Nuclear Safety Institute IBRAE Russian Academy of Sciences

Nuclear Safety Institute IBRAE Russian Academy of Sciences MASCA MAJOR FINDINGS AND THEIR IMPLICATION ON ACCIDENT PROGRESSION SCENARIO Presented by V. Strizhov MASCA Seminar France, Aix-en-Provence June 10 - 11, 2004 PRG-2 June 8 - 9, 2004 For MASCA Use only

Ingot views after tests • Corium C-22 • Carbon content – 0. 3 wt. % Small amount of carbon caused stratification of the melt PRG-2 June 8 - 9, 2004 For MASCA Use only 2

Ternary phase diagram • Experiments conducted with the mixtures of U-Zr-O oxidized above 10% did not reveal stratification • Experiments showed that corium behaviour between solidus and liquidus temperatures is of great importance – Formation of liquids phases based on the zirconium (Zr 0. 97 U 0. 03)O 0. 25 – Solid matrix (U 0. 75 Zr 0. 25)O 1. 8 PRG-2 June 8 - 9, 2004 For MASCA Use only 3

Focus of studies • Kinetics of layers formation • Steady state distribution – U/Zr ratio between upper and lower parts of the ingot – Carbon concentration • Post test examination results – Light and gray phases – Relation between phases PRG-2 June 8 - 9, 2004 For MASCA Use only 4

Kinetics of layers’ formation PRG-2 June 8 - 9, 2004 For MASCA Use only 5

Steady state distribution PRG-2 June 8 - 9, 2004 For MASCA Use only 6

Major findings • Kinetics of layers’ formation for C-32 corium containing 0. 3 wt% showed that the time for the scale of STFM facility is about 30 minutes; • Two distinct layers were found – Upper layer contains ~2 times more zirconium than the middle layer, carbon content increased up to 1. 5 wt%. – middle layer enriched with uranium with low carbon content of up to 0. 1 wt%. • Results were consistent with AW tests and allowed complete understanding of observed phenomena PRG-2 June 8 - 9, 2004 For MASCA Use only 7

Interactions of corium with steel (iron) • Small scale tests – Steel was added in a solid state – Two layers were formed during the tests • Large scale RCW test – Liquid steel was added after the corium melt was formed – Complex configuration of layers was found PRG-2 June 8 - 9, 2004 For MASCA Use only 8

Iron concentration in oxide phase PRG-2 June 8 - 9, 2004 For MASCA Use only 9

Section of phase diagram U/Zr=1. 1 – 1. 2 C 32 initial corium PRG-2 June 8 - 9, 2004 For MASCA Use only 10

Peculiarities of steel behavior in the RCW test 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. Steel charge; Zr. O 2 case for the steel charge; Upper tungsten heater; Zr. O 2 thermal insulation; Lattice of copper tubes of the upper cold crucible; Upper inductor; Zr. O 2 heat shield; Thermal insulation (Zr. O 2 powder); Zr. O 2 protective bush; Magnetic cores; Start-up tungsten heater; Thermal insulation (UO 2 and С-100 corium powder); Lattice of copper tubes of the lower cold crucible; Lower inductor; C-32 corium loading; Loading of FP simulants; Pyrometric tube; C-32 corium groats; Zr. O 2 dome PRG-2 June 8 - 9, 2004 For MASCA Use only 11

The Material Balance of the RCW experiment PRG-2 June 8 - 9, 2004 For MASCA Use only 12

The Central Cut of the Ingot 1. The corium lower non-melted briquettes; 2. The lower metallic part; 3. Zone of a partially melted corium; 4. Roundish metallic parts in the central zone; 5. The ingot oxidic part; 6. Epoxy; 7. The upper metallic part; 8. The ingot surface; PRG-2 June 8 - 9, 2004 For MASCA Use only 13

Partitioning of U, Zr and Fe in Metallic Parts along the Ingot Height PRG-2 June 8 - 9, 2004 For MASCA Use only 14

