Integrated Report

Main Resultsof the First Phaseof MASCA Project
November 2004

ORGANISATIONS PARTICIPATING
IN THE OECD/RUSSIA MASCA PROJECT:
The Association Vincotte Nuclear, jointly with Tractebel S.A., Belgium
The Atomic Energy of Canada Limited
The Nuclear Research Institute of the Czech Republic
The Fortum Engineering Ltd, jointly with
Valtion Teknillinen Tutkimuskeskus and Säteilyturvakeskus, Finland
The Commissariat á l’Énergie Atomique/Institut de Radioprotection etde Sûreté Nucléaire, France
The Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) mbH, Germany
The KFKI Atomic Energy Research Institute, Hungary
The Agenzia Nazionale per la Protezione dell’Ambiente, jointly with
the Ente per le Nuove Tecnologie, l’ Energia e l’Ambiente, Italy
The Nuclear Power Engineering Corporation, JapanThe Korea Atomic Energy Research Institute, jointly with the
Korea Institute of Nuclear Safety
The Nuclear Safety Consultancy (NSC), the Netherlands
The Consejo de Seguridad Nuclear, Spain
The Statens Kärnkraftinspektion, Sweden
The Paul Scherrer Institute, Switzerland
The Health and Safety Executive, the United Kingdom
The United States Nuclear Regulatory Commission
The Russian Federation Ministry of Atomic Energy, jointly with theFederal Nuclear and Radiation Safety Authority of Russia, and theRussian 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 theorganizations participating in the OECD MaSca Project in accordancewith the Project’s rules on information and intellectual property.

Russian Research Centre“KURCHATOV INSTITUTE”
OECD MASCA Project

Integrated Report
For MASCA Project use only

Main Resultsof the First Phaseof MASCA Project
November 2004

Presented by”RRC “KURCHATOV INSTITUTE”

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

2

MELT STRATIFICATION MECHANISMS...............................................................4
2.1
CARBON INDUCED STRATIFICATION ...........................................................................4
2.1.1
Test Apparatus and Procedure ..........................................................................4
2.1.2
Test Matrix and Results .....................................................................................5
2.2
STRATIFICATION INDUCED BY SEPARATION OF CORIUM COMPONENTS ......................9
2.2.1
Test Apparatus and Procedure ..........................................................................9
2.2.2
Test Matrix and Results .....................................................................................9
2.3
MAJOR FINDINGS ......................................................................................................11

3

INTERACTIONS OF CORIUM WITH STEEL (IRON)..........................................13
3.1
OBJECTIVES OF THE TESTS ........................................................................................13
3.2
TEST FACILITIES AND APPARATUS .............................................................................13
3.2.1
STFM Facility and test procedure ...................................................................13
3.2.2
RASPLAV-2 Facility and Test Procedure........................................................14
3.3
ANALYSIS OF EXPERIMENTAL RESULTS AND UNCERTAINTIES ..................................15
3.3.1
STFMFe Test Series.........................................................................................16
3.3.2
MA Test Series .................................................................................................19
3.3.3
Assessments of Mass Characteristics...............................................................19
3.4
DISCUSSION ON THE FINDINGS ..................................................................................24
3.4.1
Partitioning of Major Corium Components.....................................................24
3.4.2
Change of Corium Oxidation Degree ..............................................................26
3.4.3
Density of Oxide and Metal Phases .................................................................28
3.5
PECULIARITIES OF STEEL BEHAVIOR IN THE RCW TEST ..........................................28
3.5.1
The Test Facility ..............................................................................................29
3.5.2
Post-Test Material Studies...............................................................................31
3.5.3
The Ingot Splitting and Macrostructure ..........................................................33
3.6
CONCLUSIONS ...........................................................................................................40

4

FP PARTITIONING .....................................................................................................41
4.1
TEST MATRIX AND EXPERIMENTAL CONDITIONS .....................................................41
4.2
PARTITIONING OF FP SIMULANTS .............................................................................42
4.2.1
Influence of Metal Phase Composition ............................................................42
4.2.2
Influence of Temperature and Corium Composition .......................................42
4.2.3
Dependence Upon U to Zr Ratio .....................................................................45
4.2.4
Peculiarities of FP Partitioning in the RCW Test ...........................................46
4.3
CONCLUSIONS ...........................................................................................................48

5

MIGRATION OF MOLTEN METAL THROUGH POROUS OXIDE DEBRIS...49
5.1
OBJECTIVES OF THE TESTS ........................................................................................49
5.2
TEST APPARATUS......................................................................................................49
5.2.1
KORPUS Facility.............................................................................................49
5.2.2
Laboratory Tests in a Tungsten Crucible ........................................................51
5.2.3
The TULPAN T-8 Test .....................................................................................52
5.3
TEST MATRIX AND RESULTS .....................................................................................54
5.3.1
Behavior of the Carbon Steel ST-30 Melt ........................................................55
5.3.2
Behavior of Stainless Steel...............................................................................57
5.3.3
Behaviour of Stainless Steel – Zr Alloy............................................................61
i

5.4
6

DISCUSSION ON FINDINGS .........................................................................................65

MATERIAL PROPERTIES .........................................................................................67
6.1
PROPERTIES OF C-32 CORIUM...................................................................................67
6.2
THE PROPERTIES OF THE METALLIC PHASE ..............................................................68
6.2.1
The Spreading Temperature ............................................................................68
6.2.2
The Electric Conductivity ................................................................................70
6.2.3
The Thermal Conductivity................................................................................71

7

CONCLUSIONS ............................................................................................................73

REFERENCE……………………………………………………………………………….74
APPENDIX A: CONCLUSIONS AND RECOMMENDATIONS OF MASCASEMINAR 2004……………………………………………………………………………A-1

ii

LIST OF FIGURESFIGURE 2.1. SCHEMATIC OF THE EXPERIMENT (A) AND POST TEST EXAMINATION (B) ...............5
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..................................................................8
FIGURE 2.4. CHANGE OF THE AREA RATIO BETWEEN LIGHT AND GRAY PHASES ALONG THE
INGOT HEIGHT ....................................................................................................................8
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%) ..........................................................................................................................8
FIGURE 2.6. DISTRIBUTION OF THE LIGHT AND GRAY PHASE RATIO ALONG THE INGOT HEIGHT(TEST STFM#2/3) .............................................................................................................11
FIGURE 3.1. SCHEME OF PERFORMING TESTS WITH THE COLD CRUCIBLE AND FACILITY VIEW 13
FIGURE 3.2. RASPLAV-2 FACILITY SCHEMATICS EMPLOYED FOR MA-3 AND MA-4 TESTS AND
FACILITY VIEW .................................................................................................................14
FIGURE 3.3. THE VIEW OF SAMPLING FOR TESTS STFM-FE3 AND FE14 ..................................17
FIGURE 3.4. CHARACTERISTIC DISTRIBUTION OF CONCENTRATIONS IN THE OXIDE PHASE
ALONG THE INGOT HEIGHT ...............................................................................................17
FIGURE 3.5. U, ZR AND FE (STEEL) CONCENTRATIONS FOR ALL EXPERIMENTS WITH C32
INITIAL CORIUM................................................................................................................25
FIGURE 3.6. U, ZR AND FE (STEEL) CONCENTRATIONS FOR EXPERIMENTS WITH C70 INITIALCORIUM ............................................................................................................................25
FIGURE 3.7. U AND ZR CONCENTRATIONS IN A METAL PHASE AS A FUNCTION OF INITIALZIRCONIUM OXIDATION DEGREE ......................................................................................26
FIGURE 3.8. IRON CONCENTRATION IN THE OXIDE PHASE AS A FUNCTION OF FINAL ZROXIDATION DEGREE .........................................................................................................26
FIGURE 3.9. INCREASE OF THE CORIUM OXIDATION DEGREE AFTER THE INTERACTION FOR C-32
CORIUM ............................................................................................................................27
FIGURE 3.10. INCREASE OF THE CORIUM OXIDATION DEGREE AFTER THE INTERACTION FOR C60 – C-75 CORIUM ...........................................................................................................27
FIGURE 3.11. METAL PHASE DENSITY AT AMBIENT TEMPERATURE AS A FUNCTION OF
U/STEEL(FE) ATOMIC RATIO............................................................................................28
FIGURE 3.12. OXIDE PHASE DENSITY AT AMBIENT TEMPERATURE AS A FUNCTION OF OXYGENCONTENT ..........................................................................................................................28
FIGURE 3.13. THE FACILITY SCHEMATICS ................................................................................29
FIGURE 3.14. RCW FACILITY VIEW .........................................................................................30
FIGURE 3.15. LAYOUT OF THE LOWER CRUCIBLE CENTRAL SECTION ......................................32
FIGURE 3.16. THE MACROSECTION OF THE INGOT CENTRAL CUT ............................................34
FIGURE 3.17. PATTERN FOR CUTTING OUT OF THE CORIUM SAMPLES ......................................34
FIGURE 3.18. DIAGRAMS OF DENSITY DISTRIBUTION FOR THE CORIUM AND METALLIC PARTSALONG THE INGOT HEIGHT ...............................................................................................35
FIGURE 3.19. DISTRIBUTION OF U, ZR AND CHANGE OF CN IN THE CORIUM ALONG THE INGOTHEIGHT .............................................................................................................................37
FIGURE 3.20. PARTITIONING OF U, ZR AND FE IN METALLIC PARTS ALONG THE INGOT HEIGHT..........................................................................................................................................38
FIGURE 4.1. PARTITIONING OF FP SIMULANTS AS A FUNCTION OF TEMPERATURE AND FINALCORIUM OXIDATION DEGREE ...........................................................................................45
FIGURE 5.1. THE SIZE DISTRIBUTION OF THE C-32 AND C-100 CORIA .....................................50

iii

FIGURE 5.2. LAYOUT OF THE HEATING DEVICE WITH THE TEST ASSEMBLY IN THE KORPUS
FACILITY AND FACILITY VIEW ..........................................................................................50
FIGURE 5.3. TYPICAL TEMPERATURE HISTORY FOR THE SERIES OF TESTS K-1 – K-10 (TEST K5) ......................................................................................................................................51
FIGURE 5.4. THE LAYOUT OF THE TESTS D1 AND D3 ...............................................................52
FIGURE 5.5. THE LAYOUT OF THE D2 TEST ..............................................................................52
FIGURE 5.6. LAYOUT OF THE T-8 TEST SECTION ......................................................................53
FIGURE 5.7. TEST UNIT LAYOUT IN THE T-8 TEST ....................................................................53
FIGURE 5.8. MACROSTRUCTURE OF THE K-1 ASSEMBLY VERTICAL SECTION ..........................55
FIGURE 5.9. MACROSTRUCTURE AND RESULTS OF THE ELEMENT ANALYSIS FOR THE C-32
CORIUM ............................................................................................................................55
FIGURE 5.10. MACRO- AND MICROSTRUCTURE FROM THE TEST K-4 .......................................56
FIGURE 5.11. MICROSTRUCTURE OF THE C-32 BRIQUETTE UPPER PART AFTER THE TEST D1..57
FIGURE 5.12. MICROSTRUCTURE AND RESULTS OF THE ELEMENT ANALYSIS FOR THE C-100
CORIUM PARTICLES ..........................................................................................................57
FIGURE 5.13. MICROSTRUCTURE OF THE C-32 BRIQUETTE UPPER PART AFTER THE TEST D3..58
FIGURE 5.14. MICROSTRUCTURES OF THE C-32 CORIUM GROATS AND OF THE LEAKED STEEL
12X18H10T .....................................................................................................................58
FIGURE 5.15. MACROSECTION OF THE T-8 TESTS .....................................................................59
FIGURE 5.16. STEEL SPREADING IN THE CORIUM DEBRIS LOADING (THE INGOT MIDDLE PART,SAMPLE #1 IN THE AREA OF THE THERMOCOUPLE T10, T ≅ 1600°C................................59
FIGURE 5.17. MICROSTRUCTURE OF THE SAMPLE #2 (THE INGOT LOWER PART) .....................60
FIGURE 5.18. MACROSECTIONS OF CRUCIBLES AFTER THE TESTS K-6, K-7, K-8, K-9, K-5 ....61
FIGURE 5.19. MACROSTRUCTURE AND THE MICROPROBE ANALYSIS RESULTS (THE PROBE
DIAMETER = 50 µM).........................................................................................................62
FIGURE 5.20. MICROSTRUCTURE AND MICROPROBE ANALYSIS RESULTS OF THE K-6 SAMPLE(THE PROBE DIAMETER = 1 µM) ........................................................................................63
FIGURE 5.21. PENETRATION OF THE SS+ZR ALLOY INTO THE CORIUM PARTICLES ..................64
FIGURE 5.22. MICROSTRUCTURE AND MICROPROBE ANALYSIS RESULTS FOR THE K-5 SAMPLE..........................................................................................................................................64
FIGURE 6.1. VIEW OF TIGEL FACILITY ....................................................................................67
FIGURE 6.2. THE KINEMATIC VISCOSITY VERSUS THE TEMPERATURE .....................................68
FIGURE 6.3. THE TEMPERATURE DEPENDENCE OF SPECIFIC RESISTANCE ................................71
FIGURE 6.4. THE VALUES OF THERMAL CONDUCTIVITY OBTAINED IN TESTS ..........................71

