NUREG-1703

Characterization of

Radioactive Slags

U.S. Nuclear Regulatory Commission

Office of Nuclear Regulatory Research

Washington, DC 20555-0001
 NUREG-1703

Characterization of

Radioactive Slags

Manuscript Completed: February 2004

Date Published: October 2004

Prepared by

L.A. Veblen1,2

D. Farthing2

E. O’Donnell1, J.D. Randall1

1

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

2

Department of Earth and Planetary Sciences

Johns Hopkins University

Baltimore, MD 21218

 Division of Systems Analysis and Regulatory Effectiveness

 Office of Nuclear Regulatory Research

 U.S. Nuclear Regulatory Commission

 Washington, DC 20555-0001 
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iii

Abstract

Slag samples from three humid region sites located in the Eastern United States were studied in detail. Since this

investigation is generic in nature, the precise locations of the sites are not identified and are referred to as Sites A, B,

and C. Site A's slag was formed from a reprocessed Nb-Ta slag that was ground, leached and deposited in settling

ponds which were allowed to evaporate. The resulting material was a "rock like," porous, altered material that

resembled a sandstone. Site B was the most intensively studied site and it provided valuable information on the

physical and chemical form of the original Nb and Ta slag, as well as the Sn slag that was used as the "ore" material. 

Site C's slag, although not studied in as much detail as the Site B slag, is physically similar to the Site B slag and

both sites had (i) glassy slag and (ii) dense dark slags that ranged from fine grained to very coarsely crystalline. 

Detailed microanalytical data from Sites B and C provide information regarding the degradation of the

radionuclide-bearing phases. Uranium and thorium are present in glass, perovskite, calzirtite, zirconolite, and

pyrochlore. Cerium and thorium are present in hibonite. These phases are similar to phases in the proposed

high-level radioactive waste form SYNROC. Numerous studies provide experimentally determined solubility and

leach rates for these phases. These data provide insight into and bounding values for estimates of slag leach rates. 

Data reduction and a detailed petrological study provide the necessary information for guidance to the NRC's Office

of Nuclear Material Safety and Safeguards for license termination at future decommissioning sites.

v

Table of Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

1.0 Introduction ......................................................................... 1

1.1 Background .................................................................. 1

1.2 Objective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.3 Scope of the Research ......................................................... 1

1.4 Radioactive Source Term . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.5 Description of Site A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.6 Description of Site B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.7 Description of Site C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.0 Slag Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Introduction .................................................................. 7

2.2 Hand-size Sample Observation and Light Microscopy: Methods and Results ................ 7

2.2.1 Site A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.2 Site B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.3 Site C: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3 Bulk Chemical Analyses of Slag - X-ray Fluorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.3.1 Results of Bulk Chemical Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.4 Mineral Identification by X-Ray Diffraction: Methods and Results . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4.1 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4.2 X-Ray Diffraction of Site A Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4.3 X-Ray Diffraction of Site B Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.4.4 X-Ray Diffraction of Site C Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.5 Quantitative Analysis by Scanning Electron Microprobe and Electron Microprobe Analysis . . . . 10

2.5.1 Scanning Electron Microprobe and Electron Microprobe Analysis - Sample Selection and

Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.5.2 Scanning Electron Microprobe and Electron Microprobe Analysis - Analytical Methods

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5.3 Scanning Electron Microprobe and Electron Microprobe Analysis - Quantitative Analyses

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5.3.1 Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5.3.2 Spectrometer Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5.3.3 Quantitative Wavelength Dispersive Spectrometer (WDS) Analysis . . . . . . . 11

2.5.4 Scanning Electron Microprobe and Electron Microprobe Analysis Results - Site A . . . 11

2.5.5 Scanning Electron Microprobe and Electron Microprobe Analysis - Results Site B . . . 12

2.5.6 Scanning Electron Microprobe and Electron Microprobe Analysis Results - Site C . . . 12

2.6 Transmission Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.7 Surface Area Analyses: Methods and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.8 Petrology of the Slags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.0 Slag Weathering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.2 Estimating Radionuclide Release from Slags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.2.1 Estimating Leach Rates as Mass Fluxes at the Slag Surface . . . . . . . . . . . . . . . . . . . . 49

3.2.2 Release to the Surrounding Fluid and Deposition onto the Slag Surface . . . . . . . . . . . 50

3.2.3 Calculation of Bulk Leach Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.2.4 Estimation of Mass Available for Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3 Naturally Weathered Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

3.3.1 Natural Weathering Features at Site A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.3.2 Natural Weathering Features at Site B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

3.3.3 Natural Weathering Features at Site C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3.3.4 Estimation of Weathering Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
vi

3.3.5 Distinguishing Between Primary and Secondary Phases through the Use of Artificial Melts

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.4 Artificially Weathered (Leached) Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

3.5 Archaeological Slags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.5.2 Dating of Archaeological Slags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

3.5.3 Archaeological Slags Investigated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.5.3.1 Tin Slag from England and Malaysia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.5.3.2 Copper Slag from Cyprus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.5.3.3 Lead and Silver Slag from the Czech Republic . . . . . . . . . . . . . . . . . . . . . . . . 56

3.6 Summary of Weathering Mechanisms of Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

4.0 Conclusions and Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.1 Site Specific Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.1.1 Phases Present in Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.1.1.1 Site A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.1.1.2 Site B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.1.1.3 Site C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.1.2 Weathering in Slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

4.2 Generic Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

4.3 Recommendations and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

5.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

Appendix: Surface Mass-Transfer Coefficients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

A.1 Fluid Flow Regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

A.2 Idealizing Slag Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

A.3 Stagnant Fluid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

A.4 Very Slow Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

A.5 Laminar Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

A.6 Turbulent Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
vii

List of Figures

Figure 1-1. Sampling buried slag at Site B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 1-2. Site A samples. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Figure 1-3. Hand sample of Site B slag. ......................................................... 5

Figure 1-4. Hand sample of Site B slag. ......................................................... 5

Figure 1-5. Top of slag pile at Site C. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Figure 1-6. Close up of slag found at the ground surface at Site C. .................................... 6

Figure 2-1. Photomicrographs of thin section of Site A slag sample A-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Figure 2-2. Photomicrographs of thin section of Site A slag sample A-2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Figure 2-3. Photomicrographs of thin section from Site B slag sample B-8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

Figure 2-4. Photomicrographs of thin section from Site B slag sample 96TC-19. . . . . . . . . . . . . . . . . . . . . . . . . . 33

Figure 2-5. Photomicrographs of thin section from Site B slag sample B-8 showing glass, gehlenite (Ca2

Al(Al,Si)O7), and perovskite (CaTiO3). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Figure 2-6. Photomicrographs of thin section from Site B slag sample B-8 showing perovskite (CaTiO3) dendrites. 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Figure 2-7. Back scattered electron (BSE) image of a Sn slag (B-5.5) from Site B showing evidence of

devitrification and alteration of glass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Figure 2-8. Back Scattered Electron (BSE) image of Site C slag sample C-1a. . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Figure 2-9. Photomicrographs of Site C slag sample C-S in plane polarized light and BSE. . . . . . . . . . . . . . . . . 38

Figure 2-10. Ternary diagram of x-ray fluorescence data showing chemical composition for all of the slag collected.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

Figure 2-11. Compositional variation diagrams illustrating the variation of select elements when compared with TiO2.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Figure 2-12. X-ray diffraction patterns of Site A slag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Figure 2-13. X-ray diffraction patterns of Site B slag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Figure 2-14. Back Scattered Electron (BSE) image and Energy Dispersive Spectra (EDS) patterns of Site A sample

96F-A-4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Figure 2-15. Back Scattered Electron (BSE) and x-ray map images of Site B slag sample B-5.5. . . . . . . . . . . . . . 44

Figure 2-16. Site B tin slag Sample B-8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Figure 2-17. Ion-milled Site B sample B-8. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Figure 2-18. HRTEM image of Site B tin slag (sample B-8). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Figure 2-19. Phase diagram of the ZrO2 - zirconolite - TiO2 + Ca Ti O3 system (after Xu and Wang, 2000). . . . . . 48

Figure 3-1. Weathered Slags from Sites A and B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Figure 3-2. Photomicrographs of weathered slag under plane polarized light. . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Figure 3-3. SEM images showing weathering features in Site B sample B-5.5. . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Figure 3-4. TEM images of Site B tin slag B-8 showing weathering on a sub-microscopic scale. . . . . . . . . . . . . 62

Figure 3-5. Back scattered electron (BSE) image of weathered Site B slag sample 96TC-19. . . . . . . . . . . . . . . . 63

Figure 3-6. Microprobe analyses of Site B sample 96TC-19 showing variation in rare earth elements (REE) as a

function of proximity to distance from a fracture. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Figure 3-7. Schematic diagram illustrating location of elemental variation measurements from Site B sample B-5.5

as a function of proximity to a weathered edge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Figure 3-8. Chemical changes associated with weathering. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Figure 3-9. Compositional variation diagrams for Site B weathered glass. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Figure 3-10. Electron microprobe analyses of Site B samples showing the elemental variation in calzirtite as a

function of distance from a weathered edge. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Figure 3-11. Photomicrograph of a weathered edge of a dense blocky slag (sample C-2a) from Site C. . . . . . . . . 71

Figure 3-12. BSE image of remelted Site B slag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72

Figure 3-13. SEM secondary electron images of Site B Sn slag (B-5.5) showing surface of the slag leached by

PNNL with deionized water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Figure 3-14. SEM secondary electron images of Site B Nb-Ta slag sample 96TC-19 after leaching by PNNL at pH

0.5 and pH 2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Figure 3-15. Elemental variation of glass in unleached and leached Site B sample 96TC-19. . . . . . . . . . . . . . . . . 75

Figure 3-16. Elemental variation of hibonite in unleached and leached Site B sample 96TC-19. . . . . . . . . . . . . . . 76

Figure 3-17. SEM secondary electron images of Site B slag sample 96TC-19 following leaching by PNNL at pH 2

and 4. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Figure 3-18. SEM secondary electron images of Site B slag sample 96TC-19 after leaching by PNNL at pH 6, and

12. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Figure 3-19. SEM secondary electron images of Site B Nb-Ta slag sample 96TC-19 contrasting a leached sample

with an unleached one. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Figure 3-20. BSE image of thorianite grains from Site A slag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

Figure 3-21. Crift Farm tin slags, Cornwall, England. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Figure 3-22. Pribram slag pile, Czech Republic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Figure 3-23. Photomicrographs of 1,000 year old Cyprus slag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
viii

Figure 3-24. X-ray diffraction spectrum of Cu slag from Cyprus (using CuKα radiation). . . . . . . . . . . . . . . . . . . . . 83

Figure 3-25. BSE images of weathered edge of Site C sample C-1a. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

Figure 3-26. Photomicrographs of Site C slag sample C-1a in plane polarized light. . . . . . . . . . . . . . . . . . . . . . . . 85

Figure 3-27. Hypothetical plot of a slag’s surface-concentration gradient. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
ix

List of Tables

Table 2-1 Hand Specimen Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 2-2 Measurements of Samples with Detectable Radioactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 2-3 Phases Present in Samples from Sites A, B, and C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Table 2-4 Chemical Composition of Major Elements in Slag Samples (weight %) as determined by XRF. . . . . . 21

Table 2-5 Chemical Composition of Trace Elements in Slag Samples (ppm) as determined by XRF. . . . . . . . . . 22

Table 2-6 List of minerals indicated by X-Ray Diffraction analysis of Site B Sample B-5.5a. . . . . . . . . . . . . . . . . 23

Table 2-7 Modal Abundance of Phases in Representative Site A Slag Sample as determined by Electron

Microprobe Analysis (EMPA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Table 2-8 Average Chemical Composition (weight %) of Most Common Phases Site A Samples as Determined by

Electron Microprobe Analysis (EMPA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Table 2-9 Modal Abundance of Phases in Representative Samples from Sites B and C Slag as determined by

Electron Microprobe Analysis (EMPA). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 2-10 Average Chemical Composition (weight %) of Most Common Phases Site B Samples as Determined by

Electron Microprobe Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Table 2-11 Average Chemical Composition (weight %) of Most Common Phases Site C Samples as Determined by

Electron Microprobe Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Table 2-12 Weight Percent of Major Phases in Slag Samples and their Uranium, Thorium and REE (rare earth

elements) abundance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Table 3-1 Table for Estimating Leach Rate at Site B Based on Mineral Abundance in the Slag Samples. . . . . . 58

Table 3-2 Archaeological Slags investigated in this study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
1

Pickett, et al., 1998, Letter report: “Evaluation of Leach Tests for Estimating Release from Slag Sites subject to

Decommissioning - Final Report.”

x

Executive Summary

The US Nuclear Regulatory Commission (NRC) issues licences to possess nuclear material for various purposes,

one of which is the extraction of non-radioactive metals from ores that contain radioactive elements such as uranium

and thorium. Some of the materials licensees are no longer extracting metal from radioactive ore and have asked

the NRC to terminate their licenses. To terminate the licenses the NRC has to consider whether the slags left

behind from the extraction processes have sufficiently low levels of radiation, or if the radioactive material is

sequestered sufficiently to meet its license termination requirements. 

The authors of this report used a suite of microanalytical tools at the Johns Hopkins University’s Department of Earth

and Planetary Sciences to characterize the radioactive slags, physically and chemically, so that realistic

determinations of uranium and thorium leach rates from the slags can be made. Typically, leach rates are

determined experimentally on either crushed or intact materials, using standardized tests such as ANSI/ANS 16.1,

EPA 1311, or EPA 1312 leach tests. It has been reported1

 that experimentally determined leach rates range over

four orders of magnitude. An objective of the study described herein was to reduce some of the uncertainty inherent

in experimentally determined leach rates by providing the necessary detailed physical and chemical characterization

of the slags to determine “realistic” leach rates. Information provided by this study will enable NRC decision makers

and their licensees to calculate either a uranium and thorium solubility release model or a multi-phase leach rate

model, and to compare estimated releases with those determined “in-situ” from quantitative electron microprobe

analysis. 

Three humid region sites located in the Eastern United States were studied in detail. Since this investigation is

generic in nature, the precise locations of the sites are not identified and are referred to as Sites A, B, and C. Site

A’s slag was formed from a reprocessed Nb-Ta slag that was ground, leached and deposited in settling ponds which

were allowed to evaporate. The resulting material was a “rock like,” porous, altered material that resembled a

sandstone. Site B was the most intensively studied site and it provided valuable information on the physical and

chemical form of the original Nb and Ta slag, as well as the Sn slag that was used as the “ore” material. Site C’s

slag, although not studied in as much detail as the Site B slag, is physically similar to the Site B slag and both sites

had (i) glassy slag and (ii) dense dark slags that ranged from fine grained to very coarsely crystalline. Detailed

microanalytical data from Sites B and C provide information regarding the degradation of the radionuclide-bearing

phases. Uranium and thorium are present in glass, perovskite, calzirtite, zirconolite, and pyrochlore. Cerium and

thorium are present in hibonite. These phases are similar to phases in the proposed high-level radioactive waste

form SYNROC. Numerous studies provide experimentally determined solubility and leach rates for these phases. 

These data provide insight into and bounding values for estimates of slag leach rates. Data reduction and a

detailed petrological study provide the necessary information for guidance to the NRC’s Office of Nuclear Material

Safety and Safeguards for license termination at future decommissioning sites. 

The quantification (by electron microprobe analysis) of uranium and thorium depletion or enrichment in the

weathered surface of the slags provides estimates of short-term leach rates. A study of archaeological slags was

undertaken to quantify the weathering of slag over the 1000-year period of interest to decision makers. Information

from the short-term leach rates and estimated “in-situ” leach rates from the archaeological slags at various times

provides a measure of how the leach rates change as a function of time. The leach-rate histories can also be used

in decommissioning performance assessment computer programs codes such as RESRAD. 

The authors investigated weathering (dissolution and alteration) mechanisms by measuring the elemental variation

in naturally weathered decommissioning and archaeological slags and determined that the slags weather

incongruently, i.e. certain phases dissolve preferentially. It was found that weathering occurs along grain

boundaries and within particular grains and provides more penetrating access for fluids to the interior of the slags. It

is possible that over time the slags will degrade chemically, while remaining semi-intact physically, similar to a

saprolite. The authors’ data suggest that the thorium, and perhaps uranium, will remain in the resistant (refractory)

phases. Future decommissioning studies could collect x-ray fluorescence chemical data from the slag and compare

it to data from this study. If the new data fall within the range plotted for Sites B and C, then it could be assumed that

the new slag would have similar weathering properties and leach rates to the Site B and C slags. This would

eliminate the need for more detailed characterization.
xi

Foreword

The US Nuclear Regulatory Commission (NRC) issues licenses to possess nuclear material for

various purposes, one of which is the extraction of non-radioactive metals from ores that

contain radioactive elements, predominantly uranium and thorium. The leftover material from

the extraction process (slag) is enriched in radioactive elements because of the extraction of

the non-radioactive elements. Once licensees are no longer extracting metal from radioactive

ore, they will seek to terminate their licenses. If the slags are not removed, the radioactive

material in them may contaminate groundwater and surface water and may provide a health

hazard. Before terminating a license, the NRC will consider whether any slags left behind from

the extraction processes have sufficiently low levels of radiation, and if the radioactive material

is sequestered sufficiently to meet NRC license termination requirements. Staff from the NRC’s

Office of Nuclear Regulatory Research conducted the research described in this report to

provide technical support for these decisions.

Uranium and thorium are not uniformly distributed within the slags, but are present in various

mineral phases. A suite of tools was used in this research project to characterize the

radioactive slags, physically and chemically, so that realistic determinations of uranium and

thorium leach and surface-deposition rates from the slags can be made. Typically, leach rates

are determined experimentally on either crushed or intact materials, using standardized tests. 

An objective of the study described herein was to reduce some of the uncertainty inherent in

experimentally determined leach rates, by providing detailed physical and chemical

characterization of the slags to determine more "realistic" leach rates. This study provides

methods for obtaining characterization data needed to determine whether any slags remaining

at a site will meet license-termination criteria.

This report summarizes the results of research to characterize the crystalline and

non-crystalline phases in slag samples from several NRC-regulated sites that are being

decommissioned and contain isotopes of uranium or thorium. The slag was collected from

three humid region sites in the Eastern United States. The purpose of this study was to identify

the uranium- and thorium-bearing phases within the slags, and to evaluate the long-term

stability of those phases. The results of this investigation give insights into the basic weathering

mechanisms affecting the slag and the possible fate of the uranium and thorium radioisotopes.

Farouk Eltawila, Director

Division of Systems Analysis and Regulatory Effectiveness

Office of Nuclear Regulatory Research

1

1.0 Introduction

1.1 Background

The US Nuclear Regulatory Commission (NRC) 

issues licences to possess nuclear material for

various purposes, one of which is the extraction of

non-radioactive metals from ores that contain

radioactive elements such as uranium and thorium. 

Some of these firms are no longer extracting metal

from radioactive ore and have asked the NRC to

terminate their licenses. NUREG-1444 (Fauver et al.

1993 and 1995) identifies as many as 17 sites

containing radioactive slags.

The uranium and thorium in the slags described in

this report were by-products of smelting operations

that extracted niobium, tantalum, and tin. Various

waste products were disposed on the sites, and in

some cases moved off site. Solid and liquid waste

were present at some of the sites. Liquid waste

included waste water and liquids associated with

leaching processes. Solid wastes included slags,

both in their original form and also in leached,

crushed and settled forms. Three sites were

investigated in detail by the authors and are referred

to in this report as Sites A, B, and C.

Because Sites A, B, and C, are in populated areas,

and their slags tend to contain higher levels of

radioactivity than the surrounding soil, it is important

to determine the potential for release of radioactivity

from the sites. To make such determinations, one

must consider the weathering and leaching properties

of the slags. These properties determine the rates at

which elements may be released to the environment. 

The weathering properties of a slag depend on its

physical, chemical, and mineralogical composition. 

These factors, in conjunction with local climatic

conditions, control the rate at which uranium, and

thorium are released from the slag.

The NRC assessment of a licensee’s application to

decommission a site and subsequent termination of

the applicant’s license relies on performance

calculations that require consideration of the rates of

leaching of uranium and thorium from slags

containing them. Site characterization reports

submitted by licensees for Sites A, B, and C provided

information on the radiological assessment of the

slags and other waste materials at the sites. 

However, the site characterization reports contain no

information about the actual physical, chemical and

mineralogic composition of the slags. These are the

properties which will determine how easily the slags

might leach into the environment.

1.2 Objective

The purpose of this study is to identify the phases

within slags that contain uranium and/or thorium and

to determine the long-term stability of those phases. 

1.3 Scope of the Research

This is a report on the materials characterization of

radioactive slags with emphasis on three of the sites

identified in NUREG-1444. Preliminary analyses

done at the Johns Hopkins University indicate that

these slags are mixtures of glass and crystalline

phases which are unusual in terrestrial rocks. This is

due to the fact that the slags are the products from

processing exotic ores and the reprocessing of Sn

slags at extreme values of temperature and partial

pressure of CO2. The slags are composed of

minerals and glasses which resemble more closely

lunar chondrites and meteorites than terrestrial

basalts or Fe or Cu slags. 

Determining the long-term performance of these

unusual suites of minerals and glass is the focus of

this report. To determine the long-term performance

of the slags under weathering conditions typical of the

sites, it is necessary to determine the crystalline and

non-crystalline phases in the slags. This information

can be linked with (a) results from laboratory leach

tests based on ASTM standards, and (b) information

from long-term weathering behavior of archaeological

slags of similar chemical composition to understand

fully the implications of weathering on the slag and its

associated phases. 

This research includes mineralogical and chemical

data collected on samples from these sites. Much of

the analytical work that characterized the crystalline

and non-crystalline phases was done by L. Veblen

(NRC/RES) and D. Farthing (Johns Hopkins

University), at the Johns Hopkins University,

Department of Earth and Planetary Sciences as a

cooperative effort involving The NRC’s Offices of

Nuclear Regulatory Research (RES) and Nuclear

Material Safety and Safeguards (NMSS), the Pacific

Northwest National Laboratory (PNNL), and the

NRC’s Center for Nuclear Waste Regulatory

Analyses (CNWRA). 

A complete site characterization program for the

three sites was beyond the scope of this project and

the focus was on collecting material which exhibited

radioactivity in the field. A scintillometer was used for

this purpose. Determining the long-term performance

of an entire slag pile is complicated by the slag’s

compositional heterogeneity. Slag piles are usually a

mixture of soil, slag, and other waste. The slag

pieces range in size from small cm-sized pieces to

large cobbles 0.25 to 1.0 m in diameter. Based on

sampling from this study, the larger pieces of slag

were found to be the most radioactive materials on

site. The slag at Site A was placed in mounds with

minimal soil cover. Thus it lent itself to ready

sampling. Sampling at Sites B and C was more

difficult because of partial revegetation of the sites

and because the slag was covered with

approximately 0.3 to 1.0 m of soil and construction

debris. At Site B it was necessary to dig a small pit to

a depth of approximately 1.5m to collect slag samples

(Figure 1-1). 
2

Leaching studies of the slags were done by the

CNWRA2

. CNWRA analyzed slag leaching by

comparing results from different standard leach tests

[ANSI/ANS 16.1, 1986, EPA Method 1311 (TCLP),

1995, and EPA Method 1312 (SPLP), 1995]. The

CNWRA results show that the tests rank in the

following descending order of chemical

aggressiveness: TCLP > SPLP > ANSI/ANS. It

should be noted that the leach rates of uranium and

thorium vary by four orders of magnitude among the

three tests for the same slag. 

Felmy et al. (2003), at PNNL performed leaching and

solubility studies on the same slag samples as the

CNWRA. The PNNL slags were leached in solutions

with pH values ranging from 0.5 to 12 which provided

important insights on the weathering of some of the

slag samples. 

