Integrated Simulation of Rod Ejection Accidents in Boron-Free Pressurized Water Reactors Using S5K Neutronics and VIPRE-01 Sub-channel Solvers

Abstract

Transient neutronics and core-thermal subchannel simulations of rod ejection accidents were performed for the Rolls-Royce Small Modular Reactor equilibrium core design. Transient neutronics simulations were performed using SIMULATE5-K (S5K). Pin-power reconstruction capabilities in S5K allowed for predictions of the prompt Radial Averaged Peak Fuel Enthalpy (RAPFE) rise and total RAPFE encountered in rod ejection accidents. Rod ejection events were analysed across various core initial power levels, cycle statepoints, and ejected rod worths. This was done to determine relationships between fuel failure criteria and ejected rod worth, to aid future design choices about appropriate control rod worth limits.

Power shapes from the S5K simulations were then utilized in VIPRE-01 (subchannel analysis code) to obtain predictions of the Minimum Departure from Nucleate Boiling Ratio (MDNBR) in a wide range of postulated rod ejection accidents. A S5K-VIPRE interface tool was used to generate extremely detailed VIPRE-01 decks, which directly incorporated S5K predicted pin-power shapes into the models. This interface tool was used to investigate the impact of VIPRE model fidelity on the MDNBR predictions. Two VIPRE model geometries were investigated: one where the assembly containing the ejected rod was modelled in full subchannel fidelity and another where the eight assemblies surrounding the ejected rod were also modelled in full subchannel fidelity (including full S5K supplied pin-wise power profiles).

It was found that the most limiting fuel failure criterion was the MDNBR and that there were significant differences between the predicted MDNBRs in the two VIPRE model geometries. Particularly, it was found to be likely that the most limiting assembly in terms of MDNBR performance was likely to not be the assembly from which the control rod was ejected from.