the melter and offgas 

treatment system. 

10.2.1 Feed Preparation System 

Figure 10.2 shows a schematic of the IDMS 

feed preparation system. The system consisted 

of a SRAT, a process water condenser, a 

mercury decanter, and a chilled water 

condenser. The SRAT was equipped with an 

agitator, heating and cooling coils, and an air 

line that entered through a side port for purging 

the vapor space. Unlike DWPF, which has two 

tanks for processing feeds, IDMS used only one 

tank (SRAT) to process feeds. The process 

water condenser operates between 25 and 50 °C. 

The condensate passed through the mercury 

decanter and then was sent back either to the 

SRAT during total reflux or to a condensate tank

during concentration. The mercury decanter was 

designed to collect elemental mercury in a sump 

for subsequent removal from the process. The 

non-condensible offgases, after leaving the 

process water condenser, passed through a 

chilled water condenser, which was designed to 

reduce the dew point of the offgas to 10 °C, to 

minimize the release of mercury to the 

atmosphere. Prior to entering the chilled water 

condenser, vent streams from several other air

purged tanks also entered the offgas system. 

10.2.2 Vitrification System 

Figure 10.3 shows a schematic of the IDMS 

melter and offgas system. The melter system 

consisted of the melter, MFT, and a melter 

offgas system. The IDMS melter, as shown in 

figure 10.4, was a refractory-lined, cylindrical 

tank with an inner melt pool diameter of 0.61 m 

(2 ft) and melt pool depth of 0.69 m (2.3 ft). The 

melt pool surface area was one-ninth that of the 

DWPF melter. An upper and lower pair of 

diametrically opposed Inconel® Alloy 690 

electrodes provide a total of about 35 kW of 

power to maintain a glass pool temperature of 

1,150 °C. Average glass production rate was 

approximately 11.3 kg/hr (24.9 lb/hr). 

Melter feed was slowly fed through a narrow 

tube and sprayed on top of the cold-cap. Due to 

the high temperature in the melter plenum 

(800 'C), most of the water was immediately 

evaporated. The more volatile organics were 

converted to CO, CO2, and H20. Most of the 

nitrates and formic acid reacted in the cold-cap 

and defined the redox condition in the melt. 

Eventually all reaction gases exited through the 

offgas tube.

NUREG/CR-.6666 112
Pilot-Scale Vitrification Systems for High-Level Waste Plants

To Vent

Chilled 

I 

* Water 

Condenser 

Air Purges 

From Tanks 

A;r

A 

a 

I

Condensates to ultrafitters 

and Ion Exchange System

-..... Vapor Flows 

- Liquid Flows

Figure 10.2. Integrated Defense Waste Processing Facility Melter System and Feed Preparation 

Flowsheet (Ritter et al., 1991) (Permission to use this copyrighted material is granted by American 

Ceramic Society, Westerville, Ohio)

QUENCHER i- 

~. 10 .... I "~1~M~i I ~ I STEAM 

IIATOMIZED 

Q, , 1 0 c I

MELTER 

FEED 

TANK a 

CONDENSATE TO ULTRAFILTERS 

AND ION EXCHANGE SYSTEM

4LTE b6T)M j I I 

---------------------------- ~ 

r -m 

'ILJV 1-10-- TO VENT 

aHHEATER S 

HEPAS

- FEED SLURRY -- OFFGAS CONDENSATE 

Figure 10.3. Integrated Defense Waste Processing Facility Melter System and Offgas Flowsheet 

(Ritter et al., 1991) (Permission to use this copyrighted material is granted by American Ceramic 

Society, Westerville, Ohio)

-- I

I

113 NUREGICR-6666
Pilot-Scale Vitrification Systems for High-Level Waste Plants

Figure 10.4 Integrated Defense Waste Processing Facility Melter System (Hutson, 1994) (Permission 

to use this copyrighted material is granted by American Ceramic Society, Westerville, Ohio)

10.2.3 Offgas Treatment System 

The purpose of the offgas system was to 

maintain a slightly negative pressure in the 

melter plenum, provide an adequate airflow for 

combustion in the plenum (which is maintained 

at about 800 0C), treat offgases to remove 

condensibles and particulates, and provide 

vacuum required to initiate and control the 

pouring of glass. The offgas system had an 

offgas film cooler to reduce deposits at the 

entrance to the offgas system, a quencher 

(venturi scrubber) to cool offgas and remove 

large particulates, an OGCT to disengage the 

liquids and gases, a two-stage SAS to remove 

submicron particulates, a condenser to reduce 

the dew point to 10 'C, a HEME to remove 

mists and particulates, HEPA filters (including 

preheaters) to remove trace particulates, and a 

blower.

10.2.4 Process Issues and Upsets 

In order to demonstrate the DWPF process 

control and product verification strategy, the 

IDMS was operated in three discrete batches 

using simulated waste streams (Jantzen et al., 

1993). In these three test runs, all methodologies 

that were developed for use during radioactive 

operations for process control and product 

verification were followed. Results indicated 

that, by applying proper process control 

measures, DWPF can produce an acceptable 

glass product. 

Waste and waste simulants processed in the 

IDMS, as of March 31, 1993, included three 

types of SRS waste simulant [blend, HM (high 

aluminum waste at SRS), and PUREX], one 

type of Hanford waste simulant, and neutralized 

current acid waste containing noble metals 

(Hutson, 1994). A total 15.68 kg (34.5 lb) of Ru, 

5.73 kg (12.6 lb) of Pd, and 3.12 kg (6.9 lbs) of 

Rh were introduced into the melter. The IDMS 

floor samples were routinely analyzed and 

monitored along with electrical characteristics.

NUREG/CR-6666 114
Pilot-Scale Vitrification Systems for High-Level Waste Plants

The analyses of the IDMS floor samples 

indicated the presence of large clusters of 

ruthenium oxide needles, Rh oxides, spinels, 

metallic Pd, and Pd tellurides. The electrical 

characteristics of the IDMS melter also showed 

a response to this deposition of conductive 

material. After about 550 hr of operations, the 

ratio of upper melt pool resistivity to that of 

lower melt pool resistivity quickly increased. 

This increase happened at about the same time 

that the melter floor samples revealed a 

significant accumulation of conductive material. 

This ratio could be used as an effective measure 

of conductive material on the melter floor. 

In November 1994, test coupons were placed in 

the IDMS plenum and exposed to offgas 

components (Imrich and Jenkins, 1996). Fifteen

different nickel and cobalt base alloys, including 

N06690 (this is the Inconel Alloy 690 without 

the trade name), were exposed for 5 mo to 

corrosive vapors and molten glass. During the 

5 mo of testing, one melter feed/pour campaign 

was performed. Process conditions included 

temperatures exceeding 900 'C and processing 

of wastes containing formates, halides, sulfates, 

phosphates, nitrates, and mercury. Sulfates were 

less than 1,000 ppm and chlorides were less than 

3,000 ppm. The results of the study indicated 

that the alloys with substantial cobalt content did 

not perform as well as nickel-based alloys. 

Results also indicated that alloys containing Cr 

concentrations greater than 25 percent with 

additions of Si or Al performed better than the 

N06690 reference material.

115 NUREGICR-6666
11 EMERGING SOLIDIFICATION TECHNOLOGIES

Technologies described in this section have been 

proven conceptually and are either at the end of 

the experimental stage or ready for pilot-scale 

testing. Among the technologies discussed 

below, cold-crucible melting technology is at the 

most advanced level for waste solidification. 

11.1 Cold-Crucible Melting 

Technology 

The cold-crucible melting technology, also 

known as high-frequency induction melting 

technology, is an adaptation of the induction 

melting technology used in France for HLW 

vitrification. The cold-crucible skull melting 

technique was invented before World War II but 

never progressed beyond laboratory-scale. In the 

cold-crucible concept, a solidified shell of the 

same material is used as a refractory interface 

with the melt. Formation of the solidified shell is 

accomplished by providing water-cooling 

jackets around the crucible (Jouan et al., 

1996a,b; 1997). Figure 11.1 shows a schematic 

of the cold-crucible melter. 

The design uses sectored layers of water-cooled 

jackets and induction coils around the crucible. 

The joule heating is provided by the induction 

coils while the water-cooling jacket forms a 

skull layer, 1 to 10-mm thick, inside the crucible 

walls. Figure 11.2 shows a typical temperature 

distribution map in the cold-crucible. The 

formation of a skull layer assures low corrosion 

and a longer life. The crucible could be used for 

melting wastes with a variety of corrosive 

components such as P20 5 and SO 3. The skull 

layer also allows the melter to operate at 

temperatures exceeding 2,500 'C and could be 

used to melt high refractory wastes. 

The size of the crucible is limited by induction 

coils to uniformly transmit induction frequencies 

to the center of the crucible. In large-diameter 

crucibles, attenuation of induction frequencies

results in steep temperature gradients, as the 

current density diminishes exponentially from 

the outer wall to the core. In addition, if wastes 

have high resistivity, additional external heating 

sources may be required for startup. 

There are three experimental cold-crucible 

melters currently operational at Marcoule in 

France (Jouan et al., 1998). The oldest is 10 yr 

old. The crucible is 0.55 m (1.8 ft) in diameter 

and 0.70 m (2.3 ft) in height The melter 

operates at 350 kHz and is powered by a 

350 kW (1.19 million BTU/hr) power supply. 

Even after 5,000 hr of operations with a wide 

range of compositions, the crucible has not 

degraded. This facility has been used to 

demonstrate the feasibility of vitrifying HLW 

solutions from light-water reactor fuel, using 

simulated "R7T7" glass. In addition, this facility 

was also used for vitrification tests with 

solutions simulating Hanford tank wastes. There 

are two other units, 0.30-m (1-ft) and 0.55-m 

(1.8-ft) diameter, that are being operated for 

non-nuclear testing. In addition, a 0.60-m (2-ft) 

diameter crucible is used for making special 

glasses at a production rate of over 50 kg/hr 

(110 lb/hr) by a commercial glass company in 

France. 

A pilot facility for testing different waste 

streams is under construction at the 

Commissariat al'Energie Atomique Marcoule 

center (site for HLW vitrification in France). 

The pilot plant will have a 0.30-m (1-ft) 

diameter cold-crucible with an offgas treatment 

system. 

The high-frequency induction melting 

(cold-crucible) technology is also being actively 

developed in Russia at the Mayak plant in 

Chelyabinsk to vitrify HLW solutions. The 

technology is being jointly developed by the 

Moscow Institute for Research on Inorganic 

Materials and the RADON company at Zagorsk 

(Jouan et al., 1996a).

117 NUREG/CR-6666
Emerging Solidification Technologies

Scitorized crucible

CaUting nozzle Drain valve

Figure 11.1. Cold-Crucible, High-Frequency Induction Melter Schematic (Ladirat et al., 1995) (Permission to use this copyrighted material is granted by American Ceramic Society, Westerville, 

Ohio)

Inductor 

Degree °C 

I 1300 

-•120010 

F l 100

1000 

900 

800 

700 

-600 

-500 

-200

Figure 11.2. Typical Temperature Distribution in a Cold-Crucible Induction Melting (Jouan et al., 1996a) (Copyright 1996 by the American Nuclear Society, LaGrange Park, Illinois)

NUREG/CR-6666

I J

118
Emerging Solidification Technologies

Even though the technology has not been tested 

with radioactive wastes, several projects are 

under way to develop the process for radioactive 

operations world-wide (Jouan et al., 1998). In 

addition to the development of a cold-crucible 

melter (CCM) for the Hanford wastes under the 

TWRS program, two CCMs are being designed, 

one for shipment to Saluggia, Italy, to vitrify 

HLWs at the Saluggia site, and one to Taejon, 

Korea, to incinerate and vitrify LLW. 