To determine the long-term performance of the slags,

archaeological slags, similar to the decommissioning

slags, were collected from Cornwall, England, the

Czech Republic, and Cyprus (see Section 3.4,

“Archaeological Slags”). Information about how these

older slags have weathered will aid in assessing the

stability of the radioactive slags. Analytical work on

the archaeological slags was done by Dori Farthing

as a doctoral dissertation at Johns Hopkins University

(Farthing, 2002). 

1.4 Radioactive Source Term

Given that there are limited records on materials and

slag composition at these sites, it is difficult to

determine the source term. To quantify the

radionuclide source term it is necessary to gather

information regarding (1) the type and amount of ore

(source material), flux and any additional materials

used during the smelting, and (2) the smelting

process used at a site. From this information a

general characterization of the slag can be estimated. 

It is beyond the scope of this project to examine in

detail the materials used or their amounts. In

Sections 1.5, 1.6, and 1.7 which follow we shall

address in general terms the materials on site and

the smelting processes used at the three SDMP sites.

With regard to a source term, Sites A, B, and C

processed niobium and tantalum, but each used a

different process for the beneficiation and extraction

from the primary ores and/or tin slags. Accessory

phases rich in uranium, thorium, and rare earth

elements (REE), are part of the original ore, and are

concentrated in the slag which was generated by the

smelting process. These elements are incompatible

with the metallic portion of the smelt and are residual

products that are concentrated in slag. 

1.5 Description of Site A

Site A is no longer producing Nb-Ta slags and

subsequent to our sampling of the site in 1997 the

slag was transferred to a licensed radioactive waste

disposal site. 

Site A produced ferro-columbium concentrate with

ore from the Araxa mine in Brazil between 1963 and

1969. The process was an exothermic or

aluminothermic reaction in which Fe-Nb ore was

crushed and combined with Al powder, FeSi, and

Fe2O3. The mixture was ignited to produce a Fe-Nb

tap slag and Fe silicate waste slag.

The tap slag, which contained most of the Fe and Nb,

was separated from the silicate slag by crushing. The

silicate slag was then ball-milled, and water floatation

was used to separate any remaining Fe and Nb from

the slag. The waste slurry was sent to

settling/evaporation ponds, and the leachate was

recycled to the floatation slurry. Most of the Th

remained in the waste slag and ended up in the

settling/evaporation ponds, where the solid material

settled and was lithified through natural cementation

processes as the settling ponds evaporated. The

resulting slag resembles a cross-bedded sandstone. 

A reddish-brown weathered surface was observed on

samples which had been exposed to the atmosphere

and this is found in the sample illustrated in Figure 1-

2. Figure 1-3 is a photograph of this slag showing

the features that one commonly associates with

sedimentary rocks.

Prior to the removal of the radioactive material from

Site A, the radioactive material was mounded into

piles approximately 5 - 8m (15 - 25 ft) high. The

piles, located at the sites of the settling/evaporation

ponds, were covered by grasses, wild flowers, and a

20 cm soil veneer. The piles contained the

sandstone-like waste material and “failed taps” or

batches of melt that failed to produce the dense

metallic Fe-Nb layer, but instead formed a “lava-like”

mass in which the Fe-Nb was distributed throughout

the mass. The plant would have about one failed

smelt during each year. 

1.6 Description of Site B

Site B is located within 100m of a river. A slag pile

lies to the west of a decommissioned fabrication

warehouse, several hundred yards from residential

buildings. The slag pile forms a steep slope and the

pile is covered with some soil, trash, and numerous

plants and trees. The size of the slag blocks is 0.3 to

1.0 m in diameter. Hand specimens of slag collected

from the pit shown in Figure 1-1 are illustrated in

Figures 1-3 and 1-4. One sample exhibits a glassy

texture and it probably represents the outer surface of

a cooling block. The other is fine grained and

probably represents the center of a cooling block.

NRC staff made three site visits to Site B to collect

samples. The first visit of staff from RES and Johns

Hopkins University (JHU) was in September 1996. 

The second was in November, 1996, with staff from

PNNL, RES, and JHU. The third was in October,

2

 Pickett, et al., 1998, Letter report: “Evaluation of

Leach Tests for Estimating Release from Slag Sites

subject to Decommissioning - Final Report”.
3

1997, with NRC staff from RES and NMSS and the

CNWRA. 

During the first visit, it was noted that the site was

covered with tall grass, weeds, wild sunflowers,

young sycamore trees (~5 years old), older (~20

years old) sycamore, oak and maple trees, bushes,

and a wide range of debris: concrete slabs, asphalt,

wood, paper, etc. Slag samples were collected from

the surface of the pile, at the slope edge, and in the

pit dug at the location of a licensee’s borehole. The

borehole was selected because in a 1996 site

characterization report, this particular borehole was

reported to have some of the most radioactive

material at the site from a depth of 1.5 to 3.7 m. To

collect some of this material, a 1.5 m hole was dug,

during the 1996 site visit to sample buried, near

surface material. In this hole, slag, soil, concrete,

asphalt, brick and wood from the decommissioned

floor of the foundry were found to a depth of

approximately 1.2 to 1.5 m. The slag found at the site

and in this hole varied in texture and in size, ranging

from large blocks 0.5 m in diameter to sand-sized

grains. The Site B slag will be described in greater

detail along with the samples from Sites A and C in

Section 2.2, “Hand Sample Observation and Light

Microscopy: Methods and Results.” The blocks

varied from glassy to dense, dark gray to black

blocks. The site’s soil also varied in appearance and

texture. Some soil appeared to be oxidized (reddish)

while other parts of the soil remained very dark gray

to almost black. Soil samples were collected and

analyzed for chemical composition. The slag pile was

not uniformly compacted, and many large voids exist

between many adjacent blocks of slag.

During the second sampling visit in November 1996

additional samples were collected from a borehole

and at the location of several of the licensee’s other

boreholes.

The third sampling visit to the Site B was made in

October 1997. Subsequent to the November 1996

sampling visit, the top of the site was bulldozed and

cleared of debris and vegetation. The same

boreholes previously sampled were enlarged at this

time. The first was excavated to a depth of 1.5 m

(Figure 1-2), and second to depth of 0.6 to 0.9 m. At

the depth of 0.6 to 0.9 m, the original B1 borehole

was found. B1 is 18 m deep and no water was found

in the borehole. Slag and soils were collected from

both holes.

A discussion of the method for extracting Nb and Ta

from tin slags by an exothermic fusion process can

be found in Gustison and Cenerazzo (1971). This

reference provided valuable information on the

material that constituted the source term for the slag

pile. The conditions of the Source Materials License

for Site B issued by the Atomic Energy Commission

in the 1960s authorized the on-site possession of ore

(i.e. Sn slag) not to exceed 2 weight % uranium and

thorium. This is consistent with our bulk chemical

analyses of the Site B samples by x-ray fluorescence

in which the highest concentration of uranium was

1980 ppm (~0.2%) (see Table 2-7).

1.7 Description of Site C

Site C is located on a river at the edge of a town. The

site was the location of a metallurgical processing

plant for ferro-niobium and ferro-nickel alloys that

operated in the 1960s. An aluminothermic melting

process was used to extract Nb-Ta from niobium ores

and nickel scrap that contained up to 1 wt% thorium. 

Natural and depleted uranium were some

contaminants of the metal scrap. Process slag

contained thorium, uranium, and Ra-226. Little

information regarding the smelting process is

available. It is assumed that the aluminothermic

melting process is similar to that of Site A, which is

well documented.

The slag is stored in four separate sections adjacent

to a river. The slag is partially buried under

approximately 0.5 m to 1 m of soil and construction

debris (see Figure 1-5). The materials from Site C

described in this report were collected by NMSS staff

in 1997 and by RES staff in April 2001. The 1997

samples were sent to CNWRA, where they were

divided and a portion of the samples was sent to the

Johns Hopkins University for characterization of the

phases. The CNWRA used their samples for leaching

studies (Pickett et al., 1998). The Johns Hopkins

portion of the samples will be described in further

detail along with the samples from Sites A and B in

Section 2.2, “Hand Sample Observation and Light

Microscopy: Methods and Results.”

Although much of Site C’s slag was covered, several

blocks of a dense dark gray slag similar in

appearance to some of the Site B slag were found at

the ground surface. The outer surfaces of the blocks

are highly weathered and this can be seen in Figure

1-6.

 A radiological characterization of the waste and slag

storage area, done for the licensee by a contractor in

1998, indicates that, generally, the slag has a much

higher level of radioactivity than the surrounding

materials. The radioactivity of the slag ranged from

0.3 pCi/g to 6,779 pCi/g, while the radioactivity of the

soil, water, and river sediment was on the order of 1-

20 pCi/g. Samples collected by RES staff in April

2001 averaged about 1000 pCi/g.
4

Figure 1-1. Sites B and C were covered with approximately 0.3 to 1.0 m of soil and construction debris. At Site B a

small pit was excavated on top of the site to collect slag samples. The large block being pointed to had a prominent

brownish weathering rind visible on the right side of the block.

Figure 1-2. Site A samples. Note the sedimentary features of these samples. This material was produced by

crushing, leaching, and deposition of the material in a settling pond. The dark layers are enriched in thorium. The

outer 1.2 cm top surface of the slag is weathered to a red-brown phase (Ba oxyhdydroxides). (Photograph courtesy

of CNWRA).
5

Figure 1-3. Hand sample of Site B slag. This sample is dominated by glass and is apparently a cooling surface of

the slag. (Photograph courtesy of CNWRA).

Figure 1-4. Hand sample of Site B slag. This sample is more crystalline the one shown in Figure 1-3 indicating that it

was from the interior of a melt block. (Photograph courtesy of CNWRA).
6

 

Figure 1-5. Top of slag pile at Site C . The slag is found below the cleared area and it is covered with approximately

0.5 m to 1 m (1.5 ft to 3 ft) of soil and construction debris.

Figure 1-6. Close up of slag found at the ground surface at Site C. The interior is medium gray in color. The outer

rusty brown colored surface is indicative of the rapid weathering of these blocks when exposed to the atmosphere.
7

2.0 Slag Characterization

2.1 Introduction

Samples were analyzed by a number of methods to

determine the bulk chemical composition and the

phases present. No single method can be used to

identify all the phases present and their elemental

chemistry efficiently and definitively. For example,

light microscopy can sometimes be used for

identification of the larger crystalline and glassy

phases. However, the Scanning Electron

Microscope (SEM),Electron Microprobe Analyses

(EMPA), and Transmission Electron Microscopy

(TEM) are needed for identification of the phases

present in minute amounts. X-ray fluorescence is

useful for providing information on the bulk chemistry

but it can not determine the chemistry of individual

minute phases. Therefore, a combination of

analytical techniques is necessary to identify the

mineral and crystalline phases in the slag accurately.

Selected hand samples were observed under a

binocular reflected light microscope to determine

areas of interest for further investigation. Splits were

made of the samples. One portion was used for light

microscopy, SEM, EMPA, TEM. These methods

provide information on the possible phases present

and their chemistry. For these analyses, two

petrographic thin-sections were made. One was a

polished section which was used for light microscopy,

SEM, and EMPA. The other thin-section was used

for TEM and was made with Aremco Crystalbond 509

(a thermoplastic cement). The other portion of the

samples was powdered for X-ray diffraction (XRD)

and X-ray fluorescence (XRF) analyses. XRD

provided information on possible crystalline phases

present and XRF was used for bulk chemical

analyses of the samples. 

2.2 Hand-size Sample Observation and Light

Microscopy: Methods and Results

The physical appearance of hand-sized slag

specimens varied widely as shown in Table 2-1. The

greatest difference was between Site A and the other

two sites. As noted previously in Section 1.5,

“Description of Site A,” the Site A slag’s residual

waste, also commonly called “slag,” was formed by

lithification of a Th-bearing waste slurry originally

deposited in settling ponds that were allowed to

evaporate. As the ponds evaporated the waste slurry

was lithified by natural cementation processes. 

Because of this mode of origin, Site A “slag”

resembles a stratified sandstone in which angular to

sub-rounded grains are cemented together to form a

reddish brown to light tan porous, grain supported

sandstone as illustrated in Figure 1-2. In contrast,

the slags from Sites B and C more closely resemble

materials that formed from a melt. The Site B and C

slags ranged from glassy black, blue, or green blocks

similar to obsidian to dense, dark gray to black finely

crystalline phases, similar to basalt as shown in

Figures 1-3 and 1-4. The slags at all three sites

appear to weather rapidly when exposed to the

atmosphere and it is common to find light tan to

reddish brown weathering rinds on those exposed as

seen in Figure 1-6. The extent of the alteration will be

discussed in Chapter 3, “Slag Weathering.”

The first step in the hand specimen analysis was

measurement of sample radioactivity to determine

which samples merited closer investigation. This was

done in the laboratory at Johns Hopkins University with

a Biochron meter and a 2 inch pancake Geiger/Muller

probe. Results of the measurements of samples with

detectable radioactivity are shown in Table 2-2. The

highest measured doses were about 4.0 to 5.0

μSv/hour (400 to 500 μrem/hour). 

Petrographic thin sections were made of all the slag

samples. Transmitted and reflected light was used to

identify the glass and crystalline phases present and

their modal abundance was calculated by using a

JEOL Super Probe (EMPA). Silicates, silicate glass,

oxides, and metallic phases are present in all the slags

sampled, but to a differing degree depending on the

process and position relative to the outer surface

(cooling surface). A list of crystalline phases and their

chemical formulae is provided in Table 2-3. More

detailed descriptions of each site follow.

2.2.1 Site A:

As noted above, the Site A slag differs in physical

form from the slag found at Sites B and C. The Site A

slag formed by cementation of crushed slag which was

placed in settling ponds for extraction of metals

through leaching. The ponds were subsequently

drained and the resulting material which had solidified

was dumped on site. 

Common phases in the solidified material include

quartz grains, hibonite, gehlenite, perovskite, glassy

clasts, thorianite, and metallic phases (see Table 2-3

for the chemical formulas of the phases). These

phases are found in stratified layers where the darker

layers are more concentrated with metallic and oxide

phases and thus contain higher concentrations of

heavy elements (Figure 2-1). Gehlenite, glass clasts,

quartz, hibonite, and other silicates are more common

in the lighter bands. A 1.2-cm crust of red-brown

material coats the surface of the slag blocks. 

Microprobe analyses indicate that the crust is enriched

in Ba-oxyhydroxides which are believed to be a

product of weathering.

The phases observed in the settled slags are similar to

those observed in slags from Site B, namely,

gehlenite, perovskite, clinopyroxene, and zirconolite. 

However, thorianite is more prevalent in Site A

samples than in samples from the other sites. Site A

shares some of the same phases as Site C and it has

Ba aluminate, perovskite and zirconolite (Figure 2-2).

The cement that holds the grains together in the Site A

material is difficult to characterize because it is very

fine-grained and heterogeneous. Microprobe analyses

indicate that the cement contains Al, Ca, Ba, and

some Cl. 
8

The presence of large perovskite and thorianite

grains at Site A indicates that the site’s smelting

operations used a different source material from that

used at Site B. A likely source might be ore from

Brazil that often contains significant amounts of

thorite. It is also possible that the thorianite is a

secondary mineral formed by the breakdown of

pyrochlore during leaching. The presence of

thorianite as a secondary phase may be a means of

constraining the concentration of Th in the system,

with thorianite as the solubility-controlling phase.

2.2.2 Site B:

As noted earlier, soil and construction debris was

mixed in with slag blocks. The site’s soil varied in

appearance and texture. Differences in color

probably reflect the degree of exposure to the

atmosphere (oxidizing conditions). Soil color ranged

from reddish to reddish brown (most oxidized) to dark

gray to black. Slags at the site can be divided into

three groups based on physical appearance and

mineral composition will be described in the

paragraphs that follow.

The petrographic microscope was used for

examination of the rock fabric, weathering features,

and for identification of the phases present. The

dominant phases are gehlenite, glass, clinopyroxene,

perovskite, and calzirtite. To determine which phases

contained uranium and thorium, electron microprobe

analysis (EMPA) was used, and this will be described

in further detail in Section 2.5 “Quantitative Analysis

by Scanning Electron Microprobe and Electron

Microprobe Analysis. The uranium and thorium are

found in glass, perovskite, calzirtite, and in thorianite

and pyrochlore which were present in only a trace

amount (0.1%). 

Three different kinds of slag were collected from Site

B: (1) glassy slag that is grayish-green, (2) dense,

blocky, finely crystalline slag, and (3) loosely

cemented, dark gray blocks of crushed slag material. 

The dense, blocky, finely crystalline slag was the

most radioactive of the material found at Site B and is

the only one to be described here. The dark, dense,

crystalline slag is found as large angular blocks. 

These blocks might be fragmented remnants of a

melt similar in size to the melts produced at Site A:

approximately 2-3 m in diameter, and up to 0.5 m

thick. The dense blocky slags may also have been

“failed melts” in which the Nb-Ta did not coalesce and

sink to the bottom of the melt. The blocky slags are

similar in appearance to the Sn slags that were

dumped at the site. 

Within the dense, blocky slag, different textures are

observed. Surfaces that cooled quickly are glassy or

have very fine grained crystals present within the

glass (Figure 1-3). The crystal grain size increases

toward the interior of the slag due to the increased

time to cool to subsolidus temperatures. Grain size

affects the leach rate of the slag since grain

boundaries provide pathways for access by water. 

The blocky Site B slag has a thin (<20 mm) alteration

rind. 

Examination of the Site B samples under polarized

light with a petrographic microscope showed the

following: silicate glass (e.g. Figure 2-3), was found in

many samples and the glass fills interstices between

dendritic crystals of perovskite, zirconolite, rutile, and

euhedral crystals (crystals with a well defined shape)

of pyrochlore, gehlenite and clinopyroxene in Site B’s

dense, black, finely crystalline slag (samples B-5.5 and

RSP5.5).

In Site B sample 96TC-19 (Figure 2-4), the glass fills

the gaps between euhedral crystals of hibonite and

pyrochlore. The silicate minerals appear as large,

euhedral grains. Gehlenite, which is clear in plane

polarized light and highly birefringent under crossed

polars, engulfs small blue euhedral crystals of an

unknown phase (possibly hibonite), zirconolite and

perovskite dendrites (e.g. Figures 2-5 and 2-6). 

Gehlenite is also more abundant in the rapidly cooled

exteriors of the slag. Gehlenite often surrounds small

hibonite crystals, and glass. Hibonite (Figure 2-4), a

rare terrestrial mineral, with composition of CaAl12O19

is more abundant in the interior of the slag, where

higher temperatures persisted for a longer period of

time. Hibonite, a well-known phase in meteorites (e.g.

Keil and Fuchs, 1971; Smith, 1979) is thought to be

one of the first phases to crystallize from a solar

nebula at the formation of a solar system. Hibonite

comes in a variety of colors and has been used to

indicate oxygen fugacity at the time of crystallization

(Ihinger and Stolper, 1986). The hibonite in the Site B

slag is blue indicating highly reducing conditions at the

time of formation (oxygen fugacity ~10(10-12) atm).

A monoclinic phase, which is pleochroic and ranges

from light brown to green, appears to be an Al-, Ti-,

and Zr-rich clinopyroxene (Figure 2-3), similar to

fassaite (Ca1.10Mg0.39Ti0.48Al0.13(Al0.74Si1.26)O6), a phase

found in the Allende meteorite (Dowty and Clark,

1973). Wollastonite (CaSiO3) has also been identified

in sample 96TC-19.

Perovskite, pyrochlore, Fe prills, and zirconolite were

identified by reflected light microscopy. Perovskite

(CaTiO3) forms large opaque rectilinear dendrites in

transmitted light. Perovskite appears as blue

dendrites with 4-fold symmetry in transmitted light (e.g.

Figure 2-6). Pyrochlore, a Ca-Nb-Ta oxide, is found as

small cubic crystals surrounding the Fe prills (metallic

spherules). Zirconolite is observed as small (5 μm)

star shaped dendrites that are light brown in

transmitted light. Larger (10 μm) zirconolite dendrites

are opaque, but highly reflective.

Secondary phases which have probably formed as the

result of devitification and/or weathering are present in

the Site B slag. Anorthite feldspar (CaAl2Si2O8) is a

common devitrification phase, as seen in Figure 2-7. 

In addition, fine grained, feathery crystalline phases

are observed, but are too small (<5 μm) to identify

positively with the light microscope. Fine-grained

(<1μm) cerium silicates are present as a devitrification

or alteration product of the glass. Rutile needles
9

appear as a devitrification product as well. 

2.2.3 Site C:

Site C slags are heterogeneous in appearance and in

chemical composition. They fall into 3 distinct groups

based on mineral assemblage and physical

appearance. The uranium and thorium found at Site

C are concentrated in glass, calzirtite, and perovskite.

(1) Glassy Slags 

Slags in this group (e.g. sample C- 5) are highly

vitreous and often vesicular. Although all of the Site

C slags contain some glass, it is most prevalent in

samples from this group. Common crystalline phases

in this group include SiO2 (in the form of dendrites

and large rounded patches), fayalite (Fe2SiO4), and

phases containing Si, Ca, Al, Fe, K, Mn and Mg. The

SiO2 phases are likely to have crystallized originally

as tridymite or cristobalite. Tridymite and cristobalite

are high-temperature polymorphs of quartz. When

the slag cooled, the tridymite or cristobalite was no

longer stable and recrystallized as quartz which is the

phase stable at low temperature. This transformation

from a high temperature phase to a low temperature

phase involves a volume decrease and the effects of

this are seen by the prevalent cracks in the dendrites

and the large SiO2 patches (Figure 2-8)

(2) Dense Blocky Slags 

These slags are distinctly different from those in the

first group since they are more dense and blocky, and

also seem to have fewer vesicles. Slags in this

category are physically similar to basalts and to the

black, finely crystalline slag from Site B (e. g. 

samples RSP 5 and B-5.5). Hand sample (C-4) is

dark reddish-brown in color. Minerals found in this

group include hibonite, barium aluminate, periclase,

monticellite, Cr spinel, perovskite, zirconolite, 

metallic iron prills, wollastonite, wustite and an

alteration product, ardelite Ca2(SO4)2(HPO4).4H2O 

(Figure 2-9). .

(3) Green-Blue Slags

Slag in this group (e.g. C--6) is distinctly

greenish-blue in color and often has a 1 mm thick

white crust around the outside of the sample. Freshly

cut surfaces of this group’s slag reveal a

greenish-blue interior and small circular brassy

looking metallic prills are often visible to the naked

eye. Mineralogically, this group contains spinel,

periclase, monticellite, forsterite, perovskite,

zirconolite, a Ti-silicate, a Ni-oxide and metallic

chromium

Although the samples from Site C are distinct from

those studied from the Sites A and B, there are some

mineralogical similarities. For example, slag in the

first two groups both have large perovskite crystals

(Figure 2-9) as does slag from Site C.

At this site, two dominant radioactive slag types were

present. One is similar to Site A and B because it had

high aluminum phases (e.g. Ba aluminate, hibonite, or

Ca Al phases). In this slag both uranium and thorium

are in perovskite, pyrochlore, and Ti oxides such as

thorianite. The second Site C slag is rich in 2 spinels

and olivine. The dominant uranium and thorium 

bearing phases are glass, calzirtite, and perovskite

2.3 Bulk Chemical Analyses of Slag - X-ray

Fluorescence

Slags can be separated into types based upon their

bulk chemical composition. This provides a predictive

tool for regulators interested in determining leaching

characteristics of a slag from a particular site, without

necessarily going through an extended mineralogical

characterization. In the Ternary Diagram based on

chemical composition shown in Figure 2-10 the slags

and site soil can be seen to fall into groups. 