iv

LIST OF TABLESTABLE 2.1. CONDITIONS AND MAIN RESULTS OF STFM1/1-11 EXPERIMENTS...........................6
TABLE 2.2. THE MATRIX AND MAIN RESULTS OF STFM2 TEST SERIES ...................................10
TABLE 3.1. CHARGE MATERIAL COMPOSITION.........................................................................15
TABLE 3.2. GENERAL PARAMETERS OF THE EXPERIMENTS WITH THE IRON AND STAINLESSSTEEL................................................................................................................................18
TABLE 3.3. AVERAGE COMPOSITIONS OF PHASES FOR TESTS WITH IRON AND STAINLESS STEEL..........................................................................................................................................21
TABLE 3.4. MASSES OF COMPONENTS IN THE OXIDE AND METAL PHASES AFTER TESTS .........22
TABLE 3.5. MASSES OF COMPONENTS AND INTEGRAL UNCERTAINTIES FOR CORIUMCOMPONENTS ....................................................................................................................23
TABLE 3.6. INITIAL AND FINAL ZIRCONIUM OXIDATION DEGREE .............................................24
TABLE 3.7. BASIC CHARACTERISTICS OF THE RCW FACILITY .................................................31
TABLE 3.8. THE MATERIAL BALANCE OF THE RCW EXPERIMENT ...........................................33
TABLE 3.9. WEIGHTS OF METALLIC PARTS ..............................................................................35
TABLE 3.10. THE CORIUM CHEMICAL ANALYSIS RESULTS ......................................................36
TABLE 3.11. THE CHEMICAL ANALYSIS RESULTS FOR METALLIC PARTS .................................38
TABLE 3.12. THE BASIC PARAMETERS OF TESTS WITH DIFFERENT SCALES OF LOADINGS .......39
TABLE 4.1. INITIAL LOADING OF FP SIMULANTS (GRAMS) .......................................................41
TABLE 4.2. PARTITIONING OF FP BETWEEN THE OXIDIC AND METALLIC PHASES DEPENDING
ON STEEL COMPOSITION (STFM-FP-1, 6 TESTS) .............................................................42
TABLE 4.3. FP SIMULANTS CONCENTRATIONS IN OXIDE AND METAL PHASES ........................43
TABLE 4.4. MASSES OF FP SIMULANTS FOUND AFTER THE TESTS AND RELATIVE ERRORS ......44
TABLE 4.5. PARTITIONING OF FP BETWEEN THE OXIDIC AND METALLIC PHASES OF THE
STRATIFIED MELT IN THE STFM-FP-1, 3 TESTS DEPENDING ON TEMPERATURE .............44
TABLE 4.6. PARTITIONING OF FP BETWEEN THE OXIDIC AND METALLIC PHASES DEPENDING ON
U/ZR ATOMIC RATIO ........................................................................................................46
TABLE 4.7. RESULTS OF THE CORIUM PLASMA EMISSION-SPECTRUM ANALYSIS FOR THE
CONTENTS OF FP SIMULANTS ...........................................................................................46
TABLE 4.8. RESULTS OF THE PLASMA-SPECTRUM ANALYSIS FOR THE INGOT METALLIC PARTS..........................................................................................................................................47
TABLE 4.9. THE COEFFICIENT OF FP PARTITIONING BETWEEN THE INGOT METALLIC ANDOXIDIC PARTS ...................................................................................................................47
TABLE 4.10. COMPARISON OF VALUES FOR COEFFICIENTS OF PARTITIONING ..........................48
TABLE 4.11. COMPARISON OF RESULTS WITH THE RCW DATA ...............................................48
TABLE 5.1. TEST MATRIX AND MAJOR PARAMETERS OF PERFORMED TESTS ...........................54
TABLE 5.2. TYPICAL MICROSTRUCTURE OF THE T-8 MIDDLE PART (SAMPLE #1,FIGURE 5.15)60
TABLE 5.3. TYPICAL MICROSTRUCTURE OF THE INGOT LOWER PART ......................................61
TABLE 6.1. DENSITY OF C-32 CORIUM .....................................................................................68
TABLE 6.2. RESULTS ON DENSITY, CHEMICAL AND X-PHASE ANALYSES OF THE RCW INGOTSAMPLES ...........................................................................................................................69
TABLE 6.3. TEMPERATURES FOR THE RCW INGOT METALLIC PHASE SPREADING ...................69
TABLE 6.4. THE “METALLIC BODY” CHEMICAL ANALYSIS ......................................................70
TABLE 6.5. SPECIFIC RESISTANCE (ρ) AND TEMPERATURE COEFFICIENT OF THAT (α) OF THE
ALLOY AND ITS COMPONENTS ..........................................................................................70
TABLE 6.6. THERMAL CONDUCTIVITY DATA OBTAINED IN TESTS ...........................................72v

vi

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 include 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. It demonstrated that the homogeneous corium melt behaved comparably
to simulant materials in natural circulation, therefore previous evaluations based on simulantmaterial data could be scaled to prototypic reactor conditions. It was shown that such abehavior can be expected if molten pool contained oxidic materials.
However, two of the four large-scale RASPLAV tests conducted with suboxidized corium
exhibited an unexpected behaviour. Post-test examinations showed that the melt pool,
initially of a homogenous composition and density, stratified into two layers of unequal
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 to obtain
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 focus of MASCA
Project was to investigate in detail the chemical behaviour of corium in contact withstructural materials, with emphasis mainly on steel.
A total of 15 small (loading up to 500 grams), 4 medium (2 kg) and 1 large-scale tests (about
90 kg loading) were conducted. All experiments were conducted using inductive heating andcold crucible technique, which allowed maintaining integrity of the facilities. The metal andoxide samples were carefully examined after the tests.
3 of 4 medium scale tests were conducted with the C-32 corium and 1 with C-75 corium,
which compositions are shown in Table 1.1. Stainless steel was added after corium melting.

1

A special attention was paid to the analysis of the mass balance in the system oxygen
contents. Post test examination allowed measuring the compositions of both metallic andceramic ingots.
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,some differences in the final stratification and distribution of phases appeared. They will be
discussed below.
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 6 small scale,and 2 medium scale test. 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 oxide component are distributed in the oxide corium.
A series of 10 small and 1 medium scale tests were dedicated to the study of molten metal
migration through a porous corium debris bed. Most tests were carried out with 2.5 mm size
debris with the porosity about 40 %. One test was made with 1cm debris to show the effectsof surface tension and wettability. The metal consisted of either steel or a alloy of Zr andsteel. The temperature was deliberately chosen in a relatively low range (between 1900 K and2300 K) above the melting temperature of the metallic alloy, however lower than the corium
melting temperature. In some cases, the melt progression through the particle bed wasobserved whereas in other cases there was no metal penetration. Chemical interactions
between the solid particles and the liquid metal were observed and samples were analyzed tomeasure compositions.
2

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 planned. Moreover, electric conductivity and heat conductivity ofmetallic body from T-7 test were measured at low temperature.

3

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. Such relocation was clearly determined in theexperiments. 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
had significant 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 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.
After 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 the4

70

ingot 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 [2] 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- the ratio between uranium and zirconium in the middle and top• KR =U / Zr top
corium samples;
C top• KC =- the ratio between carbon concentrations in the top and middle
C middle
corium samples.

5

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

Ratio of areas
occupiedby light
Mass ratio of
the top and and grey phases
main 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

KRTop Middle

KC
Top

0.870

* - Average value** - Non-isothermal experiment

6

Middle0.100 2.2

0.05

2.5

0.43

0.5

1.2

0.4

Practicallyone layer

0.8

~ 0.6

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.
1.4

1.6

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

0.4

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).
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
with the results of AW-1 test. Detailed discussions on the phases found in course of posttest
examination can be found in the report [2]. 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 additionof 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(0.006-0.009 wt %). This increase of carbon content in the atmosphere was found to be
insignificant and did not lead to corium stratification into two layers.

7

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

b)

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
Heighta)b)

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%)

8c)

2.2 Stratification Induced by Separation of Corium Components
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
and the bottom solid layer based on (UZr)O2-x according to the model proposed in [3].
It was assumed that for the corium used in the RASPLAV tests liquid zirconium based phaseof corium may move upward through the briquettes due to porosity and plastic deformationsof the grains and compaction in corium at T = 2150 - 2300°C. The second objective was tostudy the stability of corium configuration if such kind of stratification takes place.
2.2.1 Test Apparatus and Procedure

This series of experiment was conducted using the same STF facility. Briquettes and powderof C-32 corium (U/Zr = 1.2 at./at., carbon content 0.006 - 0.009 wt%) were fabricated bypressing of the powder mixture (UO2, ZrO2, ZrH2) in accordance with the techniquedeveloped 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 STFM2/1-5 experiments tungsten crucibles were loaded withcorium 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)Sl / S g ingot middle
phases in the top and middle parts of the ingot.

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 [2]. The most important were as
follows.
9

Table 2.2. The Matrix and Main Results of STFM2 Test Series
U/Zr (at./at)
Test Heating up#
T, oC 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 greyphase based on UZrO2-x has increased in the bottom solid layer. Coefficient KS of the lightand grey phases fraction ratio in the ingot top and middle parts reached the value of 3.5. The
cut of the ingot and the 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 tablebelow: 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

Experiment #6 when 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 were insignificant, coefficient of the light and grey phase fractions
ratio in the layers was 1.7, which is 2 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 andthen heated up to 2600°C. The structure of the ingot was found to be rather uniform.

10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 threshold valueof 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 amountof carbon does not exceed 0.1 wt. %. This effect can be explaned by the formation of
zirconium carbides and oxicarbides [4]. 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 the tests (melting
temperature of oxicarbides exceeds 3000°C) and are not dissolved by the oxidic phase,
oxycarbides may separate by the density difference after the melting of the oxide 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 RASPLAVAW facility were approximately 4 – 5 times larger then expected characteristic time for largescale 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

11are in a qualitative agreement with respect to the kinetic effect and in a reasonable
quantitative agreement with respect to characteristics of layers.
Another mechanisms for melt stratification were assumed in [3]. 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 the solid phase. The liquefaction
process may lead to the relocation of metal rich liquid accumulating inside of the melt at thetemperature 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 thematrix might force liquid Zr(O)x phase upward 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 with zirconium based phase
ZrOx with relatively small amount of uranium. At this temperature level the ratio of areas
occupied by light and gray phases reached the value of 1.4 (Experiment STFM#2/3) while theaverage 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 and bulk corium.

123 INTERACTIONS OF CORIUM WITH STEEL (IRON)
3.1 Objectives of the Tests
The study of the interactions between corium and metals is an important part of theinteractions in course of accident. Due to significant melting temperature difference such aninteraction can be expected when corium is solid or liquid. Special test series involving
different experimental facilities and different experimental conditions has been performed to
study the interactions between corium and steel in liquid phase.
The objective of this experimental series was to obtain quantitative and qualitative
characteristics of uranium, zirconium and steel components partitioning between different
phases. The following facilities were used:• Small scale STFM facility with the loaded mass about 0.5 kg;• Middle scale RASPLAV-2 facility with the loaded mass of about 2 kg;• Large scale RCW facility with the loaded mass about 100 kg.
In the experiments the following parameters varied: corium composition (variation ofoxidation degree and uranium to zirconium ratio), metal composition (experiments with theiron and stainless steel). This section presents the results conducted at different facilities:
STFM-Fe (10 experiments), STFM-FP (6 experiments), MA (4 tests) and RCW test.

3.2 Test facilities and apparatus
3.2.1 STFM Facility and test procedure

Scheme of performing STFM tests using the cold crucible technology is presented in Figure
3.1. The cold crucible was formed by the set of vertically placed (with the gap of ~ 2 mm)copper tubes cooled with water.
1
2
3

4
5

1 – W tube;
7 2 – crucible;
3 – inductor;
6 4 – corium rings;
5 – corium disks;
6 – powder of UO2.
7 – protective insulation
Figure 3.1. Scheme of Performing Tests with the Cold Crucible and Facility View

13

Disks of corium selected for the experiment were put onto the metal bottom cooled withwater inside internal cavity of this crucible then corium rings were put onto the disks. Small
gaps between corium and cold crucible tubes were filled with powder of UO2. Tungsten tubeof ∅ 16 mm diameter and wall thickness of 1 mm was placed into the internal cavity formedby corium rings. The tube was used as the starting heater. Heating up to the target
temperature (2500, 2550 or 2600°C) was performed slowly during 20 – 25 min in order toensure integrity of the sintered corium. After the required temperature was achieved tungsten
tube was taken out of melt. Further maintenance of necessary temperature was performed due
to direct heating of molten corium by inductor. After exposure within ~ 5 min iron was added
to the melt of oxidic corium. No changes of the consumed energy were observed after
extraction of tungsten tube and adding iron into the melt. Exposure time after steel or ironaddition varied between 10 and 60 min. After that slow cooling was performed with the rate
~ 50°C/min. When the temperature reached ~ 1800°C high-frequency generator was turned
off and cooling down to room temperature went on for several hours.
3.2.2 RASPLAV-2 Facility and Test Procedure

The RASPLAV-2 facility with the generator HFG7-60/0.066 was employed in the tests. The
melt was prepared by the inductive heating in the cold crucible. Figure 3.2 illustrates thefurnace schematics.

