Gateway for:

Member Countries

Corium Melt Interaction with Reactor Vessel Steel

#3592


Investigation of Corium Melt Interaction with NPP Reactor Vessel Steel

Tech Area / Field

  • FIR-EXP/Experiments/Fission Reactors
  • FIR-MOD/Modelling/Fission Reactors
  • FIR-NSS/Nuclear Safety and Safeguarding/Fission Reactors

Status
8 Project completed

Registration date
15.08.2006

Completion date
04.04.2012

Senior Project Manager
Tocheny L V

Leading Institute
Research Institute of Technology, Russia, Leningrad reg., Sosnovy Bor

Collaborators

  • Forschungszentrum Rossendorf, Germany, Rossendorf\nCEA / DEN / Département de Technologie Nucléaire, France, Saint-Paul-lez-Durance\nAREVA / Areva NP GmbH, Germany, Erlangen\nEuropean Commission / Joint Research Center / Institute for Transuranium Elements, Germany, Karlsruhe\nForschungszentrum Karlsruhe GmbH, Germany, Karlsruhe\nFortum Nuclear Services Ltd, Finland, Espoo\nCEA / Institut de Radioprotection et de Surete Nucleaire, France, Saint-Paul-lez-Durance\nKorea Atomic Energy Research Institute, Korea, Yusung Taejon

Project summary

The main objective of the proposed project is the NPP reactor safety enhancement in case of a severe accident with core degradation and meltdown. Specific subject of the project is the experimental study of physicochemical phenomena occurring at the interaction between a molten corium pool and reactor vessel steel. These studies are carried out for the development and justification of the of the severe accident management concept, mainly, for predicting temperature and stress-and-strain conditions of the reactor vessel. The ISTC #833 METCOR has provided and systematized data on the vessel steel corrosion in the conditions of the melt retention inside the cooled reactor vessel (IVR).

The analysis of the uncooled reactor vessel failure and determination of the melt release scenario are critical for the management of an ex-vessel stage of the severe accident. The adequacy of the analysis and scenario depends on the reliability of data on vessel corrosion at its interaction with corium in the realistic conditions, primarily, at higher steel temperatures and different space orientation of the interaction interface. It is planned to get these data in the framework of the proposed project, which will use the approach developed and tested in the previous METCOR project.

The experimental part of the project is performed on the Rasplav-2 and Rasplav-3 test facilities employing the technique of induction melting in a cold crucible for producing uranium-bearing oxidic and metal-oxidic melts. Rasplav-2 and Rasplav-3 test facilities enable: contact-free power deposition in the melt; experiments in the inert, air and steam atmosphere; no time limit on the continuous interaction studies, and on-line monitoring of corium-steel interaction interface. These experimental facilities have been used in the METCOR, CORPHAD, CIRMAT, CIT, ENTHALPY, ECOSTAR, OECD/MASCA projects, carried out with the participation of the project team, which developed the current project proposal.

Previously the molten corium–vessel steel interaction has been studied by the team of proposed project within the ISTC Project #833 (METCOR). In its experimental program a horizontally-positioned specimen of Russian VVER reactor steel (15Kh2NMFA) was placed on the molten pool bottom, and the melt-steel interaction was studied in the stationary regimes, at which the melt composition, temperature, oxygen potential and specimen temperature on the interaction interface were kept relatively stable.

The implementation of the Work Plan of Project #833, the experimental part of which was completed in December 2005, have produced the following results important for the insight into the influence of physicochemical phenomena on the depth and kinetics of reactor vessel ablation at the IVR:


1. Quantitative characteristics of the vessel steel ablation depth and kinetics at its interaction with molten corium, depending on:
  • Corium oxygen potential sensitive to the composition of melt and above-melt atmosphere. For this purpose coria of the following compositions were produced: fully oxidized oxidic coria UO2/ZrO2 and UO2/ZrO2/FeO(Fe3O4) in an inert (nitrogen, argon) atmosphere, air or steam; suboxidized corium UO2/ZrO2/Zr UO2/ZrO2/Zr/STEEL in argon.
  • Temperatures of steel at the interaction interface, which ranged between 700 oC and 1400 oC.
  • Value of specific heat flux from the corium melt into the steel specimen, which varied between 0.3 MW/m2 and 1.2 MW/m2.
2. The main mechanisms of vessel steel ablation have been determined after the posttest physicochemical and metallographic studies of the microstructure, elemental and phase composition and other thermophysical characteristics of the corium ingot, interaction zone between corium and specimen and steel specimen near the interaction zone. It has been found that in case of a fully oxidized melt the mechanism of cooled vessel steel ablation (corrosion) is based on its oxidation, and the main diffusion barrier, which influences the process kinetics, is the FeO layer, which forms on the steel surface. In case of suboxidized oxidic and metal-oxidic molten coria, the main mechanism of the cooled vessel steel ablation is its eutectic melting (dissolution), which goes on until the boundary temperature of ~1090 oC is reached.
3. The determined mechanisms of cooled vessel steel ablation caused by the physicochemical interaction with molten corium have been used for developing approximated correlations for the numeric evaluation of corrosion (ablation) rate and depth.

