Integrated Report

Main Results of
the MASCA1 and 2
Projects
June 2007

ORGANISATIONS PARTICIPATING
IN THE OECD/RUSSIA MASCA2 PROJECT:
The Association Vinçotte Nuclear jointly with Tractebel S.A., Belgium
The Atomic Energy of Canada Limited, Canada
The Fortum Nuclear Services Ltd. jointly with Valtion Teknillinen Tutkimuskeskus
and Säteilyturvakeskus, Finland
The Institut de Radioprotection et de Sûreté Nucléaire, France
The Gesellschaft für Anlagen - und Reaktorsicherheit (GRS) mbH., Germany
The KFKI Atomic Energy Research Institute, Hungary
The Japan Nuclear Energy Safety Organization (formerly NUPEC), JapanThe Korea Atomic Energy Research Institute, Korea
The Nuclear Regulatory Authority of the Slovak Republic (UJD SR), Slovak
Republic
The Consejo de Seguridad Nuclear, Spain
The Statens Kärnkraftinspektion, Sweden
The Paul Scherrer Institute, Switzerland
The United States Nuclear Regulatory Commission, USA
The Russian Federation Ministry of Atomic Energy, jointly with the Federal Nuclear
and Radiation Safety Authority of Russia, and the Russian Research Centre«Kurchatov Institute»

Notice
The information, contained in this report is to be communicated only topersons and undertakings authorized to receive it by one of the organizations
participating in the OECD MaSca Phase II Project in accordance with the
Project’s rules on information and intellectual property.

Integrated Report

Russian Research Centre“KURCHATOV INSTITUTE”
OECD MASCA Project

For MASCA Project use only

Main Resultsof the MASCA1 and 2
Projects
D. F. Tsurikov, V. F. Strizhov*, S. V. Bechta**,
V. N. Zagriazkin, N. P Kiselev

June 2007

Presented by”RRC “KURCHATOV INSTITUTE”

TABLE OF CONTENTS
1INTRODUCTION............................................................................................................1

2

MELT STRATIFICATION MECHANISMS...............................................................5
2.1
CARBON INDUCED STRATIFICATION ...........................................................................5
2.1.1
Test Apparatus and Procedure ..........................................................................5
2.1.2
Test Matrix and Results .....................................................................................6
2.2
SEPARATION OF CORIUM COMPONENTS IN THE TEMPERATURE RANGE BETWEENSOLIDUS AND LIQUIDUS .........................................................................................................10
2.2.1
Test Apparatus and Procedure ........................................................................10
2.2.2
Test Matrix and Results ...................................................................................11
2.3
MAJOR FINDINGS ......................................................................................................12

3

INTERACTIONS OF CORIUM WITH STEEL (IRON)..........................................14
3.1
OBJECTIVES OF THE TESTS ........................................................................................14
3.2
TEST FACILITIES AND METHODOLOGY .....................................................................14
3.3
POST TEST EXAMINATION AND SOURCES OF UNCERTAINTIES ..................................23
3.3.1
STFM-Fe, STFM-FP and MA 1-6 Tests ..........................................................23
3.3.2
Oxidation Tests (STFM-Ox and MA-7, 9)........................................................24
3.4
EXPERIMENTAL RESULTS ..........................................................................................24
3.4.1
Experiments in inert atmosphere .....................................................................24
3.4.2
Experiments in oxidizing atmosphere ..............................................................31
3.4.3
Experiments on the effect of control rod material ...........................................42
3.5
DISCUSSION OF FINDINGS .........................................................................................49
3.5.1
Partitioning of Main Species ...........................................................................49
3.5.2
Change of Corium Oxidation Degree ..............................................................53
3.5.3
Density of Oxidic and Metallic Phases............................................................54
3.5.4
Partitioning of Main Species in the Presence of Boron Carbide.....................56
3.5.5
Partitioning of Fission Products......................................................................60
3.5.6
Effect of Oxidation Atmosphere on the Distribution of Main Species .............66
3.6
STEEL BEHAVIOR IN THE RCW TEST ........................................................................67
3.6.1
The Test Facility ..............................................................................................68
3.6.2
Post-Test Material Studies...............................................................................70
3.6.3
Results of Visual Observations ........................................................................71
3.6.4
Peculiarities of FP Partitioning in the RCW Test ...........................................79
3.6.5
Conclusions......................................................................................................814

MIGRATION OF MOLTEN METAL THROUGH POROUS OXIDE DEBRIS...83
4.1
INTRODUCTION AND TESTS OBJECTIVES ....................................................................83
4.2
TEST APPARATUS......................................................................................................83
4.2.1
KORPUS Facility.............................................................................................83
4.2.2
Laboratory Tests in Carbon Free Atmosphere with a Tungsten Crucible.......86
4.2.3
The TULPAN T-8 Test .....................................................................................86
4.2.4
Modifications of the RCW-Ox Facility for RCW-ST1 and RCW-Ox2, 3 Tests 88
4.3
RESULTS OF EXPERIMENTS IN THE INERT ATMOSPHERE ...........................................94
4.3.1
Behavior of the Carbon Steel ST-30 Melt ........................................................94
4.3.2
Behavior of Stainless Steel...............................................................................98
4.3.3
Behaviour of Zr Alloys ...................................................................................103
4.4
INTERACTIONS UNDER OXIDIZING ATMOSPHERE ....................................................107
4.4.1
Supporting experiments .................................................................................108i

4.4.2
Scoping Test RCW-ST1 ..................................................................................111
4.4.3
The Test RCW-Ox2 ........................................................................................112
4.4.4
The Test RCW-Ox3 ........................................................................................116
4.5
RESEARCH INTO INFLUENCE OF THE BORON OXIDE B2O3 .......................................120
4.6
DISCUSSION ON FINDINGS .......................................................................................127
5

MATERIAL PROPERTIES .......................................................................................129
5.1
PROPERTIES OF C-32 CORIUM .................................................................................129
5.2
THE PROPERTIES OF THE METALLIC PHASE ............................................................130
5.2.1
The Spreading Temperature ..........................................................................130
5.2.2
The Electric Conductivity ..............................................................................133
5.2.3
Density ...........................................................................................................134
5.2.4
Surface tension...............................................................................................138
5.2.5
The Thermal Conductivity..............................................................................140
5.3
MEASUREMENTS OF SPECIFIC HEAT CAPACITY AND THERMAL CONDUCTIVITY OF
RCW METALLIC PHASE .......................................................................................................141
5.3.1
Sampling ........................................................................................................141
5.3.2
Measurements of specific heat capacity ........................................................144
5.3.3
Determination of thermal conductivity ..........................................................147

6

CONCLUSIONS ..........................................................................................................150

REFERENCES.....................................................................................................................152
APPENDIX A: LIST OF MASCA PROJECTS REPORTS ...........................................A-1

ii

LIST OF FIGURESFIGURE 2.1. SCHEMATIC OF THE EXPERIMENT (A) AND POST TEST EXAMINATION (B) ...............6
FIGURE 2.2. CARBON CONCENTRATION (A) AND U/ZR RATIO (B) IN THE TOP AND MIDDLELAYERS VERSUS EXPOSURE TIME AT THE TEMPERATURE OF 2550OC .................................7
FIGURE 2.3. CARBON CONCENTRATION (A) AND U/ZR RATIO (B) IN THE TOP AND MIDDLELAYERS VERSUS INITIAL CONTENT OF CARBON..................................................................9
FIGURE 2.4. CHANGE OF THE AREA RATIO BETWEEN LIGHT AND GRAY PHASES ALONG THE
INGOT HEIGHT ....................................................................................................................9
FIGURE 2.5. MACROSTRUCTURE (A) AND MICROSTRUCTURE OF THE TOP LAYER (B) AND BULKCORIUM (C) OF C-32 CORIUM INGOT (STFM1/2 EXPERIMENT, CARBON CONTENT
0.36 WT%) ........................................................................................................................10
FIGURE 2.6. DISTRIBUTION OF THE LIGHT AND GRAY PHASE RATIO ALONG THE INGOT HEIGHT(TEST STFM#2/3) .............................................................................................................12
FIGURE 3.1. TEST SECTIONS OF STFM FASILITY.......................................................................15
FIGURE 3.2. RASPLAV-3 FACILITY .........................................................................................16
FIGURE 3.3. SCHEMATIC OF SYSTEMS FOR GAS/OXIDANT SUPPLY AND OFF-GAS ANALYSIS OF
THE STFM FACILITY .........................................................................................................19
FIGURE 3.4. SCHEMATICS OF GAS-AEROSOL SYSTEM OF RASPLAV-3 FACILITY ......................21
FIGURE 3.5. SAMPLING SCHEME FOR TESTS STFM-FE3 AND FE14 ..........................................25
FIGURE 3.6. CHARACTERISTIC DISTRIBUTION OF CONCENTRATIONS IN THE OXIDIC PHASE
ALONG THE INGOT HEIGHT ...............................................................................................25
FIGURE 3.7. MACROSTRUCTURES OF INGOTS AFTER MA-3 (A), MA-2 (B) AND MA-6 (C, D)
TESTS ................................................................................................................................28
FIGURE 3.8. SECTION OF METALLIC PART OF THE INGOT AND COMPOSITIONS OF SAMPLES
MEASURED BY DIFFERENT METHODS FOR MA-6 TEST .......................................................29
FIGURE 3.9. THERMOGRAM OF TEST STFM-OX NO.1 ..............................................................31
FIGURE 3.10. THERMAL FLASH ON A WATER DROP INTERACTING WITH THE MELT SURFACE
DURING THE TEST STFM-OX NO.1 ...................................................................................32
FIGURE 3.11. H2 CONCENTRATION IN THE FACILITY CHAMBER IN THE COURSE OF OXIDATIONDURING THE TEST STFM-OX NO.1...................................................................................32
FIGURE 3.12. CO CONCENTRATION IN THE COURSE OF OXIDATION IN THE FACILITY CHAMBER
DURING THE TEST STFM-OX NO.1 ...................................................................................33
FIGURE 3.13. LAYOUT OF THE INGOT PRODUCED IN THE TEST STFM-OX NO. 1......................34
FIGURE 3.14. PHOTOGRAPHS OF THE INGOT PRODUCED IN THE TEST STFM-OX NO. 1............34
FIGURE 3.15. MA-7 INGOT VIEW AND SCHEMATICS ..................................................................36
FIGURE 3.16. VIEW AND SCHEMATICS OF METALLIC PART OF MA-9 INGOT ..............................36
FIGURE 3.17. DIFFRACTOGRAMS OF MA-7 SAMPLES: A) ABOVE-MELT CRUST; B) CRUST OF
METALLIC INGOT; C) CENTRAL PART OF OXIDIC INGOT ......................................................40
FIGURE 3.18. EVOLUTION OF FLOW RATES OF REACTED OXYGEN AND NITROGEN (A) AND
INTEGRAL AMOUNT OF THE REACTED GASES (B) ...............................................................41
FIGURE 3.19. COMPARISON OF GAS ABSORPTION RATES WITH GAS COMPOSITION .....................42
FIGURE 3.20 SCHEMATICS OF THE INGOT OF STFM-SIC SCOPING TEST, AND PHOTO-PICTURES
OF THE INGOT METALLIC PARTS .......................................................................................46
FIGURE 3.21. INITIAL LOADING OF METALLIC SAMPLES...........................................................48
FIGURE 3.22. INGOTS OF MELTED STFM-SIC SAMPLES...........................................................48
FIGURE 3.23. U, ZR AND FE (STEEL) CONCENTRATIONS IN A METALLIC PHASE FOR
EXPERIMENTS WITH C≈32 INITIAL CORIUM HAVING U/ZR≈1.2 (A) AND U/ZR≈0.9 (B) ....49
FIGURE 3.24. U, ZR AND FE (STEEL) CONCENTRATIONS IN A METALLIC PHASE FOR
EXPERIMENTS WITH C≈70 INITIAL CORIUM ......................................................................50

iii

FIGURE 3.25. U (A) AND ZR (B) CONCENTRATIONS IN A METALLIC PHASE AS A FUNCTION OF
INITIAL ZIRCONIUM OXIDATION DEGREE .........................................................................50
FIGURE 3.26. IRON CONCENTRATION IN THE OXIDIC PHASE AT DIFFERENT FINAL ZROXIDATION DEGREES .......................................................................................................51
FIGURE 3.27 COMPARISON OF EXPERIMENTAL AND CALCULATED CONCENTRATIONS OF U, ZR,
AND FE IN THE FINAL METALLIC MELT FOR U/ZR~1.2 (A) AND FOR U/ZR~0.9 (B) ............52
FIGURE 3.28. INCREASE OF THE CORIUM OXIDATION DEGREE AFTER THE INTERACTION FOR C32 CORIUM .......................................................................................................................54
FIGURE 3.29. INCREASE OF THE CORIUM OXIDATION DEGREE AFTER THE INTERACTION FOR C60 – C-75 CORIUM ............................................................................................................54
FIGURE 3.30. METALLIC PHASE DENSITY AT AMBIENT TEMPERATURE AS A FUNCTION OF
U/STEEL(FE) ATOMIC RATIO ............................................................................................55
FIGURE 3.31. OXIDIC PHASE DENSITY AT AMBIENT TEMPERATURE AS A FUNCTION OF OXYGENCONTENT ..........................................................................................................................55
FIGURE 32. THE LAYOUT OF THE STFMB-3 INGOT WITH SECTIONS ANALYSED CHEMICAL AND
PHASE COMPOSITIONS ......................................................................................................56
FIGURE 3.33. PARTITIONING OF FP SIMULANTS AS A FUNCTION OF TEMPERATURE AND FINALCORIUM OXIDATION DEGREE ...........................................................................................64
FIGURE 3.34. THE FACILITY SCHEMATICS ................................................................................68
FIGURE 3.35. RCW FACILITY VIEW..........................................................................................69
FIGURE 3.36. LAYOUT OF THE LOWER CRUCIBLE CENTRAL SECTION.......................................71
FIGURE 3.37. THE MACROSECTION OF THE INGOT CENTRAL CUT ............................................73
FIGURE 3.38. PATTERN FOR CUTTING OUT OF THE CORIUM SAMPLES.......................................73
FIGURE 3.39. DIAGRAMS OF DENSITY DISTRIBUTION FOR THE OXIDIC AND METALLIC PARTSALONG THE INGOT HEIGHT ...............................................................................................74
FIGURE 3.40. DISTRIBUTION OF U, ZR AND CHANGE OF CN IN THE CORIUM ALONG THE INGOTHEIGHT .............................................................................................................................76
FIGURE 3.41. PARTITIONING OF U, ZR AND FE IN METALLIC PARTS ALONG THE INGOT HEIGHT..........................................................................................................................................77
FIGURE 4.1. THE SIZE DISTRIBUTION OF THE C-32 AND C-100 CORIA ......................................84
FIGURE 4.2. LAYOUT OF THE HEATING DEVICE WITH THE TEST ASSEMBLY IN THE KORPUS
FACILITY AND FACILITY VIEW ..........................................................................................85
FIGURE 4.3. TYPICAL TEMPERATURE HISTORY FOR THE SERIES OF TESTS K-1 – K-10 (TEST K5) ......................................................................................................................................85
FIGURE 4.4. THE LAYOUT OF THE TESTS D1 AND D3................................................................86
FIGURE 4.5. THE LAYOUT OF THE D2 TEST ...............................................................................86
FIGURE 4.6. LAYOUT OF THE T-8 TEST SECTION AND FACILITY VIEW ......................................87
FIGURE 4.7. TEST UNIT LAYOUT IN THE T-8 TEST ....................................................................88
FIGURE 4.8. VIEW OF THE RCW-ST1 FACILITY ........................................................................89
FIGURE 4.9. RCW-ST1 EXPERIMENTAL SECTION .....................................................................89
FIGURE 4.10. RCW-OX2 EXPERIMENTAL SECTION ..................................................................90
FIGURE 4.11. THERMOGRAM OF THE RCW-ST1 TEST ..............................................................91
FIGURE 4.12. VARIATION OF EXPERIMENTAL PARAMETERS DURING THE RCW-OX2 TEST ......91
FIGURE 4.13. THE TEMPERATURE IN THE CORIUM DEBRIS BED AND CONCENTRATIONS OF
WATER STEAM (AT THE SECTION INLET) AND OF HYDROGEN (AT THE SECTION OUTLET)
DURING THE RCW-OX3 TEST ...........................................................................................92
FIGURE 4.14. HYDROGEN CONCENTRATION (AT THE SECTION OUTLET) AND THE INTEGRAL
DEPENDENCE OF GENERATED HYDROGEN QUANTITY OF THE RCW-OX3 TEST ...............93
FIGURE 4.15. MACROSTRUCTURE OF THE K-1 ASSEMBLY VERTICAL SECTION ........................96iv

FIGURE 4.16. MACROSTRUCTURE AND RESULTS OF THE ELEMENT ANALYSIS FOR THE C-32
CORIUM OF K-2 TEST ........................................................................................................96
FIGURE 4.17. MACRO- AND MICROSTRUCTURE FROM THE TEST K-4........................................97
FIGURE 4.18. MICROSTRUCTURE OF THE C-32 BRIQUETTE UPPER PART AFTER THE TEST D1 ..97
FIGURE 4.19. MICROSTRUCTURE AND RESULTS OF THE ELEMENT ANALYSIS FOR THE C-100
CORIUM PARTICLES ..........................................................................................................98
FIGURE 4.20. MICROSTRUCTURE OF THE C-32 BRIQUETTE UPPER PART AFTER THE TEST D3 100
FIGURE 4.21. MICROSTRUCTURES OF THE C-32 CORIUM PARTICLES AND OF THE LEAKED STEEL
12X18H10T IN THE TEST K-10.......................................................................................100
FIGURE 4.22. K-11 CRUCIBLE MACROSECTION ......................................................................101
FIGURE 4.23. MACROSECTION OF THE T-8 TESTS ...................................................................101
FIGURE 4.24. MACROSTRUCTURE OF SAMPLE #1(THE INGOT MIDDLE PART).........................102
FIGURE 4.25. MICROSTRUCTURE OF THE SAMPLE #2 (THE INGOT LOWER PART)....................102
FIGURE 4.26. MACROSECTIONS OF CRUCIBLES AFTER THE TESTS K-6, K-7, K-8, K-9, K-5 ...103
FIGURE 4.27. MACROSTRUCTURE AND THE MICROPROBE ANALYSIS RESULTS (THE PROBE
DIAMETER = 50 µM) FOR THE TEST K-6 .........................................................................105
FIGURE 4.28. PENETRATION OF THE SS+ZR ALLOY INTO THE CORIUM PARTICLES AFTER THE
TESTS K-7 AND K-5 ........................................................................................................106
FIGURE 4.29. MICROSTRUCTURE AND MICROPROBE ANALYSIS RESULTS FOR THE K-5 SAMPLE........................................................................................................................................106
FIGURE 4.30. K-12 METALLIC INGOT .....................................................................................107
FIGURE 4.31. CRUCIBLE LOADING DIAGRAMS .........................................................................108
FIGURE 4.32. POST-TEST CRUCIBLES` LONGITUDINAL SECTIONS .............................................109
FIGURE 4.33. POST-TEST MICROSTRUCTURE OF C-100+FE2O3 CORIUM AND MICRO PROBEANALYSIS ........................................................................................................................109
FIGURE 4.34. MICROSTRUCTURE AND MICROPROBE ANALYSIS DATA FOR CORIUM C-32+FE2O3,
TUNGSTEN CRUCIBLE ......................................................................................................110
FIGURE 4.35. CRUCIBLE SECTION AND CUT-OUT DIAGRAM FOR SAMPLES ## 1, 2, 3 ...............112
FIGURE 4.36. SAMPLING SCHEME FOR METALLOGRAPHIC AND MICROPROBE ANALYSIS .......113
FIGURE 4.37. SAMPLING SCHEME FOR PLASMA SPECTRAL EMISSION ANALYSIS AND
CHROMATOGRAPHIC ANALYSIS OF OXYGEN CONTENT ..................................................113
FIGURE 4.38. MICROSTRUCTURE OF SAMPLE # 1.....................................................................114
FIGURE 4.39. MICROSTRUCTURE OF THE CORIUM PARTICLES (SAMPLE #2, PARTICLES) .........114
FIGURE 4.40. MICROSTRUCTURE OF THE CORIUM PARTICLES (SAMPLE #3, PARTICLES) .........114
FIGURE 4.41. MICROSTRUCTURE OF THE CORIUM PARTICLES (SAMPLE #3, PARTICLES) .........115
FIGURE 4.42. MACROSECTION AND THE SAMPLING MAP FOR METALLOGRAPHIC ANDMICROPROBE ANALYSES ................................................................................................116
FIGURE 4.43. THE SAMPLING MAP FOR THE PLASMA SPECTRAL EMISSION ANALYSIS ...........116
FIGURE 4.44. MICROSTRUCTURE OF SAMPLE #1 .....................................................................118
FIGURE 4.45. MICROSTRUCTURE OF SAMPLE #3 .....................................................................118
FIGURE 4.46. MICROSTRUCTURE OF SAMPLE #4 (UPPER PART)...............................................118
FIGURE 4.47. MICROSTRUCTURE OF SAMPLE #4 (LOWER PART) .............................................118
FIGURE 4.48. MELTABILITY CURVES FOR A SERIES OF OXIDES ..............................................119
FIGURE 4.49. EXPERIMENTAL ASSEMBLY DIAGRAM................................................................122
FIGURE 4.50. INTERACTION LAYER FORMED AT CRUCIBLE INNER SURFACE DURING TEST #2 .123
FIGURE 4.51. DIFFRACTION PATTERN OF THE DEBRIS BED LOWER (#1) REGION CORIUM SAMPLE........................................................................................................................................124
FIGURE 4.52. DIFFRACTION PATTERN OF THE DEBRIS BED UPPER (#2) REGION CORIUM SAMPLE........................................................................................................................................124
FIGURE 4.53. INTERACTION LAYER ON THE CRUCIBLE INTERIOR SURFACE AFTER TEST #3 ...125v

FIGURE 4.54. DIFFRACTION PATTERN OBTAINED ON THE SAMPLE CUT OUT FROM THE CRUCIBLE
BOTTOM: INTERACTION LAYER OF TEST #3......................................................................126
FIGURE 5.1. THE KINEMATIC VISCOSITY VERSUS THE TEMPERATURE FOR C-32 CORIUM ......129
FIGURE 5.2.LAYOUT OF THE STF FACILITY TO CONDUCT EXPERIMENTS TO DETERMINEMELTING TEMPERATURES OF METALLIC PHASES ...........................................................131
FIGURE 5.3. THE TEMPERATURE DEPENDENCE OF SPECIFIC RESISTANCE ...............................134
FIGURE 5.4. PHOTO PICTURE OF THE FE DROPLET WITH THE MASS OF 1.1652 G ....................135
FIGURE 5.5. PHOTO PICTURE OF THE DROPLET OF STFM-FE #20 ALLOY WITH THE MASS OF
0.4991 G .........................................................................................................................137
FIGURE 5.6. MACROSTRUCTURE OF THE DROPLET OF STFM-FE#20 ALLOY WITH THE MASS OF
0.4991 G .........................................................................................................................137
FIGURE 5.7. MICROSTRUCTURE OF THE DROPLET OF STFM-FE #20 ALLOY WITH THE MASS OF
0.4991 G .........................................................................................................................138
FIGURE 5.8. MEASUREMENT SCHEMATICS OF THE LYING DROPLET TO DETERMINE R0 AND R1
RADII ..............................................................................................................................139
FIGURE 5.9. THE VALUES OF THERMAL CONDUCTIVITY OBTAINED IN TESTS ........................140
FIGURE 5.10. BILLET OF MATERIAL CUT OUT OF THE LOWER METALLIC BODY OF RCW TEST FOR
SPECIMEN PREPARATION .................................................................................................142
FIGURE 5.11. LOCAL POROSITIES AND BILLET ASPECTS...........................................................142
FIGURE 5.12. GROUPS OF CYLINDERS CUT OUT OF A 44 × 33.5 × 13 MM SIZE BILLET ..............143
FIGURE 5.13. VARIATION OF METALLIC BODY MATERIAL DENSITY ALONG THE AXIAL AND
RADIAL DIRECTION OF RCW MACRO INGOT ACCORDING TO CALCULATIONS (RED) AND
MEASUREMENTS (BLUE) DATA ........................................................................................144
FIGURE 5.14. EXPERIMENTAL APPARATUS FURNACE FOR SPECIFIC HEAT MEASUREMENTS .....146
FIGURE 5.15. HEAT CAPACITY OF THE "LIGHT" AND "HEAVY" SAMPLES OF RCW TEST ..........147
FIGURE 5.16. COMPARISON OF SPECIFIC HEAT CAPACITY FOR "LIGHT" AND "HEAVY" SAMPLES........................................................................................................................................147
FIGURE 5.17. THERMAL DIFFUSIVITY OF "LIGHT" AND "HEAVY" SAMPLES OF RCW TEST.......148
FIGURE 5.18. COMPARISON OF THERMAL DIFFUSIVITY FOR "LIGHT" AND "HEAVY" SAMPLES OF
RCW TEST ......................................................................................................................148
FIGURE 5.19. THERMAL CONDUCTIVITY OF "LIGHT" AND "HEAVY" SAMPLES OF RCW TEST ..149vi

LIST OF TABLESTABLE 1.1. CORIUM COMPOSITIONS STUDIES IN THE MASCA PROJECT ....................................3
TABLE 2.1. MATRIX AND MAIN RESULTS OF STFM1/1-11 EXPERIMENTS ..................................8
TABLE 2.2. THE MATRIX AND MAIN RESULTS OF STFM2 TEST SERIES ...................................11
TABLE 3.1. MATRIX OF STFM-FE, STFM-B AND STFM-FP TESTS IN THE INERT CONDITIONS 17
TABLE 3.2. MATRIX OF MA TESTS IN INERT CONDITIONS .........................................................18
TABLE 3.3. MATRIX OF STFM TESTS WITH MELT OXIDATION .................................................22
TABLE 3.4. MATRIX OF MA TESTS WITH MELT OXIDATION .....................................................22
TABLE 3.5. STFM-FE AND STFM-FP EXPERIMENTAL RESULTS ON THE INTERACTION OF
MOLTEN CORIA WITH FE ..................................................................................................26
TABLE 3.6. COMPARISON OF THE DATA ON ELEMENTAL COMPOSITION OF MA-6 METALLIC
INGOT OBTAINED BY DIFFERENT METHODS........................................................................29
TABLE 3.7. COMPARISON OF THE DATA ON ELEMENTAL COMPOSITION OF MA-6 OXIDIC INGOTOBTAINED BY DIFFERENT METHODS ..................................................................................29
TABLE 3.8. MA TESTS RESULTING DATA FOR INERT CONDITIONS .............................................30
TABLE 3.9. MATERIAL BALANCE IN THE TEST STFM-OX NO.1 ...............................................33
TABLE 3.10. THE ANALYSIS RESULTS FOR THE CHEMICAL AND PHASE COMPOSITIONS OF
INGOTS IN THE TEST STFM-OX NO. 1 ..............................................................................34
TABLE 3.11. RESULTS OF TESTS OF THE STFM-OX SERIES ON THE INTERACTION OF MELTEDCORIA WITH FE (SS) .........................................................................................................35
TABLE 3.12. RESULTS OF MA TESTS WITH CORIUM OXIDATION ................................................37
TABLE 3.13. PARTITIONING OF ELEMENTS IN MA-7 TEST .........................................................38
TABLE 3.14. PARTITIONING OF ELEMENTS IN MA-9 TEST .........................................................39
TABLE 3.15. INITIAL CORIA ANALYSIS COMPOSITIONS ..............................................................43
TABLE 3.16. EXPERIMENTAL RESULTS ON THE INTERACTION OF MOLTEN CORIA CONTAINING
B4C WITH IRON .................................................................................................................45
TABLE 3.17. RESULTS OF THE INGOT COMPOSITION ANALYSES ...............................................47
TABLE 3.18 CONCENTRATIONS OF THE MAIN SPECIES IN METALLIC MELT.................................53
TABLE 19. RESULTS OF THE CHEMICAL AND PHASE COMPOSITION ANALYSIS OF THE STFMB-3
INGOT ...............................................................................................................................57
TABLE 3.20. COMPARISON OF STFM-B AND STFM-FE EXPERIMENTAL RESULTS ....................58
TABLE 3.21. PARTITIONING OF FP SIMULANTS IN THE METALLIC AND OXIDIC INGOTS OBTAINEDIN EXPERIMENTS STFM-B # 6 AND STFM-B # 9..............................................................59
TABLE 3.22. INITIAL CHARGE OF FP SIMULANTS (GRAMS).......................................................60
TABLE 3.23. PARTITIONING OF FP BETWEEN THE OXIDIC AND METALLIC PHASES DEPENDING
ON STEEL COMPOSITION (STFM-FP-1, 6 TESTS)..............................................................61
TABLE 3.24. FP SIMULANTS CONCENTRATIONS IN OXIDIC AND METALLIC PHASES.................62
TABLE 3.25. MASSES OF FP SIMULANTS FOUND AFTER THE TESTS AND RELATIVE ERRORS ....63
TABLE 3.26. PARTITIONING OF FP BETWEEN THE OXIDIC AND METALLIC PHASES OF THE
STRATIFIED MELT IN THE STFM-FP-1, 3 TESTS DEPENDING ON TEMPERATURE ..............63
TABLE 3.27. PARTITIONING OF FP BETWEEN THE OXIDIC AND METALLIC PHASES DEPENDING
ON U/ZR ATOMIC RATIO ...................................................................................................65
TABLE 3.28. PARTITIONING OF FP SIMULANTS IN THE METALLIC AND OXIDIC INGOTS OBTAINEDIN EXPERIMENTS STFM-B # 6 AND STFM-B # 9..............................................................65
TABLE 3.29. BASIC CHARACTERISTICS OF THE RCW FACILITY ................................................70
TABLE 3.30. THE MATERIAL BALANCE OF THE RCW EXPERIMENT .........................................72
TABLE 3.31. WEIGHTS OF METALLIC PARTS .............................................................................74
TABLE 3.32. THE CORIUM CHEMICAL ANALYSIS RESULTS.......................................................75
TABLE 3.33. THE CHEMICAL ANALYSIS RESULTS FOR METALLIC PARTS .................................77
TABLE 3.34. THE BASIC PARAMETERS OF TESTS WITH DIFFERENT SCALES OF LOADINGS .......78vii

TABLE 3.35. RESULTS OF THE CORIUM PLASMA EMISSION-SPECTRUM ANALYSIS FOR THE
CONTENTS OF FP SIMULANTS ...........................................................................................80
TABLE 3.36. RESULTS OF THE PLASMA-SPECTRUM ANALYSIS FOR THE INGOT METALLIC PARTS..........................................................................................................................................80
TABLE 3.37. THE COEFFICIENT OF FP PARTITIONING BETWEEN THE INGOT METALLIC ANDOXIDIC PARTS ...................................................................................................................80
TABLE 3.38. COMPARISON OF VALUES FOR COEFFICIENTS OF PARTITIONING ..........................81
TABLE 3.39. COMPARISON OF RESULTS WITH THE RCW DATA ................................................81
TABLE 4.1. TEST MATRIX AND MAJOR PARAMETERS OF TESTS WITH MOLTEN STEEL ST-30 ...95
TABLE 4.2. TEST MATRIX AND MAJOR PARAMETERS OF TESTS WITH MOLTEN STAINLESS STEEL..........................................................................................................................................99
TABLE 4.3. TEST MATRIX AND MAJOR PARAMETERS OF TESTS WITH MOLTEN ZIRCONIUM
ALLOYS ...........................................................................................................................104
TABLE 4.4. ESTIMATIONS OF CORIUM CN INDICES AT INTERACTION BETWEEN CORIUM, FE2O3,
ОAND STAINLESS STEEL AT T≥1800 C ..............................................................................111
TABLE 4.5. THE ANALYSES RESULTS FOR SAMPLES CH#1 – CH#5 (MASS%)...........................115
TABLE 4.6. THE ANALYSIS RESULTS FOR SURFACE INGOT SAMPLES......................................117
TABLE 4.7. THE ANALYSES RESULTS FOR SAMPLES CH#1 – CH#5 AND CDC .........................117
TABLE 4.8. THE DENSITIES OF SURFACE INGOT SAMPLES ......................................................117
TABLE 4.9. ELEMENTAL COMPOSITION ANALYSES AFTER TEST #3 .........................................123
TABLE 5.1. DENSITY OF C-32 CORIUM ...................................................................................130
TABLE 5.2. HELIUM PURIFICATION RESULTS ..........................................................................131
TABLE 5.3. SPREADING TEMPERATURES OF METALLIC INGOTS. EXPERIMENTS STFM-FE #17
AND STFM-FE #20 .........................................................................................................132
TABLE 5.4. RESULTS ON DENSITY, CHEMICAL AND X-PHASE ANALYSES OF THE RCW INGOTSAMPLES .........................................................................................................................132
TABLE 5.5. TEMPERATURES FOR THE RCW INGOT METALLIC PHASE SPREADING .................133
TABLE 5.6. THE “METALLIC BODY” CHEMICAL ANALYSIS OF THE T-7 TEST .........................133
TABLE 5.7. SPECIFIC RESISTANCE (ρ) AND TEMPERATURE COEFFICIENT OF THAT (α) OF THE
ALLOY AND ITS COMPONENTS ........................................................................................134
TABLE 5.8. RESULTS OF FE DENSITY MEASUREMENTS AT THE TEMPERATURE OF 1650ОC.....135
TABLE 5.9 RESULTS OF ANALYSES OF STFM-FE #20 METALLIC ALLOY ...............................136
TABLE 5.10. RESULTS OF THE DENSITY MEASUREMENT OF STFM-FE#20 ALLOY AT THE
TEMPERATURE OF 1750 ОC .............................................................................................138
TABLE 5.11. CALCULATED SURFACE TENSION RESULTS OF STFM-FE #20 ALLOY AT THE
TEMPERATURE OF 1750 ОC .............................................................................................139
TABLE 5.12. THERMAL CONDUCTIVITY DATA OBTAINED IN TESTS........................................141
TABLE 5.13. DENSITIES OF WITNESS PELLETS .........................................................................143
TABLE 5.14. MEASURED DIMENSIONS AND CALCULATED DENSITIES OF POLISHED PELLETS ...144viii

1 INTRODUCTION
The in-vessel retention of a severe accident in the vessel of a light water reactor (LWR) by
external cooling of the vessel has been recognized as an attractive accident management
measure. The problem includes different aspects such as:• Thermal hydraulic behavior of the molten pool;• Physico-chemical interactions of core components in a molten pool;• Thermal and chemical interactions of molten pool with the vessel steel;• Coolability of the pressure vessel by vessel flooding;• Mechanical behavior of the vessel.
Numerous experimental studies with simulant materials, such as COPO, ACOPO, BALI,RASPLAV-SALT and SIMECO, dealt with the problem of heat flux distribution around a
volumetrically heated molten pool at wide range of Rayleigh number. The main focus forthese studies has been the thermal loading imposed by the core melt on the reactor pressure
vessel (RPV) lower head.
Thermal hydraulic studies of the melt pool behaviour have also included cases where amolten metal (steel) layer could form on top of oxidic corium due to very limited solubility ofsteel in the oxides (the density of steel being lower than the density of core oxides). The heatfocusing effect of this molten metal layer was of concern, because the vessel walls adjacent to
the metal layer were subjected to increased heat fluxes, which could lead to substantial
localised melting of the walls.
Experimental programs have also been conducted to investigate the critical heat flux for theboiling process initiated on the outer surface of the vessel due to cavity flooding (external
vessel cooling).
Studies of the impact of real materials have been performed in the frame of OECDRASPLAV project. The RASPLAV project aimed to refine accident management strategies
during a reactor core meltdown; it was completed in June 2000. Little was known about thecomplex interactions that take place during a core meltdown, so one of the RASPLAV
project's primary goals was to develop an understanding of these processes. The information
gathered during tests at the Kurchatov Institute have allowed scientists to develop models of acore meltdown. Two aspects of the issue were considered. First, for existing reactors, where
external cooling may not be practicable, the process and time sequence before melt-throughwere studied. This was to help devlop management strategies for severe accidents. Secondly,for future and some existing reactor designs, the project aimed to determine the heat transfer
conditions under which cavity flooding can be a viable accident management option.
The project was run in two successive phases. The RASPLAV Phase-2 project investigatedthe progression of a severe accident and in particular the thermal loading imposed by acorium pool on the lower head of a Light Water Reactor (LWR) vessel. It followed an earlier
Phase-1 project dedicated mainly to the build-up of the experimental and analyticalinfrastructure.
The project objectives were to obtain relevant data on the physical and thermal behavior of
the corium in large-scale tests, to derive thermal-physical property data for various molten
core materials, and to investigate the effects of stratification of molten materials. The
programme of work involved the use of the large scale facilities available at the Kurtchatov
Institute in Russia. Four large-scale tests were carried out and were complemented by a seriesof smaller-scale experiments. It demonstrated that the homogeneous corium melt behaved
comparably to simulant materials in natural circulation, therefore previous evaluations basedon simulant material data could be scaled to prototypic reactor conditions. It was shown thatsuch a behavior can be expected if molten pool contained oxidic materials. However, two of1

the four large-scale RASPLAV tests conducted with suboxidized corium showed that themelt pool, initially of a homogenous composition and density, stratified into two layers ofunequal density. The lower layer of a denser material was richer in uranium while the upper
lighter layer was richer in metallic Zr [1]. Although further investigations showed that main
reason was in small carbon content in the melt, in light of these results the interest was toobtain more data with respect to prototypic core materials behaviour at high temperatures.
RASPLAV project dealt with the corium of different oxidation degree. Another subject ofinterest was the concern about the behavior of steel in contact with corium. As heat focusing
effect may change significantly the distribution of heat flux, interactions between molten
corium and steel under prototypic conditions is of great importance.
The project was successfully concluded in 2000.The results are set out in the Project final
report [2] which was presented and reviewed in November 2000 at the RASPLAV seminar.
The MASCA Project was a follow-up of the RASPLAV Project and investigated in-vessel
phenomena during a severe accident. In particular, it addressed the influence of the chemicalcomposition of the molten corium on the heat transfer to the pressure vessel environment.
The project addressed this by investigating stratification phenomena of the molten pool andthe partitioning of fission products (FP) within the different layers of the melt. The project
was scheduled to be completed in July 2003, but it was continued until 2006 under the
MASCA-2 Project, given the experimental needs that still exist and the quality of theexperimental work done so far.
The major goals of the MASCA Project were to:• Investigate the influence of chemical behavior of the melt in stratified molten pools of
prototypical compositions;• Investigate FP behaviour in a molten pool and in particular:o Partitioning of FP between layers in case of stratification;o Partitioning of FP between phases during melting and solidification;o Distribution of FP simulants in the melts.
• Expand the material properties database;• Develop computer models describing the relevant phenomena.
Separate effects were studied in series of small- and mid-scale experiments. Corium
tests were performed in the RASPLAV, TULPAN, TF, STF, KORPUS, and TIGEL facilities.
Salt tests were performed in the present RASPLAV-Salt facility. MASCA Seminar was heldin June 2004 to review the project results so far.
The MASCA-2 Project was an extension of the MASCA Project. It continues to be based on
experiments that make use of a variety of facilities in which corium compositions
prototypical of power reactors were tested. The tests aim to provide experimental informationon the phase equilibrium for different corium mixture compositions that can occur under
accident conditions in water reactors. In order to enhance the application of MASCA results
for reactor cases, the influence of an oxidizing atmosphere and the impact of non-uniform
temperatures (presence of solid debris) were addressed. The programme was also intended togenerate data on relevant physical properties of mixtures and alloys that are important for thedevelopment of qualified mechanistic models.
The key elements of the MASCA project were as follows:• Study of melt stratification and distribution of major species (U, Zr, O, Fe (SS))
between layers in inert atmosphere for varying iron to corium ratio;2•Study of control rod materials effects on interaction and distribution of major species(U, Zr, O, Fe (SS)) in the inert atmosphere;• Study of melt stratification and distribution of major species (U, Zr, O, Fe (SS))
between layers in oxidizing atmosphere for varying iron to corium ratio;• Investigations of molten metal alloys interactions with corium debris bed in the inert
and oxidizing atmosphere;• Expension of material properties database.
A total of 24 small (loading up to 500 grams), 7 medium (2 kg) and 1 large scale test (about
90 kg loading) were conducted in inert atmosphere. Uranium to zirconium ratio varied in thetests between 1.2 and 0.9 to represent both PWR and BWR compositions. The corium
compositions used in the tests are presented in Table 1.1. All experiments were conductedusing inductive heating and cold crucible technique, which allowed maintaining integrity of
the facilities. For each test, the temperature was raised above the liquidus temperature ofcorium. Stainless steel was added after corium melting. The exposure time for small scale
tests varies between 10 to 60 minutes. The chemical equilibrium was found to be reachedbecause of the small scale of the samples and the stirring due to induction forces. Typical
exposure time for medium scale tests was 30 minutes. Post test examination allowed
measuring the compositions of both metallic and ceramic ingots. A special attention was paid
to the analysis of the mass balance in the system and oxygen contents.
Table 1.1. Corium Compositions Studies in the MASCA Project
Coriumcomposition

Zirconiumoxidation(%)

UO2

Composition(mol. % / wt. %)
ZrO2

C-22

22

62 / 81.8

8.4 / 5.0

29.6 / 13.2

1.6

C-32

32

54.5 / 76.2

14.5 / 9.3

31.0 / 14.5

1.2

C-50

50

62 / 80.4

19 / 11.3

19 / 8.3

1.6

C-75

75

56.4/75.2

33.2/20.2

10.4/4.6

1.3

C-100

100

62 / 78.1

38 / 21.9

-

1.6

Zr

U/Zr
ratio

In a series of the medium-scale and small scale experiments it was found that corium incontact with steel separated into two immiscible layers. One of the phases is of metallic type.
It includes initial steel components but also metallic zirconium and uranium transferred fromthe suboxidized corium.
Most attention was paid to the partitioning of components in the oxide-metal layers. In aseries of small and medium scale tests the following parameters were varied:• The U/Zr molar ratio was varied between 1.29 and 1.6• The initial oxidation degree of Zr was varied between 30 % and 100 %• The amount of steel introduced into the corium varied between 2 % and 20 %• Exposure time at the temperature plateau for small-scale tests varied between 10 and
60 minutes.
A large-scale test was also conducted. Pretest analyses were performed to assess the power
input scenario, melt progression scenario and temperatures during test performance. Posttest
examination confirmed most of the results already obtained in smaller scale tests. However,3some differences in the final stratification and distribution of phases appeared. They will be
discussed below.
Studies of the influence of control materials were conducted in MASCA2. The test matrix
included 9 small scale tests to study the impact of boron carbide on stratification. The
experimental procedure was the same as for the previous test series. The impact was found to
be rather significant for small ratio of iron to corium where significant increase of the metal
phase was obtained. One experiment was conducted with the Ag-In-Cd alloy.
Another issue related to chemical interactions in the corium studied in MASCA Project waspartitioning of FP between metallic and oxide phases. The test matrix includes six small
scale, and two medium scale test. Two additional tests were conducted with the presence ofboron carbide. FP simulants were also added in the corium loading of large-scale tests. The
experiments confirmed that metal simulants such as Mo and Ru are concentrated in the metal
phase, while Ce, La are distributed in the oxide corium.
A special series of experiments dealt with the oxide and metal phases interactions in theoxidizing atmosphere. A total of 8 small scale experiments were conducted with the variationof several parameters such as the initial corium composition or the amount of steam delivered
to the melt surface. The experimental procedure included the formation of a corium pool,addition of metal (steel or iron), exposure of the oxide-metal phase during 30 minutes to getchemical equilibrium, oxidation phase to reach target corium oxidation degree, and exposure
after oxidation phase to check how fast equilibrium is established. Additionally two middle
scale oxidation tests were conducted: the first dealt with the melt the oxidation in air and thesecond with the oxidation in steam.
A series of 16 small and one medium scale tests were dedicated to the study of molten metal
migration through a porous corium debris bed in inert atmosphere. Most tests were carriedout with 2.5 mm size debris with the porosity about 40 %. One test was made with 1 cmdebris to show the effects of surface tension and wettability. The metal consisted of either
steel or an alloy of Zr and steel. To simulate the process of metal melt penetration through theporous debris the temperature was deliberately chosen in the range between 1900 K and2300 K. This temperatures were well above the melting temperature of the metallic alloy,
however lower than the corium melting temperature thus maintaining debris in a solid state.
In some cases, the melt progression through the particle bed was observed whereas in othercases there was no metal penetration. Chemical interactions between the solid particles andthe liquid metal were observed and samples were analyzed to measure compositions. Threetests were conducted in the oxidizing atmosphere.
It appeared after the first small scale tests that the metal mixture formed after phaseseparation and stratification should be characterized in detail because its physicochemical
properties would be very important to model the stratification in a reactor and the subsequent
heat transfer. Additional data on physicochemical properties were obtained. Among thosewere measurements of C-32 corium properties: spreading temperature, kinematic viscosity inthe temperature range between 2380 and 2600°C, and density in a liquid phase(2480 - 2580°C). Measurements of viscosity, thermal conductivity, and density of the liquid
metallic mixtures were also performed. Moreover, electric conductivity and heat conductivityof metallic body from T-7 test were measured at low temperature.