1 – spectral ratio pyrometer; 2 – argon-blown water-cooled pyrometer shaft;3 – water-cooled furnace cover; 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.
Figure 3.2. RASPLAV-2 Facility Schematics employed for MA-3 and Ma-4 Tests andFacility View

14

Table 3.1. Charge Material Composition
Chargecomponents
UO2
ZrO2
Zr
FP simulants
Charge mass
without steel
Steel*
Total mass

MA-1
MA-2
МА-3
МА-4
Introduced into Introduced into Introduced into Introduced into
the melt, g
the melt, g
the melt, g
the melt, g

1436.2
165.5
198.3
-

1353.8
362.7
83.5
-

1322.20
156.6
260.9
60.3

1175.2
144.3
226.9
53.6

1800.01800.01800.00

1600.00

200
2000.0200
2000.0200.002000.00

400.002000.00

Test procedure included several steps:• blow through the furnace with argon during approximately 20 minutes with a flowrate of 10 l/min;• switch on the high-frequency generator and simultaneous increase of flow rate to
20 l/min;• melting of the charge and measuring the melt temperature;• gradual addition of steel during 2 – 3 minutes with the control of melt surface
temperature;• exposure of the melt during 30 minutes;• melt crystallization in argon atmosphere during an hour.

3.3 Analysis of Experimental Results and Uncertainties
The post test examination for all tests included several steps:• Facility disassembly and visual observations of the ingot;• Removal of crust and thermal insulation separated ingot and cold crucible;• Preparation of samples for analysis of compositions of different chemical species,
namely U, Zr, Fe, Cr, Ni in both oxide and metallic parts of the ingot performed withdifferent methods, including chemical analysis, X-ray fluorescence analysis (XRF),
spark source mass-spectrometry method (SS-MS), Plasma Emission-Spectrum
Analysis (PE-SA), Mass-Spectrometry Method with Inductively Coupled Plasma(ICP MS);• Measurements of the densities of samples taken from the ingot;• Metallography and microhardness
Before discussion of experimental results it would be useful to discuss uncertainties whicharise from different sources.
1) The uncertainties associated with the measurements errors; typically these uncertainties
are not too large. The uncertainties in the determination of content of major elements withthe confidence probability γ = 0.95 were as follows:• U = ± 0.37 mass %;• Zr = ± 0.24 mass %;• Fe = ± 5 rel. %;• Cr Ni = ± 3 rel. %;• C = ± 0.014 wt %.

152) Another source of uncertainties deals with the determination of the masses of each phase.
After the disassembly the crusts between the cold crucible and ingot were removed. In
general those crusts contained the thermal insulation materials (typically UO2), materials
from corium (U, Zr), and some amount of steel or iron. Moreover between different
phases some transition layers were found materials of which can be accounted only
roughly.
1) The distribution of measured concentration was nonuniform. The reasons for thenonuniformity were different; the distribution of components during the test could benonuniform even for small scale tests, the crystallization process contributed also to the
distribution of components. The averaging was made by two different ways. For STFMFPand MA test series the average corium and metal phase probes were made. This proceduredescribed in details in the corresponding reports [6, 7, 8, 9]. For STFMFe series theaveraging was made analytically, using the experimental results on distribution ofcomponents along the height.
3) For analysis and use of experimental results initial masses and compositions of interacted
materials are required. Initial loading of different components is known however during
the test some amount of materials was evaporated, some amount interacted with thethermal insulation forming the crust. As a result initially loaded masses and theircompositions may change defining the uncertainties in the determination of initial massesof corium and steel interacted during the test.
A total of 21 tests dealt with the partitioning of main corium components were conducted.
The matrix of experiments as well as experimental conditions are presented in Table 3.2.
3.3.1 STFMFe Test Series

The ingot structure after disassembling was separated into several parts: a) the ingot
containing both oxide and metallic parts, b) the scull surrounding the ingot which presented
material which was in the contact with the molten corium, c) the powder of insulating
materials (mainly urania) which separated in the initial loading corium and cold crucible, d)the crust formed near colder parts of the melt.
In the Table 3.2 masses loaded in the crucible and masses of ingot obtained are presented.
One can see that only about 50% of initial loading finally formed the ingot. Typical sketchesof ingots obtained after the test for different masses of metal phase are presented in Figure
3.3 for Fe#3 test and Fe#14 tests. The position of metal phase in the ingot is indicated.
Approximate locations of the samples are also shown. Between 2 and 4 samples were takenfrom each ingot for analysis as it is indicated in figure as probe number (B891 for example).
Posttests examination of results indicated the dependence of the distribution of main
components along the ingot height which is illustrated in Figure 3.4. taking into account the
distribution of components the composition was averaged. The following assumptions weremade for the averaging: a) the composition of corium depends upon the height only; b) the
dependence as one can see from Figure 3.4 can be approximated by the linear fit; c) thevolume occupied by the metal phase is elliptical.
The distribution of each component in corium along the height was smoothed using 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.
As uranium and zirconium demonstrate different trends, the uranium to zirconium ratio
behave correspondingly 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 %.
16

Relative weight of each sample taken for analysis was calculated using the linear distribution
assumption and position of metal phase. These weight factors were used for 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)53
34#В893 (16,3 g)

47

4.5 g
B962*
magnetic
24.3 gB957

11#В894 (15,1 g)

6.2 g
B957*

3

5

#В890 (6,8 g)

∅17

∅27

∅30

∅30

STFM-Fe#3

STFM-Fe#14

35.6 g
B963

Figure 3.3. The View of Sampling for Tests STFM-Fe3 and Fe14
69
68
67

65
64

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

26

24

Zr, wt.%66

U, wt.%

28

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

22

20

63
18

62

16

61
0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.2

1.0

0.3

0.5

0.6

0.7

0.8

0.9

1.0

Zirconium distribution

Uranium distribution
2.0

1.6

1.8

1.4

1.6

1.2

1.4

1.0

#3
#4
#9
#14

1.2
1.0

wt.%

Fe, wt.%0.4

Height, rel. units

Height, rel. units

Cr
Ni
Fe

0.8
0.6

0.8

0.4

0.6

0.2

0.4
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 3.4. Characteristic Distribution of Concentrations in the Oxide Phase Along theIngot Height17

Table 3.2. General Parameters of the Experiments with the Iron and Stainless Steel

Test

Cn

2(C32) 33.4
3(C32) 33.4
4(C32) 32.7
5(C32) 33.4
6(C50) 50.4
7(C70) 70.6
968(C100)9(C32) 32.7
10(C70) 70.6
14(C32) 32.7
15(C32) 32.7
32.3
FP1
32.8
FP2
32.3
FP3
32.3
FP6
56.2
FP7
65.4
FP8
Ma-1(C33) 33
Ma-2(C75) 75
Ma-3(C33) 33
Ma-4(C33) 33
RCW(C32) 32∗

Total Mass of Crusts Total
Exp.
Metal Corium
mass
and
ingot
loadedMetal time, T, oC initial, loading,mass, (Oх+Mе), skull afetermingram grams
grams mass, g test
gram

Iron
Iron
Iron
Iron
Iron
Iron
Iron
Iron
Iron
Iron
SS
Iron
Iron
Iron
SS
Iron
Iron
SS
SS
SS
SS
SS

10
30
30
60
10
10
10
30
30
30
30
15
15
15
15
15
15
30
30
30
30
25

2500
5.4
408.5 567.9
2500
5.3
433.3 598.3
2500
15.0
424.5 611.0
2500
5.2
457.2 618.4
2550
5.3
425.0 600.7
2600
5.2
443.0 615.9
2650
6.1
416.4 491.5
2500
29.3
417.4 606.9
2600
15.1
436.6 604.7
2500
48.5
426.6 627.1
2500
29.9
459.7 637.6
2600
15.0 453.1∗ 623.9
2600
15.0 459.1∗ 604.0
2700
15.0 464.5∗ 621.8
2600
15.0 456.9∗ 622.4
2600
15.0 468.8∗ 633.2
2600
15.0 461.6∗ 635.6
2500 200.0 1800.0 2000.0
2500 200.0 1800.0 2000.0
2500 200.01800∗ 2000.0
2500 400.5 1600.0 2000.5
2500 4640.0 66985.0 97938.0

CoriumCorium
mass
mass
Metal
Mst
∆m,
beforeafter
final,(Mcor+Mst)
gram
gram interaction, interaction,
gram
gram

281.5 283.8 565.3 -2.6 18.8
306.6 288.6 595.2 -3.1 19.3
329.8 277.7 607.5 -3.5 54.2
327.9 284.3 612.2 -6.2 15.4
304.0 294.2 598.2 -2.5 18.2
315.0 298.6 613.6 -2.3 12.3
291.35 183.0 474.35 -17.2 5.45
302.6 300.6 603.2 -3.7 102.6
304.7 296.8 601.5 -3.2 23.8
313.7 307.7 621.4 -5.7 117.0
333.5 300.2 633.7 -3.9 87.0
348.0 273.0 621.0 -2.9 56.3
362.2 239.4 601.6 -2.4 46.7
349.3 269.1 618.4 -3.4 58.2
346.3 273.9 620.2 -2.2 47.4
364.3 265.8 630.1 -3.1 28.7
333.0 299.8 632.8 -2.8 22.4
1594.2 397.4 1991.6 -8.4 355.1
1748.1 250.4 1998.9 -1.1 213.7
1680.9 318.3 1999.1 -1.1 466.8
1854.1 144.5 1998.6 -1.9 673.0
53180.0 43819.0 96999.0 -939.0 7930.0

This mass includes the mass of FP simulants also (see Chapter 4).

18

262.7
287.3
275.6
312.5
285.8
302.7
285.9
200.0
280.9
196.7
246.5
291.7
315.5
291.1
298.9
335.6
310.6
1265.41535.51214.11181.145250.0

276.1
301.3
314.8
322.7
298.7
309.8
285.3
273.3
289.6
265.2
303.6
333.0
347.2
334.3
331.3
349.3
318.0
1434.21560.21503.11483.7
48537

1.84
1.59
4.36
1.29
1.57
1.52
1.86
9.51
4.28
15.18
7.97
3.89
3.33
3.73
3.86
2.82
2.77
11.22
10.69
11.24
19.48

3.3.2 MA Test Series

After the disassembly of the facility, the charge components and the molten products were
weighed (0.1 g accuracy). In general the same structure of the ingot as in the STFMFe tests
was found. The ingot containing oxide and metal phases was separated from the crusts andnot reacted materials the sum of which is shown in Table 3.2 (between 15 and 20 % of initialloading).
On preparing samples for the analysis all procedures were performed in argon atmosphere. At
first, the appropriate parts (the ingot oxidic and metallic parts, crust, and skull) were grinded
down to a grain size of not more than 2 mm. Then, the average sample was produced byquartering and sintered again down to a particle size of not more than 50 µm.
3.3.3 Assessments of Mass Characteristics

The ingot mass was evaluated after facility disassembly. All materials were separated in two
different parts: 1) the materials interacted during the test which were separated into corium
and metal parts; 2) materials such as crust, skull, thermal insulation (urania), aerosols, etc.
The second portion of materials was assumed not participating in the interaction. These
masses are shown in Table 3.2.
After separation of metal and oxide phases both parts of ingot were weighted, their masses
are summarized in the same table. General mass disbalance as a difference between loaded
and unloaded masses for most part of experiments do not exceed 0.5%. Corium mass beforeMeMeinteraction was calculated as the difference M oOx = M Ox
f + M f − M o . The indices here
represented final (f) and initial (o) states.
Concentrations of different components after the test are presented in the Table 3.3. This
table presents the results for average composition of the metal and oxide phases. Note that for
STFMFe tests the averaging was made analytically, while for STFMFP and MA tests theresults of chemical analysis of the average probe were used.
Using masses of oxide and metal phases after interaction one can find masses of each
, Me
,Me
, Me
component contained in each phase mOx= C OxM Ox, and the mass of each component
f ,j
f ,j
f
Ox
Me
Me
Ox ,Meinteracted which can be found as m f , j = C Ox
is the concentration
f , j M f + C f , j M f . Here C f , j

of j component in the oxide or metal phase (j=U, Zr, O, Fe, Cr, Ni) after the test.
If the mass of each component interacted is known then corium and metal masses before
interaction can be found as M Ox
∑ m f , j and M Mefo = ∑ m f , j . These formulae provide
fo =j∈Coriumj∈Steelassessments of the initial masses from the final state.
On the other hand the masses of each component before interaction can be obtained from theinitialcompositions
of
oxide
and
metal
phasesas
Ox ,MeOx Ox
Me Memo , j = M o Co , j + M o Co , j , j ∈ Corium, Steel . The values Co , j
are the concentrations ofcomponents in the oxide or metal phase from the initial loading. The integral uncertainties ofm − mo , jinteracted masses can be calculated as ∆ j = f , j.
mo , jTable 3.4 presents mass characteristics for each test calculated in accordance with theprocedure described above, namely interacted masses mf,j assessed from final compositionMeand masses for each component in the oxide ( mOx
f , j ) and metal ( m f , j ) phases.