11.2 Hybrid, Plasma-Induction, 

Cold-Crucible Melter 

This technology was developed in Russia in the 

mid-1970&s and was used for producing 

chemically active high purity metals such as 

zirconium, titanium, uranium, and rare-earth 

metal alloys. PICCM combines the advantages 

of plasma pyrolysis with temperature control 

aspects of induction heating. The cold-crucible 

eliminates concerns about refractory corrosion 

in metal/glass melts and eventual disposal of

worn refractories. In addition, it provides a clean 

separation of glass from the metal phase. 

Because it is Russian proprietary technology, 

only a brief description of the process is 

available for discussion (Schumacher and 

Bickford, 1995). 

A melter schematic is shown in figure 11.3. 

Induction heating in addition to plasma torch 

heating creates intense stirring within the metal 

portion of the melt. The metal melt is 

water-cooled to allow formation of a skull layer. 

Separate taps allow the casting of metal from the 

bottom and the glassy phase from the side of the 

melter. This allows a clean separation between 

metal and glassy phase. A plasma torch is 

inserted from the top to supply enough energy to 

pyrolyze organics and melt glassy phase above 

the metal melt. The control of induction heating 

provides a balance between the formation of 

cold-cap above the metal phase and pyrolysis of 

organics, thus reducing volatilization of the 

radionuclides and other species.

Tonrh!". j I

Induction 

Heating 

Coils

Figure 11.3. Hybrid Plasma/Induction Cold-Crucible Melting System (Schumacher and Bickford, 

1995)

119 NUREG/CR-6666
Emerging Solidification Technologies

Under a technology transfer program, a PICCM 

pilot test facility currently is being installed at 

the Georgia Institute of Technology by the 

Westinghouse Savannah River Center. 

11.3 Submerged Combustion 

Melter 

The submerged combustion melter is a patented 

natural-gas fired technology (Olabin et al., 

1996) and is proposed to be used for producing 

mineral wood and cement, roasting ores, melting 

glass and scrap, and vitrifying waste and ashes. 

In submerged combustion melting, natural gas 

mixed with enriched air or oxygen is fired 

directly into and under the surface of the bath of 

RECU••RATMMR 

SEPARMATM 

ZONE 

MELT 

REMOVAL

the material being melted. Combustion products 

bubbling through the bath provide very effective 

heat transfer and mixing. Enriched air or oxygen 

help to combust organics in the feed stock. 

A process schematic of the submerged 

combustion melter is shown in figure 11.4. 

Submerged combustion melters of 2,800 and 

3,300 kg/hr (6,160 and 7,260 lb/hr) capacity 

using 315 °C preheated air are in commercial 

operation in Ukraine and Belarus for mineral 

wool production. An advanced pilot-scale 

submerged combustion melter test facility was 

built at the Institute of Gas Technology, Des 

Plaines, Illinois, and is being used to 

demonstrate the production of a number of 

mineral wool melts. However, to date, no studies 

have been conducted to explore waste 

vitrification. 

STACK 

FEER 

MELT BATH

ý= BURNERS

Figure 11.4. Submerged Combustion Melter Schematic (Olabin et al., 1996) (Permission to use this copyrighted material is granted by American Ceramic Society, Westerville, Ohio)

NUREG/CR-6666 120
12 NON-VITRIFICATION SOLIDIFICATION PROCESS 

EXPERIENCE

In 1979, the National High-Level Waste 

Technology Program initiated research and 

development on waste forms at fourteen 

laboratories, three universities, three industrial 

laboratories, and several DOE sites 

(U.S. Department of Energy, 1982; Hench, 

1995; Hench and Clark, 1984) for solidifying 

HLW in the United States. Between 1979 and 

1981, seventeen different waste forms were 

evaluated and ranked. The peer review panel 

documented that borosilicate glass was the best 

developed and most viable waste form for both 

defense and commercial HLW. Synethic rock 

(Synroc) was selected as an alternative waste 

form. Other waste forms that reached the 

semifinal stage included tailored ceramic, 

high-silica glass (formed under elevated 

temperature and pressure), concrete-coated 

sol-gel particles, and glass marbles in a lead 

matrix. In the United States, most of the work on 

non-vitrified waste forms like glass ceramics, 

Synroc, and vitreous ceramics is being 

conducted at the INEEL and PNNL. At this 

time, research is focused on developing the 

waste form and not on designing the production 

facility. 

The only production facility for the non-vitrified 

waste form for solidifying HLW into Synroc is 

located in Australia. This non-radioactive, 

commercial-scale facility has a production 

capacity of 10 kg/hr (Jostsons, 1994). To date, a 

production plant for radioactive operations for 

non-vitrified waste forms has not been 

developed anywhere in the world. Reviews of 

cement/concrete solidification and polymer 

encapsulation are not included here because 

these waste forms are not suitable for Hanford 

waste. The Q-CEP by Molten Metal Technology 

Incorporated (MMT) is a new solidification 

process. Due to the proprietary nature of the 

technology, there is no information available in 

the literature on its performance. The following

sections describe Synroc and Q-CEP 

solidification technologies. 

12.1 Synroc Demonstration Plant, 

Australia 

Synroc was first proposed as a waste form for 

HLW in 1978. After more than a decade of 

research and development on Synroc materials 

in Australia, Synroc production, using 

non-radioactive simulants, was demonstrated on 

a commercial scale in a 1 0-kg/hr (22-lb/hr) 

production unit at the Synroc Demonstration 

Plant (SDP) in Australia (Jostsons, 1994). 

Construction of the plant was completed in 1987 

and it has been operating in short programs to 

optimize processing and provide design input 

for the radioactive plant. As of 1996 (National 

Research Council, 1996), SDP has produced 

6,000 kg (13,200 ib) of Synroc. 

Synroc can be produced either by a conventional 

ceramic sintering process or by a hot-pressing 

process. The hot-pressing method was chosen 

for processing waste into Synroc. A Synroc 

process flowsheet is shown in figure 12.1. In a 

typical process, HLW solutions are mixed with 

Synroc precursor powders as shown in 

table 12.1. The mix is calcined at 700 *C for 

1-2 hr in a rotary calciner (similar to one used in 

the AVM process in France). The calcination is 

done under a 3.5 percent H2/N2 reducing 

atmosphere. The calcined powder is blended 

with 2 percent Ti metal powder to control redox 

potential and loaded into stainless steel bellows 

containers. The powder mixing and filling 

process is shown in figure 12.2. The steel 

containers are cold-pressed, followed by 

hot-pressing at 14-21 MPa (2,000-3,050 psi) for 

2 hr between 1,150-1,200 'C. The bellows 

(with Synroc) are then cooled and loaded in 

standard disposal canisters. The space between

121 NUREG/CR-6666
Non-Vitrification Solidification Process Experience

Water

Simulated 

High Level Wastes 

in 2M HN03 To Stack

Filter

T10 2 

Z02 

A120 3 

CaO 

BaO

Storage

(2%)

Hot 

Pressing 

14 MPa 

1150-1200°C

Figure 12.1. Synroc Process Flowsheet (Ringwood et al., 1988) (Reprinted from Radioactive Waste 

Forms for the Future, W. Lutze and R.C. Ewing, eds. Copyright 1988, figure 39, p. 310, with 

permission from Elsevier Science Publishing Company, Oxford, United Kingdom) 

Table 12.1. Composition of Synroc-C (Jostsons, 1994) (Permission to use this copyrighted material 

is granted by Institution of Engineers, Sidney, Australia) 

Composition wt % 

Tn0 2 57.1 

ZrO2 5.3 

A120 3 4.3 

BaO 4.5 

CaO 8.8 

HLW 20

NUREG/CR-6666 122
Non-Vitrification Solidification Process Experience

Powder from 

Calciner 

Storage 

Bins 

Bellows Feed 

Hopper 

(65 kg)

-Ti Powder

•J) j Vibratory 

Feeder 

Conical Screw 

Blender 

(65 kg) 

"Rotary 

Valve

(•.._i- Rotary Valve

Powder 

Filling 

Booth

Figure 12.2. Powder Mixing and Filling Process During Synroc Manufacture (Ringwood et al., 

1988) (Reprinted from Radioactive Waste Forms for the Future, W. Lutze and R.C. Ewing, eds. 

Copyright 1988, figure 41, p. 316, with permission from Elsevier Science Publishing Company, 

Oxford, United Kingdom)

123 NUREG/CR-6666
Non-Vitrification Solidification Process Experience 

the bellows is filled with a good heat transfer 

medium (e.g., lead). Figure 12.3 shows the size 

of the bellows before and after hot-pressing. 

Major process improvements include using 

larger bellows. Bellows of 30-40 cm (1-1.3 ft) 

diameter have been successfully processed. In 

addition, development of a more reactive 

precursor has resulted in the elimination of the 

cold-pressing step prior to hot-pressing. 

The results have indicated that Cs and Ru 

volatility below 0.15 percent of the inventory. 

Also the quality of the product was not 

influenced by large variations in HLW and 

precursor concentrations. However, at this time, 

there is not much information available on the 

processing and safety issues related to Synroc. A 

recent study (Vance et al., 1996), using

simulated Hanford HLW liquid waste, has 

shown that Synroc will meet durability 

requirements for the Hanford HLW. In addition, 

Tc volatilization losses were less than with other 

solidification technologies. 

Synroc is the most durable waste form, but the 

technology has not reached a maturity and 

confidence level to be adopted for radioactive 

operations. 

12.2 Quantum-Catalytic Extraction 

Process 

The Catalytic Extraction Process (CEP) is a 

new, innovative, flexible, proprietary technology 

of MMT in Waltham, Massachusetts 

(Herbst et al., 1994; Valenti, 1996). This 

technology is used for processing hazardous

Figure 12.3. 400-mini-diamneter Synroc Bellows Before and After Hot Pressing (Ringwood et al., 1988) (Reprinted from Radioactive Waste Forms for the Future, W. Lutze and R.C. Ewing, eds. Copyright 1988, figure 44, p. 319, with permission from Elsevier Science Publishing Company, Oxford, United 

Kingdom)

NUREG/CR-6666 124
waste streams, allowing the organic, 

organometallic, and inorganic waste streams to 

be recycled into useful resources. The Q-CEP is 

an adaption of the CEP technology for handling 

radioactive and mixed waste streams. The 

Q-CEP, in addition to destroying hazardous 

components, allows partitioning of 

radionuclides. The partitioning allows recycling 

of a large number of waste components into 

commercial products and solidification of 

radionuclides, either in glass/ceramics or metals. 

In both CEP and Q-CEP, the molten bath 

(typically iron or nickel) acts as a homogeneous 

catalyst and as a solvent in dissociation of feeds, 

synthesis of products, and concentration of 

radionuclides in a desired phase. In the Q-CEP 

process, various radioactive mixed wastes are 

piped into a sealed bath of molten metal called a 

Catalytic Processing Unit. The bath is a sealed 

vessel lined with a ceramic refractory. Induction 

coils at the bottom of the bath are used to supply 

heat and keep the bath in the molten state. 

Proprietary chemicals are fed through tuy~res at 

the bottom of the reactor with the waste streams 

to reconfigure elements from the bath into 

useable raw materials. The reactor can handle 

feeds of most physical forms such as gases, 

liquids, and slurries. The catalytic properties of 

the high-temperature metal breaks down the 

chemical compounds to their primary elements.

Non-Vitrification Solidification Process Experience 

The gases (i.e., H2, CO2, etc. ) are sent to a 

hermetically sealed gas handling system. The 

elements (cations) are extracted as ceramics or 

alloys. The radionuclides from the wastes are 

encapsulated into ceramic or metal phases in the 

bath. The ceramic end product floats on the top 

of the molten metal and is skimmed off by 

tapping the bath. Metal byproducts remain in the 

bath as ferroalloy and are removed as needed. 

According to the authors (Herbst et al., 1994), 

Q-CEP offers superior capabilities to those of 

melt refining. Q-CEP allows oxygen to be 

injected into the molten metal bath, greatly 

enhancing oxidation of the radioactive 

components. In addition, turbulence caused by 

gas injection into the metal bath facilitates mass 

and heat transfer, leading to active partitioning 

of radionuclides. 

