B4C oxidation modelling in severe accident codes: Applications to PHEBUS and QUENCH experiments
Résumé
Boron carbide is used in many nuclear power plants like BWR, VVER, some PWR, and EPR as a neutron absorber material. Consequently, it is important to assess its role in the core degradation phenomena during a severe accident (SA), as well as that of the carbon gas released from its degradation on the fission products behaviour. This paper describes the progresses achieved in the frame of the Network of Excellence SARNET concerning the B4C control rod degradation modelling in Severe Accident codes, such as ATHLET-CD, ICARE2, ASTEC MELCOR and MAAP. These new developments complete improvements made during the European Union 5th Framework COLOSS project. Starting from basic modelling derived from available tests reported in the literature, large improvements of the kinetic correlation for B4C oxidation were obtained from analytical experiments performed at FzK (Germany) and IRSN (France), mostly in the temperature range above 1400 K. The new modelling was considered in the analysis of experiments involving a B4C control rod in small fuel rod assemblies, such as Phebus FPT3 in-pile experiment, as well as out of pile experiments Quench 07 and 09, aimed at studying the course of severe accidents. Regarding the hydrogen generation, the results given by different code simulations are consistent with the experimental values. Concerning the control rod degradation, SA codes such as ICARE2 and ATHLET-CD, using suitable modelling of B4C oxidation, predicted rather well the total carbon release. The results of the MELCOR code, involving initially a B4C oxidation model designed to be used for BWR control blades, have been largely improved in the most recently released version, with a model extended for PWR B4C control rods. Codes still have some difficulties to reproduce the final degradation of fuel bundles involving B4C rods. Spreading of molten materials from the control rods onto fuel rods of the bundle is suspected, suggesting that the main effect of the B4C control rod materials on the bundle behaviour during degradation is connected with B4C-Stainless Steel (SS) eutectics formation and B4C-SS-Zry liquid mixture relocation. These phenomena are not accounted for in the SA codes. The need for further code developments of the early phase of core degradation is recognized, involving the absorber rod material behaviour. The BECARRE experiments, on-going in the framework of the International Source Term Program, are designed to provide in-depth understanding of these phenomena and help improving their modelling. © 2009 Elsevier Ltd. All rights reserved.
Mots clés
Code development
Code simulation
Core degradation
European Union
Experimental values
Fuel bundle
Fuel rods
Germany
Hydrogen generations
In-depth understanding
Liquid mixture
Main effect
Material behaviour
Neutron absorber material
Oxidation model
Severe accident
Severe accident codes
Source terms
Temperature range
Total carbon
Accidents
Boiling water reactors
Boron
Boron carbide
Degradation
Experiments
Fission products
Fuels
Hydrogen production
Metal melting
Molten materials
Nuclear energy
Nuclear industry
Nuclear power plants
Oxidation
Piles
Pressurized water reactors
Simulated annealing
Simulators
Small nuclear reactors
Stainless steel
Control rods