19

The concentration of oxygen in the probes was not measured, therefore oxygen concentrationin the phases was calculated from the balance CO = 1 − ∑ C j both for the initial and final
jstate. The uncertainty in the oxygen content may be assessed as δCO = ∑ δC j . For oxidejphase one can get that δC = 0.7 wt.%, for metallic phase δC = 1÷ 2 wt.%. Relative errorsof oxygen content in oxide phase may reach 5 – 6%, and for the metal phase the value of 30 –60% depending on its concentration. Determining oxygen concentrations for STFMFP test
series, and MA-3 and MA-4 tests oxygen bound by the fission products was taken into
account.
When masses of each component interacted were calculated one can assess the corium
parameters such as U to Zr ratio and zirconium oxidation degree. The uncertainties of U/Zr
ratio was about 1%.
Zirconium oxidation degree in the oxide phase was calculated as follows:
Ox
O

Cn =

Me
O

mO / 2 AO − mU / AU
. The uncertainty of Cn was mainly due to uncertainty of oxygenm Zr / AZrconcentration, which might reach the value of 6 - 7 %. In the cases when content of freezirconium was measured these data were compared with the data obtained by the balancemethod.
Table 3.5 presents assessments of interacted masses calculated from final corium state andfrom initial loading as well as relative uncertainties of masses for major components
interacted. In most cases the uncertainty for uranium is positive and do not exceed 2 %, forzirconium uncertainty is higher, amount of steel assessed from final state is always less thenit was put as some amount of steel formed crusts and scull. Two last column in the tableshows masses of oxide and metal phase which are used as interacted masses.
Table 3.6 shows changing corium oxidation degree during FP test series.

20

Table 3.3. Average Compositions of Phases for Tests with Iron and Stainless Steel
Test
Fe2
Fe3
Fe4
Fe5
Fe6
Fe7
Fe8
Fe9
Fe10
Fe14
Fe15
FP1
FP2
FP3
FP6
FP7
FP8
Ma-1
Ma-2
Ma-3
Ma-4
RCW×

U

65.83
65.96
64.39
64.74
66.72
65.58
68.51
64.22
64.95
64.84
66.44
65.95
70.60
66.12
66.87
65.98
66.01
67.22
64.40
63.20
63.22
66.01

Composition of the oxide ingot, wt.%
Zr
O
Fe
Cr
Ni

21.22
21.13
22.68
22.81
20.05
20.68
16.67
20.64
21.10
20.76
19.08
19.57
16.07
19.83
19.15
18.55
18.20
18.00
19.30
20.10
17.74
19.95

12.34
11.93
12.52
11.77
12.77
13.34
14.72
14.13
13.49
13.93
13.17
12.60
12.19
12.60
12.19
14.56
14.81
13.10
13.60
12.23
13.27
13.04

0.51
0.88
0.31
0.57
0.36
0.30
0.00
0.90
0.36
0.37
0.87
0.90
0.42
0.53
0.50
0.31
0.30
1.30
1.30
0.80
0.90
0.27

0.28

Other

∗

0.07

0.082

0.057

0.3
0.4
0.09
0.29
0.13

0.1
0.3
0.05
0.05
0.08

0.861
0.633
0.826
1.012
0.531
0.594
2.840
2.280
0.451

∗

The sum of FP content in the oxide phase
The sum of FP content in the metal phase×
The data for RCW ingot are taken from the central part of the ingot
+

21

U

56.10
62.90
53.22
57.15
56.22
47.21
0.14
51.87
38.88
41.46
44.05
51.40
49.70
52.56
47.68
37.03
33.00
43.20
20.20
41.70
31.26
41.31

Composition of the metal ingot, wt.%
Zr
O
Fe
Cr
Ni

20.50
21.50
19.34
23.51
20.10
19.50
0.16
19.17
8.46
15.18
14.38
19.70
17.63
19.70
18.77
14.40
12.35
10.50
5.50
16.80
15.32
16.25

3.00
3.45
2.54
3.60
3.12
1.82
0.40
2.69
2.17
3.30
0.75
4.98
5.22
4.68
7.09
1.75
0.83
0.30
0.93
1.31
0.61
2.38

20.40
12.15
24.90
15.74
20.56
31.47
99.30
26.27
50.49
40.06
30.05
21.43
23.87
21.30
15.78
42.20
48.90
31.20
50.90
27.89
35.73
26.70

8.28

Other+

2.49

5.05

2.52

9.4
15.1
5.2
8.93
6.90

5.4
7.2
3.36
5.18
4.82

2.495
3.585
1.759
3.112
4.622
4.925
3.74
2.97
1.64

Table 3.4. Masses of Components in the Oxide and Metal Phases after Tests
Test

U

Masses of component in the oxide phase, g
Zr
O
Fe
Cr
Ni
Other

172.94 55.74 32.41
1.35
Fe2
189.49 60.72 34.28
2.53
Fe3
177.45 62.51 34.50
0.87
Fe4
202.32 71.30 36.78
1.79
Fe5
190.69 57.30 36.48
1.03
Fe6
198.52 62.59 40.37
0.92
Fe7
195.87 47.66 42.08
0.00
Fe8
127.43 43.03 27.04
2.30
Fe9
182.43 59.27 37.91
1.01
Fe10
127.55 40.83 27.41
0.72
Fe14
163.77
47.02
32.47
2.13
Fe15
192.38 57.09 36.74
2.63
FP1
222.74 50.70 38.47
1.33
FP2
192.48 57.73 36.67
1.54
FP3
199.87 57.24 36.85
1.49
FP6
221.43 62.25 48.85
1.04
FP7
205.03 56.53 45.99
0.94
FP8
850.35227.77164.5017.72
Ma-1
990.47 296.83 219.93 19.99
Ma-2
767.29 244.03 160.01
9.96
Ma-3
740.19 215.90 164.05 10.63
Ma-4
RCW× 28607.00 8027.35 7710.84 121.37

×

0.69

0.17
2.51
2.00
2.40
3.02
1.78
1.84

0.25

0.17

3.80
6.15
2.55
3.43
59.33

1.27
4.61
1.46 29.02
0.59 47.84
34.19 209.51

The data for RCW ingot are taken from the central part of the ingot

22

U

10.55
12.14
28.85
8.80
10.23
5.81
0.01
53.22
9.25
48.51
38.33
28.94
23.21
30.59
22.60
10.63
7.39
133.8442.53
194.6410.55
12.14

Masses of component in the metal phase, g
Zr
O
Fe
Cr
Ni
Other

3.85
4.15
10.48
3.62
3.66
2.40
0.01
19.67
2.01
17.76
12.51
11.09
8.23
11.47
8.90
4.13
2.77
48.33
11.54
78.42
3.85
4.15

0.56
3.84
0.67
2.34
1.38 13.50
0.55
2.42
0.57
3.74
0.22
3.87
0.02
5.41
2.76 26.95
0.52 12.02
3.86 46.87
0.66 14.18
7.25
2.15
2.80 12.07
1.40
2.44 11.15
1.67
2.72 12.40
1.02
3.36
7.48
2.39
1.19
1.48
0.50 12.11
1.33
0.18 10.95
1.10
18.12 106.92 32.21 18.51
1.99 108.77 32.20 15.39
6.11 130.18 24.27 15.68 17.46
0.56 238.58 53.84 39.10 19.99
0.67 2117.00 547.10 382.44 130.05

Table 3.5. Masses of components and integral uncertainties for corium components
Test
Fe2
Fe3
Fe4
Fe5
Fe6
Fe7
Fe8
Fe9
Fe10
Fe14
Fe15
FP1
FP2
FP3
FP6
FP7
FP8
Ma-1
Ma-2
Ma-3
Ma-4
1

Masses of components
interacted (mf,j), gU
ZrO
Steel1

183.49201.63206.29211.12200.92204.33195.88180.65
191.69176.05208.90221.31245.95223.07222.47232.06212.421002.21033.0961.92943.07

59.59
64.87
73.00
74.92
60.96
64.99
47.67
62.70
61.28
58.59
63.70
68.18
58.93
69.19
66.14
66.39
59.30
253.63308.37322.44310.00

32.97
34.94
35.87
37.33
37.05
40.59
42.11
29.80
38.42
31.27
34.08
40.85
41.45
40.44
39.59
52.31
48.93
171.86221.92168.92196.95Masses of components
loaded (mo, j), g
U
Zr
O
Steel

5.19 182.69
4.87 199.49
14.36 208.95
4.22 214.01
4.78 196.70
4.79 201.60
5.41 196.33
29.25 180.01
13.03 189.55
47.59 175.23
26.57 202.10
13.38 214.42
11.93 224.85
12.89 216.04
13.16 212.91
10.20 229.04
9.14 208.74
180.42 1004.12
187.12 1036.78
184.10 922.55
346.17 927.99

61.09
66.71
69.18
71.56
63.08
64.93
46.68
60.64
61.05
59.03
68.08
70.20
73.62
70.73
69.71
74.01
67.66
254.49308.19309.26303.13

31.55
34.45
36.20
36.95
38.52
42.87
42.36
31.78
40.31
30.94
35.68
39.82
41.76
40.12
39.76
47.44
44.07
169.07218.33163.64166.94

Including Fe, Cr, Ni as main components of the steel

23

5.4
5.3
15.0
5.2
5.3
5.2
6.1
29.3
15.1
48.5
29.9
15.0
15.0
15.0
15.0
15.0
15.0
196.5
196.5
196.5
393

Mass uncertainties(∆j ), %
U
Zr
O
Steel

0.21 -1.25
0.56 -1.38
-0.58
2.74
-0.57
2.39
1.10 -1.66
0.69
0.06
-0.04
1.12
0.15
1.64
0.87
0.50
0.37 -0.25
2.16 -2.82
1.18 -1.86
4.36 -11.20
1.36 -1.34
1.73 -3.10
0.86 -5.22
1.29 -6.17
0.59
0.52
0.19
0.40
1.15
1.15
0.67
0.98

2.20
0.74
-0.40
0.61
-1.90
-2.71
-0.23
-3.25
-2.08
0.66
-1.79
0.89
-0.50
0.17
-0.68
5.09
5.64
1.51
1.19
0.65
8.11

-2.02
-4.23
-2.17
-10.45
-5.21
-4.12
-5.98
-0.08
-7.38
-0.94
-5.89
-5.71
-11.42-7.56-6.52-19.07-24.26
-4.27
-2.44
-3.26
-5.68

Oxide and metal
masses interacted
MOx
MMet

276.05301.44315.16323.37298.94309.91285.65273.15291.39265.91306.68330.35346.34332.70328.21350.75320.651427.68
1563.291453.291450.02

5.29
5.09
14.68
4.71
5.04
4.99
5.76
29.28
14.06
48.05
28.24
14.19
13.46
13.95
14.08
12.60
12.07
188.5
191.8
190.3
371.9

Table 3.6. Initial and Final zirconium Oxidation Degree

Zirconium oxidation degree (Initial) Zirconium oxidation degree (Final)
C no

Based on Zrfree

C nfinBased on Zrfree

Fe2

32.6

33.4

46.8

39.6

Fe3

32.6

33.4

41.3

33.2

Fe4

33.4

32.7

48.5

57.0

Fe5

32.6

33.4

38.3

38.1

Fe6

54.6

50.4

54.0

Fe7

69.2

70.6

62.3

Fe8

97.5

100.0

94.2

Fe9

35.6

32.7

76.0

Fe10

69.2

70.6

64.4

Fe14

35.6

32.7

71.7

80.3

Fe15

35.6

32.7

63.3

59.0

FP1

44.6

54.3

59.6

FP2

44.6

47.9

57.2

FP3

44.6

53.3

59.2

FP6

45.5

47.6

60.8

FP7

59.5

87.4

78.3

FP8

62.4

92.9

82.6

Ma-1

38.1

73.3

53.6

Ma-2

73.0

83.3

72.3

Ma-3

36.5

66.1

64.4

Ma-4

39.7

85.2

70.2

70.1

3.4 Discussion on the Findings
3.4.1 Partitioning of Major Corium Components

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 anduranium transfer to metallic phase from suboxidized corium. This process leads to theincrease of metal phase mass and its density and consequently to the change of layer
configuration. Uranium transport to the metallic phase is closely related to the existence of
free zirconium in the initial suboxidized corium. The amounts of U and Zr atoms transferredto the metallic phase depend upon metal mass and corium oxidation degree. In Figure 3.5 theresults for U and Zr concentrations in a metal phase obtained for experiments with C32
corium are presented as a function of relative metal addition. The results demonstratereasonable consistency between different experimental series.
24