The Q-CEP plant at Scientific Ecology Group in 

Oak Ridge, Tennessee, is providing treatment 

for low-level radioactive ion-exchange resins 

from nuclear power plants. Other Q-CEP plants 

at the M4 Environmental facility at Oak Ridge 

are providing treatment for converting uranium 

hexafluoride to stable uranium compounds and 

commercial products, and for treatment of 

mixed wastes at DOE sites. In addition, several 

other plants are being constructed.

125 NUREG/CR-6666
13 TECHNOLOGICAL ASSESSMENTS

Solidification technologies presented in this 

survey can be broadly classified into the 

following categories: Inconel*-based, 

joule-heated melters; high-temperature, 

joule-heated melters; induction melting, 

cold-crucible induction melters; plasma 

melters; combustion melters; microwave 

melters; molten metal technology; and Synroc 

technology. The following sections summarize 

strengths and weaknesses of each technology. 

13.1 Joule-Heated Inconel® Melters 

Strengths 

(i) Mature Technology: Two HLW plants 

are currently operating in the United 

States and one in Japan. The Pamela 

plant in Mol, Belgium, successfully 

completed operations. Numerous 

pilot-scale nonradioactive plants are in 

operation or have completed successful 

operations in the last 20 yr. 

(ii) Low Volatility: This technology provides 

extremely low volatility for radioactive 

components such as Tc and Cs due to its 

operation in a cold-top mode. 

(iii) Homogeneity: Due to the long residence 

time in the melter, the glass product is 

homogeneous. 

(iv) Waste/Glass/Melter Component 

Interactions: The interactions between the 

waste, glass, and melter components have 

been well established for certain types of 

wastes. 

Weaknesses 

(i) Low Glass Production Rate: Glass 

production rates are limited by an upper 

operating temperature of 1,150 *C. The

production rate for the DWPF melter is 

100 kg/hr. Production rates depend upon 

the size of the melter but, for the same 

size melter, the production rates are 

significantly higher compared to the 

induction-melters but lower compared to 

the high-temperature, joule-heated 

melters and plasma melting systems. 

Production rate enhancements can be 

achieved by bubblers and stirrers that 

may cause increased erosion of melter 

components. 

(ii) Failure due to Noble Metal 

Accumulation: Several melters have 

experienced loss of electrical resistance in 

the melter due to the accumulation of 

noble metals. The melter at the Pamela 

plant in Mol, Belgium, failed due to 

noble metal accumulation on the melter 

floor. New sloped-wall designs, like 

those at the WVDP and at the TVP in 

Japan, and use of bottom drains, like 

those in the Duratek and Envitco melters, 

allow the noble metals to drain 

periodically, minimizing the probability 

of melter failure. 

13.2 High-Temperature, Joule-Heated 

Melters 

Strengths 

(i) High Glass Production Rate: Glass 

production rates are higher than those for 

most solidification systems but lower than 

those for plasma systems. Production rate 

enhancements can be achieved by 

bubblers and stirrers but may result in 

increased erosion of melter components.

127 NUREG/CR-6666
Technological Assessments

Weaknesses

(i) Developed Technology but Not Mature: 

The transportable vitrification system and 

Fernald high-temperature melter have 

failed during operations. However, pilot 

scale plants at Clemson University 

(Envitco melter), CUA-VSL (Duratek 

melter), and PNNL are still operating. 

(ii) High Volatility: Due to high operating 

temperatures, the volatility is higher 

compared to joule-heated Inconel° 

melters. No data were available on 

volatilization of radioactive components 

such as Tc and Cs. 

(iii) Homogeneity: Because of high melting 

temperatures and high production rates, 

the melt has a short residence time in the 

melter and, therefore, the glass product 

may or may not be homogeneous. 

(iv) Waste/Glass/Melter Components 

Interactions: a high-temperature melter 

system requires the use of molybdenum 

electrodes. These electrodes corrode in 

the presence of elements such as lead or 

nickel, which may cause premature 

melter failures. The melter at the Fernald 

site failed due to incompatible materials 

used in the melters. More research is 

needed to ensure waste material 

compatibility with the components of the 

high-temperature melter system. 

(v) Additional weaknesses include those 

similar to the joule-heated Inconel° 

melters.

13.3 Induction Melting Technology 

Strengths 

(i) Mature Technology: Two HLW plants, 

T7 and R7, are operating in France and 

one in Sellafield, United Kingdom. The 

HLW Plant at Marcoule, France, 

successfully completed radioactive 

operations. The technology has been in 

existence for 20 yr. 

(ii) Low Volatility: This technology provides 

extremely low volatility from the melter 

for radioactive components such as Tc 

and Cs, due to the use of calcined feed. 

(iii) Homogeneity: The glass product is 

homogeneous, due to the long residence 

time and limited melting volume in the 

melter. 

(iv) Waste/Glass/Melter Components 

Interactions: The interactions that may 

cause corrosion between the waste, glass, and the Inconel crucible have been well 

established for certain types of wastes. 

The melter uses the Inconel" crucible as a 

melting pot. Due to corrosion, however, 

the pots should be replaced after 2,000 to 

3,000 hr of operations. 

(v) Noble Metal Accumulation: The entire 

contents of the crucible are regularly 

drained, avoiding metal accumulation. In 

addition, a bottom drain helps to flush 

noble metals on a frequent basis. The 

alloying of the noble metals with the 

Inconel' crucible is a potential problem 

and crucible replacements are necessary 

every 2,000 to 3,000 hr.

NUREG/CR--6666 128
Technological Assessments

Weaknesses 

(i) Low Glass Production Rate: Glass 

production rates are limited by the size of 

the melt crucible and the upper operating 

temperature of 1,150 °C. The size of the 

melter cannot be increased because 

attenuation of induction frequencies 

results in steep temperature gradients as 

the current density diminishes 

exponentially from the outer wall to the 

core in large-diameter crucibles. The 

production rate for the induction melter 

are lower, and production rate 

enhancements are not possible with 

current designs. 

(ii) Feeding Mode: Due to the design of the 

system, only calcined feeds can be 

processed in the melter. 

(iii) Crucible Life: Short crucible life and 

frequent replacement is an issue. 

13.4 Cold-Crucible Induction Melting 

Technology 

Strengths 

(i) Waste/Glass/Melter Components 

Interactions: The interactions and 

consequent corrosion between the waste, 

glass, and the Inconel' crucible are 

significantly minimized by the formation 

of a skull layer at the glass-crucible 

interface by using sectored water-cooled 

jackets around the crucible. 

(ii) Crucible Life: Due to the formation of 

the skull layer, crucible life is much 

longer than in induction melting systems. 

(iii) Noble Metal Accumulation: The entire 

contents of the crucible are regularly

drained, decreasing metal accumulation. 

In addition, a bottom drain helps to flush 

noble metals on a frequent basis. The 

alloying of noble metals with the Inconel* 

crucible is reduced due to the formation 

of the skull layer. 

(iv) Glass Production Rate: Melting 

temperatures are not restricted by the 

operating temperatures for Inconel* 

crucibles. The formation of a skull layer 

allows for much higher operating 

temperatures. Glass production rates are 

higher compared to induction melting 

systems due to the higher operating 

temperatures in the melting chamber. The 

production rate could still be limited by 

the size of the melt crucible. The size of 

the melter cannot be increased due to the 

inability of the induction heating to reach 

a uniform temperature in large-diameter 

crucibles. 

(v) Homogeneity: Due to a long residence 

time and limited melting volume in the 

melter, the glass product is 

homogeneous. 

Weaknesses 

(i) Developing Technology: This technology 

is an adaptation of the more mature 

induction-melting technology. Several 

pilot-scale tests are being conducted in 

France and Russia. There are no full

scale, cold-crucible induction melting 

systems in operation at this time. 

(ii) Feeding Mode: Due to the design of the 

system, only calcined feeds can be 

processed in the melter. 

(iii) Volatility: The volatility could be higher 

than with the induction melting systems 

due to higher operating temperatures.

129 NUREG/CR-6666
Technological Assessments

13.5 Plasma Melting Technology 

Strengths

(i) Glass Production Rate: Glass production 

rates are the highest among all 

solidification technologies. 

(ii) Due to high operating temperatures of the 

plasma systems, this technology is well 

suited for hard-to-vitrify heterogeneous 

wastes. 

Weaknesses 

(i) Developing Technology: Even though 

this technology has been used in the 

commercial steel industry for a long time, 

its adoption to waste solidification is quite 

recent. Several pilot-scale plants are 

operating, and some testing has been 

performed using mixed low-level 

radioactive wastes. Full-scale radioactive 

testing has not been performed. 

(ii) High Volatility: Due to high operating 

temperatures, the volatility is extremely 

high; it is the highest among all 

solidification technologies. Due to high 

volatilization, the final composition of the 

glass may not meet the target 

composition. Volatilization may be 

reduced if the system could be operated 

in a cold-top mode. 

(iii) Homogeneity: Because of high melting 

temperatures and high production rates, 

the melt has a short residence time in the 

melter. As a consequence, glass 

homogeneity is a concern. 

(iv) Waste/Glass/Melter Components 

Interactions: Plasma systems, due to their 

extremely high operating temperatures, 

cause extensive erosion on refractories

and melter components. The formation of 

a skull layer at the glass-refractory 

interface, by using water-cooled jackets 

around the melter shell, can minimize 

refractory corrosion. 

(v) Short Plasma Torch Life: Short plasma 

torch life is a major concern in the 

plasma systems for radioactive 

operations. Depending upon the 

conditions, the life of the torch could 

range from a few hours to a few days. 

(vi) Failure due to Noble Metal 

Accumulation: This issue has not been 

evaluated for the plasma systems. 

13.6 Combustion Melting Technology 

Strengths 

(i) Glass Production Rate: Glass production 

rates are high. 

Weaknesses 

(i) Developing Technology: Even though the 

technology has been used in the 

commercial glass, coal, and steel 

industries for a long time, its adoption to 

waste solidification is quite recent. 

Several pilot-scale plants are operating 

but radioactive testing has not been 

performed. 

(ii) High Volatility: Due to high operating 

temperatures, the volatility is extremely 

high. Volatility is comparable to plasma 

melting technologies. Volatilization may 

be reduced if the system could be 

operated in a cold-top mode. 

(iii) Homogeneity: Because of high melting 

temperatures and high production rates, 

the melt has a short residence time in the

NUREG/CR-6666 130
Technological Assessments

melter. As a consequence, glass 

homogeneity is a concern. 

(iv) Waste/Glass/Melter Components 

Interactions: Combustion systems, due to 

their extremely high operating 

temperatures, cause extensive wear on 

refractories and melter components. The 

formation of a skull layer at the 

glass-refractory interface, by the use of 

water-cooled jackets around the melter 

shell, can minimize refractory corrosion. 

In addition, air/fuel interactions with 

wastes are unknown. 

(v) Failure due to Noble Metal 

Accumulation: This issue has not been 

evaluated for the combustion melter 

systems. 

13.7 Microwave Melting Technology 

Strengths 

(i) One-Step Process: The waste can be 

melted and disposed of in the same 

container. 

(ii) Low Volatility: Due to low melting 

temperatures, volatility is low. 

Weaknesses 

(i) Experimental Technology: There are no 

full-scale radioactive operating 

microwave solidification systems. Some 

studies have been conducted using pilot

scale melter systems. 

(ii) Low Glass Production Rate: Glass 

production rates are limited by the size of 

drums that could be placed in a 

microwave melter. The size of the drum 

is limited by the inability of the 

microwaves to uniformly heat large

quantities. This is not a continuous 

process. 

(iii) Homogeneity: Due to non-uniform 

melting, the glass product is not 

homogeneous. 

(iv) Waste/Glass/Melter Components 

Interactions: The interactions and 

corrosion between the waste, glass, and 

container have not been established. In 

some instances, formation of hot spots 

has led to glass-flow from the containers. 