4

2 MELT STRATIFICATION MECHANISMS
Experiments and analysis performed in course of RASPLAV project showed thatstratification phenomenon was featured for corium containing some amount of carbon.
Special series of small-scale experiments performed with the carbon free coria have notrevealed any stratification into two liquids regardless to the oxidation degree of zirconium.
That allowed concluding that for corium containing U-Zr-O miscibility gap is outside thepractical compositions, which are characterised by the zirconium oxidation degree exceeding
30 %.
However the same set experiments showed that corium behaviour between solidus andliquidus temperatures may be of great importance. Sub-oxidised corium independently on
carbon content in the temperature range between solidus and liquidus separated into two
different phases. At temperatures above solidus temperature (which is about 1900°C) thephase formed by zirconium with some amount of oxygen ZrOx becomes liquid, while ceramicphase (UZr)O2-x remains solid. Average composition of liquid formed in the intermediate
temperature range (1900 - 2300°C) is characterized by formula (Zr0.97U0.03)O0.25 while solid
phase has average composition of (U0.75Zr0.25)O1.8. Amount of liquid decreased with theincrease of zirconium oxidation degree.
Liquid phase formed during heating up process may migrate through pores, cracks, and gaps
between the briquettes due to the gravity. As corium was a multicomponent mixture low
melting temperature species may be pushed to the top of the corium loading due to capillaryeffects and corium sintering. Such relocation was clearly determined in the experiments.
Moreover, if liquid phase meets metal structures (tungsten and or stainless steel) it interacts if
the temperature is high enough to allow such an interaction. Tungsten protector hadsignificant local corrosion as the result of interactions with liquid zirconium phase. The
processes of separation of corium components and further migration of the liquid phases cantake place in the course of a severe accident, especially during the heating up of the debris
bed if zirconium metal is not oxidised completely.
To resolve the peculiarities of corium behavior in the frames of MASCA project a special
attention was paid to the possibilities of melt stratification and separation. Two mechanisms
were considered:• Stratification induced by small amounts of impurities (carbon);• Separation of components in the solidus – liquidus temperature interval.

2.1 Carbon Induced Stratification
2.1.1 Test Apparatus and Procedure
The experiments [3] were performed at the STFM test facility in a hot tungsten crucible withdimensions ∅14 × 0.5 mm in diameter and 70 mm high. Figure 2.1(a) presents schematic of
STF facility. Temperature was measured by LMP – 066 micro-pyrometer through a hole in
the heater with the accuracy of ± 20°C. The boundary conditions along the crucible height
during experiments were isothermal within the same range of uncertainties.
In all experiments the maximum exposure temperature was T = 2550°C. Heating up rate wasfast and slow: 500 – 600o/min or 8o/min correspondingly. Cooling down rate was about
50o/min. All experiments except of those which studied stratification kinetics exposure timewas 60 min. For studying kinetic effects the exposure time varied in the range 5, 15 and60 min.

5

70After experiment the ingot was cut into two equal halves along the vertical axis. One half wasused for preparation of a macro-cross-section to assess dimensions of the top and bottom
layers, then micro-cross-sections were prepared to study microstructure along the ingot
height. Samples for chemical and micro-probe analyses were cut out of the second half of theingot from the regions of the top layer, main layer, and bottom part of the ingot as it is shownin Figure 2.1(b).

14

a)

b)

Figure 2.1. Schematic of the Experiment (a) and Post Test Examination (b)2.1.2 Test Matrix and Results
A series of 11 small-scale experiments [3] were made to quantify the effect of corium
stratification due to small additives of carbon found during RASPLAV project. Conditions
for each experiment and main experimental results are presented in Table 2.1. Experiments
were conducted using C-32 corium and carbon content in the corium varied between 0.006 to
0.69 wt%. Moreover the heat up rate in the solidus-liquidus temperature range ∆TS-L/∆τvaried between 8-10°C/min and 0.13°C/min.
The errors of evaluation of concentrations with the confidence level 0.95 were as follows:• U = ± 0.37 wt%, Zr = ± 0.24 wt%, U/Zr = ± 0.03 at.%/at.%;• C = ± 0.05 wt% in the range of 0.9-1.6 wt%;
• C = ± 0.025 wt% in the range of 0.2-0.4 wt%;
• C = ± 0.014 wt% in the range of 0.6-0.15 wt%;
• The ratio of the light and grey phases ± 0.1.
Several parameters were used to identify the stratification effect. In the course of RASPLAV
Project it was found that the most important characteristics which define the ratio between
metallic and oxide phases in both layers are carbon content and uranium to zirconium ratio in
the layer. To quantify the stratification effect the following parameters were used:
U / Zr middle• KR =- the ratio between uranium and zirconium weight concentrations
U / Zr top
in the middle (bulk) and top corium samples;6

C top- the ratio between carbon weight concentrations in the top andC middle
middle (bulk) corium samples.
KC =•Another characteristic which characterizes stratification effect was the ratio between gray andlight phases. As one may see from Figure 2.5 amount of gray and light phases was
significantly different for top layer and bulk corium. Light phase was enriched with metal
zirconium and carbon, while gray phase contains almost completely oxidized species. The
ratio of areas occupied by light and gray phases is presented in last column of Table 2.1.
Stratification kinetics of the corium containing 0.3 – 0.36 wt% of carbon was studied in 4
experiments – STFM1/1, 2, 3, and 7. The amount of carbon is approximately corresponded to
the RASPLAV AW-1 test. Results of layers formation kinetics are shown in Figure 2.2. One
can see that carbon content in the upper layer increases gradually and decreases
simultaneously in the bulk corium. U/Zr ratio exhibits similar behavior. The kinetics is fastenough, after first 15 minutes the process slows down significantly. It is important to notethat no impact of the heating rate (8°C/min and 500°C/min) onto the process of carbon
redistribution was revealed.
1.6

1.4

1.2

2

1.2

Top layer
Middle layer

17

Top layer
Middle layer

1

U/Zr

Carbon concentration (mass%)

3

0.8

1

0.8

3

7

0.4
0.6

2
0.4

0
0

20

Time (min)

40

60

a)

0

20

Time (min)

40

60

b)

Figure 2.2. Carbon Concentration (a) and U/Zr Ratio (b) in the Top and Middle Layers
versus Exposure Time at the Temperature of 2550oC

Other experiments with the exposure time of 60 minutes were used to analyze the final state
as a function of initial carbon content which varied in the range between 0.085 wt% and0.69 wt%. Results of experiments are shown in Figure 2.3. One can see that if carbon content
in the initial corium was less than 0.2 wt%, carbon redistribution and change of U/Zr ratio in
the corium along the ingot height melt was insignificant. However if carbon concentration in
the initial corium exceeded 0.2 wt% significant redistribution of corium components andchange of U/Zr ratio along the ingot height was observed: increase of carbon and zirconium
concentrations in the top layer and decrease in the middle layer. This was accompanied by thegrowth of the light phase fraction (zirconium based phase) in the upper layer (See Figure 2.4
and Figure 2.5).

7

Table 2.1. Matrix and Main Results of STFM1/1-11 Experiments
C (mass%)

Ratio of areas
occupiedby light
Mass ratio of
the top and and grey phasesbulk layers Top
Bulk
layer Corium

STF-M

Exposure∆TS-L/∆τ,time(oC/min)
(min)

U/Zr

C
(mass%)

1

0.360

5

8 - 100.89

1.09 1.22 0.970

0.210 4.6

2

0.360

60

8 - 100.57

1.27 2.23 1.350

0.060 22.5

3

0.360

60

0.13 0.71

1.33 1.87 1.480

0.090 16.4

4

0.690

60

0.13 0.74

1.58 2.14 1.620

0.120 13.5

5

0.300

60
0.1vol%CO

8 - 100.83

1.30 1.57 1.250

0.100 12.5

6

0.006

60
0.1vol%CO

8 - 101.23

1.22 1.00 0.110

0.060 1.8

7

0.300

15

0.13 0.74

1.30 1.76 0.980

0.100 9.8

8

0.085

60

0.13 1.11

1.26 1.14 0.110

0.080 1.32

90.200*

60

0.13

1.05

1.26 1.20 0.275

0.072 3.8

Crucible # 1

1.08

1.30 1.20 0.265

0.062 4.2

Crucible # 2

10**

0.300

60

0.13 0.83

1.30

0.086

11

0.160

60

0.13 1.11

1.28 1.16 0.220

KC

KR
Top

Top Middle

0.870* - Average value** - Non-isothermal experiment

8

Middle0.100 2.2

0.05

2.5

0.43

0.5

1.2

0.4

Practicallyone layer

0.8

~ 0.6

The average composition of the light phase was found to be (Zr0.97U0.03)O0.29C0.23, while grayphase can be represented by formula (U0.74Zr0.26)O1.99. The results were rather compatible withthe results of AW-1 test. Detailed discussions on the phases found in course of posttest
examination can be found in the report [3]. From the Table 2.1 one can see that increase ofcarbon content in the corium led to the growth of mass ratios between the top and bulk layers.
Two experiments – STFM1/5 and 6 were performed in the argon environment with addition of
CO gas (0.1 vol.%) to study the process of corium carbidization from the gas atmosphere. No
noticeable increase of the carbon content in the top layer was observed in C – 32 corium
containing 0.36 wt% of carbon in comparison with the similar experiment #2 without CO gas
addition. Some increase of the carbon content in the top layer (up to 0.1 wt %) and in the bulk
corium volume (up to 0.06 wt %) took place in the test #6 with low initial carbon content (0.0060.009 wt %). This increase of carbon content in the atmosphere was found to be insignificant anddid not lead to corium stratification into two layers.
2

1.6

3

6

5

1.2

2

1.2

Top layer
Middle layer

1011

U/Zr at/at

Carbon concentration (mass%)

4
1.6

0.8

9
Top layer
Middle layer

8
10
0.8

3
4

7

0.4

6

8

11

9
2
0.4

0
0

0.2

0.4

0.6

0.8

Initial carbon concentration (mass%)

0

a)

0.2

0.4

0.6

Initial carbon concentration (mass%)

0.8

Figure 2.3. Carbon Concentration (a) and U/Zr Ratio (b) in the Top and Middle Layers
versus Initial Content of Carbon
White/grey phases ratio
0.2

0.4

0.6

0.8

1

1.2

White/grey phases ratio
1.4

0

0

0

10

10

Height (mm)Height (mm)

0

20

30

0.2

0.4

0.6

0.8

1

1.2

1.4

20

30

a)

40

b)

40

STFM1/4
C = 0.69 (wt%)

STFM1/8
C = 0.085 (wt%)

Figure 2.4. Change of the Area Ratio between Light and Gray Phases along the Ingot
Height

9

b)

a)

b)

c)

Figure 2.5. Macrostructure (a) and Microstructure of the top layer (b) and bulk corium (c)of C-32 Corium Ingot (STFM1/2 Experiment, Carbon Content 0.36 wt%)

2.2 Separation of Corium Components in the temperature range
between solidus and liquidus
The objective of this work was to study possibility of C - 32 corium (U/Zr = 1.2 at./at.)stratification in the temperature range of TSol - TLiq into the top liquid layer based on Zr(O)x andthe bottom solid layer based on (UZr)O2-x according to the model proposed in [4].
It was assumed that for the corium used in the RASPLAV tests liquid zirconium based phase ofcorium may move upward through the briquettes due to porosity and plastic deformations of thegrains and compaction in corium at T = 2150 - 2300°C. The second objective was to study thestability of corium configuration if such kind of stratification takes place.
2.2.1 Test Apparatus and Procedure

This series of experiment [3] was conducted using the STF facility. Briquettes and powder of
C-32 corium (U/Zr = 1.2 at./at., carbon content 0.006 - 0.009 wt%) were fabricated by pressingof the powder mixture (UO2, ZrO2, ZrH2) in accordance with the technique developed for
RASPLAV-AW-200-4 experiment.
A total of 6 experiments were performed as it was described in the previous section (section
2.1.1). The tungsten crucibles with the following dimensions: ∅ = 14 mm, h = 70 mm,
δ = 0.5 mm were used. In STFM2/1-6 experiments tungsten crucibles were loaded with corium
particles (1 - 4 mm), porosity of the loading was 30-50%.
Heating up in all of the experiments was performed with the rate of 500 - 600°C/min at the
T > TSol. Cooldown rate to TSol was 50°C/min.
After the experiment the ingot was visually inspected and cut into two equal halves along theaxis. One half was used to prepare a macro-cross-section, the other one – for cutting out samples
along the ingot height for chemical, micro-probe, and X-ray analyses. The following parameters
were analysed: the area ratios between the light and grey phases, and the U/Zr ratio in the top
and middle parts of the ingot.
The following quantitative parameters were used to specify the separation of components during
heating up and melting:U / Zr middle- the atomic ratio which characterizes redistribution of corium
U / Zr top

•

KR =

•

components along the ingot height;
Sl / S g ingot top
KS =- the area ratio occupied by the light (Sl) and grey (Sg) phases
Sl / S g ingot middle
in the top and middle parts of the ingot.
10

2.2.2 Test Matrix and Results

To simulate a variety of conditions the experimental heating and exposure scenarios included(see matrix of experiments and major experimental parameters in the Table 2.2):• Three tests with constant temperature 2150°C (test #4), 2200°C (test #3), and 2300°C(test #1);
• Two tests were conducted with the heating to the liquidus temperature 2500°C, thencooling to 2300°C (test #2) and to 2200°C (test #6) and exposure at this temperaturelevel;• One test with the heating and exposure at the temperature of 2200°C (test #5), thenheating to the 2600°C.
The obtained results were discussed in details in the report [3]. The most important ones were as
follows.
Table 2.2. The Matrix and Main Results of STFM2 Test Series
U/Zr (at./at)
Test Heating upo#T, C τ (min) Ingot top Ingot middle

KR

Sl/Sg
Ingot top Ingot middle

KS

2/1

2300

60

1.13

1.16

1.03

0.42

0.55

0.76

2/2

2500
2300

10
60

1.14

1.12

0.98

~0.5

~0.6

0.8

2/3

2200

60

0.96

1.35

1.4

1.4

~0.4

3.5

2/4

2150

60

2/5

2200
2600

60
30

0.6

~0.5

1.2

2/6

2500
2200

10
60

0.5

0.3

1.7

No sintering of solid corium
1.28

1.15

0.9

For isothermal tests there was no onset of stratification at temperature 2150°C and 2300°C. For
the test STFM#2/3 conducted at the temperature of 2200°C post test examination exhibited
increase of the light phase Zr(O)x concentration and corresponding decrease of U/Zr ratio in the
top layer. Enrichment of the top liquid layer with zirconium has occurred simultaneously.
Concentration of the light phase has decreased and the fraction of the grey phase based on
UZrO2-x has increased in the bottom solid layer. Coefficient KS of the light and grey phases
fraction ratio in the ingot top and middle parts reached the value of 3.5. The cut of the ingot andthe distribution of ratio between light and grey phases for test STFM#2/3 are presented in Figure
2.6. One can see that amount of light phase gradually increases to the top of the ingot.
The compositions of phases found during the post test examination are shown in the table below:
the light phase mainly consists of zirconium Zr(O)x phase with small amount of uranium while
grey phase presents oxide phase (UZr)O2-x.
Phase

Concentration (wt%)
U

Zr

O

Phase molecular formulaLight

9.21 85.19

4.79(Zr0.96U0.04)O0.30

Gray74.29 12.10 13.70(U0.70Zr0.30)O1.93

In the experiment #6 corium was heated first to 2500°C and then cooled down to 2200°C andexposed for 60 minutes did not indicate significant differences in light phase composition in the
top and middle layers, ratio of the light and grey phase fractions in the layers was 1.7, which is 2
11

times less than in the #3 experiment. Similar results were observed for the case when corium was
exposed first at the temperature of 2200°C and then heated up to 2600°C. The structure of theingot was found to be rather uniform.
White/grey phases ratio
0

0.2

0.4

0.6

0.8

1

1.2

1.4

0

Height (mm)

10

20

30

40

Figure 2.6. Distribution of the Light and Gray Phase Ratio along the Ingot Height (test
STFM#2/3)

2.3 Major Findings
Different possibilities for corium stratification were studied in a series of small-scale
experiments. The major findings were as follows.
• Stratification observed in the large scale AW-1 test can be explained by small amount ofcarbon 0.3 - 0.4 wt. %. Small-scale experiments indicated that there is some thresholdvalue of about 0.2 wt. % above which carbon produce a top layer enriched with zirconium.
Carbon content in the upper layer increases up to 1.5 wt. %, while in the bulk oxide phase
the amount of carbon does not exceed 0.1 wt. %. This effect can be explaned by theformation of zirconium carbides and oxicarbides [2]. These conclusion was also confirmed
in [5] where analysis with the GEMINI code was performed which showed that zirconium
oxycarbides (ZrCxOy) may form at rather low temperature. As they do not melt during thetests (melting temperature of oxicarbides exceeds 3000°C) and are not dissolved by theoxidic phase, oxycarbides may separate by the density difference after the melting of theoxide phase.
• The heating up rate does not influence on characteristics of the top layer if exposure timeexceeds 1 hr. For low content of carbon in the initial corium the presence of CO gas in theenvironment above corium increases carbon content from 0.006 wt. % up to 0.06 wt. %after 1 hr of exposure. This amount is too small to lead to formation of the upper layer.
• Stratification kinetics was fast enough; the characteristic time assessed from data presented
in Figure 2.2 varies between 6 and 12 minutes. As characteristic dimensions of the
RASPLAV-AW facility were approximately 4 – 5 times larger then expected characteristic
12

•time for large-scale tests might be 20 times larger (between 2 and 4 hours). Note that the
duration of AW-1 test was 2 hours and AW-2 test about 1 hour. Thus results of the STFM
tests and AW tests are in a qualitative agreement with respect to the kinetic effect and in areasonable quantitative agreement with respect to characteristics of layers.
Another mechanisms for melt stratification were assumed in [4]. One of the mechanisms
was the influence of the heating up rate which may lead to separation of corium
components. The analysis made with the GEMINI2 code, which predicted that at
temperature 2200°C there was approximately 25 % of the liquid phase and 75 % of thesolid phase. The liquefaction process may lead to the relocation of metal rich liquid
accumulating inside of the melt at the temperature range between 2200°C to 2300°C.
During the temperature increase the porous matrix based on (UZr)O2-x begun to soften.
Simultaneous consolidation and sintering of the matrix might force liquid Zr(O)x phaseupward through the pores and gaps of the corium producing the stratified
configuration.This mechanism was partially confirmed by experiments performed in the
STFM-2 series. At the temperature of 2200°C the upper part of ingot was enriched withzirconium based phase ZrOx with relatively small amount of uranium. At this temperaturelevel the ratio of areas occupied by light and gray phases reached the value of 1.4(Experiment STFM#2/3) while the average value at higher temperature was about 0.5 - 0.6.
However experiments showed that this stratification was unstable; increase of temperature
up to 2500°C led to practical disappearance of the differences between upper layer andbulk corium.

13

3 INTERACTIONS OF CORIUM WITH STEEL (IRON)
3.1 Objectives of the Tests
It is well known [6, 7] that practically all scenario-specific compositions of corium molten pool
formed at the late phase of LWR accident correspond to the domain of miscibility gap of U-ZrFe-O phase diagram. Two immiscible liquids, the first with low content of oxygen (metallic
liquid) and the second with high content of oxygen (oxidic liquid) will have nearly equilibriumcompositions in the pool after a certain time. Previously U, Zr, Fe and O repartitioning was
found both theoretically [8] and experimentally [9] during formation of such a pool, i.e. at theinteraction of molten core materials with steel of in-vessel structures and reactor vessel.
Chemical equilibrium in molten corium pool was experimentally studied in the frame of
MASCA-1 OECD program with practical interest to molten pool configuration, heat transfer andpossible focusing effect in the IVR conditions [10]. Results of STF [11, 12] and MA [13, 14, 15]series with chemically prototypic corium indicate sensitivity of pool structure, position, mass andcomposition of coexisting stratified layers to corium composition, particularly to melt oxidationindex (C), U/Zr ratio and corium/steel mass ratio [16].
Experimental study was extended in the 2nd phase of MASCA program in order to quantify
partitioning phenomena in a wider range of test parameters and to check an effect of the pooloxidation. The program has been recently finished and the objective of this chapter is to present
integrated results of the 1st and the 2nd phases.

3.2

Test Facilities and Methodology

Small and medium scale STF and MA test series have been carried out correspondingly in
LUCH and in NITI with corium masses of 0.2 - 0.3 and 1 – 2 kg. At present the induction
melting in a cold crucible (IMCC) [17] is the only method applicable for the preparation ofhighly corrosive oxidic/metallic corium melts having prototypic composition, which prevent
melt pollution by the crucible material. This method was used in the both test series.
One large scale test was performed in the RRC KI by using RCW facility with IMCC, thefacility methodology and the test results are presented in section 3.6 of the report.
Figure 3.1 presents the schemes of STFM and Figure 3.2 - MA test sections. Both furnaces were
designed to produce stratified metallic/oxidic pool in the water-cooled crucible (2 and 8) by
induction heating from the coil (3 and 5). Pool surface temperature was measured by different
color pyrometers (1) combined with video cameras for the melt surface monitoring.
In STFM tests corium disks (5) selected for the experiment were charged into the crucible, afterthat corium rings (4) were put onto the disks. Small gaps between corium and cold crucible tubes
were filled with powder of UO2. Tungsten tube (1) of 16 mm diameter and wall thickness of
1 mm was initially placed into the internal cavity formed by corium rings. The tube was used as
the starting heater. In MA tests the charge was made from oxide powders and compacted Zr
metal, the startup heating was made directly by inductive heating of Zr placed in the central zoneof the charge.

14

1 – W tube; 2 – crucible;
3 – inductor; 4 – corium rings;
5 – corium disks; 6 – powder of UO2
7 – protective insulation

View of the STFM Facility

Figure 3.1. Test sections of STFM Fasility15

1 – spectral ratio pyrometer; 2 – argon-blown watercooled pyrometer shaft; 3 – water-cooled furnace lid;
4 – quartz tube; 5 – inductor; 6 – corium melt; 7 –crust;
8 – pipes of the crucible sections;
9 – stainless-steel pellets; 10 – sluice; 11 – damper;
12 – bottom calorimeter.

View of the RASPLAV-3 Facility

Figure 3.2. RASPLAV-3 facility

In accordance with the experimental procedure of STFM series the sintered corium briquettes ofa specified composition were slowly heated (20...25 min.) up to the required temperature,
2500...2600°C, at which they were exposed during ∼ 10 min. After this pure iron (in some tests stainless steel) was added, the resulting oxidic-metallic melt was exposed during 10...60 min,after that the temperature was slowly reduced to ∼ 1800°C and heating was disconnected.
STFM-FP tests [18] were different from STFM-Fe by the initial charge composition, which had
simulants of low-volatile FP (Mo, Ru, Sr, Ba, La, Ce) in the form of Mo, Ru, SrCO3, BaO2,
La2O3 and CeO2 with the concentration of 0.4 wt.% from the total mass of the charge.
In accordance with the experimental procedure of the MA series the charge was molten andsuperheated up to 2400...2500°C, stainless steel was introduced into the melt stepwise, the melt
was exposed during 30 min., after which the heating was disconnected. In MA-3, MA-4 thecharge included FP simulants: Mo, Ru, La2O3, CeO2 and Ba, Sr ‘metazirconates’ prepared from
BaO and Sr(OH)2⋅8H2O by their sintering with ZrO2. The content of each FP stimulant was ∼ 0.5
w% from the total mass of the charge.
Tests in inert conditions were carried out in argon of high purity (impurity content was: O2 –2 ⋅ 10-4, CO2 – 2 ⋅ 10-5, H2O - 3 ⋅ 10-4 vol. %) to prevent melt oxidation.
The experimental matrixes of STFM and MA series are presented in Table 3.1 and Table 3.2.
16

Table 3.1. Matrix of STFM-Fe, STFM-B and STFM-FP tests in the inert conditions
Test

T, °C

STFM-Fe4

2500

STFM-Fe6

2550

STFM-Fe7

2600

STFM-Fe92500

STFM-Fe10

2600

STFM-Fe15

STFM-Fe18

Iron

30
SS
60

2500

2600

120

120

Iron

STFM-Fe21
STFM-Fe22
STFM-Fe23

2500

60

STFM-Fe24
STFM-FP1
STFM-FP2
STFM-FP3

2600
2700

STFM-FP6
STFM-FP7

Iron
15

2600

SS
Iron

STFM-FP8

U/Zr,
at

Cn

1.15

32.6

1.16

33.4

4.36

1.15

32.6

1.29

1.20

54.6

1.57

1.19

69.2

1.52

1.14

35.6

9.52

1.19

69.2

4.30

1.14

35.6

M st0 / M Σ , %
1.84
1.59

15.22
11.26

30.7

15.95

30.9

14.78

1.27

29.9

22.77

0.95

29.8

1.07

1.23

66.6

12.81

0.96

29.7

4.48

0.98

32

9.57

0.92

32

0.63

0.97

25.7

21.01

1.29

32.2

3.89

1.68

32.8

3.33

1.28

32.3

3.73

1.34

32.3

3.86

1.36

56.2

2.82

1.39

65.4

2.77

1.25

60

STFM-Fe19
STFM-Fe20

10

2500

STFM-Fe16
STFM-Fe17

30
60

STFM-Fe5

STFM-Fe14

Metal

10

STFM-Fe2
STFM-Fe3

Exposition
time,
min

STFM-B3

2550

30

1.17

31.8

2.04

STFM-B4

2550

30

1.15

32.5

5.91

STFM-B5

2550

30

1.15

32.5

10.66

STFM-B7

2600

30

1.2

70.2

1.63

STFM-B8

2550

30

0.94

30.7

6.00

STFM-B6

2550

60

1.15

32.5

4.69

STFM-B9

2550

60

0.94

30.7

4.78

Iron

Iron + FP
17

Table 3.2. Matrix of MA tests in inert conditions
U/Zr,
at

Cn

M st0 / M Σ , %

MA-1

1.45

33.1

11.42

MA-2

1.28

74.5

10.80

1.14

36.5

10.78

1.17

39.7

19.09

0.89

31.5

9.89

Test

T, °C

Metal

30

MA-3
MA-4

Expositiontime, min

2500

SS

MA-5
MA-5b

120

0.90

30.5

7.05

MA-6

30

1.14

29.1

29.71

It is evident from the tests matrixes that the melt temperature, exposition, composition of corium(U/Zr and Cn), composition of steel (pure iron and stainless steel) and relative initial mass ofsteel ( M st0 / M Σ ) were varied in the tests.
Study of transient effects at melt oxidation was carried out in parallel with STFM [19] and MA[20] tests. For oxidation tests both installations were upgraded in order to supply steam to themolten pool surface and to measure off-gas composition and oxidation rate. In STFM tests meltoxidation by steam was achieved by drop-wise supply of water on the molten pool surface. Dry
air and saturated steam supply to the furnace was used for melt oxidation in MA tests.
Schematics of systems for gas/oxidant supply and off-gas analysis are presented in Figure 3.3.
It is possible to monitor the melt oxidation by steam in the on-line mode using hydrogengeneration kinetics. In STFM, the chromatographic technique was chosen for off-gas
composition measurements. Due to the interaction of hydrogen and oxygen with carbon of steel,CH4 and CO may be also generated. The method of gas chromatography allowed to separate themulti-component mixture, identify and determine each component with a high sensitivity andaccuracy. The column separating the analyzed gas mixture into components is thechromatograph main part. The separated components coming out of the column with the gascarrier enter the heat-conductivity detector (katharometer) that measures the components andsends a signal to the recorder and integrator for the quantitative determination of impurities. The
columns and detector are thermostatically controlled at the level of 70.0 ± 0.2ºC. Strict
requirements (hermetical sealing, and clean lines of the sample delivery to the analyzer, purity of
the gas-carrier, device calibration and so on) were followed for the gas analysis instrumentation.
Possible measurement range of hydrogen is 2·10-2 - 100 vol.% and 1·10-3 - 100 vol.% for theother components. The delivery line was manufactured from the stainless steel pipe(∅ 3 x 0.5 mm) with polished inner surface. The line was 7 meter long, delivery time was ~ 5 sat the gas-carrier flow rate of 0.5 nl/min. A filter with a high-efficiency exsiccant made ofmagnesium perchlorate [Mg(ClO4)2 ⋅ 3H2O], which is quite neutral to the analyzed gases, wasmounted at the inlet.
In presence of steam, the discreteness of H2 and O2 determination is defined by the time for the
separation of components and is equal to ~ 45 seconds. Time for the complete analysis of allgases in analyzed mixtures did not exceed 200 seconds.

18

7

H2O

He

Ar

B

15. – gas analysis system:
1. - STFM chamber;
8. – rotameter;
chromatographs: Tsvet-4, Tsvet-6,
2. - cold crucible;
9. - gas cylinders;
Tsvet-100, Tsvet-100A;
3. – inductor;
10. – reducer;
4. - vacuum pump;
11. - purification system; gas analyser «Lazurite»;hydrometers: «Baikal and G-2-72)
5. –gas meter;
12. – manometer;
6. - fine adjustment valve; 13. – oxidiser supply;
16. – trap;
7. – filter;
14. – sampler;
17. – exsiccant.
Figure 3.3. Schematic of systems for gas/oxidant supply and off-gas analysis of the STFM
facility

Chromatographs were calibrated by the method of the absolute calibration. Precisely preparedgas mixture containing the analyzed components in known quantities was fed into thechromatographic column before (5 times) and after (5 times) the main analysis. This calibration
method [21] provides relative error of measured concentrations:- 10% for concentrations of ~ 10-3 vol.%,- 5% for concentrations of ~ 10-2 vol.%,- 3% for concentrations of ~ 10-1 vol.%,- 1% for concentrations of ~ 1 - 100 vol.%.

19

In course of the tests, gas was continuously sampled from the chamber upper part by a watercooled probe of ~ 5 mm diameter. Helium was provided for the fast intermixing of generated
gases (H2, CO) in the facility volume and for hydrogen safety. The constant rate of helium
sparging (17 nl/min) limited hydrogen concentration, which did not exceed 5 vol.%. Results ofconcentration measurements for generated interaction products (H2 and CO) were plotted
automatically in the form of discrete peaks with a time interval of ~ 15 seconds.
In MA tests the systems for gas/oxidant supply and off-gas analysis were combined with thesystem of aerosol sampling. Figure 3.4 presents schematics of gas-aerosol system, which had
different configurations in MA-7 (Figure 3.4 -a) and MA-9 (Figure 3.4 -b) tests because ofdifferent oxidants.
In MA-7 an electrochemical sensor and mass-spectrometer EMG-20-1 were installed at the
furnace exit in order to provide the on-line monitoring of oxygen concentrations in the off-gas.
EMG-20-1 is a time-of-flight mass-spectrometer of the mass-reflectron type. Its operation
principle is based on the ion separation into mass groups depending on the time of their flight in
the field-free vacuum drift space. Main technical parameters of EMG-20-1 mass spectrometer:••

registered mass range, a.e.m.
1…150;
range of measured volumetric fraction of components in the gas phase,
vol.% 0.005…100;• detection limit (2σ) of argon volumetric fraction, vol.%, below
0.005.
Before the measurements the electrochemical sensor and mass-spectrometer were calibratedusing the reference gas mixtures.
Argon was supplied into the furnace in the startup regime of melt generation, stainless steel
adding and exposition of the oxidic-metallic melt during 30 min. The gas-aerosol mixture wasdischarged into the startup line, in which a MAF filter (6) was installed for aerosol collection.
After the 30-minute exposition of oxidic-metallic melt the argon supply was stopped, and the air
supply started at a steady flow rate. Silica gel column (1) was installed at the furnace entrance
for the complete gas drying. For the evaluation of aerosol amount the disposable alternatively
switched analytical Petrianov filters F1 (2) were installed at the furnace exit. Gas for thecomposition analysis went from the same point to a mass spectrometer and an oxygen sensor.
Before that the gas was cleaned by aerosol filters (2). For the gas flow monitoring (flow rate,pressure and temperature) electromechanic flow rate meters of the OR-40/C type (G1,G2),
rotameters (G3,G4), ‘Motorola’ pressure gauges and thermocouples of L type were placed in thedenoted points of the scheme.
Similarly to MA-7 in MA-9 during the startup stage aerosol mixture went to the startup line
through the medium-area filter and hydrolock (15, 16). After the 30-minute exposition of oxidicmetallic melt the argon supply was stopped, and the steam supply was started with a steady flowrate from steam generator (2) via the supply choke and heated steam line (3). Simultaneously themain analytical steam-gas line was connected. In the line the following devices were sequentiallyconnected: condenser (5), large area filter – LAF (9) and drier filled with silica gel (10).
Condensate amount was determined using the weight sensor (18), equipped with condensate
collector (6). High-purity nitrogen was used as the carrier of uncondensed gases. Ejector (8) wasused for gas collection from the condenser. Gas composition was determined using massspectrometer (14), oxygen and hydrogen sensors (12) installed in the sampling line. Gas samples
were also taken to evacuated burettes (17) and subjected to the chromatographer analysis.
Matrixes of STFM and MA oxidation tests are presented in Table 3.3 and Table 3.4.