U
Zr
C32
U
Zr
Fe

70

Concentration, wt.%

60

50

40

30

20

10
0.00

0.05

0.10

0.15

0.20

Mst/(Mst+Mcor)

Figure 3.5. U, Zr and Fe (Steel) Concentrations for all Experiments with C32 Initial
Corium80

Concentration, mass %

70
60

U
Zr
Fe(Cr,Ni)

50
40
30
20
10
0
0.00

0.02

0.04

0.06

0.08

0.10

0.12

Mst/(Mst+Mcor)

Figure 3.6. U, Zr and Fe (Steel) Concentrations for Experiments with C70 Initial
CoriumSteel concentration increases with the increase of the relative steel content in the corium
while the concentrations of uranium and zirconium decrease with the increase of the steel(iron) content in 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).
The zirconium oxidation degree varied in these tests from 32 % to 100 %;• Steel (iron) content about 5 wt. % (Experiments STFMFe#4, FP#1, FP#2, FP#3,
FP#6, FP#7, and FP#8).
25

• Steel (iron) content about 10 wt. % (Experiments Fe#9, Fe#15, MA2 and MA3).
Data in a form of graphs are presented in Figure 3.7. Concentrations of U and Zr decrease
with the increase of zirconium oxidation degree. There is also some dependence upon metal
content, however this dependence is rather weak.
70~2% Fe~5% Fe~10% Fe

60

20

50

15

Zr, wt.%40

U, wt.%~2% Fe~5% Fe~10% Fe

25

30

10

20

5
10

0

0
30

40

50

60

70

80

90

100

30

110

40

50

60

70

80

90

100

110

Initial Zr oxidation degree

Initial Zr oxidation degree

Figure 3.7. U and Zr Concentrations in a Metal Phase as a Function of Initial
Zirconium Oxidation Degree

Figure 3.8 presents concentration of steel components in oxide phase. A very small amount ofiron goes into the oxide phase (typically about or less than 1 % wt.). Experiments conductedat the STFM facility with small amount of iron (about 2 wt. %) demonstrate regulardependence upon oxygen content in the melt (black squares). Note that final oxidation degree
was calculated on the base of Zrfree content in the melt. One may also conclude from thisfigure that iron concentration is increasing with the increasing of the mass of metal added tothe melt.
~2% Fe~5% Fe~10% Fe

1.4

Fe concentration, wt.%

1.2
1.0
0.8
0.6
0.4
0.2
0.0
30

40

50

60

70

80

90

100

Final oxidation Zr degree

Figure 3.8. Iron Concentration in the Oxide Phase as a Function of Final Zr Oxidation
Degree3.4.2 Change of Corium Oxidation Degree

After interaction, the degree of oxidation of the oxide phase is increased. Increasing of steel
to corium ratio leads to increasing of oxide phase oxidation degree approaching almost
26complete oxidation for large amounts of steel (iron) content in the test. Two methods wereused for analysis. 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 3.9 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. Test MA-1 excluded as U/Zr ratio was 1.5. Both sets of data demonstrated
reasonable agreement.
110
100

Mass balance
ZrfreeFinal Zr oxidation,%

90
80
70
60
50
40
30
0

5

10

15

20

Mst/(Mst+Mcor)

Figure 3.9. Increase of the Corium Oxidation Degree after the Interaction for C-32
CoriumFor C-60 – C-75 corium the data are shown in Figure 3.10. The following tests were used forthis plot (STFMFe#7 and #10, STFMFP#7 and #8, and MA2 test). Oxidation degree obtained
from the measurements of the Zfree were supplemented by data obtained from the massbalance. Again, the uncertainties in the oxidation degree for STFMFP series were large
enough, however the range of Mst/(Mst + Mcor) variation sufficiently large.
100

Mass balance
Zrfree95

Oxidation degree, %

90
85
80
75
70
65
60
55
0

2

4

6

8

10

12

Me/(Me+Ox), wt.%

Figure 3.10. Increase of the Corium Oxidation Degree after the Interaction for C-60 –C-75 Corium

27

3.4.3 Density of Oxide and Metal Phases

The density measurements were performed at the room temperature for the samples takenfrom the metallic and oxide phases. The results of measurements are presented in Figure 3.11
and Figure 3.12. The most relevant factor for the metal phase density is the ratio of U to steel
or iron, while oxide phase density depends mainly upon corium oxidation degree.
11.0
10.5
10.0

Density, g/cm

3

STFM
MA
RCW

9.5
9.0
8.5
8.0
7.5
0.0

0.2

0.4

0.6

0.8

1.0

1.2

U/Fe at.units

Figure 3.11. Metal 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 3.12. Oxide Phase Density at Ambient Temperature as a Function of OxygenContent

3.5 Peculiarities of 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°С 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.

28

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 [10].
3.5.1 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 3.13 and Figure 3.14. 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 С-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 3.13. The Facility Schematics

29

Figure 3.14. 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 и 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°С. 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 °С).
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.

30

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°С. The RCW
Facility Basic Characteristics are shown in Table 3.7.
Table 3.7. Basic Characteristics of the RCW Facility

Number of test sections

2

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

UO2 plates (kg)1.48

Thermal insulation 50 % of С-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 (°С)

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.
3.5.2 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;31•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.
Results of Visual Observations
Figure 3.15 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 3.15. 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 a32

rectangular mould that was filled up with epoxy for the further cutting out. Table 3.8 presents
the material balances for components loaded in the test section before the test.
Table 3.8. The Material Balance of the RCW Experiment
The initial loading

Mass
(kg)

Components collected after the test

Mass
(kg)

The С-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 С-100 corium + 50 % 24.830
the С-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.%
3.5.3 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 3.16.
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 3.16 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

33

are filled with metal migrated from metallic parts. Areas with the increased content ofmetallic 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 3.16. The Macrosection of the Ingot Central Cut
Cutting out of Samples for the Investigations
Samples for the chemical, metallographic and microprobe analyses (Figure 3.17) were cut outof the ingot oxidic part by a tubular diamond drill. Metallic parts were drawn out of the ingot
and weighed (Table 3.9).

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 3.17. Pattern for Cutting out of the Corium Samples

34

Table 3.9. 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 3.18. 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

200

160

#1
#2
120

#3
80

#4

40

#5
#6

0

7

Bottom metallic part

8
0

-40

8.8

9

9.2

9.4

Density (g/cm3)

9.6

Density distribution of the corium

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 3.18. Diagrams of Density Distribution for the Corium and Metallic Parts Alongthe Ingot Height35

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 3.17. 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 3.10 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 3.19 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) С-32
corium [11].
Table 3.10. 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

36

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

The degree of zirconium oxidation (Сn) was assessed basing on the results of determinationof free zirconium (Zrfree) and total zirconium (Zrtotal) contents according to the formula:Сn = (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 3.19 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
Сn = 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 [10] 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.

37

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 3.11 presents the chemical analysis results for these samples for six elements (uranium,
zirconium, iron, chromium, nickel, and carbon). Figure 3.20 shows the distribution ofcomponents in the metallic parts versus the level of their location in the corium melt after the
test.
Table 3.11. 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 3.20. Partitioning of U, Zr and Fe in Metallic Parts along the Ingot HeightAs Table 3.11 and Figure 3.20 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(Мss - ∑МSSmet.part)/Мcorium in ingot
is ~ 3.7 mass %. Oxygen content in the metallic parts was 0.8 – 1.2 mass %.

38

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 МА-1, 3 tests and ~ 150 times higher than those in the tests of the
STF series. Table 3.12 lists the data for three tests with different scales of loadings (the C-32
corium). The mass ratio Мsteel/Мcorium ≅ 0.1.
Table 3.12. The Basic Parameters of Tests with Different Scales of Loadings

CoriumSteel

Exposition
time,
min

RCW

45250

4643

22

STFMFe-15

273

29.9

STFMFe-3

287.3

MA-3

1480.8)Test*

Mass, gComposition of the lower
metallic part, mass%

Mst./Mcor

U

Zr

Steel

52.0

20.1

23.6

38.9

13.5

45.7

30

44.1

14.4

40.8

0.11

5.3

30

62.9

21.5

12.2

0.02

200

30

41.7

16.8

36.4

0.13

0.10

*) – In all listed tests, the mentioned metallic parts relocated onto the pool bottom.
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 МА-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 30 – 40 mass % of uranium which is enough to increase the density
above corium density) and partial detaching from the top 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 thecomposition of the lower part should be comparable with the results of STFM tests with theadded mass of about 2% (test STFMFe 3, where the uranium concentration was even higher63%). Measured concentrations are higher than the values measured in the tests MA-3 and
STFMFe-15 conducted with the steel.
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 МА-1 test, in
39

which 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 Сn = 65 %. After the
RCW test was completed, Сn ≅ 53 % that was significantly less than the equilibrium value.

3.6 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.
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 МА.
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.

40

4 FP PARTITIONING
4.1 Test Matrix and Experimental Conditions
Experiments with the corium and steel performed in the frames of MASCA Project showedsignificant redistribution of major corium components such as uranium and zirconium
between metallic and oxide phases. Significant part of uranium, zirconium and oxygen passinto the steel melt due to the interaction of structural steel with a suboxidised corium. The FP
series of tests was aimed at the investigations of mechanisms associated with the partitioningof low-volatile fission products between stratified melts of the metallic and oxidic phases.
Mo, Ru, Sr, Ba, Ce and La were chosen as the simulants of fission products.
Three experimental facilities were used for study of FP behavior, large scale RCW facility,
two middle scale tests MA-3 and MA-4, and a series of small scale experiments performed at
the STFM facility. 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 4.1. As
FP were added in a different chemical forms in the brackets the recalculated amount for thepure species are shown. Material balances made up for each test allowed to demonstrate thatthe 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 4.1. Initial Loading 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 [9,12]. 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 loading, a part of sintered coria was grinded
under nonoxidizing conditions down to the groats 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 coriumoxidation degree, 1 – 3 minute exposition and the inductor switching off.
U, Zr, Zrfree, Fe were determined by the standard chemical methods. Fission products
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 (SeeTable 3.2.
The following parameters varied in the STFMFP tests:• The 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
41

oxidation degree was about 65% for tests #1 through #6, and 80 – 85% for tests #7
and #8.
• One tests #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 otherexperiments the ratio was 1.2.
Two MA experiments were the STFM series was unique, thus it was assumed that the errors
in the FP test series are conducted at lower temperature about 2500oC and amount of steel
added to the corium was higher about 10 % and 20 %.
The following parameters were chosen as main parameters which determine the partitioningof FP between phases: temperature, final corium oxidation degree, and U/Zr ratio. Moreoverthe dependence upon steel (iron) to corium ratio was analyzed. Partitioning coefficient wascalculated as ratio of concentrations between metal and oxide phases.
It is difficult to assess the uncertainties associated with the nonuniformity of FP distribution
as each experiment conducted in the same as for RCW test. The 95 % confidence levelassume the error equal 3⋅σ.

4.2 Partitioning of FP Simulants
4.2.1 Influence of Metal 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 the temperature of 2600oC. The results are shownin the Table 4.2.
Table 4.2. 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

Data from this demonstrate that the steel composition impact is weak with respect to Mo andRu partitioning coefficients. Although partitioning coefficients for other FP are subjected to a
noticeable influence at the transition from practically pure Fe to stainless steel. The value of
ko is reduced 2.3 times for Sr, 5.2 times – for Ba, and 8 times – for La. However thisconclusion can not be made as the number of tests was too low and uncertainties were toolarge to make a distinct conclusion.
4.2.2 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 4.3). Final corium oxidation degree for those tests
was approximately similar and ranged between 55 and 60%. Masses of FP simulants foundafter the tests and relative errors are shown in Table 4.4. Another series of tests with the final
corium oxidations degree of about 85% was formed of MA4, FP#7, and FP#8 tests. In the
Table 4.5 calculated partitioning coefficients are presented.