(v) Failure due to Noble Metal 

Accumulation: This issue has not been 

evaluated for the microwave systems. 

13.8 Molten Metal Technology 

Q-CEP technology is proprietary in nature, and 

not enough information is available to 

accurately evaluate its strengths or weaknesses. 

13.9 Synroc Technology 

Strengths 

(i) Excellent waste form for disposal. 

(ii) Homogeneity: Hot-pressing makes the 

product homogeneous. 

(iii) Low Volatility: Volatility during 

processing is low. 

(iv) Waste/Glass/Melter Components 

Interactions: Since no melting is 

required, corrosion issues are minimal. 

The waste is hot-pressed in stainless steel 

bellows, which become a part of the 

waste form itself.

131 NUREG/CR-6666
Technological Assessments

(v) Noble Metal Accumulation: This is not 

an issue. All components are retained in 

the Synroc. Each time, the process starts 

with a fresh batch. 

Weaknesses 

(i) Developing Technology: There are no 

operating Synroc radioactive materials

solidification plants. A pilot plant has 

been operating in Australia for 

several years. 

(ii) Low Production Rate: Synroc production 

rates are limited by the size of 

hot-presses. This is not a continuous 

process.

NUREG/.CR-6666 132
14 PROCESS SYSTEM DESIGN CONSIDERATIONS

The TWRS waste solidification plant should be 

designed to solidify wastes stored at the Hanford 

site without posing undue radiological safety 

and health risks to workers, the public, and the 

environment from solidification operations. 

Because of limited understanding of the 

composition, physical characteristics, and 

interactions among wastes in different tanks of 

the TWRS waste at the Hanford site, the task is 

challenging. However, incorporating lessons 

learned from the design and operations of 

commercial glass industries and existing waste 

vitrification plants should significantly reduce 

radiological safety and health risks. The review 

of key process system design considerations 

related to radiological safety and health risks 

from operations can be broadly classified into 

four categories: (i) tank farms and feed 

preparation, (ii) waste solidification and storage, 

(iii) offgas treatment and release, and 

(iv) remote operations and human factors. The 

significance and ramifications of process upsets 

and incidents and their relationship to process 

and system design are discussed in the following 

sections.. 

14.1 Tank Farms and Feed 

Preparation 

The radioactive wastes stored at the Hanford site 

are complex mixtures of solids and liquids, 

incompletely characterized, consisting of 

liquids, slurries, saltcakes, and sludges. More 

than 99 percent of the waste volume and a 

majority of the total radionuclide inventory is 

contained in 149 SSTs and 28 double-shell tanks 

(DSTs). Summary descriptions of the wastes in 

SSTs and DSTs are provided in reports by 

Gephart and Lundgren (1995) and Cragnolino et 

al. (1997), and adetailed assessment of tank 

waste contents is provided by Agnew (1997). 

The total volume of SST waste is approximately 

132 million L (35 million gal.), of which

66 percent is wet saltcake consisting of 

crystallized nitrate and other salts, and 

34 percent is sludge consisting of solids formed 

after waste neutralization with sodium 

hydroxide addition. The solids and dissolved 

constituents of the SSTs are -90 percent sodium 

nitrates and nitrites, with the remainder 

consisting mostly of phosphates, carbonates, 

hydroxides, and sulfates. Radioactivity in the 

SSTs is dominated by Sr-90 (75 percent) and 

Cs-137 (24 percent); Sr is mostly in the sludge, 

whereas Cs is chiefly in the saltcake and 

interstitial liquids. The DST waste, with an 

approximate total volume of 76 million L 

(20 million gal.), is dominated by supernatant 

liquids transferred from SSTs and, thus, is 

85 vol % water. The constituents of the DST 

waste, like those of the SSTs, are dominated by 

sodium nitrate and nitrite (-70 percent), with 

-20 percent metal hydroxides and the rest as 

phosphates, carbonates, oxides, and sulfates. 

The DST waste radioactivity is 72 percent from 

Cs-137, which is mostly in the slurry and 

interstitial liquid, and 27 percent from Sr-90, 

which is mostly in the sludge. 

The storage of wastes at the Hanford site poses 

safety hazards of generation and combustion of 

flammable gases (Johnson et al., 1997) such as 

hydrogen or ammonia in the tanks, energetic 

reactions between organics and nitrates/nitrites 

(Turner et al., 1995) that could release 

radioactivity to the environment, and high heat 

loads from radionuclide decay (Wang, 1997) 

that could damage the waste tank concrete 

structures. These issues are unique to the TWRS 

and are being evaluated independent of the 

TWRS privatization program. In addition, 

several tanks that have potential for causing 

onsite or offsite radiation exposure to workers 

through uncontrolled release of fission products 

have been identified as watch list tanks in 

accordance with Public Law 101-510, 

Section 3137, Safety Measures for Waste Tanks

133 NUREG/CR-6666
Process System Design Considerations 

at Hanford Nuclear Reservation (Hanlon, 1997) 

to ensure that safety of the tanks is maintained 

during storage. The following discussion 

focuses on safety issues related to waste 

retrieval, transfer, homogenization, 

pretreatment, feed preparation, and reductant 

addition. 

14.1.1 Waste Transfer Lines 

Several safety issues are of potential concern 

during mixing and transfer of TWRS wastes. 

The foremost concern is rupture of transfer lines 

resulting from changes in waste slurry rheology. 

Depending on rheological behavior, the waste 

either could become more fluid or more viscous 

during mixing or transfer. High viscosity causes 

a potential for transfer line plugging that could 

result in accumulation of flammable 

radiolytically produced gases such as hydrogen. 

A rise in flammable gas concentration in a 

plugged transfer line above the LFL increases 

the probability of explosion, particularly in the 

presence of air or nitrous oxide in the waste. In a 

transfer line containing waste with high heat 

load due to radioactive decay of Sr-90 and 

Cs-137, plugging could result in a severe 

temperature excursion and problems such as 

thermal expansion and distortion or rupture of 

the line. Analogous problems could arise from 

waste that has a potential for energetic or 

exothermic reactions due to organic oxidation 

mechanisms. 

14.1.2 Waste Transfer and Mixing 

Precipitation of specific components, 

exothermic reactions, rheological changes, and 

flammable gas generation due to mixing of 

different wastes potentially could damage the 

mixing and transfer pumps. A transfer of waste 

to an empty tank poses no process safety hazard 

provided the tank material is compatible with the 

waste characteristics. However, the process

hazard exists in transfer to tanks that already 

contain other materials. For example, at the 

WVDP, a highly acidic THOREX waste was 

transferred to a carbon-steel tank containing an 

alkaline PUREX washed sludge. Prior to 

transfer, laboratory tests using both simulated 

and actual waste, coupled with fluid flow 

modeling to study mixing behavior, were 

conducted and a plan was developed to ensure 

proper tank safety during transfer (Gray and 

Westerman, 1995). A detailed analysis of waste 

compatibility prior to waste transfer should 

address potential problems and incidents relating 

to incompatibility of wastes and tank material. 

14.1.3 Waste Homogeneity 

Waste homogeneity during mixing and transfer 

is another important safety issue. While 

criticality is not an issue for the Hanford Site 

HLW tanks (Bratzel et al., 1997), potential for 

the accumulation of fissile material should be 

addressed during normal processing campaigns. 

The potential for criticality resulting from 

preferential segregation of fissile material or 

from failure of mixing pumps should be 

addressed before the retrieval and transfer of 

tank contents is initiated. In addition, waste 

inhomogeneities could result in waste batches 

that are unacceptable for processing, leading to 

production of nonconforming waste forms or to 

the return of these batches to the tank farm. 

Although production of nonconforming waste 

forms is not in itself a safety hazard, excessive 

concentrations of minor components, such as 

sulfates and phosphates, could result in 

increased corrosion of melter components, and 

accumulation of noble metals could cause 

formation of a conductive sludge on the melter 

floor and electrical shorting of the melter.

NUREG/CR-6666 134
14.1.4 Waste Pretreatment 

Waste pretreatment is an important part of the 

solidification process. Pretreatment allows 

separation of highly radioactive species, such as 

Sr-90, Tc-99, Cs-137, and transuranic waste 

(TRU) from the supernatant Understanding the 

chemistry of the wastes and the separation 

media helps ensure a safe pretreatment process. 

Each component of the waste, before 

pretreatment to extract radionuclides, should be 

compatible with the pretreatment media. For 

example,. in the WVDP pretreatment. process, Pu 

solubility in the supernatant was found to 

increase due to dilution of the waste in the waste 

tank with time. This increase in Pu solubility 

resulted in Pu being released with the effluent 

into the evaporator (Dalton, 1992)'. Another 

example is the evolution of benzene during the 

ITP process at the SRS facility that caused TPB 

to precipitate Cs as Cesium TPB. This is a safety 

issue and currently is being addressed by the 

DOE as a result of the safety assessment of the 

ITP process by the Defense Nuclear Facilities 

Safety Board (1996). 

14.1.5 Feed Preparation for 

Solidification 

The feed preparation flowsheet, like the 

pretreatment flowsheet, should provide a safe 

operating environment. Each component of the 

glass-forming chemicals, before it is mixed with 

waste, should be compatible with the waste. 

Chemicals added other than in an established 

sequence could result in temperature excursions 

due to exothermic heat of reaction, 

agglomeration, or the release of NO.. As an 

example, in the WVDP flowsheet the addition of 

1Dalton, W.J. 1992. Qualification Testing of 

Titanium-Treated Zeolite for Sludge Wash Processing. 

White paper prepared for the U.S. Department of Energy. 

West Valley, NY: West Valley Nuclear Services.

Process System Design Considerations 

a polyglycol-based antifoam agent after nitric 

acid addition results in NO. fumes and, 

therefore, it is added last to minimize NO, 

generation (Firstenberg et al., 1995). In addition, 

the WVDP process flowsheet requires an acidic 

feed to ensure that proper rheology is 

maintained during mixing and feeding to a 

melter. When acidic glass formers are added to 

an alkaline waste, the rheology of the mix 

changes very rapidly. The mix rheology exhibits 

a maximum in slurry viscosity between a pH of 

6 and 8 and decreases at lower or higher pH. 

Detailed laboratory studies on rheological 

behavior as a function of pH allowed the WVDP 

to prepare feed batches within processable 

bounds. In the DWPF feed preparation 

flowsheet, adjustments are made using frit and 

formic acid (Ramsey et al., 1994). The DWPF 

frit is much more erosive than the chemicals 

used at the WVDP, which has resulted in 

erosion of the DWPF feed tank components 

(Gee et al., 1996). These examples highlight the 

importance of using the proper chemicals to 

ensure that the design life of the components is 

attained. Other potential safety issues related to 

the feed preparation process include erosion and 

corrosion of on-line instruments that could result 

in the malfunction of on-line pressure, 

temperature, and level instruments. For 

example, erosion of level measurement tips 

could lead to erroneous reading and could result 

in overfill or underfill of the feed preparation 

tanks. Measures to mitigate such problems will 

help minimize any remote or hands-on 

maintenance work that needs to be done. 

14.1.6 Selection of Reducing Agents 

One of the major concerns in developing a feed 

preparation flowsheet is the selection of 

reducing agents and a proper understanding of 

noble metal-catalyzed reactions. For example, if 

a formic acid-based flowsheet is selected for 

TWRS waste (similar to the DWPF flowsheet), 

the potential for hydrogen generation due to

135 NUREG/CR-6666
Process System Design Considerations

catalyzed decomposition of formic acid by noble 

metals should be addressed to ensure hydrogen 

generation does not exceed the LFL. In addition, 

noble metal-catalyzed formic acid reduction of 

nitrite and/or nitrate to ammonia is a significant 

safety concern because of ammonium nitrate 

formation, which is an explosion hazard (King 

and Bhattacharya, 1996). For the TWRS waste 

feed preparation flowsheet, the effects of 

organics and organometallic compounds that are 

introduced from the waste should also be 

addressed to ensure that they do not result in 

formation of hazardous compounds. 

