Preliminary interpretative analysis of the QUENCH-18 bundle test using SCDAPSim/mod3.5

Highlights: • Accelerated oxidation due to nitrogen. • Steam and oxygen starvation phenomena. • Formation and oxidation of zirconium nitride. • Reflood excursion and quench. • Sensitivity to modelling assumptions - Abstract: A preliminary analysis of the bundle reflood experiment QUENCH-18 is performed with the SCDAPSim/Mod3.5/da code containing the PSI-developed model for oxidation in the presence of air. The simulation follows on from pre-test planning and prediction calculations using the same code and input model. The starting point for the post-test calculations differs from the pre-test only in respect of using the actual boundary conditions. Comparison with measured data enables several aspects of the experiment to be studied. Various treatments of steam and air oxidation kinetics investigate the effect of nitrogen on the oxidation and its continuing influence when air is no longer present. Concerning degradation, different assumptions on failure of the oxide crust indicate how the exposure of relocated metallic melt can enhance the oxidation excursion during reflood. Some modelling and knowledge limitations are identified, particularly regarding oxidation in steam-air mixtures, the roles of nitrogen and zirconium nitride as chemically active species. Several observed features of the facility operation remain unresolved. Simulations suggest that damage to the shroud affected the reflood progression. The bundle may also have been in a highly damaged state, with further impacts on reflooding. Interpretation is therefore provisional, pending more information on the bundle final state. However, the simulation results have significant implications for reactor calculations.