Bulk chemical analyses of the slags were obtained

using X-ray fluorescence (XRF) spectroscopy. These

analyses were done with a Philips PW2400

spectrometer at Franklin and Marshall College,

Lancaster, Pennsylvania. The results of the bulk

chemical analyses of the major elements from XRF

are reported in Table 2-4 and trace elements are in

Table 2-5. Major elements are recorded in weight

percent oxide (wt%) and trace element concentrations

in parts per million (ppm). 

Difficulties arise when analyzing slag samples,

because they are enriched in trace elements relative to

the standards used for “normal” rock analyses. The

relatively high concentrations of thorium and uranium

in the samples result in line overlaps with other

elements on the spectra (notably in electron shells

Zrkα, Ykα, Srkα, Nbkα, Bakα etc.) which causes a

problem in quantifying the other trace elements. Trace

elements which are present but have serious line

overlaps are represented by a question mark (?) in

Table 2-5. 

2.3.1 Results of Bulk Chemical Analyses

The slags sampled from the three sites vary in

chemical composition, but can be classified in terms of

major element oxides CaO, FeOT, Al2O3, and SiO2. 

Figure 2-10 depicts the range of slag compositions

within the (CaO + Na2O), (TiO2 + FeOT + Al2O3) , SiO2

ternary system. The samples cluster in three areas: 

(1) soils which are high in SiO2 ; (2) reprocessed slag

from Site A which is high in FeOT , Al2O3 ; and (3)

slags from Site B and Site C which have equal

concentrations of CaO, (FeOT + Al2O3) and SiO2. The

slag from Site B can be further categorized into a Sn

slag and a Nb/Ta slag that formed after the Sn slag

was re-smelted. 

It is also instructive to look at the compositional

variation diagrams representing the slag’s bulk

chemical composition (Figure 2-11). These diagrams

indicate that slags from Site B and C fall along a

continuum of composition. Sn slag (the original feed
10

stock for the smelting operations) is found at Site B,

thus contributing to the range of composition seen in

the ternary diagram (Figure 2-10). The composition

of slag from Site B plots within a tighter compositional

range than Site C. This indicates that either the

processing or the original stock material was more

homogeneous than materials used at Site C, or that

no original feed stock was disposed at these sites. It

is clear from Figure 2-10 that slag from Site A was

subject to different processing. Chemical

composition plotted in a ternary diagram such as

Figure 2-10 could be used for future slag analyses if

the slags plot within a similar range to Site B’s and

Site C’s slags.

2.4 Mineral Identification by X-Ray Diffraction:

Methods and Results

2.4.1 Methods

Powder x-ray diffraction was used in conjunction with

electron microprobe analyses to determine the

identity of minerals within the slags. EMPA was also

used to investigate the compositional variability of the

slag phases. XRD analyses were done with a Philips

diffractometer equipped with Materials Data, Inc.,

automation. Usually, a preliminary scan of a crushed

sample was run from 5° to 60° or 70° 2θ at a step

size of 0.05 degrees/step. A second, more detailed

scan with smaller step sizes and longer counting

times was then run. Phase identification was done by

peak matching the collected data to known

crystallographic data in the International Center for

Diffraction Data (ICDD) database using the software

program JADE (Materials Data, Inc.). Initially

samples of the “whole-rock” slag were analyzed,

however, the number of phases within the slag made

it difficult to interpret the powder x-ray diffraction

pattern of “whole rock” samples. (See Table 2-6

which shows the number of crystalline phases found

in a single slag sample.) Analyses of slag separates

were significantly easier to interpret. The separation

was done by hand and by density differences of the

individual phases. Some of the common crystalline

phases found include gehlenite, hibonite, metal

carbides, oxides of uranium and thorium, perovskite,

and zirconolite. 

2.4.2 X-Ray Diffraction of Site A Slag

XRD spectra for Site A, Figure 2-12, samples indicate

the presence of quartz, zirconolite, perovskite, metal

carbides and oxides of uranium and thorium. 

Numerous additional phases are present but have not

been verified. These spectra have not been

background subtracted and the flatness of the

spectra indicates the presence of little, if any glass in

these particular samples. 

2.4.3 X-Ray Diffraction of Site B Slag

XRD scans of bulk slag samples from Site B are

significantly simpler than those of Site A due to a

smaller number of minerals present in the Site B

slags. This can be seen in Figure 2-13 which shows

x-ray spectra for two Site B samples. The slag at the

top at the top (96 TC-19) is thought to be waste

product following Nb/Ta extraction from a Sn slag, and

the slag at the bottom is thought to be Sn slag prior to

Na/Ta extraction. In the former, hibonite appears to be

the dominant phase and in the latter gehlenite appears

to be the dominant phase. Spectra of Site B’s group 1

slags (glassy slags) indicate the presence of quartz

(SiO2) and a significant amorphous component (glass). 

Spectra of Site B’s group 2 slags (dense blocky slags)

indicate the presence of spinel, periclase, monticellite,

and forsterite. SEM and EMPA (see below) indicate

the presence of perovskite and zirconolite. However

their concentration is low, thus, their peaks could not

be positively identified by XRD. Spectra for slags from

Site B’s group 3 indicate the presence of spinel.

2.4.4 X-Ray Diffraction of Site C Slag

As is the case with the slags from the other sites, there

is a large number of phases present in the Site C slag.

Mineral separates (either by hand picking specific

grains or by density separation) were used to identify

specific phases. XRD spectra of samples from Site C 

indicate the presence of gehlenite, zirconolite, rutile,

calzirtite and pyrochlore.

2.5 Quantitative Analysis by Scanning Electron

Microprobe and Electron Microprobe Analysis

While light microscopy and XRD help identify the

major phases within a slag, they cannot determine the

chemical composition of the various phases, and

therefore it is difficult to identify which phases

contribute most to the radioactivity. The importance of

linking the radioactivity to the phase is illustrated by

the autoradiograph shown in Figure 3.1 in “Evaluation

of Leach Tests for Estimating Release from Slag Sites

Subject to Decommissioning Final Report” (Pickett et

al., 1998.) The autoradiograph shows uranium and

thorium disseminated throughout the slag. However

Pickett et al., 1998, note that grain size limits the

effectiveness of autoradioagraphy on a micron scale. 

This was confirmed by our microprobe analyses which

show that U and Th are found in specific phases rather

than being uniformly disseminated throughout the slag. 

It should be noted that these phases will weather at

different rates, and therefore the release of U and Th

will vary over time.

2.5.1 Scanning Electron Microprobe and Electron

Microprobe Analysis - Sample Selection and

Preparation

Following examination with the petrographic

microscope, samples were selected for further study

with the Electron Microprobe, based on their

mineralogy, chemical composition, and possible

radioactivity. 

Slag samples were prepared in several ways. For

observation of surface topography, samples were

mounted on brass or carbon platforms using carbon

paint. For slab samples, hand samples were cut and

polished with diamond and alumina polishing
11

compounds to a mirror finish. Slabs were mounted in

the electron microprobe using a universal holder. 

Grain mounts were made with embedded particles in

2.5 cm rings using Epotech 301 epoxy and were

subsequently polished. Samples were also made

into doubly-polished thin sections mounted with the

heat-sensitive adhesive Crystalbond. All samples

were coated with 50-100 Å of carbon in a carbon

evaporator before their insertion into the electron

microprobe. 

2.5.2 Scanning Electron Microprobe and

Electron Microprobe Analysis - Analytical 

Methods

Electron microprobe analyses were done with a JEOL

8600 Superprobe equipped with four wavelengthdispersive spectrometers (WDS) and a Tracor

Northern TN-5500 energy dispersive spectrometer

(EDS). The microprobe was operated at 25 keV with

variable beam currents. Surface topography was

investigated using secondary electron imaging (SEI). 

Backscattered electrons (BSE) were used for

compositional imaging on polished thin sections or

slabs.

X-ray maps were generated with both energy

dispersive and wavelength dispersive spectrometers 

while rastering the beam over a stationary sample. 

Eight 128 x 128 pixel X-ray maps were collected

simultaneously for different elements including Si, Ca,

Ti, Zr, Ce, Ta, Th, and U. The beam was held at 30

nA and 25 keV with a dwell time of 2 seconds. Each

set of maps took approximately 9 hours to collect.

2.5.3 Scanning Electron Microprobe and

Electron Microprobe Analysis - 

Quantitative Analyses

2.5.3.1 Standards

Obtaining the proper standard for each element is of

primary importance in the quantitative analysis of

materials through the use of EMPA. As the bonding

environment can affect the production of X-rays by

the electron beam, standards must be chosen to

have structures similar to that of the unknown

material. Since the slag material contains many

different types of materials (i.e., metal, oxides,

silicates), a different standard for each material is

recommended. However, this is not always possible. 

For the slag investigation, metal standards were used

for all metal phases. Oxide and silicate phases were

analyzed using oxide and silicate standards except

for the rare-earth elements, which were standardized

using rare-earth phosphates.

2.5.3.2 Spectrometer Considerations

Since many of the phases found in slags contain

numerous elements in relatively high concentrations,

overlap between X-ray peaks is a serious problem. 

Therefore, the peak used for quantification must be

the most intense peak with the least interference from

any other possible element. In some cases, an

overlap correction factor must be used when two

elements cannot be completely separated. In addition,

the position of background collection is very important

for the detection of trace amounts of elements. The

backgrounds chosen were determined to be free of

interference with characteristic peaks for forty

elements found in major to minor concentrations in the

slag samples. This stringent requirement forced some

background positions to be unusually far apart, so that

linear background extrapolations are inappropriate. In

these cases, a non-linear background correction factor

was determined, based on the curvature of

backgrounds in standards with similar mean atomic

number.

2.5.3.3 Quantitative Wavelength Dispersive

Spectrometer (WDS) Analysis

Quantitative wavelength dispersive spectrometer was

used in conjunction with SEM and EDS to determine

the chemical composition of the phases present in the

slags. WDS analysis of slag materials were carried

out using a 25 keV beam with 20 to 50 nA current and

a beam diameter of 1μm. Peak and background

counts were collected for 60 seconds for each

element. Selected elements that were found in low

concentration but were deemed important were

analyzed repeatedly over the course of one analysis. 

Uranium and lead were analyzed with six 60 second

replicates and thorium with five 60-second replicates. 

The total time for one analysis was 45 minutes. Data

were reduced by the program CITZAF (Armstrong,

1989).

2.5.4 Scanning Electron Microprobe and Electron

Microprobe Analysis Results - Site A

The major portion of the waste material at Site A site

was a recycled slag. As described earlier, the original

slag was crushed and leached, and allowed to settle in

an evaporation pond. This material is now in the form

of a sedimentary rock made up of angular clasts of

glass, hibonite, Fe-bearing pyroxene (esseneite,

CaFe+3AlSiO6), perovskite, quartz (SiO2), thorianite

(ThO2) and thorite ((Th,U)SiO4). (Ideal chemical

formulas of key phases are found in Table 2-3). It

should be noted that uranite (UO2) forms a series with

thorianite (ThO2).

The major Th-bearing phase is perovskite which has a

general “idealized” formula of AMO3, where A = Ca,

lanthanides, Ce, REE; and M = Al, Ti, V, Mn, Th. 

These perovskites have compositions of: 

Ti-rich perovskite:

(Ca0.82Zr0.04La0.03Ce0.08Nd0.03)(Ti0.85Th0.01Al0.09)O3

Al-rich perovskite: 

(Ca0.44Ce0.30La0.04Pr0.01Nd0.03Gd0.02Nd0.05REE0.05)(Al0.56

Ti0.46Th0.08)O3.

Thoriantite is present as (Th0.83Ce0.11Ti0.02Zr0.02)O2. 
12

Table 2-7 shows the results of a modal analysis by

EMPA of the phases in A-2 a sample representative

of the crushed recemented slag. The analysis

employed a count of 1760 points on a thin section. 

The uranium and thorium are found in a Ca-Ti

perovskite which makes up 2.3% of the sample, in 

Nb, Th, Ce phases (perovskite, zirconolite,

pyrochlore, thorianite) which make up 9.3% of the

slag. The other dominant, but non-radioactive

phases include quartz or cristobalite (5.1%), a

hibonite-like Al-Ca phase (24.2%), ground mass

containing primarily Si, Ca, and Al (32.4%), and void

spaces (20.2%).

Quantitative WDS analyses of the phases in the Site

A samples are presented in Table 2-8. Samples 96FA-4 (unleached) (Figure 2-14) and sample 96F-A-1-

727 (leached in deionized water) depict crushed Site

A slag provided by A. Felmy, PNNL. Splits of these

samples were used in the PNNL experiments. The

samples, both unleached and leached , were

subsequently mounted in epoxy, polished and

analyzed by SEM and EDS at Johns Hopkins

University to investigate changes to the leached

slags.

Figure 2-14 shows a BSE image and EDS spectra of

unleached sample from Site A’s crushed and

recemented slag (96F-A-4). Perovskite is found as

small white dendritic crystals within a large grain of

glass. Hibonite appears as a large (400 μm) lath in

the lower center of the BSE image and it encloses

bright crystals of pyrochlore.

2.5.5 Scanning Electron Microprobe and Electron

Microprobe Analysis - Results Site B

Modal abundance of major phases in a representative

sample of Site B slag was determined by counting

2000 points per slide, from four slides using a JEOL

Super probe. They are as reported in Table 2-9. The

dominant phases in these slides are gehlenite 

38.8%, glass 36.3%, clinopyroxene 10.1%,

perovskite 9.5%, and calzirtite 4.4%. Uranium and

thorium were found in the glass, perovskite, calzirtite,

a weathering rind of Ba aluminate, and in thorianite

and pyrochlore which were present in only a trace

amount (0.1%). Void space is significantly lower than

the Site A sample and in this case it makes up 2.1%

of the sample. In contrast the void space made up

20.2% of the Site A sample which represented the

crushed recemented slag. Table 2-10 shows the UO2

and ThO2 content of selected phases from Site B

samples.

The principal silicate phases in Site B’s Sn and Nb-Ta

slags consist of silicate glass, gehlenite, and

clinopyroxene. Of the silicate phases, only glass

contains any significant amount of radioactive

elements (U, Th). The glass also contains abundant

rare earth elements (REE). In several samples, the

glass appears to be altered to wollastonite, feldspar

and/or perhaps a high Al-pyroxene and a Ce, Thsilicate (Figure 2-7). These secondary silicate

phases are less than 1μm and are too small to be

accurately analyzed by the microprobe at Johns

Hopkins University.

The principal oxide phases in the Site B slags are

rutile, hibonite, spinel, perovskite, calzirtite or

zirconolite, and pyrochlore minerals. U and Th are

contained in the perovskite, calzirtite and pyrochlore. 

Hibonite contains Ce. The chemical compositions of

the major crystalline phases are reported in Table 2-

10. X-ray maps (Figure 2-15) provide striking visual

evidence for the presence or absence of uranium and

thorium in the various slag phases. The maps clearly

indicate that thorium is concentrated in perovskite,

calzirtite, and glass, while uranium is concentrated in

calzirtite and glass.

Several of the phases analyzed do not fit

stochiometrically into any mineral formula, making

conclusive identification difficult with the microprobe

alone. The Ti-Zr- high Al- clinopyroxene does not fit

the formula of 4.00 cations per 6.00 oxygens for a

stoichiometric pyroxene. These pyroxenes have total

cations that range from 3.51 to 3.90. However, optical

properties, diffraction patterns, and d-spacings all

indicate the presence of a pyroxene in the slag. For

this reason, selected samples were analyzed with the

TEM for more accurate phase identification and TEM

analysis proved the unknown phase to be pyroxene.

2.5.6 Scanning Electron Microprobe and Electron

Microprobe Analysis Results - Site C

Site C slags (C-1a, C-1b, C-5a, C-5b, C-6a, C-S) were

analyzed by SEM, coupled with EDS. Site C’s Group 1

slag (glassy slag) is composed primarily of a Ca-Si-Al

glass and SiO2 in the form of dendrites and large

(~300 μm) rounded blebs. Cracks formed by the

change in volume as the SiO2 was transformed from

cristobalite to quartz are quite noticeable in microprobe

images (Figure 2-8). The silicate glass, rich in Ca, Al

and Fe (with accessory amounts of K, Mn and Mg), is

devitrified in certain areas and has recrystallized as

wollastonite and Fe oxide and Ni silicates.

A modal analysis of phases present in a representative

sample from Site C is shown in Table 2-9. Uranium

and thorium are found in glass which makes up only

0.3% of the sample, and perovskite-pyrochlore which

is 9.2% of the sample. Other major phases include an

unidentified alteration product (25.3%), wollastonite

and monticellite (27.9%), spinel (22.6%), (periclase

9.4%). Void space represents 5.0% of the sample.

Site C’s Group 2 slag (dense blocky slag) contains

primarily spinel, olivine and oxides. The red cubic

crystals in Figure 2- 9 are Al-Mg-Cr spinels. 

Perovskite, thorianite and other oxide phases are also

present. A small amount of thorium is contained in the

perovskite. The dense blocky samples also contain

pyrite nodules and Fe-Ni prills. Perovskite appears to

be the only mineral containing Th. The slag is oxidized

and the glass, which is normally clear, has been

altered from blue or green to brown and contains small

microlites of Fe oxyhydroxides.
13

EMPA was used for detailed chemical analysis of the

dominant phases in samples C-4 and C-6 and the

results are shown in Table 2-11. Analyses were done

of the core and the altered edges of several of the

phases chosen to see patterns associated with

alteration of the material.

2.6 Transmission Electron Microscopy

Slag from the three sites was investigated using a

Philips 420 Transmission Electron Microscope and

Philips 300 kV field-emission-gun TEM. The TEM

was used to obtain high magnification and high

resolution images, electron diffraction patterns, and

small scale elemental analyses for the samples. Two

methods of sample preparation were employed. The

first method involved crushing a small portion of the

slag in alcohol to make a suspension and then

dipping a sample grid with a carbon support film into

the suspension. The second method involved gluing

a copper TEM grid to a small area of a thin section

and then ion-milling a portion of the sample to

electron transparency. Figures 2-16 and 2-17 provide

an illustration of how a thin section is prepared as a

TEM sample. Figure 2-16 shows three images of a

portion of thin section. In (a), an area of interest is

identified in plane-polarized light, (b) the same area

in cross-polarized light, and (c) provides chemical

information in the BSE image. Figure 2-17 is a plane

polarized light micrograph of the same area in Figure

2-16, to which a TEM grid was glued to the thin

section and a hole was made in the center of the grid

with an ion-mill. A-A’ is a cross-section of the TEM

sample. Sample sizes for both methods are either

2.3 or 3.0 mm in diameter depending on the grid

used. Because of the small grid size, TEM

observations are not meant to be applied to the bulk

sample, but are extremely helpful in identifying and

describing individual phases. Electron diffraction

patterns of unknown phases can be measured and

converted to lattice spacings which are then

compared to crystallographic databases, thus

identifying unknown components of the slags. These

identifications are supported by an analytical system

(EDS) that provides elemental spectra for the small

grains in the sample. Devitrification of glass and

alteration along grain boundaries can be explored

with TEM, as in Figure 2-18.

The electron microprobe provides excellent chemical

analyses of points >1.0 micrometer in size. However,

it provides no information on the crystal structure, and

hence is difficult to determine mineral chemistry. A

combination of EMPA, XRD and TEM provides the

necessary information to determine crystal chemistry

and structure.

The TEM has been used in this study to positively

identify hibonite, Ba aluminate, Ti-Al-Zr

clinopyroxene, perovskite, calzirtite and zirconolite. 

Hibonite contains Ce and trace amounts of Th. 

Perovskite, calzirtite, and zirconolite contain varying

amounts of U and Th. TEM analyses of the glass

provided data on the elemental composition of small

areas now devitrified. Devitrification of the glass

produced small crystallites of Ce silicates, wollastonite,

and voids.

Future TEM studies could investigate the phase

interfaces as well as the presence of structural defects

within individual crystals. Crystalline defects often

serve as conduits for leaching fluids. Crystal defects 

are also a starting point for the breakdown of a phase. 

Information about the mineralogy as well as the types

of defects and their relative concentrations should

provide insight into the leaching rates and solubility of

U and Th in the slag.

2.7 Surface Area Analyses: Methods and Results

Surface area analyses were performed by L. Veblen at

the Johns Hopkins University to assist A. Felmy,

PNNL, in interpreting the slag solubility and leaching

studies performed by PNNL. The surface area of

powdered slags was measured using a BET Surface

Area Analyzer. Three of the Site B samples

(96TC19F, 96TC20F, and 96TC21F) provided by

PNNL were analyzed. These were powdered

(crushed) samples that they used in the PNNL

leaching experiments. The surface area varied among

samples and it ranged from 0.166 m2

/g for 96TC19F to 

0.6 m2

/g for 96TC20F to approximately 1.0 m2/g for

96TC21F. Intact samples could be measured, with a

new sample holder that allows larger (cm sized)

samples.

2.8 Petrology of the Slags

Based on the XRF bulk chemical analyses and the

mineralogy of the Sn, and Nb-Ta slags, it is possible to

classify the slag into discrete chemictypes as seen in

the Ternary Diagram (Figure 2-10). Additional

analyses by the microprobe, XRF, and TEM data

should enable one to define petrologically a “typical”

Nb-Ta slag, and thereby classify additional Nb-Ta

slags based solely on bulk chemical analyses. A

similar method of classification is commonly used in

igneous petrology. This classification, based on bulk

chemical composition, may be used in the future by

licensees to place bounds on the weathering

characteristics of the materials on site without the

detailed solid phase characterization done in this

report. For example: if one had a phase diagram for

the production of slag, given a bulk composition, the

crystalline phases could be predicted. This

information could be used along with individual mineral

weathering or degradation information to estimate

leach rates.

Table 2-12 summarizes the principal phases and their

corresponding amounts present in the slags at the

three sites. This table also provides information on the

weight percent of U and Th in each phase which can in

turn be used in estimating leach rates. Sites A, B, and

C share many phases. All three sites contain hibonite,

perovskite, and Site A contains considerable

thorianite. There is not much glass at Site A but this is

understandable because the Site A material was

crushed and processed with acid. Under such

conditions the glass is not expected to survive. 
14

The Site B slags that contribute the greatest

radioactivity to the source term have been

characterized chemically and mineralogically. There

appear to be two slag types that contribute most to

radioactivity at the site. These are the Sn slag (e.g.

B-5.5) which had been brought from Malaysia and

the Nb-Ta slag which was a waste product following

the extraction of Nb-Ta from the original Sn slag. 

Both slags have similar chemical composition, and

hence similar crystalline phases. The primary

silicates of both are gehlenite (a Ca rich melilite),

clinopyroxene (there appear to be two distinct high Al

pyroxenes), wollastonite, and silicate glass. The

secondary minerals include feldspar, Ce silicate, and

quartz, and they formed by devitrification of the glass

or from alteration of the glass and primary crystalline

phases. Quartz is also included as a xenocryst from

the physical process of smelting. The original feed

materials sat on a quartz-sand bed. Zirconolite and

hibonite were most likely the first phases to crystallize

from the melt (Figure 2-19). Hibonite, spinel,

perovskite, calzirtite and/or zirconolite, and pyrochlore

are the oxides present and appear to be the primary

phases. Rutile is present, but may be a secondary

alteration phase of perovskite. Perovskite is not

stable under low temperature, atmospheric

conditions, and will eventually alter to rutile or

anatase. The alteration proceeds via incongruent

dissolution of the Ca ions, followed by a transitional

CaTiO phase according to the following reactions

(Banfield and Veblen, 1992):

2

2

3

05 25

2 2

CaTiO

Ca TiO Ca O

perovskite

aqueous aqueous

( )

. . ( )( )

→

+ + + −

and

2

2 22

05 25

2 2

2 5

2 2

Ca TiO Ca O

TiO Ca O

aqueous aqueous

anatase aqueous aqueous

. . ( )( )

.( ) ( ) ( ).