14.2 Waste Solidification and Storage 

Initial research on the development of 

vitrification processes for HLW started in the 

early 1960s. Since then, vitrification has gained 

wide acceptance as a viable technology for 

solidification and disposal of HLW stored 

around the world, and has also been 

recommended for disposal of LLW and mixed 

low-level radioactive and hazardous wastes in 

the United States. In a typical HLW vitrification 

process, waste is mixed with glass-forming 

chemicals and the resultant mixture is melted at 

temperatures greater than 1,100 'C in a melter 

inside a shielded cell. The melting process fuses 

the various components of the mixture into a 

glass melt that is poured and cast into stainless 

steel canisters. Since the operation of the first 

HLW vitrification plant at Marcoule, no 

catastrophic accident involving HLW 

vitrification technology (i.e., accidents that 

resulted in any significant radioactive dose 

exposure to the public or release to the 

environment) has occurred. This does not mean 

that vitrification technology is free from 

problems and issues that could result in failures. 

In fact, over the past 20 yr, several operational 

incidents such as breach of a melter vessel 

allowing molten glass to leak/drop out of the 

melter, uncontrolled release of offgas from the 

melter, failure of the joule-heating system, and

plugging of the glass pour drain, have occurred 

at various IHLW and LLW vitrification facilities. 

Table 14.1 provides a summary of some of the 

published melter-related incidents, corrective 

actions, and lessons learned from these 

incidents. 

These examples clearly indicate that an in-depth 

understanding of melter design and of 

compatibility between melter and waste 

components is extremely important to minimize 

melter incidents. Even though the melter 

incidents that have occurred to date have not 

resulted in deaths or injuries, they did pose a 

threat to worker health and safety. In a robustly 

designed and built melter, control of the waste 

and glass chemistry under defined melter 

temperature and pressure ranges is a key to safe 

operations. Parameters such as chemical 

reactions, redox control, formation of 

conductive sludges, and glass properties such as 

viscosity, phase separation, and liquidous 

temperature should be addressed for safe 

operations. These parameters are discussed in 

the following sections. 

14.2.1 Chemical Reactions 

In a typical waste vitrification process, as slurry 

is fed to a melter, water evaporates and leaves 

behind a crusty layer of waste and glass-forming 

components. Organic and inorganic salts present 

in the slurry decompose in the crusty layer, 

releasing oxides of nitrogen, carbon, and sulfur. 

These reaction gases, along with water vapor, 

exit the melter through an offgas port. 

Knowledge of possible exothermic and 

endothermic chemical reactions that could occur 

within the crusty layer and formation of reaction 

gases such as ammonia, cyanides, and aromatic 

hydrocarbons is necessary to ensure that the 

process does not exceed safe concentration 

levels of flammable or explosive gases.

NUREG/CR--6666 136
Process System Design Considerations

Table 14.1. Summary of Melter-Related Incidents

Incident Corrective Action Lessons Learned 

Joule-heated melter at the high- Facility was shut down and Melters with sloped walls were 

level waste vitrification facility replaced by an identical melter. designed at the West Valley 

at Pamela, Mol, Belgium, Demonstration Project and the 

electrically shorted due to noble Tokai Vitrification Plant to 

metal accumulation on the minimize the effect of noble 

melter floor. (Elliott et al., metal accumulation. In 

1994) addition, bottom drains are 

routinely provided to drain 

noble metals at regular 

intervals. 

At the Defense Waste The discharge chute was Future melter designs should 

Processing Facility, wicking of modified to reduce wicking account for wicking of glass 

the glass stream during pouring and, in addition, remote pour streams. 

resulted in plugging of the equipment was installed to clean 

discharge orifice on a regular a plugged orifice. 

basis. (Carter et al., 1997) 

At the Pamela facility in Mol, Glass composition was modified Glass composition should be 

Belgium, high viscosity and to reduce liquidous temperature designed to meet both disposal 

crystallization of glass melt and viscosity, and processing constraints. 

resulted in plugging of the 

discharge orifice. (Wiese et al., 

1988) 

At the Tokai Vitrification Plant The discharge orifice was Melter design should evaluate 

in Japan, radioactive glass remotely modified and the the placement of measuring 

accumulated in the coupling location of the thermocouples instruments. 

device between the melter and was changed to provide better 

canister. This was attributed to control of temperature near the 

extremely low temperatures at discharge section. 

the discharge orifice due to the 

lack of proper temperature 

control in the region. 

(Tomikawa and Yoshioka, 

1996)

137 NUREG/CR-6666
Process System Design Considerations

Table 14.1. Summnary of Melter-Related Incidents (cont'd) 

Incident Corrective Action Lessons Learned 

At the West Valley The nozzle liners were Selection of materials and 

Demonstration Project, the redesigned. design of components should 

ceramic nozzle liners failed due undergo evaluation prior to 

to insufficient thermal radioactive operations. 

expansion allowance. 

At the West Valley A flow-reducing orifice was Operating limits for airflow 

Demonstration Project, installed to reduce airflow, rates, pressure, and temperature formation of glass fibers in the should be established prior to 

discharge section led to the the start of the process. 

blockage of the discharge 

orifice. This was a result of 

high air-inflow through the 

discharge orifice to the melter. 

(Hamel et al., 1998) 

At the Mayak vitrification The facility was shut down and Redundancy and stricter 

facility in Russia, the high-level a similar melter with improved controls to operate within 

waste melter suffered an electrode cooling system was specification should be provided electrode failure due to installed. for critical systems. excessive current load that Performance of the melter 

caused accelerated corrosion should be continually assessed 

and eventual failure of during operations via quality molybdenum electrodes. This assurance programs and safety 

was partially attributed to the audits. 

failure of cooling systems for 

the electrodes. (Bradley, 1992) 

The mixed low-level radioactive The melter is in the process of Corrosivity of the melt and its 

vitrification facility in the being replaced. compatibility with the 

M-area at the SRS site was shut components should be 

down due to the formation of established before startup. hot spots in the melter. The Performance of the melter 

formation of hot spots was should be continually assessed 

attributed to chemical additions during operations via quality that were made during the assurance programs and safety 

startup. (GTS Duratek, 1997) audits.

NUREG/CR-6666 138
Process System Design Considerations

Table 14.1. Summary of Melter-Related Incidents (cont'd) 

Incident -- Corrective Action Lessons Learned 

AtAt the Fernald site, a The facility was shut down and Corrosivity of the melt and its 

nonradioactive melter failed, no decision has been made to compatibility with the 

spilling 6,000 kg of glass onthe restart the facility, components should be 

floor, due to degradation of the established before melter 

melter components caused by operations. Performance of the 

incompatible feed chemicals. melter should be continually 

(FERMCO, 1997) assessed during operations via 

quality assurance programs and 

safety audits. 

At the low-level waste in-situ The recommendations for Melters should have safeguards 

vitrification plant at the Oak corrective actions included designed not only to account for 

Ridge National Laboratory, a diversion of standing water normal operating conditions, 

release of offgas and an around the pit, installation of but also for abnormal 

expulsion of 20,000 kg of glass flow monitors and curved vent conditions such as steam 

caused glass fragments to be pipes beneath the melt to explosion. 

thrown up to 100 m from the provide alternate paths for 

melter site. In addition, several steam, sub-melt pressure 

small fires resulted from measurement, and video 

escaping hot gases and molten monitoring of the melt surface. 

glass. This event was a result of 

pressurized steam venting 

rapidly through the melt. (Oak 

Ridge National Laboratory, 

1996)

14.2.2 Glass Redox 

The control of redox, defined as 

reduction-oxidation potential of the melt, is 

probably the most important process control 

parameter in the melter. The redox is dependent 

on both feed and melting systems, and is a result 

of the combined effects of reducing agents such 

as organics; oxidizing agents such as nitrates 

and nitrites; the waste concentrations of 

transition metal ions such as Fe, Cu, Mn, Ni, and 

Cr; and melter operating conditions such as

bubbler flow rates, melter temperature, pressure, 

and air in-leakage into the melter. Both bubblers 

and air in-leakage introduce extra oxygen in the 

system and could potentially shift redox 

equilibrium. A redox response within a specified 

range is necessary to avoid process upsets that 

eventually could lead to permanent melter 

damage. 

Because iron is abundant in most wastes, the 

redox response in most waste vitrification 

systems is measured by analyzing the Fe2÷/Fe3

" 

ratio. The melter environment is defined as

139 NUREG/CR-6666
Process System Design Considerations 

being extremely reducing if the Fe2

+/Fe3+ ratio is 

greater than 1. Under such conditions, sufficient 

accumulation of conductive metals and metal 

sulfides could short-circuit the melter. If the 

Fe24/Fe3

+ ratio is less than 0.01, the melter is 

defined as extremely oxidizing. The quantitative 

lower limit of Fe2

+/Fe3+ is not known; existing 

redox measurement equipment cannot analyze 

Fe2+/Fe3+ less than 0.01 with confidence. Under 

extreme oxidizing conditions, foaming is 

observed in the melter. Foam creates an 

insulating layer of gas bubbles between the 

cold-cap and the melt, disrupting the thermal 

gradients in the melter. Typically, foaming, if 

controlled at its onset, slows the glass 

production rate but, in most cases, is not an 

issue. However, if foaming is not controlled, 

conditions can arise that could lead to melter 

shutdown. For instance, the foam could fill the 

melter plenum, cause the melter offgas line to 

plug, and eventually pressurize the melter. If the 

melter pressure exceeds the pressure differential 

needed for glass pouring, a sudden uncontrolled 

discharge of the melt will occur. The discharge 

could result in complete blockage of the 

discharge chute with solid glass and lead to 

thermal shock of the melter due to a sudden loss 

of thermal mass in the melt chamber. 

Because no on-line redox measurement tools are 

available, the redox response is usually forecast 

based on some empirical relationship between 

the feed characteristics and melter response. The 

WVDP and the DWPF use empirical models to 

forecast the redox state of the glass melt based 

on measured concentrations of the reducing and 

oxidizing components of the feed. The empirical 

models were developed based on extensive 

laboratory and pilot-scale testing of simulated 

feeds. At the WVDP, the redox is controlled by 

adding sugar to the feed, the amount of which is 

based on the nitrates, TOC, and % TS in the 

waste (Barnes and Jain, 1996). At the DWPF, 

the redox is controlled by addition of either 

formic acid or nitric acid to the feed using an

empirical relationship based on the nitrate and 

formate analysis (Ramsey et al., 1994). Both the 

WVDP and DWPF melters operate at oxidizing 

conditions to avoid the formation of conductive 

sludges. 

Redox monitoring of wastes containing high 

concentrations of MnO 2 and CuO is much more 

complex. Since both MnO2 and CuO transitions 

occur at the oxidizing end of the Fe2÷/Fe3

+ 

couple, the redox response cannot be correlated 

to the Fe2/Fe3

" ratio. In addition, reboiling 

occurs under oxidizing conditions. Reboiling is 

attributed to the shift in the redox state of the 

melt with a shift in temperature. The higher the 

temperature, the more reducing the melt 

becomes. Under reboiling conditions, MnO2 is 

reduced to MnO, releasing significant amounts 

of oxygen. The reboil event behaves similarly to 

foaming, but is dependent on temperature 

changes in the melter. 

The sensitivity of the redox response as a 

function of reductant concentration is extremely 

important in defining the redox range for any 

melter system. An acceptable range for redox 

response in the DWPF system is limited by the 

presence of CuO in the DWPF waste. In the 

melter, CuO could precipitate as Cu metal at 

moderately reducing conditions (e.g., at 

Fe2+/Fe3

" greater than 0.5). On the other hand, an 

acceptable range for redox response in the 

WVDP system is very sensitive to the presence 

of nitrites. The operating range for redox is 

limited by the amount of nitrites present in the 

WVDP feed. Both DWPF and WVDP 

operations are limited to an extremely narrow 

redox range. The situation for TWRS waste is 

much more complex due to the presence of 

significant amounts of different types of 

organics. Melter incidents resulting from redox 

control problems for the TWRS waste should be 

addressed to determine the key parameters that 

will control the redox response.