20

1 – drier; 2 – Petrianov filters (F1); 3 – mass-spectrometer;
4 – vacuum receiver; 5 – oxygen sensor OXYC; 6 – middle area filter (MAF);
7 – hydrolock; G1,G2 – gas rate meters; G3,G4 – rotameters;
P – pressure gauges; T –L-type thermocouples.
(a) in the MA-7 test with melt oxidation by air

1 – tanks with Ar,N2; 2 – steam generator; 3 – heated steam line; 4 – steam supply quartz tube; 5– steam condenser; 6 – condensate collector; 7 – condensate collector; 8 – ejector; 9 – large area
filter (LAF); 10 – silica gel drier; 11 – medium area filter (MAF); 12 – O and H electrochemicalsensors; 13 – vacuum receiver; 14 – mass-spectrometer; 15 – medium area filter (MAF); 16 –hydrolock; 17 – evacuated gas burettes; 18 – weight strain sensor.
(b) in the MA-9 test with melt oxidation by steam
Figure 3.4. Schematics of gas-aerosol system of RASPLAV-3 facility

21

Table 3.3. Matrix of STFM Tests with Melt Oxidation
Expositiontime**,(τ1/τ2) min

Oxidationindex of final
corium [19],Cnf

STFM-OX-1

30/0

STFM-OX-2

Test

T,
°C

Melt characteristics before
oxidation:
U/Zr,
at

Cn0

M st0 / M Σ , %

69

1.23

32

3.94

30/30

72

1.24

31

4.02

STFM-OX-3

30/30

54

1.21

37

4.85

STFM-OX-4

30/0

84

1.21

29

4.57

30/0

75

1.21

32

9.03

30/0

92

1.25

27

8.06

STFM-OX-6

30/0

88

1.18

30

6.90

STFM-OX-7

60/0

73

0.96

23

4.30

STFM-OX-8

55/0

87

0.98

18

2.87

STFM-OX-5
STFM-OX-6*

2600

Metal

Iron

30/0
87
SS
1.21
26
5.82
*) Calculated for oxidic part of the ingots**) τ1 – melt exposition after steel addition and oxidation start; τ2 - melt exposition after
oxidation
STFM-OX-9

Table 3.4. Matrix of MA Tests with Melt Oxidation
Exposition
time τ1,min

Melt characteristics before oxidation:
U/Zr,
Metal
Cn0
M st0 / M Σ , %
at
air
1.17
31.5
8.97
MA-7
2500
30
SS
steam
1.17
27.2
9.12
MA-9
Basic test procedure consisted of two stages. Firstly corium molten pool was prepared after acertain melt exposition, like in previous tests in inert conditions. Molten pool composition before
the oxidation start, in particular U/Zr atomic ratio and oxidation index of initial oxidic corium,
steel composition and content, exposition time of metal/oxide pool were varied. Configuration of
the resulting pool structure and composition of coexisting oxidic and metallic liquids werepredictable from the results of previous tests.
For the second, oxidation, stage the varied parameters were oxidant composition for MA tests
and oxidant amount for STFM tests. Test termination criterion for MA-7 was secondary
inversion of oxidic and metallic layers, while for MA-9 test it was a significant decrease of meltoxidation rate and hydrogen production. Then the induction heating of the pool was
disconnected, melt was crystallized and the ingot was cooled to the room temperature. STFM
tests were terminated after reaching a certain oxidation index of corium. Additional exposition ofmolten pool after oxidation during time τ2 of 30 minutes was applied in the tests STFM No. 2
and 3. After that, the melt temperature was gradually (~ 50 degree/min) decreased down to~ 1600ºC. Then, helium sparging was terminated; the high-frequency generator was switched
off, and the facility was cooled down to the room temperature during several hours.
Test

T, °C

Oxidant

22

3.3

Post Test Examination and Sources of Uncertainties

3.3.1 STFM-Fe, STFM-FP and MA 1-6 Tests

It was of primary importance to identify the directly interacting molten part of the charge inorder to fulfill the experimental tasks and determine quantitative characteristics of species
partitioning between oxidic and metallic liquids.
In the STFM tests the ingot and crusts were weighed. The charge mass, which had notparticipated in the interaction, was determined with adequate accuracy. After that the ingot was
split and its metallic part was extracted, separated from the corium shell and weighed. In thisway the total mass of interacted melt and, separately, masses of oxidic and metallic parts, which
had been formed in the process of interaction, were determined.
In the MA tests the total mass of interacting melt was determined as a difference between themass of initial charge and collected and weighed products, which had not taken part in theinteraction, i.e. unmolten charge and crusts. The separate masses of metallic and oxidic parts of
the ingot were determined in a way similar to the STFM tests. In all the tests the mass imbalance
between the charge and the molten products collected after the test did not exceed 1.5%.
The initial composition of corium melt was calculated from the concentration of components inthe oxidic and metallic ingot parts determined from the analyses.
In the STFM tests the content of main species was determined using standard chemical methods,
and oxygen content – from the residue. In some tests Zr oxidation degree in the oxidic part wasdetermined by direct measurements of free Zr. In the STFM-FP tests the content of FP simulants
was determined by the spectral plasma emission method.
In the MA tests the composition of main species was determined by the XRF (for the oxidic part– from the average sample), oxygen in the oxidic part of the ingot – from the residue, and in themetallic part of the ingot – by the spark source mass spectrometry (SS MS). In the MA -3, MA-4
tests the content of FP simulants was determined by the mass spectrometry with inductively
coupled plasma (ICP MS).
The errors of experimental data result from the errors of direct measurements, of the interacting
corium and steel mass and composition identification. In MA tests the error of the melt surface
temperature measurements does not exceed 30 C as the influence of aerosols on the pyrometerreadings was minimized by using pyrometer shaft. Bulk temperatures of the melt were notmeasured in the both test series.
The density of separate ingot parts was measured by picnometer, its relative error was 2%.
Methodological errors could be associated with different phenomena:• The species partitioning between the oxidic and metallic corium during ingot
crystallization should be mentioned. The thermodynamic evaluation of oxygen content in
the conditions of equilibrium between phases at 2600°C gives higher values of oxygen
concentration in the metallic phase as compared to the post-test analysis data. This can be
explained by the decreased oxygen solubility, when the melt temperature is reduced, and,especially, at the metallic melt crystallization. During this oxygen can migrate from themetallic melt both to gas and oxidic phases, were it oxidizes such active reducers as U
and Zr.
• The distribution of measured concentration was nonuniform. The reasons for this weredifferent; the distribution of components during the test could be nonuniform even forsmall scale tests, as the crystallization process contributed also to the distribution ofcomponents. The averaging was made by two different ways. For STFMFP and MA test
series the average corium and metallic phase probes were made. This procedure is
described in detail in the corresponding reports [11, 13, 14, 15]. For STFMFe series theaveraging was made analytically, using the experimental results on the vertical
distribution of components.
23•For analysis and use of STFM experimental results masses and compositions ofinteracted materials are required. Initial charge of components is known, but during thetest some amount of materials evaporated, some interacted with the crust-forming
insulation. As a result, initially charged masses and their compositions may change,
which causes uncertainties in the determination of initial masses of corium and steel
interacted during the test.
• A temperature difference between metallic and oxidic layers of molten pool was notmeasured directly in the tests, but it could influence chemical equilibrium.
As there was no direct methods for measurements of oxygen content it was not possible toquantify methodological errors for oxygen content in metallic melt. This question should be
addressed in future studies.
3.3.2 Oxidation Tests (STFM-Ox and MA-7, 9)

In STFM and MA oxidation tests an additional objective of post test analysis was to determine
amount of oxygen reacted with the melt components, i.e. the shift of corium oxidation index, andrelease of hydrogen. In STFM test the final oxidation index was determined by themeasurements of Zr-free content in the oxide part of the ingot. Hydrogen release was determinedby chromatography of off-gas with the error of 1 relative %.
An attempt was made to determine kinetic data of gas absorption/release in MA tests. Results of
on-line measurements of gas/steam flows and compositions were used. The transport delays ofmass-spectrometer, oxygen and hydrogen sensors were determined and taken into account. The
resulting relative error of kinetic data does not exceed 10 %.

3.4 Experimental Results
3.4.1

Experiments in inert atmosphere

STFM Tests
The ingot structure (Figure 3.5) after disassembling was separated into several parts: a) ingot
containing both oxidic and metallic parts, b) scull surrounding the ingot, which represented thematerial in the contact with the molten corium, c) powder of insulating materials (mainly urania),d) crust formed at the periphery.
In the Table 3.5 masses and compositions of corium before and after interaction are presented.
One can see that only about 50% of initial charge finally formed the ingot. Typical sketches ofingots obtained after the test for different masses of metallic phase are presented in Figure 3.5 forFe#3 test and Fe#14 tests. The position of metallic phase in the ingot is indicated. Approximate
locations of the samples are also shown. Between 2 and 4 samples were taken from each ingot
for analysis; it is indicated in figure as a probe number (B891 for example).
Posttests examination of results indicated the dependence of vertical distribution of main
components, which is illustrated in Figure 3.6. Taking into account the measured distribution ofcomponents the composition was averaged. The following assumptions were made for the
averaging: a) the composition of corium has only vertical dependence; b) the dependence, as one
can see from Figure 3.6, can be approximated by the linear fit; c) the volume occupied by themetallic phase is elliptical.
The vertical distribution of each component in corium was generalized using the linear
regression method. Uncertainties of the fit associated with the non-uniform distribution ofcomponents were evaluated. It is necessary to mention that the correlation factor for linear
approximation was about 0.95. Average error of the fit was about 0.5 – 0.8 wt.% for U, 0.2 –0.4 wt.% for Zr.

24

As uranium and zirconium demonstrate different trends, the uranium to zirconium ratio behavescorrespondingly varying from 1.2 - 1.4 in the bottom to 0.95 - 1.05 in the top. Integral
uncertainty of U to Zr ratio was assessed to be about 3 %.
Relative weight of each sample taken for analysis was calculated using the linear distribution
assumption and position of metallic phase. These weight factors were used for the averaging ofexperimental results.
36.4 gB965
magnetic

∅12
∅9
14

46.3 gB966
nonmagnetic#В891 (46,7 g)

43.1 gB962#В892 (38,7 g)

4.5 g
B962*
magnetic

53
34

47

#В893 (16,3 g)

24.3 gB957

5 11#В894 (15,1 g)

6.2 g
B957*

3

#В890 (6,8 g)

∅17

∅27

∅30

∅30

STFM-Fe#3

35.6 g
B963

STFM-Fe#14

Figure 3.5. Sampling Scheme for Tests STFM-Fe3 and Fe14
69

#3
#4
#9
#14
#5
#15

68
67

#3
#4
#5
#9
#14
#15

26

24

Zr, wt.%U, wt.%

66

28

65
64

22

20

63
18

62
16

61

0.2

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.3

0.4

0.5

1.0

0.6

0.7

0.8

0.9

1.0

Height, rel. units

Zirconium distribution

Height, rel. units

Uranium distribution
1.6
2.0

1.4

1.8

1.2

1.6

1.0

#3
#4
#9
#14

1.2
1.0

wt.%

Fe, wt.%

1.4

Cr
Ni
Fe

0.8
0.6

0.8
0.6

0.4

0.4

0.2

0.2
0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0

1.0

0.2

Height, rel.units

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Height, rel. units

Iron distribution

STFM-Fe#15 test

Figure 3.6. Characteristic Distribution of Concentrations in the Oxidic phase Along theIngot Height

Summarized in Table 3.5 below are the overall results on the interaction of molten coria with Fe
in the first (Experiments ##2 - 7, 9, 10, 14) and the second series (Experiments ##16 - 24).

25

Table 3.5. STFM-Fe and STFM-FP Experimental Results on the Interaction of Molten Coria with Fe
Experiment #/initial composition1)

2/276.3 gm C-32.6+ 5.2 gm Fe, k=1.2
3/301.5 gm C-32.6+ 5.1 gm Fe, k=1.2
5/323.7 gm C-32.6+ 4.2 gm Fe, k=1.2
4/315.4 gm C-33.4+ 14.4 gm Fe, k=1.2
9/273.3 gm C-35.6 +29.3 gm Fe, k=1.2
14/266.0 gm C-35.6+ 47.7 gm Fe, k=1.2
16/281.5 gm C-30.7+ 53.4 gm Fe, k=1.2
17/293.5 gm C-30.9+ 50.9 gm Fe,k=1.2
18/281.5 gm C-29.9+ 83.0 gm Fe, k=1.2
6/299.2 gm C-54.6+ 4.8 gm Fe, k=1.2
7/310.1 gm C-69.2+ 4.9 gm Fe, k=1.2
10/291.6 gm C-69.2+ 13.1 gm Fe, k=1.2
20/303.8 gm C-66.6+ 44.6 gm Fe, k=1.2

Temperature/Mass/duration,
density of theoxidic phase,оC/min
g/(g/cm3)

Composition of the oxidic phase,
at. %

Mass/
density of themetallic phase,g/(g/cm3)

U

Zr

Composition of the metallic phase, at.
%

Fe

C

O3)

U

Zr

Fe

C

O3)

2500/10262.7/9.24

18.8/10.47

21.37 18.00 0.72

-

59.91

23.26

22.18

36.05

-

18.51

2500/30

287.3/9.29

19.3/10.89

21.762 18.97 1.36

-

57.91

28.32

25.27

23.31

-

17.30

2500/60

312.5/9.18

15.4/10.41

21.34 19.52 0.80

-

58.34

23.90

25.65

28.05

-

22.40

275.6/8.97

54.2/10.29

20.59 19.25 0.45

-

59.71

21.50

20.38

42.86

-

15.26

200.0/8.77

102.6/9.85

19.473 17.159 1.498

-

61.87

20.43

19.70

44.10

-

15.76

196.7/8.83

117/9.87

17.76 15.81 0.54

-

65.89

13.78

13.16

56.74

-

16.31

212/8.82

123/9.68

20.50 14.89 0.98 0.055 63.57

13.82

14.00

61.73

0.09

10.36

229.6/9.0

114.8/9.93

20.56 15.32 1.91 0.049 62.16

14.07

13.94

60.85

0.08

11.06

186.1/8.63

178.4/9.16

20.75 13.88 1.28 0.055 64.03

11.66

11.85

61.28

0.09

15.12

2550/10285.8/9.11

18.2/10.41

21.22 17.55 0.53

-

60.69

23.16

21.61

36.10

-

19.12

2600/10302.7/-

12.3/-

20.41 17.50 0.47

-

61.61

18.21

19.62

51.73

-

10.44

2600/30280.9/8.75

23.8/9.44

19.62 16.74 0.51

-

63.13

12.61

7.16

69.77

-

10.47

2600/120282.6/8.87

65.8/8.64

19.63 15.55 1.08 0.05 63.69 6.762)

2500/30

2500/60

2500/12026

7.372) 77.502) 0.072) 8.292)

Experiment #/initial composition1)

19/283.3 gm C-29.8+ 3.1 gm Fe, k=0.9
21/276.8 gm C-29.7+ 13.0 gm Fe, k=0.9
22/267.4 gm C-32
+28.3 gm Fe, k=0.923/300.3 gm C-32
+ 1.9 gm Fe, k=0.9
24/271.3 gm C-25.7+ 72.2 gm Fe, k=0.9
FP1/ 333.0 gm C-44.6 +14.7 gm Fe, k=1.29
FP2/ 347.2 gm C-44.6 +12.5 gm Fe, k=1.68
FP3/ 334.3 gm C-44.6 +13.9 gm Fe, k=1.28
FP6/ 331.3 gm C-45.5 +8.97 gm Fe, k=1.34
FP7/ 349.3 gm C-59.5 +13.2 gm Fe, k=1.36
FP8/ 318.0 gm C-62.4 +11.9 gm Fe, k=1.39
1)
2)
3)

Mass/
Temperature/density of theduration,oxidic phase,оg/(g/cm3)
C/min

2500/60

Mass/density of themetallic phase,g/(g/cm3)

Composition of the oxidic phase,
at. %

U

Zr

Fe

C

O3)

Composition of the metallic phase, at.
%

U

Zr

Fe

C

O3)

273.9/9.06

12.5/10.35

20.985 21.939 0.825 0.046 56.205 22.036 27.451 19.759 0.140 30.614

230.6/8.96

59.2/10.29

19.75 20.15 1.20 0.06 58.83

189.2/8.90

106.5/9.89

19.38 18.16 1.20 0.056 61.20 19.672) 24.112) 42.282) 0.122) 13.812)

293.4/9.24

8.8/10.02

22.29 24.32 0.81 0.06 52.52

25.48

28.57

6.56

0.09

39.27

172.2/8.73

171.3/9.22

19.31 17.24 1.40 0.06 61.99

12.01

15.22

56.24

0.09

16.43

56.3/10.16

21.27 16.47 1.24

-

60.47

18.74

18.74

33.31

-

27.02

46.7/10.24

23.78 14.12 0.60

-

61.08

17.53

16.22

35.88

-

27.39

58.2/10.13

21.38 16.73 0.73

-

60.62

19.59

19.16

33.83

-

25.95

47.4/10.28

22.09 16.50 0.70

-

59.90

15.39

15.81

21.71

-

34.04

27.8/10.02

19.79 14.52 0.40

-

64.98

12.71

12.90

61.74

-

8.94

22.4/9.40

19.63 14.12 0.38

-

65.52

11.10

10.84

70.08

-

4.15

21.90

24.36

31.55

0.13

22.05

2500/60

2600/152700/15

2600/15

291.7/
8.94-9.09
315.5/
9.12-9.19
291.1/
8.92-9.04
296.2/
8.93-9.03
336.2/
8.84-9.01
315.4/
8.58-8.75

Typical initial concentrations of carbon were the following: coria ~0.007wt%; iron ~0.05wt%; SS ~0.12wt% (see [11, 12, 18] for details).
the major part of the metallic phase;by subtraction from 100 %.

27

MA Tests
For all tests of MA series except MA–2 and 6 the metallic part was located in the bottom part ofthe pool. Figure 3.7 shows principally different ingot structures after the tests and illustrates
different configuration of molten pool having metallic layer at the bottom and on top.

(a)- axial chip(b) - reconstruction(c) – side view(d) – view of metal/oxide boundary

Figure 3.7. Macrostructures of ingots after MA-3 (a), MA-2 (b) and MA-6 (c, d) tests

Results of different posttests analysis of MA-6 metallic part are presented in Figure 3.8 for oneof the sections of metallic ingot. A small difference of SS MS, XRF and chemical analysis data
should be mentioned (see also average values in Table 3.6, Table 3.7). XRF data for the main
species and SS MS data for oxygen and admixtures were selected as more reliable. Non-uniform
composition affected the density values, which are in the range 8.71 - 8.78 g/cm3.
The differences in composition of the average sample produced from the oxidic part are smaller.
XRF data for the oxidic part were also used as final values.
It should be mentioned that application of different methods in the elementary analysis produces
additional difficulties for data interpretation, but significantly increases the reliability of results.
28

dn=8.71
U: 16.9-25.7
Zr: 12.1-13.0
Fe: 49.1-42.1
U=20.9-22.9
Zr=12.6-13.8
Fe=45.8-43.5

U=23.0-24.1
Zr=13.6-11.9
Fe=42.6-44.4
U=23.3
Fe=42.5

U=22.7-24.4
Zr=13.8-14.1
Fe=43.3-42.4

dn=8.73
U=22.8-23.5
Zr=13.9-12.7
Fe=43.3-43.9-SS MS, mass %;-XRF, mass %;-ChA, mass%;-bottle densimetry, g/см3

U= 21.6
Zr=13.9
Fe=42.6

dn=8.78

Figure 3.8. Section of metallic part of the ingot and compositions of samples measured bydifferent methods for MA-6 test
Table 3.6. Comparison of the data on elemental composition of MA-6 metallic ingot
obtained by different methods
Element
U
Zr
Fe
Cr
Ni
Mn
Ti
O
Admixtures

SEM/EDX

XRF

SS MS

Chemical analysis

22.69
13.15
44.04
9.51
7.94
0.38
0.29
0.73
1.27

23.31
41.57
-

mass%

20.12
10.81
45.28
11.36
7.75
-

21.19
14.04
44.13
11.17
7.30
0.45
0.17

4.68

1.56

-

Table 3.7. Comparison of the data on elemental composition of MA-6 oxidic ingot obtainedby different methods
Element

XRF

Chemical analysis

SEM/EDX

mass %
64.04
63.23
62.50
U
19.48
18.48
Zr
1.45
1.72
0.60
Fe
0.24
0.36
Cr
0.02
Ni
14.77
18.06
Admixtures and O
Resulting data of MA tests in the inert conditions are integrated in Table 3.8.

29

Table 3.8. MA tests resulting data for inert conditions

Experiment #/initialcomposition

Mass/
Temperature/ density ofthe oxidic
duration,
phase,
о
g
C/min
g/cm3

Mass/
density ofthemetallic
phase,
g
g/cm3

Composition of the oxidic phase, at. %

U

Zr

Fe
Cr
Ni

O

Impurites(Mn,
Ti)

Composition of the metallic phase, at. %

U

Zr

Fe
Cr
Ni

O

Impurites
(Mn,
Ti)

MA-1
1.75
48.72
355.1
1265.41434.2 gm C-33
0.43
21.25 14.84
61.60
15.83 10.04 15.76
1.64+ 184.9 gm SS,
8.50
9.00-9.40
0.13
8.02
k=1.45
MA-2
1.65
61.73
213.7
1535.51560.2 gm C-75
0.54
19.20
63.30
5.65
4.01
1.25
7.90/8.70- 19.16 14.98+ 188.8 gm SS,
8.61
8.90**
0.36
8.16
2500
k=1.28
MA-3
30
1.05
44.06
466.8
1214.11503.1*gm C-6.5
0.13
8.82
19.46 16.15
61.88
1.28
15.46 16.25
7.22
3.13
+ 181.6gm SS,
8.64-8.69
9.93***
0.06
5.05
k=1.14
MA-4
1.21
50.77
1181.1673.01483.7*gm C-39.7
0.42
20.01 14.65
62.49
1.15
10.42 13.33 13.63
3.03
1.83
+ 350.1 gm SS,
8.15-8.25
9.00-9.30
0.06
7.00
k=1.17
MA-6
4.13
53.27
1037.7806.4
2700
1296.3 gm C-29.1
0.91
20.95 13.84
60.05
0.08
6.05
10.47 14.52
6.47
0.81
+547.8 gm SS,
30
8.86
8.71-8.78
0.04
8.40
k=1.14*) – Including mass of fission products.
**) – Difference in densities indicates non-uniform composition of metal ingot. The highest values correspond to the ingot bottom, the lowers –to thetop.
***) This is the average density of metal. Preparation of the average sample from the metal ingot was possible for MA-3 only.
30

3.4.2

Experiments in oxidizing atmosphere

STFM Tests
The complete set of STFM-Ox results is presented in [19]. For a better understanding of theintegrated results presented in this section let us consider more detailed results of a test, e.g.
STFM-Ox No.1.
Corium C-32 (U/Zr = 1.2) was used in the test. The test was planned so that the obtained results
may be compared with the tests STFM-Fe No.4 and STFM-Fe No.10 performed earlier. At thefirst stage, which was performed in the inert atmosphere, the main test conditions including themass ratio of iron and oxidic corium mFe/mΣ ~ 0.05 were similar to the conditions of the test
STFM-Fe No.4. At the second stage, free zirconium in the corium was oxidized up to the level
corresponding to the initial corium C-70. According to the test conditions, this stage
corresponded to the test STFM-Fe No.10 with the only difference that the oxygen content in the
corium composition similar to the initial corium C-70 in the test STFM-Fe No.10 was produced
at the end of oxidation. The test temperature was increased up to 2600ºC in relation to theassumed increase of the liquidus temperature due to the corium additional oxidation.
Based on the experience of all tests of the STFM-Fe series performed earlier with the same
geometry of the cold crucible, the corium melt mass was ~ 300 g. For the same mass in theplanned test, the amount of water required for the oxidation of chosen corium C-32 up to C-70
was estimated. The estimates showed that about 10 g of water would be required for this. Figure
3.9 demonstrates the thermogram of STFM-Ox No.1.
3000

Fe

Start

2600oC

Stop

Temperature, OC

2500

2000

1500

H 2O
10 gm
~300 dropsτ1=30 min
1000

500

0
0

10

20

30

40

50

Time, min

60

70

80

90

Figure 3.9. Thermogram of Test STFM-Ox No.1

At the first stage, after 15.1 g of Fe was introduced into the corium, the exposition τ1 lasted 30
minutes. At the oxidation stage, the estimated 10 g of water in the form of ~ 300 drops weredelivered onto the melt surface during the total time of 15 minutes with the average frequency 1
drop per 3 seconds. On each drop reaching the melt surface, bright flashes associated with theoxidation of free zirconium (Figure 3.10) were noted and recorded by a digital video camera.

31

Figure 3.10. Thermal Flash on a Water Drop Interacting with the Melt Surface during thetest STFM-Ox No.1

Chosen frequency for the delivery of drops and their mass together with helium sparging
provided a fast dissipation of exothermal heat. Due to this the melt temperature did not increaseat the set electric power.
Figure 3.11 presents the averaged schedule for the variation of hydrogen volumetricconcentration in the course of oxidation in the facility chamber.
5

H2, %vol.

4

ΣH2 - 10.8 l

3

2

1

0
55

60

65

Time, min

70

75

80

Figure 3.11. H2 Concentration in the Facility Chamber in the Course of Oxidation during
the Test STFM-Ox No.1

At the initial time interval from 55 min up to ~ 61 min with constant helium sparging at the flowrate of 17 nl/min, relative accumulation of hydrogen takes place in the chamber up to the steadystate equilibrium. After the completion of the oxidation stage, hydrogen generation ends and on
continued helium sparging, hydrogen residual concentration in the chamber drops. The
measurements showed that a total amount of generated hydrogen was 10.8 liters. It follows that
only 8.68 g of the total mass (10 g) of water delivered on the melt surface in the form of drops
reacted with the molten surface. The difference of 1.32 g stayed the interaction area in the formof steams. The coefficient of water assimilation was 86.8 %. Oxygen mass reacted with the melt
was 7.71 g and caused the oxidation of 21.98 g of free Zr up to ZrO2.
Simultaneously with hydrogen, CO generation was observed in the composition of the helium
atmosphere. Figure 3.12 demonstrates the dynamics of CO concentration variation in time at thestage of oxidation.
It can be seen that the level of generated CO concentration is much lower than that of hydrogen.
After the oxidation stage completion, CO concentration in the chamber is rapidly reduced down32

to the detectability level due to the temperature decrease. The integration along the concentration
curve in Figure 3.12 with regard to the constant velocity of helium sparging (17 nl/min) and to afree chamber volume (~ 25 l) with helium operating pressure of 1.4 atm allowed to evaluate a
total amount of generated CO as ~ 8.4⋅10-3 nl.
The test material balance presented in Table 3.9 was made after the cold crucible dismantlingand the ingot cutting.
10-3

CO, vol.%

8
6
4
2
0

10

5

15

Time, minFigure 3.12. CO Concentration in the Course of Oxidation in the Facility Chamber during
the test STFM-Ox No.1
Table 3.9. Material Balance in the Test STFM-Ox No.1
Put into crucible, gCollected after test, g

C-32 corium (U/Zr=1,2) 467.7 UO2 powder

128.5

UO2 powder

169.6 Upper skull

148.9

Fe

15.1 Ingot lining slag

84.3

Σ

652.4 Oxidic part of ingot 269.2∆m

Metallic part

27.6

Σ

658.5
+6.1

After the separation of the upper and side crusts from the ingot and the ingot splitting into oxidic
and metallic parts, it was established that the metallic ingot mass was 27.6 g. The mass of theoxidic part was 269.2 g. Figure 3.13 and Figure 3.14 demonstrate the layout and photographs of
the ingot produced in the test STFM-Ox No. 1.
Data in Table 3.9 show that the mass of the molten oxidic corium was ~ 274 (269.2+27.6-15.17.71) g. The actual ratio of the introduced Fe mass to the melted corium mass, mFe/mcor, was
0.055. Due to the additional oxidation of 21.98 g of free Zr, the corium (according to Zr
oxidation degree) actually shifted from C ~ 32 up to C ~ 69. Table 3.11 contains the analysis
results for the chemical and phase compositions.
Measurements of the bottle density in the oxidic and metallic ingots resulted in the following: theoxidic corium density γ = 9.12 g/cm3, the metallic ingot density γ = 10.00 g/cm3.

33

1

50

2

14

1.Upper crust;
2.Oxidic phase;
3.Metallic phase.

3

19

3

~∅19
∅30

Figure 3.13. Layout of the Ingot Produced in the Test STFM-Ox No. 1

b)

a)

c)

a) – the ingot top view; b) – the ingot vertical chip; c) – the ingot metallic part
Figure 3.14. Photographs of the Ingot Produced in the Test STFM-Ox No. 1
Table 3.10. The Analysis Results for the Chemical and Phase Compositions of Ingots in the
Test STFM-Ox No. 1
Sample
#

C

5.27

-

(U, Zr)O2(a=0.535-0.546)++incl.α-Zr(O)+ α-Fe+incl.(U, Zr)Fe2

0.33

-

(U, Zr)O2(a=0.535-0.546)+α-Zr(O)B 1278 Metallic part 2.5 g 100.00--

B 1275U

Zr

ZrfreeMagnetic
15.8 gpart

Phase composition andparameter of lattice, nm

Fe

B 1274
1

Chemical composition, wt.%

Non-magnetic
part 130.6 g

2 B 1276 65.19 19.77 2.95

0.81 0.006 (U, Zr)O2(a=0.535-0.543)+ α-Zr(O)3 B 1277 44.88 18.54

31.85 0.043-(U, Zr)Fe2(a=0.705)++(U, Zr)O2(a=0.532-0.542)

Integrated results of all STFM-Ox tests are presented in Table 3.11. It should be mentioned that
6th and 9th tests were repeated because of technical problems. In STFM - Ox No.9 the ingot wasdestructed by the cooling water penetrated into the crucible after damage of cold crucible tubes
and posttest analysis of this test have not been done. In STFM - Ox No.6 the situation was not sodramatic, posttest analysis indicates positive results and data of the both tests were included into
Table 3.11.
34

Table 3.11. Results of Tests of the STFM-Ox Series on the Interaction of Melted Coria with Fe (SS)Test No.:
initial composition1)
1: 278.1 g C-32 (k=1.23)+10.97 g Fe
2: 267.0 g C-31 (k=1.24)+10.73 g Fe
3: 270.3 g C-37 (k=1.21)+13.11 g Fe
4: 253.0 g C-29 (k=1.21)+11.56 g Fe
5: 274.1 g C-32 (k=1.21)+24.74 g Fe62): 312.8 g C-26.6 (k=1.25)+25.22 g Fe
6: 321.6 g C-30 (k=1.18)+22.19 g Fe
7: 287.6 g C-23 (k=0.96)+12.35 g Fe
8: 382.2 g C-18 (k=0.98)+ 10.97 g Fe
9: 357.7 g C-26 (k=1.21)+20.83 g SS
1)
2)
3)

Time
τ1/τ2,min

Mass ofreactedsteam,
g

30/0

8.7

30/30

9.2

30/30

5.1

30/0

11.1

30/0

9.9

30/0

16.0

30/0

15.0

60/0

12.2

60/0

21.5

30/0

16.6

Oxide
mass/
density,
g
g/cm3

Metal
mass/
density,
g
g/cm3

269.2
9.12
260.7
9.10
251.2
9.10
258.8
8.90
263.2
8.98
323.4
8.87
331.7
8.90
285.1
8.83
396.1
8.72
365.9
9.00

27.6
10.00
25.2
9.90
36.8
10.10
17.6
8.60
44.5
8.77
28.8
8.03
25.4
7.90
25.7
9.64
16.2
8.66
28.4
9.05

Oxidic phasecomposition, at.%
U

Zr

Fe

C

Metallic phasecomposition, at.%
O

U

Zr

Fe

O

19.64 15.54 1.04 0.04 63.74 14.98 16.15 45.31 0.28 23.27
19.54 15.38 1.19 0.03 63.86 14.24 16.00 45.99 0.30 23.46
20.04 16.05 1.11 0.04 62.76 15.34 16.69 42.47 0.20 25.30
19.20 15.71 0.84 0.01 64.23 5.79 7.09

70.27 0.05 16.79

19.37 15.68 1.38 0.03 63.53 8.87 8.94

57.59 0.21 24.40

19.48 15.47 0.59 0.006 64.45 1.40 2.19

74.20 0.21 22.06

19.22 16.13 0.44 0.01 64.19 1.48 2.21

75.98 0.26 20.07

18.05 18.72 0.85 0.02 62.36 12.30 13.78 54.92 0.26 18.74
17.95 18.28 0.47 0.01 63.28 6.03 8.36

64.17 0.36 21.08

19.02 15.64 1.642) 0.018 63.69 7.30 7.37 64.353) 0.37 20.61

Typical initial concentrations of carbon were the following: coria ~0.007wt%; iron ~0.05wt%; SS ~0.12wt% (see [19] for details).
Fe-1.04; Cr-0.31; Ni-0.29 at.%Fe-50.09; Cr-5.78; Ni-8.48 ат.%35

C

MA tests
Results of MA tests with melt oxidation are presented in [20]. Figure 3.15 and Figure 3.16 shows
the photographs of the MA-7 and 9 ingots, and schematics of ingot macrostructure.

1 – side and bottom crust; 2, 4 – pores and voids; 3 – oxidic part; 5 –metallic part; 6 – above-melt crust.
Figure 3.15. MA-7 Ingot view and schematics

A

A

Figure 3.16. View and schematics of metallic part of MA-9 ingot

According to the presented macrostructures of both ingots, the pool configuration with surface
metallic layer has been achieved after corium oxidation in both tests. Results of posttest analysisof the ingots are included into Table 3.12.
It should be explained that compositions of oxidic part were obtained by XRF of average
samples. Oxygen in these samples was determined by chemical analysis of free Zr. After thatmetallic part compositions were calculated from the element-specific mass balance.
Compositions of coexisting oxidic and metal liquids after interaction of initial corium melt withstainless steel and after melt oxidation are presented in Table 3.13 and Table 3.14. We usedgeneralized data of MASCA-1 tests for calculating compositions of stratified liquids before
oxidation. The mass of stainless steel in the melt is a little lower than the added mass (200 g) due
to the fact that some metal was found in the crusts and some amount of steel was evaporated(typically 3 – 5 grams).
In MA-9 test U and Zr of metallic liquid were completely oxidized and their oxides released into
the oxidic melt. Also part of Fe, Cr, Ni - steel components got oxidized. Significant oxidation
resulted in the dramatic reduction of mass of metallic part of corium - from 500 to 127 g.

36

Table 3.12. Results of MA tests with corium oxidation
Oxidant/reacted
Test No.:
initial composition mass,
g

Oxide
mass/
density,
g
g/cm3

Metal
mass/
density,
g
g/cm3

MA-7:
1753.5g C-32
(k=1.17)+172.7 g SS

Air
46.4

1753.88.59-8.61

215.4
8.39

MA-9:
1758.6g(k=1.17)+176.5 g SS

C-27 Steam
140.9

1925.3
8.78

Oxidic phase composition, at.%
U

Zr

Fe

Cr

Ni

Mn

19.27 16.11 1.36

0.90

0.28

126.5
16.78 14.30 1.61
7.70-7.93

1.41

0.16

37

Metallic phase composition, at.%
O

U

Cr

Ni

Mn

O

0.04 62.03 6.97

9.53 63.64 9.02

9.07

0.13

1.65

0.07 65.67

076.46 6.45 12.04

0

5.06

0

Zr

Fe

Table 3.13. Partitioning of elements in MA-7 test
Initialcomoposition

Metal

1753.47172.73

U, at. fraction

0.54

-

Zr, at. fraction

0.46

Fe, at. at. fraction

-

0.73

Cr, at. fraction

-

0.16

Ni, at. fraction-

Mn, at. fraction
O, at. fraction

Element

Mass, g

Molten corium before
oxidation
Oxidic liquid

Metallic
liquid1426.2

500.0

0.54

0.19

0.45

0.18

0.00

0.46

0.00

0.10

0.11

0.00

-

0.00

1.37

-

+

-

Ar

23980C→

Ingot after melt oxidation andcooling
Oxidic part

Metallic part

1753.8

215.4

0.51

0.07

0.42

0.10

0.04

0.65

0.02

0.09

0.07

0.01

0.09

0.00

0.00

0.00

0.00

1.49

0.31

1.63

0.02

38

Air

26450C→Table 3.14. Partitioning of elements in MA-9 test
Initialcompositi
on

Metal

1758.60

176.50U, at. fraction

0.54

-

Zr, at. fraction

0.46

Fe, at. at. fraction

-

0.71

Cr, at. fraction

-

0.18

Ni, at. fraction-

Mn, at. fraction
O, at. fraction

Element

Mass, g

Molten corium before
oxidation
Oxidic liquid

Metallic
liquid1435.1

503.0

0.54

0.19

0.45

0.18

0.00

0.46

0.01

0.10

0.10

0.00

-

0.01

1.33

-

+

-

Ar

23760C→

Ingot after melt oxidation andcooling
Oxidic part

Metallic part

1925.30

126.50

0.49

-

0.42

-

0.05

0.81

0.04

0.06

0.07

0.00

0.13

0.00

0.00

0.00

0.00

1.42

0.33

1.91

0.05

39

Steam

25130C→There are distinct peculiarities of melt oxidation behavior in MA-7 and MA-9 due to different
oxidants, i.e. air and steam respectively.
The posttest analysis of MA-7 corium samples has shown that in the course of interaction
between the air and melt the reaction of zirconium with oxygen:
Zr + O 2 = ZrO 2 ,(3-1)

is accompanied by a reaction with nitrogen to form zirconium nitride and/or oxinitride:
2Zr + N 2 = 2 ZrN(3-2)

2Zr + N 2 + O 2 = 2ZrNO(3-3)

Refractory nitrides produced crust on the melt surface, which formed at the end of oxidation
phase of the test. The presence of Zr nitride (oxinitride) in the oxidic part is confirmed by the
XRF results (Figure 3.17).

Figure 3.17. Diffractograms of MA-7 samples: a) above-melt crust; b) crust of metallic
ingot; c) central part of oxidic ingot

Mass rates of reacted oxygen and nitrogen calculated from gas mass balance and integral
masses of the reacted gases are presented in Figure 3.18.

40

0.16

35
0

3

0

2

3

2

30

0.12

Absorbed mass (mα ), g

Mass rate of absorption (Gα,ab), g/s

0.14

0.1
N2

0.08

O2

0.06
0.04

25

20

15

10
N2

5

0.02
0

O2

0
3400

3500

3600

3700 3800
Time (t), s

3900

4000

4100

3400

3500

3600

3700 3800
Time (t), s

3900

4000

(a)

4100

b)

Figure 3.18. Evolution of flow rates of reacted oxygen and nitrogen (a) and integral
amount of the reacted gases (b)These plots show very well the periods of accelerated absorption rate during the absence of
crust on the melt surface (from the 0-th to 3-rd time mark) and reduced gas absorption when
the slag crust appears on the surface (from the 3-rd to 2-nd time mark). Figure 3.18b also
shows subsequent reduction of absorption after the heating is disconnected (after time mark2). Masses of absorbed components are 13.5 and 32.9 g of oxygen and nitrogen, respectively.
Note that the total mass of absorbed components (46.4 g) agrees rather well with the mass
imbalance (43.0 g) of solid components (Table 3.1).
In order to understand at which stage (kinetic, diffusion or convective) the air – meltinteraction actually takes place we can construct the plot of ratio between molar absorption
rates of oxygen and nitrogen. It is shown in Figure 3.19 (curve 4). The same figure shows theratio of molar concentrations of oxygen and nitrogen in the air supplied into the furnace(straight line 5) and in the furnace gas (curve 6). It can be seen from Figure 3.19, that during
first 200 s of the process after the oxidation start (excluding the very first 20…30 s, for which
concentrations were determined by extrapolation) the ratio of O2 and N2 absorption rates
actually coincides with the ratio of their molar concentrations in the supplied air (≈ 0.268),
but not in the furnace gas. From this also follows that melt interaction with both components
takes place at the transport, to be more exact – convective stage, when the rate of the process
is limited (controlled) by the mechanism of convective transport of O and N to the melt
surface, but not by Zr in the melt. It is also confirmed by the observation that the drop of Zr
concentration in the melt in the course of interaction is not accompanied by the corresponding
decrease of absorption rate. This mechanism is typical of the time interval without crust on
the melt surface.
After solid crusts containing zirconium nitrides started to form on the melt surface theinteraction rate had a considerable decrease (Figure 3.18), and the interaction shifted to alarger absorption of oxygen in comparison with nitrogen.
In MA-9 test during the melt oxidation by steam its reaction with zirconium:
Zr + 2H2O = ZrO2 + 2H2(3-4)
was accompanied by the redox reactions with other components of metallic melt:41

2Fe + 3H2O = Fe2O3 + 3H2,
2Cr + 3H2O = Cr2O3 + 3H2,
Ni + H2O = NiO + H2,
1.2

(3-5)
(3-6)
(3-7)
0

3

2
4
5
6

[ (GO2,ab/MO2) / (GN2,ab/MN2) ]

1

0.8

0.6

0.4

0.2

0
3400

3500

3600

3700 3800
Time (t), s

3900

4000

4100

4- ratio of oxygen and nitrogen absorption rates, 5 – ratio of molar concentrations of oxygenand nitrogen in air, 6 – ratio of molar concentrations of oxygen and nitrogen in off-gas.
Figure 3.19. Comparison of gas absorption rates with gas composition

Only reactions with components having a considerable content are taken into account here.
Kinetic data for this test were determined separately from measured mass balance ofsteam/condensate and from hydrogen content in off-gas. Steam absorption flow rate in steammetal reactions calculated from water mass balance was practically constant - 0.12 (±10%)g/s. The flow rate error basically results from the steam generator flow-rate oscillations andcondensate flow-rate pulses caused by pressure pulses in the condenser. Comparison of
integral water absorption determined from water mass balance with the value calculated fromingot post test analyses (i.e. from the masses of oxidized Zr, Fe, Cr, Ni) indicates ≈5 %difference. The value determined from hydrogen release was 0.11 (±20%) g/s, but this is aless accurate value.
All mentioned steam – metal reactions belong to the transport stage, i.e. when the interaction
rate is limited not by the kinetics of chemical reaction itself and not by the supply of metals in
the melt, the concentration of which reduces during the interaction, but by steam supply tothe melt surface. That is why melt oxidation rate is not influenced by Zr concentration in themelt for MA test conditions.
It should be noted that differently from MA-7 in MA-9 the mass flow of oxidant directed tothe melt surface was influenced by the counter-current flow of hydrogen produced during
steam – metal reactions.
3.4.3

Experiments on the effect of control rod material

STFM tests with boron carbide
In case of a severe accident control rods fabricated with a use of boron carbide may be one of
the basic boron sources for the corium melt in the reactor vessel. A total mass of B4C, as it
follows from the reference [22], may range for different reactor types from 270 kg (VVER-

42

1000) up to 900 kg (BWR). This corresponds to 0.25 and 0.5 mass% of the core mass foreach case. Apart from the control rods containing boron carbide, boric acid H3BO3 dissolvedin the primary circuit coolant can be considered as a potential source of boron in PWRs’
severe accident corium melts. Some data on the quantities of boron that could remain in thereactor pressure vessel prior to core melting down in case of severe accident were presented
in [23, 24]. Earlier [14], on studying carbon impact on the behaviour of the suboxidized C-32
corium, it was shown that the melting and further freezing resulted in the stratification of that
into two layers at the concentration of carbon in the initial corium higher than 0.2 mass%.
The nature of the stratification process is probably associated with the peculiar features ofphase diagrams for the system Zr(O) - UO2 [25, 26], with the lack of noticeable carbon
dissolubility in oxides UO2 and ZrO2 and in their solid solutions as well as with a high carbon
affinity of Zr. It is known that zirconium has also a high affinity with other interstitial
elements (B, N). This is reflected in the comparison of melting temperatures of theappropriate stoichiometric compounds. The melting temperatures for ZrC, ZrB2, ZrN are
3420, 3245, and 3250 °C [27], respectively.
Therefore, the existence of similar stratification phenomena in the corium may be also
expected in the presence of boron. On studying the impact of boron carbide, it is of anadditional interest to determine the opportunity for carbon and boron combined impact on thestratification processes in the suboxidized corium and interaction with the steel as well as thepartitioning of boron and carbon between the oxide and metallic melt phases of the core. In
addition to these, in two experiments of the series the influence of boron carbide on thepartitioning of FP simulants between the oxidic and metallic melts was investigated.
In the experiments [28, 29] different coria were used with the uranium to zirconium ratio of
U/Zr = 1.2 and U/Zr = 0.9 and zirconium oxidation degree 32%. One test was conducted with
the C–70 corium and ratio U/Zr = 1.2. The results of theses analyses are presented in Table
3.15.
Table 3.15. Initial coria analysis compositions
Corium
C-32
U/Zr=1.2
C-32+0.5%B4C
U/Zr=1.2
C-32
U/Zr=0.9
C-32+0.5%B4C
U/Zr=0.9
C-70
U/Zr=1.2
C-70+0.5%B4C
U/Zr=1.2

SampleChemical composition, wt. %BU
Zr Zrfree C

B1194 66.52 21.92 14.80 0.008
B1195 66.33 21.76-

0.12 0.37

B1196 63.12 25.74 17.83 0.007
B1197 62.81 25.51

-

-

-

0.11 0.39

B1213 64.98 21.16 6.30 0.005
B1214 64.55 20.96--

0.08 0.38

Phase composition and lattice
parameter, nm(U, Zr)O2(a=0.5423) ++ α-Zr(O)(U, Zr)O2(a=0.5431)++α-Zr(O)+ZrB2+Zr(C,O)(U, Zr)O2(a=0.5318)++α-Zr(O)(U, Zr)O2(a=0.5429)++α-Zr(O)+ZrB2+Zr(C,O)(U, Zr)O2(a=0.5350)++α-Zr(O)(U, Zr)O2(a=0.5490)++α-Zr(O)+ZrB2+Zr(C,O)

For Experiments # 6 and # 9 [29], in addition to 0.5 wt. % of boron carbide FP simulants
including Mo, Ru, SrO, BaO, CeO2, La2O3 were introduced in the form of fine powder into
the fabricated powder of C-32 corium to the extent of 0.5 wt. % with respect to the metalelement. These mixtures were sintered in tungsten containers in a protective atmosphere atsome 2150 ºC for 15 minutes. The chemical composition of the iron used (sample B 859)
featured the following impurities: Mn – 0.21; Si – 0.15; C – 0.05 wt. %.