+ +→

+ +

+ −

+ −

Site C has two main types of slag based on

mineralogy. One characteristically contains Ba

aluminate, hibonite, and perovskite. Radioisotopes of

Th are typically found in the perovskite. The second

slag type contains spinel, olvine, and perovskite. Site

C is similar to Site A in that there is Ba aluminate

present at both sites along with hibonite or a hibonitelike phase, perovskite and thorianite. Site C has

some glass. As noted above, there is little glass at

Site A because the acid leaching during refining of

the material would remove glass.
15

Table 2-1 Hand Specimen Description

Sample

Number

Description

Site A Samples

A-1 Crushed, naturally cemented slag, specific gravity approximately that of sandstone. 

Resembles a sandstone with cross bedding and some graded bedding. Grain size ranges

from <1mm to 1mm. Has some voids <1mm which are probably depositional structures rather

than voids caused by weathering. Fresh surfaces are white (N/8/0) and light gray (N/7/0). 

Has a thin goethite like weathering rind typically 1 - 2mm thick but up to 10mm in places.

Color of rind ranges from gray (*Munsell color 10YR/6/1) to brownish yellow (*Munsell color

10YR/6/6) to strong brown (*Munsell color 7.5YR/4/6). Radioactivity 3200 counts/minute.**

A-2 Crushed, naturally cemented slag. Almost identical to A-1 except that portions of the

weathering rind are greenish gray (*Munsell color 10G/6/1) in addition to the colors noted

under A-1. Radioactivity 4000 counts/minute.**

Site B Samples

B 1-4 Slag, high specific gravity, surface vitreous, vesicles <1mm and make up 5% of sample. 

Elongate crystal laths greenish gray in color stand out on a weathered surface. Color is

greenish gray (*Munsell color 10Y/5/1). 

B- 5-5 Slag, high specific gravity, very glassy. Numerous crystals stand out on weathered surface. 

Fresh surfaces are very dark gray (*Munsell color N/3/0). Outer weathered surface is pale

yellow (*Munsell color 2.5Y/7/3). Gamma Radioactivity 4000 counts/minute.**

B- 5.5b Slag, high specific gravity, crystalline with the crystals. Small vesicles <1mm are visible on

weathered edge and make up ~5% of volume. Color of fresh surface is gray (*Munsell color

N/5/0) weathers to light gray (*Munsell color 10YR/7/2). Radioactivity 4000 counts/minute.**

B- 5-8 Slag, high specific gravity. Numerous vesicles <1mm to 10mm in diameter toward one

surface. Make up ~ 40% of the volume at that surface. Numerous euhedral crystals 1 - 2mm

on the edge with vesicles. Color is gray (*Munsell color N/5/0/N) on fresh surface and on

weathered surfaces it is light brownish gray (*Munsell color 10YR/6/2). Radioactivity 3600

counts/minute.**

B- 5-9 Slag, high specific gravity, fine grained, dense. Fresh surface dark gray (*Munsell color

N/4/0). Weathers to a light gray (*Munsell color 2.5Y/7/2). Radioactivity 3000

counts/minute.**

B- 5-10 Slag, high specific gravity, finely crystalline. Very dark gray (*Munsell color N/3/0/N) weathers

to light gray (*Munsell color 2.5Y/7/2). Large vesicles ranging in size from < 1mm to >100mm

on one surface. Radioactivity 2000 counts/minute.**

B- 5-11 Slag, high specific gravity, finely crystalline with abundant glass particularly close to cooling

surface. Fresh surfaces are dark bluish gray (*Munsell color 5B/3/1). Weathering surface is

red (*Munsell color 10R/4/6). Radioactivity 2200 counts/minute.**

B- 5-11 Slag, high specific gravity, finely crystalline with an abundance of metallic prills <1mm to

~5mm. Has tiny vesicles <1mm throughout and represent ~10% of volume. Color on fresh

surfaces is dark gray (*Munsell color N/4/0). Weathering surface is red (*Munsell color

10R/4/6). Radioactivity 2200 counts/minute.**

B- 1 Possibly crushed slag. Very fine grains <1mm in size with quartz and slag fragments 1 - 2mm

in diameter. Color is dark greenish gray (*Munsell color 10Y/4/1)

B- 2 Possibly crushed slag. Poorly lithified. Contains about 5% of light colored angular fragments

which are possibly crushed fire brick or concrete up to 10mm in diameter. Color is gray

(*Munsell color N/5/0) .

B- 3 Fine grained dense metal. Silvery metallic on fresh surfaces some of which oxidized after

cutting to a light red (*Munsell color 10R/6/8). Fresh surfaces are gray (N/5/0) and weathered

surfaces are dark reddish brown (*Munsell color 2.5Y/3/3).
Table 2-1 Hand Specimen Description

Sample

Number

Description

16

B- 4 Ore like material. Very fine grained with grains <1mm. Has a 10mm think lens of very fine

<1mm euhedral pyrite crystals. Lens is centimeters long. Color is very dark gray (*Munsell

color N/3/0) and the sample weathers to a yellowish red (*Munsell color 5YR/4/6) goethite like

material.

B- 5.3 May be a weathered slag. High specific gravity which is made up of fairly uniform very fine

grained grains <1mm in diameter. Vesicles <1mm in diameter make up <1% of sample. 

Fresh surfaces are very dark gray (*Munsell color 5YR/3/1) and the outer surfaces is

weathered to a goethite like material which is light brown (*Munsell color 7.5YR/6/4).

B- 5-5 Slag, high specific gravity. Glassy and fine grained made up of subhedral to euhedral black

grains. Has minor amount (<1%) round metallic prills and some of them have weathered to a

goethite like material. Color is dark gray (*Munsell color N/4/0). 

B- 6 Possible ore, high specific gravity. On a polished surface there were numerous filament like

silvery metallic veins 2 - 5mm thick and cm long. Matrix between the veins is dark reddish

gray (*Munsell color 10R/4/1). Fresh surfaces are gray (*Munsell color N/5/0) and the outer

weathered surface is a weak red (*Munsell color 10YR/5/3). Very small vesicles <1mm make

up ~5% of the material.

B- 7 Slag, specific gravity about that of sedimentary rock. About 3% of sample consists of sub

angular to rounded quartz grains <1mm in diameter. Has thin glassy rind about 10mm thick. 

Many vesicles <1mm are present and make up 15% of the sample. The outer surface has

vesicles 1 - 2mm in diameter. Cut surface is dark metallic gray (*Munsell color N/4/0) and the

outer surface is gray (*Munsell color N/5/0).

96T-C-9C Crushed sample with angular fragments approximately 2mm in length. Looks like a devitrified

slag. Some glass fragments present. Unweathered surfaces are black. Overall color is

grayish brown (*Munsell color 10YR/5/2) and probably represents weathered surfaces. 

96 T-C-19C Crushed sample of slag with angular fragments 2 - 20mm in length. Made up of subeuhedral

to euhedral dark glassy grains. Vesicles <1mm in diameter are about 2% of sample. Needle

like laths stand out in relief on weathered surfaces. Color is dark gray (*Munsell color N/4/0)

and the material weathers to gray (*Munsell color 5YR/5/1).

96 T-C-20C Crushed sample of slag consisting of angular fragments 2 - 10mm in length. It’s very glassy. 

Vesicles <1mm in diameter make up ~3% of sample. Unweathered surfaces range from very

dark gray (*Munsell color N/3/0 to black (*Munsell color N/2.5/0). Weathered surfaces are

weak red (*Munsell color 10R/4/2). 

Site C Samples

C-1 Slag, very high specific gravity. Large lath like crystals up to 10 mm long visible in cut surface

and the ends of the laths are very prominent on weathered surface where they stand in relief. 

Has other smaller crystals <1mm at cooling surface. Fresh surfaces are dark bluish gray

(*Munsell color 10B/4/1) to bluish black (*Munsell color 10B/2.5/1). There is a weathering rind

1 to 5mm thick with a range of colors including lightish green gray (*Munsell color 10Y/8/1),

grayish brown (*Munsell color 2.5Y/5/2), to white (*Munsell color N/8/0). Radioactivity 5000 -

8000 counts/minute.** 

C-2 Slag, very high specific gravity. Very fine grained. Lots of metallic prills <1mm which are

about 20% of sample. Fresh surface is dark bluish gray (*Munsell color 5PB/4/1). There is a

thin weathering rind <1mm thick with a range of colors including dark gray (*Munsell color

5Y/4/1), light gray (5Y/7/2), light gray (*Munsell color 2.5Y/7/2) and light gray (*Munsell color

2.5Y/7/1). Radioactivity 8000 counts/minute.**

C-3 Slag, high specific gravity. Description the same as C-2 except the metallic prills are missing

and large laths stand in relief on weathering surfaces. Radioactivity 600 counts/minute.**
Table 2-1 Hand Specimen Description

Sample

Number

Description

17

C-4 Slag, specific gravity similar to sandstone. Resembles C-6. Made up of equant grains <1mm

in diameter. Has some vesicles up to 3mm in diameter and they represent ~5% of the

sample. Metallic prills along the edge are <1mm in diameter and make up around 1% of the

area. Exterior surface very fractured and it resembles mud cracks (dehydration cracking). 

Fresh surface is light bluish gray (*Munsell color 5PB/7/1). Weathered pretty much throughout

the sample and the outer surface is light greenish gray (*Munsell color 10Y/7/1) and almost

white (*Munsell color N/8/1). Radioactivity 650 counts/minute.**

C-5 Slag, resembles green obsidian. Specific gravity the same as obsidian. Has some prills. On

the edge they are up to 2mm in diameter. Inside they are <1mm. Fresh surfaces is grayish

green (*Munsell color 5G/4/2). It is devitfified along the edge and the color is light greenish

gray (*Munsell color 5GY/7/1). Other portions of the weathered edge is pale brown (*Munsell

color 10YR/6/3) and yellowish brown (*Munsell color 10YR/5/4). Radioactivity 100

counts/minute.**

C- 5a Sample chips, angular 2mm to 10mm in diameter. Fine grained with vesicles <1mm making

up about 5% of volume. Color is gray (*Munsell color N/5/0). Weathers to light gray (*Munsell

color 2.5Y/7/2).

C-6 Slag, specific gravity moderate, highly weathered. Similar in appearance to C-4. Exterior

surface very fractured and it resembles mud cracks (dehydration cracking). Has some prills

<1mm in diameter throughout the sample. Very fine grained with grains <1mm. Has very fine

equant crystals green in color. Fresh surface is light greenish gray (*Munsell color 5GY/8/1)

and the exterior weathered surface is close to white (*Munsell color N/8/0). Radioactivity 2000

counts/minute.**

* Color description is based on Munsell Soil Color Charts, 1994 revised edition, Macbeth Division of Kollmorgen

Instruments Corporation, New Windsor, New York. The order of description is hue/value/chroma.

** Radioactivity measured with a 5 cm (2") pancake type Geiger Muller probe.
18

Table 2-2 Measurements of Samples with Detectable Radioactivity

Site and Sample number Bichron μSv/hour Contact counts per

minute

Laboratory background 0.05

Site A

A-1 3200

A-2 (alteration product) 3.0 4000

A-2 7000

A-2 4000

Site B

B-1 100

B-5 3000

B-5.5 4.0 4000

B-5.5 4.5 4000

B-5.8 4.0 3600

B-5.9 3000

B-5.10 2000

B-5.11 2500

B-5.11 2200

Green slag 100

Site C

C1 5.0 5000-8000

C2 3.5 8000

C3 2.5 600

C4 0.25 650

C5 0.05 100

C6 2000
19

Table 2-3 Phases Present in Samples from Sites A, B, and C

Phase Ideal formula Site A Site B Site C

Calzirtite* (Ca,U,Th)ZrTi2O7 x xx

Zirconolite* (Ca,Th,Ce)Zr(Ti,Nb)2O7 x xx

Perovskite* CaTiO3 x xx

Perovskite (Loparite)

 

(Ca,Ce,Th)(Ti,Nb)O3 x x

Zirconolite (Ca2.45Th0.03REE0.04)Zr0.34Al0.30Ti1.24

Nb0.15Ta0.12O7

x x

Zirconolite (Ca 1.18 Ce0.61Th0.11REE0.26) 

Zr0.02Al1.17Ti1.14Nb0.08O7

x x

Pyrochlore (Betafite) (Ca,U,Ce)2(Nb,Ti)2O6 x x

Pyrochlore (Ca,Th,Ce)2(Nb,Ti)2O6 x xx

Glass Si,Al,Ca,Ti,U,Th trace x x

Hibonite (Ca,Ce)9(Ti,Al)12O19 x xx

Barium Aluminate* Ba,Ce(Ti,Al)12O19 x x

Spinel* AB2O4 A = Co, Fe, Mg, Mn, Ni

 B = Al, Cr, Fe, Mg, Mn, V 

x xx

Thorianite ThO2 (note: forms a series with uranite

(UO2)

x ??

Thorite (Th,U)SiO4 x ??

Periclase MgO x

Wustite FeO x

Monticellite (olivine) CaMgSiO4 x

Gehlenite Ca2Al(AlSi)O7 x xx

Melilite (Ca,Na)2(Mg,Fe,Al,Si)307 x

Clinopyroxene Ca(Zr,Ti,Al)(Al,Si,Ti)2O6 x

Feldspar CaAl2Si2O8 x xx

Wollastonite CaSiO3 x

Ce-Silicate Ce,Si x

Ce-Aluminosilicate Ce,Si,Al x

Barium Aluminosilicate BaAl2Si2O8 x

Gypsum Ca(SO)4 x
Table 2-3 Phases Present in Samples from Sites A, B, and C

Phase Ideal formula Site A Site B Site C

20

Tridymite SiO2 x xx

Groundmass Ca,Ba,Al; Ca,Ba,S,Cl,Al x

indicates some radioactivity may be present

* indicates SYNROC phases 
21

Table 2-4 Chemical Composition of Major Elements in Slag Samples (weight %) as determined by XRF.

(Blank = not analyzed; ? = undefined amount; n.d. = not detected)

Site A Site B A-1 A-2 B-5 B-5.5 B-8(1) B-5-5A B-5-5B B-5-5C

avg

B-5-8 B-5-11 B-4

(soil)

B-9 (soil

SiO2 18.01 10.73 25.47 25.85 38.42 26.40 28.59 28.76 29.79 26.50 62.26 50.1

TiO2 2.06 2.75 8.61 11.40 1.04 13.62 6.81 6.79 6.43 9.86 0.54 1.46 Al2O2 41.87 46.65 14.11 13.35 23.54 15.10 16.78 16.90 16.30 14.38 10.51 5.64 

Fe2O3 n.d. n.d. 0.66 0.22 20.65 0.57 0.06 0.05 0.01 2.20 10.90 4.48 

FeO 1.22 0.54 0.62 1.78 7.68 N.d. n.d. n.d. n.d. n.d. 1.03 3.13 MnO 0.07 0.05 0.61 0.54 0.15 0.49 0.51 0.51 0.69 0.74 0.01 0.24 MgO 0.22 0.24 1.54 1.51 0.80 1.52 1.76 1.76 2.27 1.77 0.52 2.05 CaO 11.53 11.32 28.92 26.80 3.50 29.17 32.65 32.72 31.77 27.81 0.10 9.71 Na2O 0.72 0.80 0.17 0.13 0.68 0.11 0.09 0.10 0.11 0.14 0.30 0.33 K2O 0.06 0.07 0.22 0.23 1.24 0.16 0.19 0.20 0.29 0.22 1.65 0.96 P2O5 0.01 0.01 0.04 0.21 0.45 0.05 0.01 0.01 0.01 0.26 0.05 0.16 CuO 

ZnO SO2 

Total 75.77 73.16 80.97 82.02 98.15 87.19 87.45 87.78 87.67 83.88 87.87 78.3LOI 20.53 23.24 0.20 -3.80 0.43 5.35 -0.32 -0.33 -0.37 -3.11 9.69 20.7Fe2O 3 total 1.36 0.60 1.35 2.20 29.19 0.57 0.06 0.05 0.01 2.20 12.04 7.96
22

Table 2-5 Chemical Composition of Trace Elements in Slag Samples (ppm) as determined by XRF.

(Blank = not analyzed; ? = undefined amount; n.d. = not detected)

Site A Site B 

A-1 A-2 B-5 B-5.5 B-8(1) B-5-5A B-5-5B B-5-5C

avg

B-5-8 B-5-11 B-4 (soil) B-9 (

Rb 45.0 67.0 15.0 ? 63.0 24.0 23.0 21 33.0 ? 86.0 22.0 Sr 373 474 258 240 1570 230 271 269.5 265 245 74 134 Ba 40600 61800 905 1125 1530 1240 1300 1325 1300 1250 472 127 Y 96 109 3850 ? 60 3250 5400 5300 5300 ? 23 91 Zr ? ? 21000 ? 217 18000 41000 40250 40000 ? 157 940 V 75 105 102 275 230 183 55 50 54 234 61 192 Ni 20 21 57 52 86 40 44 41.5 43 42 54 155 Cr ? ? 154 387 223 233 19 1 24 395 39 584 Nb 12400 12200 1150 7000 25.8 1250 12? 13.5? 5? 6400 12.3 1026 Ga n.d. n.d. 3.2 139 21.6 18.0 ? ? ? 145 28.9 33.6 Cu 45 39 ? ? 102 ? ? ? ? ? 42 2420 Zn 37 23 15 16 117 11 ? ? ? ? 60 4390 

U 213 280 180 1980 9.1 225.0 ? ? ? 1920 2.9 18.0 Th 10250 13250 3960 3650 22.8 3640.0 4400.0 4225 4200 4100 1.8 84.5 

Co ? ? ? ? 28 4 1 2 3 2 31 18 La 2600 2750 ? ? 67 3900 5000 4725 4600 4600 41 138 Ce 6400 6500 ? ? 168 ? ? ? ? ? 120 351 Sc Pb 34 ? ? ? ? ? ? 
23

Table 2-6 List of minerals indicated by X-Ray Diffraction analysis of Site B Sample B-5.5a.

Mineral Formula

Gehlenite Ca2Al2SiO7

Schorlomite Ca3(Fe,Ti)2[(Si,Ti)O4]3

Kimzeyite Ca3(Zr,Fe,Ti)2[(Al,Si,Fe)O4] 3

Grossular Ca3Al2(SiO4)3 

Goldmanite Ca3 (V,Fe,Al)2(SiO4)3 

Epidote Ca2Al2Fe(SiO4)(Si2O7)(O,OH)

Rutile (Ti,Nb,Ta,Fe)O2

Struverite (Ti,Ta,Fe)O2

Ilmenorutile Fex(Nb,Ta)2xTi1-xO2

Ashanite (Nb,Ta,U,Fe,Mn)O2

Wodgnite (Ta,Mn,Sn,Nb)O2

Srilankite (Ti,Zr)O2 (synthetic)

Ixiolite (Ta,Fe,Sn,Nb,Mn)O2

Alumotantite AlTaO4

Thorite (Th,U)SiO4

Aeschynite (Ce,Ca,Fe,Th)(Ti,Nb)2(O,OH)6 

Vigezzite (Ca,Ce)(Nb,Ta,Ti)2O6

Tantalaeschynite (Y,Ce,Ca)(Ta,Ti,Nb)2O6

Niobo-aseschynite (Ce,Ca,Th)(Nb,Ti)2 (O,OH)6

Rynersonite Ca(Ta,Nb)2O6

Staringite (Sn,Fe)(Sn,Ta,Nb)2O6

Manganotapiolite (Mn,Fe)(Ta,Nb)2O6

Ferrotapiolite (Fe+2,Mn+2)(Ta,Nb)2O6

Fersmite (Ca,Ce,Na)(Nb,Ta,Ti)2(O,OH,F)6

Ferrocolumbite (Fe,Mn)(Nb)2O6

Euxenite (Y,Ca,Ce,U,Th)(Nb,Ta,Ti)2O6 

Zirconolite CaZrTi2O7

Yttropyrochlore (Y,Na,Ca,U)1-2(Nb,Ta,Ti)2(O,OH)7

Calzirtite CaZr3TiO9

Tantalcarbide (Ta,Nb)C

Kamacite (Fe.Ni)

unnamed mineral WC

Moissanite SiC

The JADE database was used for this peak based search. This list indicates the complexity in identifying mineral phases based solely on x-ray

diffraction data. The JADE data are for pure phases. However, the phases in the slags are not pure end members. They contain elements such

as thorium, uranium, and REE(rare earth elements) that are not present in the materials within the JADE database.
24

Table 2-7 Modal Abundance of Phases in Representative Site A Slag Sample as determined by Electron

Microprobe Analysis (EMPA).

Phase Modal Abundance (1760 points)

Si only (Quartz or Cristobalite) 5.1%

Al-Ca phases (Hibonite-like) 24.2%

Ca-Al phases (small amounts of Cl) 3.7%

Si-Al, Ca) (glass and silicates) 2.5%

Ca-Ti phases (Perovskite)* 2.3%

Nb, Th, Ce-phases (Perovskite, Zirconolite,

Pyrochlore, Thorianite) *

 9.3%

Fe phases (Ba, Al, Si, Ca, Ti, Mg, Ce, Cl) 0.4%

Groundmass (contains primarily Si, Ca, and Al) 32.4%

Voids and fractures 20.2%

 * uranium and thorium bearing phases in bold type
25

Table 2-8 Average Chemical Composition (weight %) of Most Common Phases Site A Samples as

Determined by Electron Microprobe Analysis (EMPA).

 Perovskite

 (average of 13 samples)

 Thorianite

 (average of 5 samples)

Calzirtite

 (average of 2 samples)

 Barium Aluminate 

(average of 4 samples)

Na2O 0.83 0.01 0.42

MgO 0.03 0.03 0.13 0.06

Al2O3 0.83 0.04 1.01 5.02

SiO2 0.00 0.00 0.34 0.11

P2O5 -------- -------- -------- --------

K2O 0.09 0.14 0.10 0.11

CaO 16.12 0.10 19.31 11.84

TiO2 23.21 0.71 29.07 17.67

Cr2O3 0.00 0.00 0.00 0.00

MnO 0.01 0.001 0.18 0.06

FeO 0.01 0.05 0.001 0.02

Y2O3 0.08 0.05 0.98 0.37

ZrO2 0.68 0.45 33.1 11.41

BaO 0.34 0 0.21 0.18

La2O3 3.13 0.38 1.44 1.65

Ce2O3 22.51 5.63 4.01 10.76

Pr2O3 1.18 0.40 0.55 0.71

Nd2O3 2.82 0.95 1.84 1.87

Sm2O3 0.20 0.08 0.26 0.18

Gd2O3 1.65 0.45 0.57 0.89

Dy2O3 0.04 0.01 0.23 0.09

Er2O3 0.01 0.01 0.14 0.05

Yb2O3 0.00 0.001 0.12 0.04

HfO2 0.00 0 0.75 0.25

Ta2O3 0.00 0 0.01 0.003

PbO 0.02 0.02 0.09 0.04

ThO2 11.42 94.31 1.49 35.74

UO2 0.12 1.99 0.72 0.94 

Total 98.55 105.81 96.65 100.48

UO2 +

ThO2

11.54 96.30 2.21 36.68
26

Table 2-9 Modal Abundance of Phases in Representative Samples from Sites B and C Slag as

determined by Electron Microprobe Analysis (EMPA).

Phase Modal Abundance) Site B Modal Abundance Site C 

B-5 (?) 

1794 points

B-5 (?) 