NUREG/CR-6666 140
Process System Design Considerations

14.2.3 Conductive Sludges and Noble 

Metals 

Conductive sludges in the melter are defined as 

precipitated conductive metal sulfides or metals 

in the glass melt which are formed under 

reducing conditions. In addition, noble metals 

such as Rh, Pd and Ru, because of their very 

limited solubility in glass, tend to settle on the 

melter floor, forming highly electrically 

conductive paths. Most of these metals cannot 

be pretreated or extracted easily. Both formation 

of conductive sludges and settling of noble 

metals could potentially result in electrical 

shorting of a joule-heated melter. The melter 

design and feed composition should address the 

risk of premature melter failure, such as the one 

that occurred at the Pamela melter, due to 

settling of noble metals. An example of these 

studies is the extensive testing that was done at 

the PNNL to determine the influence of noble 

metals on the operational life of a reference 

Hanford Waste Vitrification Plant melter (Elliott 

et al., 1994). 

14.2.4 Glass Properties 

The proper design of glass composition is 

important to ensure that the glass can be 

processed within the operating range of the 

melter and will not degrade the melter design 

life. The properties that dictate glass processing 

include glass viscosity, electrical resistivity, 

liquidous temperature, and phase separation. 

These properties are discussed in the following 

sections. 

14.2.4.1 Viscosity 

Control of glass viscosity, similar to control of 

waste rheology, is extremely important for 

continuous and safe melter operations. HLW 

glass melters operate between a viscosity of 20 

and 100 poise at their operating temperatures.

The convective currents in the melt, due to the 

thermal gradients in the melter, are responsible 

for homogenization of waste and glass-forming 

chemicals. At a viscosity higher than 100 poise, 

homogenization of the waste components is 

delayed because higher viscosity slows the 

kinetics of convective mixing. In addition, high 

viscosity makes the flow of glass from the 

melter into the canister slow and difficult. Also, 

an extremely high viscosity could result in the 

formation of a glass plug at the discharge 

section. On the other hand, a viscosity less than 

20 poise results in increased volatilization of 

radioactive components, such as Tc-99 and 

Cs-137, and of alkali oxides such as Na 2O, Li2O, 

and K20 and boron oxide. This could result in 

accumulation of radionuclides in the offgas 

filters instead of encapsulating in glass. The 

filters may need frequent replacement to avoid 

plugging. In addition, low glass viscosity 

increases refractory erosion. 

14.2.4.2 Electrical Resistivity 

The control of electrical resistivity of the glass 

melt is extremely important for joule-heated 

melter operations. The electrical resistivity is 

mostly due to ionic conductivity and is a 

function of glass composition, largely the 

concentrations of alkali oxides such as Na2O, 

Li20, and K20. Joule-heated HLW glass melters 

operate at electrical resistivities between 

I and 10 ohm-cm at melter operating 

temperatures. High resistivity of the glass melt 

results in high voltage potential across the 

electrodes that allows conduction to occur 

within the refractory walls that could cause 

dissipation of heat in refractory walls and 

increase power requirements. On the other hand, 

low resistivity of the glass melt results in high 

current density that could exceed the current 

carrying capacity of the electrodes required to 

maintain the melt at the desired temperature and, 

thus, result in electrode failure.

141 NUREG/CR-6666
Process System Design Considerations

14.2.4.3 Liquidous Temperature 

Whenever a glass melt is cooled below the 

liquidous temperature, crystal formation occurs 

in the melt. The crystals that are formed could 

settle on the melter floor as do noble metals. 

However, unlike noble metals, the crystals may 

not be conductive. If the melter is maintained 

below the liquidous temperature for an extended 

period of time, these crystals could continue to 

form and eventually block the glass pour drains 

and make the melter non-functional. Depending 

upon the nature of these crystals, they may or 

may not redissolve in the melt upon reheating to 

above liquidous temperatures. 

14.2.4.4 Phase Separation 

Phase separation occurs whenever a component exceeds its solubility limit in the glass melt. The 

glass melts have very limited solubility for 

components such as noble metals and sulfur. 

The issues related to noble metals have been 

discussed in a previous section. The issues 

related to sulfur solubility are discussed next. 

At the WVDP, the sludge was washed several 

times to reduce the concentration of sulfates, 

measured as SO3, below 0.25 weight percent. The SO 3 solubility in WVDP reference glass is 

0.25 weight percent; SO3 concentrations above 

this value may result in formation of an 

immiscible molten sulfate (gall) layer on top of 

the cold-cap. This gall behaves like a foam. 

Also, at the Pamela vitrification facility, the 

amount of waste that could be processed into the 

glass was limited by the amount of sulfur in the 

waste (Wiese et al., 1988). In addition, the 

sulfates, in the presence of chlorides in the 

waste, dramatically corrode stainless steel and 

Inconel® between 500 and 900 'C. Glass 

composition should address the phase separation issues during melting. Potential for molten 

sulfate formation in Hanford waste is discussed 

in a paper by Sullivan (1995). 

NUREG/CR-6666 142

14.3 Offgas Treatment and Release 

There are two major functions of the offgas 

treatment system: fir'st, to ensure that offgas 

released to the atmosphere meets local, state, 

and federal guidelines; and second, to maintain a 

negative pressure in the vitrification facility. A 

typical offgas exiting the melter contains water 

vapor and oxides of nitrogen, carbon, and sulfur, 

along with entrained feed particles and volatile 

components. As the offgas is cooled, the 

entrained feed particles and volatile components 

deposit along the offgas pipeline. A robust 

design allows the entrained particles to either 

drop back into the melter or be carried over into 

another vessel without depositing along the 

offgas pipeline. If the design is not capable of 

meeting functional requirements, the deposits eventually build up, blocking the offgas flow. 

Under such conditions, the offgas released from 

the cold cap in the melter will pressurize the 

melting chamber. If the melter pressure exceeds 

the pressure differential needed for glass 

pouring, a sudden uncontrolled discharge of the 

melt similar to foaming events could occur. The 

discharge could result in a complete blocking of 

the discharge chamber with solid glass that 

would cause thermal shock of the melter due to 

a sudden loss of thermal mass in the melt 

chamber. In addition, overpressurization may 

cause the melter to release offgas outside the 

cell. 

A generic offgas treatment consists of a 

quencher to cool the offgas, a series of scrubbers 

to remove NO, and entrained particles, a HEME 

for removing mist, and HEPA filters for 

containment of radioactive particulates. In 

addition, the offgas system could include 

specific treatment components depending upon the nature of the offgas. 

In most vitrification facilities, the offgas system controls the pressure in all the feed vessels and 

the melter. A slightly negative pressure is
required to contain radioactivity within the 

system. Failure of the offgas blower or process 

upsets such as foaming, plugging of offgas lines, 

or plugging of the glass discharge chamber, 

could disrupt the offgas pressure control. During 

such upsets, it is likely that certain parts of the 

system could go slightly positive and release 

radioactivity outside the cell. 

14.4 Remote Operations and Human 

Factors 

In a HLW vitrification facility, all process 

components and equipment are enclosed within 

heavily shielded concrete walls because of the 

highly radioactive environment. A large amount 

of equipment is operated and maintained using 

MSMs and in-cell cranes. The complexity of the 

in-cell layout of equipment requires long hours 

of activity for expert MSM and in-cell crane 

operators. Human factors therefore, are an 

integral part of safe operations. Designing

Process System Design Considerations 

low-maintenance, MSM-friendly equipment and 

training operators to efficiently and effectively 

operate the MSMs will significantly reduce 

human errors, such as misalignments of pipes 

and hoses and incomplete closure of valves, that 

could lead to offgas releases or slurry leaks. 

Training of operators using mock-ups would 

also reduce downtime and incidents involving 

human error. An example of the use of 

MSM-friendly equipment is the redesign of a 

slurry sample station at the WVDP to simplify 

valve operations. This redesign reduced the 

sampling time and minimized radioactive slurry 

drips on the sample station floor (Hamel et al., 

1998). Similarly, at the WVP at Sellafield, 

improvements to in-cell cranes and MSMs 

alleviated most of the in-cell operating problems 

and dramatically reduced downtime (Millington, 

1995).

143 NUREG/CR-6666
15 SUMMARY AND RECOMMENDATIONS

This report provides a current status of 

high-temperature solidification technologies that 

have been used or proposed throughout the 

world. The report includes a survey of 

technologies used or proposed in the waste 

management (chemical and nuclear materials) 

industries, and relevant information on 

commercial technologies used in the ceramic 

and glass industries. This report has been 

prepared to assist the NRC in: 

"* determining whether sufficient 

information exists to assess safety 

considerations regarding the Hanford 

TWRS, 

"* determining whether current regulatory 

guidelines are adequate for controlling 

implementation of the technology to 

solidify Hanford waste, 

identifying the existence of technical 

uncertainties, 

assessing where future guidance may be 

warranted, and 

evaluating a "first-of-a-kind" process that 

is being proposed or utilized. 

The survey of waste solidification processes 

used in the United States and abroad indicates 

that, although major accomplishments have 

occurred in the area of vitrification in the last 

20 yr, developments are continuing in other 

solidification technologies such as 

glass-ceramics, Synroc, and, more recently, in 

molten metal processes. The technologies can be 

broadly classified into the following categories: 

Inconel®-based, joule-heated melters; 

high-temperature, joule-heated melters; 

induction melters; cold-crucible induction 

melters; plasma melters; combustion melters; 

microwave melters; molten metal technology;

and Synroc technology. The development of a 

successful solidification process requires a 

reasonable level of characterization of the 

wastes that need to be solidified before a 

technology can be selected. This selection 

should be based on the nature of the waste and 

the final waste form that will provide effective 

encapsulation. 

The joule-heated Inconel® melter technology 

and induction melting technology are the two 

most widely accepted and mature technologies 

for vitrifying HLW. Glass melting via joule 

heating is used in the United States, Japan, 

Russia, and Belgium, while glass melting via 

induction-type heating is used in France, 

England, and India for vitrifying HLW. Both 

technologies have successfully encapsulated 

HLW into glass and provide extremely low 

volatility for alkalis, boron, and radioactive 

components such as Tc and Cs, and a 

homogeneous product due to long residence 

time in the melter. While the joule-heated melter 

can be designed to meet production rate 

requirements [vitrification plant in Mayak, 

Russia, has a production capacity of 1,200 kg/hr 

(2,640 lb/hr) while the vitrification plant in 

Tokai, Japan, has a production capacity of 

9 kg/hr (19.8 lb/hr)], the size of the induction 

melter cannot be increased due to the attenuation 

of induction frequencies in large-diameter 

crucibles. In addition, the life of a joule-heated 

melter ranges from 3 to 5 yr, while the melting 

crucible for the induction melting is considered 

a disposable item with replacement time ranging 

from 1,500 to 4,000 hr. Due to the design of the 

induction melting system, only calcined feeds 

can be processed in the melter while the 

joule-heated melter can easily adopt to wet or 

dry feeding. Other technologies that are 

discussed in the report are either in the 

development stage or have never demonstrated 

solidification of radioactive wastes.