43

At the stage of preparation of the C-32 corium samples with uniformly partitioned carbide(0.5 mass%), the initial pressed mixtures of powders (UO2 - ZrO2 – Zr - B4C) were sinteredunder the temperature 1830 °C (15 minutes), 2150 °C (10 minutes), and 2400 °C (5 minutes).
It was determined by the further X-ray analysis that in all cases the phase composition ofsynthesized samples was the same one, that is, a solid solution based on UO2, α - Zr(O),
Zr(CO), and ZrB2. It was also determined that the temperature of the beginning of the C-32
corium intensive melting was increased up to ~ 2400 °C due to the addition of 0.5 mass% of
B4C. It is evident that this was a result of the reduction of metal Zr fraction due to thegeneration of more refractory zirconium boride ZrB2 and zirconium oxycarbide Zr(CO). The
system Zr(O) - UO2 - B4C practically was not studied earlier experimentally. The study is
known of B4C - Zr interaction [30] that has shown that the reaction starts under thetemperature higher than 1250 °C and results in the generation of ZrB2 and ZrC in theinteraction area. A fast acceleration in the interaction kinetics takes place under thetemperatures higher than 1650 °C resulting in zirconium meltback. The interaction ofoxidized α - Zr(O) with boron carbide was not investigated earlier [31]. Therefore, theinvestigation of boron carbide behaviour contacting with the suboxidized corium at thetemperature range up to the complete melting of the whole system must precede theperformance of tests studying B4C impact on the stratification. The volatile boron oxide
generation was not expected taking into account a low chemical potential of oxygen in thechosen C-32 corium. The initial pressed billets were sintered in three different modes within
the temperature range from 1830 °C up to 2400 °C. However, the phase compositions of
produced samples were found to be the same in all cases. Zirconium oxycarbide and diboride
appeared in the composition of the produced samples. Boron carbide did not remain as anindependent phase. This shows that B4C interaction with α - Zr(O) as for Zr starts under thetemperature lower than 1800 °C. The obtained result is in agreement with the data of ref.
[30].
Experimental research into the influence of B4C addition were carried out on the STF (Small
Tungsten Facility), all in high-purity argon (impurity content was: O2 – 2 ⋅ 10-4, CO2 –2 ⋅ 10-5, H2O - 3 ⋅ 10-4 vol. %), at a slight overpressure of some 0,2 atm [28, 29]. A 250 kW
installed capacity 66 kHz high-frequency generator was used as a power source. The cold
crucible technology was used in all of the experiments.
It was observed in all of the experiments of the reported series that the Fe-based metallic
phase and the oxidic corium are immiscible in liquids. The interaction leads to a saturation of
the molten Fe with uranium, zirconium, boron, carbon and oxygen. The metallic ingots
obtained in Experiments ## 3-6, 8, 9 were found in the lower part of the cold crucible and thisprovides the evidence of that the specific weights of the molten metallic phases were higher
than those of the molten oxidic coria. For the metallic and oxidic ingots at room temperature,a similar density ratio was observed. The metallic phase was found in the upper part of theingot obtained in Experiment # 7 with C-70 corium. Apparently, the specific weight of themetallic melt was slightly higher than that of the corium melt in this experiment where theinversion of the specific weight ratio took place in the course of cooling down andcrystallization. The mean density of the metallic ingot (8.41 gm/cm3) was found to be lower
than that of the oxidic ingot (9.00 gm/cm3).
A pattern of concentartion of boron and carbon predominantly in the metallic melt hasfeatured the additional post-cristallisation ZrB2 and Zr(C, O) phases. Table 3.16 comprises
the results on the interaction of molten coria containing 0,5 wt. % of B4C with iron.

44

Table 3.16. Experimental results on the interaction of molten coria containing B4C with ironExperiment #/corium

3/240 gm C-32 +0.5%B4C +5.0 gm Fe, k=1.24/240.4 gm C-32 +0.5%B4C +15.1 gm Fe, k=1.25/251.4 gm C-32 +0.5%B4C +30 gm Fe, k=1.2
7/307.8 gm C-70 +0.5%B4C +5.1 gm Fe, k=1.28/235.2 gm C-32 +0.5%B4C+15 gm Fe, k=0.9
6/308.6 gm C32+0.5%B4C +FP+
15.2 gm Fe, k=1.2
9/302.3 gm C-32 +0.5%B4C +FP+
15.2 gm Fe, k=0.9

Mass/oxidicphase
density,g/ g/cm3

Mass/metallic
phase
density,gm/
gm/cm32550/30

197.2/9.37

47.8/9.43

22.37 14.20 0.13 0.89 0.49 61.92 20.68 27.51 12.09 10.84 2.17 26.69

2550/30196.8/9.30

58.7/9.47

21.59 15.99 0.52 0.84 0.28 60.77 16.88 21.05 26.08 9.15

1.65 25.19

2550/30

196.5/9.28 84.9/10.07 21.32 16.28 1.62 0.56 0.16 60.06 16.38 17.57 49.08 4.91

1.81 10.25

2600/30

295.8/9.00

17.1/8.41

19.06 15.39 0.40 0.58 0.01 64.56 16.77 29.51 22.64 11.09 0.28 10.71

2550/30

194.6/9.06

55.6/9.43

19.52 18.21 2.09 1.02 0.45 58.70 17.26 26.35 24.66 6.57

1.60 23.56

2550/60

252.5/9.02 71.3/10.20 22.70 16.53 0.89 0.74 0.14 58.05 15.82 20.08 24.40 5.75

1.36 30.00

2550/60

242.2/9.06 75.3/10.05 20.02 18.00 0.92 0.70 0.13 59.27 17.18 24.27 24.72 6.54

1.40 23.10

Temperature/
duration,о

C/min*) – by subtraction from 100 %.

45

Oxidic phase composition, at. %Metallic phase composition, at. %

U

U

Zr

Fe

B

C

O*

Zr

Fe

B

C

O*)

STFM test with Ag – In – Cd Alloy
The control rods with Ag, In, Cd (SIC) absorbing material are used in many western PWR.
To study material impact on interactions and distribution of main components (U, Zr, O,
Fe(SS)) the special experimental matrix have been prepared. However it was desided to
conduct the only STFM-SIC scoping test. The objectives of this test were to check technicalfeasibility of the experiments and to identify phenomenology. It was agreed that member
countries concerned by SIC effect should propose continuation of tests.
The scoping test was carried out in the STFM facility earlier described in [11, 12]. The test
conditions were the following:
U/Zr = 1.2
mSS/mcor = 0.05
mAg, In/mSS = 0.16
mcor = 485.9 g
mSS = 30.2 g;metal – SS;
T = 2500°C;
Exposition time 30min.
After test section disassembling and dividing of the ingot into oxide and metallic parts it was
found out that the ingot contained two metallic parts (Figure 3.20) with masses of 28 g (top)and 52 g (bottom).

Figure 3.20 Schematics of the Ingot of STFM-SIC Scoping Test, and Photo-Pictures of
the Ingot Metallic Parts

Table 3.17 lists the results of the ingot chemical and spectral analysis.
Data of the chemical analysis indicate:
1. Ag, In are mainly located in the top skull and are practically absent in the ingot oxide
part. This is probably due to their evaporation and condensation on the surfaces withlower temperatures.
2. Content of U, Zr, and Fe in the metallic parts of the melt in this test is the same as
STFMFe-4 experiment carried out at same conditions [11, 12], i.e. Ag, In do not
influence the content of U, Zr, and Fe in metallic bodies.

46

Table 3.17. Results of the Ingot Composition Analyses

Sample code

Element composition, mass%
method

Zr

Fe

Ni

In

Ti

-

0.06

Chem.
53.28 24.87 14.11 3.10 2.50 B1135
top metallic bodySpectral 54.50 24.20 14.70 3.06 2.64 0.100 0.057

-

-

-

0.19

-

-

Spectral 51.90 20.90 19.20 3.98 3.40 0.015 0.059

-

0.15

Chem.

-

-

B1136
corium*)Chem.

50.67 21.22 18.81 4.20 3.55 -66.82 18.40 0.43

Spectral 67.10 19.80 0.71*)

0.17 0.11 --Cd

1.28 0.68 0.410 0.200

Spectral 61.88 16.70 4.03

1.38 0.63 -

Ag

-

B1134
bottom metallic
body

62.30 16.47 4.20

Cr

-

B1137
top skull

Chem.

U

-

-

0.16 0.13 0.003 <0.003 -0.05– sample is averaged within the total corium volume.
In STFMFe test series [12] when no Ag or In were added to the melt formation of one
metallic body was observed in every experiment. In STFM-SIC Scoping Test when Ag andIn were added formation of two metallic bodies was observed. It is possible to assume that
Ag and In increase the surface energy of oxide phase, when being added to U-Zr-Fe, andaccelerate separation of the almost spherical metallic body. It is known [32] that Ag does not
mix with U and Zr. Due to this reason Ag that was on the surface of the melt metallic parts
also contributed to the quick formation of the spherical metallic body. This takes place prior
to saturation of the whole iron layer with uranium and zirconium. After separation of the firstmetallic body further saturation of the remaining top iron layer with uranium and zirconium,
as well as formation of the second metallic body take place.
Ag and In content in the skull is higher than in the metallic bodies and corium. This isconnected with higher pressure of vapours of these elements at the temperature of 2500°С,
upward migration of Ag and in vapours, and condensation of them on the surfaces with lesstemperatures (skull). This is probably the reason why the content of these elements is higher
in the top metallic body and not in the bottom one.
On recommendation of PRG-2 и MB-2 (The 2-nd MASCA 2 PRG Meeting Aix-enProvence, France, June 8-9, 2004) the experiment on melting the metallic samples taken fromdifferent parts of the formed metallic parts has been carried out.
The test was performed basing on the technique for the spread temperature determination
described in detail in report MP-TR-12 [33]. The samples of metallic parts have been laid on
substrates from UO2 (Figure 3.21).

47

Figure 3.21. Initial Loading of Metallic Samples

The technique consist in supervision through an optical micropyrometer behind a condition ofsamples during a warming up before their full fusion. The temperature raised with constantrate 8 oC/s. The maximal temperature of a warming up has made 1740 oC. It is as a result
received (Figure 3.22), that two lower samples were completely alloyed in single ingot, andothers two have kept visible border of partition.
Single ingot of twolower metallic samples

Visible border between
lower
and
upper
metallic samples

Figure 3.22. Ingots of Melted STFM-SIC Samples

Chemical composition of the upper and lower metallic parts on the basic components areclose (Table 3.17). Visible essential difference consists in concentration of Ag: The upper
metallic part contains it in 10 times more, than lower. It is possible to note also, that total
concentration of the elements having the greatest affinity to oxygen, in chemically less active
upper metallic part on 5 mass% it is more, than in more active lower metallic part.
It is necessary to note, that results of one performed test have not given a full picture of thenature and the mechanism of formation of two spatially divided parts of a metal phase.
However, the PRG considered that the priority for such a determination was not very high.
PRG4 and MB4 recommended canceling the planned SIC tests in Matrix 2.2 due to STFMSIC scoping test result that SIC did not affect the total amount of metal phase.

48

3.5

Discussion of Findings

3.5.1 Partitioning of Main Species

In a series of small and middle scale tests with iron and stainless steel it was clearly
demonstrated that in contact with molten steel some amount of metallic zirconium and uranium
is transferred to metallic phase from suboxidized corium. This process leads to the increase of
metallic phase mass and its density and sometimes to the change of layer configuration. Uraniumtransport to the metallic phase is closely related to the existence of free zirconium in the initialsuboxidized corium. The amounts of U and Zr atoms transferred to the metallic phase depend
upon metal mass and corium oxidation degree. In Figure 3.23 and Figure 3.24 the results for U,
Zr, Fe concentrations in a metallic phase obtained for experiments with C32 and C70 corium arepresented as a function of relative Fe (SS) content.
70

60
U
Zr
Fe (Cr,Ni)

Concentration, wt.%50

40

Cn0=30 - 40
U/Zr =1.0 - 1.3

30

20
#16

#17

#18

Ма-6

10

0
0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Mst/(Mst+Mcor)

(a)

70
#23

60

Concentration, wt.%

U
Zr
Fe (Cr,Ni)

#22

#19

50

Cn0= 30 - 40
U/Zr = 0.90 - 0.98

#21

Ma-5b

40

#24

Ma-5

30

20

10

0
0

0.05

0.1

0.15
Mst/(Mst+Mcor)

0.2

0.25

(b)

Figure 3.23. U, Zr and Fe (Steel) Concentrations in a metallic phase for Experiments withC≈32 Initial Corium having U/Zr≈1.2 (a) and U/Zr≈0.9 (b)

49

90
80

Concentration, wt.%

70
60
U
Zr
Fe (Cr,Ni)

Cn0 = 65 -75
U/Zr= 1.0 - 1.3

50
40
30

#20

20
10
0
0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Mst/(Mst+Mcor)

Figure 3.24. U, Zr and Fe (Steel) Concentrations in a metallic phase for Experiments withC≈70 Initial Corium

In these figures we’ve noted the points obtained in MASCA-2 program by indication of the tests
numbers, e.g. captions “#17” and “Ma-6” point to the results of STFMFe#17 and MA-6 tests.
The results demonstrate reasonable consistency between different experimental series.
Steel concentration increases with the increase of the relative steel content in corium while theconcentrations of uranium and zirconium decrease with the increase of the steel (iron) content
during the tests.
To find the dependence of concentration of major component upon the corium oxidation degree
the data were grouped by the amount of steel (iron) added to the corium melt. The following
groups were composed:• Steel (iron) content about 2 wt. % (Experiments STFMFe#2, #3, #5, #6, #7, and #8).
• Steel (iron) content about 5 wt. % (Experiments STFMFe#4, FP#1, FP#2, FP#3, FP#6, FP#7,
and FP#8).
• Steel (iron) content about 10 wt. % (Experiments Fe#9, Fe#15, MA-2 and MA-3).
The zirconium oxidation degree varied in these tests from 32 % to 100 %. Data in a form ofgraphs are presented in Figure 3.25.
30

70
2% Fe
5% Fe
10% Fe

60

2% Fe
5% Fe
10% Fe25

U/Zr= 1.0 -1.3

U/Zr =1.0 - 1.3

50

40

Ma-3

Zr, wt.%U, wt.%

20

30

Ma-3
15

#20

#20

10
20
5

10

0

0
20

30

40

50

60

70

80

90

100

20

30

40

50

60

70

80

Initial Zr oxidation degree

Initial Zr oxidation degree

(a)

(b)

Figure 3.25. U (a) and Zr (b) Concentrations in a Metallic Phase as a Function of Initial
Zirconium Oxidation Degree

50

90

100

Concentrations of U and Zr in a metallic phase decrease with the increase of zirconium oxidation
degree. There is also some dependence upon metal content, however this dependence is rather
weak.
Figure 3.26 presents concentration of steel components in the oxidic phase. A very small amountof iron goes into the oxidic phase (typically about or less than 1 % wt.). Experiments conductedat the STFM facility with small amount of iron (about 2 wt. %) demonstrate regular dependence
upon oxygen content in the melt (black squares). Note that final oxidation index was calculatedusing Zr-free content in the melt. One may also conclude from this figure that iron concentration
is increasing at the increase of the mass of metal added to the melt.
2% Fe
5% Fe
10% Fe

1.40
1.20

#20

Fe, wt.%

1.00
0.80
0.60
0.40
0.20
0.00
30

40

50

60

70

80

90

100

Final oxidation Zr degree

Figure 3.26. Iron Concentration in the Oxidic Phase at Different Final Zr Oxidation
Degrees

It would be interesting to compare MASCA results with equilibrium concentrations of the main
species calculated by a thermodynamic code with supporting database, though the tests were notanalytical and were not directly focused on phase diagram study. We have made athermodynamical modeling of STF and MA tests applying GEMENY 2 code and NUCLEA06_1 database. Experimentally determined integral compositions of molten components wereused as the input data for modeling of every test. It was found in the supporting GEMENY
calculations, that mass fractions and compositions of the liquids in miscibility gap are notsensitive to superheating of melt above the liquidus temperatures for the MASCA compositions
with Fe (SS). This has been experimentally confirmed in MASCA-1 tests [10]. Therefore thecalculations were done for the liquidus temperature of the test specific composition. Isobaric
calculations were done at 1 bar of pure argon. Strategy of miscibility gap was chosen in themenu options. Results are presented in Figure 3.27 and in Table 3.18 in comparison with theexperimental data.
Qualitative agreement and constant components in the quantitative deviations of the calculated
and experimental data are evident form the figures. In the metallic melt calculated U
concentrations are systematically higher than those measured, while Zr concentrations are lower.
The apparently non-systematic deviation in Fe concentration can be explained by a notcompletely adequate interpretation of MA tests with stainless steel. The alloying element
concentrations(Cr, Ni, Mn et al) were simply included into iron concentration. That is why some
calculated points are lower than experimental.
Currently it is difficult to identify the source of deviation; it could be explained by amethodological (experimental) error and by a modeling inadequacy.

51

Better agreement of MA test results with calculated data could be mentioned from Table 3.18 incomparison with STFM results, especially for oxygen concentrations.
70

Cn0=30 - 40
U/Zr =1.0 - 1.3

60

ExperimentU
Zr
Fe (Cr,Ni)
GEMENY2 + NUCLEA-06_1

Concentration, wt.%

50

40

30

20
#16

#17

#18

Ма-6

10

0
0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

Mst/(Mst+Mcor)(a)80
Cn0=30-40
U/Zr = 0.90-0.98

70
#23

60
Concentration, wt.%

#19

U
Zr
Fe (Cr,Ni)
GEMENY 2 + NUCLEA-06_1
Experiment

321

50

#22
Ma-5b

40

#24

Ma-5

30
20
10
0
0

0.05

0.1

0.15

0.2

0.25

Mst/(Mst+Mcor)(b)
Figure 3.27 Comparison of experimental and calculated concentrations of U, Zr, and Fe in
the final metallic melt for U/Zr~1.2 (a) and for U/Zr~0.9 (b)52Table 3.18 Concentrations of the main species in metallic melt
Experiment
Test

Cn

0

U/Zr

U

Zr

Fe

Calculation
O

U

Zr

Fe

O

wt.%
2

32.6

1.18

56.10

20.5

20.40

3.00

62.02

17.39

20.26

0.33

3

32.6

1.15

62.90

21.5

12.15

3.45

64.13

18.26

17.25

0.36

4

33.4

1.07

53.22

19.34

24.90

2.54

59.16

17.46

23.06

0.32

5

32.6

1.09

57.15

23.51

15.74

3.6

65.03

19.92

14.64

0.41

9

35.6

1.10

51.87

19.17

26.27

2.69

52.37

15.40

31.95

0.28

14

35.6

1.10

41.46

15.18

40.06

3.3

41.38

11.58

46.79

0.25

15

35.6

1.3

44.05

14.48

40.82

0.75

49.06

16.55

33.87

0.51

16

30.7

1.25

40.21

15.61

42.14

2.03

42.35

11.11

46.26

0.25

17

30.9

1.25

40.86

15.51

41.46

2.16

43.64

11.53

44.55

0.25

18

29.9

1.27

36.9

14.37

45.5

3.22

35.88

8.93

54.93

0.23

Mа-1

33.0

1.45

43.20

10.5

46.00

0.3

43.15

12.98

43.46

0.41

Mа-3

36.5

1.14

41.70

16.8

36.45

1.31

44.42

15.17

40.03

0.38

Mа-4

39.7

1.17

31.26

15.32

49.84

0.61

26.30

9.81

63.48

0.41

20

66.6 1.23

23.86

9.97

64.19

1.97

30.87

7.26

61.61

0.22

19

29.8 0.95

56.14

26.8

11.81

5.24

65.98

22.91

10.52

0.56

21

29.7 0.96

54.58

23.27

18.45

3.69

58.23

19.75

21.53

0.42

22

19.8 0.98

49.47

23.24

24.95

2.33

53.94

17.58

28.14

0.30

23

15.2 0.92

62.74

26.96

3.79

6.5

68.17

26.66

4.11

1.00

24

25.7 0.97

37.36

18.14

41.05

3.44

38.93

13.12

47.66

0.27

Mа-5

31.5 0.89

37.1

25.4

34.3

2.70

47.09

19.68 32.77

0.46

Mа5b

30.5 0.9

42.9

27.5

27.3

2.00

52.47

20.79 26.29

0.45

3.5.2 Change of Corium Oxidation Degree

After interaction, the oxidic phase oxidation degree is increased. Increasing of steel to corium
ratio leads to the increase of oxidic phase oxidation degree approaching almost complete
oxidation for large amounts of steel (iron) content in the test. Two methods were used foranalysis. The first was based on the measured values for free zirconium content in the corium,
and the second utilized the balance method, when oxygen content in the corium was calculated.
In Figure 3.28 the final oxidation degree for the tests with the C-32 corium with the U/Zr ratio of
about 1.2 are shown. The list of tests shown in the figure includes STFM 2-5, 9, 14,15, MA3,MA4 of MASCA-1 and indicated tests of MASCA-2. Test MA-1 is excluded as U/Zr ratio was
1.5. Both sets of data demonstrated reasonable agreement except results of MA-6 test. In [13] it
was supposed, that chemical equilibrium was not reached in the test due to incomplete expositionof melt having the highest mass of steel.
53

For C 60 – C 75 corium the data are shown in Figure 3.29. The following tests were used for thisplot: STFMFe#7 and #10, STFMFP#7 and #8, MA-2 test. Oxidation degrees obtained from themeasurements of free Zr were supplemented by data obtained from the mass balance. Again, theuncertainties in the oxidation degree for STFMFP series were large enough, however the rangeof Mst/(Mst + Mcor) variation sufficiently large.
100
ZrfreeMass balance

90

Final Zr oxidation degree

Cn0 = 30- 40

#18

80
#16
70

#17

Ма-6

60

50

40

30
0

5

10

15

20

25

30

35

Mst/(Mst+Mcor)

Figure 3.28. Increase of the Corium Oxidation Degree after the Interaction for C-32
Corium105
Mass balanceZr free

Cn0= 60-75

Final Zr oxidation degree

95

85

#20

75

65

55

#6

45
0

2

4

6

8

10

12

14

Mst/(Mst+Mcor)

Figure 3.29. Increase of the Corium Oxidation Degree after the Interaction for C-60 – C-75
Corium
3.5.3 Density of Oxidic and Metallic Phases

The density measurements were performed at room temperature for samples taken from themetallic and oxidic phases. The results of measurements are presented in Figure 3.30 and Figure
3.31.

54

11
10.5
U/Zr=1.0 - 1.3

Density, g/cm3

10

17
16

9.5
9

18
Ма-6
20

8.5

Masca
RCW
STFM
Correlation (3-8)
Model of ideal mixing of molar volumes

8
7.5
0.0

0.2

0.4

0.6

0.8

1.0

1.2

U/Fe, at.unit
11
U/Zr= 0.90 - 0.98
#19

#21
10
3

Density, g/cm

#23

STFM
Masca

#22
Ma-5b
Ma-5
#24

Correlation (3-8)
Model of ideal mixing of molar volumes

9

8

7
0

0.5

1

1.5

2

2.5

3

3.5

4

U/Fe, at.units

Figure 3.30. Metallic phase Density at Ambient Temperature as a Function of U/Steel(Fe)Atomic Ratio
9.5

STFM
MA
RCW

Density, g/cm

3

9.0

8.5

8.0
11.5

12.0

12.5

13.0

13.5

14.0

14.5

15.0

15.5

16.0

Oxygen concentration, wt.%

Figure 3.31. Oxidic phase Density at Ambient Temperature as a Function of OxygenContent

The most relevant factor for the metallic phase density is the ratio of U to steel or iron, while
oxidic phase density depends mainly upon corium oxidation degree. The results of STFM test
#23 seems to be out from other data because of very small iron thus the uranium to steel ratio
may not be a governing parameter for this case.
55

Empirical correlation for calculation of metal alloy density at ambient temperature as a functionof the alloy composition has the following form:
1
(3-8)
ρ=
5/6
⎡⎛ U ⎞ 6 / 5 ⎛ Zr ⎞ 6 / 5 ⎛ Fe ⎞ 6 / 5 ⎤⎟⎟ ⎥⎟⎟ + ⎜⎜⎟⎟ + ⎜⎜⎢⎜⎜⎢⎣⎝ ρU ⎠⎝ ρ Zr ⎠⎝ ρ Fe ⎠ ⎥⎦where U, Zr, Fe, ρU = 19.04 g / cm 3 , ρ Zr = 6.45g / cm 3 , and ρ Fe = 7.87 g / cm 3 [34] are atomic
fractions of elements and densities of the metals at ambient temperature. In the range of U
atomic fraction from 0.05 to 0.50, Zr atomic fraction from 0.05 to 0.50 (U/Zr = 0.90 - 0.98 and
1.0 – 1.3) and Fe atomic fraction from 0.9 to 0 the deviation of calculated results from theexperimental data is not higher than 5 % (Figure 3.30). Dramatic difference of experimental datawith the results calculated by the model of ideal mixing of molar volumes should be stressed.
3.5.4 Partitioning of Main Species in the Presence of Boron Carbide

The first test SFTFM-B3 showed some peculiarities. After the side and lower skulls wereseparated of the ingot it was found out that the basic part of the metallic phase was located in theingot lower part. The mass of that was ~ 26.5 g. A porous phase smaller in size of ~ 21.3 g mass
was located on the metallic phase top. A dense metallic phase and upper porous one wereindividually ground and separated according to their magnetic properties. It was revealed that
22.3 g of the material (#B1019) from the dense metallic phase had strongly pronounced magnetic
properties and 4.2 g of the material (#B1019*) had no magnetic properties. 7.1 g of the material(#B1018) from the upper porous phase were magnetic and 14.2 g of the material were nonmagnetic. Figure 32 illustrates the layout of the ingot with sections for which the chemical andphase compositions were analysed.
#B1018 (21.3 g)#B1019 (26.5 g)#B1017 (43 g)

38

Metallic body locationin similar STFMFe #3#B1020 (20.2 g)#B1021 (36.7 g)

30
Figure 32. The Layout of the STFMB-3 Ingot with Sections Analysed Chemical and Phase
Compositions

Table 19 contains the analysis results of the chemical and phase compositions of the ingots fromthe STFMB-3 test.

56

Table 19. Results of the Chemical and Phase Composition Analysis of the STFMB-3 Ingot
Sample Sample
code
mass, g

B1017

B1018

B1018*

43.0

7.1

14.2

Chemical composition, mass%
U

70.72

52.99

66.40

Zr

Fe

B

Phase composition and the lattice
parameter, nm

C15.96 0.009

Solid solution of
0.11 0.008 (U,Zr)O2(0.539)+α-Zr(O)+tracesof Zr(C,O)+traces of ZrB2

30.53 8.660

Solid solution of(U,Zr)O2(0.541)+solid solution of
0.28(U,Zr)Fe2 (0.714)+α-Zr(O) ++Zr(C,O)+ZrB2(a=0.316;c=0.352)

20.61 1.830

1.25

0.51

B 1019

22.3

51.29

34.06 12.160

1.87

B 1019*

4.2

59.43

26.86 3.110

1.56

B1020

20.2

68.03

18.60 0.200

0.12

B1021

36.7

69.56

17.27 0.130

0.15

0.22

Solid solution of(U,Zr)O2(0.541)+traces of(U,Zr)Fe2+α-Zr(O)+Zr(C,O)++traces of ZrB22 solid solutions of (U,Zr)O2(0.543
and 0.537)+solid solution of(U,Zr)Fe2 (0.714) +0.37+ZrB2(a=0.316;c=0.352) ++Zr(C,O)+traces of α-Zr(O)2 solid solutions of (U,Zr)O2(0.542
0.27 and 0.537)+ ZrB2+Zr(C,O)+tracesof (U,Zr)Fe2 + traces of α-Zr(O)Solid solution of (U,Zr)O2(0.539)
0.18+α-Zr(O)+traces of Zr(C,O)
Solid solution of
0.10 (U,Zr)O2(0.539)+α-Zr(O)+tracesof Zr(C,O)+traces of ZrB2

It should be added to the results of the X-ray phase analysis shown in Table 19 that lines of the(U, Zr)Fe2 phase at the roentgenograms of the B1018 and B1019 samples were quite fuzzy.
Widening of lines in this case is associated with a chemical non-homogeneity of the solid
solution based on (U, Zr)Fe2. The pycnometric density measurements of the oxidic corium andmetallic phase resulted in the following. The oxidic corium density in the surface area wasγ = 9.37 g/cm3. The metallic phase density was at the average γ = 9.43 g/cm3.
The test resulted in the production of a metal ingot of an unusual shape consisting of two parts
different in composition. The ingot lower part, larger in size and mass, contains at the average
more zirconium (~ 32.9 mass%) and less uranium (~ 52.6 mass%) than the ingot upper part (Zr~ 24 mass% and U ~ 62 mass%). The total mass of the metallic ingot produced in the STFMB-3
test is ~ 47.8 g. The generation nature for this metallic ingot consisting of two parts requires toperform additional tests. We may assume that on cooling and crystallization of the metallic andoxidic melts, mutual volumetric variations may contribute definitely into the final result.
The conditions of the STFMB-3 [28] test were to the maximum close to those of the STFMFe-3
test [11] performed earlier in which there were no additions of boron carbide. The comparison of
the STFMB-3 test results with those of the similar STFMFe-3 test allows to reveal at least threebasic differences in the test with boron:••

larger mass (~ 2.5 times) and volume (~ 2.8 times) of the metallic ingot;
less (by ~ 1.5 g/cm3) density of the metallic ingot;57

• a greater shift of the oxidic corium composition to that of the C-100 corium.
In comparison to experiment STFM-Fe # 3 performed earlier with no B4C added to the melt [11]and almost identical initial mass ratio Mmet/Mcor ≈ 0.02, the metallic phase appeared to be
additionally enriched in boron, carbon and zirconium. The coefficient of partitioning defined as
the ratio of the element averaged atomic concentration in the metallic phase to that in the oxidic
phase i.e. ko = xmet/xcor, was 12.2 and 4.4 for boron and carbon, respectively. With boron carbide
introduced into the C-32 corium composition, the coefficient of partitioning of zirconium
increased from 1.3 in experiment STFM-Fe # 3 to 1.9 in experiment STFM-B # 3. These led to anotable decrease of the specific weight of the metallic ingot. In the presence of boron carbide theinitial C-32 corium composition was shifted further to the C-100 corium composition. Taking
into account the boron to carbon mass ratio in boron carbide one would expect boron to have themajor effect on the quantitative modification of the results of the interaction between Fe andC-32 corium.
Metallic inclusions of variable degree of dispersion were found in the upper oxidic part of theingots of experiments STFM-B ## 5 – 9 [29]. For this reason, the compositions of the metallic
ingots obtained in these experiments and displayed in Table 3.16 were averaged, account being
taken for the separate parts of the metallic phase. The results have revealed that the residual
concentration of boron in the oxidic ingots is not a strong function of the Mmet/Mcor mass ratio. It
is not a strong function of the U/Zr atomic ratio or of the degree of oxidation of zirconium (Cn)either. Its average value was found to be 0.8 ± 0.2 at. %. As is seen from Table 3.16, the densityof the oxidic ingots is not a strong function of the experimental parameters either and its average
was found to be 9.2 ± 0.2 gm/cm3. Alternatively, one can see from the results of Experiments
STFM-B # # 3-5 that the residual concentration of carbon in the oxidic ingots exhibits a steadydecrease with increasing Mmet/Mcor mass ratio. The coefficient of partitioning of boron which isdefined as the ratio ko = xmet/xcor of atomic concentrations of boron in the metallic and oxidic
phases decreases as the Mmet/Mcor ratio increases and amounts to 12.2 at Mmet/Mcor = 0.021 forExperiment STFM-B # 3 and to 8.8 at Mmet/Mcor = 0.12 for Experiment STFM-B # 5.
Accordingly, the coefficient of partitioning of zirconium decreases from 1.94 to 1.08.
Let us compare the results of some of the STFM-B series experiments with those of similar
STFM-Fe series experiments. As an example, Table 3.20 presents a comparison of some of theexperimental results obtained under similar conditions (Mmet/Mcor, T, τ) within the STFM-B and
STFM-Fe experiments [11] where the main difference consisted in the presence of boron carbide
in the oxidic corium of the STFM-B compositions.
Table 3.20. Comparison of STFM-B and STFM-Fe experimental results
Mass of Experiment # Composition, mass and density of the metallic ingot
Fe, gmU/Zr
B, at. %Mass, gm
ρ, gm/cm3
~5
~ 15
~ 30

STFM-B # 3

0.75

STFM-Fe # 3 1.12
STFM-B # 4

0.80

STFM-Fe # 4 1.05
STFM-B # 5

0.93

STFM-Fe # 9 1.04

10.84

47.8

9.43

-

19.3

10.89

9.15

58.7

9.47

-

54.2

10.29

4.91

84.9

10.07

-

102.6

9.85

One can see in this table that as the mass of Fe added to the corium melt of a C-32 + 0.5 % B4C(k = 1.2) composition increases, the metallic melt exhibites a decreased fraction of boron andzirconium atoms. The average densities of the metallic ingots as measured after Experiments
STFM-B # # 3 - 5 at room temperature rise steadily (9.43; 9.47 and 10.07 gm/cm3). The mass58difference between the formed metallic ingots exhibits a decrease. Since so, it may be concluded
that the influence of the B4C additive on the process of the interaction of a corium melt with irondecreases with increasing Mmet/Mcor mass ratio when the metallic phase is diluted with iron.
Resulting from a high chemical activity of oxygen, a release of carbon, aparently in the form ofcarbon monoxide, took place when sintering C-70 + 0.5 % B4C corium at some 2150 oC. In thecourse of Experiment STFM-B # 7 at 2600 oC, there was an additional release of carbon from themelt. The residual concentration of carbon in the oxidic ingot was found to be about 50 times
lower than that determined in Experiment STFM-B # 3 with C-32 + 0.5 % B4C corium. Consider
the results of Experiments STFM-B # 7 and STFM-Fe # 7 [11], the only difference between
them being the presence of boron carbide in the initial composition of the former. It can be notedthat for corium C-70 with boron carbide the mass of the obtained metallic ingot is slightly
increased (compare 17.1 gm in Experiment STFM-B # 7 with 12.3 gm in Experiment STFM-Fe# 7). Hence, it can be concluded that the influence of the boron carbide additives on the processof the interaction of an oxidic corium melt with iron decreases with increasing oxidation degreeof zirconium.
In a comparison of the masses of the oxidic and metalic ingots obtained in Experiments STFM-B# 4 and STFM-B # 8 one can observe that a decrease in the U/Zr atomic ratio in the coria used
(1.2 and 0.9) had a negligeable effect on the results of the interaction.
For the coria featured in Experiments STFM-B # 6 and STFM-B # 9, there were additional FP
simulants introduced in these compositions. This research concerned the influence of boron
carbide additives to C-32 corium on the distribution of the selected low-volatility FP simulants
under the interaction with iron. presents the post experiment data on the partitioning of Mo, Ru,
Sr, Ba, Ce and La between the oxidic and metallic ingots.
Table 3.21. Partitioning of FP simulants in the metallic and oxidic ingots obtained inExperiments STFM-B # 6 and STFM-B # 9
Experiment

STFM-B # 6

STFM-B # 9

Ingot

Element concentration (at. %) and partitioning
coefficient
Mo

Ru

Sr

Ba

Ce

La

Metallic

1.25

1.21

0.03

0.03

0.05

0.03

Oxidic

0.13

0.09

0.12

0.13

0.25

0.23

ki = xmet/xcor

9.62

13.44

0.25

0.23

0.20

0.13

Metallic

1.26

1.38

0.04

0.03

0.04

0.03

Oxidic

0.13

0.06

0.16

0.15

0.23

0.22

ki = xmet/xcor

9.69

23.00

0.25

0.20

0.17

0.14

The data presented in Table 3.21 corroborate the earlier observed phenomena of a predominantconcentration of Mo and Ru in the metallic phase and of a such of Sr, Ba, Ce and La in theoxidic phase [18]. Hence, the presence of boron carbide in the oxidic corium has no effect on thepartitioning of FP between the oxidic and metallic melts. Besides, Table 4 indicates that in therange 0.9 – 1.2 the partitioning of the FP simulants reveals no dependence upon the U/Zr ratio.
Conclusions
1. The STFM-B series of experiments focused on the research into the effect of B4C introducedin the U-Zr-O corium on the interaction of the former with Fe-based melts has been
completed.
2. It has been found that as a result of such interaction both boron and carbon tend toconcentrate predominantly in the metallic phase of the melt.
59

3. It has been found that the influence of B4C additives on the interaction of the corium meltwith Fe decreases as the Mmet/Mcor mass ratio in the metallic phase undergoing dilution withiron increases.
4. From a comparison of the results obtained in Experiments STFM-B ## 3, 7 with those of
STFM-Fe ## 3, 7 performed earlier [11] it can be concluded that the influence of boron
carbide additives on the interaction of the corium oxidic melt with Fe decreases as the degreeof oxidation of zirconium increases.
5. It has been demonstrated that the presence of boron carbide in the corium composition has aminor effect on the partitioning of FP simulants between the oxidic and metallic melts. As iswith no boron carbide introduced, both Mo and Ru tend to concentrate in the metallic phase
whereas Sr, Ba, Ce and La tend to concentrate in the oxidic phase of the melt. The
partitioning of the FP simulants reveals virtually no dependence upon the atomic mass ratio
within the range of U/Zr = 0.9 – 1.2.
3.5.5 Partitioning of Fission Products

Experiments with corium and steel performed in the frame of MASCA Project showedsignificant redistribution of major corium components, such as uranium and zirconium, between
metallic and oxidic phases. Significant part of uranium, zirconium and oxygen pass into the steel
melt due to the interaction of structural steel with a suboxidised corium. The FP series of tests
was aimed at the investigation of mechanisms associated with the partitioning of low-volatile
fission products between stratified melts of the metallic and oxidic phases. Mo, Ru, Sr, Ba, Ce
and La were chosen as simulants of fission products.
Three experimental facilities were used for the study of FP behavior, large-scale RCW facility[35], two middle-scale tests MA-3 and MA-4 [15], and a series of small-scale experiments
performed at the STFM facility [18]. In the tests several parameters such as zirconium oxidation
degree, ratio of iron (steel) introduced in the corium and U/Zr atomic ratio were measured.
Initial loading of different components for different experiments are shown in Table 3.22. As FP
were added in different chemical forms in the brackets the recalculated amount for the purespecies are shown. Material balances made up for each test allowed to demonstrate that the
average concentration for each FP in the STFMFe test series was 0.4 ± 0.03 mass %, For MA
tests it was slightly higher, about 0.53 mass % for each FP simulant.
Table 3.22. Initial Charge of FP Simulants (grams)
Facility

SrO

BaO

La2O3

CeO2

STFM-FP 1.36(1.15) 1.30 (1.15) 1.34 (1.15) 1.41 (1.15)
MA-3 10.64 (9.0) 10.05 (9.0) 10.55 (9.0) 11.06 (9.0)
9.46 (8.0) 8.93 (8.0) 9.38 (8.0) 9.83 (8.0)
MA-4135 (80) 89 (72.4)
94 (80)98 (80)
RCW

Mo

Ru

1.15
9.0
8.0
80

1.15
9.0
8.0
71

The experimental procedure for performed experiments is described in the specific reports [18].
Major characteristics of the melt, mass balances and distribution of major corium components
have been discussed earlier.
In the STFMFP series to ensure the uniform FP charge, a part of sintered coria was grinded
under nonoxidizing conditions down to the grains of ~ 500 µm typical size, into which chosenelements were added in the form of a powder mixture of Mo, Ru, BaO2, CeO2, SrCO3, La2O3
in the quantity of 0.4 mass % scaling to metals. Powder mixtures of corium with FP stirred
uniformly were loaded into tungsten crucibles of 30 mm inner diameter and ~ 130 mm height.
The experiments were conducted in the argon atmosphere in the following mode: fast heating to
the temperature of complete melting, 2400 – 2600°, depending on the corium oxidation degree, 1– 3 minute exposition and the inductor switching off.
60

U, Zr, Zrfree, Fe were determined by the standard chemical methods. Fission product simulants
were determined by the plasma emission-spectrum method. The method sensitivity is as follows:
Mo 0.001; Ru 0.002; Ba and Sr 0.0002; Ce 0.01; and La 0.001 mass %. The relative error is≤ 10 %. Simultaneously, the x-ray structural phase analysis was performed.
In all STFM FP tests the amount of steel (iron added into the corium) was 15 grams (Table 3.5)
The following parameters varied in the STFMFP tests:• Zirconium oxidation degree. Initial oxidation degree of the corium mixture before the test
was about 45% for tests #1, #2, #3, #6, and about 70 for tests #7 and #8. Final oxidation degree
was about 65% for tests #1 through #6, and 80 – 85% for tests #7 and #8.
• One test #6 was performed with the stainless steel.
• Temperature in the tests varied between 2600oC and 2700.
• One test was conducted with the U/Zr ratio equal to 1.6 (FP#2), in all other experiments theratio was 1.2.
The following parameters were chosen as main parameters, which determine the partitioning of
FP between phases: temperature, final corium oxidation degree, and U/Zr ratio. Moreover the
dependence upon steel (iron) and corium ratio was analyzed. Partitioning coefficient wascalculated as ratio of concentrations between metallic and oxidic phases.
It is difficult to assess the uncertainties associated with the nonuniformity of FP distribution aseach experiment conducted in the same as for RCW test. The 95 % accuracy level assumes theerror equal 3⋅σ.
Influence of Metallic phase Composition
The influence of metal composition was studied in two tests performed at the STFM facility,
namely FP#1 and FP#6 tests performed at 2600oC. The results are shown in Table 3.23.
Table 3.23. Partitioning of FP Between the Oxidic and Metallic Phases Depending on Steel
Composition (STFM-FP-1, 6 Tests)
Test number

Mo

Simulants of fission products
Ru
Sr
Ba
Ce

La

1 (Fe)

Partitioning coefficient, ko
Relative errors,

16
40

16
40

0.16
60

0.13
60

<0.05
60

0.42
60

6 (SS)

Partitioning coefficient, ko
Relative errors, %

14.2
40

23.7
40

0.07
60

0.025
60

<0.04
60

0.05
60

These data demonstrate that the steel composition influence on Mo and Ru partitioning
coefficients is weak . Although partitioning coefficients for other FP are noticeably influenced at
the transition from practically pure Fe to stainless steel. The value of ko is reduced 2.3 times forSr, 5.2 times – for Ba, and 8 times – for La. However this conclusion can not be made as thenumber of tests was too low and uncertainties were too large to make a distinct conclusion.
Influence of Temperature and Corium Composition
The temperature dependence was analyzed for the following tests: STFM FP#1 (2600oC), FP#3
(2700oC), and MA3 (2500oC) (Table 3.24). Final corium oxidation degree for those tests wasapproximately similar and ranged between 55 and 60%. Masses of FP simulants found after thetests and relative errors are shown in Table 3.25.. Another series of tests with the final coriumoxidations degree of about 85% was formed of MA4, FP#7, and FP#8 tests. In the Table 3.26
calculated partitioning coefficients defined as the ratio of wight concentrations in the metallic
and oxidic phases are presented.