2000 points 

C- 4 

357 points

C- 4B

778 points

C- 4

947 points

Gehlenite 37.0% 38.8%

Glass * 36.4% 36.3% 0.3%

Wollastonite and

Monticellite

28.3% 30.1% 27.9%

Spinel 21.3% 23.0% 22.6%

Clinopyroxene 9.6% 10.1%

Perovskite * 9.3% 9.5% 9.0% 9.8% 9.2%

Calzirtite +

zirconolite*

4.8% 4.4%

Periclase 26.3% 22.5% 9.4%

Rutile 0.2% 1.9%

Fe prills 0.1% 0.3%

Pyrochlore * 0.1% 0.1%

Alteration product 0.5% 0.4% 5.0% 14.3% 25.3%

Voids and 

fractures

2.2% 2.1% 5.0%

 * uranium and thorium bearing phases in bold type
27

Table 2-10 Average Chemical Composition (weight %) of Most Common Phases Site B Samples as

Determined by Electron Microprobe Analysis (EMPA).

Perovskite Pyrochlore) Calzirtite

Zirconolite

 Melilite

 Gehlenite

 Spinel Hibonite Pyroxene Glass

Na2O 0.04 2.04 0.05 0.09 0.04 0.01 0.02 0.11

MgO 0.64 1.24 2.58 3.76 4.07 2.41 4.16 0.50

Al2O3 9.43 16.37 34.51 35.08 27.42 82.64 22.54 37.99

SiO2 22.09 1.86 11.31 22.49 3.74 0.091 24.96 16.61

P2O5 0.04 0.004 0.01 0.01 0.01 0.008 0.006 0.01

K2O 0.43 0.48 0.13 0.33 0.12 0.02 0.002 0.28

CaO 31.97 12.56 18.22 24.92 10.35 7.45 23.40 35.99

TiO2 18.60 31.90 15.13 14.39 38.39 3.81 15.38 1.96

Cr2O3 0.01 0.05 0.03 0.04 0.10 0.00 0.021 0.002

MnO 0.48 0.45 0.40 0.67 0.85 0.08 0.48 0.08

FeO 0.01 0.24 0.007 0.006 0.008 0.01 0.002 0.003

Y2O3 0.25 0.32 0.23 0.14 0.20 0.03 0.11 0.29

ZrO2 6.61 27.10 15.49 1.97 14.46 1.36 7.18 0.58

BaO 0.13 0.48 0.27 0.22 0.58 0.04 0.25 0.08

La2O3 0.86 0.18 0.24 0.14 0.02 0.81 0.07 0.36

Ce2O3 2.56 0.49 0.56 0.35 0.12 1.55 0.23 0.97

Pr2O3 0.30 0.07 0.06 0.04 0.008 0.16 0.05 0.13

Nd2O3 1.05 0.17 0.21 0.18 0.04 0.43 0.12 0.49

Sm2O3 0.21 0.06 0.06 0.05 0.04 0.06 0.02 0.12

Gd2O3 0.32 0.05 0.05 0.04 0.03 0.05 0.03 0.11

Dy2O3 0.07 0.05 0.06 0.05 0.04 0.02 0.03 0.07

Er2O3 0.04 0.06 0.04 0.01 0.03 0.01 0.22 0.03

Yb2O3 0.04 0.06 0.05 0.03 0.05 0.004 0.21 0.03

HfO2 0.03 0.94 0.52 0.00 0.59 0.005 0.22 0.00

Ta2O3 ---- 3.51 0.21 0.04 0.34 0.01 0.00 0.00

PbO 0.03 0.12 0.07 0.003 0.08 0.01 0.04 0.01

ThO2 0.45 1.27 0.30 0.06 0.20 0.130 0.01 0.22

UO2 0.08 0.39 0.16 0.005 0.11 0.007 0.01 0.03

Total 96.77 102.514 100.957 105.114 102.036 101.215 99.771 97.055

UO2+

ThO2

0.53 1.66 0.46 0.065 0.31 0.137 0.02 0.25
28

Table 2-11 Average Chemical Composition (weight %) of Most Common Phases Site C Samples as Determined by Electron Microprobe An

C-6Pyrochlore

C-4-Pyrochlore

C-6-Alteration

C-6 Spinelcore C-6 Spinel- rim C-4 Spinel- core C-4 Spinel-rim C-6- Peric

MgO 3.525 1.147 0.204 29.128 19.048 27.813 25.330 Al2O3 4.092 4.190 17.170 68.088 45.283 50.590 54.653 

K2O 0.064 0.081 0.048 0.045 0.046 0.045 0.045 

CaO 50.491 35.507 36.250 0.081 12.142 0.097 4.107 TiO2 19.515 25.773 0.157 1.732 1.295 1.997 1.675 

Cr2O3 0.069 1.431 0.014 0.662 6.903 20.033 9.199 

MnO 0.055 0.039 0.052 0.137 0.157 0.281 0.191 FeO 0.002 0.031 0.066 0.007 0.078 0.160 0.082 ZrO2 2.725 7.593 0.001 0.013 0.030 0.076 0.040 NbO2 0.038 5.380 0.000 0.001 0.008 0.022 0.010 BaO 0.219 0.211 0.000 0.010 0.007 0.011 0.009 Ce2O3 0.036 0.071 0.013 0.010 0.009 0.004 0.008 Sm2O3 0.004 0.003 0.000 0.004 0.001 0.000 0.002 Er2O3 0.001 0.002 0.011 0.002 0.008 0.012 0.007 HfO2 0.000 1.517 0.000 0.000 0.000 0.000 0.000 Ta2O3 0.000 10.250 0.000 0.000 0.000 0.000 0.000 ThO2 0.009 2.290 0.001 0.004 0.003 0.004 0.003 UO2 0.007 0.008 0.006 0.003 0.004 0.003 0.003 La2O3 0.020 0.012 0.000 0.000 0.003 0.008 0.003 Pr2O3 0.011 0.010 0.023 0.010 0.011 0.000 0.007 Nd2O3 0.013 0.021 0.000 0.011 0.004 0.000 0.005 Gd2O3 0.005 0.009 0.001 0.004 0.002 0.000 0.002 PbO 0.014 0.018 0.026 0.012 0.018 0.016 0.015 SiO2 15.677 0.000 3.110 0.000 1.037 0.000 0.346 

P2O5 0.011 0.000 0.008 0.004 0.005 0.003 0.004 

Y2O3 0.013 0.000 0.021 0.013 0.014 0.006 0.011 

Dy2O3 0.004 0.000 0.019 0.014 0.021 0.030 0.022 

Yb2O3 0.001 0.000 0.000 0.002 0.002 0.003 0.002 Total 96.620 95.595 57.199 99.996 86.136 101.212 95.781 UO2+ThO2 0.0016 2.298 0.007 0.007 0.007 0.007 0.006 
29

Table 2-12 Weight Percent of Major Phases in Slag Samples and their Uranium, Thorium and REE (rare

earth elements) abundance.

Phase Modal Abundance Weight % Uranium and

Thorium

Weight % REE

Glass Site A: 3%

Site B: 36

Site C: 2%

All: 0.6% All: 10%

Gehlenite Site A: 0-10%

Site B: 39%

Site C: 29%

Site A:0

Site B:0.065%

Site C:0

Site A:  1%

Site B: 3%

Site C:

Calcium Aluminate

(hibonite)

Site A: 30%

Site B: 1%

Site C: 

Site A: 0.2%

Site B: 0.137%

Site C: 

Site A: 4%

Site B: 1%

Site C:

Clinopyroxene Site A: 0.0%

Site B: 10%

Site C: 

Site A: 0.0%

Site B:1%

Site C: 

Site A: 0.0%

Site B: 1%

Site C:

Ce-Silicate Site A: ?

Site B: 5-10%

Site C: 

Site A: ?

Site B:2%

Site C: 

Site A: ?

Site B:18%

Site C: 

Calzirtite - Zirconolite Site A: 

Site B: 9.4%

Site C: 4.0%

Site A: 2.2%

Site B: 0.46%

Site C: 

Site A: 

Site B:

Site C:

Perovskite - Pyrochlore Site A: 11.6%*

Site B: 9.5%

Site C: 5.7

Site A: 11.5%

Site B: 2.19%

Site C:

Site A: ?

Site B:

Site C: 

Thorianite - Th oxides Site A: 

Site B:

Site C: 

Site A: 96.2%

Site B:

Site C:

Site A: 

Site B:

Site C: 

Spinel Site A: 0.0%

Site B: 

Site C: 22.6%

Site A:

Site B:0.31%

Site C:

Site A: 

Site B:

Site C: 

* For Site A calzirtite, perovskite, pyrochlore, and thorianite are combined.
30

Figure 2-1. Photomicrographs of thin section of Site A slag sample A-2. Uranium and thorium are present in

perovskite and thorianite. These photomicrographs show a layer enriched in perovskite and thorianite. See also

Figure 1-2 for photograph of hand sample. Upper image shows the sample in plane polarized light. The perovskite

and thorianite layer is black and it extends across the thin section from left to right toward the middle of the image.

The same layer appears as small white grains in the lower image which was taken in reflected light.

Plane polarized light

Reflected light

Perovskite

thorianite

layer

Perovskite

thorianite

layer
31

Ba, Al cement perovskite

Figure 2-2. Photomicrographs of thin section of Site A slag sample A-2. See also Figure 1-2 for photograph of hand

sample. Upper image shows the sample in plane polarized light. The fine grained cement, which is a complex Ba, AI

compound, is medium gray. The uranium and thorium is found in perovskite which is the black opaque material under

plane polarized light. The lower image is reflected light . The fine grained cement is more evident in this image. The

medium gray colored grains with cleavage are Ba hibonite. Light gray grains are thorianite and perovskite.

Plane polarized light

Reflected light

Ba, Al cement

thorianite

Ba hibonite

Ba hibonite

perovskite

thorianite
32

 plane polarized light

 cross polarized light

Figure 2-3. Photomicrographs of thin section from Site B slag sample B-8. The upper image is plane polarized light.

The lower image is cross polarized light (crossed nicols). In the upper image, the brown euhedral crystals are

clinopyroxene (Ca, Al, Zr, Ti, Si2O6). The dark brown feathery dendritic crystals are calzirtite (Ca2Zr5Ti2O16 ). It is

difficult to distinguish between glass and gehlenite (Ca2 Al(Al,Si)O7) under plane polarized light because both are

white. However, under cross polarized light (lower image) gehlenite is pinkish yellowish and the glass is bluish black

thus making it easy to differentiate between them.

glass

glass

gehlenite

gehlenite

perovskite

perovskite

calzirtite

calzirtite
Figure 2-4. Photomicrographs of thin section from Site B slag sample 96TC-19. Upper image is plane polarized light,

lower image is cross polarized light (crossed nicols). Uranium and thorium are found in perovskite and glass. When

viewed in color under the polarizing microscope, hibonite [(Ca,Ce)(Ti,Al)12O19] appears as bluish white laths under

both plane polarized and cross polarized light. Gehlenite also appears as light coloroed laths under plane polarizing

light but it is golden under cross polarized light. The gehlenite is visible in the lower image as medium gray laths

trending from the upper right toward the lower left part of the photomicrographs. The grayish white laths are hibonite

and trend almost 900 to the gehlenite. In the lower image, the black phases are oxides, perovskite, calzitite, and

glass.

33
Figure 2-5. Photomicrographs of thin section from Site B slag sample B-8 showing glass, gehlenite Ca2Al(Al,Si)O7,

and perovskite (CaTiO3). Uranium and thorium are found in the perovskite and glass. Under plane polarized light

(upper image) the glass has a mottled texture and is white to colorless. However under cross polarized light (lower

image) the glass appears black. Gehlenite is a white crystal with cleavage under plane polarized light (upper image)

and is visible in the lower left corner. Under cross polarized light (lower image) the gehlenite is medium gray to

white. Perovskite is green blue to black in plane polarized light and it is black in cross polarized light (lower image). 

Finally, note the weathered greenish brown fracture toward the top of the upper image.

34

gehlenite

gehlenite

glass

glass

weathered fracture

perovskite

Plane polarized light

Cross polarized light

perovskite

perovskite
35

Figure 2-6. Photomicrographs of thin section from Site B slag sample B-8 showing perovskite (CaTiO3) dendrites.

Uranium and thorium is found in the perovskite. Perovskite is is medium gray to black in plane polarized light and

although perovskite is black in cross polarized light (see Figure 2-5) it is white in reflected light (lower image).

Perovskite

Glass

Perovskite

Glass
Figure 2-7. Back scattered electron (BSE) image of a Sn slag (B-5.5) from Site B showing evidence of devitrification

and alteration of glass. This sample has weathered naturally. The glass devitrified to form wollastonite (CaSiO3) and

the Ce and Th from the glass form Ce-silicate. There is some void space created due to decrease in volume during

devitrification which would increase the porosity.

36
37

Figure 2-8. Back Scattered Electron (BSE) image of Site C slag sample C-1a. The large dark grains are quartz

pseudomorphs after cristobalite. Cracks formed by the change in volume as the SiO2 transformed from cristobalite to

quartz are quite noticeable particularly in the large grains at the top.

Quartz

Crack
38

Figure 2-9. Photomicrographs of Site C slag sample C-S in plane polarized light and BSE. Upper image is plane

polarized light and the bottom one is a Back Scattered Electron image (BSE) of a different part of the sample. In the

plane polarized light image, the large rhombohedral crystals are Al, Cr, Mg spinel (Al-Sp), the dendrites are Mg,Fe

spinel (Mg-sp), the black dendrites and interstitial phases are perovskite, and the white phase is glass. In the BSE

image, the dark gray euhedral crystals are Al, Cr,Mg spinel (Al-sp), the medium gray dendrites are Mg,Fe spinel, the

light gray is glass (G), and the white phase is perovskite (Pv).
Figure 2-10. Ternary diagram of x-ray fluoresence data showing chemical composition for all of the slag collected. 

This includes the three decommissioning sites A, B, and C, the Czech Pb slag from Pribram, the UK Sn slag from

Crift Farm and Trereife, and the UK Cu slag at Hayle (H). Site B and Site C slag fall along same trend as the

Pribram slag. Note however that the Pribram >100 year old slags have a slightly different trend line than Pribram’s 1

- 50 year old slags. Legend:  = Site A, Nb-Ta slag, 40-50 years old. = Site B, Nb-Ta slag, 40-50 years old. 

 = Site B, Sn glass, 40-50 years old.  = Site C, Na-Ta slag, 40-50 years old.  = Pribram, Pb slag 1-50 years

old.  = Pribram, Pb slag > 100 years old.  = Hayle, Cu slag, ~ 300 years old.  = Crift Farm, Sn slag, 500 -

1000 years old.  = Trereife, Sn slag 100-400 years old.

39

Bulk Chemistry

TiO2 + FeOT +

MnO + MgO

CaO + Na2O

SiO2 + Al2O3

Site C

Site B

Site A

Pribram < 100 years old

Pribram > 100 years old

Site B
40

 Weight Percent Weight Percent

 Weight Percent TiO2

Figure 2-11. Compositional variation diagrams illustrating the variation of select elements when compared with TiO2.

Site A slag - solid square, Site B glass – open circle, Site C slag – triangle, Site C slag – solid circle.

0 5 1 0 1 5 2 0

0.0

0.5

1.0

1.5

2.0

P2 O5

TiO 2

0

5

1 0

1 5

2 0

MnO

0 5 1 0 1 5 2 0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

K2 O

TiO2

10

20

30

40

50

60

SiO 2

0

10

20

30

40

50

Al2O3

0

5

10

15

20

MgO

0

5

10

15

20

Fe2O3+FeO

0 5 10 15 20

10

20

30

40

CaO

TiO2
41

Figure 2-12. X-ray diffraction patterns of Site A slag. Note the complexity of the patterns due to the number of

phases present. It is difficult to distinguish between many of the phases by this means. Consequently a suite of

analytical techniques was required to identify the phases.
42

Figure 2-13. X-ray diffraction patterns of Site B slag. Note the compositional differences. The slag at the top is

probably the result of Nb - Ta processing and the slag at the bottom is thought to be the parent tin slag prior to Nb -

Ta extraction.
43

Figure 2-14. Back Scattered Electron (BSE) image and Energy Dispersive Spectra (EDS) patterns of Site A sample

96F-A-4. In the back scattered electron image, the bright phases have a higher mean atomic number. The large,

central grain consists of glass and perovskite and it contains Al, Nb, Th, Ca, Ti, Ce, and Th (indicated by upper EDS

spectra). The medium gray grain in the lower right of the BSE image is hibonite which contains Al, Ti, and Ce (lower

EDS spectra) and it surrounds a bright sphere of pyrochlore which contains Nb, Th, Ca, Ti and Ce (lower left EDS

spectra).
Figure 2-15. Back Scattered Electron (BSE) and x-ray map images of Site B slag sample B5.5. The upper one is a

BSE image of slag in which the bright crystals are uranium/thorium bearing perovskite and calzirtite. The dark

phases are gehlenite and Ti clinopyroxene, and the medium gray interstitial phase is glass. The eight smaller images

are x-ray maps of elements: Si, Ca, Ta, Th, Ti, Ce, Zr, and U. Brightness of the phase indicates greater abundance

of the element in the image. It can be seen from these images, that uranium is concentrated in the calzirtite and

thorium is concentrated in the perovskite.

44
Figure 2-16. Site B tin slag Sample B-8. The top image (a) represents plane-polarized light. The central medium

gray euhedral crystal is a Ti - Zr pyroxene. Under plane-polarized light, gehlenite and glass are both light gray and

are difficult to distinguish. Perovskite is a black dendrite and calzirtite is present as small feathery dendrites. Image

(b) is the same view as (a) under cross-polarized light, the gehlenite crystals appear white and are easily

distinguished from the black glass. The pyroxenes are visible as medium gray, zoned crystals. (c) is a BSE image of

the same view as above. Pyroxene is medium to dark gray, gehlenite is dark gray, the perovskite dendrites are

white, and the small white stars are calzirtite. The circle in image (a) was drawn in ink on the thin section and it is

the area selected for TEM study. See Figure 2-17 for a further explanation of the methodology.

45
Figure 2-17. Upper image shows a TEM grid of Site B slag sample B 8 with an ion milled hole. The euhedral crystal

in the lower center is the Ti- Zr pyroxene seen in Figure 2-16. The black dendrites are perovskite and the gray

material around the edge of the view consists of gehlenite and glass. Ion milling was used to make a section thin

enough to be used in the high resolution TEM (HRTEM). The HTEM patterns are seen in Figure 2-18. The lower

image is a schematic cross-section of the ion milled sample.

46

 

 

 

 

 

 

 

 

A A’

10 μ

Plane polarized light

A A’

gehlenite

and glass

Cross section

ion milled void

void
47

Figure 2-18. HRTEM image of Site B tin slag (sample B-8). The upper image shows the grain boundary between

gehlenite and calzirtite. Note that the glass is devitrifying forming an unidentified crytalline material. The lower image

is a magnification of the lower left corner of the upper image and it shows the contact of the gehlenite and calzirtite.
Figure 2-19. Phase diagram of the ZrO2 - zirconolite - TiO2 + CaTiO3 system (after Xu and Wang (2000)).

48
49

3.0 Slag Weathering

3.1 Introduction

Slag is a multiphase material, containing glass and

crystalline phases. Corrosion of glasses and

ceramics has been studied for many years by both

industry and the nuclear waste community (Bates, et

al., 1982, Byers, et al., 1985, Grambow, 1992,

Vandiver, 1992), and this information is available to

bound the leaching behavior of the glass found in the

slags.

Degradation of a slag is controlled by the phases

most susceptible to corrosive attack by the

environment, which depends on the composition,

concentration and the temperature of the attacking

substances, and the glass or crystalline phases. For

example, experimental weathering of a borosilicate

glass indicates that the effect of boiling, concentrated

hydrochloric acid on borosilicate glass is almost

negligible. However borosilicate glass is severely

corroded by interaction with dilute hydrofluoric acid at

25 degrees C (Parry, 1992). In contrast, phosphate

glasses are resistant to HF, but are corroded by HCl

(Parry, 1992).

Susceptibility to corrosion by water and acid solutions

increases with the alkali content of the glass (Parry,

1992). Parry also showed that the higher the

coulombic force between the divalent cations and a

singly charged oxygen ion the lower the solubility. 

Jantzen (1992, p 20) discussed a thermodynamic

approach to glass corrosion and noted that

“...hydration thermodynamics has wide applicability to

predict the durability of ... modern, and nuclear waste

glass...” This approach may have utility with respect

to the corrosion of the glass phase in the SDMP

slags. 

In addition to dissolution of the glass, devitrification

may lead to increase in overall porosity of the slag,

and provide access for fluids to the interior of a slag

block, thus allowing potential corrosion and alteration

of glass and radionuclide-bearing phases. Glass

from the naturally weathered samples at the SDMP

sites have been analyzed and results are discussed

below.

3.2 Estimating Radionuclide Release from Slags

Chemical composition and elemental variation in the

SDMP slag glasses can be determined and provide

the basis for useful alternatives to standardized leach

tests. As was pointed out in Section 1.3 there can be

up to four orders of magnitude of difference in the

results among the ANSI/ANS 16.1, the EPA Method

1311 (TCLP) and EPA Method 1312 (SPLP) leach

tests. 

One can use the data discussed in Sections 2 and 3

of this report to calculate leach rates as mass fluxes

of radionuclides3

 from the slags analyzed to the fluids

surrounding the slags. Depending on nature of the

surrounding fluid, the radionuclide species released

may be soluble in the fluid and may deposit on the

surface of the slag. The volumetric concentrations of

dissolved species and the mass fluxes of deposited

species depend on species solubility in the flow

regime of the surrounding fluid. The data taken in

this report allow for the calculation of mass flux

releases for individual radionuclides. Cruder

estimates that lump radionuclides and their species

together also are possible.

An alternative to using mass fluxes is to estimate the

total amount of radionuclide species that could be

available from the slag and use this quantity as a

concentrated source in a transport calculation.

3.2.1 Estimating Leach Rates as Mass Fluxes at

the Slag Surface

Radionuclide species were observed as compounds,

e.g. UO2, in phases, e.g. perovskite, in slags. Massbased concentrations of radionuclide species (mass

of species/mass of phase) were measured as weight

percents. Volume fractions of phases (volume of

phase/volume of slag) were measured as modal

abundances. For purposes of estimating releases of

radionuclides from slags, one needs to know the

concentration of radionuclide r occurring as species k

in phase p in units of mass of radionuclide r/volume

of slag. This concentration can be expressed in

terms of the observable quantities and quantities

available in handbooks as

C C k p

s

ppk

m

r rp , , = μ ρ (1)

where

Ck p the volumetric concentration of species k s

r , =

containing radionuclide r in phase p in the slag in

units of (mass of species k containing radionuclide

r/volume of slag),

μ the modal abundance (volume fraction) of p =

phase p in the slag in units of (volume of phase

p/volume of slag), measured in the laboratory;

ρ the density of phase p in the slag in units of p =

(mass of phase p/volume of phase p), available from

handbooks of properties of materials; and 

C the mass-based concentration of the kth k

m

r p, =

species containing radionuclide r in phase p in units

of (mass of constituent k/mass of phase p), measured

in the laboratory.