145 NUREG/CR-6666
Summary and Recommendations

Initial research on the development of the 

joule-heated vitrification process for LHLW 

started in the early 1960s. The first vitrification 

plant that was operational in the United States 

was commissioned for radioactive operations in 

December 1985 to produce 32 heat and radiation 

source canisters for the FRG. This facility was 

shut down in March 1987. The melter was 

developed at the PNNL as a part of a bilateral 

agreement between the Bundesminiterium fur 

Forschung und Technologie and the DOE. The 

first joule-heated radioactive vitrification plant, 

the Pamela vitrification facility in Mol, Belgium, 

was commissioned in 1985. In the United States, 

vitrification plants at the WVDP and the DWPF 

were commissioned in 1996. The TVP in Japan 

was commissioned in December 1995 and 

employs a joule-heated melter system similar to 

the WVDP, West Valley, NY and the DWPF at 

the SRS. Major differences exist in the capacity 

of the TVP melter, which is only 0.3 m3 

(10.6 W), compared to 0.9 m3 (31.8 ft) at the 

WVDP and 2.2 m3 (77.7 ft) at the DWPF, and 

the canister size, which is 300 kg (660 lb) for the 

TVP, and 2,000 kg (4,400 lb) for the WVDP and 

the DWPF. In addition, the average melt rate for 

the TVP is 9 kg/hr (19.8 lb/hr), compared to 

30 kg/hr (66 lb/hr) for the WVDP and 100 kg/hr 

(220 lb/hr) for the DWPF. 

Induction melting technology using calcined 

wastes for encapsulating HLW was first 

developed in France. The AVM in Marcoule, 

France, was commissioned in 1978 to vitrify 

fission product solutions from the UP1 

reprocessing plant in France. The success of the 

AVM led to the startup of two similar facilities, 

R7 and T7. The R7 and the T7 facilities were 

commissioned in 1989 and 1992, respectively, at 

the La Hague reprocessing plant. The R7 and the 

T7 are identical plants, each having three 

vitrification lines and are being used to vitrify 

UP2 and UP3 reprocessing plant wastes, 

respectively. Each vitrification line is designed 

for a production rate of 30 kg/hr (66 lb/hr).

Furthermore, the United Kingdom adopted this 

technology at the Sellafield site in 1981. The 

WVP at Sellafield was commissioned in 1991. 

The WVP consists of two vitrification lines and 

has a higher production rate than the AVM. 

Cold-crucible induction melting technology is 

an adaptation of the mature induction melting 

technology. Several pilot-scale tests are being 

conducted in France and Russia. Even though 

the technology has not been tested with 

radioactive wastes, several projects are under 

way to develop the process for radioactive 

operations worldwide (Jouan et al., 1998). In 

addition to the development of a CCM for the 

Hanford wastes under the TWRS program, two 

CCMs are being designed for shipment to 

Saluggia, Italy, to vitrify HLWs at the Saluggia 

site, and to Taejon, Korea, to incinerate and 

vitrify low-level reactor wastes. The interactions 

and corrosion between the waste, the glass, and 

the Inconel crucible are significantly 

minimized by the formation of a skull layer at 

the glass-crucible interface, which provides a 

longer crucible life compared to induction 

melting systems. In addition, melting 

temperatures are not restricted by the operating 

temperatures for the Inconel® crucibles, which 

results in higher glass production rates. 

The Hanford low-level vitrification test program 

was initiated to test and evaluate promising 

commercial vitrification technologies to support 

selection of reference technologies for the 

Hanford site LLW vitrification. This program 

evaluated seven different technologies: the 

cyclone combustion melter; the joule-heated 

Inconel® electrode melter; the carbon electrode 

melter; the plasma torch-fired cupola furnace; 

and three different types of high-temperature, 

joule-heated melters. The evaluation and 

assessment of the technologies determined that 

the joule-heated melter technology operating in 

a cold-top mode is the most reliable.

NUREG/CR-6666 146
Summary and Recommendations

The MWFA was established by the DOE to 

develop and facilitate implementation of 

technologies required to meet DOE 

commitments for characterization, treatment, 

and disposal of mixed wastes. Under MWFA 

sponsorship, a number of thermal treatment 

technologies to stabilize mixed wastes have 

evolved. Several technologies such as the 

plasma hearth process, plasma arc process, 

high-temperature, joule-heated melters, 

alternating current arc melters, microwave 

melters, and combustion melting systems are 

extensions of the existing commercial 

technologies, while cold-crucible technology 

and the Q-CEP are new technologies that were 

evaluated by the MWFA. 

The CEP is an innovative, flexible, proprietary 

technology of MMT, Incorporated, in Waltham, 

Massachusetts. This technology is used for 

processing hazardous waste streams. It allows 

the organic, organometallic, and inorganic waste 

streams to be recycled into useful resources. The 

Q-CEP is an adaption of the CEP technology for 

radioactive and mixed waste streams. In addition 

to the destruction of hazardous components, the 

Q-CEP allows partitioning of radionuclides. The 

partitioning allows recycling of large numbers 

of waste components into commercial products 

and the solidification of radionuclides, either in 

glass/ceramics or metal. In both the CEP and the 

Q-CEP, the molten bath (typically iron or 

nickel) acts as a homogeneous catalyst and as a 

solvent in dissociation of feeds, synthesis of 

products, and concentration of radionuclides in 

the desired phase. The Q-CEP was selected by 

the DOE as the alternative to joule-heated melter 

technology for the Phase I TWRS privatization 

program. Unfortunately, due to the proprietary 

nature of the process, not much information is 

available regarding operating experience with 

Q-CEP technology. 

The high-temperature, joule-heated melter 

technology offers glass production rates higher

than all other solidification technologies except 

for plasma and combustion melting systems. 

The production rate can be further enhanced by 

the use of bubblers and stirrers. Both the TVS 

and the Fernald high-temperature melter have 

failed during operations. The TVS failed due to 

thermal shock resulting from the heatup and 

cool-down cycles that the melter underwent 

during its operating life while the melter at 

Fernald failed as a result of excessive high

temperature corrosion of melter components and 

refractory erosion. More research is needed to 

ensure waste material compatibility with the 

components of the high-temperature, joule

heated melter. 

The only operational, joule-heated, low-level 

radioactive mixed waste vitrification plant is 

situated in the M-Area at the SRS. The plant is 

in the process of vitrifying approximately 

2,500 m3 (660,500 gal.) of low-level radioactive 

mixed waste stored at the M-area. GTS Duratek 

completed the construction of the vitrification 

plant in January 1996, and commissioned it to 

process low-level radioactive mixed wastes in 

October 1996. The, Duramelter® 5000 is 

currently the largest low-level radioactive melter 

system operating in the world. This is also a 

one-third size prototype of the melter system to 

be built for the TWRS in Richland, Washington. 

On March 31, 1997, waste processing was 

temporarily suspended after observing possible 

signs of erosion and corrosion on the melter 

components. Visual inspections indicated the 

formation of hot spots in the melter due to 

refractory corrosion by molten glass, especially 

at the refractory joints. The facility was restarted 

after installation of a new melter. 

Plasma melting technology has been widely 

used in the commercial steel industry for a long 

time, and its adoption to waste solidification is 

quite recent. Several pilot-scale plants are 

operating and some testing has been performed 

using low-level radioactive mixed wastes.

147 NUREG/CR-6666
Summary and Recommendations

Full-scale radioactive testing has not been 

performed. The technology is well suited for 

hard-to-vitrify heterogeneous wastes due to high operating temperatures. Short plasma torch life 

is a major concern in plasma systems for 

radioactive operations that, depending upon 

operating conditions, could range from a few 

hours to a few days. 

Combustion melting technology, like plasma 

melting technology, is a developing technology. 

Even though this technology has been used in 

the commercial glass, coal, and steel industries 

for a long time, its adoption to waste 

solidification is quite recent. Several pilot-scale plants are operating, but radioactive testing has 

not been performed. In both plasma melting and 

combustion melting technology, glass production rates are high, but volatility due to 

high operating temperatures and inhomogeneity 

resulting from short residence time are a 

concern. In addition, extremely high operating 

temperatures cause extensive erosion and 

corrosion of refractories and melter components. 

Microwave melting is an experimental 

technology. There are no full-scale radioactive 

operating microwave solidification systems. 

Some radioactive waste vitrification studies 

have been conducted using pilot-scale melter 

systems. Microwave melting is a one-step 

process. The waste can be melted and disposed in the same container. Due to low melting 

temperatures, volatility is low. Glass production 

rates are limited by the size of drums that can be 

placed in a microwave melter. The size of the 

drum is limited by the inability of the 

microwaves to uniformly heat large quantities and due to nonuniform melting, and the glass product is inhomogeneous. The interactions and 

corrosion between the waste, the glass, and the 

container have not been established. In some 

instances, the formation of hot-spots has led to 

glass flow from the containers.

Synroc technology has been around for a long 

time, but it is still a developing technology. 

There are no operating Synroc solidification 

plants processing radioactive materials. A pilot 

plant has been operating in Australia for several 

years with nonradioactive feed. Synroc is an 

excellent waste form for disposal. Hot-pressing 

makes the product homogeneous and reduces 

volatility during processing. Since no melting is 

required, corrosion issues are minimal. The 

waste is hot-pressed in stainless steel bellows, 

that become a part of the waste form. Noble 

metal accumulation is not an issue. All 

components are retained in the Synroc: each 

time, the process starts with a fresh batch. 

Synroc production is a batch process. The 

production rates are limited by the size of 

hot-presses. 

Based on the review of the solidification 

technologies, technological assessments, and 

evaluation of the process and operational safety 

issues, a number of areas associated with the 

solidification options for TWRS activities that 

will require evaluation have been identified. 

These include: 

"* Explosion hazard due to flammable gas 

(hydrogen and ammonia) generation. 

"* Criticality hazard from plugging and 

accumulation of radionuclides in transfer 

and offgas pipes. 

"* Uncontrolled chemical reactions, leading 

to radionuclide release, due to mixing of 

incompatible wastes and chemicals. 

0 Corrosion of melter components, leading 

to melter failure, due to processing of 

wastes containing corrosive chemicals. 

0 Gap formation in the melter, due to 

thermal stresses and thermal shock during 

heatup and cool-down cycles.

NUREG/CR-6666 148
Summary and Recommendations

Precipitation of metals, leading to 

electrical shorting of the melter, due to 

improper redox control methodology or 

settling of noble metals. 

Failure of the process components, 

leading to radionuclide release, due to 

incompatibility of the materials-of

construction with waste and feed 

chemistry.

In summary, it is important that the plant be 

designed to accommodate possible variables in 

the wastes to ensure safe operations. Because of 

repeat limited knowledge regarding waste 

compositions at the Hanford site, that task is 

challenging, but lessons learned from the design 

and operations of commercial industries and the 

radioactive and nonradioactive plants in 

operation around the world will be useful in 

minimizing process and safety risks.

149 NUREG/CR-6666
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159 NUREG/CR-6666
APPENDIX A 

PRIMER ON GLASS PRODUCTION

NUREG/CR-6666
A.1 Introduction 

Radioactive wastes can be encapsulated 

into several solid forms such as cement, mortar, 

concrete, crystalline ceramics, glass, and metals. 

Glass has developed as a preferred waste form 

among all available waste forms for 

encapsulation of high-level waste (Hench, 

1995). The sections below define glass as a 

waste form and discuss basic technologies 

involved in transforming radioactive waste into 

glass. 

A.2 Glass 

Glasses are generally considered as solids 

with liquid-like structure, noncrystalline solids, 

or simply as amorphous solids (Varshneya, 

1994) and are broadly categorized into 

inorganic, organic, or metallic glasses. The 

discussion on the formation of organic and 

metallic glasses is beyond the scope of this 

review. The inorganic glasses are further 

classified into oxide and nonoxide glasses. The 

oxide glasses are predominantly used for 

encapsulating radioactive wastes. 

Oxide glasses are either single-component 

or multi-component. In most multi-component 

oxide glass forming systems, silica is a major 

component. Depending on the remaining 

components, oxide glasses relevant to 

radioactive waste vitrification can be classified 

as alkali silicates (glasses containing a mixture 

of alkali oxides and silica), alkali borosilicates

(glasses containing a mixture of alkali oxides, 

boron oxide, and silica), alkali aluminosilicates 

(glasses containing a mixture of alkali oxides, 

aluminum oxide, and silica), and alkali 

boroaluminosilicates (glasses containing a 

mixture of alkali oxides, boron oxide, aluminum 

oxide, and silica). The non-silica based oxide 

glasses include phosphate glasses. The glass 

compositions most widely used for the low-level 

waste and high-level waste vitrification fall into 

the category of alkali boroaluminosilicates. 

