B 4C oxidation modelling in severe accident codes: Applications to PHEBUS and QUENCH experiments

Abstract

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 B 4C 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 B 4C 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 B 4C 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 B 4C oxidation, predicted rather well the total carbon release. The results of the MELCOR code, involving initially a B 4C 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 B 4C control rods. Codes still have some difficulties to reproduce the final degradation of fuel bundles involving B 4C rods. Spreading of molten materials from the control rods onto fuel rods of the bundle is suspected, suggesting that the main effect of the B 4C control rod materials on the bundle behaviour during degradation is connected with B 4C-Stainless Steel (SS) eutectics formation and B 4C-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.