42

Table 4.3. FP Simulants Concentrations in Oxide and Metal Phases
Concentration of FP
simulants in the oxide phase

Test conditions
Exposure Oxidation,
Metal T, oC Mo
time
final

Ru

Sr

Ba

Ce

La

Concentration of FP
simulants in the metal phase
Nb

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

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

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

MA4
RCW, Av.
RCW, σ1

1

25

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

43

NbTable 4.4. 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 4.5. 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°С

16

16

0.16

0.13

<0.05

0.42

FP#6 2600°С

14.2

24

0.069

0.025

0.037

0.046

FP#3 2700°С

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°С

61.7

76.2

0.13

0.1

<0.14

0.24

FP#8 2600°С

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 4.1). The partitioning coefficient for metal FP such as Mo and Ru is decreasing withthe growth of temperature. This is due to the fact that the concentration of Mo and Ru in
metal phase decreases, while concentrations in oxide phase increase. In contrast, temperature
increase from 2500°С to 2700°С in results in the general increase of partitioning coefficients
Sr, Ba, Ce and La.
Increasing of the corium oxidation degree change significantly the partitioning of Mo and Ru.
However partitioning coefficients of Sr, Ba, Ce and La remain at approximately the samelevel.

44

Final corium oxidation degree of 85%

Final corium oxidation degree of 65%
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

o

2600

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

2550

2600
o

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 4.1. Partitioning of FP Simulants as a Function of Temperature and Final
Corium Oxidation Degree
4.2.3 Dependence Upon U to Zr Ratio

Experiment FP#2 was conducted with the ratio of uranium to zirconium equal to 1.6.
Comparison of partitioning coefficients for tests FP#1 and FP#2 is presented in Table 4.6,
which demonstrates that the increase in U/Zr atomic ratio impacts noticeably Mo and Ru

45

partitioning for which ko is increased by ~ 2 times. For Sr, Ba and La, changes in the valuesof ko do not exceed 30 %.
Table 4.6. 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°С

16

16

0.16

Atomic ratio of U/Zr = 1.68
FP#8

2600°С

35

32

0.115

4.2.4 Peculiarities of FP Partitioning in the RCW Test
Partitioning of Fission Products 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 4.7 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.
Table 4.7. 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

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 4.8).

46

Table 4.8. 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

Because the distribution of FP was not uniform the measurement errors were evaluated by theaveraging of results. For plasma emission spectrum method the measurement the dataobtained at the RCW facility were used. Results of measurements presented in Table 4.7 wereaveraged. Because they do not indicate any spatial distribution, the average value wascalculated and standard data deviations were assessed. The integrated uncertainties inconcentrations (standard deviation) for RCW ingot are shown in Table 4.4.
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 4.9) that characterizes the ratio of concentrations
in two phases being in the equilibrium state with each other under isothermal conditions.
Table 4.9. 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

Table 4.9 employs FP concentrations in the samples of the oxidic corium containing steel
Xst. < 1 mass % (Table 3.10).
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 might bereleased with dissolved in that Zr and U precipitated onto the pool bottom. This process wasobserved in the STF tests. It was also possible that the sinking metallic part captured Ru fromthe corium. This may explain a significant difference between ko for ruthenium in the lower
metallic body and 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:
the chemical composition, oxygen concentration, Cn, density. Let us compare values of ko
obtained in different tests STFMFP-1, МА-3 and RCW, in which the C-32 corium was used
and the ratio Мsteel/Мcorium ≤ 0.1. These data are presented in Table 4.10.

47

Table 4.10. 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 [13] presents data on FP partitioning in the melt UO2 - iron (Table 4.11).
Table 4.11. Comparison of Results with the RCW Data
Ru

Mo

Ba

La

Ce

FP contents in iron, mass %0.21 - 1.14

1

7⋅103

1⋅10-40.02

FP contents in UO2, mass %

0.01

0.1

0.51

0.35

0.68

ko [13]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 4.11 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.

4.3 Conclusions
Initial series of tests dealing with the distribution of FP between metallic and oxide phases
demonstrate that metallic fission products such as Mo and Ru are concentrated in the metallic
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 - 2.7 times with the temperature increase
from 2600°С up to 2700°С.
• The increase in the corium Cn (from С-65 up to С-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 rangeof ~ 50 % for Sr, Ba, Ce, and La.
Fe replacement by SS does not practically influence ko for Mo and Ru but decreases
significantly (2 - 9 times) that for Sr, Ba, and La.

48

5 MIGRATION OF MOLTEN METAL THROUGH POROUS OXIDEDEBRIS
5.1 Objectives of the Tests
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 [14].
In case of accident, it is assumed that relocated molten materials comes into water andfragmented into debris of quite small sizes. This process was studied in detail in the FARO
facility [15]. At the average, 50 % of particles had a size less than 4 mm in all tests.
According to the data of the reference [16], 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 [17, 18 and 19] and it was found that thisprocess may influence the formation of molten materials.
Thus the objectives for the tests was to study migration of molten metals formed in course of
accident through the porous corium debris. The parameters varied were the major parameterswhich may determine the interaction process, such as:• sizes of particles;• compositions of metallic melts;• the temperature variation rate, gradient, and maximum value;• the corium composition (suboxidised, completely oxidized);• the test scale.
To achieve experimental goals a total of 10 small-scale tests were conducted. Moreover 3
supporting tests were conducted using the STF facility. In addition one larger scale test T-7
was conducted using TULPAN facility.

5.2 Test Apparatus
5.2.1 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.
The molten metals were presented by the steel of the St301 quality, stainless steel
12X18H10T2, 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 were1
2

Fe - 99.59 wt.%, Mn - 0.21 wt.%, Si - 0.15 wt.%, C - 0.05 wt.%
Fe - 67.5 wt.%, Cr- 20.37 wt.%; Ni- 10.43 wt.%, Other: 1.7 wt.%49

prepared by the use of two riddles with openings of 1.2 and 3.5 mm. Figure 5.1. illustrates thesize distribution of the corium particles.

C-100

C-32
Figure 5.1. The Size Distribution of the C-32 and C-100 Coria

Most tests were performed in the KORPUS facility in the argon atmosphere (Figure 5.2). 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.
1. graphite heater;
T1
T2
2. Ta cup;
3. steel;
4. W crucible;
8
5. corium loading;10
6. ZrO2 (or UO2) crucible;
7.
W support;
1
8. carbon-fibrous thermal insulation;
2
9. graphite disc;10. Mo casing with ZrO2 cladding.

5
6

70

D 28

30

4

~20

3

ПИР. 5

D 30

7
9
D 40

8

Ar

Figure 5.2. Layout of the Heating Device with the Test Assembly in the KORPUS
Facility and Facility View

50

As a rule, one test assembly was placed into the heating device in each test. In one case threetest 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 [20]. Typical temperature history for all tests isshown in Figure 5.3 using as an example temperature measurements in the K-5 test.
2500
T1
T2
Pyr 5

Temperature (oC)

2000

1500

1000

500

0
0

60

120
Time (min)

180

240

Figure 5.3. Typical Temperature History for the Series of Tests K-1 – K-10 (Test K-5)
5.2.2 Laboratory Tests in a Tungsten Crucible

As K test series was conducted in a carbon heater three supporting tests were conducted to
study the possible carbon influence on the test results. The tests were conducted with the C32 corium briquette of the size of 15 × 15 × 15 mm. Steel St-30 or SS ring plate (See Figure5.4) 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 5.5).

51

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 5.4. 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 5.5. The Layout of the D2 Test
5.2.3 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 porous
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 5.6 illustrates the test section layout and test unit itself is shown in Figure 5.7.
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.

52

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

2400°C

q
7

q

q

6

Figure 5.6. Layout of the T-8 Test Section
7

8

6
340
240
216
154

1 – intermediate bottom;
17

16

14
13

3

11

12

5

4

120

90

1
2

10

6
7

15
8

9

18

1

2

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 5.7. Test Unit Layout in the T-8 Test

The following parameters were monitored in addition to the temperatures:• voltage, current, and power with regard to cos ϕ in the inductor;• argon flow rate and pressure;53• CO content in the atmosphere of the gas-vacuum chamber;• cooling water flow rate and temperature in the inductor coils.
Temperatures in TU were monitored by the system of thermocouples consisting of 8 Cr/Al
thermocouples and 10 W/Re thermocouples. Figure 5.7 shows the schematics for the locationof 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).

5.3 Test Matrix and Results
Table 5.1 demonstrates the basic parameters for the performed tests. In the tests different
parameters varied such and temperature, porosity of the debris and ratio of calculated liquid
metal volume to the pores volume in corium. As a rule, the steel melt volume exceeded thevolume of pores in the corium loading except of three D tests, which were conducted with thecorium briquettes. The experimental matrix and major parameters of the test are presented inTable 5.1.
Table 5.1. Test Matrix and Major Parameters of Performed Tests
CoriumMass, Porosity,
Type
g
%

#

Metal
Mass, V
QualityVgliq. Me
pore

Test
Metal
temperature, penetration
into the loading°СInteraction of steel St-30 with corium debris
К-1 С-32 117.00
К-2 С-32 80.00
К-4 С-32 81.00
D-1 C-32
К-3 С-100 90.30

38.0
~50.0
48.0
10.0
43.0

St-30
St-30
St-30
St-30
St-30

95.50
95.50
64.80
14.6
96.00

1.70
1.33
0.96
0.5
1.48

1600
1600
1800
1800
1600

down to bottom~ 2.8 mm
down to bottom
down to bottom~ 0.2 mmInteraction of stainless steel with corium debris
D-3 C-32
К-10 С-32
T-84) C-32

248.9
4026

10.0 12X18H10T 14.6
51.2 12X18Н10Т 259.46
51.0 X18H10T 3450

0.25
1.11
1.0

1800
2050
2050

down to bottom
down to bottom
down to bottom

Interaction of 85SS/15Zr with corium debris

51.0 85SS/15Zr2) 43.00 1.01
1800
1.27 mm
К-7 С-32 45.37
49.0 85SS/15Zr1) 63.70 0.922050
3.2 mmК-5 С-32 80.20
85SS/15Zr
0.25
1800
PartialD-2 C-32
2)
3)
85SS/15Zr
44.00 0.98
1800
0.71 mm
К-8 С-70 46.33 46.0
85SS/15Zr2) 46.15 1.06
1600
1.4 mmК-6 С-100 65.90 41.02)3)85SS/15Zr
44.00 0.98
1800
0.34 mm
К-9 С-100 67.86 41.0
1) Briquette pressed from the mixture of zirconium and stainless steel powders(85 mass % of SS + 15 mass % of Zr) hot-extruded in the protective casing.
2) Alloy of 85 mass % of SS and 15 mass % of Zr.
3) Assemblies K-7, K-8 and K-9 were placed inside one heater and were heatedsimultaneously.
4) Non isothermal test conditions.
3)54

After test was 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
aiming to obtain images with a large magnification and to determine the element compositionof individual phases.
The methods of optical microscopy (Neophot 30) with a quantitative element analysis in the
micro analyzer Comebax 50SX were employed for the study.
5.3.1 Behavior of the Carbon Steel ST-30 Melt

Four experiments were conducted to study the interaction of St-30 steel with the debris madeof C-32 corium, and one test utilized C-100 corium. In the test K-1, the corum briquettes
were used and molten steel penetrated into the gaps between the fragments of the C-32
corium briquettes in all layers and reached the crucible bottom at relatively low temperatureof 1600oC. The minimum gap in which steel was able to penetrate was about 0.2 mm (SeeFigure 5.8.

Figure 5.8. 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 5.9. Macrostructure and Results of the Element Analysis for the C-32 Corium

In the test K-2 the debris was formed of particles (See Figure 5.1), molten steel penetrated attemperature of 1600OC into the corium C-32 loading at a depth of ~ 2.8 mm (Figure 5.9).
Temperature increase to 1800oC (Test K-4 with the same corium particles as in the test K-2)
caused penetration to the bottom of the crucible as it is shown in Figure 5.10. This figure55

demonstrates also the microstructure of samples cut out of the ingot upper and lower parts
and the microprobe analysis results. It can be seen that a partial interaction of steel with thecorium particles took place in the crucible lower part resulted in the mixture melting. This isindicated by the ingot structure containing the iron matrix with dendrites of intermetallic
compounds Fe3Zr and Fe3U with the impurity of zirconium.

Figure 5.10. Macro- and Microstructure from the Test K-4

Similar metallic phases were found in the interaction zone of the K-2 test (Figure 5.9).
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.
Steel St-30 penetrates through the pores of the corium briquettes with the porosity of about
10 %, pores characteristic size as one can see from Figure 5.11 was about 20-30 micrometers.
The metallographic study demonstrated vertical streams of the light metallic phase (Figure5.11) observable throughout the volume of the C-32 corium.
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 5.12) the following basic phases were observed:• of the (U0.9Zr0.1)O2.0oxidic type with the impurity of Fe and• of iron with the impurity of oxygen (3 - 8 at%).