These glasses offer excellent durability and 

thermal stability for long-term storage. 

A.3 Glass Properties 

The control of glass properties such as 

viscosity, electrical resistivity, and liquidous 

temperature, within the range shown in 

Table A-1, is extremely important in 

encapsulating waste into glass. The electrical 

resistivity determines whether joule heating will 

be effective in dissipating heat while viscosity 

ensures that the glass will easily flow and 

homogenize in the melter. The liquidous 

temperature establishes the'lowest temperature 

below which melter operations for extended 

time will crystallize the melt. 

A.4 Vitrification 

The process of fusing components of the 

waste and glass forming oxides into a glassy 

structure is called "vitrification." Even though 

glasses have an ability to incorporate most of the

Table A-1. Process constraints (Hrma et al., 1994) 

Parameter Lower Acceptance Bound Upper Acceptance Bound 

Viscosity at 1150 'C (Poise) 20 100 

Glass electrical resistivity at 10 

1150 °C (ohm-cm) 

Liquidous temperature N/A 1,050

A-1 NUREG/CR-6666
Appendix A 

elemental constituents of the radioactive waste 

into a glassy matrix, it requires temperatures in 

excess of 1,100 TC. The upper operating 

temperature depends upon the choice of 

electrodes. In melters having Inconel® 

electrodes, the maximum operating temperature 

is 1,150 'C to avoid creep in the electrodes 

while for molybdenum electrodes, the maximum 

operating temperature is 1,500 'C. 

The vitrification temperature depends 

upon amount of flux present in the composition. 

Fluxes are defined as the components, such as 

alkali oxides and boron oxide, that lower the 

fusing temperature of the glass. It should be 

noted that the higher the amount of fluxes, the 

lower the durability of the waste form. The 

solubility of various components in glass 

depends on the composition of the glass as well 

as the chemical composition of the radioactive 

waste. Special consideration is required for 

low-solubility components such as sulfates and 

noble metals, and volatile components such as 

halides, during process flowsheet development. 

A.5 Process Chemistry 

In a typical radioactive waste vitrification 

process, a liquid slurry (in the Idaho National 

Engineering Laboratory case, calcined waste 

mixed with dry or wet glass formers) containing 

radioactive wastes mixed with glass-forming 

chemicals is fed into a melting vessel. As the 

slurry falls on the top of the molten glass 

contained in the melter, water evaporates 

leaving behind a crusty layer of salts. This 

crusty layer of salts is called "cold-cap". The 

formation of a stable cold-cap is extremely 

important to minimize volatilization of 

radioactive species such as Cs-137. The 

components of the cold-cap decompose as the 

heat from the molten glass pool is transferred 

into the cold-cap. The inorganic salts, which are 

predominantly a mixture of nitrates, nitrites, and 

sulfates, and organic compounds such as sugar 

and/or formic acid, decompose leaving behind

calcined metal oxides. The decomposition gases, 

consisting mostly of NO. from nitrates and 

nitrites, CO/CO2 from organics and SO, from 

sulfates, exit the melter through an offgas line 

and are treated to remove entrained solids, 

volatile components such as Cs-137, and 

hazardous components prior to releasing to the 

atmosphere. The calcined oxides fuse together to 

form a glassy melt. The process is optimized by controlling the parameters such as melt 

temperature, melter pressure, slurry feed rate, 

redox chemistry, and glass pour rate. 

A.6 Vitrification Technology 

Fossil fuel combustion and electric 

heating are the primary sources of thermal 

energy used for vitrification. While fossil fuel 

combustion is a primary mechanism for making 

glass in the traditional glass industry, the 

radioactive waste industry depends largely on 

electric heating for vitrification. The electric 

heating mechanisms that are currently used or 

are being developed include joule heating, 

induction heating, microwave heating, plasma 

heating, and electric arc heating. 

In joule heating, a liquid slurry is fed to a 

refractory lined ceramic melter and a set of 

Inconel® or molybdenum electrodes supplies 

electric current through a melt. The internal 

resistance offered by the melt dissipates energy 

in the form of heat energy. The joule heating is 

most effective when the resistivity of the melt is 

between 5 and 10 ohm-cm at 1,100 °C. 

In induction heating, calcined waste and 

glass-forming oxides are placed in an 

induction-coil-lined Inconel® melter and 

subjected to an alternating electromagnetic field. 

The induced currents produced in the calcined 

waste dissipate electric energy in the form of 

heat by the joule effect. Note that in the 

induction melters an additional pretreatment 

process step to calcine the slurry is required 

prior to vitrification. Induction melters are not

NUREG/CR-6666 A-2
Appendix A

suitable for water-based slurries. The 

high-frequency cold-crucible induction melters, 

in addition to normal induction heating have a 

series of sectored water cooling coils that cool 

the surface of the melter and form a skull layer 

around the melter. A "skull layer" is defined as a 

frozen vitrified solid layer of the melt around the 

crucible-melt interface. This significantly 

prolongs the melter life. 

Microwave heating is a form of dielectric 

heating in which waste and glass-forming oxides 

are polarized in an alternating, microwave

frequency electric field. As the electric field 

alternates, distortion of the polarized molecules 

causes heat dissipation. 

Plasma heating is accomplished by 

converting gases, such as nitrogen, oxygen, 

noble gases, air or mixtures of these gases, into 

plasma by an electric arc. An electric arc can 

either be generated by direct current or 

alternating current. In a non-transferred arc 

process, plasma is generated by passing gas 

between the electric arc that is generated 

between two internal electrodes within the 

plasma torch. In a transferred arc process there 

is only one electrode in the torch while the other 

electrode is the waste material itself. Plasma is 

generated as the gas passes through an electric 

arc between the torch and the waste material. In 

electric-arc heating, which is slightly different 

from plasma heating, heat is transferred to the

waste and glass-forming oxides by ionized gases 

generated by creating an electric discharge 

between two electrodes. 

In summary, selection of glass 

composition, type of melting technology and 

pretreatment requirements depends largely on 

the chemical composition and the characteristics 

of the waste, and the final set of waste form 

properties required for disposal. 

A.7 References 

Hrma, P.R. et al. Property/Composition 

Relationships for Hanford High-Level 

Waste Glasses Melting at 

1150 °C-Volume 1: Chapters 1-11. 

PNL-10359. Volume 1. Richland, WA: 

Pacific Northwest National Laboratory. 

1996. 

Hench, L. L. The 70s: from selection of alternate 

waste forms to evaluation of storage 

system variables. Ceramic Transactions 

61. Westerville, OH: American Ceramic 

Society. 1995. 

Varshneya, A. K. Fundamentals of Inorganic 

Glasses. San Diego, CA: Academic Press, 

Inc. 1994.

A-3 NUREG/CR-6666
APPENDIX B 

GLOSSARY

NUREG/CR-6666
Calcined waste-Powdered waste in which the 

salts such as nitrates, carbonates, and hydroxides 

have decomposed to their oxide form. 

Cold-cap--A semi-solid crusty layer of waste 

components floating on top of the molten glass 

in a melter. Cold-cap is responsible for 

minimizing volatilization of components such as 

Tc, Cs, Na, Li, K and B. 

Decontamination factor-The factor by which 

the concentration of radioactive contaminants is 

reduced, measured by the ratio of initial 

radioactivity to that after decontamination. 

Double-shell tank-The newer one-million

gallon underground waste storage tanks 

consisting of a concrete shell and two concentric 

carbon steel liners with an annular space 

between the liners. 

Float-glass-Flat glass that has been formed on 

molten metal. 

High-efficiency particulate air-A filter 

designed to achieve 99.995 percent minimum 

efficiency in the containment of radioactive 

particulates greater than 0.3 micrometer in size. 

High-level waste--HLW means: (i) irradiated 

reactor fuel; (ii) liquid wastes resulting from the 

operation of the first cycle solvent extraction 

system, or equivalent, and the concentrated 

wastes from subsequent extraction cycles, or

equivalent, in a facility for reprocessing 

irradiated fuel; and (iii) solids into which such 

liquid wastes have been converted. 

Neutralized current acid waste-The primary 

HLW stream from the PUREX process. It is a 

liquid waste, high in Cs, Sr, and TRU content, 

and is the most radioactive of the waste streams 

from the reprocessing facility. 

Salt cake-At Defense Waste Processing 

Facility, the high-level supernatant is 

concentrated by evaporation beyond saturation. 

The concentrate is then transferred to the tank 

and is called salt cake. The salt cake and its 

saturated solution contain most of the 

radioactive Cs-137 and traces of Sr-90. At 

Hanford, crystallized nitrate and other salts 

deposited in HLW tanks, usually after active 

measures are taken to remove moisture. 

Skull layer-Layer of cold glass formed 

between crucible and molten glass in a 

cold-crucible melter. 

Sludge-Solids consisting mostly of precipitated 

metal hydroxides except alkali metals. These 

solids are formed after waste neutralization with 

sodium hydroxide solution. 

Transurauic waste--Wastes containing 

elements with atomic numbers greater than that 

of uranium.

B-1 NUREG/CR-6666
NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER 

(2-89) (Assigned by NRC, Add Vol., Supp., Rev., 

NRCM 1102. and Addendum Numbers, If any.) 

3201, 3202 BIBLIOGRAPHIC DATA SHEET 

(See instructions on ft reverse) NUREG/CR-6666 

2. TITLE AND SUBTITLE CNWRA 98-005 

Survey of Waste Solidification Process Technologies 3. DATE REPORT PUBLISHED MONTH YEAR 

February 2001 

4. FIN OR GRANT NUMBER 

J5164 

5. AUTHOR(S) 6. TYPE OF REPORT 

V. Jain Technical 

7. PERIOD COVERED (inclusive Dates) 

3/1/97 - 4/1/98 

8. PERFORMING ORGANIZATION - NAME AND ADDRESS (If NRC, provide Division, Office or Region, U.S. Nuclear Regulatory Commission, and mailing address; if contractor, 

provide name and mailing address.) 

Center for Nuclear Waste Regulatory Analyses 

Southwest Research Institute 

6220 Culebra Road 

San Antonio, Texas 78238-5166

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (If NRC, tye 'Same as above; i#contractor, provide IRGt• vmwn•, U•.lea Reutr •.... commission,... ......... 

and mailing address.) 

Division of Fuel Cycle Safety and Safeguards 

Office of Nuclear Material Safety and Safeguards 

U.S. Nuclear Regulatory Commission 

Washington, DC 20555-0001

10. SUPPLEMENTARY NOTES

R.E. Shewmaker, NI-t Project Manager

11. ABSTRACT (200 words or less)

This report provides a current status of the high-temperature solidification technologies that have been used or proposed 

in the United States and abroad. The technologies presented in this report can be broadly classified into the following 

categories: Inconel®-based,joule-heated melters; high-temperature,joule-heated melters; induction melters; cold-crucible 

induction melters; plasma melters; combustion melters; microwave melters; molten metal technology; and Synroc 

technology. The operating experience and safety issues associated with major solidification technologies are summarized. 

12. KEY WORDSIDESCRIPTORS (List words or phrases that will assist researchers in locating the report) 13- AVAILABILITY STATEMENT 

Glassunlimited 

Glass 14. SECURITY CLASSIFICATION 

Vitrification TsPae 

High-Level Waste unclage) 

Low-Level Wasteunclassified 

Melters (This Report) 

Tank Waste Remediation System unclassified 

Hanford 15. NUMBER OF PAGES 

16. PRICE

NRC FORM 335 (2-89)
Federal Recycling Program
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NUCLEAR REGULATORY COMMISSION 

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PENALTY FOR PRIVATE USE, $300