61

Table 3.24. FP Simulants Concentrations in Oxidic and Metallic Phases
Concentration of FP
simulants in the oxidic phase, wt.%

Test conditions

Exposition Oxidation,
Metal T, oC Mo
time
final

Ru

Sr

Ba

Ce

La

Nb

Concentration of FP
simulants in the metallic phase, wt%
Mo

Ru

Sr

Ba

Ce

La

FP1

15

59.6

Iron 2600 0.082 0.069 0.130 0.200 0.190 0.190

1.310 1.100 0.021 0.026 0.010 0.028

FP2

15

57.2

Iron 2600 0.059 0.044 0.130 0.180 0.100 0.100

2.040 1.390 0.015 0.016 0.010 0.024

FP3

15

59.2

Iron 2700 0.120 0.078 0.140 0.250 0.100 0.110

0.730 0.860 0.018 0.024 0.010 0.017

FP6

15

60.8

SS

2600 0.110 0.057 0.058 0.200 0.270 0.280

1.560 1.350 0.004 0.005 0.010 0.013

FP7

15

78.3

Iron 2600 0.036 0.029 0.110 0.200 0.071 0.075

2.220 2.210 0.014 0.020 0.010 0.018

FP8

30

82.6

Iron 2600 0.052 0.027 0.140 0.170 0.100 0.092

2.630 1.980 0.030 0.026 0.030 0.019

MA3

30

64.4

SS

2500 0.020 0.030 0.350 0.380 0.750 0.670

1.540 1.640 0.020 0.005 0.017 0.013

MA4

30

70.2

SS

2500 0.030 0.020 0.390 0.320 0.630 0.490

1.110 1.120 0.010 0.010 0.020 0.010

RCW, Av.1

25

SS

0.016 0.031 0.110 0.15 0.067 0.078

0.14 0.019 0.026 0.025 0.026 <0.01

SS

0.005 0.028 0.02 0.035 0.032 0.044

RCW, σ1

For RCW facility concentrations in the oxidic phase were averaged along the height, concentrations for metallic phase were taken for middle part of the ingot

62

NbTable 3.25. Masses of FP Simulants Found after the Tests and Relative Errors
Masses of FP simulants
detected after the test, g
Mo

Ru

Sr

Ba

Ce

Relative errors in masses, %

La

Nb Mo

Ru

Sr

Ba

Ce

La

FP1 0.977 0.821 0.391 0.598 0.560 0.570

15

29

66

48

51

50

FP2 1.139 0.788 0.417 0.575 0.320 0.327

1

31

64

50

72

72

FP3 0.774 0.728 0.418 0.742 0.297 0.330

33

37

64

36

74

71

FP6 1.068 0.810 0.175 0.600 0.812 0.843

7

30

85

48

29

27

FP7 0.758 0.732 0.373 0.677 0.241 0.257

34

36

68

41

79

78

FP8 0.751 0.527 0.442 0.534 0.317 0.290

35

54

62

54

72

75

MA3 7.917 8.687 8.652 6.336 9.185 8.195

17

11

34

30

2

9

MA4 7.825 7.774 5.382 5.855 7.930 7.036

15

3

33

27

12

12

Nb

Table 3.26. Partitioning of FP between the Oxidic and Metallic Phases of the Stratified
Melt in the STFM-FP-1, 3 Tests Depending on Temperature
Test number

Simulants of fission products
Mo

Ru

Sr

Ba

Ce

La

Final corium oxidation Cn = 65
MA3 2500oC

82.0

58.0

0.057

0.013

0.093

0.090

FP#1 2600°C

16

16

0.16

0.13

<0.05

0.42

FP#6 2600°C

14.2

24

0.069

0.025

0.037

0.046

FP#3 2700°C

6.1

11

0.13

0.1

<0.1

0.15

Final corium oxidation Cn = 85
MA4 2500oC

37

56

0.026

0.031

0.032

0.021

FP#7 2600°C

61.7

76.2

0.13

0.1

<0.14

0.24

FP#8 2600°C

50.6

73.3

0.21

0.15

0.3

0.02

The following conclusions can be made from the comparison of presented data and graphs(Figure 3.33). The partitioning coefficient for metallic FP such as Mo and Ru is decreasing
with the growth of temperature. This is due to the fact that the concentration of Mo and Ru in
metallic phase decreases, while concentrations in the oxidic phase increase. In contrast,
temperature increase from 2500°C to 2700°C results in the general increase of partitioning
coefficients Sr, Ba, Ce and La.
Increasing corium oxidation degree changes significantly the partitioning of Mo and Ru.
However partitioning coefficients of Sr, Ba, Ce and La remain at approximately the samelevel.

63

Final corium oxidation degree of 65%

Final corium oxidation degree of 85%

Mo
Ru

30

Mo
Ru

95
90

25

85
80
75

Partitioning

Partitioning

20

15

70
65
60
55
50

10

45
40
5

35
2500

2550

2600

2650

30

2700

2500

o

Temperature, C

2550

2600

o

Temperature, C

Partitioning coefficient of Mo and Ru
Sr
Ba

Sr
Ba
0.30
0.20
0.25
0.20

Partitioning

Partitioning

0.15

0.10

0.05

0.15
0.10
0.05
0.00

0.00
2500

2550

2600

2650

2700

2500

o

2550

2600
o

Temperature, C

Temperature, C

Partitioning coefficient of Sr and Ba
Ce
La

0.25

Ce
La

0.45
0.40

0.20

0.35

Partitioning

Partitioning

0.30

0.15

0.10

0.25
0.20
0.15
0.10

0.05

0.05
0.00

0.00
2500

2550

2600

2650

2500

2700

2550

2600
o

o

Temperature, C

Temperature, C

Partitioning coefficient of Ce and La
Figure 3.33. Partitioning of FP Simulants as a Function of Temperature and Final
Corium Oxidation Degree
Influence of U to Zr Ratio
Experiment FP#2 was conducted with the U-Zr ratio of 1.6. Comparison of partitioning
coefficients for tests FP#1 and FP#2 is presented in Table 3.27, which demonstrates that theincrease in U/Zr atomic ratio noticeably influences Mo and Ru partitioning, for which ko isincreased by ~ 2 times. For Sr, Ba and La, changes in the values of ko do not exceed 30 %.

64

Table 3.27. Partitioning of FP between the Oxidic and Metallic Phases Depending on
U/Zr Atomic Ratio
Test number

Simulants of fission products
Mo

Ru

Sr

Ba

Ce

La

0.13

<0.05

0.42

0.089

<0.1

0.24

Atomic ratio of U/Zr = 1.29
FP#1

2600°C

16

16

0.16

Atomic ratio of U/Zr = 1.68
FP#8

2600°C

35

32

0.115

For the coria featured in Experiments STFM-B # 6 and STFM-B # 9 [29], there wereadditional FP simulants introduced in these compositions. This research concerned theinfluence of boron carbide additives to C-32 corium on the distribution of the selected lowvolatility FP simulants under the interaction with iron presents the post experiment data on
the partitioning of Mo, Ru, Sr, Ba, Ce and La between the oxidic and metallic ingots.
Table 3.28. Partitioning of FP simulants in the metallic and oxidic ingots obtained inExperiments STFM-B # 6 and STFM-B # 9
Experiment

STFM-B # 6

STFM-B # 9

Ingot

Element concentration (at. %) and partitioning
coefficient
Mo

Ru

Sr

Ba

Ce

La

Metallic

1.25

1.21

0.03

0.03

0.05

0.03

Oxidic

0.13

0.09

0.12

0.13

0.25

0.23

ki = xmet/xcor

9.62

13.44

0.25

0.23

0.20

0.13

Metallic

1.26

1.38

0.04

0.03

0.04

0.03

Oxidic

0.13

0.06

0.16

0.15

0.23

0.22

ki = xmet/xcor

9.69

23.00

0.25

0.20

0.17

0.14

The data presented in Table 3.28 corroborate the earlier observed phenomena of apredominant concentration of Mo and Ru in the metallic phase and of a such of Sr, Ba, Ce
and La in the oxidic phase [18]. Hence, the presence of boron carbide in the oxidic corium
has no effect on the partitioning of FP between the oxidic and metallic melts. Besides, Table
4 indicates that in the range 0.9 – 1.2 the partitioning of the FP simulants reveals no
dependence upon the U/Zr ratio.
Conclusions on FP behavior
Initial series of tests dealing with the distribution of FP between metallic and oxidic phases
demonstrate that metallic fission products, such as Mo and Ru, are concentrated in themetallic phase, while Sr, Ba, Ce, and La - in the oxidic phase.
The FP partitioning coefficient between the oxidic and metallic phases Ko=Xmet/Xcor, depends
on the melt parameters such as:• Temperature, Ko for all FPs decreases 1.2 to 2.7 times with the temperature increase from
2600°C up to 2700°C.

65• The increase in the corium Cn (from C-65 up to C-86) and in U/Zr atomic ratio (from1.29 up to 1.68) increases Ko (2, 5) times for Mo and Ru and changes Ko in the range of ~ 50
% for Sr, Ba, Ce, and La.
Fe replacement by SS does not practically influence Ko for Mo and Ru, but decreases it
significantly (2 - 9 times) for Sr, Ba, and La.
3.5.6 Effect of Oxidation Atmosphere on the Distribution of Main Species

It was experimentally established that the configuration of molten pool produced at theinteraction of suboxidized corium melt with molten Fe (SS) continuously changes duringoxidation. The main qualitative phenomenon determined in the oxidation experiments is thesecondary inversion of oxidic and metallic melts caused by the oxidation of components andcorresponding changes in the compositions and densities of coexisting oxidic and metallic
liquids. As the corium oxidation degree is increased, the mass of molten metallic phase andits density are reduced. The inversion of layers takes place on achieving the critical degree ofthe corium oxidation due to the decrease of the metallic phase density lower than that of theoxidic one. Oxidation of steel components takes place in parallel to the oxidation of free Zr
and U. When concentrations of Zr and U concentrations in the melt decrease, the oxidation
rate of steel components increses. Considerable steel oxidation can influence mass andthickness of surface metallic layer of corium pool. This may be of importance in the context
of possible focusing effect.
MASCA results indicate that the pool transient behavior should be analyzed for the IVR.
Long term retention of the melt in the reactor vessel under the oxidation conditions, inprinciple, results in the pool configuration with the metallic layer on top. But the oxidation
rate is very sensitive to the amount of available steam and pool surface conditions, e.g.
surface crust formation. Certainly, the performed small and medium scale experiments were
not directly focused on the kinetic aspects of the oxidation process. But several qualitative
effects observed in the tests could be relevant for reactor application.
In MA [20] tests it was found that, if there is no crust on the pool surface, melt oxidation rate
corresponds to the transport (starvation) stage of the oxidation process, i.e. the rate is limited
not by the kinetics of chemical reaction itself and not by the supply of reducers (Zr, Fe, Cr
and Ni) to the melt surface, the concentration of which decreases during the interaction, butby the transport of the oxidant (steam) to the melt surface. Therefore the integral oxidation
rate is govern by the steam supply and does not depend upon concentrations of separate
species. Unlike MA, in STF [19] test series the steam absorption rate at the dropwise watersupply to the melt surface was decreased as the corium oxidation index grew. The apparent
contradiction can be explained by the peculiarity of the dropwise water supply to the melt
surface, which was significantly more effective. At higher flow rates of water to the poolsurface (at the kinetic stage) the oxidation rate is influenced by Zr content in the melt. It
should be mentioned, that it is very difficult to determine the specific steam absorption ratenormalized for a unit of melt surface in STF experiments. Additional experiments could berecommended for a better insight and generalization of the STF experimental results. In theexperiment a layer of water will be maintained on the melt surface, for this the cold crucible
test facility and methodology [36] can be recommended. The most efficient steam supply tothe melt surface can be provided in this test.
Due to the oxidant starvation conditions the two competing reactions with free Zr wereidentified in MA test in air: oxidation and nitration. The second reaction accelerates spendingof Zr and its mass transfer from metal to oxide melt. The ratio of O2 and N2 absorption rates
actually coincides with the ratio of their molar concentrations in the supplied air (≈ 0.268)
when the melt surface is free. Refractory zirconium nitrides (oxynitrides) promote surface
66

crust formation, after that the interaction rate has a considerable decrease, and the interaction
shifts to a larger absorption of oxygen in comparison to nitrogen. It can be explained by ahigher capability of oxygen to penetrate through the crust layers in comparison with nitrogen.
It is clear that the process enters a still slower, diffusion phase, when the interaction rate islimited by the diffusion of components through the solid crust. A peculiarity of this stage is aconsiderable fluctuation of the absorption rate related to the irregularity in the crust growthrate, its possible cracking, chaotic opening of the melt surface, etc. In the test the oxidation
was terminated before the total free Zr had reacted. But for the long-term oxidation we expect
the start of ZrN oxidation after reduction of Zr concentration in the melt up to a certain level.
Formation of the surface crust on the molten pool switches oxidation process to the diffusion
stage both in air and in steam. This stage is characterized by the drop of oxidation rate, whichis expected to be controlled mainly by crust thickness and temperature. As the situation withsurface crust is very typical for the IVR, the corresponding tests proposed for CORTRAN areof high importance.
Surface heating of corium melt has been observed in MA tests. Unfortunately it was notpossible to quantify the effect due to the following reasons:- “Chemical” heating of the melt took place against additional inductive heating
induced by the change of melt resistivity due to the shift of melt composition atoxidation.
- Melt surface temperature was measured only during oxidation in air, because all usedcolor pyrometers are not operable in steam due to the selective absorption of light atworking wavelengths.
Note that effect of surface heating can be calculated, if an adequate model of oxidation
kinetics is developed.
In both MA experiments the molar rate of air and steam absorption by the melt without crust
was practically the same: 1.7(±0.2) mol./m2c, while the molar flow rates of these reagents at
the furnace entrance were substantially different: 1.79(±0.03) and 3.3(±0.2) mol/(m2⋅s),respectively. It could be an indication that oxidation kinetics is influenced by Zr
concentration at the melt surface, but as it was explained above, this is not the case. This canbe explained by the counter-flow of hydrogen, which reduces the inflow of steam to the melt.
In order to check this hypothesis it is recommended to make an additional experiment on themelt oxidation by air at the same flow rate as for steam.
Coming back to the plant application of the results, it should be noted that oxidation of meltby air-steam mixtures is relevant for the IVR after LOCA, particularly if the containmentpressure is not high. Taking into consideration the discovered differences of oxidation bysteam and air, future tests with air-steam mixtures should be also proposed as topical tests.

3.6 Steel Behavior in the RCW Test
The large-scale RCW facility test was successfully performed on September 17, 2002. During
the test, the components of the lower section loading (uranium oxide, zirconium oxide,zirconium, simulants of fission products) were heated up to 2500°C and melted. In contrast to
all tests in a smaller scale, stainless steel was melted in the upper section and then introduced
into the prepared oxidic melt.
This section is dedicated to the discussion on the major results obtained in the course of the
post-test material studies and calculation analyses.
Detailed description of the test power history and post test examination results can be foundin the report [35].
673.6.1 The Test Facility

For the conduction of RCW test the same approach of cold crucible similar to the scale tests
was used. The facility consists of two sections each heated separately by its own inductor as
it is shown in Figure 3.34 and Figure 3.35. Each of the melting compartments is locatedinside the bearing supporting cylindrical lattice made from water-cooled copper tubes andcopper headers for water input-output that form a cold crucible.
17

1
2
3
4
5
6

water

7
8
19
9

10
11
18
12
16
13
14

water

15

1. Steel charge;
2. ZrO2 case for the steel charge;
3. Upper tungsten heater;
4. ZrO2 thermal insulation;
5. Lattice of copper tubes of the upper cold crucible;
6. Upper inductor;
7. ZrO2 heat shield;
8. Thermal insulation (ZrO2 powder);9. ZrO2 protective bush;10. Magnetic cores;
11. Start-up tungsten heater;
12. Thermal insulation (UO2 and C-100 corium powder);13. Lattice of copper tubes of the lower cold crucible;
14. Lower inductor;
15. C-32 corium loading;
16. Loading of FP simulants;
17. Pyrometric tube;
18. C-32 corium groats;
19. ZrO2 dome

Figure 3.34. The Facility Schematics

68

Figure 3.35. RCW Facility View

The depth of the melting compartment in the lower section is 700 mm; the inner diameter is
176 mm.
The lower section loading consisted of 26 layers of the C-32 corium ring briquettes and of theloading of FP simulants. The corium briquettes were put on the powder thermal insulation
40 mm thick (the mixture of UO2 and the C-100 corium powders). The loading of FP
simulants was located in the middle part between the levels of 146 and 342 mm from thesection bottom.
A layer eight millimeters thick of the powder thermal insulation (the mixture of UO2 and the
C-100 corium powders) was filled between the loading and the cold crucible pipes. The same
mixture filled up all clearances between the cold crucible pipes and inductor.
A pipe of the start-up tungsten heater 65 mm long located along the compartment axis was
put into the loading down to the depth ~ 100 mm. Tungsten heater was heated by the inductor
field with the frequency 8 – 10 kHz up to the temperature 2500 – 2600°C. It heats, in its turn,the surrounding briquettes by the radiationally-conductively up to the formation ofelectrically closed rings and the occurrence of the necessary electric conductivity in those (atf = 10 kHz, Tinitial heat generation ≈ 1500 °C).
A melting compartment of the upper section is formed by a tungsten pipe-heater insulated
from the cold crucible pipes by the layers of Al2O3 coating (the layer is 10 mm thick) and of
ZrO2 powder (the layer is 14 mm thick). The compartment is supported by heat-insulatingrings made from ZrO2 ceramics.
Inside the compartment, a stainless steel charge (a pipe ~ 9 mm thick) is placed into the casemade of ZrO2 ceramics. The case prevents the contact of melting steel with the heater
tungsten.

69

The corium briquettes and FP simulants were heated and melted in the lower section. After
that, steel preform was melted in the upper section, and steel runs down through the discharge
channel onto the corium melt in the lower section where they interact.
W/Re thermocouples used in the test survive up to the temperature 1800 - 1900°C. The RCW
Facility Basic Characteristics are shown in Table 3.29.
Table 3.29. Basic Characteristics of the RCW Facility

Number of test sections

2

The C-32 corium mass (U/Zr=1.2) (kg)66.40

UO2 plates (kg)1.48

Thermal insulation 50 % of C-100
corium + 50 % of uranium dioxide (kg)24.830

Stainless steel mass (kg)4.64

Mass of fission products (kg)0.57

Protective medium

Ar

Melting compartments

upper

lower

Operating temperature (°C)

2200 - 2400

2600 –
2650

Power supply frequency (kHz)

2.4

10

Inductor power (kW)Up to 50

up to 50

Test section power (kW)up to 150

up to 250

The power input scenario was estimated in a series of the pre-test calculations before the test
in order to assess the characteristic power input and time table of events, which allowed tofollow the heating and melting processes. The test was conducted in several phases, namely:
1. Heating up at the moderate level of power up to the target tungsten temperature ofabout 2200oC;
2. Removal of the starter heater from the crucible after the melt front reached cold walls;
3. Sustained heating of corium at the appropriate power level to provide the corium
heating and melting;
4. Heating of steel in the upper section and pouring of that into the melted corium;
5. Quasi-steady state operation.
The starter tungsten heater was removed after the beginning of the radial melting. The
sustained heating of the corium loading provided downward motion of the melting front. The
thermocouples located in the peripheral area indicated rapid temperature increase. As theheating efficiency increased gradually, the input power was reduced during the third phase tomaintain approximately constant melting front velocity.
3.6.2 Post-Test Material Studies

The objectives of the post test material studies were as follows:• splitting of the frozen ingot;• determination of the melt quantity and shape assessing the impact of the cold crucible
insulation on the corium composition at the melting moment;

70

•
•

determination of the structure, phase and element composition of the oxidic andmetallic formations;determination of the distribution of the corium density and metallic formations overthe ingot cross-section.

3.6.3 Results of Visual Observations

Figure 3.36 illustrates the layout of the lower crucible central section. The layout shows theconfiguration and location of non-melted (2 - 2.5 layers) and partially melted (a layer
5 - 20 mm thick) briquettes of the C-32 corium (1). Above the melt surface (3), the crust
remains (2) were found at the depth of 302 mm from the top of the cold crucible. The ingot
surface (3) is located ~ 30 mm below the crust.

302

336

1

2

260

700

3

1
4

1 – non-melted and partiallymelted briquettes;
2 – crust;3 – the melt surface;
4 – thermal insulation lower
layer (C-100 and UO2)

Figure 3.36. Layout of the Lower Crucible Central Section

60 g of frozen metal drops of steel not penetrated into the molten corium were collected
above the crust. The ingot (a part of sintered and molten loading) from the melt surface up to
the end of lower non-melted briquettes was retracted from the crucible, placed into a71

rectangular mould that was filled up with epoxy for the further cutting out. Table 3.30
presents the material balances for components loaded in the test section before the test.
Table 3.30. The Material Balance of the RCW Experiment
The initial loading

Mass
(kg)

Components collected after the test

Mass
(kg)

The C-32 corium

66.415 The ingot mass

53.180UO2 plates

1.480

Oxidic part45.250FP simulants

0.570

Metallic part

7.930

Mass of components outside the ingot

43.819Stainless steel 12X18H10T*) in
4.643
the upper section

Including:Non-melted briquettes and UO2
19.929platesStainless steel
Including:
Thermal insulation
Thermal insulation (50 % of
(50 % of C-100 corium + 50 % 24.830
the C-100 corium + 50 % ofof uranium dioxide)
uranium dioxide)
Total weight

97.938 Total weight

Disbalance

0.060
23.83096.9990.939*) Fe - 67.5 wt.%, Cr - 0.37 wt.%; Ni - 10.43 wt.%, other: 1.7 wt.%
The Ingot Splitting and Macrostructure
A cylindrical ingot was cut into two parts along the central cross section. A macrosection of
the ingot is shown in Figure 3.37.
The following typical macrozones of the structure are observed:
1) four horizontal layers of non-melted briquettes in the cut lower part. A well-definedboundary between the lower metallic part (2) the non-melted corium layer (1) isobserved. Typical traces of metal leakages can be seen in the gaps between thebriquettes and cracks;
2) the zone of the oxidic and metallic materials completely melted extends from thelower boundary of the lower metallic part up to the surface (8) and is about 260 mm
thick. The zone (3) in Figure 3.37 is hypothetically the corium partially molten within
the temperature range Ts-Tl.
The zone of the melted ingot oxidic part (5) is of a grey colour and is divided by cracks
produced during corium cooling phase.
In contrast to the STFM and MA tests the metallic phase was separated into three different
types:
1) the lower metallic part with a flat top and a dumbbell-like lower surface the contourof which follows the isotherm of the corium melting;2) the upper metallic part is also of a dumbbell-like shape with bulges near the cruciblewalls;
3) a cluster of roundish metallic parts is located along the ingot central axis.
Practically all metallic parts both the roundish and plate ones contain interstices mainly in thecentral part produced by the gas pressure (presumably CO). In the bulk corium, some cracks
are filled with metal migrated from metallic parts. Areas with the increased content of72

metallic impurities in the form of fine spheres, dendrites, crystal clusters are observed in theoxidic corium. This is mainly observed near coarse metallic parts.

1.
2.
3.
4.
5.
6.
7.
8.

The corium lower non-melted briquettes;
The lower metallic part;
Zone of a partially melted corium;
Roundish metallic parts in the central zone;
The ingot oxidic part;
Epoxy;The upper metallic part;
The ingot surface;

Figure 3.37. The Macrosection of the Ingot Central Cut
Cutting out of Samples for the Investigations
Samples for the chemical, metallographic and microprobe analyses (Figure 3.38) were cut outof the ingot oxidic part by a tubular diamond drill. Metallic parts were drawn out of the ingot
and weighed (Table 3.31).

Corium samples for chemical analysis and density
measurements
Samples for metallography and microprobe analysis
Samples of roundish metallic parts («column»)
Metallic part samples for density measurements

Figure 3.38. Pattern for Cutting out of the Corium Samples

73

Table 3.31. Weights of Metallic Parts
Metallic part location

Metallic parts weight, (g)

Crust

410

Upper one

2440

Middle roundish parts (6 pcs.)

735

Lower one

4345

Total amount of metal phase7930

Density of the Corium and Metallic Parts
The results of density measurements for oxide and metal parts are shown in Figure 3.39. As
one can see from the figure the corium density along the ingot height varies within the range
from 9.17 up to 9.03 g/cm3. Samples ## 7, 8 contain fine metallic impurities which increases
slightly their density.
The density of metallic parts varies more significantly accordingly to the location in theingot:• the upper metallic part – 9.07 g/cm3.
• metallic parts located in the ingot centre (samples do not contain sealed interstices) –9.67 ± 0.04 g/cm3.
• the lower metallic part – 10.69 g/cm3.
240

240

10
200

1

Heightfrom upper surface of lower metalic part (mm)Heightfrom upper surface of lower metalic part (mm)Upper metallic part

11

13

2
160

12
14

120

4

5

80

15
40

7

200

160

#1
#2
120

#3
80

#4

40

#5
#6

0

Bottom metallic part

8
0

-40

8.8

9

9.2

9.4

Density (g/cm3)

9.6

Density distribution of the oxidic part

9

9.2

9.4

9.6

9.8

10

10.2

Density (g/cm3)

10.4

10.6

10.8

Density distribution of metallic parts

Figure 3.39. Diagrams of Density Distribution for the Oxidic and Metallic Parts Alongthe Ingot Height74

Distribution of major components
The corium samples for the chemical analysis were cut out by a tubular diamond drill inaccordance with the layout presented in Figure 3.38. Samples were analysed according to thestandard technique applied earlier in the RASPLAV project for seven elements: uranium,
zirconium, free zirconium, iron, chromium, nickel, and carbon.
The corium samples were taken out of the ingot different structure zones. Table 3.32 contains
the corium chemical analysis results. All corium samples contain stainless steel components(Fe, Cr, Ni) in different ratios. Steel content in the corium is in a definite association with thelocation of samples in the ingot. The least content of steel was revealed in the peripheryzones, it is 0.1 mass % in samples ## 14 and 15, which are most distant from metallic parts.
Nine samples contain less than 1.0 mass % of steel. In the samples located much closer tolarge metallic parts and in those located in the areas of metallic part clusters, steel content
achieves several per cents. The maximum value of the content (~ 12 mass %) falls on thezone with the visible background of metallic impurities above the column of roundish
metallic parts (sample # 12).
In all samples, carbon content was in the range of 0.02 – 0.06 mass % except for the sample#13 (~1 mass %).
Figure 3.40 presents the distribution of basic elements (U and Zr) in the corium along theingot height (samples were taken with steel content ≤ 0.8 mass %). Quite a homogeneouscontent of these elements is seen with a weak reduction of uranium content and similar
increase of zirconium content in the corium upper layer. The similar uranium and zirconium
behaviour was revealed in the test AW-200-4 performed with a pure (without steel) C-32
corium [37].
Table 3.32. The Corium Chemical Analysis Results
Sample#Element concentration (mass%)
U

CFe+CCr+CNi

U/Zr Cn = 1 - Zrfree
Zrtotalat/at

Zrtotal Zrfree

Fe

Cr

Ni

1

64.34 18.32 13.12

3.550

0.72

0.39

4.70

1.35

60

2

66.43 20.01 10.54

0.530

0.15

0.11

0.80

1.27

52

3

66.68 19.88

9.58

0.260

0.11

<0.05

0.40

1.29

54

4

66.42 20.09

9.60

0.250

0.09

<0.05

0.40

1.27

55

5

67.03 19.85

9.45

0.290

0.11

<0.05

0.40

1.29

55

6

66.78 17.70 11.60

2.160

0.32

0.25

2.70

1.45

53

7

67.58 19.11 10.70

0.380

0.16

0.12

0.70

1.36

48

8

66.22 18.43 11.98

1.890

0.28

0.30

2.50

1.38

51

9

67.51 18.96

6.61

0.110

<0.05 <0.05

10

61.91 20.16 11.86

0.600

0.18

0.13

0.91

1.18

49

11

65.23 20.30 12.20

1.070

0.28

0.21

1.60

1.23

49

12

56.53 15.02 29.01 12.330

3.52

2.09

17.90

1.44

54

13

64.68 20.64

9.63

0.060

0.32

0.17

0.60

1.20

57

14

66.29 20.20

9.03

0.014

<0.05 <0.05

0.10

1.26

56

15

67.06 19.64

9.63

0.030

<0.05 <0.05

0.10

1.31

52

75

non-melted briquettes

200

in area of large
metallic parts

13
2

160

120

13

11
2

160

Height (m m )

14
4

80

1

10

200

U

Zr

3
Height (cm)

240

3

14

120

15

80

7

40

4

5
15

40

7

0

8

0

10

20

30

40

50

60

Concentration (mass%)

U and Zr

70

40

50

Cn

60

Cn = 1 – CZrfree/CZrtotal
CZrfree was corrected for Fe, Cr,
Ni

Figure 3.40. Distribution of U, Zr and Change of Cn in the Corium along the Ingot
Height

The degree of zirconium oxidation (Cn) was assessed basing on the results of determinationof free zirconium (Zrfree) and total zirconium (Zrtotal) contents according to the formula:
Cn = (Zrtotal – Zrfree)/Zrtotal ⋅ 100%On Zrfree chemical analysis, the presence of Fe, Cr, Ni free atoms in the corium resulted in the
spurious overestimation of Zrfree content value. Therefore, the calculated value (Zrfree)calc.
equal to the difference between the measured value of Zrfree content and the sum of steel
component (Fe, Cr, Ni) concentrations with regard to Fe, Cr, Ni valent states was employed
as the value of free zirconium content for Cn calculation. Figure 3.40 demonstrates the
distribution of the degree of free zirconium oxidation along the corium ingot height. A
compact distribution of the corium oxidation degree is observed in the range of values
Cn = 53±2 %.
Cn values determined from the samples ## 10 and 11 located between the upper metallic partand the central zone of metallic parts as well as from the samples ## 7 and 8 located near the
lower metallic part are somewhat biased to the reduction of the corium oxidation degree(densities of samples ## 7, 8, 10, 11 are included into determination of the average value).
One sample from each of the upper, lower, and middle (# 4) metallic parts was taken from theingot metallic component.
It followed from the previous reports [35] that partitioning of elements throughout thevolumes of metallic parts was non-homogeneous that led to a significant scattering of results
on local sampling. To improve the analysis confidence representative samples were taken
including all typical volumes of metallic parts. For illustration, four cross-sections with a
total mass ~ 16 % of the part mass were splitted in 90о from the lower metallic part of a torus
shape.

76

To prepare the average representative sample, the ingot cut out parts wetted preliminary withalcohol were crushed into fine parts of ~ 1 – 1.5 cm3 size by a mechanical press. Then, theproduced pieces also wetted with alcohol were grinded to powder in a molybdenum mortar.
The produced powder was thoroughly stirred in a glass pot by a china paddle during not lessthan 15 minutes. The further averaging was performed by quartering. Then powder in theglass was evened and a sample of 20 g weight was taken by a bulb with a narrow and long
throat of 10 mm diameter.
Table 3.33 presents the chemical analysis results for these samples for six elements (uranium,
zirconium, iron, chromium, nickel, and carbon). Figure 3.41 shows the distribution ofcomponents in the metallic parts versus the level of their location in the corium melt after the
test.
Table 3.33. The Chemical Analysis Results for Metallic Parts
Element (mass%)

Sample

U

Zr

Fe

Cr

Upper metallic part (melt surface)

23.09

10.20

43.20

Metallic part #4 («column»)

38.89

13.53

Lower metallic part

51.96

20.10

Ni

C

SS

11.55 8.01

0.058

62.76

31.60

8.36 5.43

0.042

45.76

16.60

4.04 2.93

0.190

23.57

3
Top

U

Zr

SS

2
MiddleBottom
1
0

10

20

30

40

Concentation (mass%)

50

60

70

Figure 3.41. Partitioning of U, Zr and Fe in Metallic Parts along the Ingot HeightAs Table 3.33 and Figure 3.41 show, the U/Zr ratio in the upper metallic part is less (0.87)
then that in the middle and lower parts (1.0 – 1.1). It may be assumed that at the initial stage
zirconium is dissolved in a more intensive manner than uranium. The average concentrationof steel (SS) contained in the corium (outside the metallic parts) estimated from the relationof masses(Mss - ∑MSSmet.part)/Mcorium in ingot
is ~ 3.7 mass %. Oxygen content in the metallic parts was 0.8 – 1.2 mass %.

77

Discussion of Results
The main distinction of the RCW test from the series of tests STF and MA lies in the scale
factor. The mass values of the corium and steel interacted in the RCW tests are 20 - 30 times
higher than those in the MA-1, 3 tests and ~ 150 times higher than those in the tests of the
STF series. Table 3.34 lists the data for three tests with different scales of loadings (the C-32
corium).
Table 3.34. The Basic Parameters of Tests with Different Scales of Loadings
Final corium
Mass, goxidation
degree, % Corium Steel

)

Test*

RCW

Exposition
time,
min

Composition of the lower
metallic part, mass%
U

Zr

Steel

52.0

20.1

23.6

38.9

13.5

45.7

Mst./Mcor

50 – 55

45250

4643

22

STFMFe-15

60

273

29.9

30

44.1

14.4

40.8

0.11

STFMFe-3

38

287.3

5.3

30

62.9

21.5

12.2

0.02

STFMFe-4

48

315.4

14.4

30

54.6

19.7

25.6

0.046

STFMFe-6

55

299.2

4.8

10

56.2

20.0

20.6

0.016

MA-3

71

1503.1

181.6

30

41.7

16.8

36.4

0.13

64

1434.2

184.9

30

36.8

7.8

55.1

0.11

MA-1
)

0.10

* In all listed tests, the mentioned metallic parts relocated onto the pool bottom.
**) There was a bottom oxide (UO2+ZrO2 powder mixture) blanket with mass of 500 g.
Considering the results presented in the Table and in the previous sections one may conclude
that the scale factor influences the interaction kinetics.
In the tests with low masses (~ 300 g of corium), the interaction process as it was shown insection 3.4 is completed within τ ≤ 15 min, and the metallic part absorbed uranium up to theconcentration 50 mass % relocates onto the pool bottom.
In the MA-1 test with the corium mass ~ 1500 g, during the interaction time τ ≤ 30 min themetallic part that absorbed uranium up to the concentration ~38 mass% relocates onto thepool bottom.
In the RCW test with the molten corium mass ~ 45250 g, the lower part seems to be saturated
with the uranium up to the concentration 40 – 50 mass %. Total mass of uranium migrated
into steel was 3.1 kg. An additional absorption of ~ 2.2 kg of uranium (as it is shown below)
is necessary to achieve uranium concentration ~ 50 mass % in all metallic parts.
It is necessary to note that the method of steel introduction into the melt (in the solid or liquid
forms, in one part or several parts) as well as the ratio of steel mass to the surface of thecontact with the corium may also influence the kinetics of the corium interaction with metal.
Judging from the ingot structure, one may assume that the process of steel interaction in the
RCW test was discrete. If the steel layer was formed at the top of the corium pool then the
diffusion process led to the increase of uranium and zirconium in the lower part of steel layer
was (presumably, up to 40 - 50 mass % of uranium which is enough to increase the density
above corium density) and partial detaching from the top metal layer. Accordingly to theconcentration and mass of the lower layer the mass of steel relocated onto the pool bottom(without uranium and zirconium) was about 1 kg. If such behaviour took place the78composition of the lower part should be comparable with the results of STFM tests with theadded mass of about 2% (test STFMFe6, where the uranium concentration and final coriumoxidation degree were very comartible as one may see from the Table 3.34). The same can besaid for the test STFMFe-4 with the added iron mass about 5% which is characterized by thesimilar corium oxidation degree. Measured concentrations are higher than the values
measured in the tests MA-3 and STFMFe-15 conducted with the steel which arecharacterized by higher final corium oxidation degree.
The roundish metallic parts had uranium concentration ~ 38 – 40 mass % and density~ 9.7 g/cm3 and were to relocate onto the pool bottom as it was observed in the MA-1 test, inwhich the metallic part relocated onto the bottom had uranium concentration ~ 41 mass %.
However, in the RCW case, these parts did not relocate onto the bottom as they contained
sealed cavities with CO gas (that is most probable). The location of these parts along thecentral vertical line is associated with the extrusion of metallic parts by the corium
crystallization front on cooling.
The upper metallic body did not sink as it had a significantly less content of uranium(23 mass %) and density 9.07 g/cm3 that is a bit less than that of the corium (9.1 – 9.2 g/cm3).
The following progression of the process in the RCW test may be predicted for the extensionof the exposition time.
In the STFMFe-15 and MA-3 tests, the process of the corium–steel interaction was completed
and Zr oxidation degree in the corium achieved the equilibrium value Cn = 65 %. After the
RCW test was completed, Cn ≅ 53 % that was significantly less than the equilibrium value.
3.6.4 Peculiarities of FP Partitioning in the RCW Test

The analysis of fission products in the corium and the ingot metallic parts was performed by
the plasma emission-spectrum method. Table 3.35 presents the measurement results for thecontents of FP simulants in the corium. The evaluation inaccuracy is not higher than 10 %.
Ba, Sr and La accumulation and practical lack of Ru and Mo is observed in the oxidic
corium.
The analysis of FP concentration in the ingot metallic parts showed a high content of Mo in
all three groups and of Ru in the lower metallic part (Table 3.36).
Because the distribution of FP was not uniform the measurement errors were evaluated by theaveraging of results. For plasma emission spectrum method the samples taken from the RCW
facility ingot were used. Results of measurements presented in Table 3.35 were averaged.
Because they do not indicate any spatial distribution, the average value was calculated andstandard data deviations were assessed.
FP simulants Ba, La, Ce having oxides more stable thermodynamically than Mo, Ru aremainly contained in the oxidic corium and to a significantly less extent – in metals. Ru andMo are contained in the metallic parts. This can be clearly seen from the values of thepartitioning coefficient ko = xmet/xcor (Table 3.37) that characterizes the ratio of concentrations
in two phases being in the equilibrium state with each other under isothermal conditions.