56

Upper part of the ingot

Middle part of the ingot

Metal

Figure 5.11. Microstructure of the C-32 Briquette Upper Part after the Test D1

С-100 particles

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

100 µ mFigure 5.12. Microstructure and Results of the Element Analysis for the C-100 Corium
Particles
5.3.2 Behavior of Stainless Steel
Results of Supporting and Small Scale Tests
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.
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.
At temperature of 1800oC molten steel leaked through the corium briquettes with the porosity
about 10% as it is shown in Figure 5.13. 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 so that corium particles became uncovered in the upper part.
(Figure 5.14a).

57

Figure 5.14b 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 5.14c 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 those
produced in the test K-2.

The C-32 briquette upper part

The C-32 briquette middle
part

Figure 5.13. Microstructure of the C-32 Briquette Upper Part after the Test D3

c)

a)

b)

Figure 5.14. Microstructures of the C-32 Corium Groats and of the Leaked Steel
12X18H10T
Results of middle scale tests TULPAN T-8
Additional information on steel behavior was obtained from the test TULPAN T-8. As it was
mentioned this test was conducted in the conditions of the vertical temperature gradients. The
cross section of ingot obtained after the test is shown in Figure 5.15.
On examining the macrosection of the ingot central section, the following was noted:• 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 form a layer ~ 8 mm thick. ZrO2 crucible walls
were wetted by steel (Figure 5.15), the wetting angle Θ ≅ 70°.

58

•
•

Separate portions of particles with a light surface similar to those observed in the K-5
test were observed in the loading in the ingot lower part below the lower boundary ofsteel penetration.
A thickness of the corium particle layer was reduced after the test by ~ 12 mm1(~ 20 % of the initial thickness).

Figure 5.15. Macrosection of the T-8 Tests

Thermocouples T7 and T9 were located approximately 3 mm lower than the lower front of
the leaked alloy and showed the temperatures 1170°C and 1310°C, respectively, thatapproximately by 100 - 250°C lower than Tmelt of steel.
Figure 5.16 shows the macrostructure of the sample #1 (Figure 5.15). Table 5.2 contains the
microprobe analysis results.
Two basic phases were observed in the sample #1 (T ≅ 1600°C):
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
composition observable only in electron rays (1.1.2). These two phases did notcontain oxygen.
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 5.16. Steel Spreading in the Corium Debris Loading (the Ingot Middle Part,
Sample #1 in the Area of the Thermocouple T10, T ≅ 1600°C

1

Average porosity reduced between 7 to 20 % usually.

59

Table 5.2. Typical Microstructure of the T-8 Middle Part (Sample #1,Figure 5.15)
#

Fe

Zr

U

C

Ni

Cr

Concentration (mass%)O

Sum Point Comment

Composition

Averaged according to the changes in three points

1 69.95 0.06 0.04 0.11 5.41 23.17 0.00 99.23

1.1.

Grains in
the light
field

Fe0.7Cr0.25Ni0.05

Impurities
at the2 35.66 23.43 19.55 0.00 16.52 3.41 0.00 98.45 1.1.2*)
Fe0.48Ni0.21Cr0.05Zr0.2U0.06
boundaryof grains
0 0.01 6.86 20.76 0.00 98.98

1.2

Light
grainsFe0.7Cr0.23Ni0.06U0.01

4 0.05 7.64 79.54 0.00 0.05 0.00 12.97 99.93

1.3

Dark
grains(U0.8Zr0.2)O2.0

3 70.33 0.00*) impurities 1.1.2 were segregated only by the microprobe analysis and were not observablein the optical microscope.
Figure 5.17 illustrates the microstructure of the sample #2 (Figure 5.15). Table 5.3 presents
the microprobe analysis results for the ingot lower part (Sample #2)

Figure 5.17. Microstructure of the Sample #2 (the Ingot Lower Part)

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.
Light impurities 2.1 consisted of two phases: 2.1.1 and 2.1.2 (Table 5.3) and wereapproximately equal in areas (could be segregated by the microprobe analysis). Grey
impurities were close in composition to U6Fe.

60

Table 5.3. Typical Microstructure of the Ingot Lower Part
#

Fe

Zr

U

C

Ni

Cr

Concentration (mass%)O

Sum Point Comment

Composition

5 10.22 33.47 49.29 0.00 6.07 0.03 0.00 99.73 2.1.1*)

Light
impurities

6 36.15 42.93 7.58 0.00 2.43 10.83 0.01 99.82 2.1.2*)

Light
Zr U Fe Cr Ni
impurities 0.33 0.02 0.47 0.15 0.03

7

Grey
impurities

3.54 0.35 95.54 0.00 0.26 0.00 0.00 99.44

2.2

Zr0.43U0.24Fe0.21Ni0.12

U0.84Fe0.14Zr0.01Ni0.01*) impurities 2.1.1 and 2.1.2 were differed only by the microprobe analysis.
5.3.3 Behaviour of Stainless Steel – Zr Alloy

Remaining experiments were conducted with the alloy made of 85 mass % of
SS + 15 mass % of Zr. Figure Figure 5.18 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 the corium
particles in neither of the tests. Wetting of the ZrO2 crucible walls by the melt was not also
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 at the same temperature of 1800oC, the penetration depth decreases with the increase ofoxidation degree: from 1.3 mm for C-32 to 0.71 and 0.34 for C-70 and C-100
correspondingly.
К-7
С-32

К-5
С-32

К-8
С-70

К-6
С-100

К-9
С-100

1800°С

2050°С

1800°С

1600°С

1800°С

Figure 5.18. Macrosections of Crucibles after the Tests K-6, K-7, K-8, K-9, K-5

Let us consider the interactions of alloys with the coria of different types.
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.

61

Figure 5.19 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.

Fe

Ni

Cr

Zr

U

C

O

Concentrations of elements (mass%)Sum

Dz
point Calculation formula(µm)

54.14 8.52 13.56 17.10 4.04 0.32 0.00 97.68

50

1[(Fe0.71Cr0.19Ni0.10)0.87
Zr0.12U0.01]×С0.02

28.24 4.26 7.90 38.15 11.23 1.76 8.63 100.17

50

2[(Fe0.69Cr0.20Ni0.11)0.61
Zr0.35U0.04]×С0.12O0.39

12.69 1.88 3.50 18.81 54.22 0.00 9.92 101.02

50

3[(Fe0.69Cr0.21Ni0.10)0.44
Zr0.27U0.29]×O0.8

0.03 0.03 0.00 21.93 62.66 0.00 14.48 99.13

50

4(U0.50Zr0.50)O1.8

0.03 0.02 0.00 21.28 65.22 0.00 13.04 99.59

50

5(U0.55Zr0.45)O1.6

Figure 5.19. Macrostructure and the Microprobe Analysis Results (the Probe
Diameter = 50 µm)

The element composition of layers was studied by the microprobe analysis employing
different diameters of electronic probes. Table in Figure 5.19 presents the microprobe
analysis results obtained by a large diameter probe (∅ ~ 50 µm). 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.

62

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.
The analysis of the corium phases by the point probe (∅ ~ 1 - 2 µm) is illustrated in Figure
5.20. A light phase close to SS and a dark oxidic phase containing uranium and zirconium
with the ratio of U/Zr = 0.14 at % (in the initial corium U/Zr = 1.2 at %) were detected in the
corium area #2 that confirmed uranium migration into the melt.
Area #2

Area #4

Area #3

Figure 5.20. Microstructure and Microprobe Analysis Results of the K-6 Sample (theprobe diameter = 1 µm)

The area #3 contained 2 phases:
I phase presented a grey oxidic base of the ~ (U0.8Zr0.2)O1.7-1.8 composition with the
reduced quantity of zirconium as compared to the initial C-100 corium.
II phase was of a white colour and presented zirconium oxycarbide.
The areas ## 4 and 5 far from the melt boundary presented a one-phase corium close in thecomposition to the C-100 corium with the unbroken initial U/Zr ratio.
Interactions with the C-32 Corium
The SS+Zr alloy penetration into the porous corium loading (Figure 5.21, Figure 5.18) 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 (Figure635.22a) as well as by the mechanism “vaporization – condensation” (Figure 5.21 and Figure
5.18, the test K-5).
К-7

К-5

Figure 5.21. Penetration of the SS+Zr Alloy into the Corium Particles

The metallic sample upper partThe corium C-32 gritsa)b)
The metallic sample lower part

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 5.22. 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 5.22a) which included the following:64•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;• a grey oxidic phase (U1.3Zr)O2.
Figure 5.22b,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.

5.4 Discussion on Findings
The corium (UZr)O2-x particles were employed in the tests with different contents of oxidized
zirconium from 32 % up to 100 % (Cn = 32 - 100). Particles were sized within the range
1.2 - 3.5 mm. Steel St-30, stainless steel 12X18H10T and alloy of stainless steel (85 mass %)with zirconium (15 mass %) were used as liquid metals.
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 are in turn depend upon composition and temperature;
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.
3. In some cases (under the temperature higher than 1800°C) the metal migration by themechanism “vaporization-condensation” on the surface of particles located deeply inthe loading was foundMolten steel demonstrated good ability to penetrate into the corium if the temperature of steel
exceeds its melting point by 350 K. At the temperature of 1600°C steel penetrated only if thesize of pores is large enough. However at temperature higher than 1800°C, steel penetrate not
only in the debris formed by particles of 1.2 - 3.5 mm1, but also through the corium briquettes
with the low porosity (about 10%). TULPAN T-8 test conducted in the vertical temperature
gradient indicated that the penetration of steel was observed up to the temperature about its
melting point.
However the composition containing the alloy of stainless steel and zirconium(85 % SS + 15 % Zr ) demonstrated weak wettability and consequently the penetration ability
even in the cases when superheating above the melting point was 400 – 500°. The wetting
angle was assessed to be ~ 150° (See Figure 5.18).
The most probable reason for the lack of wetting 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 [21] 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 may explain the experiments with steel St30 at the temperature of 1600°C if the Fe3C film with the melting temperature of 1650°C [22]is formed on the surface of molten steel.
1

Particle’s size after interactions remained practically unchanged.

65This hypnoses was confirmed in the test K-6. As one can see in Figure 5.9, a heightenedcarbon 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 %).
The contact interaction of the corium with any Cn value with liquid metals was observed
under all test temperatures (starting from 1600°C). In principle, the interaction leads to the
diffusion release of U and Zr from the contacting layer of the 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 complicated compounds of the metal type. They are oxygen-free orwith low oxygen content. The thickness of the interaction layers is increased with thedecrease of Cn 1. However, the interaction was observed also in the completely oxidized
corium (Cn = 100).
The oxidic phase of the corium particles (UZr)O2-x varies to a less degree. This may include
metal atoms of steel up to ~ 10 at % with respect to U + Zr (for example, [U0.7Zr0.2Fe0.1]O2-x).
Improvement of understanding about the zirconium and stainless steel alloy penetration into
the porous medium of the corium particles, it seems expedient to continue studies in the lineof the temperature range increase (up to 2300°C) and employment of other alloys'
composition.

1

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.

66

6 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. The experimental
techniques (Most tests were performed in TIGEL facility shown in ) and the full set of dataobtained were presented in the report MP-TR-12 [23].

Figure 6.1. View of TIGEL Facility

6.1 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 [24].
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 6.2 presents the dependence of kinematic viscosity upon temperature. It

67should be noted that measured viscosity for C-32 corium is very close to that measured
earlier for the C-22 corium [24].
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 6.2. The Kinematic Viscosity Versus the Temperature

For density measurements a modified hydrostatic weighing method was chosen [24]. The
results of measurements are shown in Table 6.1.
During the RASPLAV Project the C-32 density was also measured simultaneously with thesurface tension [25]. 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.
Table 6.1. 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

6.2 The Properties of the Metallic Phase
6.2.1 The Spreading Temperature

The technique for Tspread determination consists in the observation of the sample condition
through the optical micropyrometer which allows the observation of the liquid phaseappearance in the form of a film or drops, trickling of that on the surface up to complete
melting. The experiment was conducted in the inert atmosphere to prevent the change ofchemical composition of samples. The temperature increased with a constant rate .
The pyrometer was calibrated by the melting point Tmelt of reference metals Ni, Zr, Mo. The
samples of the metallic phase were taken from the ingot obtained at the RCW facility. Totalof three samples from different parts of the ingot were prepared in the form of pyramid fromthe ingot fragment in the argon protective medium.

68The composition of the metallic samples used in the tests is presented in Table 6.2. The
temperature rise is terminated and heating is switched off after the exposure of ~ 60 s. Table
6.3 demonstrates the obtained results.
Table 6.2. 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
middlepart

9.85

38.89 13.53 31.6 8.36 5.43 0.042#3bottom

10.72

51.96 20.10 16.6 4.04 2.93 0.19

U

Zr

Fe

Gr

Ni

C

Table 6.3. 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

2140
2130

2420
2430

30
30

Sample#3#4bottomThe 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-melted
part.
From the Table 6.3 one can see that the spreading temperature varied from 1770 in the top
sample to 2430 for the samples taken from the bottom of the ingot.