3

The data could be used to calculate the release of nonradioactive contaminants as well.
50

If x=0 represents the slag surface and the x

coordinate is directed into the slag, the surface

gradient of the volumetric concentration is

∂

∂ μ ρ

∂

∂

C

x

C

x

k p

s

x

p p

k p

m

x

r r , , 













 = 













 = = 0 0

(2)

An analysis of the slag data collected for this report4

has shown that the slags’ phases dissolve and their

surfaces recede at a dissolution rate vp (length/time),

which can be estimated from a knowledge of the

history of the slag’s shape or from the slag’s age and

the thickness of the rind around the slag. The

volumetric concentration of the species k containing

radionuclide r in phase p in the slag near a receding

surface follows approximately the steady-state law

C x

C C

v x

D

C

k p

s

k p

s

k p

s p

k p

s k p

s

r

r r

r

r

,

,, ,,

,

, ,

( )

( )exp( )

=

0 − −+ ∞ ∞

(3)

where and denote surface and far-from- 0 ∞

surface values of concentration, and is the Dk p

s

r ,

diffusion coefficient for component k containing

radionuclide r in phase p, which one can estimate if vp

is known by fitting Equation (3) to measured

concentration profiles. The surface mass flux of

component k containing radionuclide r in phase p

from the slag is

m D

C

x

vC C

kp kp

s k p

s

x

p kp

s

k p

s

r r

r

r r

,

''

,

,

,, ,, ( ).

= 



 





= −

=

∞

∂

∂

0

0

(4)

The mass flux of species k containing radionuclide r

from the slag at its surface may be deposited on the

slag surface, dissolved in the fluid adjacent to the

slag and carried away, or both deposited on the slag

surface and dissolved and carried away. The surface

mass flux for radionuclide r, released by all skpdies

containing the radionuclide, is

m M

m

M rp r

k p

k k

r

r r

,

'' ,

''

=  , (5)

where M denotes molecular weight. The amount of

species k containing radionuclide r taken away from

the slag by the adjacent fluid depends on the flow

regime of the fluid and whether the radionuclide

species being released is soluble in the fluid. Given

the slags’ outdoor locations on or below the ground

surface, the most common adjacent fluids are

expected to be air and water. Radionuclide species

are not expected to be soluble in air, but they may be

soluble in water.

If the concentration of species k containing

radionuclide r in the adjacent fluid and far from the

slag is zero, then conservation of mass at the slagfluid interface can be expressed as

HC m k k

F

k kp

p

rr r r + = ∆  ''

,

''

(6)

where

H the surface mass-transfer coefficient for kr =

species k containing radionuclide r in the fluid

adjacent to the slag in units of (length/time) and is

described in more detail in this report’s Appendix,

C the volumetric concentration of species k k

F

r =

containing radionuclide r in the fluid adjacent to the

slag in units of (mass of species k containing

radionuclide r/volume of fluid), and

∆ surface deposition flux of species k kr

'' =

containing radionuclide r in the fluid adjacent to the

slag in units of (mass species k containing

radionuclide r/area/time).

3.2.2 Release to the Surrounding Fluid and

Deposition onto the Slag Surface

The right-hand side of Equation (6) gives the

available mass flux of released species from the slag. 

The left-hand side gives the distribution of released

species between fluid dissolution and deposition on

the slag surface. Depending on whether the species

released at the slag surface is soluble in the fluid

adjacent to the slag, Equation (6) provides a means

to calculate either the concentration of the species in

the fluid adjacent to the slag or the surface deposition

flux onto the slag.

If the fluid concentration, , of the released Ck

F

r

species is less than the species’s solubility, , Skr

there is no surface deposition and Equation (6) gives

C mH k

F

k p

p

k r rr =  ,

'' . (7)

4

“Mathematical and Geochemical Modeling of Radionculide

Leaching Rates from Radioactive Slags,” Chen Zhu,

University of Pittsburgh, Letter Report to the NRC,

September 2002.
51

For dose calculations in performance assessments,

one needs the concentration of radionuclide r in the

fluid,

C M

C

M r

F

r

k

F

k k

r

r r

=  . (8)

If the calculated value of in Equation (7) turns Ck

F

r

out to be greater than , then one has to set Skr

C equal to and use Equation (6) to estimate k

F

r Skr

the surface deposition flux of the species:

∆k kp

p

k k r r rr m HS ''

,

'' = −  . (9)

The deposition flux of radionuclide r is

∆

∆

r r

k

k k

M

M

r

r r

''

''

=  . (10)

Equations (7) - (10) account for the possibility that for

a particular radionculide, there may be some

dissolution and some deposition. This could be the

case for release into water. For release into air,

S for all radionuclides, and only deposition kr = 0

occurs at a rate given by

∆k kp

p

r r m ''

,

'' =  (11)

During field surveys for slag samples, α radiation was

observed on slag surfaces. Given that α radiation is

easy to block and that U and Th are α emitters, these

observations suggest that surface deposition of U or

Th may have taken place on the slags.

3.2.3 Calculation of Bulk Leach Rates

One can estimate a bulk mass-flux leach rate of all

radionuclides in phase p or a slag by combining all of 

the radionuclides into a composite radionuclide, R. 

The slag-surface mass flux for the composite

radionuclide is estimated using the equation

mv C

M

M pR p p p k p

m

r

r

k

r

R

r

,

'' = ,



 μ ρ (12)

The summations in Equation (12) define the

composite radionuclide R. Section 3.3.4 gives an

example of the use of this equation.

3.2.4 Estimation of Mass Available for Transport

An alternative to estimating mass fluxes directly from

the slag as boundary conditions for transport

equations is to specify an amount of mass of

radionuclide available for transport. This mass could

be treated in the appropriate transport equation as a

concentrated initial condition represented by a Dirac

delta function or an approximation to it. If all of a

constituent kr

 containing radionuclide r in phase p in a

slag is assumed to be available directly for transport,

it could be represented as a mass source in the

transport equations as

C r CV r k p

s

p p kp

m

s r r , , (,) ()   0 = μρ δ (13)

where Vs is the volume of the slag and δ is the Dirac

delta function or a product of Dirac delta functions. 

The table below describes how Vs and δ depend on

the dimensionality of the transport analysis in

Cartesian (x, y, z) coordinates.

Dimensions Vs δ( ) 

r

1 Slab

thickness δ( ) x

2 Slab crosssection

δ()( ) x y δ

3 Slab volume δ()( )() xyz δ δ

If a finite-difference method is used to solve the

transport equations, δ(x), δ(y), and δ(z) would be

1/∆x, 1/∆y, and 1/∆z, where ∆x, ∆y, and ∆z are the

dimensions of the cell where the initial contaminant

mass is concentrated.

3.3 Naturally Weathered Samples

Many of the slags collected have light tan to medium

reddish-brown weathered surfaces (Figure 3-1). This

surficial alteration is believed to be caused by natural

weathering processes. The alteration is not

unexpected since the slags were formed under

strongly reducing conditions. When the slag located

at or close to the ground surface is exposed to

oxidizing conditions, alteration may take place

because the slag is no longer in an environment in

which it is chemically stable. The alteration must be

considered in long-term estimates of the slag’s 

leaching behavior and the potential for release of

radionuclides. To determine the physical and

chemical changes taking place in the slag with time 

an investigation into the slag’s weathering was done

on a few of the samples to determine what physical
52

and chemical changes were occurring. 

The slag’s weathering was rapid and it took place

some time between the slag’s formation which began

in the 1950s and continued through the mid 1970s at

Site B and the 1990s at Sites A and C (i.e. ~ 40

years or less). Although this is not a long time by

regulatory standards for radioactive waste disposal,

weathering has taken place and it must be

considered in long-term estimates of the slag’s

leaching behavior and the potential for release of

radionuclides.

In hand specimens the weathering manifests itself as

a surface rind of a few mm to > 1 cm thick. However,

the weathering extends further into the samples than

is apparent to the unaided eye. Optical and electron

microscopy was particularly useful for clarifying the

extent of alteration and for determining the

mineralogic changes associated with alteration (i.e.

what disappears and what secondary minerals form

due to alteration). Microscopy revealed that the

alteration extended well into the fabric of the slag

particularly along fractures and grain boundaries (e.g.

see Figure 3-2). 

Sample porosity may play a role in the extent of

weathering and a preliminary analysis was done. 

Given that the slags have all been sitting for about

the same amount of time (20 to 40 years) there are

slight differences in the thickness of the weathering

rind. They are often >1.0 cm thick on the Site A

slags, and ~0.2 cm thick on the Site B and Site C

slags (Figure 3-1). The difference in the thickness of

rind appears to be a function of sample porosity. 

3.3.1 Natural Weathering Features at Site A

Samples from Site A often feature a ~1.0 cm

yellowish-brown rind (Figure 3-1). The mineralogy of

this rind is complicated and has not been fully

determined. It is apparent though that the rinds are

enriched in barium sulfates (barite), iron oxides and

hydroxides. SEM studies have also identified

secondary clays on the surfaces of the slag. Clays

are a common secondary mineral formed after the

alteration of silicate minerals. Detailed studies of

chemical variation around cracks and grain

boundaries have not been carried out. The clastic

nature and high porosity of the Site A slags makes

this a difficult task. The task is complicated further

because it is likely that all of the clasts have been

altered to some degree due to the manner in which

the original slag was crushed, chemically processed,

and then allowed to settle in evaporation ponds

where natural cementation of the processed material

took place. Because of this processing, it is difficult

to determine what is a primary mineral assemblage of

the slag as opposed to chemical alteration caused by

the Site operator’s processing of the slag, or by

weathering processes which may have altered

minerals and resulted in leaching of phases. 

3.3.2 Natural Weathering Features at Site B

Alteration rinds on Site B slag are typically ~0.2 cm

thick and they are composed of secondary iron

oxides and hydroxides. Microscopy of the slag shows

that the alteration is actually more pervasive than

what is visible with the unaided eye and the alteration

actually penetrates tens of centimeters into the slag. 

The alteration concentrates along grain boundaries

and fractures in the slag. This is confirmed by EMPA

elemental analyses which show a depletion in several

elements along fractures. It is not clear, at this time,

whether the depletion is due to primary crystallization

or to leaching from along fractures. One could

determine the cause of the depletion with additional

studies of the interfaces with the TEM.

SEM images of weathered surfaces of Site B slag

reveal: (1) dissolution of the glass, (2) leaching of

glass and crystalline phases, (3) devitrification of the

glass, (4) precipitation of secondary phases, and (5)

possible microbial activity. Some examples of these

processes are shown in Figures 3-3, 3-4, and 3-5. 

Figure 3-3 shows a weathered edge of Site B sample

B-5.5. Perovskite, calzirtite and gehlenite are

observed (BSE image 0004, Figure 3-3a) protruding

from the glass along the weathered edge of some

Site B slag, indicating that the glass has been

dissolved from around the more resistant crystalline

phases. Glass is also devitrified near the edge of

some samples. SEM image 0006, sample B-5.5

(Figure 3-3b) of a weathered surface shows small,

round precipitates that are high in Fe content, on the

surface of a calc-silicate. This is possibly the result of

microbial activity.

A sample of Sn slag (B-5.5) from Site B provided

leaching information over a longer period than

represented by the site’s Nb-Ta slag (e.g. samples B5.5, B-8, B-11). Sn slag was imported from Malaysia

in the mid-1960’s and it was the parent material from

which Nb-Ta was extracted. However, it has not

been produced in Malaysia for the last 50 – 100

years. Therefore the Sn slag found at Site B is at

least 50 years older than the site’s Nb-Ta slag. 

Devitrification of the glass in the Sn slag is evident in

BSE images of B-5.5 (Figure 2-7). The feathery

features radiating from gehlenite and clinopyroxene

grains are wollastonite (CaSiO3) and Ce-silicates. 

Also evident is the decrease in volume of the

devitrified materials relative to the original glass. 

Voids and fractures have been introduced into the

slag from devitrification. A fracture filled with Casilicate and Fe-oxides is also observed in the Snslag, indicating the leaching of Ca and transport of Ca

and Fe through the slag. The calzirtite and zirconolite

in the slag appears to be altered. This could be

investigated further with TEM which might bring to

light variations in the Th and U content of calzirtite

and structural degradation in the crystals and along

grain boundaries.

The uranium and thorium are not present as uranium

or thorium oxides, but as elements in (minor or trace
53

amounts) in silicate or oxide minerals and glass. 

Table 2-12 shows the relative amounts of U and Th in

the minerals as well as the modal abundance of the

minerals in the slag. These quantities can be used in

Equations (1) and (4) to estimate boundary conditions

or in Equation (13) to estimate a concentrated initial

condition for solving transport equations.

Analysis of a Site B glass provides an estimate of the

surface-concentration gradient that can be used to

estimate the long-term leach rate in Equation (4). 

Figure 3-6 is a graph of microprobe data for the

variation of REE with distance from a fracture in a

glass. REE were used as a surrogate for U and Th

because their low concentration made it difficult to

detect them. In Figure 3-6, the concentration of REE

increases with distance from a fracture indicating that

leaching has been removing REE from the region

near the fracture, which also is shown schematically

in Figure 3-7. The linear-logarithmic regressions of

the data points in Figure 3-8 provides a way to

calculate surface concentration gradients that can be

used in Equation (4) to estimate of the 25-30 year

leaching rates.

It is apparent from EMPA data and BSE images that

the chemical composition of the weathered glass in

Site B samples B-5.5 (Sn slag) and 96TC19 (Nb-Ta

slag) varies with distance from fractures and grain

boundaries (Figure 3-9 and Figure 3-10). Leaching of

specific elements from the glass may have occurred

in these areas. Microprobe analyses along a traverse

crossing the fractures indicate a change in glass

chemistry. Glass near the fractures tends to be

depleted in Th and U relative to glass several

hundred microns from the fractures (see Figure 3-10). 

There is some scatter in the data and the plot is not

smooth, however, there are several variables that

have not been addressed by these analyses. It is not

certain whether the glass initially had these chemical

variations (i.e. whether the variations are due to initial

crystallization). Another possible explanation for the

scatter in the data is the inability to determine the

distance of a point analyzed from a surface or

fracture in the third dimension. These uncertainties

could be addressed in the future with additional

analyses.

Glass, clinopyroxene, and perovskite from four

samples from Site B’s Nb-Ta slag (sample B-5.5)

were analyzed in detail by EMPA for chemical

variation as a function of distance from the weathered

surface. Samples B-5.5a, B-5.5b, B-5.5c, and B-5.5d

were cut from one sample, B-5.5a taken from the top

weathered surface, with B-5.5d taken from the lower,

presumed unweathered surface (Figure 3-7). The

data for glass are plotted in Figure 3-8. The outer 10

mm of the sample show a depletion of Si, Ca, Ce,

Mn, Zr, Th and U with respect to the interior of the

sample. Al, Ti, and Mg all increase in the outer 10

mm of the sample.

Glass analyses from B-5.5a and B-5.5b have been

plotted as compostional variation diagrams Figure 3-

9, comparing the variation of key elements with SiO2 . 

Data from sample B-5.5d (unweathered), all indicate

a degree of correlation for Al, Ti, Zr, U with SiO2. 

However, the data for B-5.5a (weathered) indicate U,

Zr, and Ca are not as well correlated with SiO2. This

may be indicative of leaching of U and Ca from the

glass in the weathered sample.

The electron microprobe was used to determine

elemental variation in calzirtite as a function of

distance from a weathered edge in samples from Site

B and the results are shown in Figure 3-10. In the

weathered slag (B5.5a) the weight percent of Ca and

Si declines towards the weathered edge whereas the

weight percent of Zr, Ti, Ce, Th, and U increases. In

two samples of unweathered slag (B5.5c) data were

collected throughout the thin sections and there is

little variation in values of the aforementioned

elements. It is interesting to note that the weathered

samples contain higher values of both Th and U

compared to the unweathered samples.

3.3.3 Natural Weathering Features at Site C

Alteration rinds were common in the Site C slag. This

is not unexpected as the Site C samples had been

collected at the ground surface where they were

exposed to weathering processes. 

(1) Glassy Slags

Slags in this group (e.g. sample C-2a) are highly

vitreous and often vesicular. Weathered slags of this

category are devitrified, transformed from a high

temperature glass phase to a low temperature

crystalline phase involves a volume decrease and the

effects of this are seen by the prevalent cracks in the

dendrites and the large SiO2 patches (Figure 2-11). A

weathered rind is often seen, consisting of iron oxides

and clay-like materials.

(2) Dense Blocky Slags 

These slags are distinctly different from those in the

first group since they are more dense and blocky, and

also seem to have fewer vesicles. Slags in this

category are physically similar to basalts and to the

black, finely crystalline slag from Site B (e. g. 

samples B-5 and B-5.5). The exterior surfaces of

these slags are readily weathered to a greenish-gray

fine-grained, clay-like material with deep fractured

surface. These fractures appear to be hydration

cracks. The fresh surface of hand sample (C-4) is

dark reddish-brown in color. Minerals found in this

group include hibonite, barium aluminate, periclase,

monticellite, Cr spinel, perovskite, zirconolite, 

metallic iron prills, wollastonite, wustite and an

alteration product, ardelite Ca2(SO4)2(HPO4).4H2O.

(3) Green-Blue Slags

Slag in this group (e.g. C-6) is distinctly greenish-blue

in color and often has a 1 mm thick white crust

around the outside of the sample. The exterior
54

surfaces of these slags are deeply indurated from

hydration cracking. These surfaces contain clay-like

materials. Freshly cut surfaces of this group’s slag

reveal a greenish-blue interior and small circular

brassy looking metallic prills are often visible to the

naked eye. Mineralogically, this group contains

spinel, periclase, monticellite, forsterite, perovskite,

zirconolite, a Ti-silicate, a Ni-oxide and metallic

chromium

Samples in Site C’s blue-green slag often have a fine

powdery coating of secondary minerals consisting of

CaCO3, Ca alumino sulfate, and MgCO3. 

A sample of the site’s dense blocky slag (C-s) was

studied in detail to describe the alteration features

observed. Alteration was not visible with the unaided

eye, but thin sections of the sample showed a distinct

zone of alteration (Figure 3-11). The zone is

characterized by a ~0.4 mm thick reddish-brown

alteration rind. The minerals are too fine grained to

be determined with a petrographic microscope. At

the SEM scale, the weathering features became quite

apparent. In the unweathered interior, periclase

dendrites are overgrown by spinel and these phases

are usually surrounded by monticellite with the

occasional forsterite. Moving towards the edge of the

sample, a transition zone is encountered where intact

periclase dendrites disappear, leaving an empty

dendritic shape. At the very edge of the sample,

periclase is completely weathered away leaving

empty dendritic forms that are partially infilled by

secondary iron oxides and iron hydroxides. Also,

within the weathered zone, metallic iron prills are

rimmed with secondary iron oxides and hydroxides. 

Secondary iron phases also fill cracks in the

weathered rind and rim the euhedral spinel crystals.

3.3.4 Estimation of Weathering Rate

As mentioned in the text preceding Equation (3), the

weathering rate vp can be estimated if one has

information on the history of the slag’s shape. For

example, a slag sample at Site C weathered to a

depth of .035 cm over 50 years, so that vp = 0.007

cm/yr. Equation (12) can be used to estimate the

bulk release of U and Th (combined molecular weight

of 470 g/mol) from the slag. U and Th occurr in this

sample as UO2 and ThO2 (combined molecular

weight of 534 g/mol) in perovskite (density of 4.1

g/cm3

) with no surface weight percent, a combined

bulk weight percent of 0.2, and a modal abundance of

4.6%. Using Equation (12), one can estimate the

bulk mass flux ro be 230 ng/cm2

-yr.

3.3.5 Distinguishing Between Primary and

Secondary Phases through the Use of

Artificial Melts

To distinguish between the primary phases that

formed during the initial cooling of the slag and the

secondary phases which formed after the slag

weathered, we melted a powdered Sn slag from Site

B in a Burrell electric furnace. It required a

temperature of around 1525o

C to melt the powdered

slag. Following cooling of the melt, microprobe

analyses of the resulting material indicated the

presence of perovskite, gehlenite, clinopyroxene

along with glass Figure 3-12. By comparing this

laboratory produced slag with Sn slag collected at

Site B (sample B-5- 5.5) we concluded that the

secondary phases consist of Ce silicate,Ce

aluminosilicate, feldspar, wollastonite, barium

aluminosilicates , clays, and gypsum.

3.4 Artificially Weathered (Leached) Samples

It is recognized that weathering and therefore

potential leaching of U and Th from the slags may

occur at different rates depending on the solution

chemistry. In an attempt to quantify this effect, A.

Felmy and D. Rai, PNNL, (Felmy, et al., 1999)

performed a series of leaching experiments on splits

from several slag samples collected at Sites A and B. 

The slags were leached with deionized water and with

solutions at pH ranging from 0.5 - 12. Splits of the

solid portion from the PNNL slag leaching studies

were investigated at JHU. Samples of unleached

slags, slags leached with deionized water, and slags

leached under different pH conditions were

characterized by SEM and quantitative electron

microprobe analyses to compare the effects of

leaching on the degradation of the various phases. 

The samples were prepared in several ways that

allowed the authors to examine the leached surfaces,

and to determine changes in mineral and glass

chemistry due to leaching. Pieces of the leached

solids were mounted on microprobe sample holders,

carbon coated and analyzed by Secondary Electrons

to view the surface morphology, thereby providing a

qualitative image of alteration. Other pieces were

mounted in epoxy, then polished, carbon coated

which allowed for detailed elemental analyses by

wave-length dispersive X-ray analysis (WDS) of the

crystalline and glass phases.

SEM images of a Sn slag (B-5.5) from Site B 

leached by deionized water are shown in Figure 3-13. 

The glass in these images has been partially

dissolved. Perovskite and gehlenite protrude slightly

from the glass surface. Fine grained needle-like

precipitates litter the glass. The secondary

mineralization is too fine grained for accurate analysis

by electron microprobe.

SEM micrographs of slag surfaces of a Nb-Ta slag

(96TC-19) from Site B leached under varying pH

conditions are shown in Figure 3-14. Figure 3-14 is

an image of a glass surface leached at pH 0.5 and

pH 2. The glass exhibits dehydration cracks, pitting,

and crystal precipitation. Calzirtite, perovskite and

hibonite crystals protrude from the glass. Crystallites

of secondary phases are precipitated on the glass

surface. Pitting of the glass occurs at pH 0.5. The

(100) surface of hibonite crystals appears to be

relatively unaffected by the acidic solution, however

the (010) surface shows signs of dissolution
55

perpendicular to the axis (Figure 3-14). It is likely that

the Ca ions which occupy sites between octahedral

and tetrahedral sheets in the hibonite have been

leached or exchanged at this pH (Figure 3-14). This

is reflected in the chemistry of the glasses and

minerals. A plot of microprobe data from leached

glasses of 96TC19 shows a depletion in CaO and

Al2O3 at pH< = 4, accompanied by an increase in

SiO2 (Figure 3-15).

Figure 3-16 shows an analysis of the effect of

leaching hibonite in Nb-Ta slag sample 96TC19 over

a pH range from 0.5 to 12. The UO2 content of the

hibonite was low and its UO2 made up < 0.02 wt % of

the hibonite. Little of the UO2 appears to have been

leached from the hibonite between pH 0.5 to pH 12. 

ThO2 was slightly more abundant and it made up to

0.19 wt % of the hibonite. The most significant

leaching of the ThO2 was between pH 0.5 and pH 2.

The data indicate that Al2O3, TiO2, and ThO2 are

depleted in the low-pH range, 1-4. MgO and CaO

content has increased in the low pH range. SEM

images of the above analyzed samples are seen in

Figures 3-17, 3-18, and 3-19. A BSE image of

thorianite in this sample, Figure 3-20, indicates a

similar trend, i.e. depletion of certain elements in the

low pH range (Figure 3-16). These data may help to

explain some of the leaching and solubility data

obtained by PNNL for these slags.

3.5 Archaeological Slags

3.5.1 Introduction

Since standardized leaching tests give information

only about short-term processes, archaeological

slags are being studied to determine their long term

leaching behavior and weathering characteristics. 

Comparison of the leaching behavior and weathering

characteristics of slags covering a span of ages will

yield information about how certain phases withstand

exposure to weathering over the long term. To

perform this comparison, a small suite of samples

was collected from archaeological sites (Table 3.1). 