79

Table 3.35. Results of the Corium Plasma Emission-Spectrum Analysis for the Contents
of FP Simulants
Mass portion of elements, %Sample
#

Sr

Ba

Ru

Mo

La

Ce

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

0.10
0.11
0.14
0.13
0.12
0.10
0.12
0.10
0.092
0.12
0.15
0.086
0.094
0.097
0.096

0.16
0.18
0.18
0.18
0.18
0.14
0.17
0.15
0.12
0.18
0.18
0.11
0.10
0.10
0.10

0.010
0.026
0.038
0.026
0.032
0.018
0.038
0.13
0.014
0.028
0.026
0.016
0.023
0.023
0.018

0.009
0.091
0.020
0.020
0.014
0.016

0.080
0.073
0.17
0.095
0.074
0.041
0.052
0.069
0.017
0.094
0.17
0.043
0.026
0.098
0.073

0.084
0.052
0.11
0.071
0.045
0.032
0.033
0.058
<0.01
0.071
0.14
0.052
0.020
0.084
0.080

Table 3.36. Results of the Plasma-Spectrum Analysis for the Ingot Metallic Parts
SampleMass portion of elements, %
Sr

Ba

Ru

Mo

La

Ce

1. Upper

0.015

0.013

0.017

0.15

0.020

<0.01

2. Middle #4

0.026

0.025

0.019

0.14

0.026

<0.01

3. Lower

0.002

0.003

1.33

1.10

0.015

<0.01

Table 3.37. The Coefficient of FP Partitioning Between the Ingot Metallic and Oxidic
Parts
FP

ko of the metallic part
upper

middle

lower

Sr

0.14

0.24

0.20

Ba

0.09

0.17

0.02

La

0.26

0.33

0.19

Ce

≤0.02

≤0.15

≤0.15

Ru

0.55

0.61

42.90

Mo

5.36

5.00

39.29

80

Table 3.37 employs FP concentrations in the samples of the oxidic corium containing steel
Xst. < 1 mass % (Table 3.32).
For the value of ko in the upper and middle metallic body, the same dependence is kept,
however, low values of ko for Ru require explanations.
On the corium melting before the introduction of steel, Ru heavy metallic particles may sinkwith some amount of dissolved Zr and U onto the pool bottom. This process was observed in
the STF tests. It was also possible that the sinking metallic part captured Ru from the corium.
This may explain a significant difference between ko for ruthenium in the lower metallic bodyand those in the middle and upper ones. It should be noted that the contents of all FP along
the oxidic corium height differ insignificantly as well as other corium properties: thechemical composition, oxygen concentration, Cn, density. Let us compare values of ko
obtained in different tests STFMFP-1, MA-3 and RCW, in which the C-32 corium was used
and the ratio Msteel/Mcorium ≤ 0.1. These data are presented in Table 3.38.
Table 3.38. Comparison of Values for Coefficients of Partitioning
Test

Ru

Mo

Ba

La

Ce

Sr

RCW (lower metallic part)

42.9

39.29

0.02

0.19 <0.150

0.020

STFMFe-1

15.8

15.80

0.13

0.40

0.050

0.160

MA-3

20.8

34.00

1.10-3 3.4⋅10-2

0.036

0.021

Table demonstrates that described above conformities for the values of ko are observed in all
tests. The paper [38] presents data on FP partitioning in the melt UO2 - iron (Table 3.39).
Table 3.39. Comparison of Results with the RCW Data
Ru

Mo

Ba

La

Ce

xmet (FP contents in iron), mass %0.21 - 1.14

17⋅10-3

1⋅10-40.02

xcor (FP contents in corium), mass %

0.01

0.1

0.51

0.35

0.68

ko = xmet/xcor[38]21 - 114

10

1.3⋅10-2 2.8⋅10-4

0.03

ko (RCW: lower metallic part)

26

22

10-2

0.27

0.25

Table 3.39 shows that FP behaviour in the system UO2 – iron is similar to that in the system
corium – steel.
It should be noted that data on FP contents in the metallic part containing U and Zr wereobtained in the RCW large-scale test. This is of a great technical interest for the assessmentof heat generation in the analysis of severe accident scenarios.
3.6.5 Conclusions

1. The performed RCW test gave the opportunity to study individual stages in the progressof the oxidic corium interaction with steel:• The metallic part large mass (content of U ≅ 52 mass %, density = 10.69 g/cm3,
mass = 4.3 kg) relocated onto the liquid pool bottom.
• The medium mass of the metallic part (content of U ≅ 23 mass %,density = 9.07 g/cm3, mass = 2.44 kg) was settled on the melt surface.
• A small mass of metallic parts of the spherical form (content of U ≅ 39 mass %,density = 9.7 g/cm3, mass = 735 grams) was settled in the melt central part.
81

2. The oxidic corium has uniform properties (U/Zr ≅ 1.29 ±0.03; Cn ≅ 53 ± 0.03;
density = 9.0 - 9.2 g/cm3). Clusters of steel crystals with a variable content of U and Zr
are observed in the oxidic corium near the metallic parts. It may be assumed that steel
clusters may generate new metallic parts or join the existing ones.
3. The scale factor allowing to observe different stages of steel fragmentation, saturation ofthat with uranium and zirconium and relocation of fragments onto the liquid pool bottom
is the peculiar feature of the RCW test in comparison with the tests STF and MA.
4. The test confirmed the behaviour of FP simulants in the corium melt observed earlier,namely: Ba, Sr, Ce, La are concentrated in the corium oxidic part, Ru and Mo – in themetallic parts.

82

4 MIGRATION OF MOLTEN METAL THROUGH POROUS OXIDEDEBRIS
4.1 Introduction and tests objectives
The report containing the generalization of test results on investigation the metal alloy
behavior in the corium particle bed was issued based on recommendations of the PRG-4 [39].
In 1998 an expert group from the NEA/OECD recognized that the migration of the moltenmaterials was one of the domain of essential uncertainties due to limited base of experimental
data and consequently insufficient knowledge had an effect on the simplification of models
employed in the codes assessing the consequences of severe accidents [40].
In case of accident, it is assumed that relocated molten materials come into water andfragmented into debris of quite small sizes. This process was studied in detail in the FARO
facility [41]. At the average, 50 % of particles had a size less than 4 mm in all tests.
According to the data of the reference [42], from 3 up to 10 % of elements had a size lessthan 1 mm after the accident at the TMI-2.
Thus, the matter concerns the significance of studying some phenomena taking place underrelatively low temperature (ranging from 1200°C up to 2000°C) when that of the corium
particles is still far off the melting temperature but liquid metallic phases are alreadyappearing. Relocation of molten metals in the lower head and its penetration may accompany
by the interactions with the corium debris.
The question of interactions between molten zirconium and uranium dioxide is one of theconcerns. Such interactions were studied earlier [43, 44 and 45] and it was found that thisprocess may influence the formation of molten materials.
Thus, the objectives for the tests were to study migration of molten metals formed in courseof accident through the porous corium debris. The parameters varied were the major
parameters, which may determine the interaction process, such as:• environment composition (inert or oxidizing)• sizes of particles;• compositions of metallic melts;• the temperature variation rate, gradient, and maximum value;• the corium composition (suboxidized, completely oxidized);• the test scale.
To achieve experimental goals a total of 16 small-scale tests [46, 47] were conducted by now.
Moreover supporting tests were conducted using the standard laboratory furnace withtungsten ohmic heater (D1 – D3) and STF facility. The modifications of the RCW-Ox
facilities were used to study the oxidation environment effects [47, 48, 49, 50]. In addition
one larger scale test T-8 was conducted using TULPAN facility [46].

4.2 Test Apparatus
4.2.1 KORPUS Facility

Coria of different compositions namely C-32, C-70, C-100 compositions with the atomic
ratio of U/Zr = 1.2 were employed in the tests. Most part of experiments was conducted withthe corium particles loading with the sizes of particles 1.2 - 3.5 mm, and one test (the K-1
test) was with the corium fragments of ~ 8 × 8 × 10 mm size.
83

The molten metals were presented by the steel of the St302 quality, stainless steel
12X18H10T3, and by the alloy of stainless steel (85 mass %) and zirconium (15 mass %).
The corium fragments and particles were produced by cutting or crushing of the corium
sintered briquettes (the C-32 corium was sintered under the temperature 1700°C, the C-70
and C-100 coria were sintered under the temperature 2000°C). The particle fractions wereprepared by the use of two riddles with openings of 1.2 and 3.5 mm. Figure 4.1 illustrates thesize distribution of the corium particles.

C-100

C-32
Figure 4.1. The Size Distribution of the C-32 and C-100 Coria

Tests were performed in the KORPUS facility in the argon atmosphere (Figure 4.2). The
heating device consisted of a resistive graphite heater of a cylindrical geometry and up to
20 kW power. A tantalum cup in which studied test assemblies were placed was located in
the heater. The protection of test facility from the influence of the graphite heater externalcarbon atmosphere was provided by the maintenance of excessive pressure under this cup.
test assemblies, K-7, K-8, and K-9, were tested simultaneously in one heating device. Test
conditions for these assemblies were the same ones. Energy to heat a test assembly wastransferred by the radiation from the graphite heater to the tantalum cup and from the Ta cup
to the test assembly.
Temperature was measured within the temperature range 1400°C – 2300°C by W/Re
thermocouples (TCs) and optical pyrometer calibrated for the tungsten radiating surface. The
measurement error was ± 20°C. Pressure in the protection chamber was measured by adifferential sensor with the measurement range ∆p = 760 mm Hg. Readings from all sensors
were recorded by the automatic measurement system.
The technique for all tests conduction was similar and consisted in several steps, which aredescribed in details in the report MP-TR-5 [46]. Tests K-11 and K-12 [47] of the MASCA
project second stage were performed basing on the same technique. Typical temperature
history for all tests is shown in Figure 4.3 using as an example temperature measurements in
the K-5 test.

2
3

Steel St30 composition(wt.%): 0.27-0.35 C, 0.5-0.8 Mn, 0.25 Cr, 0.25 Cu, and Fe base
Stainless Steel 12X18H10T composition(wt.%): 70 Fe, 18.1 Cr, 10.2 Ni, 0.78 Mn, 0.63 Si, 0.41 Ti, 0.12 C

84

T1

T2

8
10
1
2

30

D 28

5

70

4

~20

3

ПИР. 5

D 30

6
7
9

D 40

8

Ar

1 - graphite heater; 2 - Ta cup; 3-steel;
4 - W crucible; 5 - corium loading; 6 - ZrO2 (orUO2) crucible; 7 - W support; 8 - carbon-fibrousthermal insulation; 9 - graphite disc; 10 - Mo
casing with ZrO2 cladding.

Facility View

Figure 4.2. Layout of the Heating Device with the Test Assembly in the KORPUS
Facility and Facility View

As a rule, one test assembly was placed into the heating device in each test. In one case three
2500
T1
T2
Pyr 5

Temperature (oC)

2000

1500

1000

500

0
0

60

120
Time (min)

180

240

Figure 4.3. Typical Temperature History for the Series of Tests K-1 – K-10 (Test K-5)85

4.2.2 Laboratory Tests in Carbon Free Atmosphere with a Tungsten Crucible

As K test series was conducted in a carbon heater three supporting tests were conducted in
the laboratory furnace with a tungsten heater [46] to study the possible carbon influence on
the test results. The tests in carbon free atmosphere were conducted with the C-32 corium
briquette of the size of 15 × 15 × 15 mm. Steel St-30 or SS ring plate (See Figure 4.4) was
placed onto the briquette. The space between the steel ring plate and a tungsten wall was
filled with the C-32 corium particles. The debris temperature was measured by the W/Re
thermocouple.
Test D-1 through D-3 were performed under the temperature T = 1800°C in argon
atmosphere with the overpressure 0.3 atm. The exposition time under the temperature
T = 1800°C was 30 minutes.
In the D - 2 test the mixture of SS (85 mass %) and Zr (15 mass %) was used. The sample
was of a pyramid shape with the area contacting with the corium ~ 1.5 cm2 (Figure 4.5).

6

1

2

1.
2.
3.
4.
5.
6.

3
4
5

Steel St-30 or SS 12X18H10T;
C-32 particles;
C-32 briquette;
Tungsten crucible;
Tungsten heater;
W/Re thermocouple.

Figure 4.4. The Layout of the Tests D1 and D3
6
1

1.
2.
3.
4.
5.
6.

2
3
4
5

SS(85 mass %) and Zr(15 mass %);C-32 particles;
C-32 briquette;
Tungsten crucible;
Tungsten heater;
W/Re thermocouple.

Figure 4.5. The Layout of the D2 Test
4.2.3 The TULPAN T-8 Test

The T-8 test was performed in the TULPAN facility to study the interaction of the C-32
corium particles with stainless steel in non-isothermal conditions and in a larger scale. One of
the goals was to determine the depth to which molten steel may penetrate through the porous86debris. The goals for the posttest examination was also to study interactions occurred duringpenetration.
The TULPAN facility represents a gas-vacuum induction furnace with a graphite heater. The
inductor power at the frequency 1000 Hz is up to 250 kW. The temperature gradient wasmaintained by the radiation from the outer surface of the test unit.
Figure 4.6 illustrates the test section layout and test unit itself is shown in Figure 4.7.
The intermediate steel bottom (1) was located in the steel protective casing (2). The
protective casing of 340 mm diameter and 120 mm high was placed on the support (3). Twotiers of the C-100 corium briquettes (5) serving for radial thermal insulation were put on theintermediate bottom. The C-32 corium grits (8) were loaded into the gap between thebriquettes and protective casing. ZrO2 crucible of 174 × 10 mm diameter and 90 mm high (4)was located inside the cavity between the C-100 briquettes. The crucible was filled with alayer of the C-32 corium particles (6) 55 mm thick and of 154 mm diameter. A stainless steel
X18H10T plate ~ 25 mm thick and of 152 mm diameter (7) was situated on this layer.
The temperatures in the facility were monitored by the system of thermocouples measuring
the temperature on the structures, in thermal insulation and in the gas medium.

1
2
8
3
4
5

2400°C

q
7

q

q

6

1 – graphite pyrometer tube;
2 – inductor;
3 – graphite heater;4 – soot;
5 – thermal insulation (TKM);
6 – Test Unit;
7 – the inductor non-operated section;
8 – thermocouple;

Figure 4.6. Layout of the T-8 Test Section and Facility View

87

7

8

6
340
240
216
154

17

16

14
13

3

11

12

5

4

120

90

1
2

10

6
7

15
8

9

18

1

2

1 – intermediate bottom;
5 2 – protective casing;3 – support;
4 – crucible;
4
5 – C-100 corium briquettes;
6 – C-32 corium particles;
7 – steel plate;
8 – C-32 corium grits;3• - thermocouples

Figure 4.7. Test Unit Layout in the T-8 Test

Temperatures in test unit were monitored by the system of thermocouples consisting of 8
Cr/Al thermocouples and 10 W/Re thermocouples. Figure 4.7 shows the schematics for thelocation of thermocouples. Thermocouples (TCs) were located symmetrically relative to the
TU central axis approximately in one and the same diametral plane:• 9 W/Re TCs 5-20 were located in the C-32 corium particles at 3 levels along theheight and at 3 radii;• 1 W/Re TC 5-20 was located in the opening in the steel plate.
In the test course, the steel maximum temperature achieved ~ 2050°C. The temperature of thecorium particles loading at the lower level of thermocouples was ~ 1300°C (T9).
4.2.4 Modifications of the RCW-Ox Facility for RCW-ST1 and RCW-Ox2, 3
Tests

Tests studying the interaction of the steel melt with the corium particles in the oxidation
medium were performed in the facilities RCW-ST1 (in argon-air atmosphere) [47] and in
RCW-Ox2 (in argon-steam atmosphere) [49] with the C-32 corium in which the sizes ofparticles were 1.2 – 3.5 mm.
Experimental sections of these facilities (Figure 4.9, Figure 4.10) were located in theprotective chamber RASPLAV of ~ 8.5 m3 volume filled with argon. The subject under study
is the heated ceramic crucible loaded with the corium particles, which with the inlet andoutlet ceramic pipes forms a vertically oriented operating channel of the experimental
section.
In the test RCW-ST1, the flow of argon-air mixture was fed into the experimental cell in topdown direction (Figure 4.9) and in the test RCW-Ox2, argon-steam mixture was fed in thebottom-up direction (Figure 4.10).
For the mixture uniform penetration through the crucible bottom and particle layer, thecrucible ZrO2 – ceramic bottom was made multislit.
The section heater components – the inductor and tungsten heater coaxial with the operating
channel – were separated from the flow of argon-steam mixture by walls of the oxidezirconium channel.

88Figure 4.8. View of the RCW-ST1 Facility
D 78

A-A

Air+Ar

D 45

Steel

A

A

Figure 4.9. RCW-ST1 Experimental Section

89

1

8

2
3
4

9
10
11
12
13
14

5
6
7

C-32

Tcor1
Tcor2

UO2

15
16
17
18
19
20

1. Heat exchanger;
2. Steel charge;
3. View channel
4. Water;
5. Tungsten heater
6. ZrO2 crucible;
7. ZrO2 powder;
8. Nichrome wire
9. Nb tube
10. ZrO2 outlet tube

1. Nb lid;
2. UO2 powder;
3. Crucible lid (ZrO2);
4. C-32 corium particles;
5. UO2 particles;
16. W/Re thermocouples;
17. Crucible bottom;
18. Inductor;
19. ZrO2 support plate;
20. ZrO2 inlet pipe

Figure 4.10. RCW-Ox2 Experimental Section

In the test RCW-ST1, the steel load was settled in the corium particle bed and in the test
RCW-Ox2, the load of stainless steel was hanged by a nichrome wire in the heat exchanger
central pipe (Figure 4.10). At the set moment of time, the wire was burnt through and steel
was dropped on the corium particles.
The temperature of the corium particle bed was measured in the test RCW-ST1 by the
pyrometer, and in the test RCW-Ox2, two W/Re thermocouples measured the temperature in
the volume of particles (at the 6 mm distance from the inner surface of the crucible
cylindrical part) at 6 mm (Tcor1) and 28 mm (Tcor2) beneath the surface of particles.
Gas flows were controlled by differential pressure gages, gas flowmeters, thermocouples andsensors of concentration of water steam, hydrogen, carbon oxide, and oxygen.
Figure 4.11 illustrates the course of the corium temperature of argon and air flow rates in thetest RCW-ST1. Figure 4.12 and Figure 4.13 presents the temperature in the corium particles

90and concentrations of water steam and hydrogen in gas mixture in the course of the tests
RCW-Ox2 [49] and RCW-Ox3 [50], correspondingly.
2400

0.3

2000

Temperature, oC

1800
1600

0.2

1400
1200

Ar+air

1000
800

Gas rate, l/s

Pyr
W/Re T/C
Gas rate

2200

0.1

600

Ar

400

Steel melting

200
0

0
7200

7800

8400

9000

9600

10200 10800 11400 12000 12600 13200

Time, sec

Figure 4.11. Thermogram of the RCW-ST1 Test
Tcor2
Tcor1
H2Oin

2000
Steel addition

H2Oout
H2out

1200

4

800

2

H2, rel%

6

020
16
12
8
4
0

400

0
40

60

80

100

120

140

Time, min

160

180

200

H2O, rel%Temperature, oC

1600

220

Figure 4.12. Variation of Experimental Parameters during the RCW-Ox2 Test

For both tests the crucible assembly with the C-32 corium debris bed heated by the radiationof the tungsten heater up to the temperature 1800°C is blown by the flow of the steam-argon
mixture through the slotted bottom that results in oxidation of free zirconium in the debris
bed. After the expiration of the pre-set time, a stainless steel charge is dropped onto the debris
bed, and the process of debris and steel melt oxidation is continued in the steam-argon
mixture blowing through.

91

Time from heating start, sec.
5400

7200

Tcor1
Tcor2

2000

Temperature, Co

3600

10800

12600

Steel addition
T

Pyr
H2O
H2

1500

9000

CH2

10

1000

5
0

500

10

jH2O

5
0

0
0

30

60

90

120

150

CH2, vol.%

1800

180

jH2O, %

0
2500

210

Time from heating start, min.
Figure 4.13. The Temperature in the Corium Debris Bed and Concentrations of Water
Steam (at the Section Inlet) and of Hydrogen (at the Section Outlet) during the
RCW-Ox3 Test

Initial conditions for tests RCW-Ox-2 and RCW-Ox-3 were similar:
Parameter

RCW-Ox2

RCW-Ox3

1.2 – 3.5 mm

1.2 - 3.5 mm

Mass of corium debris
Mass of free zirconium in debris, g

630 g

650 g

91.3

94.3

Bottom layer of UO2 particles, g
Steel type
Steel mass

108
12X18H10T
48 g

110 g
12X18H10T
48 gCorium C-32 debris size

Scenarios of tests RCW-Ox-2 and RCW-Ox-3 were as follows:
Parameter

RCW-Ox2

RCW-Ox3

Argon flow rate- at the inlet:

0.264 g/s

0.165-0.17 g/s

Argon flow rate- at the outlet

0.528 g/s

0.34-0.35 g/s

Time of steam supply, min

122

98

0.012 g/s

0.015 g/s

Duration of steam supply, min

50

112

Operating temperature, °C

1800

1800

Steam flow rate, g/s

92

On the steam-argon mixture incoming into the experimental section, the concentration ofwater steam is controlled by the H2O–sensor of humidity, and on the gas mixture coming outof the section, the H2–sensor of the measuring unit measures the concentration of hydrogengenerated due to oxidation processes in the crucible assembly loading (Figure 4.13 andFigure 4.14).

30000

Time from DAS start, sec.
32000

34000

36000

12

9

CH2

8

8

6

7

4
2

6

0

5
4

H2

H2, mole

CH2, vol.%

10

10

3
2
1
0

0

10 20 30 40 50 60 70 80 90 100 110 120 130

Time from steam addition, min.
Figure 4.14. Hydrogen Concentration (at the Section Outlet) and the Integral
Dependence of Generated Hydrogen Quantity of the RCW-Ox3 Test

On steam supplying, the increase in the debris temperature due to the additional heatgeneration on the corium C-32 oxidation by steam in the following reaction can be assessedusing the reaction:
Zr + 2H2O = ZrO2 + 2H2 + 625 kJ/mole ZrO2.
This should be expected as the steam-zirconium reaction power in this test was limited by thevelocity of steam supply into the reaction area. At the rate of steam supply equal to 0.67
mmlos/s for Ox2 test and 0.833 mmole/s for Ox3 test, the rate of ZrO2 generation was about
0.33 and 0.416 mmole/s, that limited the chemical reaction power of 206 and 260 W
correspondingly. This value allowed stable control of the reaction thermal effect.
For test RCW-Ox2 the corium C-32 mass was equal to 630 g and contained about 91 g of
Zrfree (the content of Zrfree in the corium C-32 is ~ 14.5 % wt). To oxidize this quantity ofZrfree up to ZrO2 1 mole of O2 is required that is equal to 32 g of O2. Oxygen consumption
from water steam in the test course is approximately equal to 36.8 g that exceeds by 15% thenecessary oxygen quantity for the debris oxidation up to C-100.

93

For RCW-Ox3 test 1.04 mole of free zirconium and 0.866 mole of stainless steel components
approximately 2.07 moles and 0.945 moles of steam are respectively required. The oxidation
times at flow rate 0.833 mmole/s were 42 and 19 minutes, respectively.
As one can see from Figure 4.14 the oxygen consumption rate and correspondingly theoxidation rate was ~ 2.5 times reduced and kept the tendency towards the further.
A total quantity of hydrogen generated by the test end exceeds the estimated hydrogen
quantity generated on the oxidation of debris and stainless steel by 45 - 50%. This is possibly
caused by the interaction of water steam with some structural materials of the experimental
section.

4.3 Results of Experiments in the Inert Atmosphere
Tests objectives were to study the molten metals progression through the corium porous
debris and interactions between molten metal and the solid corium particles under variableconditions. In the tests different parameters varied such and temperature, porosity of thedebris and ratio of the liquid metal volume to the pores volume in corium. As a rule, the steel
melt volume exceeded the volume of pores in the corium loading except of several tests,
which were conducted with the corium briquettes.
After tests were completed, crucibles were filled up with epoxy and cut along the diameter inthe vertical direction. A macrosection was prepared from one crucible half in which a general
pattern of the metallic melt interaction with the corium particles corium was studied.
Microsections of individual interaction zones were prepared from the second crucible half toobtain images with a large magnification and to determine the element composition ofindividual phases. The methods of optical microscopy (Neophot 30) with a quantitative
element analysis in the micro analyzer Comebax 50SX were employed for the study.
Three different series of experiments were conducted in the inert atmosphere with the molten
steel ST30, stainless steel and zirconium steel alloys. For each series the corium composition
varied between C32 and C100. Special series of experiments was conducted in the oxidizing
atmosphere when steam or air was supplied to the test facility.
4.3.1 Behavior of the Carbon Steel ST-30 Melt

The test matrix for the steel ST-30 is presented in Table 4.1 [51]. In frame of MASCA1
program four experiments were conducted to study the interaction of St-30 steel with thedebris made of C-32 corium, and one test utilized C-100 corium.
In the test K-1, the corium pellets were used and molten steel penetrated into the gaps
between the fragments of the C-32 corium briquettes in all layers and reached the cruciblebottom at relatively low temperature of 1600oC. The minimum gap in which steel was able topenetrate was about 0.2 mm (See Figure 4.15).
In the tests K-2 and K4 the debris was formed of particles (See Figure 4.1). At temperature of
1600OC molten steel penetrated into the C-32 corium loading to the depth of ~ 2.8 mm(Figure 4.16). Temperature increase to 1800oC caused penetration to the bottom of the
crucible as it is shown in Figure 4.17. This figure demonstrates also the microstructure ofsamples cut out of the ingot upper and lower parts and the microprobe analysis results. One
can see that partial interaction of steel with the corium particles took place in the crucible
lower part resulted in the mixture melting. This is indicated by the ingot structure containing
the iron matrix with dendrites of intermetallic compounds Fe3Zr and Fe3U with the impurityof zirconium

94

Table 4.1. Test Matrix and Major Parameters of Tests with molten steel ST-30
Corium
#Cn initial(Cn final)

Metal

Mass, Porosity,
Mass, V
QualityVg%gliq. Mepore

Testtemperature, Environment
°C

Metal
penetration into theloading

K-1

C-32

117.00

38.0

St-30

95.50 1.70

1600

Ar

down to bottom

K-2

C-32

80.00

~50.0

St-30

95.50 1.33

1600

Ar

~ 2.8 mm

K-4

C-32

81.00

48.0

St-30

64.80 0.96

1800

Ar

down to bottom

D-1

C-32

10.0

St-30

14.6

0.5

1800

Ar

down to bottom

K-3

C-100

43.0

St-30

96.00 1.48

1600

Ar

~ 0.2 mm

184

St-30

15

2160 max

Ar+air~ 10 mm

650

St-30

58

1750 max

Ar+steamno data

C-32(Cfin=65-75)
C-32
RCW-ST2(Cfin= - )RCW-ST1

90.30

95

Initial carbon
content:
Metal;
Corium,mass% (g)Me: 0.3 (0.29g);
C: 0.01 (0.01g)
Me: 0.3 (0.29g);
C: 0.01 (0.01g)
Me: 0.3 (0.19g);
C: 0.01 (0.01g)
Me: 0.3 (0.04g);
C: 0.01 ( )Me: 0.3 (0.29g);
C: 0.01 (0.01g)
Me: 0.3 (0.05g);
C: 0.01 (0.02g)

Figure 4.15. Macrostructure of the K-1 Assembly Vertical Section
21

Fe0.7Zr0.3

2.8

24

15

(U0.7Zr0.2Fe0.1)O1.7

20 µ mFigure 4.16. Macrostructure and Results of the Element Analysis for the C-32 Coriumof K-2 test

96

Figure 4.17. Macro- and Microstructure from the Test K-4

Similar metallic phases were found in the interaction zone of the K-2 test (Figure 4.16).
Thus, the temperature increase by 200°C in comparison with that in the test K-2 resulted in
the complete leaking of liquid steel St-30 through the C-32 corium particles in a partial
melting of interacted products in the crucible lower part.
As experiments were conducted in the graphite crucible to tests possible influence of graphite
additional experiment D1 was performed in the tungsten crucible. A piece of steel was put on
the C32 corium briquette with the porosity about 10%. It was found that steel St-30
penetrates through the pores of the corium briquettes, which characteristic size as one can see
from Figure 4.18 was about 20-30 micrometers. The metallographic study demonstrated
vertical streams of the light metallic phase (Figure 4.18) observable throughout the volume of
the C-32 corium.

Upper part of the ingot

Middle part of the ingot

Figure 4.18. Microstructure of the C-32 Briquette Upper Part after the Test D1

97

MetalPost test examination showed that during interaction eutectic Fe3Zr melt was formed withsome amount of Fe3(ZrxU1-x) phase as one can see from the Figure 4.17.
Experiment K-3 conducted with the C-100 corium demonstrated significantly less penetrationof the steel in the corium (0.2 mm at 1600oC). The penetration pattern was not the continuous
filling of pores but steel diffusion over surfaces of the corium particles. Steel did not
penetrate deeper into the particles and the latter freely spilled out of the UO2 crucible. The
reason may be that there is no wettability of solid UO2 by molten steel.
In the test with the C-100 corium (Figure 4.19) the following basic phases were observed:• of the (U0.9Zr0.1)O2.0 oxidic type with the impurity of Fe and• of iron with the impurity of oxygen (3 - 8 at%).

С-100 particles

Fe(U0.9Zr0.1)O2.0

100 µ mFigure 4.19. Microstructure and Results of the Element Analysis for the C-100 Corium
Particles

From Table 4.1 one can see that in inert argon atmosphere the steel ST30 melt penetrated tothe bottom of crucible in the C32 debris when temperature exceeded 1800oC (Tests K-4 andD-1). At lower temperature T=1600oC in the test K-2 penetration was small ~3 mm. The
reason was formation of iron carbides Fe3C at the melt surfaces which melting point was
Tm=1650oC which is higher then test temperature in the test K-2. Experimental results above
1800oC demonstrated consistency of data obtained in K and D test series. Special experimentconducted at STFM facility to determine the threshold of steel ST-30 penetration throughdebris bed showed that at temperature 1670oC molten steel penetrated through the debris bed.
This temperature threshold is slightly higher then the melting point of iron carbide.
The penetration observed in test K-1 at the same conditions is conditioned by larger particle
size since in this experiment pellets made of C32 corium were used instead of debris
particles.
Increase of oxidation degree led to the decrease of wettability of corium particles. Thisphenomenon explained low penetration observed in the test K-3 with C-100 corium particles.
4.3.2 Behavior of Stainless SteelThe test matrix of experiments conducted with the stainless steel is presented in Table 4.2[51]. Two of four experiments were conducted at temperature 1800oC and two otherexperiments at the temperature 2050oC.
Stainless steel of the following chemical composition given in mass% was employed in thetest K-10: Cr - 18.1; Ni - 10.2; Mn - 0.78; Si - 0.63; Ti - 0.41; Mo < 0.01; Fe – the rest part.

98

Table 4.2. Test Matrix and Major Parameters of Tests with molten stainless steel
Corium
#Cn initial Mass, Porosity,(Cn final)
g
%

D-3

C-32

K-10

C-32

T-81)
K-11

Quality

Mass, VVgliq. Mepore

Testtemperature, Environment
°C

Metal
penetration into theloading

10.0

12X18H10T 14.6

0.25

1800

Ar

down to bottom

248.9

51.2

12X18H10T 259.46 1.11

2050

Ar

down to bottom

C-32

4026

51.0

12X18H10T 3450

1.0

2050

Ar

down to bottom

C-32

259.6

~45

12X18H10T 227.7

-

1800

Ar

3 – 5 mm

C-32
(Cfin= - )C-32
RCW-Ox2(Cfin=100)
RCW-Ox1

RCW-Ox3

Metal

C-32

700

12X18H10T

45

1800

Ar+airno data

630

12X18H10T

48

1800

Ar+steam~ 20 mm

650

12X18H10T

48

1800

Ar+steam

down to bottom

1) Non isothermal test conditions.

99

Initial carbon
content:
Metal;
Corium,mass% (g)Me: 0.12 (0.02g);
C: 0.01 ( )Me: 0.12 (0.31g);
C: 0.01 (0.02g)
Me: 0.12 (4.14g);
C: 0.01 (0.40g)
Me: 0.12 (0.27g);
C: 0.01 (0.02g)

Me: 0.12 (0.06g);
C: 0.01 (0.06g)
Me: 0.12 (0.06g);
C: 0.01 (0.06g)

In general the behavior of stainless steel observed in the tests K-10 and D-3 was similar to the
behavior of St-30 steel.
In the test D3 conducted in the tungsten crucible molten steel leaked through the corium
briquettes with the porosity about 10% at temperature of 1800oC as it is shown in Figure
4.20. In the test K-10 molten steel under T = 2050°C leaked through the C-32 corium
particles and partially leaked out of the crucible due to the crucible bottom degradation(Figure 4.21a).
Figure 4.21b shows the C-32 corium particles that partially interacted with steel. In the area“1”, a good wettability of the corium particles by liquid steel (the wetting angle ~ 45°C) isseen well. Figure 4.21c demonstrates the interface between the corium C-32 particle andliquid steel. A partial steel penetration into the corium is also seen clearly as well as the
produced intergranular streaks of intermetallic compounds of Fe-U-Zr similar to thoseobserved in the test K-2.
In experiment K-11 conducted at 1800°C a non-uniform penetration of liquid stainless steel
into the near-surface layer of corium debris was observed. The penetration depth was in therange of 3 - 12 mm (Figure 4.22).

The C-32 briquette upper part

The C-32 briquette middle part

Figure 4.20. Microstructure of the C-32 Briquette Upper Part after the Test D3a)b)

Figure 4.21. Microstructures of the C-32 Corium Particles and of the Leaked Steel
12X18H10T in the test K-10

100

c)

Figure 4.22. K-11 Crucible Macrosection

Additional information on steel behavior was obtained from the test TULPAN T-8 [46]. This
test was conducted in the conditions of the vertical temperature gradients. The cross sectionof ingot obtained after the test is shown in Figure 4.23.

Figure 4.23. Macrosection of the T-8 Tests

On examining the macrosection of the ingot central section, it was found that steel was
melted and leaked through the corium particles down to the depth ~ 35 mm from the lower
surface of initial steel filling the porosity among particles. Approximately 25 % of initial steel
formed a layer of about 8 mm thick. ZrO2 crucible walls were wetted by steel (Figure 4.23),
the wetting angle Θ ≅ 70°. The thickness of the corium particle layer was reduced after thetest by ~ 12 mm4 (~ 20 % of the initial thickness).
Thermocouples T7 and T9 located approximately 3 mm lower than the lower front of theleaked alloy showed the temperatures 1170°C and 1310°C, respectively, that approximatelyby 100 - 250°C lower than Tmelt of steel.
Figure 4.24 shows the macrostructure of the sample #1 (Figure 4.23) exposed at temperature
T ≅ 1600°C Two basic phases were observed in the sample #1:
1. The light one among the corium particles (1.1.) was of the steel composition withsome abnormal Cr/Ni ratio and with striations of the (Fe0.48Cr0.05Ni0.21)Zr0.2U0.06

4

Average porosity reduced between 7 to 20 % usually.

101composition observable only in electron rays. These two phases did not containoxygen.
2. The spotty phase representing the corium particles consisted of grey grains (1.3)
typical of the corium (U0.8Zr0.2)O2 of the oxidic phase and of the light metal phase(1.2) presenting steel with a low impurity of U.
Figure 4.25 illustrates the microstructure and the microprobe analysis results for the ingot
lower part (Sample #2).
Filling of gaps among the corium particles was not observed in the lower sample but separate
corium particles were observed that interacted with steel according to the mechanism“vaporization-condensation-diffusion” generating oxygen-free intermetallic compounds(UZrFeCrNi) as it was proposed for the explanation of results of the ACRR-DCC experiment.
Grey impurities were close in composition to U6Fe.
From Table 4.2 it is seen that molten stainless penetrated into C-32 corium debris up to the
crucible bottom at temperature T=2050oC (the experiments K-10). At lower temperature
T=1800oC the melt penetrated well in the C32 debris in the experiment installation withtungsten heater (the experiment D-3) and demonstrated smaller penetration in the test K-11
conducted in the graphite crucible. One of possible reasons is formation of chromium
carbides which meting temperature was Tm=1890oC. Formation of carbides at the melt
surface may change the wettability of corium particles by molten steel.

1.1 Fe0.7Cr0.25Ni0.05; Fe0.48Ni0.21Cr0.05Zr0.2U0.06
1.2 Fe0.7Cr0.23Ni0.06U0.01
1.3 (U0.8Zr0.2)O2.0

Figure 4.24. Macrostructure of Sample #1(the Ingot Middle Part)a.
b.

2.1. Zr0.43U0.24Fe0.21Ni0.12 ; Zr0.33U0.02Fe0.47Cr0.15Ni0.03
2.2. U0.84Fe0.14Zr0.01Ni0.01

Figure 4.25. Microstructure of the Sample #2 (the Ingot Lower Part)102

4.3.3 Behaviour of Zr Alloys

Several experiments as it is shown in Table 4.3 [51] were conducted with the alloy made of
85 mass % of SS + 15 mass % of Zr. Figure 4.26 illustrates macrosections from the tests K-7,
K-5, K-8, K-6, and K-9. It can be seen that the metallic melt (SS + Zr) leaked through thecorium particles in neither of the tests. Wetting of the ZrO2 crucible walls by the melt was notalso observed.
The penetration depth depended upon temperature and corium composition. For example, in
the K-7 test the penetration was about 1.3 mm while with temperature increase thepenetration increased also, 3.2 mm for the K-5 test at 2050oC. Comparing K-7, K-8 and K-9
tests conducted at the same temperature of 1800oC, the penetration depth decreases with theincrease of oxidation degree: from 1.3 mm for C-32 to 0.71 and 0.34 for C-70 and C-100
correspondingly.
K-7
C-32

K-5
C-32

K-8
C-70

K-6
C-100

K-9
C-100

1800°C

2050°C

1800°C

1600°C

1800°C

Figure 4.26. Macrosections of Crucibles after the Tests K-6, K-7, K-8, K-9, K-5
Interactions with the Highly Oxidized Corium
Wetting of the C-70, C-100 coria particles by the melt and the melt leakage into the gaps
among the particles were not observed in all three tests. The melt interaction with a highoxidized corium C-100 was studied in more detail in the test K-6 taken as an example.
Figure 4.27 shows the macrostructure of the melt (SS + Zr) boundary layer with 2 - 3
particles of the C-100 corium (the test K-6, T = 1600°C). Three layers separated by clear
boundaries being “parallel” to the surface of the contact with the melt were observed in the
C-100 corium particle located in the direct contact with the SS + Zr melt. The layers are 0.5,
0.9, 0.7 mm thick beginning from the melt boundary.