The produced results enable to predict thermal and mechanical behavior of the reactor vessel bottom during the IVR. The major findings from the preformed investigations have been presented in the following papers and reports:

  • Bechta S.V., Khabensky V.B., Vitol S.A. et al., Experimental Study of Oxidic Corium Interaction with Reactor Vessel Steel Samples // RASPLAV Seminar 2000, Munich, Germany, 14-15 Nov., 2000
  • Bechta S.V., Khabensky V.B., Vitol S.A. et al., Experimental studies of oxidic molten corium-vessel steel interaction // Nucl. Eng. Des., 210 (2001) 193-224
  • Bechta S.V., Khabensky V.B., Granovsky V.S., Krushinov E.V., Vitol S.A., Gusarov V.V., Almjashev V.I., Lopukh D.B., Tromm W., Bottomley D., Fischer M., Cognet G., Kymalainen O., New Experimental Results on the Interaction of Molten Corium with Reactor Vessel Steel // Proceedings of ICAPP’04, Pittsburgh, P.A. USA, June 13-17, 2004, Paper 4114
  • Bechta S.V., Khabensky V.B., Granovsky V.S., Krushinov E.V., Vitol S.A., Gusarov V.V., Almjashev V.I., Mezentseva L.P., Petrov Yu.B., Lopukh D.B., Fischer M., Bottomley D., Tromm W., Barrachin M., Altstadt E., Piluso P., Fichot F., Hellmann S., Defoort F., CORPHAD and METCOR ISTC projects // The first European Review Meeting on Severe Accident Research (ERMSAR-2005), Aix-en-Provence, France, 14-16 November, 2005
  • Bechta S.V., Khabensky V.B., Vitol S.A., Krushinov E.V., Granovsky V.S., Lopukh D.B., Gusarov V.V., Martinov A.P., Martinov V.V., Fieg G., Tromm W., Bottomley D., Tuomisto H., Corrosion of vessel steel during its interaction with molten corium. Part 1: Experimental // Nucl. Eng. Des., 236 (2006) 1810-1829
  • Bechta S.V., Khabensky V.B., Vitol S.A., Krushinov E.V., Granovsky V.S., Lopukh D.B., Gusarov V.V., Martinov A.P., Martinov V.V., Fieg G., Tromm W., Bottomley D., Tuomisto H., Corrosion of vessel steel during its interaction with molten corium. Part 2: Model development // Nucl. Eng. Des., 236 (2006) 1362-1370
  • Bechta S.V., Khabensky V.B., Granovsky V.S., Krushinov E.V., Vitol S.A., Gusarov V.V., Almjashev V.I., Lopukh D.B., Tromm W., Bottomley D., Fischer M., Piluso P., Miassoedov A., Altstadt E., Willschufz H.G., Fichot F., Experimental Study of Interactions Between Suboxidized Corium and Reactor Vessel Steel // Proceedings of ICAPP’06, Reno, NU USA, June 4-6, 2006, Paper 6054.

Still, certain specific features of the corium – reactor vessel interaction remain unclear. Among them:
  • Transients accompanying the steel temperature and oxygen potential growth in the system when the neutral atmosphere is replaced by the oxidizing (steam) atmosphere. They can cause the melt temperature rise, pool structure transformation due to major component repartitioning and corresponding increase of heat fluxes into the vessel (focusing effect).
  • Interactions at vertical orientation of steel specimen surface, in this case the gravity effects could be critical.
  • Behavior of the European reactor vessel steel

The proposed Project will provide the following data and results:
  • Physicochemical phenomena which are critical for determining vessel steel ablation in the severe accident conditions.
  • Experimental data on the vessel steel corrosion kinetics and depth for the steady state and for melt oxidation transients.
  • Qualitative and quantitative characteristics of the European vessel steel corrosion.
  • Correlations for calculating corrosion kinetics in the studied parameter range.

The results will be used for:
  • modeling physicochemical phenomena of severe accidents;
  • code verification;
  • specifying temperature condition and mechanical behavior of the reactor vessel;
  • development and justification of different concepts of the core melt localization.


Back