Peculiarities of RCW test • Initially only part of steel interacted with corium – Mass of steel interacted about 1. 02 kg – Mass of corium 50. 3 kg – Steel to corium mass ratio 0. 02 (similar to STFM tests with addition of about 5 g of steel) • Remaining steel interacted with the corium of about 50% of zirconium oxidation • Transient character of interactions PRG-2 June 8 - 9, 2004 For MASCA Use only 15

Comparison of small scale tests data and RCW test data C-32 Data PRG-2 June 8 - 9, 2004 C-70 Data For MASCA Use only 16

Some conclusions • Steady state characteristics of corium steel interactions were studies in a series of small and medium tests (STFM and MA series up to 20% of steel content in corium) • RCW test indicated that transient condition may play an important role during relocation of molten steel – Steel addition methods (solid/liquid) – Role of diffusion – Jet fragmentation PRG-2 June 8 - 9, 2004 For MASCA Use only 17

Partitioning of FP simulants • Small scale tests – – Influence of metal phase composition Influence of Temperature and corium composition Dependence upon U to Zr ratio Peculiarities of FP Partitioning in the RCW test • Large scale RCW test – Distribution of FP in oxide phase – Distribution of FP in metal phase PRG-2 June 8 - 9, 2004 For MASCA Use only 18

Influence of temperature and corium composition • Partitioning coefficient for metal FP such as Mo and Ru decreases with temperature increase • Partitioning coefficients Sr, Ba, Ce and La increases • Number of tests was too small to make definite conclusion PRG-2 June 8 - 9, 2004 For MASCA Use only 19

Partitioning coefficient of Mo and Ru Final corium oxidation degree of 65% PRG-2 June 8 - 9, 2004 Final corium oxidation degree of 85% For MASCA Use only 20

Partitioning coefficient of Sr and Ba Final corium oxidation degree of 65% PRG-2 June 8 - 9, 2004 Final corium oxidation degree of 85% For MASCA Use only 21

Results of FP analysis for RCW test La and Ce Sr and Ba • Distribution of FP was not uniform through the ingot • Change of corium composition • Non uniform temperature distribution PRG-2 June 8 - 9, 2004 For MASCA Use only 22

Peculiarities of FP Partitioning in the RCW test Partitioning coefficient (2500 – 2600 K) Ru, Mo: 15 – 25 Sr, Ba: 0. 07 – 0. 15 Ce, La: 0. 05 – 0. 15 PRG-2 June 8 - 9, 2004 For MASCA Use only 23

Implications of MASCA to VVER accident progression scenarios • Analysis of core degradation scenarios – Determine degree of zirconium oxidation – Determine masses of materials relocated to the lower head • The list of scenarios includes – Station blackout scenario – LOCAs scensrios • Parameters of coria – Zirconium oxidation degree was bound to be between 30 and 40% – Mass of corium about 95% of total mass – Mass of steel 40 – 50 tons PRG-2 June 8 - 9, 2004 For MASCA Use only 24

Structure of VVER-1000 lower head Supporting plate Assembly support Perforated bottom Pressure vessel PRG-2 June 8 - 9, 2004 For MASCA Use only 25

Phenomena in the lower head • Debris bed – Melting and relocation of low temperature components (assemblies support) – Filling of pores with molten steel – Melting of corium – Formation of layered structure • Molten pool – Steady state conditions (Applicable for IVR case) – Transient condition PRG-2 June 8 - 9, 2004 For MASCA Use only 26

Summary • Experiments with the molten steel and corium revealed important peculiarities of interactions – Extraction of uranium from suboxidized corium – Different layer’s configuration – Transient conditions may play a role (formation of three layers) • • Debris experiments indicated that molten steel spreads through the porous debris if superheating above the melting point is sufficient (about 200 K) 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, the effect is not significant for reactor application. PRG-2 June 8 - 9, 2004 For MASCA Use only 27