103

Table 4.3. Test Matrix and Major Parameters of Tests with molten zirconium alloys
CoriumMetal

Cn initial
Mass, Porosity,(Cng%final)#

Quality

Mass, VVgliq. Mepore

Testtemperature, Environment
°C

Metal
penetration into theloading

Initial carbon
content:
Metal;
Corium,mass% (g)Interaction of zirconium alloys with corium debris
K-72)

C-32

45.37

51.0

85SS/15Zr1) 43.00 1.01

1800

Ar

1.27 mm

K-5

C-32

80.20

49.0

85SS/15Zr 63.70 0.92

2050

Ar

3.2 mmD-2

C-32

85SS/15Zr

0.25

1800

Ar

Partial3)

K-82)

C-70

46.33

46.0

85SS/15Zr 44.00 0.98

1800

Ar

0.71 mm

K-6

C-100

65.90

41.0

85SS/15Zr 46.15 1.06

1600

Ar

1.4 mmK-92) C-100

67.86

41.0

85SS/15Zr 44.00 0.98

1800

Ar

0.34 mm

K-12 C-32

247.2

~45

99Zr+1Nb

2050

Ar~1.5 mm

41.2-1) Alloy of 85 mass % of SS and 15 mass % of Zr.
2) Assemblies K-7, K-8 and K-9 were placed inside one heater and were heated simultaneously.
3) A part of the metal load enriched in carbon remained at the surface of the corium briquette.

104

Me: 0.1 (0.04g);
C: 0.01 (0.005g)
Me: 0.1 (0.06g);
C: 0.01 (0.008g)
Me: 0.1 ( );C: 0.01 ( )Me: 0.1 (0.04g);
C: 0.01 (0.005g)
Me: 0.1 (0.04g);
C: 0.01 (0.007g)
Me: 0.1 (0.04g);
C: 0.01 (0.007g)
Me: 0.1 (0.04g);
C: 0.01 (0.02g)

1.
2.
3.
4.
5.

[(Fe0.71Cr0.19Ni0.10)0.87Zr0.12U0.01]×C0.02[(Fe0.69Cr0.20Ni0.11)0.61Zr0.35U0.04]×C0.12O0.39[(Fe0.69Cr0.21Ni0.10)0.44Zr0.27U0.29]×O0.8(U0.50Zr0.50)O1.8(U0.55Zr0.45)O1.6

Figure 4.27. Macrostructure and the Microprobe Analysis Results (the Probe
Diameter = 50 µm) for the Test K-6

The element composition of layers was studied by the microprobe analysis employing
different diameters of electronic probes. The microprobe analysis results obtained by a large
diameter probe (∅ ~ 50 µm) listed in the right column of Figure 4.27. They indicated that the
crystallized melt (area #1) near the boundary with the C-100 particle consisted of stainlesssteel (~ 87 at %), carbon, zirconium (~ 12 at %) that was close to the alloy initial
composition. Besides, approximately 1 at% of uranium was found out in the alloy.
Stainless steel (up to 60 at %) was found out in the solid C-100 corium (area #2) with theinitial ratio of elements including carbon.
In the third area, the content of stainless steel was reduced down to ~ 40 at %. The quantities
of uranium and zirconium in the form of oxides with the reduced content of oxygen wereabout 60 at %. Contents of the area #4 of the C-100 corium particle being in the contact withthe melt and of the particle in the area #5 not contacting with the melt were close to that of
the C-100 corium.
Interactions with the C-32 Corium
The SS+Zr alloy penetration into the porous corium loading (Figure 4.28, Figure 4.26) was
observed in the test K-7 and especially in the test K-5. However, this penetration took place
not by the leakage of the liquid melt through the gap system in the corium loading particles
but, apparently, by the “surface” diffusion over the surfaces of the corium particles (Figure
4.29a) as well as by the mechanism “vaporization – condensation” (Figure 4.28 and Figure
4.26, the test K-5).

105

K-7

K-5

Figure 4.28. Penetration of the SS+Zr Alloy into the Corium Particles after the Tests
K-7 and K-5

The metallic sample upper partThe corium C-32 gritsb)
The metallic sample lower parta)

1. (Fe0.66Cr0.07Ni0.27)0.8Zr0.20
2. Fe0.70Cr0.25Ni0.05
3. (Fe0.70Cr0.17Ni0.13)0.72Zr0.28
4. (Fe0.72Cr0.18Ni0.10)0.69Zr0.31
5. (U0.6Zr0.4)O2
6. (Fe0.71Cr0.25Ni0.04
7. (Fe0.73Cr0.12Ni0.15)0.76Zr0.24c) 8. (Fe0.66Cr0.70Ni0.27)0.74Zr0.22U0.04
Figure 4.29. Microstructure and Microprobe Analysis Results for the K-5 Sample

The melt first relocated by the mechanism “vaporization – condensation” then penetrated bydiffusion in depth of the particles loading, as this was discussed in detail in the test K-6, andproduced a two-phase system (Figure 4.29a) which included the following phases:• a light metallic phase containing stainless steel of the composition close to the initial
one and zirconium the quantity of which approximately 2 times exceeded that in theinitial alloy;106• a grey oxidic phase (U1.3Zr)O2.
Figure 4.29b,c shows the structure of the metallic SS+Zr alloy cut out of the metallic ingot
upper (b) and lower (c) parts.
The melt was crystallized into a two-phase system:• the one close in composition to stainless steel (Fe0.7Cr0.25Ni0.05)• the intermetallic compound of the (SS3Zr) approximate composition.
Up to 4 at % of uranium was observed in the metal phase in the ingot lower part near the
corium particles that coincided with the investigation results for the test K-6.
In Experiment K-12 molten alloy 110 (Zr + 1 % Nb) assumed the shape of a globule, and thepenetration of the melt into the debris was a one-layer deep. A tight contact ('welding-up’)
between the globular surface and corium particles was observed (Figure 4.30).

Metal

CoriumTop view

Side view

Figure 4.30. K-12 Metallic Ingot

In the test D-2, conducted in the furnace with tungsten heater good penetration of alloy SS/Zr
in the porous briquette of C-32 corium was observed at temperature T=1800oC. It is seen thatalloy SS/Zr wets well C-32 corium and gets into pores, the Zr content in the alloy increases
up to 30% due to partial transition of free zirconium from C-32 corium to molten metal alloy
85SS/15Zr.
Approximately 1/3 of the composition 85SS/15Zr was not melted because of presence of theoxygen and carbon in source composition. This part contained the phases ZrO2, ZrC0.14 that isconditioned by high ability zirconium to oxygen and carbon chemisorption fromsurroundings.
Small penetration of 85SS/15Zr and 99Zr/1Nb melts in interval of the temperature interval of1800 ÷ 2050oC, observed in experiments K-6 ÷ K-9, K-12, conducted in the facility withgraphite heater was due to formation of zirconium carbide on the melt surface and due todecrease of wetting of C-32 corium. Small penetration of 85SS/15Zr melt in the corium
debris made of C-70 (K-8) and C-100 (K-6, K-9) coria was conditioned also by highoxidation degree of corium as wettability of corium particles is reducing with the increase of
the oxidation degree, since oxidized corium, for instance C-100, is not wetted by the
85SS/15Zr melt.

4.4 Interactions under Oxidizing Atmosphere
According to the experimental matrices five tests have been performed during the MASCA2
project. Two tests RCW-ST1 [47] and RCW-Ox1 [47] were performed on the oxidation by107the mixture of argon and air and two tests RCW-ST2 [48], RCW-Ox2 [49] and RCW-Ox3[50] were performed in the oxidizing atmosphere generated by the mixture of argon andwater steam. During tests RCW-Ox1 and RCW-ST2 destructions of the experimental section
components took place and data on the interaction of molten metals with the corium particles
were not obtained.
4.4.1 Supporting experiments

The objectives of these experiments [47] were, first, to investigate the redistribution ofoxygen stored in Fe2O3 and in corium of different degree of oxidation (Cn) and, second, tostudy the interaction between the phases thus formed. The initial compositions in theseexperiments simulate the product compositions, which may be formed in the mixture of steel
and corium particles at different oxygen contents stored in corium and iron (in the form of
Fe2O3). The experiments were conducted at the same temperature of 1800°C. The
experimental matrix and the initial arrangement of materials in crucibles are presented in
Figure 4.31 (and in table under this figure). The experiments were conducted under the inert
atmosphere of argon with the use of corium C-32 and C-100. The experiments wereconducted as well as early D1 - D3 tests (see 4.2.2) in standard laboratory furnace equippedwith a tungsten heater and Mo shields. Iron and oxygen were introduced in the form of a
Fe2O3 pellet. The experiments were conducted in a tungsten furnace equipped with a tungsten
heater and Mo shields.

Fe2O3
Fe2O3
#1

Experiment
#1
#2

#2
C-100

C-32

CoriumMetal

C-100; m = 18 gm Fe2O3; m = 2 gm
C-32; m = 18 gm Fe2O3; m = 2 gm

CrucibleW
W

Figure 4.31. Crucible loading diagrams

The post-experiment longitudinal sections of crucibles are presented in Figure 4.32.

108

Experiment #1

Experiment #2

C-100 + Fe2O3
C-32 + Fe2O3
tungsten crucible tungsten crucible
Figure 4.32. Post-test crucibles` longitudinal sections
4.4.1.1 Experiment #1
The interaction between the C-100 corium debris bed and oxidized iron powder (Fe2O3)
produced shrinkage and compaction of the debris. As it is seen from the metallographic
results and structural analysis of the ingot (Figure 4.33), the post-test grey matrix of corium(the upper part of the crucible) is enriched in iron (up to 1.7 – 2.2 wt.%) with the uranium tozirconium atomic ratio being increased up to 2.1 as a result of precipitation of fine inclusionsof Zr-U-Fe oxides rich in Zr. In the lower part of the crucible the uranium to zirconium
atomic ratio remained at about initial level (about 1.2) with larger inclusions of reduced iron.

Fe

Zr
U
Ni Cr Mo W
Element concentration (wt.%)O

Sum

dz Pointµm (meas.#)Comment

1.88 12.62 70.34 0.02 0.02 0.06 1.37 14.44 100.76 1

3 (av.)

Grey matrix

3.21 18.97 59.80 0.01 0.01 0.08 2.62 15.10 99.80 20

1 (av.)

Grey matrix

89.60 0.04 0.00 0.01 0.02 0.19 10.06 0.00 99.91 1

2 (av.) Light inclusion

Figure 4.33. Post-test microstructure of C-100+Fe2O3 corium and micro probe-analysis

109During experiment a liquid phase of U, Zr, and Fe oxides was formed during the interaction
between Fe2O3 and corium C-100 at 1800°C. The liquid phase wets the remaining porous bedof C-100 corium particles and interacts simultaneously with corium. On cooling down avirtually monolithic structure is formed (Figure 4.33). The major phase in this structure is thegrey matrix constituted of corium C-100 and dissolved iron (iron oxide). Minor ironinclusions (light phase) containing tungsten at impurity level were observed. Apparently,
corium C-100 with a composition of (U Zr)O2-x reduced some Fe2O3 to Fe and augmented theoxygen potential.
4.4.1.2 Experiment #2
In the case of interaction between the oxidized iron (Fe2O3) with sub-oxidized corium C-32
complete oxidation of the upper part of the corium debris bed and its melting took place(Figure 4.34). Corium was enriched in iron (up to 2 wt.%). The most part of corium particles(12 g) of the lower part spilled-out through the orifice drilled in the bottom at dismantling.
Large light inclusions in the upper part of the sintered mass represent iron reduced fromFe2O3 containing crucible tungsten at impurity level. The iron inclusions are smaller in thelower part of the sintered mass, and their compositions are less homogeneous.

2

1

3
3

Fe

Zr
U
Ni Cr Mo W
Element concentration (wt.%)OSum

80.81 0.00 0.00 0.00 0.03 0.08 18.82 0.00 99.75
1.27 19.22 62.84 0.04 0.02 0.04 1.68 14.82 99.93
77.80 0.16 0.00 0.00 0.02 0.04 22.88 0.17 101.07
1.29 21.30 60.43 0.00 0.04 0.00 2.74 14.92 100.73
81.91 0.15 0.00 0.00 0.04 0.10 17.86 0.00 100.06
0.70 19.49 63.81 0.02 0.00 0.04 0.52 14.63 99.22

dz Pointµm (meas.#)

1
1
1
1
1
1

Comment

1 (av.) White inclusion
2 (av.)
Grey matrix
3 (av.)
Light phase
4 (av.)
Grey matrix
5 (av.)
Light matrix
6 (av.)
Grey matrix

Figure 4.34. Microstructure and microprobe analysis data for corium C-32+Fe2O3,
tungsten crucible

110

In this experiment the interaction between Zrfree stored in corium C-32 with Fe2O3 proceeds
according to the following equation
Fe2O3 + 3/2 Zrfree = 2Fe + 3/2 ZrO2,
with iron oxide being reduced almost entirely according to the micro probe-analysis data.
As a result, a Fe-phase is formed and corium C-32 undergoes oxidation and Cn increases.
The average corium Cn would equal 75 if iron oxide had reacted with the entire volume ofcorium. As there was no bulk equilibrium the corium Cn index can be close to Cn = 100 in
the corium upper part and remain at virtually unchanged level in the corium lower volume.
These processes allowed observation of iron permeation areas (light phase) in the gaps of thecorium grey matrix of Cn=90-100 in the upper part of the micro section (Samples 1 and 2 inFigure 4.34). The lower part of corium C-32, which did not interact with Fe2O3, spilled out
through the bottom orifice pierced post experiment.
Hence, the results of these experiments indicate that the phase composition of the products ofinteraction in the system (U - Zr - O) + iron depend upon the oxygen potential of the system.
For the excess of oxygen at 1800°C, a monolithic oxidic phase is formed with U, Zr, Fe
oxides. For oxygen contents which are insufficient to oxidize corium to C-100 and iron to
Fe2O3 a variety of phases is observed in the ingot on cooling down, coria of Cn indices in therange Cn = 32 – 100, all being subject to the amount of oxygen introduced in the system. The
estimations of the corium Cn indices, amount and composition of the liquid and solid phases
during the interaction between Fe2O3 and corium particles at T≥1800оC are presented inTable 4.4.
Table 4.4. Estimations of corium Cn indices at interaction between corium, Fe2O3, andstainless steel at T≥1800оC
Products formed
Liquid phase
Solid phase
Test
Quantity,
Quantity,
Quantity,
Composition
Composition
Composition
mol
mol
mol
Initial materials#1#2

Corium
Cn=100Fe2O3
Corium
Cn=32
Fe2O3

0.088
0.01250.1
0.0125(UZrFe)O2-x

0.025

Fe

0.025

(UZrFe)O2-x

0.025

Corium
Cn=100Cn=75 (averagecomposition)
Cn=90-100 (upper
part)
Cn=32 (lower part)

0.063

0.1

4.4.2 Scoping Test RCW-ST1

The principal objective of this test was to verify the workability of the employed method of
research into the interaction between the metallic melt and corium particles under the flowthrough oxidizing conditions. The aim was also to verify the protection of the tungsten heaterfrom the oxidizing media as well as to obtain data on the interaction in the system C-32
corium particles – steel.
The facility design is described in detail in section 4.2.4 and in the report [47] in which thetest course and post-test studies are also presented in detail.
The photograph of the crucible vertical section is presented in Figure 4.35. On dismantling of
the experimental section it was observed that all of the steel (15 gm) melted down entirely,
penetrated through some volume of corium particles and formed a 38 mm in dia., 10 mm high111

ingot with a closed porosity of about 26 %. Moreover it is important for analyses that thetemperature reached in the upper part of the heater exceeded the limiting one foraluminoferous cements (Tмах = 1750°C); this incurred decomposition of certain cement
components, partial evaporation, generation of carbon and tungsten oxides and their ingress
through the joint leaks into the operating channel.
Three specimens were cut out of the crucible for metallographic and micro probe analyses.
The specimen cut-out diagram is presented in Figure 4.35 (Samples # 1 - 3).

Figure 4.35. Crucible section and cut-out diagram for Samples ## 1, 2, 3

The post-test analyses furnished the following results providing description of the process ofoxidation when heating C-32 corium with a small St30 specimen.
Air flow-through made oxidize some of the Zr grains in C-32 corium particles rather
uniformly over the bed volume up to Cn = 65 – 75. It should be noted that these values
obtained by micro probe analysis are slightly underestimated, the lowest Cn being observed in
the St30 specimen placement zone.
The St30 cylindrical specimen melted down at 1530°C, or 1.5 min. after the air feed was
activated. The molten steel occupied the central zone in the inner crucible and penetrated thevoids between the grains and the open pores of the corium particle bed.
The lower boundary of the St30 melt is an arc defined presumably by an isotherm
corresponding to the steel melting point and be wetting corium by molten steel.
The estimations of corium oxidation degree assuming that all oxygen transported by theargon-air mixture was consumed for oxidation of Zrfree showed that Cn = 68 which is in areasonable agreement with experimental assessments.
4.4.3 The Test RCW-Ox2

After the completion of the test RCW-Ox2 [49], the operating section was retracted from thefacility and the tungsten heater was withdrawn. The crucible was cut into two halves along
the vertical axis. A macrosection was prepared from one half of the crucible that
demonstrated that a thin monolithic layer of molten steel X18H10T was located in the upper
part. Molten steel penetrated into pores of the corium particles at the depth of ~ 18 - 20 mm.
A plate 10 mm thick was cut off of the other crucible half from which samples were taken forthe metallography and for the microprobe analysis (Figure 4.36). Another plate 10 mm thick
was cut off of the remained part from which 5 samples were taken for the Plasma Spectral
Emission Analysis and Chromatographic Analysis of Oxygen Content (Figure 4.37).

112

Figure 4.36. Sampling Scheme for Metallographic and Microprobe Analysis

Figure 4.37. Sampling Scheme for Plasma Spectral Emission Analysis andChromatographic Analysis of Oxygen Content

Figure 4.38 - Figure 4.41 illustrate the microstructure of samples #1, #2, #3 and present
formulas obtained basing on the results of the microprobe analysis of points under study.

113

1. grey grains of Fe0.70Cr0.26Ni0.04

1. white inclusions Fe0.72Cr0.22Ni0.06

2. leakages of Fe0.47Cr0.04Ni0.24U0.09Zr0.16

2. grey matrix (U0.72Zr0.28)O1.80

Molten Steel

Corium Particles

Figure 4.38. Microstructure of sample # 1

1. grey matrix (U0.51Zr0.22)O1.64
Figure 4.39. Microstructure of the Corium Particles (sample #2, particles)

2. grey matrix (U0.53Zr0.47)O1.58
Figure 4.40. Microstructure of the Corium Particles (sample #3, particles)114

1. grey matrix UO1.70
Figure 4.41. Microstructure of the Corium Particles (sample #3, particles)

Table 4.5 presents the results of the plasma spectral emission analysis of metal elements andof the chromatographic analysis of oxygen content by the method of the reduction smelting of
homogeneous powder samples cut off performed according to the scheme (Figure 4.37).
Oxygen content in samples of the corium particles and of steel along the height is as follows:
sample # 2 – 48 at.%, sample # 3 – 64 at.%, sample # 4 – 65 at.%. Sample # 5 is the bed of
UO2 powder. Calculations demonstrate that oxygen supply was sufficient to oxidize C-32
corium completely. Results of the test showed that corium was oxidized to C-91-93 (sample #2) and up to C-100 (samples ## 3. 4). Thus, in the course of test, corium was oxidized to C100 except of the upper layer ~20 mm thick.
The same results were obtained by the microprobe analysis of the corium composition (greymatrix) in all samples ## 1, 2, 3 taken according to the scheme (Figure 4.36), where thecorium oxidation degree along the height was C ≈ 95÷100. This is also denoted by the
structure analysis: the corium structure is one-phase-oxide (grey phase) that is typical of thecorium C-100; the light phase Zr(O) typical of the corium C-32 is lacking in the corium
structure.
During initial stage of the test steel penetrated into the corium particles at the depth of 1820 mm which corresponds to the depth of completely oxidized corium. These results are
consistent with the results of K-3 , further penetration was ceased as iron (the basic steel
component) did not practically penetrate into the corium C-100. It should be noted that the
corium bed temperature over the whole volume exceeded significantly the temperature ofsteel melting, and the observed ceasing of steel penetration was not associated with steel
crystallization.
Table 4.5. The Analyses Results for Samples ch#1 – ch#5 (mass%)

Sample

U

Zr

Fe

Ni

Cr

Ti

Ch#1

49.0

14.9

12.9

1.72

3.43

0.091

7

89.04

Ch#2

48.9

13.9

13.2

2.28

3.43

0.10

10.2

92.01

Ch#3

63.8

17.5

1.59

0.30

1.24

0.049

14.7

98.18

Ch#4

66.7

15.3

0.076

0.022

0.005

0.056

14

96.67

Ch#5

79.3

3.17

0.075

0.010

0.012

0.040

13.2

95.77

115

O

Σ

4.4.4 The Test RCW-Ox3

The facility RCW-Ox3 [50] is practically similar to the facility RCW-Ox2 [49] thedescription of which is given above [see section 4.2.4]. The only difference was that theamount of steam was increased to allow oxidation of both corium debris and steel.
After facility was disassembled it was seen that the corium bed particles formed a monolithic
ingot. Crucible was cut along the vertical axis into two parts. Samples were cut off from one
plate for the metallographic and microprobe analyses. Figure 4.42 illustrates the pattern for
cutting off the samples. Samples for the element analysis were cut off from the other plate.
Figure 4.43 demonstrates the pattern for cutting off the samples.

Figure 4.42. Macrosection and the Sampling Map for Metallographic and Microprobe
Analyses

Figure 4.43. The Sampling Map for the Plasma Spectral Emission Analysis

Results of the plasma spectral emission analysis of the content (weight %) of elements U, Zr,
Fe, Ni, Cr and of O content determined by the method of the reduction smelting of samples
taken from the ingot surface are presented in Table 4.6 and of samples cut off according to
the pattern shown in Figure 4.43 are presented in Table 4.7.

116

Table 4.6. The Analysis Results for Surface Ingot Samples
Sample U

Zr

Fe

Ni

Cr O

center 4.35 50.5 12.5 4.32 1.73 24
border 36.9 32.5 5.95 2.55 0.93 21
Table 4.7. The Analyses Results for Samples ch#1 – ch#5 and CDC
Sample U

Design formula

Fe

Ni

Cr

ch#1

52.0 23.7 3.90

1.19

ch#2

52.8 21.9 4.84

0.25

0.91 18.0 (U0.37Zr0.44Fe0.12Ni0.03Cr0.03)O1.92
1.85 17.0 (U0.38Zr 0.41Fe 0.15Ni 0.01Cr 0.06)O1.80

ch#3

67.1 14.5 2.32 0.024 0.37 15.0 (U 0.58Zr 0.33Fe 0.08Cr 0.01)O1.91

ch#4

50.6 23.1 5.10

ch#5

Zr

O2

2.58

0.25 16.0 (U 0.35Zr 0.42Fe 0.15Ni 0.07Cr 0.01)O1.77
79.3 5.78 1.32 0.014 0.18 12.5 (U 0.79Zr 0.15Fe 0.06Cr 0.01)O1.94

CDC 63.8 20.4 <0.01 <0.01 <0.01 15.4

CDC is the sample taken from the non-cohesive area the volume of which is < 20 % of theingot volume. The analysis after the test performance of the oxygen content in samples (seeTable 4.7) over the volume of the corium bed and steel showed that in this test, the corium
C-32 was oxidized up to the corium C-100 and steel components (Fe, Cr, Ni) weretransferred into oxides. Iron oxide interaction with the corium particles determines a goodwetting of these particles which in turn determine the penetration into the corium bed down
to the bottom layer. Small quantity of iron oxide penetrated into UO2 loading on the cruciblebottom.
At the same time inhomogeneous iron content (from 0.5 weight % up to 10 weight %) in theingot volume (the grey matrix), a high porosity from 14 % up to 30 % in the upper layer(Table 4.8) and 12 - 22% in the ingot main volume and a relatively small quantity of steel inthe corium bed (msteel/mcorium) = 0.074 allow to conclude that the system “corium + steel” inthis test has not been completely melted at the temperature of 1800ºC and consisted of thecorium C-100 particles and the liquid phase of U, Zr, Fe, Ni, Cr oxides.
Density and porosity were measured at the samples taken from the ingot surface (the center,border) by the method of hydrostatic weighing in kerosene. Table 4.8 shows the results.
Table 4.8. The Densities of Surface Ingot Samples
Sample γ. g / cm3 d. g / cm3 Po. %center

5.00

5.83

14.2

border

5.13

7.47

31.3

where: γ is the density with regard to open pores;
d is the density without open pores;
Po is the open porosity.
The structure of the system “debris + steel” macrosection (Figure 4.42) after the oxidation bysteam became one-type consisting of grey and dark areas with small inclusions having themetallic luster. The structure had a significant porosity; the shape of pores is close to the oval
one.
117

In all sections along the assembly height (samples # 1, 3, 4), the microstructure wascharacterized by a high porosity ≅ 12 - 22 % (Figure 4.44, Figure 4.45, Figure 4.46, Figure4.47).

Figure 4.44. Microstructure of Sample #1

Figure 4.45. Microstructure of Sample #3

Figure 4.46. Microstructure of Sample #4(upper part)

Figure 4.47. Microstructure of Sample #4(lower part)

The microprobe analysis allowed to determine the composition of the basic phases observedon the microsections:• the oxide phase (grey matrix) is the solution of U-Zr-Fe-O(U0.48-0.16Zr0.47-0.70Fe0.05-0.14)O1.82-1.67;• the oxide phase of a darker colour is the solution of U-Zr-Fe-Cr-O(U0.15-0.86Zr0.31-0.02Fe0.35-0.02Cr0.19-0.01)O1.62-2.0;• the metallic phase based on Fе with Ni (Fе0.80-0.31Ni0.20-0.69).
118

Sizes of the metallic phase particles are mainly ranging up to 200 µm, several larger metallic
particles (up to 2 mm) are observed in the ingot upper area.
It is known that iron oxides interact with U and Zr oxides, for illustration, iron oxide with thezirconium oxide content up to 70 moll% at T = 1800ºC is in the liquid condition (Figure 4.48)[52]. Consequently, Fe oxide melt at the temperature of 1800ºC interacts with the corium
C-100 particles producing the melt of U, Zr, Fe oxides, for example, (U0.08Zr0.53Fe0.39)O1.88,
this melt mass is proportional to Fe oxide mass as Fe oxide mass is significantly less than the
corium part mass.

CeO2

Temperature, oC

2600

Cr2O3

2400
2200

NiO
Al2 O
3
Co
O

2000

Zn

1800

TiO2
Mn
3O
1600
4

1400

O

Fe2O3
0

20

40

60

80

100

ZrO2, %(mol)Figure 4.48. Meltability Curves for a Series of Oxides

It can be seen from microprobe analysis of the samples (See details in the report MP-TR-26[50]) that iron in the forms of oxide and element is found out at all levels along the height ofthe system “the corium bed + steel” after this system oxidation and cooling. The round shapeof pores (Figure 4.46) testifies the availability of the liquid oxide phase in the corium bed.
The availability of the liquid oxide phase led to the adhesion of the corium bed particles andto the production of a porous oxide ingot (the grey matrix) over the bed whole volume except
for some volume in the area of the corium bed central part.
Thus, Fe oxide melt with additions of Ni and Cr interacting with the oxidized corium
particles penetrated through the bed pores down to the crucible bottom. Due to the interactionof oxides, the melt of U, Zr, Fe, Ni, Cr oxides was generated with different contents of theseelements in different regions of the corium bed.
On the system “corium + oxidized steel melt” cooling, the following basic phases areproduced:• oxide ones based on U, Zr with Fe impurity, for illustration, (U0.15Zr0.77Fe0.08)O1.66
and (U0.82Zr0.16Fe0.02)O2.0, which represent the corium C–100 (the initial corium C–32
is oxidized up to the corium C–100) with Fe impurity;

119

•

oxide ones based on U, Zr, Fe, for illustration, (U0.08Zr0.53Fe0.39)O1.88 and(U0.48Zr0.38Fe0.14)O1.68, which are the product of the corium C-100 interaction withiron oxide;• oxide ones based on Cr with Fe addition, for illustration, (Fe0.38Cr0.62)O1.41, where Cr
oxide with dissolved Fe is separated into an autonomous phase;• metal phases based on Fе with dissolved Ni, Cr, for illustration, Fе0.55Ni0.43Mо0.02,
Fе0.99Cr0.01, Fе0.3Ni0.7.
Metallic phases based on iron were apparently produced on FeO cooling and decomposition
into Fe and FeOх.
Steel components Cr and Ni transferred in the following way: Ni – into the metal phase(solution of Fe - Ni) and Cr – into the oxide phase (Cr0.62Fе0.38)O1.41.
According to the analysis data (Table 4.7), the area in the corium bed central part that did not
become a ingot and crumbled out on preparing macrosections did not contain iron and ironoxide.
The penetration of iron oxide melt into this volume did not take place and the corium
particles were not adhered. It may be assumed that the bed higher density in this region may
cause the prevention of iron oxide penetration into the corium bed central area.
According to the previous test RCW-Ox2 data, steel penetrated into the corium bed only
down to the depth of 18 - 20 mm, the penetration did not take place deeper due to the corium
C-32 oxidation up to the corium C-100 particles of which were not wetted by iron.
In this test, iron oxide interacts with the corium C-100 particles, wets those and penetrates
deep into the corium bed down to the crucible bottom for τ ≅ 50 - 60 minutes.
Thus considering the results of oxidation tests one can conclude that in tests RCW-Ox2
partial oxidation of debris bed was obtained, while in the test Ox-3 corium and partiallystainless steel were oxidized. The melt of (Fe, Cr, Ni)Ox oxides wetting well particles of theoxidized debris at the temperature of 1800ºC penetrated through the corium bed pores down
to the bed whole depth. The availability of a liquid phase at the temperature of 1800ºC isdetermined by the lower melting temperature of iron oxide. This is testified by the roundshape of the ingot pores (the grey matrix). However, a part of the corium did not melt
completely during interaction with the molten oxides of steel component that is testified by
the iron inhomogeneous content over the ingot volume and by the presence of the ingot
volumes with low (~ 0.5 weight %) iron content. This is a consequence of a small quantity ofsteel (msteel/mcorium = 0.074) in the test RCW-Ox3.

4.5 Research into Influence of the Boron Oxide B2O3
With debris being overheated in the lower plenum of the reactor pressure vessel, one can
expect formation of a melt on the basis of boron oxide following evaporation of the residual
borated water and subsequent boric acid decomposition into B2O3. The molten B2O3 would
then interact with the sub-oxidized corium. It can be shown that to oxidize completely thezirconium in a C-30 corium composition of U/Zr = 1.2 according to the interaction(1.2 UO2+0.3 ZrO2+0.7Zr)+B2O3→ (1.2 UO2+ZrO2)+B
as low as 7.65 wt.% of boron oxide would be needed for the corium to attain the C-100
degree of oxidation. With smaller amounts of B2O3 introduced, interaction between a fractionof the elementary boron formed and a fraction of the sub-oxidized zirconium is possible andinvolves formation of ZrB2. Thus, ZrO2, ZrB2 and elementary boron represent the products of
the interaction between sub-oxidized coria and boron oxide. The elementary boron can be120

engaged in interaction with reactor pressure vessel steel and cause its borating and, hence,
facilitate its embrittlement. It should be noted here that Fe forms an eutectic with Fe2B at atemperature of 1177 ºC [53], and this together with the low-temperature eutectics of Fe with
U and Zr, which would be likely to occur, would bring down the lower phase boundary of theliquid metallic phase should the latter occur on the reactor pressure vessel interior surface.
To study effect of boron oxide three experiments were conducted. In the experiments thecorium particles obtained through crushing of sintered briquettes of C-32 corium of thefollowing composition were used: U – 66.5; Zrtotal – 22.16; Zrfree – 15; C – 0.008 wt. %. The
debris particle size was in the range 0.8 – 3.5 mm.
In the first test 53.82 g of fine-dispersed powder of boron acid of 99.8 % purity and put into
the crucible of iron with the carbon content of 0.17 mass %. Total mass of the crucible withloading was 3061.82 g.
In experiment #2 the crucible overall loading was 816.23 grams. The crucible cover had atube to insure removal of water vapour on heating up and during boric acid decomposition.
The experimental assembly was heated up to 1250°C with a rate of 0.25 – 0.35 K/sec and wasthen held at this temperature level for 60 min.
Similar to the experiment mentioned above, the experimental assembly in Experiment #3
employed a carbon steel crucible (carbon content 0.3 %). A loading of 3204.7 gm of C-32
corium debris mixed with 116.6 gm of boric acid powder was put in the crucible. It had been
considered that the weight fraction of boron oxide after dehydration would amount to 9 % of
the mass of the corium. After the temperature level of 1390 – 1400 ºC was attained, a 4 hr.
heating hold of the assembly was conducted.
The experiments were conducted in two stages. During the first stage the boric acid wasdehydrated by heating up of the experimental assembly to 95 – 110°C. The completion of
dehydratation was indicated by the fall of the system residual pressure to below 0.5 mm of
mercury. During the second stage the chamber was pressurized with argon to some 1.05 atm5.
The reported experiments were conducted under the inert atmosphere (high-purity argon) of
the KORPUS facility [54, 55], which employs for heating a heater positioned in a watercooled chamber (Figure 4.49).

5

Technical atmospheres, 1 at = 0.980665 bar.

121

8

Ar + H2O

1

7
2
Ar

3

4

5

1 – W/Re thermocouple;
2 – Heater;
3 – Jacket (Mo);4 – C-32+H3BO2 corium particles;
5 – Steel crucible;
6 – Support;
7 – Thermal insulation;
8 – Gas release line.

6
7

Ar

Figure 4.49. Experimental assembly diagram

The interaction of C-32 corium was investigated in a scoping test in a low-carbon steel
capsule. In the first experiment with 3 wt. % of B2O3 [54] at a relatively low temperature of~ 1000 oC exposure time of 1 hr it was observed that the interaction between α-Zr(O) of C-32
corium and boron oxide leading to formation of ZrB2 and elementary boron takes place
already at such low temperatures. The elementary boron formed thereby was observed topartially interact with the crucible wall and incur formation of a thin and fragile layer of aphase, presumably Fe2B, of high hardness. Further experiments at 1250 and 1400oC [55] also
demonstrated that elemental boron caused borating of the internal walls of steel crucibles.
Metallographic studies of the crucible and ingot after test #2 [55] revealed an interaction
layer thinner than 10 µm on the crucible interior surface. Its photograph is presented in Figure
4.50. The microhardness in this layer couldn’t be obtained owing to its minuteness andfragility.

122Figure 4.50. Interaction layer formed at crucible inner surface during Test #2

The post-test phase structure of the corium was studied with the use of a DRON-46 X-ray
diffractometer linked to an IBM PC for automated data acquisition and registration. For the
analyses, monochromatic copper Kα X-rays were used. Four phases were determined:(U, Zr)O2-х, α-Zr(O), ZrO2 and traces of ZrB2.
Three samples were taken for analyses after test #3 [55]: 1 – corium powder from the lower
region of the debris bed; 2 – corium powder from the upper region of the debris bed; 3 –deposits on the interior surface of the crucible.
The elemental composition of the samples was determined through the plasma spectral
emissivity analysis (U, Zr, Fe, B) and through the reduction melting (O). The results of theses
analyses are presented in Table 4.9.
Table 4.9. Elemental composition analyses after Test #3
Sample #Element, wt. %
U

Zr

B

Fe

O

1

63.2

18.8

0.5

< 0.003

14

2

62.1

22.3

0.9

0.03

12.5

3

62.9

20.1

1.2

0.07

18

The post-test phase composition analyses were carried out using a DRON7-4 X-ray
diffractometer. Shown in Figure 4.51and Figure 4.52 are the diffraction patterns of corium
samples taken from the lower (#1) and the upper (#2) region of the debris bed.

6
7

DRON (English transcription for a Russian made type of diffractometer).
DRON (English transcription for a Russian made type of diffractometer).

123

Figure 4.51. Diffraction pattern of the debris bed lower (#1) region corium sample

Figure 4.52. Diffraction pattern of the debris bed upper (#2) region corium sample

The diffraction patterns shown above exhibit lines for three phases only: (U, Zr)O2-х, ZrO2,
and ZrB2. The α-Zr(O) phase lines typical of the initial C-32 corium composition have almost

124disappeared. The upper sample has revealed a somewhat higher content of ZrB2 compared tothat of the lower sample.
Samples made of the crucible wall, bottom, and cover were subjected to the metallographic
analysis. The analysis data revealed an interaction layer featuring a sharp boundary (seeFigure 4.53) all over the crucible wall height and also on the crucible bottom. The thicknessof this layer varies from about 1.6 mm in the upper part of the crucible to about 0.75 mm in
the lower part and is about 2 mm on the crucible bottom. There is no interaction layer on thespecimen taken from the crucible cover.

Figure 4.53. Interaction Layer on the Crucible Interior Surface after Test #3

As one can see, the structure of the interaction layer resembles a typical dendritic one and thisis indicative of the fact that a liquid phase component was present in the interaction zone athigh temperatures.
To determine the phase composition of the interaction layer a sample was cut out of the flat
crucible bottom for X-ray analyses. The diffraction patterns obtained thereon have featured
the α-Fe phase and, presumably, the Fe3 (B~0,7 C~0,3) phase. The corresponding diffraction
pattern is shown in Figure 4.54.

125

Figure 4.54. Diffraction pattern obtained on the sample cut out from the cruciblebottom: interaction layer of test #3

The metallographic analyses data from samples of the crucible interior surface have revealed
that a wide interaction layer, from 0.75 mm on the crucible wall to 2 mm on its bottom, wasformed in Experiment #3. The zone of interaction formed on the interior surface of the steel
crucible has a two-phase structure, which is not typical of the diffusion-controlled processes.
Taking into account the particularities of the Fe-B phase diagram it can be assumed that thesteel wall of the crucible was subjected to enrichment with elementary boron during theannealing at 1400 ºC until the δ-Fe solubility limit was reached The solubility limit of boronin δ-Fe at temperatures typical of the experiment is some 0.5 at.% [53, 56]. On reaching thislimit the surface underwent primary melting.
According to our estimates, since the total concentration of the elementary boron formed
could amount to no more than 2.3 wt.% of the total debris loading, the concentration of boron
at the boundary, which in turn defines the diffusion flux into the wall was low. Hence, theamount of the primary liquid phase formed on the crucible interior surface was insignificant
either. Solubility of boron in γ-Fe is far lower than that in δ-Fe. Account taken of this fact, themajor fraction of the liquid phase was supposedly formed in the interaction zone at cooling
down presumably through a catatectic transformation of the kind δFe → γ-Fe + liq [56] indescending below 1381 ºC. The composition of the liquid phase undergoing formation varied
continuously while the temperature decreased to the eutectic temperature of 1177 ºC.
Retained from downcoming the liquid phase was owing to the surface tension as well as tothe debris bed. Due to carbon of the steel used in this experiment, it was not Fe2B but rather
the Fe3 (B~0,7 C~0,3) phase that was formed in the interaction layer. Together with the phase
diagrams of Fe-B, Fe-C, Fe-U, Fe-Zr [53] the reported experimental data point on both thetemperature and the degree of oxidation of corium as the key parameters governing theinteraction between the reactor pressure vessel steel and corium debris. Typical of all of thephase diagrams mentioned above are the eutectics with the following melting temperatures:
1055 ºC for Fe-U; 1153 ºC for Fe-C; 1177 ºC for Fe-B; and 1335 ºC for Fe-Zr. For compound126

melting of several components the eutectic temperature would only decrease. For this reason,
the probability of a local reactor vessel steel melting off would increase in case of theinteraction in a sub-oxidized corium mixture (corium C-32, for instance) containing B2O3 attemperatures around 1400 ºC. At low boron oxide concentrations the thermodynamic
activities of U and Zr would remain high enough to insure eutectics Fe-U and Fe-Zr through adiffusion-controlled mechanism. At higher boron oxide concentrations total oxidation of freeZr occurs. In this case, as it was observed in Experiment #3, the liquid phase can be formed
due to the formation of sufficient quantities of elementary boron.
Thus, the mechanism of appearance of the boron in the oxidic corium as a result of reductionof B2O3 by suboxidized zirconium Zr(O). Boron formed as a result of this reaction may leadto the borating of the internal surfaces of the rector vessel and together with the uranium andzirconium may lead to its eutectic liquefaction.

