Abstract
(Englisch)
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The project complies with the 'Nuclear fission programme' on Severe Accidents and Management Measures. The objective of COLOSS is to improve the safety of European reactors. Therefore additional research on core degradation is proposed combining experiments, model developments and SA code improvement packages that address uncertainties which significantly limit the understanding of both existing or future plant behaviour (PWR, BWR, VVER, EPR) under Severe Accident conditions. Risk?relevant topics selected are: BC4 oxidation and control rod degradation, high burn?up UO2 and MOX behaviour, oxidation of U?O?Zr mixtures and liquefaction?collapse of fuel rods.
Specific objectives are.
- Experiments on selected topics at different scales for code developments and validation,
- Model development and coupling in SA codes used by Utilities, Industry and Safety Authorities,
- Evaluation of the consequences of results on safety, feedback on SAM for different plant designs.
Studies on risk?relevant core degradation topics are proposed for different plant designs: PWR, BWR, VVER and EPR. Experiments at different scales up to integral tests (bundles) are proposed on the following topics:
a) High burn?up UO2 and MOX dissolution by molten Zr and melting point of resulting U-O-Zr mixtures and of a TMI?2 corium sample.
b) Simultaneous dissolution of UO2 and ZrO2 by molten Zr and rod collapse conditions for prototypic PWR and VVER fuel rods (clad failure and loss of rod geometry due to UO2 and ZrO2 dissolution phenomena).
c) Oxidation of B4C alone (pellets/powder from different plant designs) and degradation-oxidation of prototypic B4C control rods representative of PWR, BWR and VVER rods. The key point is the measure of gas and aerosols produced, in particular H2 and CH4 highly risk-relevant for safety (H2?risk in containment, formation of Organic Iodine gas which cannot be trapped in filters). Two integral tests are planned in QUENCH and CODEX facilities with a fuel rod bundle and a central B4C control rod representative of PWR/BWR and VVER designs.
d) Oxidation of U?O?Zr mixtures responsible of H2 production during late stages of core degradation.
The aim of this experimental effort is to enable the development and validation of models (B4C, MOX, U?O?Zr) which will be implement in European integral SA codes (ASTEC, ICARE/CATHARE and ATHLET?CD/KESS). This analytical work is favoured by work-packages, which include experimental and analyst teams. Finally emphasis has been put on plant applications with different SA codes (ASTEC, ICARE/CATHARE, SCDAP/RELAP5, MELCOR, MAAP?4), different plant designs sequences for PWR?1300, VVER?1000, BWR, EPR and the TMI?2 reference accident. Calculations will be run by Designers, Utilities, R&D and Safety authorities enabling benchmarks.
Plant calculations will be carried out to quantify the impact of new data and models on the safety and to evaluate the consequences on risks and accident management measures. Special emphasis will be put on H2?risk (oxidation of B4C and U?O?Zr), corium risk (burn?up effect, MOX) and source term (CH4 ?Organic Iodine production).
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