Such characterization of archaeological materials is

necessary for understanding and defining the

mechanisms of long term alteration and weathering of

a slag, which, in turn is important for assessing the

hazards that slag presents to its surrounding

environment. Defining these mechanisms may also

be pertinent to determining the safety and

effectiveness of glass and ceramic waste forms. The

weathering mechanisms that affect slags are closely

linked to the individual phases and their composition. 

Before the specifics of alteration can be defined, all

phases present in the system must be identified. The

processes of alteration and weathering ultimately

determine the relationship between slag piles and

their surrounding environments. Understanding

these processes may signal the need to take a closer

look at the current uses of slag (e.g. as building

material and as a soil additive) and current

remediation techniques for large slag piles. 

Most slag studies associated with archaeological

sites look to the composition and morphology of slags

for information about ancient smelting processes

rather than their long term weathering characteristics

(e.g. Franklin et al., 1976; Mascaro et al., 1995). 

Slags and metallic artifacts are often the only

indicators for the smelting conditions and the original

ingredients of the smelt which may be of interest to

those reconstructing the history of an area. For

example, excavations in Göltepe, Turkey, found a

small piece of vitreous material (slag) melted to the

wall of a crucible (Vandiver et al, 1992). Further

studies and a comparison to a medieval tin slag from

Cornwall, England, indicated that the Göltepe sample

was formed by a tin smelting process (Adriaens,

1996). 

Unfortunately, detailed, full-scale mineralogical

surveys which would provide information on the long

term weathering of slag are rare in investigations of

ancient slags. Compositional zoning and trace

element content are not usually included in

petrochemical descriptions of the phases (e.g.

Maskall et al., 1995; Adriaens, 1996; Gee et al.,

1997). Knowledge of such small-scale information

may not be obviously useful for archaeological

studies, but is very important for understanding the

possible contamination emanating from a smelting

site. With rising concerns about the pollution

associated with ancient and modern metallurgical

sites, archaeologists and geologists have begun

examining elemental migration (e.g. Mascall et al.,

1995). In the Czech Republic, lead and other heavy

metals are being leached from large slag heaps and

are polluting nearby streams and soils. Lead leached

from ancient metal working sites has also become a

concern for drinking water sources in Greece

(Voutsinou-Taliadouri et al., 1995; Sharp, 1997). 

More recently, slags that were crushed and added to

soils to raise the pH have been pinpointed as the

source of vanadium that poisoned herds of cattle

(Frank et al., 1996). Again, many of these studies

present bulk chemical data but do not present

detailed information about the individual phases

present in the slag.

3.5.2 Dating of Archaeological Slags

At many archaeological sites, slags serve as the sole

“witnesses of metallurgical activity” (Mascaro et al,

1995). At some locations they are surficial indicators

for unexcavated metal working facilities, as was the

case at Crift Farm, Cornwall, England (Figure 3-21). 

Dating of such material can be challenging. 

Obtaining an absolute age (age in years before the

present) is often impossible due to the lack of organic

material (such as charcoal or pieces of unburned

wood) that can be dated by 14 C methods. When

such material is absent, relative dating techniques

are used. Relative ages are based on the relative

position of material within a vertical column and

association with other datable artifacts. In an

undisturbed pile of slag the material at the base of the

pile is older than the overlying material. If the
56

layering of a pile has been disturbed or re-worked, or

if the deposition surface was not horizontal, such age

correlations are more complicated, and in some

cases, totally unreliable. For example, at the Crift

Farm site in Cornwall, England, small pieces of

glassy slag are dispersed throughout the soil. The

soil has been tilled and re-worked and thus the small

pieces of slag, the rare potsherd, and even larger

artifacts such as a large rock that appears to be a

grinding stone, elude dating.

The juxtaposition with datable material can also yield

a relative age for a slag. Potsherds (with chemical

dates or relative dates due to their style and

decoration) incorporated into a slag pile can give a

relative age for the slag. In Cyprus, the slag pile at

Skouritossa mine has Roman-aged potsherds

associated with the lower section of the pile which

implies an age of approximately 2,000 years before

present for the base of the pile. 

All of the samples associated with this study of

archaeological slags have relative dates except for

the Cornish slags, which have a C14 date of 1125 A.D.

(Earl, 1998). In some cases, sample ages are

constrained by knowledge of how long a smelting site

was in operation (as is the case with the Cu slags

from Hayle, Cornwall, UK). Samples from the Czech

Republic have relative ages based on their MgO

content, since there is documentation about the

changes in the composition of the ore and flux over

time.

3.5.3 Archaeological Slags Investigated

Table 3.1 contains a list of archaeological slags

investigated in this study. They include tin slag from

Malaysia and Great Britain (Figure 3-21), Late Bronze

age copper slag from Cyprus, and lead and silver

slag from the Czech Republic Figure 3-22. These

slags range in age from ~ 2 years old to over 2,000

years old. Transmission electron microscopy,

electron microprobe analyses, X-ray spectroscopy,

and traditional optical microscopy were used to

identify and describe slags’ components. In

particular, the distribution of rare earth, radioactive,

and/or toxic elements among primary and secondary

minerals was determined by Farthing (2002).

3.5.3.1 Tin Slag from England and Malaysia

Tin slags from England and Malaysia were collected

for comparison to the slag from Sites A, B, and C. 

The tin slag from Malaysia is quite complex

mineralogically, with some samples featuring more

than 20 individual phases. Still it is hoped that the

mineral assemblages of the Malaysian and English

slag will be comparable, and that textures and phases

associated with alteration will be more prevalent in

the older samples from England and Malaysia than in

the samples from Sites A, B, and C. To assist in

determining the effects of weathering, cassiterite

(SnO2) ore from one of the Cornish tin mines could be

smelted to create an ‘age-zero’ unweathered slag. 

The phases in this ‘age-zero’ slag can be compared

with the older Cornwall Slag to determine the

changes due to weathering of an ancient slag and to

project what might have been lost with time. The fact

that we were unsuccessful in finding archaeological

Sn slags from the UK much older than 1000 years

may be the result of devitrification and other

processes that have consumed the very old slags.

3.5.3.2 Copper Slag from Cyprus

A sample of a Late Bronze Age copper slag from

Skouritossa, Cyprus, was incorporated into the study

of the long-term weathering and leaching of slag. 

Although the composition of the Cyprus slag is not

analogous to that of the tin slags, it may offer an

insight to the behavior of glass. A thin-section of the

sample features long crystals of olivine (Figure 3-23). 

Glass is usually the host material for slag crystals, but

in the Cyprus sample, the area between the olivine

laths is no longer glass. Complete devitrification of

the interstitial glass is observable, and is also

supported by a flat background in powdered XRD

spectra of the bulk sample which is seen in Figure 3-

24. The composition and identity of the alteration

product are unknown at this time. The lack of glass

observed in the Cyprus slag raises the question of

whether the absence of glass is consistent

throughout the slag pile. Examination of other

samples from this site would indicate if this was

indeed the case. Also, the material around the olivine

laths should be identified to help define the

mechanisms and reactions that might have caused

the breakdown of interstitial glass. 

3.5.3.3 Lead and Silver Slag from the Czech

Republic

Our investigation of lead and silver slag from

Pribram, in the Czech Republic, was only preliminary. 

The bulk chemistry of the Pribram slag differs from

the Site A, B, and C slags (Figure 2-10). Careful

investigation of the phases present in the Pribram

slag would need to be done for comparison with the

slags from Sites A, B, and C. Because pitchblende

(uraninite-UO2) was inadvertently incorporated into

the Pribram smelting process there may be

mineralogical similarities of Pribram with Sites A, B,

and C. With regard to weathering, the samples from

Pribram range in age from 2 to >100 years old. The

ore and the processing techniques have been

relatively constant over this span of time, making the

Pribram slag an ideal candidate for investigating

weathering through time. 

3.6 Summary of Weathering Mechanisms of Slag

The following weathering mechanisms are evident in

the slag from sites A, B, and C: (1) formation of

weathering rinds through oxidation, (2) devitrification

of the glass accompanied by formation of voids, (3)

leaching of glass and crystalline phases along

fractures and grain boundaries, (4) precipitation of

secondary phases, and (5) possible microbial activity. 
57

All of the above lead to the formation of secondary

minerals. As noted previously, the slags were

formed under strongly reducing conditions. When

exposed to oxidizing conditions, such as found at or

near the ground surface, weathering is rapid and

many if not most of the samples collected had

weathering rinds up to several cm thick. Weathering

is also visible on a microscopic scale and glass

corrosion could be seen along fractured surfaces as

seen in the upper photograph in Figure 3-2. This is

an image of an archaeological slag, several hundred

years old and the glass alteration along the fractures

is quite evident. Devitification also occurs and this is

accompanied by a negative change in volume that

results in voids within the slag ( Figure 2-7). In the

case of natural glasses, weathered basaltic glasses

form palagonite, a vitreous, red, yellow or brown, finegrained mixture of clays and silicates (Jervinovic and 

Ewing, 1992). Likewise, we see evidence of

palagonite formation in the Site B and Site C slags. 

The outer tens of microns of material in the Site C

slag show a preferential dissolution of periclase, a

Mg-Fe-oxide (Figure 3-25). Dissolution extends into

the slag along and within the spinel grains. This

indicates grain boundary diffusion of corroding fluids

into the interior of the slag. Glass in the Site C slag

(Figure 3-26) has oxidized and partially recrystallized

to Fe-oxyhydroxides. Weathering along grain`

boundaries is fairly common in humid regions. With

the passage of enough time, granitic and basaltic

rocks often form saprolites in humid, near-surface

environments. Saprolite is a material that has all the

textural features of the original rock, but, the minerals

have been altered to clays, and oxides. This material

is friable and does not have the structural integrity of

the original rock. This is a likely scenario for the longterm degradation of the slag in an Eastern humid

environment. The fact that we have been

unsuccessful in finding archaeological Sn slags from

the UK much older than 1000 years may support this

conclusion.

The weathered slag provides information that can be

used for estimation of a leach rate. For example,

naturally weathered glass from Site B (Figure 3-8)

shows a loss of 0.2 weight percent Th and 0.05

weight percent U from the outer 10 millimeters of the

slag over a 30 year period. This information can be

used in Equation (4) to estimate a 30-year leach rate. 

Leach rates calculated using Equation (4) or (12)

require determining the weathering rate for each

phase. Although the weathering process appears to

follow a steady-state law (cf. Equation (4)), the law is

at best a quasi-steady state law. As Figure 3-27

shows schematically, the surface-concentration

gradient will vary with time and from Equation (4), the

degradation rate, vp, also will vary with time. An

estimation of this temporal variability requires an

examination of young (e.g. SDMP) and

archaeological slags.
58

Table 3-1 Table for Estimating Leach Rate at Site B Based on

Mineral Abundance in the Slag Samples.

Phase Modal

Abundance

Wt% (U +

Th) in

phase

Wt% in Slag

Gehlenite 38.8 -- --

Glass 36.3 ~ 0.25 ~0.09

Clinopyroxene 10.1 –0.02 –0.002

Perovskite 9.5 ~0.53 ~0.05

Calzirtite 4.4 ~0.46 ~0.02

Rutile 1.98 -- --

Pyrochlore 0.05 ~1.66 ~0.001

Alteration 0.4 --

Cracks and holes 2.1 -- --

Table 3-2 Archaeological Slags investigated in this study.

Location Metal Extracted Age

SW Great Britain

(Cornwall and Devon)

Crift farm

Trereife*

Lamb and Flag

Seleggan*

Chy-an-Dour*

Charlestown

Week Ford

Eylesbarrow

Hayle

Sn

Sn

Sn

Sn

Sn

Sn

Sn

Sn

Cu

1100 - 1600 a.d.

1710 - 1896 a.d. 

1715 - 1883 a.d.

1887 - 1931 a.d.

1660 - 1910 a.d. 

1834 - 1884 a.d. 

640 - 17th century

1822 - 1831 a.d. 

~300 years old

Czech Republic

Pribram

Pb and Ag ~2 to 100 years old

Cyprus

Skouritossa

Cu Late Bronze Age

(>1,000 years old)

Malaysia* Sn 50-100 years old

* indicates radioactive slag
59

Figure 3-1. Weathered Slags from Sites A and B. Top photograph is of a Site A sample after it was split into two

pieces. The contrast between a weathered and unweathered surface can be seen in the block in the foreground. The

outer weathered surface is visible on the right edge of the block and it is a reddish brown material several mm thick.

The unweathered interior is visible on the left and the alternating light and dark layers show up well. The lower

photograph is of Site B sample B-5.5 after being split in the field. The outer weathered surface is visible on the top of

the block on the left. It is a light tan color and is several mm thick. The unweathered interior is very dark gray in

color.
Figure 3-2. Photomicrographs of weathered slag under plane polarized light. The upper image is archeological slag

from Lamb and Flag, UK (200 years old). The lower image is Site B slag ~35 years old. Alteration is seen along

fractures in even this young slag. The small star shaped crystallites have a calzirtite composition.

60

200 year old slag 

from England

35 year old slag

 (Site B)

alteration

alteration
61

Figure 3-3. SEM images showing weathering features in Site B sample B-5.5. Sample is from PNNL and it was

leached with deionized water. In the upper image perovskite and calzirtite protrude from weathered glass. In the

lower image, round precipitates are visible on the glass surface. The precipitates have a high Fe content and might

be the result of microbial activity.
Figure 3-4. TEM images of Site B tin slag B8 showing weathering on a sub-microscopic scale. (a) HRTEM image of

devitrification in glass. Small clusters of lattice fringes are visible and represent nano-crystals of gehlenite and

calzirtite formed as the glass devitrified. (b) TEM image showing crystals of calzirtite nucleated on a gehlenite

crystal.

62

glass

nano crystals

gehlenite

calzirtite

glass
Figure 3-5. Back scattered electron (BSE) image of weathered Site B slag sample 96TC-19. The BSE image is at

the top and the x-ray maps showing elemental variation are at the bottom. In the x-ray maps, the higher the relative

concentration of a given element, the lighter the color. Thorium is associated with tantalite, pyrochlore, or zirconolite.

63
64

Figure 3-6. Microprobe analyses of Site B sample 96TC-19 showing variation in rare earth elements (REE) as a

function of distance from a fracture. As can be seen in this graph, there is a depletion in the rare earth elements

closer to the fracture.
Figure 3-7. Schematic diagram illustrating location of elemental variation measurements from Site B sample B-5.5 as

a function of proximity to a weathered edge. See Figure 1-1 for a photograph of B-5.5 being collected. Samples B5.5a, B-5.5b, B-5.5c, and B-5.5d were cut from one sample with B-5.5a taken from the top weathered surface and B5.5d from the bottom presumably unweathered surface. B-5.5b and B-5.5c are intermediate between the weathered

edge and the interior.

65
Figure 3-8. Chemical changes associated with weathering. These graphs of microprobe analyses show elemental

variation associated with weathering. The sample is a Site B glass in which B-5.5a represents the outside of the

sample and B-5.5d the unweathered interior. Note the elemental changes in the weathered sample. In approaching

the weathered edge, there is an increase in the relative amount of Al, Ti, and Mg accompanied by a decrease in Zr,

Ce, Mn, Th, and U. Ca and Si exhibit little change in amount. In contrast, elemental variation is negligible in the thin

section of the unweathered sample and there is no depletion of Th and U.

66

Site B Glass - 9 Traverses Across Thin Section

Weathered Edge Unweathered Edge Weathered Edge Unweathered Edge

Weight Percent Oxide

Distance from edge of thin section (mm)

100

10

1

0.1

0.01

100

10

1

0.1

0.01

y = 0.0278x + 32.862

y = -0.0999x + 8.7227

y = -0.0417x + 3.5089

y = - 0.0208x +0. 2936

y = 0.0624x + 36.493

 y = 0.0298x + 5.0565

y = 0.0039x + 1.1986

y = 0.0045x + 0.5213

y = 0.0009x + 0.0841

y = 0.0148x + Ce

2.2237

0 10 20 30 40 0 10 20 30 40

Ca

Al

Ti

Mg

Si

Zr

Mn

Th

U
Figure 3-8 (cont’d). Chemical changes associated with weathering. These graphs of microprobe analyses show

elemental variation associated with weathering. This sample is a Site B glass in which B-5.5d represents the

unweathered interior of a slag block. Note that in this sample, B-5.5d, elemental variation is negligible in the thin

section and there is no depletion of Th and U. In contrast, the elemental changes in the weathered sample (B-5.5a,

pg 66), in approaching the weathered edge, there is an increase in the relative amount of Al, Ti, and Mg

accompanied by a decrease in Zr, Ce, Mn, Th, and U. Ca and Si exhibit little change in amount.

67

Site B Glass - Unweathered Thin Section

Weight Percent Oxide

Distance (mm)

100

10

1

0.1

0.01

100

10

1

0.1

0.01

y = 0.0372x + 33.154

y = 0.005x + 11.491

y = 0.0472x + 3.4085

y = 0.0254x + 0.868

Ce

y=0.0411x+1.6357

y = -0.1078x + 38.063

y = 0.0598x + 5.5935

y = -0.0091x + 1.096

0 10 20 30 40 0 10 20 30 40

Ca

y = -0.0004x + 0.3661

Al

y = -0.0001x + 0.0676

Ti

Mg

Si

Zr

Mn

Th

U
68

Figure 3-9. Compositional variation diagrams for Site B weathered glass. Note the degree of correlation of Al, Ti, Zr,

and U with SiO2.
69

Figure 3-9 (continued). Compositional variation diagrams for Site B weathered glass. Note the degree of correlation

of Al, Ti, Zr, and U with SiO2.
Figure 3-10. Electron microprobe analyses of Site B samples showing the elemental variation in calzirtite as a

function of distance from a weathered edge. In the weathered slag (B5.5a) the weight percent of Ca and Si declines

towards the weathered edge whereas the weight percent of Zr, Ti, Ce, Th, and U increases. In the two samples of

unweathered slag (B5.5c) data were collected throughout the thin sections and there is little variation in values of the

aforementioned elements. It is interesting to note that the weathered samples contain higher values of both Th and

U compared to the unweathered samples.

70
71

Figure 3-11. Photomicrograph of a weathered edge of a dense blocky slag (sample C-2a) from Site C. Alteration

was not visible to the naked eye, but thin sections of the sample showed a distinct zone of alteration. The alteration

zone is characterized by a ~0.4 mm thick reddish-brown alteration rind. The minerals of this zone are too fine grained

to be determined with a petrographic microscope. At the SEM scale, the weathering produce of this zone was

identified as ardelite (Ca2(SO4)2(HPO4).H2O and periclase (MgO). The dark black material in the middle of the slide is

perovskite. Other phases present in the sample are Ba aluminate, periclase, monticellite, Cr spinel, zirconolite, and

metallic prills.

Alteration layer

Plane-polarized light

Cross-polarized light

Reflected light

Cr spinel

Ba aluminate

zirconolite
72

gehlenite

calzirtite

perovskite

glass clinopyroxene

Figure 3-12. BSE image of remelted Site B slag. Note that there is no evidence of devitrification. The large dark

gray crystals are gehlenite, the medium gray groundmass is glass, and the dark areas within the glass are

clinopyroxene. The very light phases are perovskite which appear as dendrites, and calzirtite which form small star

shaped clusters.
73

Figure 3-13. SEM secondary electron images of Site B Sn slag sample B-5.5 showing surface of the slag leached by

PNNL with deionized water. Image (a) shows perovskite dendrites (white) protruding from the glass (dark gray).

Smaller needle-like precipitates of clay (?) Are visible on the glass surface. Images (b) and (c) have abundant

needle-like precipitates on the glass surface.
74

Hibonite

Glass

Calzirtite

Glass

Dehydration

crack

Secondary

precipitate

Figure 3-14. SEM secondary electron images of Site B Nb-Ta slag sample 96TC-19 after leaching by PNNL at pH

0.5 and pH 2. The upper image (a) shows the surface after leaching at pH 0.5. There is evidence of glass corrosion,

dehydration cracking and attack on hibonite. The lower image shows the slag after leaching at pH 2. The glass

exhibits dehydration cracks, pitting and crystallites of secondary phases are visible on the surface of the glass.
75

Figure 3-15. Elemental variation of glass in unleached and leached Site B sample 96TC-19. Leaching was done by

PNNL from pH 0.5 through pH 12. The greatest effect on CaO, Al2O3, and possible Ce2O3 are from pH 2 and below.

There appears to be leaching in that range. However the pattern with Th, U, and Zr is not as simple.

Unleached Glass Data

0.000

0.001

0.010

0.100

1.000

0 10 20

Sample Number

Weight Percent Oxide

PbO

ThO2

UO2

Variation of oxide in glass with pH

0.000

0.001

0.010

0.100

1.000

0 2 4 6 8 10 12 14

pH

Weight Percent Oxide

PbO

ThO2

UO2

Variation of Oxide in leached glass as 

function of pH

10.000

15.000

20.000

25.000

30.000

35.000

40.000

45.000

0 2 4 6 8 10 12 14

pH

Weight Percent Oxide

Al2O3

SiO2

CaO

Unleached Glass Data

10.000

15.000

20.000

25.000

30.000

35.000

40.000

45.000

0 5 10 15 20

Sample Number

Weight Percent Oxide

Al2O3

SiO2

CaO
Figure 3-16. Elemental variation of hibonite in unleached and leached Site B sample 96TC-19. Leaching was done

by PNNL from pH 0.5 through pH 12. The greatest effects are from pH 2 and below, and there appears to be

leaching in that range.

76

Weight Percent Oxide

Sample Numbers pH

Hibonite - Unleached

Hibonite - Leached,

pH-dependent

Al2O3

CaO

ThO2

TiO2

MgO
77

Figure 3-17. SEM secondary electron images of Site B slag sample 96TC-19 following leaching by PNNL at pH 2

and 4. The upper image shows the slag after leaching at pH 2. Dehydration cracking of the glass indicates that glass

corrosion has taken place (medium gray, layered appearance). Calzirtite, perovskite (both small white needles) and

hibonite crystals (dark gray prismatic crystals) protrude from the glass. Crystallites of secondary phases are

precipitated on the glass surface. The lower image shows the slag after leaching at pH 4. The glass (medium gray)

is not nearly as corroded. However the calzirtite and perovskite still protrude from the glass.
78

Figure 3-18. SEM secondary electron images of Site B slag sample 96TC-19 after leaching by PNNL at pH 6, and

12. The upper image shows the glass surface after leaching at pH 6. Crystallites of secondary phases are visible on

the surface of the glass. The lower image shows the glass surface after leaching apt pH 12. Calzirtite is seen

protruding from the glass and it does not appear to be as affected by the leaching as was the glass.
79

Hibonite basal

parting

Hibonite

Figure 3-19. SEM secondary electron images of Site B Nb-Ta slag sample 96TC-19 contrasting a leached sample

with an unleached one. The upper image was made after leaching by PNNL at pH 2. There is evidence of corrosion

along basal partings in hibonite. It is likely that the Ca ions which occupy sites between the octahedral and

tetrahedral sheets in the hibonite have been leached or exchanged at this pH. The lower image shows an unleached

sample and there is little evidence of corrosion.
80

Thorianite

Figure 3-20. BSE image of thorianite grains from Site A slag. The top image shows an unleached grain of thorianite.

This polished surface appears to have no defects. The lower image is a thorianite grain leached with deionized

water. Note the pitting that has occurred in the interior of the grain.
81

Figure 3-21. Crift Farm tin slags, Cornwall, England. According to radiocarbon dating, these slags date to 1200 A.D.

The slags are very glassy. Because of deep chemical weathering of the slag, it is very difficult to locate sites such as

this one.