4.6 Discussion on Findings
In the series of debris tests conducted in the inert and oxidizing atmosphere the penetration ofiron (St-30), stainless steel (12X18H10T) and zirconium alloys (85 mass % of SS and
15 mass % of Zr) were studied. The corium (UZr)O2-x particles employed in the tests
represent different coria with the contents of oxidized zirconium ranged from 32 % to 100 %(Cn = 32 - 100).
Results of experiments demonstrated several mechanisms for the penetration of molten metalin the porous debris:
1. Filling of the open porosity among particles by liquid metal. This process depends
upon the debris particle size, the wettability of the corium particles and surface
tension which in turn depend upon composition and temperature. The threshold
temperature for penetration depends upon impurities such as carbon content in themelt. For iron (steel St30) which contain 0.27 – 0.35 wt.% of carbon the threshold
temperature was found to be 1670oC, slightly above the melting point of ironcarbide (Tm=1650oC [57]). Stainless steel demonstrated similar behavior, attemperature 1800oC steel penetrated through corium briquettes, however
experiment conducted at the same temperature in the graphite furnace showedvery limited penetration of stainless steel. One of possible reasons is the increased
carbon content in the metal melt and formation of chromium carbides which
melting temperature was Tm=1890oC. However the composition containing thealloy of stainless steel and zirconium (85 % SS + 15 % Zr ) demonstrated weak
wettability and consequently the penetration ability even in the cases whensuperheating above the melting point was 400 – 500°. The wetting angle wasassessed to be ~ 150° (See Figure 4.26). The most probable reason for the lack ofwetting is as follows. In contact with the gas or liquid on the surface of the
85 % SS + 15 % Zr liquid films of low-volatile and insoluble materials [58] may
be formed which increases sharply the limiting wetting angle. Carbon contained in
SS may form refractory zirconium carbide that comes to the surface due to“adsorption” and increases the wetting angle. This hypothesis was confirmed in
the test K-6. As one can see in Figure 4.27, a heightened carbon concentration(~ 2 at % - in metal and ~ 8 at % - in the corium) is near the boundary “liquid
metal – the C-100 corium” (the carbon content in the initial alloy
85 % SS + 15 % Zr does not exceed 0.5 at %).
2. The interactions between molten metals and corium particles near the contact area
between phases. The depth of such penetration did not exceed ~ 3.2 mm. The
127

contact interaction of the corium with any Cn value with liquid metals wasobserved under all test temperatures (starting from 1600°C). In principle, theinteraction leads to the diffusion release of U and Zr from the contacting layer ofthe corium particles into liquid metal (up to ~ 5 at %) and to the diffusion of steel
components Fe, Cr, Ni into the corium solid particles forming complicatedcompounds of the metal type. They are oxygen-free or with low oxygen content.
The thickness of the interaction layers is increased with the decrease of Cn8. Someinteraction was observed also in the completely oxidized corium (Cn = 100). In
the test K-12 alloy 110 with zirconium as its major component penetrated slightly
between the corium particles. A well-known fact is that zirconium readilyinteracts with UO2 and dissolves about 50 - 70 wt.% of UO2 in τ = 1 min. time attemperatures in the range of 1950 - 2050°C [59]. Alloy 110 being molten at theexperimental temperature of 2050oC demonstrated insignificant penetration of themelt between the corium particles. This can be explained by the presence of the
carbon impurity in the gas phase and also in corium particles. This carbon allows
for the formation of carbides of Zr and Nb on the molten alloy free surface at theexperimental temperature. These reduced drastically the molten alloy’s wetabillity
of corium particles and hindered the penetration of the melt into the debris bed.
The presence of carbon is apparently due to the insufficient insulation of graphitewith regards to crucible as well as to the traces of impurities in both corium
particles and alloy. Nevertheless, it should be noted that carbon makes part ofreactor structural materials made of stainless steel and also in control rods inseveral types of LWR. Carbon can thus play the same role in penetration ofstainless steel and/or zirconium melts into the corium debris bed under theconsidered accident conditions.
3. Under temperature gradient conditions molten iron not oxidized during the test
penetrates on depth, corresponding to isotherm of melting temperature. In case ofcarbon presence this temperature may exceed the melting temperature due toformation of carbides. The second reason preventing penetration is the coriumoxidation degree. As wettability is reducing with the increase of corium oxidation,
the oxidation to C75 – C100 prevent further penetration. This was confirmed by
the oxidation tests RCW-Ox2 where the steel melt penetration in the C-32 debris
was only to the depth of 18-20 mms. Further oxidation of steel components led tointeraction between steel components oxides and oxidized corium debris. As
wettability of oxides is much better molten oxides easily penetrated up to thebottom of the crucible. It is necessary to note that oxidation of steel components
occurs after complete corium oxidation.
4. Experiments with boron acid showed that elementary boron formed as a result of
this reaction may lead to the borating of the internal surfaces of the rector vessel
and together with the uranium and zirconium may lead to its eutectic liquefaction.

8

For example at 1600C steel penetrated into depth of 2.8 mm for C-32 corium while for C-100 thepenetration depth was only 0.2 mm. The same tendency was observed for 85SS/15Zr melt composition. At
1800C the penetration decreases for C-32, C-70, and C-100 as 1.27, 0.71, and 0.34 mm.

128

5 MATERIAL PROPERTIESThe focus of the material properties measurements was the extension of the database obtained
during RASPLAV Project. New data were obtained for the corium properties, namely C-32
corium viscosity and density were measured. Moreover, some properties of the metallic phasewere considered such as electric conductivity, thermal conductivity, viscosity andtemperatures for phase transitions as well as on the spreading temperature.

5.1 Properties of C-32 Corium
The viscosity was measured using the method of damping of torsional oscillation of acylinder filled with studied liquid similarly to the measurements performed during the
RASPLAV project. The technique theoretical basis, test facility, test performance and theexperimental data processing were described earlier [60].
Up to 2200°C the system behaves as a solid body and is characterized by a low damping
factor that is practically independent of the temperature. Under the temperatures higher than
2370°C, the system is characterized by a high damping factor that decreases slowly with thetemperature increase. This behavior is typical of normal liquids. Within the temperature range
2200 - 2370°C, the system is characterized by a noticeable growth of the damping factor withthe temperature increase. This temperature range may be considered as a simultaneous
existence of solid and liquid phases.
The values of kinematic viscosity for the corium C-32 were obtained at temperatures higher
than 2370°C. Figure 5.1 presents the dependence of kinematic viscosity upon temperature. It
should be noted that measured viscosity for C-32 corium is very close to that measured
earlier for the C-22 corium [60].
0.85
0.80
0.75

V*E6 (m2/s)

0.70
0.65
0.60
0.55
0.50
0.45
2350

2400

2450
2500
Temperature (oC)

2550

2600

Figure 5.1. The Kinematic Viscosity Versus the Temperature for C-32 Corium

For density measurements a modified hydrostatic weighing method was chosen [60]. The
results of measurements are shown in Table 5.1.
During the RASPLAV Project the C-32 density was also measured simultaneously with thesurface tension [61]. The following value for density 7.49 ± 0.44 g/cm3 for the temperature of
2470 - 2580 oC which is consistent with the current data.
129

Table 5.1. Density of C-32 Corium
Temperature (oC)

Density (g/cm3)

Dispersion (g/cm3)

2482

7.40

0.17

2482

7.61

0.24

2534

7.42

0.21

2585

7.39

0.19

5.2 The Properties of the Metallic Phase
5.2.1 The Spreading Temperature

The technique for Tspread determination [33] consists in the observation of the sample
condition through the optical micropyrometer which allows the observation of the liquid
phase appearance in the form of a film or drops, trickling of that on the surface up tocomplete melting. The experiments were conducted in the inert atmosphere to prevent thechange of chemical composition of samples. The temperature increased with a constant rate.
Objective of the research was to determine the temperature range when the metal phaseingots containing refractory oxides and intermetallides pass into the state of homogeneous
melt. The method of determining the temperature of complete spreading - Tspread lies inobserving through the optical micro-pyrometer the state of the specimen located on the W
undercoat. Temperature increases at the constant rate. The specimen is in the weak flow of
the pure inert gas. It is necessary to protect it from changing of its chemical properties due tothe impurities coming from the chamber volume, and to remove gas generated during melting– as this makes it more difficult to observe the item. The temperature of appearing of theliquid phase in the form of film or droplets is registered, as well as the process of its
spreading along the surface, and the total ingot melting down. Layout of the STF facility todetermine the spreading temperature is presented in Figure 5.2. Specimen (8) of the tested
material is placed inside the heater (6) onto the support of thin-wall tube (3). There are holes
for the passage of gas so that the supporting shield (9) was by 3 mm lower than the throughpyrometer hole (11). The specimen is to cover ∼2/3 of the hole area (a zone) not covering partof the area (b zone) to have the gap. Shields (7, 9) in the heater isothermal zone allow
approaching the cavity, where the specimen is located, to the black body model. Corrugatedfoil (4) in the tube (3) provides heating of the inert gas at the melting zone inlet.

130

6
7
8
10
130

5

А

9
60

4
3
2
1

A

b

11
a

He

1 – Support (Mo); 2 – receiver bushing (Mo); 3 – W tube (φ 14х0,4mm); 4 – corrugated W
foil; 5 – micro-pyrometer; 6 – W heater (φ 36х2mm); 7 – W shield; 8 – tested specimen; 9 –W shield - support; 10 – inductor; 11 – hole (φ 3mm)
Figure 5.2.Layout of the STF Facility to Conduct Experiments to Determine Melting
Temperatures of Metallic Phases

All of the tests were conducted in a weak flow (∼2 nl/min) of specially purified helium.
Multi-stage structure for additional purification of ”A” class helium was used. It consisted ofa system of dehumidifiers, freezing out with liquid nitrogen, molecular screens, and hightemperature catchers based on (Cu+CuO) active at the temperature of ~ 350oC, and based on
Zr – at the temperature of ~ 600oC. Chromatography method was used to monitor helium
purity. Purification results are presented in Table 5.2.
Table 5.2. Helium Purification Results
HeliumVolumetric concentration 10-4 % vol.
H2O

CO2

O2

N2

“A” class

5

0,2

1,0

1,0

After purific.

0.6

0.04

0.2

0.1

Dew point for the purified helium was 80оC.
Specimens were heated up by irradiation. W heater was located inside the inductor (10) that
operates due to the high frequency generator. To watch the spreading process a holeφ = 3 mm was made in the W heater. Brightness temperature is measured with LPM-066

131

pyrometers (instrumental error ± 1 %), and the controlling pyrometer - EOP-66 (instrumental
error ± 0,4 %). Prior to performing the measurements the facility was calibrated against themelting temperatures of pure metals (Ni, Zr, Mo). More details about the methods of the
facility preparation and of the measurements of the metal ingot melting temperatures are
given in the report [33].
The remaining fragments of metallic ingots resulting from experiments STFM-Fe #17 and
STFM-Fe #20 with significant difference in densities at room temperature (9.93 and8.64 g/cm3, respectively [11]) were used. In the course of experiments spreading
temperatures were determined for these specimens. They are listed in Table 5.3.
Table 5.3. Spreading Temperatures of Metallic Ingots. Experiments STFM-Fe #17 and
STFM-Fe #20

Specimen

Spreading temperature, оC

Metallic phase composition, wt. %
U

Zr

Fe

C

beginningend

STFM-Fe #17

40.85

15.52

41.46

0.012

1630

1680

STFM-Fe #20

23.87

9.97

64.18

0.013

1440

1470

In both cases complete spreading was observed. There were no refractory inclusions. Thatmeans that refractory components of metallic ingots from experiments STFM-Fe #17 and
STFM-Fe #20 are completely dissolved in the more easily melted matrix in the narrow
enough temperature range.
The obtained results indicate that the specimens of metallic ingots that are pure enough inrespect to carbon can pass into the completely melted state at the temperatures acceptable formeasurements with the lying droplet method.
Earlier experiments [33] were conducted using samples from the ingot obtained at the RCW
facility. Total of three samples from different parts of the ingot were prepared in the form ofpyramid from the ingot fragment in the argon protective medium.
The composition of the metallic samples used in the tests is presented in Table 5.4. The
temperature rise is terminated and heating is switched off after the exposure of ~ 60 s. Table
5.5 demonstrates the obtained results.
Table 5.4. Results on Density, Chemical and X-Phase Analyses of the RCW Ingot
Samples
Chemical composition, mass%

SampleDensity,g/cm3#1 (top)

9.03

23.09 10.20 43.2 11.55 8.01 0.058#2 (middle part)9.85

38.89 13.53 31.6 8.36 5.43 0.042#3 (bottom)

10.72

51.96 20.10 16.6 4.04 2.93 0.19

U

132

Zr

Fe

Cr

Ni

C

Table 5.5. Temperatures for the RCW Ingot Metallic Phase Spreading
Spreading temperature, oC
beginningend

Non-melted part,%#1 top

1580

1770

21

#2 middle part

1760

1870

13

#3 bottom#4 bottom

2140
2130

2420
2430

30
30

SampleThe basic phase of the escaped liquid is (UZr)Fe2. The non-melted part of samples consists
mainly of [Zr(C,O) + solid solution of UO2]. Their ratio depends mainly on the quantity ofcarbon in the initial sample. The remained oxycarbide hull is ~ 30 % of the sample mass andit does not spread even under the temperature ~ 2700°C if the content of Zr(C,O) is ranged
between 0.3 and 0.5 mass% (samples #3 and #4 from the ingot bottom part).
The sample #4 taken from the ingot bottom part was tested with stops and observations under
the intermediate temperatures (2240°C and 2330°C) between the spreading beginning andend. The mass of the escaped liquid phase was 15 - 20 mass% after the exposure of 5 minutes
under the temperature 2240°C and drops appeared on the surface. The exposure (T = 2330°C,
τ = 5 min) led to the liquid phase escaping (40 - 50 mass %), drops at the surface werelacking. about 30 mass % remained non-melted (as in the sample #3) under T = 2430°C. The
basic phase of the escaped liquid phase and in drops is (UZr)Fe2 and Zr(C,O) of non-meltedpart.
In these experiments no complete melting down was achieved at the temperature of up to
2450оC. The reason of differences in the fusibility of metallic specimens from theexperiments STFM-Fe and RCW is in the much higher (~ 5 - 10 times) carbon content in thelatter specimens. Due to this reason oxycarbide phases of (U, Zr)(C, O) type with the melting
temperature higher than 2700оC were formed in these specimens
5.2.2 The Electric Conductivity

Electric conductivity is one of the most important characteristics of the material. The
measurement of electric conductivity and its temperature dependence allows to judge about
the nature of chemical linkage between the components of the studied material. The sample
of 9 × 10 × 21 mm3 size cut out of the metallic part of the ingot obtained in the T-7 test wasused for the investigation. Table 5.6 demonstrates the results of chemical analysis.
Table 5.6. The “Metallic Body” Chemical Analysis of the T-7 Test
Sample composition (mass %)U

Zr(Zr+Fe+U)free

Fe

C

Nb

O (diff.)

U/Zr(at./at.)

27.2

13.26

*

59.37

0.143

*

0.03

0.79

* - not measured
The method employed for the study consists of the measurements of the Volt-Ampere
characteristic of the sample [33]. Figure 5.3 demonstrates the temperature behaviour of thespecific resistance. The linear growth of specific resistance is observed which is typical of
metals.

133

The temperature coefficient for specific resistance was determined by the least-squares
method. Table 5.7 presents the specific resistance of the studied sample under 20°C and thetemperature coefficient. Figures in round brackets are the measurement errors (dispersion).
The same table contains also the values of for pure iron [62], zirconium [63], and uranium[64]. It is seen that specific resistance of the alloy under study is very close to that ofzirconium and the temperature coefficient of specific resistance – to that of uranium.
Table 5.7. Specific Resistance (ρ) and Temperature Coefficient of that (α) of the Alloyand Its Components
Material

T (oC)

ρ ⋅ 108(Ohm ⋅ m)α ⋅ 103 (1/K)

Alloy under study20

43 (4.3)

2.38 (0.17)

Fe

20

9.8

6.2

Zr

27

43.5

3.8

U

25

25 - 34

2 – 2.8

1.4

R(t)/R(20)

1.3

1.2

1.1

1

0.9
0

40

80

120

Temperature (oC)

160

200

Figure 5.3. The Temperature Dependence of Specific Resistance
5.2.3 Density

In all of the experiments, performed with suboxidized corium (C-32, C-50, C-70) in the inert
atmosphere, molten Fe formed as a result of interaction became saturated with uranium,
zirconium, and oxygen. Its density becomes higher than the density of molten oxidic coria
and inversion of metallic and oxide layers takes place. The objective of this work isdevelopment of the methodology to experimentally determine densities of metallic phases
formed during interaction of steel with suboxidized corium. These studies dealt mainly withthe demonstration of the feasibility of measurements of metallic phase densities.
Dependencies of the metallic phase densities upon concentration were not studied.
The methodology of experiments and choice of undercoat materials were considered indetails in the report [65]. During the preparatory work it was found that undercoat made ofY2O3 did not practically interact within 10 minutes up to the temperature of 2000оC.

134

In the course of compatibility testing alloy loadings with the mass of 0.5 to 3 grams wereused. For the density measurement the droplet mass was selected such that it was possible toobserve it, to register its sizes, and that it did not exceed 1 g. After testing change of thedroplet mass did not exceed 10–2 % at the temperature of ~ 1750oC. Greater loss of mass wasobserved due to evaporation of mainly Fe was noted at higher temperatures. For instance,
when heated to the temperature of 2200oC within 10 min, the droplet mass of STFM-Fe #20
alloy was reduced by 12 %. Evaporation of Fe at higher temperatures makes the situation
more difficult due to the changes in the droplet material composition. Shape of the droplets
formed in the temperature range of up to 2100оC testifies to bad wetting (θ > 90o) by themolten droplets of Y2O3 undercoat. Along with the temperature growth interaction with theundercoat and wetting between them increase up to complete spreading. This circumstanceintroduces limitations to the temperature range of application of the lying droplet method tomeasure density and surface tension of the items of interest. Based on the results ofcomparative testing Yttrium oxide was selected as the main undercoat material.
Mercury specimen with the mass of 0.3705 g on the Y2O3 undercoat formed the droplet at thetemperature of 20оC. The droplet volume is determined by numerical integration within thearea limited by the obtained curve rotation figure. As a result it was found that mercury
density was 13.5609 g/cm3. If this value is compared to the tabular value (13.5461 g/cm3[66]) then the error in determining the mercury density is 0.1 %.
Final certification of the method was performed by measurement of the density of the molten
iron. The chemical analysis results indicated that there were some impurities in it: C-0.02,
Si-0.023, Mn-0.015 and S-0.02 mass%. Three series of tests were performed at thetemperature of 1650оC. Pieces of Fe with the masses 1.1652, 1.1808 and 1.2254 grams wereused. In the course of conducting experiments it was found out that unlike STFM-Fe alloys
iron interacts more actively with the Y2O3 undercoat. That is why alumina oxide (Al2O3) wasused as the undercoat material in this case. After all of the adjustments and readjustments
were performed and the working temperature was reached the formed droplets were
photographed and their heights were measured. For example, Figure 5.4 presents the photopicture of the Fe droplet with the mass of 1.1652 g.

Figure 5.4. Photo Picture of the Fe Droplet with the Mass of 1.1652 g

The result related to measurements of iron density are listed in the Table 5.8.
Table 5.8. Results of Fe Density Measurements at the Temperature of 1650оC

135

#

Mass, Droplet
DropletDensity,3gheight, mm volume, mm g/cm3.

1 1.1652

3.84

168.4759

6.916

2 1.1808

4.00

169.3137

6.974

3 1.2254

4.08

175.253

6.992

Arithmetic average for the density is 6.961 g/cm3. In accordance with the known reference
data [67] temperature dependence of Fe density is described with the following equation:
ρ = 7.0505 – 7.06·10-4 ·(t – 1535oC), g/cm3
о

(5.1)
3

Hence for the temperature of 1650 C we get the value ρ1650 = 6.9693 g/cm . Difference in the
results is 0.12 %.
Thus testing experiments under prototypic conditions indicated that the developed technologyof the lying droplet method ensures getting satisfactory results.
The fragments of STFM-Fe #20 alloy were used for the measurements of the metal phasedensities. Those fragments were repeatedly analyzed for their element and phasecomposition. Results of the analyses are listed in Table 5.9.
Table 5.9 Results of Analyses of STFM-Fe #20 Metallic Alloy
Element Mass% At.%

Phase composit. and lattice parameter, nm

U

23.74

6.725

Zr

10.03

7.413

Fe

64.27 77.586 (U,Zr)O2 (a=0,531 and a=0.543)+(U,Zr)Fe2 (a=0.703)

C

0.012 0.0674

O*

1.948

8.209

*) – on difference up to 100%.
Average bottle density of STFM-Fe #20 alloy at room temperature was 8.64 g/cm3.
Density measurements were performed with different loadings of the alloy at the temperatureof 1750оC. Holding time in all of the experiments was ~ 20 min. Figure 5.5 presents photopicture of the molten droplet of STFM-Fe #20 alloy with the mass of 0.4991 g.

136

Figure 5.5. Photo Picture of the Droplet of STFM-Fe #20 Alloy with the mass of 0.4991 g

Cross-section of this droplet was studied after crystallization process was over. Homogeneous
phase distribution and insignificant shrinkage shells in the peripheral areas are seen in themacro cross-section (Figure 5.6).

Figure 5.6. Macrostructure of the Droplet of STFM-Fe#20 Alloy with the mass of
0.4991 g

Typical homogeneous structure formed as a result of melting down and slow crystallizationof the alloy is seen in the micro photo pictures of this cross-section (Figure 5.7).

137

Figure 5.7. Microstructure of the Droplet of STFM-Fe #20 Alloy with the mass of
0.4991 g

All of the formed droplets were photographed and their heights were measured. The results ofdata processing related to the five samples taken from the STFM-Fe#20 ingot are presented inTable 5.10.
Table 5.10. Results of the Density Measurement of STFM-Fe#20 Alloy at theTemperature of 1750 оC#

Mass, Droplet
DropletDensity3gheight, mm volume, mm g/cm3.

1 0.4693

4.01

61.5058

7.630

2 0.4991

4.21

65.62197.605

3 0.6832

4.45

89.76347.611

4 0.8174

4.58

106.61197.667

5 0.8629

4.66

112.0348

7.702

Arithmetic average value of STFM-Fe#20 alloy density at the temperature of 1750оC is
7.643 g/cm3. Standard deviation (error) of the mean value is 0.036 g/cm3 and thus themeasurement relative error is 0.5 %. Averaging obtained data allowed to determine thedensity of the metal phase which composition is shown in the Table 5.9 at the temperature of1750оC as:ρ1750оC = 7.64 ± 0.04 g/cm3
5.2.4 Surface tension

The lying droplet method allows to measure density and surface tension of the melt
simultaneously [65]. The main equation of the lying droplet method is the Young-Laplacian
equation in the form [67]σ · (1/R1 +1/R2) = ρ·g·z + Po

(5.2)

where: σ –surface tension, R1 and R2 – main radii of the curvature, and z – droplet heightfrom its top in the reviewed point on the droplet surface, ρ – droplet density, g – gravity
acceleration, Po – integral pressure in the droplet due to the surface tension forces. Let’s write138down equation (5.2) for the droplet top, that means close to the point with z=0. The surface inthis place is spherical, and therefore it is possible to write
2 σ/R0 =Po(5.3)

Deducting equation (5.3) from equation (5.2) and making transformations we will get theformula to calculate the surface tension value
σ = ρ·g·z / (1/R1 +1/ R2 – 2/R0)(5.4)

expressed in terms of the radii of the droplet curvature on top and in the point with thearbitrary z coordinate. It is convenient to choose the height at which the droplet has the
maximum radius R2 as the z value. Figure 5.8 presents the droplet schematic with indicationof the parameters measured to determine R0 and R1 radii.
a0

a0

h0

h1
R0

a1

R1

R2
Z

Figure 5.8. Measurement Schematics of the Lying Droplet to Determine R0 and R1 Radii

Applying the known definition of the point degree relative to this circle it is possible to write
down that
R0 = (a02 + h02)/2h0(5.5)

R1 = (a12 + h12)/2h1(5.6)

The testing of the methodology was performed using results of measuring the mercury
droplet shape. After corresponding radii were determined the value for the mercury surface
tension was found to be σHg = 0.469 N/m. Known reference values of the mercury surface
tension at room temperature are: σHg = 0.465 N/m according to the method of the maximum
pressure in the droplet; 0,480 N/m – according to the method of the maximum pressure of thegas bubble; 0.478 N/m – according to the method of the lying droplet [68]. Reference [66]recommends the value of 0.465 N/m. The results obtained in present experiments are in good
agreement with the known values.
Droplet profiles of the molten iron were analyzed in the similar way. This allowed to find thatthe molten iron surface tensions obtained in three tests were σFe = 1.54 N/m, σFe = 1.57 N/m,and σFe = 1.59 N/m. The average value was found to be σFe = 1.57 N/m.
The same measurements were performed for the samples obtained from the STFM-Fe #20
ingot. Results of calculations results are presented in Table 5.11.
Table 5.11. Calculated Surface Tension Results of STFM-Fe #20 Alloy at theTemperature of 1750 оC

139

Droplet Droplet#
height, mm

R0, m

R1, m

R2, m

Z, m

Density,Surface3
kg/m tension, N/m

1

4,01

0,002765 0,001992 0,002586 0,002406

7630

1,081

2

4,21

0,002747 0,001876 0,00264 0,002425

7605

0,985

3

4,45

0,003318 0,002082 0,002974 0,002688

7611

0,939

4

4,58

0,003723 0,002278 0,003183 0,002789

7667

0,972

5

4,66

0,003389 0,002102 0,003232 0,002918

7702

1,131

The mean value of the surface tension for STFM-Fe #20 alloy at the temperature of 1750оC is
σm = 1,02 N/m. Standard deviation of the mean value determined by the formula (5.4) is
0.4 N/m, and therefore, relative measurement error is ~ 4 %. Thus finally the surface tensionof the molten metal phase of STFM-Fe #20 alloy at the temperature of 1750оC isσ1750оC = 1.02 ± 0.04 N/m.
5.2.5 The Thermal Conductivity

The sample was placed between the heater and heat receiver. The heater and heat receiver
were fabricated from copper to minimize the temperature gradients. The copper heater washeated by an electric coil. Four thermocouples were mounted in the assembly, that is, one –on the heater, one – on the receiver and two ones – on the sample.
The technique for thermal conductivity measurement is based on the solution of the nonsteady-state heat conductivity equation and presented in the report [33].
The tests were performed with different power applied to the heater. In Figure 5.9 the sample
thermal conductivity values are presented. It is seen that thermal conductivity does notpractically depend on the temperature within the used temperature range. Averaged data areshown in the Table 5.12.

Thermal conductivity (W/m/K)

16

12

8

4
POWER of HEATER
P=89 W
P=35W
P=44 W

0
80

120

160

Temperature (oC)

200

240

Figure 5.9. The Values of Thermal Conductivity Obtained in Tests140

Table 5.12. Thermal Conductivity Data Obtained in Tests
#

o

T( C)

ThermalDispersion
Dispersion due to
Total
Number of
conductivity experimental due to statistic systematic error dispersion
points(W/(m⋅K))(W/(m⋅K))(W/(m⋅K))(W/(m⋅K))

1 113 - 201

13.68

989

0.39

1.3

1.3

2 49 - 134

12.32

2075

0.98

1.3

1.6

3 152 - 216

13.39

353

0.21

1.3

1.3

4 84 - 174

13.53

1381

0.42

1.3

1.4

84 - 216

13.53

2753

0.41

1.3

1.4

5.3 Measurements of specific heat capacity and thermal
conductivity of RCW metallic phase
5.3.1 Sampling

In order to prepare specimens for properties tests [69], a billet (see Figure 5.10) was cut outof the lower metallic body produced during RCW test. The material was found to be rather
conducting, but revealed locally porous non-homogeneous aspects (Figure 5.11). The billet
surface was then conditioned for specimens cutting out.

141

RCW Macro Ingot Components:
1 – non-molten corium;
2 – lower metallic body;
3 – partially molten corium;
4 – metallic bodies in the mid part of the oxidic ingot;
5 – oxidic ingot;
6 – epoxy;7 – upper metallic body;
8 – free surface.

Billet cut out of the lower metallic
body of the RCW macro ingot.

2

3

1

Figure 5.10. Billet of material cut out of the lower metallic body of RCW test for
specimen preparation

Figure 5.11. Local porosities and billet aspects

The conditioned billet was segmented into two sections, and two groups of metallic cylinders
were cut out by the electroerosion technique, Group A along the Radial Direction and GroupB along the Axial Direction of the RCW macro ingot (see Figure 5.12). This was done in theattempt to capture variations of the metallic body material thermal diffusivity along two
different directions.

142

The cylinders were cut into specimens in the following manner. Each specimen (pellet,
hereinafter) received the following designation: GroupLine-Cylinder-Pellet. Each time, twopellets were cut out of a cylinder: one for high-temperature tests (last digit = 1) and an
adjacent witness one (last digit = 2) for room-temperature density measurements andmicrostructure analyses. Additionally, microstructure and microprobe analyses were
performed on some of the annealed pellets.
Radial Direction
B2-2

A1-1

B2-1

…

…

A2-2

…
…

Axial Direction

B1-1

A2-1

A1-2

Section ‘A’Section ‘B’Side of billet
against the
Figure 5.12. Groups of cylinders cut out of a 44 × 33.5 × 13 mm size billet

The density of the billet was determined by weighing it in kerosene at room temperature. The
density of this macro section was found to be 10.3 g/cm3. This is in good agreement with [35]where density of the lower metallic body is reported to be 10.69 g/cm3.
Densities of ten witness pellets were determined through weighing them in distilled water andare reported in Table 5.13. For Group A, the densities are in the range 10.27 – 11.01 g/cm3.
The density exhibits more scatter for Group B being in the range 9.32 – 10.69 g/cm3. All inall, these appear to be in fair agreement with the macro density of the material.
Table 5.13. Densities of witness pellets
Pellet Density, g/cm3 Open porosity, %

10.27
1.28
A1-2-110.40
1.35
A1-3-111.01
0A1-4-1
10.49
3.90
A2-2-1
10.69
1.35
A2-3-19.45
0B1-2-1
10.44
3.03
B1-3-110.11
0B1-4-19.32
1.35
B2-1-110.69
0B2-3-1For high-temperature tests pellets, their densities were calculated assuming their rightcylindrical shape (see Table 5.14). It can be seen from a comparison between Table 5.14 and143

Table 5.13 data that the calculations are always somewhat overestimating the densities(Figure 5.13). These are reported in [65] and were tracked throughout the program ofproperties tests.
Table 5.14. Measured dimensions9 and calculated densities of polished pellets
Pellet Ø, mm Thickness, mm Mass, mg Density, g/cm3
A1-2-2A1-2-3A1-3-2
A1-4-2A2-2-2A2-3-2B1-2-2B1-3-2B1-4-2B1-4-3B2-1-2

2.982
3.127
2.984
3.008
3.006
3.009
3.012
3.004
3.008
2.993
2.943

0.876
1.023
0.960
0.964
0.984
0.943
0.984
0.973
0.972
1.245
0.968

60.20176.10471.26872.24371.18969.82766.26768.23569.38581.46862.390

9.84
9.69
10.62
10.55
10.19
10.41
9.41
9.90
10.05
9.30
9.48

Specimen failure
B2-3-2
1.325
74.9939.29
B2-3-3 2.785
As density of samples depend upon concentration ratio of uranium to zirconium, for dataprocessing all samples were grouped on the base of their densities. The first group (lightsamples) was formed from the samples A1-2-2, A1-2-3, B1-3-2, and B2-3-3. The second
group was formed from the samples A1-3-2, A1-4-2, A2-2-2, and A2-3-2.
11ρ, g/cm3

ρ, g/cm3

11
10
10

From center tocrucible wall

From top tobottom5

10

Axial, mm

510Radial, mm

Figure 5.13. Variation of metallic body material density along the Axial and Radial
Direction of RCW macro ingot according to calculations (red) and measurements (blue)data
5.3.2 Measurements of specific heat capacity

The specific heat capacity measurements of pellets cut out of the lower metallic material
body obtained after RCW test were made using the method of differential scanningcalorimetry with a commercial Netzsch DSC 404 C apparatus. The schematics of theapparatus furnace chamber are presented in Figure 5.14.
9

Average values out of 10 measurements.

144

The furnace (Figure 5.14) is made of an electrical resistance heater winded around a ceramic
tube. The sample carrier system is installed inside this tube in a mirror surface shaft. It
consists of a tube hosting wires of type S thermocouples. This tube carries the measuring
head, or sample carrier, comprising two compartments for two PtRh crucibles with corundumbottoms. A sample is put in only one crucible and is covered with a lid. The other crucible isalways kept empty as a reference. The experimental procedure was as follows. Three
measurements with identical physical conditions were performed each time in a serious of
immediate succession on the same day. First, a baseline measurement with empty crucibles
was carried out each time for each pellet according to a preset temperature scanning program
to establish the baseline. Second, a measurement on a 99.99 % pure synthetic sapphire
standard was carried out against the baseline. Finally, a pellet measurement against thebaseline was carried out over the same temperature range.
The specific heats of pellets were then recalculated according to the following formula.
DSC Sample − DSC Baselinem
C P = S tan dard ⋅⋅ C PS tan dard
(5.7)
m Sample DSC S tan dard − DSC Baseline
where
CP

- specific heat of sample at temperature T;

CPStandard- specific heat of standard at temperature T;

mStandard- mass of the standard;

mSample- mass of the sample;

DSCSample - value of DSC signal at temperature T from the sample curve;
DSCStandard - value of DSC signal at temperature T from the standard curve;
DSCBaseline - value of DSC signal at temperature T from the baseline curve.
The accuracy of a Cp measurement is dependent on the reproducibility of the baseline
measurement. Asymmetries of the measuring system can be seen as a baseline drift. Different
from the classical ratio method, the ASTM method improves this by correcting the drift witha linear term.
The apparatus was verified against NBC molybdenum and corundum reference data withboth methods employed and presented the report MP-TR-30 [69].
The results of measurement of specific heat capacities for two groups of samples are shownin Figure 5.15. Results of measurements were averaged for both groups. Comparison of
averaged results is presented in Figure 5.16. One can see that the averaged values are very
close.

145

Ar

Mirror wall
Ceramic shaft

Sample carrier
Furnace winding

Ar

Figure 5.14. Experimental apparatus furnace for specific heat measurements

146

2.5

A1-2-2A1-2-3B1-2-2B1-3-2Average value

2.5

2.0

A1-3-2
A1-4-2A2-3-2Average value

2.0

Heat capacity

1.5

Heat capacity

1.5

1.0

1.0

0.5
0.5

0.0

0.0
200

300

400

500

600

700

800

900

1000

200

1100

300

400

500

600

700

800

Temperature, C

Temperature, C"light" samples"heavy" samples

900

1000

1100

Figure 5.15. Heat capacity of the "light" and "heavy" samples of RCW test
Light samples
Heavy samples
2.0
1.8

Heat capacity, J/gK

1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
400

500

600

700

800

900

o

Temperature, C

Figure 5.16. Comparison of specific heat capacity for "light" and "heavy" samples
5.3.3 Determination of thermal conductivity

The method employed for the experimental thermal diffusivity measurements was the laser
flash transient technique. The general idea of this method consists in delivering an energy
impulse by resort to a laser flash device to the front surface of a specimen and simultaneous
tracking of the temperature variation due to this impulse at the rear end surface. The dataobtained includes information related to the thermal diffusivity of the specimen [70].
The experimental procedure was as follows. A 0.25 mm colloid graphite layer was applied to
the front end surface of a pellet to ensure minimum reflectance of beam energy. The preparedspecimen was installed in the working chamber by means of the positioning tube. The laser
beamer was installed 1.25 ± 0.25 m before the pellet, and a laser flash sight on the pellet
surface was 3 ± 0.1 mm. A 0.01 Pa vacuum was established in the internal volume of theworking chamber. The external volume of the working chamber was filled with argon and
147

-6

8.0x10-6

7.5x10-6

7.0x10-6

6.5x10-6

6.0x10-6

5.5x10-6

5.0x10-6

4.5x10-6

4.0x10-6200

A1-2-2A1-2-3B1-2-2B1-3-2Average value2

8.5x10

Thermal diifusivity, m /c

2

Thermal diffusivity, m /cpressurized to 1.01325 × 105 Pa. The pyrometer was installed 1 m away from the rear end
surface of the specimen, with the pyrometer sight diameter being 2 mm.
The specimen was then heated up to a specified temperature level with the rate of 20 K/min.
For the majority of pellets, during each experiment eight isothermal temperature levels in therange of 300 – 1000ºC were established with an accuracy of some ± 0.25ºC for
measurements. At each temperature level a number of measurements were taken. Whenattained a specified temperature level, 2 ms laser flashes of some 1.8 - 2 J at 1.053 µm wereshot 5 times, and five temperature response profiles were registered for each pellet at eachtemperature level.
The measured data of the thermal diffusivity are presented in Figure 5.17. Results varysignificantly from sample to sample, however comparison of averaged data for both groups ofsamples demonstrate rather close results (See Figure 5.18)

300

400

500

600

700

800

900

1000

1100

8.0x10-6

7.5x10-6

7.0x10-6

6.5x10-6

6.0x10-6

5.5x10-6

5.0x10-6

4.5x10-6

4.0x10-6

A1-3-2
A1-4-2A2-2-2A2-3-2Average value

200

300

400

o

500

600

700

800

Temperature, C

Temperature, C"light" samples"heavy" samples

900

1000

2

Thermal diffusivity, m /s

Figure 5.17. Thermal diffusivity of "light" and "heavy" samples of RCW test

8.0x10-6

7.0x10-6

6.0x10-6

5.0x10-6

Light samples
Heavy samples

400

500

600

700

800

900

1000

o

Temperature, C

Figure 5.18. Comparison of thermal diffusivity for "light" and "heavy" samples of
RCW test

148

1100

Using average data one may calculate the heat conductivity. The results of calculations areshown in Figure 5.19.
Light samples
Heavy samples
20

Heat conductivity, W/mK

19
18
17
16
15
14
13
12
400

500

600

700

800

900

o

Temperature, C

Figure 5.19. Thermal conductivity of "light" and "heavy" samples of RCW test

The results of earlier measurements of thermal conductivity using non-steady-state method
and presented in section 5.2.5. The data obtained here are in a good quantitative agreementwith the results presented earlier.

149

6 CONCLUSIONS
1. Development of test methodology. During the MASCA Project the technology of
conduction experiments using induction melting in a cold crucible was developed. This
technology allowed performance of experiments using different materials such as steel
and corium with the limited undesirable interactions between materials.
2. Pertitioning of main components. Several series of experiments have been conductedto study behavior and interactions of corium of different compositions and steel (iron)both in intert and oxidizing atmospheres. The results may be summarized as follows:• Inversion of metal and layers. Experiments with the molten steel and corium revealed
important peculiarities of interactions. Interactions between molten steel and corium
results in the mass exchange between phases when some amounts of metallic uranium
and zirconium initially located in the suboxidized corium pass into metallic phase,leading to the change of density and sinking of metal layer. In the medium and smallscale tests final state is assumed to be in the equilibrium conditions. Transient interactions
as it was observed in the RCW test indicated that for reactor case the interaction may be
more complex. Equilibrium and transient conditions should be considered if in-vessel
retention concept is allied as an accident management procedure for existing and future
NPPs.
• Melt oxidation and secondary layers' inversion. Experiments in the oxidizing
atmosphere revealed fast oxidation of the melt limited by steam or air supply only. The
consumption of the oxygen lasted until the oxidation degree of corium reached the valueof 85 – 90%. The mass transfer processes between metallic and oxide phases was fast,
thus the composition and density of metal phase varied simultaneously with the change ofcorium oxidation degree. This led to the flip of metal and oxide phases in the tests.
Comparison of the final state after oxidation with the tests in the inert atmosphere showedreasonable agreement with the experimental data with respect to the composition ofphases, mass ratios, etc. This allows conclusion that final state was close to equilibrium
conditions.
• Influence of control rod materials. Addition of boron carbide showed that there was no
significant influence of carbon on the partitioning of main species between oxide andmetal phases except of the case when amount of iron was small enough. However thiscase is less prototypic for reactor applications. Addition of Ag – In – Cd seems not toaffect the total amount of metallic phase, however evaporation and condensation of
species led to increase of these elements in the skull and upper metal body.
• FP partitioning between layers. Partitioning of fission products revealed that metallic
FPs are concentrated in the metal phase while oxidic FP concentrated in the oxide phase.
The partitioning likely depends upon temperature, however the amount of tests seems to
be not enough to establish dependence of FP partitioning as a function of temperature.
3. Migration of moltem metal through debris. In the temperature range between corium
solidus and liquidus temperatures molten steel sufficiently overheated above the melting
point (about 200 K) easily penetrated through the corium debris with the characteristic
size between 2 and 3 mm. This process depends upon the debris particle size, thewettability of the corium particles and surface tension which are in turn depend upon
composition and temperature. The limiting temperatures were determined. Penetration
pattern depends upon small amount of impurities. Steel –zirconium metal melt does notleak through the debris due to formation of zirconium carbides and oxycarbides whichchanges significantly the wettability of corium particles. Oxidation experiments showed150

that after almost complete corium oxidation of iron and formation of iron oxides led totheir penetration and formation of conglomerate debris in the test section.
4. Extension of material properties database. Еxperiments were conducted to determine
the properties of oxide and metal phases. The most significant was development of themethodology for measuring the densities using the lying drop method. This allowed todetermine the density of the metal phase obtained in the corium – iron interaction tests.

151

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