Modelling of B₄C oxidation by steam at high temperatures based on separate-effects tests and its application to the bundle experiment QUENCH-07

Abstract

A new model on B 4 C oxidation in steam at temperatures above 800 °C is developed by the Nuclear Safety Institute of Russian Academy of Sciences, Moscow (IBRAE) based on the analysis of the BOX Rig tests performed at Forschungszentrum Karlsruhe (FZK). The model considers the surface reaction kinetics and mass transport in the gas phase as rate determining steps of the oxidation process. A full set of independent chemical reactions is considered in the new B 4 C oxidation model. Correspondingly, the full set of mass action laws for equilibrium gas reactions either on the surface or in the gas bulk, is used. For non-equilibrium surface reactions a semi-empirical correlation for the reaction rate is deduced from the analysis of the BOX Rig test results. In this approach, the main inconsistency of many other B 4 C oxidation models is eliminated. As a result, a more precise solution for hydrogen release and additionally, a chemical composition of the outlet gas mixture, can be calculated by the advanced model. The new module of the B 4 C oxidation in steam has been implemented in the SVECHA/QUENCH (S/Q) code and verified against the BOX Rig tests. The SVECHA/QUENCH code was further applied to simulation of the bundle test QUENCH-07 with the central absorber B 4 C rod. The simulation was performed within the framework of the 'effective channel approach' using corrected average temperature field. Data concerning release rate of B 4 C oxidation products were analysed. The new model might be recommended for implementation in the various integral codes for adequate simulation of the absorber rods oxidation during severe accidents in the NPP. A simplified version of the model has been already implemented in the Russian SA code RATEG/SVECHA and allowed reasonable predictions in plant calculations.