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Reactor Core Melting


Modelling of Reactor Core Behaviour under Severe Accident Conditions. Melt Formation, Relocation and Evolution of Molten Pool

Tech Area / Field

  • FIR-MOD/Modelling/Fission Reactors
  • FIR-ENG/Reactor Engineering and NPP/Fission Reactors

8 Project completed

Registration date

Completion date

Senior Project Manager
Tocheny L V

Leading Institute
Institute of Safe Atomic Power Engineering Development, Russia, Moscow


  • CEA / Institut de Radioprotection et de Surete Nucleaire, France, Saint-Paul-lez-Durance\nEuropean Commission / Joint Research Center / Institute for Transuranium Elements, Germany, Karlsruhe\nForschungszentrum Karlsruhe GmbH, Germany, Karlsruhe\nKorea Atomic Energy Research Institute (KAERI), Korea, Yuseong

Project summary

Study of the processes which take place in reactor core under severe accident conditions is one of the main aspects of reactor safety validation. Currently a rather extended set of experimental data on corium melt formation and relocation is already elaborated, and new experiments are under preparation. On the other hand, the corresponding models for melt behaviour are generally oversimplified and cannot adequately describe various complicated phenomena associated with corium melt formation. A systematic study and analysis of all available and new experimental data in this area and their self-consistent modelling in the mechanistic approach, are strongly requested and required in order to make progress.

Good understanding of the details of reactor core degradation will make it possible to predict and to control the accident development, to reduce its consequences or even prevent an accident. Such knowledge can also help to improve fuel technology and its operation.

The objective of the project is to perform the modelling work on reactor core molten materials behaviour at consecutive stages of an accident development. This will run from the early stage, when the core is mostly intact and the first Zr cladding melting occurs, to the late stage, when the core is completely degraded and a molten pool is formed in lower head of the PRV.

It is proposed to model the following processes

1. Melt formation, onset of melt relocation:

- Simultaneous dissolution of ZrO2 crust and UO2 fuel (fresh and irradiated) by molten Zircaloy,
- Cladding oxide shell failure,
- Release of U-Zr-O mixture from the cladding breach

2. Candling process: flowing down in the form of drops and rivulets during the first stage of melt relocation,

3. Formation of massive coolant channel blockage (slug), its oxidation and downward relocation in the course of the second stage of melt relocation process,

4. Thermal hydraulic behaviour of molten pool in the lower head of the RPV.

State of the art. In the main system codes, which are used presently for the analysis of reactor core degradation under severe accident conditions the models dealing with melt relocation and physico-chemical interactions are oversimplified and are not able to give the adequate description of the corresponding phenomena. Moreover there are still gaps in the phase diagram data (e.g. sub-oxidized melts of Zr-U-O), and often difficulties in interpretation of the data that already exists.

Impact of the proposed project on the progress in the field of severe accident modelling. The proposed modelling work should significantly improve the understanding of the initial melting processes as well as enabling better use of the currently existing and future thermodynamic data. This, in turn, will improve the modelling of reactor core molten materials behaviour under severe accident conditions. It should provide a description of the melt relocation, heat- and mass-exchange at all the stages of the accident

- from the single point of view regarding the physico-chemical processes at work,
- with the help of uniform material properties data base,
- with a consistent level of accuracy.

Competence of the project teams. The IBRAE specialists have gained considerable experience in the above-specified area of research. During the last 10 years original investigations have been performed in IBRAE concerning melt relocation, physico-chemical interactions (such as liquid Zr oxidation, ZrO2 and UO2 dissolution), molten pool heat exchange.

The proposed project work will use previously obtained results: theoretical models of melt relocation at different accident stages, numerical modules of ZrO2 and UO2 dissolution, numerical SVECHA code (developed in IBRAE), giving the detailed description of an intact rod under severe accident conditions, numerical CONV code (developed in IBRAE) describing 3-d turbulent hydrodynamics and heat exchange of the molten pool.

Expected results and their application. It is assumed that physical models and numerical modules describing reactor core molten materials behaviour at the initial and intermediate stages of the accident (down to debris bed formation phase) will be updated, improved and then implemented in the SVECHA code. A set of verification calculations with the newly developed SVECHA/MELT code and comparison with available experimental data will be performed. The analytical support of the part of COLOSS experimental program carried out in ITU Karlsruhe (tests on irradiated and MOX duel liquefaction by molten Zr and tests on U-Zr-O mixture melting point determinations) with the help of the SVECHA/MELT code will be provided. The experimental results obtained in these tests will be used for the improvement of the SVECHA/MELT code material properties data base. The code will be also validated against the large-scale bundle tests CORA and QUENCH (FZK, Germany), in which liquefaction of the core materials was observed.

The CONV code will be improved and applied to the analysis of the thermal hydraulic behaviour of molten pool in the lower head of the RPV. The solution of this task will be complementary to the LIVE (Late In-Vessel phase Experiments) project in FZK, Karlsruhe and thus will provide analytical support of the LIVE test. In particular, the 3-d heat flow distribution to the lower head will be calculated and compared with the experimental measurements. The CONV code will be verified against the LIVE experimental data.

The developed and verified models will be used for benchmarking of simplified models and will be ready for implementation in a system code such as ICARE/CATHARE, MELCOR, or the new ASTEC suite that is currently under development by IRSN/CEA in France. The proposed work is complementary to the SARNET Project (6th FP), the obtained results and new models can be used also in this Project.


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