69

6.2.2 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 6.4 demonstrates the results of chemical analysis.
Table 6.4. The “Metallic Body” Chemical Analysis
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 consist of the measurements of the Volt-Ampere
characteristic of the sample. Figure 6.3 demonstrates the temperature behaviour of thespecific resistance. The linear growth of specific resistance is observed which is typical of
metals.
The temperature coefficient for specific resistance was determined by the least-squares
method. Table 6.5 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 [26], zirconium [27], and uranium[28]. 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 6.5. 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)

Reference

Alloy under study20

43 (4.3)

2.38 (0.17)

This work

Fe

20

9.8

6.2

[37]

Zr

27

43.5

3.8

[38]

U

25

25 - 34

2 – 2.8

[39]

70

1.4

R(t)/R(20)

1.3

1.2

1.1

1

0.9
0

40

80

120

160

Temperature (oC)200

Figure 6.3. The Temperature Dependence of Specific Resistance
6.2.3 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 [23].
The tests were performed with different power applied to the heater. In Figure 6.4 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 6.6.

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 6.4. The Values of Thermal Conductivity Obtained in Tests

71

Table 6.6. Thermal Conductivity Data Obtained in Tests
#

o

T( C)

Dispersion
ThermalDispersion 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

72

7 CONCLUSIONS
1. During the MASCA Project the technology of conduction experiments using induction
melting in a cold crucible was developed. This technology allowed performance ofexperiments using different materials such as steel and corium with the limited
undesirable interactions between materials.
2. Experiments with the molten steel and corium revealed important peculiarities ofinteractions. Interactions between molten steel and corium results in the mass exchange
between phases when some amounts of metallic uranium and zirconium initially locatedin the suboxidized corium pass into metallic phase, leading to the change of density andsinking of metal layer. In the medium and small-scale 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 andtransient conditions should be considered if in-vessel retention concept is allied as anaccident management procedure for existing and future NPPs.
3. In the temperature range between corium solidus and liquidus temperatures molten steel
sufficiently overheated above the melting point (about 200 K) easily penetrated throughthe corium debris with the characteristic size between 2 and 3 mm. However steel
zirconium metal melt does not leak through the debris due to formation of zirconium
carbides and oxycarbides.
4. 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.
5. All tests were performed under a non-oxidizing atmosphere. A steam atmosphere might
have a strong impact on the phase separation and stratification process.

73

REFERENCE
1 RASPLAV Application Report, OECD RASPLAV seminar, Munich, 2000.
2 V. G. Asmolov, Ye. V. Astakhova, Yu. G. Degaltsev, Ye. K. Dyakov, et al. Study of carbon
and separation effect on suboxidized corium stratification. Report MP-TR-2, June 2001.
3 K.Froment, The stratification phenomena in the RASPLAV experiments, DTA/CEREM,
Draft Report attached to E-mail massage of 23 May 2000.
4 RASPLAV Final Report. Behaviur of the corium molten pool under external cooling.
Edited by V. G. Asmolov, V.F.Strizhov, Yu.G.Degaltsev, Moscow, 2000.
5 CSNI/NEA RASPLAV Seminar 2000, Summary and Conclusions, Munich, November
2000, NEA/CSNI/R(2000)23.
6 V.G. Asmolov, Ye.V. Astakhova, V.Yu. Vishnevsky, Yu.G. Degaltsev, Ye.K. Dyakov,
V.N. Zagryazkin,
A.Yu. Kotov,
V.M. Repnikov,
T. V. Trushkina,
Yu.M. Utkin,
V. O. Chernyshov, Study of the Liquid Steel Behaviour in the Molten Corium in STFMFe
Test Series, MP-TR-8, February 2003.
7 V.G. Asmolov,
V.B. Khabensky,
S.V. Bechta,
V.V. Gusarov,
S.A. Vitol,
Ye.V. Krushinov, Yu.G. Degaltsev, V.V. Vlasov, Yu.M Utkin, MA-1 Experiment: Post-Test
Analysis Results, MP-TR-1, November 2001.
8 V.G. Asmolov,
V.B. Khabensky,
S.V. Bechta,
S.A. Vitol’,
E.V. Krushinov,
S.Yu. Kotova, E.K. Kalyago, N.E. Kamensky, A.V. Lysenko, V.G. Bliznyuk, V.R. Bulygin,
E.V. Shevchenko, V.S. Granovsky, V.V. Martynov, V.V. Gusarov, V.I. Al’myashev,
M.D. Tolkachyov, M.R. Pavlov, V.V. Vlasov, Yu.G. Degaltsev, Yu.M. Utkin The MA-2
Experiment:Zirconium and Uranium Partitioning Between Oxide and Metallic Phases of Molten
Corium, MP-TR-6, March 2002.

9 V.G. Asmolov, V.B. Khabenski, S.V. Bechta, E.V. Krushinov, S.A. Vitol, V.V. Gusarov,
S.Yu. Kotova, V.I. Almiashev and Yu. G. Degaltsev, MA-3 and MA-4 tests: Zirconium andUranium Partitioning Between Oxidic and Metallic Phases of Molten Corium, MP-TR-9,
February 2003.
10 V.G. Asmolov, S.S. Abalin, Yu.A. Veselkin, V.Yu. Vishnevsky, V.V. Vlasov,
B.L.Gershman, Yu.G. Degaltsev, Ye.K. Dyakov, I.F. Isaev, A.N. Kiselev, N.P. Kiselev,
A.M.Kovalev, A.G. Ol’khovsky, K.V. Pechalin, I.M. Semenov, V.F. Strizhov,
T.V.Trushkina, V.S. Uglov, Yu.M. Utkin, V.V.Chudanov, A.E.Aksenova, V.A.Pervichko,
I.M. Khazanivich, RCW Post-Test Analysis Results, Mp-TR-11, July 2003.
11 V. Asmolov, S. Abalin, I. Isaev, B. Gershman, K. Pechalin, V. Strizhov, V. Chudanov,
A. Aksenova, Yu. Veselkin, V. Vishnevsky, V. Vlasov, Yu. Degaltsev, A. Kiselev,
A. Kovalev, T. Trushkina, Yu. Utkin, V. Chernyshov, V. Churin, RASPLAV-AW-200-4
Post-Test Analysis, RP-TR-43, April, 2000
12 V.G. Asmolov, Ye.V. Astakhova, V.Yu. Vishnevsky, Ye.K. Dyakov, V.N. Zagryazkin,
A.Yu. Kotov, V.M. Repnikov, V.S. Uglov, Yu.G. Degaltsev, Yu.M. Utkin, T.V. Trushkina,
Partitioning of fission products between the corium melt metallic and oxidic phases resulting
from the STFM-FP tests, MP-TR-10, 2003.
13 D.A.Powers. Thermochemistry of Core Debris: Partitioning of Uranium and Fission
Products Among Condensed Core Debris Phases. RASPLAV Seminar 2000. Munich,Germany, Nov. 14 - 15, 2000
14 Molten Material Relocation into the Lower Plenum: A Status Report.
NEA/CSNI/R(97)34, September, 1998.
74

15 D. Magellon et. al., Debris and Pool Formation/Heat Transfer in FARO-LWR:
Experiments and Analyses. Work shop Proceedings, March 3-6 1998.
16 H. Unger et. al., Key Phenomena of Late Phase Core Melt Progression, AccidentManagement Strategies and Status Quo of Severe Fuel Damage Codes. Workshop
Proceedings, March 3-6 1998.
17 P. Hofmann et. al., Dissolution of Solid UO2 by Molten Zircaloy and its Modelling.
International Symposium on severe accidents in Muclear Power Plants. Sorrento, Italy, 21-25
March 1988.
18 K.T.Kim, D.R.Olander. Dissolution of uranium dioxide by molten zircaloy, J.Nuclear
Materials, 1988, v.154, pp.85 - 115.
19 Hayward P.J., George I.M. Dissolution of UO2 in molten Zircaloy-4. Part 4: Phase
evolution during dissolution and cooling of 2000 to 2500 oC specimens. J. Nuclear Materials,
1996, v.232, N 1, p.13-22.
20 V.G. Asmolov, Yu.G. Degaltsev, O.Ya. Shakh, S.S. Abalin, I.M. Semenov, et al. Study of
the Molten Metal Migration in the Oxide Porous Matrix. MP-TR-5, July 2003.
21 L.L. Kunin, Surface Phenomena in Metals, M., 1955 (rus)
22 G.V. Samsonov, Refractory Compounds, Metallurgizdat, M., 1963 (rus)
23 V.G. Asmolov, S.S. Abalin, F.V. Merzlyakov, V.Yu. Vishnevsky, A.Yu. Kotov, V.M.
Repnikov. Measurements of Material Properties, RP-TR-12, Moscow, 2003.
24 S.S. Abalin, V.G. Asmolov, V.D. Daragan, Ye.K. D’yakov, A.V. Merzlyakov,
V.Yu. Vishnevskiy, Kinematic viscosity measurement of C-100 and C-22 corium. OECDRASPLAV Project, RP-TR-18, 1996.
25 V. Asmolov, S. Abalin, A. Merzliakov, Corium Surface Tension Measurement,
RP-TR-45 April, 2000.
26 N.I. Koshkin, M.G. Shirkevich, Handbook on Elementary Physics, Moscow, Nauka,
1972, (Russ.)
27 V.E. Peletzkii, E.A. Belskaya, Electrical Resistance of Refractory Metalls, Moscow,“Energoizdat”, 1981. (Russ.)
28 J.J. Katz, E. Rabinovitch, The Chemistry of Uranium, v.1, NY, 1951.

75

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APPENDIX A: CONCLUSIONS AND RECOMMENDATIONS OF
MASCA SEMINAR 2004
(Aix-en-Provence, France, 10-11 June 2004)
During the MASCA Seminar, results from MASCA experiments were presented to the audience.
In addition to presentations made by the Operating Agent, participants presented their views on
theoretical analyses and applications of the MASCA results.
The organizing committee considered that one of the key values of the Seminar was to gather
experts from project member countries. Those experts, not directly involved in the project
operation, mainly addressed the analytical aspects of molten corium behavior in the lower head of
the reactor pressure vessel. The project greatly benefited from experts’ opinion which was
focused on experimental research, therefore helping to identify future needs in order to bridge thegap between experimental research and applications.
The conclusions presented hereafter are divided into 2 parts: (a) MASCA integrated reportconclusions on experiments; (b) conclusions drawn by the Organizing Committee from thepresentations and discussions with experts participating in the seminar.
(a) MASCA integrated report conclusions
1. 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. 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 small-scale 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 theinteraction 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 andfuture NPPs.
3. In the temperature range between corium solidus and liquidus temperatures molten steel
sufficiently overheated above the melting point (about 200 K) easily penetrated through thecorium debris with the characteristic size between 2 and 3 mm. However steel zirconium
metal melt does not leak through the debris due to formation of zirconium carbides andoxycarbides.
4. 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 dependenceof FP partitioning as a function of temperature.
5. All tests were performed under a non-oxidizing atmosphere. A steam atmosphere might havea strong impact on the phase separation and stratification process.

(b) Conclusions and recommendations from participants in the seminar
Experimental results:
1. Seminar participants recognized the high quality of the experiments performed and of datadelivered during the MASCA project;

A-1

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2. Experimental facilities employed in the MASCA project are an important source of data formolten corium pool thermochemical behavior;
3. Participants stressed the importance of the results of this study in support of their work.
Theoretical analyses:
1. Various thermochemistry modeling presented using available data base seems to reasonably
account for gross features of many of the MASCA experiments;
2. Although quite challenging to obtain, accurate thermochemical and physical property data athigh temperatures of interest are necessary for thermochemical analyses and predictions ofsevere accident progression. However the level of accuracy required for adequate modelingof scenarios for reactor applications is not as high as it is for a satisfactory description of all
the thermochemical details. It can be considered now that remaining uncertainties in current
thermochemical databases have been sufficiently reduced to lead to errors in the predictionof heat flux on the vessel wall comparable to errors induced by other sources of uncertainties(eg initial composition of debris, modeling of species transport in the lower plenum). It must
pointed out that this conclusion is valid only around U/Zr = 1.2 for variable steel content andlevel of Zr oxidation where databases have been recently explored and assessed;
3. Melt progression modeling with chemical interactions in porous corium debris bed is an open
issue. No presentation was made by any participants on this subject.
Applications to reactor cases:
1. Several presentations addressed scaling to reactor conditions;2. The main source of uncertainty has been recognized to be in-vessel melt progression andtherefore the initial composition of the relocated melt in the lower head;
3. High temperature properties, especially liquid oxidic and metallic phase densities, still remaina large source of uncertainty and therefore need to be addressed.
Further recommendation
Diffusion of species including oxygen in a molten pool of a large scale reactor should be
addressed in order to assess its effect on stratification.

A-2