Figure 3-22. Pribram slag pile, Czech Republic. Pribram mine was the site of Pb, Ag, mining and smelting. This is a

10 - 30 year old slag pile. The surface weathers within several years to a buff or white color.
Figure 3-23. Photomicrographs of 1,000 year old Cyprus slag. Complete devitrification of the interstitial glass, as a

result of weathering, is observable with a light microscope and is also supported by a flat background in powdered

xray diffraction spectra of the bulk sample (see Figure 3-24). The composition and identity of the alteration product

are unknown at this time.

82
Figure 3-24. X-ray diffraction spectrum of Cu slag from Cyprus (using CuKα radiation). This scan has not been

background subtracted. The peaks observed in the spectrum correspond with olivine ((Fe,Mg2)SiO4). The absence

of a broad hump signifies the absence of glass. During weathering, the glass was altered to form crystalline phases.

83
Figure 3-25. BSE images of weathered edge of Site C sample C-1a. MgO, periclase dendrites are dissolved at the

outer, weathered edge of the sample (upper image). Toward the interior of the sample (bottom image), the periclase

is not dissolved. In the upper image, wustite (FeO) has replaced some of the periclase dendrites.

84
Figure 3-26. Photomicrographs of Site C slag sample C-1a in plane polarized light. Large cristobalite - quartz grains

are surrounded by laths of quartz. The glass which appears brown under the polarizing microscope has oxidized in

some areas to form Fe oxides.

85
Figure 3-27. Hypothetical plot of a slag’s surface-concentration gradient. The figure shows the trend of how the

surface-concentration gradient might vary with time, suggesting that the weathering rate and leach rate may vary

with time (cf. Equations (4) and (5)).

86
87

4.0 Conclusions and Recommendations

Slag is a multi-phase material containing glass and 

crystalline phases: oxides, silicates, carbides, and

metals. The release of uranium and thorium from

slags depends on the phases of the slags and their

durability when subject to weathering. Prior to this

study, very little was known about the mineralogy,

petrology, and the results of weathering of SDMP NbTa slag. As a result of the study we have identified

the dominant phases in slag from three SDMP sites

and we have been able to determine the phases in

which uranium and thorium reside. These phases

weather at different rates, and therefore the uranium

and thorium will be released at different rates over

time. Identification of the uranium- and thoriumbearing phases will enable the calculation of their

release from the slag. In addition to identification of

the phases, the weathering characteristics of the slag

were identified and the elemental variation which

resulted from weathering was determined. The

conclusions and recommendations based on this

study, have been divided into Site Specific

Conclusions, Generic Conclusions, and Future work.

4.1 Site Specific Conclusions

4.1.1 Phases Present in Slag

4.1.1.1 Site A 

Slag from Site A differs from the two other sites in

that it is the product of the reprocessing of a Nb-Ta

slag. Following cooling of the Nb-Ta slag, it was

ground and subjected to chemical reprocessing and

then disposed in evaporation ponds. Natural

cementation took place and the slag presently

resembles a sandstone. The reprocessed slag is

more permeable than slag from sites B and C, so

fluid access to the radionuclide-bearing phases is

potentially greater. 

The dominant phases present at Site A are: hibonite,

barium aluminate, spinel, pyrochlore, and perovskite,

and are similar to those found at Site C. Glass is

present to a lesser extent than at Site B or C. In

contrast, Ba aluminate, thorianite, and spinel are

absent from Site B. 

The radioactivity in the Site A slag is in discrete

phases. The uranium- and thorium-bearing phases

in the waste material are: perovskite, thorianite

(ThO2), thorite (ThSiO4), perhaps thorogummite

(Th,U,Ce)SiO4 )), and uranophane

(Ca(UO2)2(SiO3OH)2.5H2O). Uranium-bearing phases

are a minor constituent of the Site A waste. 

4.1.1.2 Site B

The dominant phases are gehlenite, Zrclinopyroxene, glass, hibonite, and perovskite. 

Uranium and thorium are contained within discrete

phases. The uranium- and thorium-bearing phases

in the waste material are perovskite, pyrochlore,

glass, calzirtite, hibonite, cerium silicate (CeSiO4),

and barium aluminate. 

4.1.1.3 Site C

The dominant phases present at Site C are hibonite,

barium aluminate, spinel, olivine, perovskite, wustite

and periclase. At this site, two dominant slag types

were present. One is similar to Site A and B

because it had high aluminum phases (e.g. Ba

aluminate, hibonite, or Ca Al phases). In this slag

both uranium and thorium are in perovskite,

pyrochlore, and Ti oxides such as thorianite. The

second Site C slag is rich in two spinels and olivine. 

The dominant uranium- and thorium-bearing phases

are glass, calzirtite, and perovskite. 

4.1.2 Weathering in Slag

Glass and perovskite appear to be the most unstable

phases in oxidizing, near-surface conditions.

Perovskite may weather to anatase (TiO2) and

thoriantite (ThO2) a stable form of Th, in which case

the release of Th may be limited by the relatively low

solubility of ThO2. Thorium release will likely be

controlled by the solubility of perovskite, thorianite

and thorite. 

Glass weathers by several means: dissolution, ion

exchange, and devitrification. These studies have

shown examples of all three mechanisms. Evidence

of glass dissolution is visible in several of the SEMBSE images presented in this report. Quantitative

EMPA data provides evidence of preferencial

leaching or ion exchange in the form of chemical

gradients in the near surface glass environment. 

Devitrification is evident in SEM-BSE and TEM

images. These weathering mechanisms are

important to understanding the degradation of the

slag. The glass is the last phase to form and it fills

interstices among all the other phases. Its

destabilization will in turn effect all the other phases. 

Thus, water and other corrosive elements will be

more accessible to the radioactive phases.

From examination of the weathering of Site C sample 

C-s we calculated a bulk release rate of 230

ng/yr/cm2

 of uranium and thorium as was explained in

Section 3.3.4. This is similar to the experimentally

determined leach rates of Site A (Felmy et al., 1999).

The slags at all three sites have weathered over the

30 years since disposal. It appears from the EMPA

analyses that some uranium (and perhaps thorium)

has been released from the glass within the slag at

Site B. Quantitative analyses of elemental variation

within the weathered slags have been completed, and

provide estimates of leach rates to be compared with

PNNL (Felmy et al., 1999) and CNWRA (Pickett et

al., 1998) leach and solubility tests. The leach rate

from 96TC-19 for uranium is 3.8 μg/yr/cm2

, and

thorium is 32.1 μg/yr/cm2

.

Examination of samples by TEM of experimentally
88

leached slags from Sites A and B show evidence of

degradation. The EMPA data on elemental variation

in the slag as a function of pH provides information

for interpretation of the leaching data from the PNNL

and CNWRA studies. The greatest changes in

elemental composition occurred between pH 0.5 to 4. 

In this range, Ca, Al, U, and Th were depleted, while

Si, Mg, and Ti increased. 

4.2 Generic Conclusions

As a result of this study’s characterization of the

chemical composition and phases in slags from three

SDMP sites in the humid Eastern United States, One

can make the following conculsions.

Although the three sites produced slag from different

processes for different metals they all produced NbTa slag from a Sn slag parent material. Any NRClicensed site that processed Nb-Ta slag may have

similar materials to Site B. Therefore the degradation

of the slag at this site could be used as a model for

other Nb-Ta slag sites.

Uranium and thorium are not uniformly distributed

within the slags, rather they are present in discrete

phases such as glass, perovskite, calzirtite, and

pyrochlore. Microanalytical (petrographic and electron

microprobe) information provided in this report will

help guide decision makers in identification of slag

phases at additional sites.

A bulk dissolution or leach rate does not provide an

accurate representation of slag leaching over long

times (1000 years). It overestimates the release of

radionuclides to the environment. Slag phases

weather at different rates, and congruent dissolution

of the slag does not occur. 

Demonstrable slag weathering occurs on the time

scale of 25 - 30 years. Preferential dissolution of

certain phases allows fluid access to the interior of

the slag. Uranium and thorium are contained within

glass and perovskite, which are both unstable at

near-surface, humid conditions. However, thorium

from perovskite appears to form secondary thorianite

which is stable. The alteration products of the glass

may sequester the thorium and to a lesser extent the

uranium in Ce- and Th-silicates.

The slags studied have been characterized into two

similar groups based on bulk chemical composition

and the crystalline phases as is shown in the ternary

phase diagram of slag compositions in Figure 2-10. 

The end points of the ternary diagram represent

octahedral and tetrahedral sites and coordinating

cations. Such characterization provides a means for

classification of additional sites, that fall within the

bounds of this study. Site B and C slags fall within a

tight range on the compositional diagram, indicating a

similarity in composition. This means that similar

phases will exist in these slags, and therefore will 

most likely weather at the same rate. Site A slag

has a different composition because of the additional

processing. For archeaological slags with the

passage of time there is a depletion in calcium and

sodium. This is evidenced by samples of Sn glass

(40-50 years old), Crift Farm Sn slag (500 - 1000

years old), and Treriefe Sn slag (100 - 500 years old)

which all fall along a trend line of CaO + Na2O < 10

%. The Pribram samples older than 100 years also

fall along the <10 % CaO and Na2O. Site B and Site

C slag fall along same trendline in Figure 2-10 as the

Pribram 150 year old slag.

4.3 Recommendations and Future Work

To better understand the release of uranium and

thorium from all the phases present in the slags

studied requires a more sophisticated approach than

assuming a bulk dissolution rate. The phases are

similar to SYNROC, vitrified wastes, natural glasses,

and naturally occurring perovskite, calzirtite, and

pyrochlore. Data gleaned from the literature relating

to degradation of these phases can be used to

enhance our understanding of slag leach rates. 

Analogous archaeological slags have been identified

and sampled based on similarity of bulk chemistry,

crystalline phases, and glass chemistry. Primary slag

phases and alteration products have been identified. 

Quantification of the modal abundance of primary and

alteration phases has been completed and presented

in Farthing, 2002. 

This study has provided the phase characterization

data necessary for calculation of solubility-limited

dissolution of slag phases, and calculation of a

realistic leach rate (Equation (4)). The data

necessary for a solubility limited release rate model

are:

(a) radionuclide bearing phase (for solubility

calculation) and its density

(b) amount of phase present (modal abundance)

(c) amount of U or Th in phase (mass fraction

from EMPA chemical analysis)

(d) estimate of the weathering rate of a particular

phase (m/y)

Given this information, a realistic leach rate could be

calculated from Equation (4).

A cost-effective means of site characterization might

be as follows:

a) Review information on the smelting operation,

the processing of the slag and final waste

products to refine calculations based on the

amount of material present at the site. 

(b) Collect representative radioactive material

present on the site and obtain bulk chemical

analysis, and obtain modal abundance of

radioactive phases. (This could be done with X-
89

ray diffraction, petrographic microscope and

point counts.) It should not be necessary for a

licensee to provide the level of micro analytical

detail provided in this report. With this

information: a radionuclide release model could

be developed based on the phase solubility and

weathering rates obtained in this study.
90

5.0 References

Adriaens, A. 1996. “Elemental composition and

microstructure of early Bronze Age and

Medieval tin slags,” Mikrochim. Acta 124:

89-98.

ANSI/ANS-16.1-1986. Measurements of the

Leachability of Solidied Low-Level

Radioactive Waste by a Short-term Test

Procedure. American National Standard

American Nuclear Society, Pub. by

American Nuclear Society, 555 North

Kensington Avenue, La Grange Park, IL

60525, 1986, pp 35.

Armstrong, 1989. Quantitative analysis of silicate

and oxide minerals: Comparison of Monte

Carlo, ZAF and Phi-rho-z procedures. In:

Newbury, D.E. (Ed.) Microbeam Analysis -

1988. San Francisco, CA: San Francisco

Press, pp. 239-246. 

Banfield, J.F. and D.R. Veblen, 1992, “Conversion of

perovskite to anatase and TiO2: A TEM

study and the use of fundamental buillding

blocks for understanding relationships

among the TiO2 minerals,” American

Mineralogist, 77:545-557.

Bates, J.K., L.J. Jardine, and M.J. Steindler. 1982. 

Hydration aging of nuclear waste glass. 

Science 218: 51-54.

Byers, C.D., M.J. Jercinovic, R.C. Ewing, and K Keil. 

1985. Basalt glass: an analogue for the

evaluation of the long-term stability of

nuclear waste form borosilicate glasses. 

Materials Research Society Symposium

Proceedings 44: 583-590.

Dowty, E. and J.R. Clark, 1973. Crystal Structure

Refinement and Optical Properties of a Ti3+

Fassaite from the Allende Meteorite, Am.

Min., 58:230-242.

EPA 1311, July 1995, Toxicity Characteristic

Leaching Procedure, Method 1311,. EPA

Manual SW-846 Washington DC: US

Environmental Protection Agency.

EPA 1312, July, 1995, Synthetic Precipitation

Leaching Procedure,Method 1312, EPA

Manual SW-846 Washington DC: US

Environmental Protection Agency.

Felmy, A.R., V.L. LeGore, S.A. Hartley, “Solubility and

Leaching of Radionuclides in Site

Decommissioning Management Plan

(SDMP) Soil and Ponded Wastes,” 

NUREG/CR-6821, United States Nuclear

Regulatory Commission, Washington, DC,

May 2003.

Farthing, D.J., 2002, “The Mineralogy of Tin Slags”,

dissertation submitted to Johns Hopkins

University, Baltimore, Maryand in conformity

with the requirements for degree of Doctor

of Philsophy.

Fauver, D. N., M. F. Weber, T. C. Johnson, and J. D.

Kinneman, 1995, “Site Decommissioning

Management Plan," NUREG-1444,

Supplement 1, U.S. Nuclear Regulatory

Commission, Washington, DC 20555, 95

pp.

Frank, A., A. Madej, V. Galgan, and L.R. Petersson. 

1996. Vanadium poisoning of cattle with

basic slag. Concentrations in tissues from

poisoned animals and from a reference,

slaughterhouse material. The Science of

the Total Environment 181: 73-92.

Franklin, U.M., J.C. Grosjean, and M.J. Tinkler. 1976. 

A study of ancient slags from Oman. 

Canadian Metallurgical Quarterly 15 (1): 29-

35.

Gee, C., M.H. Ramsey, J. Maskall, and I. Thornton. 

1997. Mineralogy and weathering

processes in historical smelting slags and

their affect on the mobilization of lead. 

Journal of Geochemical Exploration 58: 249-

257.

Grambow, B., 1992. Geochemical Approach to Glass

Dissolution. In Corrosion of Glass,

Ceramics and ceramic Superconductors,

edited by D.E. Clark and B.K. Zoitos, (Noyes

Publications, Park Ridge, New Jersey) pp.

124-152.

Gustison, R.A. and J.A. Cenerazzo, 1971, 

Exothermic Fusion of eastern tin slag

carbides to a tantalum-columbium

concentrate. Journal of Metals, pp 45-48.

Ihinger and Stolper, 1986. The color of meteoritic

hibonite: an indicator of oxygen fugacity,

Earth Planet. Sci. Lett., 78:67-79.

Jantzen, C.M., 1992, “Thermodynamic Approach to

Glass Corrosion”, in Corrosion of Glass,

Ceramics and Ceramic Superconductors,

Princil\ples, Testing,characterization and

Applications”, edited by Clark, D.E and B.K.

Zoitos (Noyes Publications, Park Ridge,

New Jersey) pp 153-215.

Keil, K. and L.H. Fuchs, 1971. Hibonite

Ca2(Al,Ti)24O38 from the Leoville and Allende

chondritic meteorites. Earth Planet. Sci. Lett.

12:184-90.

Mascaro, I., M. Benvenuti, and G. Tanelli. 1995. 

Mineralogy applied to archaeometallurgy: an
91

investigation of medieval slags from Rocca

San Silvestro (Campiglia M.ma, Tuscany).

Science and Technology for Cultural

Heritage 4 (1): 87-98.

Maskall, J., K. Whitehead, I. Thornton. 1995. Heavy

metal migration in soils and rocks at

historical smelting sites. Environmental

Geochemistry and Health 17: 127-138.

Smith, J.V., 1979. Mineralogy of the planets: a

voyage in space and time. Mineral. Mag.

43:1-89.

Vandiver, P. 1992. Corrosion and conservation of

ancient glass and ceramics. In In Corrosion

of Glass, Ceramics and ceramic

Superconductors, edited by D.E. Clark and

B.K. Zoitos, (Noyes Publications, Park

Ridge, New Jersey) pp. 393-427.

Vandiver, P., K.A. Yenner, and L. May. 1992. Third

millennium B.C. tin processing debris from 

Göltepe (Anatolia), p. 545-570. In:

Vandiver, P.B. et al. (eds.), Materials Issues

in Art and Archaeology III. Materials

Research Society, Pittsburgh, PA.

Voutsinou-Taliadouri, F., R. Psyllidou-Giouranovits,

and E. Georgakopoulou-Gregoriadou. 

1995.Active growth of a submarine slag

disposal ore deposit in the northern

Euboekos Gulf, Greece. Fresenius

Environmental Bulletin 4: 195-200.

Xu, H. and Y. Wang, 2000. Crystallization sequence

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Journal of Nuclear Materials 279: 100-106.
92

Appendix: Surface Mass-Transfer Coefficients

There is an extensive chemical engineering literature

on mass-transfer coefficients. Some examples of

these coefficients are given in Sherwood, Pigford,

and Wilke (1975). The surface mass- transfer

coefficient depends on the fluid flow regime around

the slag and on the geometry of the slag. 

A.1 Fluid Flow Regimes

The flow regimes that may occur around slags

include stagnation, very slow flow such as

groundwater flow, laminar flow, and turbulent flow. 

The flow regime that has to be considered depends

on how the surround fluid’s inertial forces compare

with its viscous forces, as expressed by the fluid’s

Reynolds number,

Re = UL

ν

(A1)

where U is a characteristic fluid velocity, L is a

characteristic length of the piece of slag under

consideration, and is the fluid’s kinematic viscosity ν

or momentum diffusivity. Mass-diffusion and

momentum-diffusion near the slag is are compared

using the Schmidt number,

Sc

D k

k

r F

r

= ν

(A2)

where is the fluid diffusion coefficient for Dk

F

r

species k containing radionuclide r. Mass transfer

theory has shown that the Sherwood number for

species k containing radionuclide r,

Sh

H L

D k

k

k

r F

r

r

= , (A3)

depends on the Reynolds and Schmidt numbers. 

This functional relationship, which depends on the

fluid flow regime and the slag geometry, allows one to

estimate the surface mass-transfer coefficient . Hkr

A.2 Idealizing Slag Shapes

It may be convenient to view large slabs of slag as

flat plates and smaller chunks of slag as spheres or

disks. In this study, long cylindrical slags were not

observed. Cylindrical shapes may not be suitable

idealizations of pieces of slag.

A.3 Stagnant Fluid

In this case, the Reynolds number is zero,

momentum diffusion plays no role, and mass transfer

in the fluid surrounding the slag is diffusional. 

Sherwood, Pigford, and Wilke (1975) show that for a

spherical slag of diameter in a stagnant fluid, d

H d

D

k

k

F

r

r

= 2. (A4)

Stagnant-fluid Sherwood numbers, such as that in

Equation (A4), are the minimum values for the

geometries considered.

A.4 Very Slow Flow

In very slow flows such as groundwater flows, the

Reynolds number is finite but very small. Momentum

diffusion is negligible in such flows and

H U kr = (A5)

A.5 Laminar Flow

In laminar flows, inertial fluid accelerations are large,

but not enough to destabilize the flows. In the

boundary layers near slag surfaces, momentum- and

mass-diffusion effects are important. For flow over

flat plates (Sherwood, Pigford, and Wilke (1975)),

H L

D

UL

D

k

k

F

k

F

r

r r

= 



 



 













 0 664

1

2

1

3

. ν

ν (A6)

where U is the flow velocity parallel to the flat plate. 

Equation (A6) is valid for values of Reynolds number

up to about 300,000 to 500,000, after which the flow

over the plate becomes turbulent.

For laminar flow over a sphere of diameter , d

H d

D (4 1.21 UdD ) k

k

F

k

F

2/3

r 1/2

r r

= +















 (A7)

which is valid for Ud Dk

F

r / ,. <10 000

A.6 Turbulent Flow

Turbulent flows are chaotic and characterized by high

Reynolds numbers. For turbulent flows over flat

plates, 

H L

D

UL

D

k

k

F

k

F

r

r r

= 



 



 













 0 036

0 8

1

3

.

.

ν

ν (A8)
BIBLIOGRAPHIC DATA SHEET

(See instructions on the reverse)

NRC FORM 335

(2-89)

NRCM 1102,

3201, 3202

U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER

 (Assigned by NRC, Add Vol., Supp., Rev.,

 and Addendum Numbers, if any.)

NUREG-1703

DATE REPORT PUBLISHED

MONTH

October

YEAR

2004

4. FIN OR GRANT NUMBER

2. TITLE AND SUBTITLE

Characterization of Radioactive Slags

5. AUTHOR(S)

Linda A. Veblen(1), Dori Farthing(2), Edward O'Donnell(1), John D. Randall(1)

(1) U.S. Nuclear Regulatory Commission (See Block 8)

(2) Department of Earth and Planetary Sciences; Johns Hopkins University; Balti more, MD

21218

6. TYPE OF REPORT

Staff report

7. PERIOD COVERED (Inclusive Dates)

1997-2002

8. PERFORMING ORGANIZATION - NAME AND ADDRESS (If NRC, provide Division, Office or Region, U.S. Nuclear Regulatory Commission, and mailing address; if contractor,

Division of Systems Analysis and Regulatory Applications

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (If NRC, type "Same as above"; if contractor, provide NRC Division, Office or Region, U.S. Nuclear Regulatory Commission, 

Same as above

provide name and mailing address.)

and mailing address.)

10. SUPPLEMENTARY NOTES

11. ABSTRACT (200 words or less)

Slag samples from three humid region sites located in the Eastern United States were studied in detail. Since this

investigation is generic in nature, the precise locations of the sites are not identified and are referred to as Sites A, B, and C. 

Site A's slag was formed from a reprocessed Nb-Ta slag that was ground, leached and deposited in settling ponds which were

allowed to evaporate. The resulting material was a "rock like," porous, alter ed material that resembled a sandstone. Site B

was the most intensively studied site and it provided valuable information on t he physical and chemical form of the original Nb

and Ta slag, as well as the Sn slag that was used as the "ore" material. Site C's slag, although not studied in as much detail

as the Site B slag, is physically similar to the Site B slag and both sites had (i) glassy slag and (ii) dense dark slags that

ranged from fine grained to very coarsely crystalline. Detailed microanalytica l data from Sites B and C provide information

regarding the degradation of the radionuclide-bearing phases. Uranium and thor ium are present in glass, perovskite, calzirtite,

zirconolite, and pyrochlore. Cerium and thorium are present in hibonite. Thes e phases are similar to phases in the proposed

high-level radioactive waste form SYNROC. Numerous studies provide experimenta lly determined solubility and leach rates

for these phases. These data provide insight into and bounding values for esti mates of slag leach rates. Data reduction and

a detailed petrological study provide the necessary information for guidance to the NRC's Office of Nuclear Material Safety and

Safeguards for license termination at future decommissioning sites.

12. KEY WORDS/DESCRIPTORS (List words or phrases that will assist researchers in locating the report.)

Slags, uranium, thorium

14. SECURITY CLASSIFICATION

13. AVAILABILITY STATEMENT

unlimited

(This Page)

unclassified

(This Report)

unclassified

15. NUMBER OF PAGES

16. PRICE

NRC FORM 335 (2-89) This form was electronically produced by Elite Federal Forms, Inc.

3.

Office of Nuclear Regulatory Reearch

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001
Printed 

on recycled 

paper

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