Coupled DNS/RANS Simulation of Fission Gas Discharge During Loss-of-Flow Accident in Generation IV Sodium Fast Reactor

The objective of this paper is to give an overview of a multiscale modeling approach to three-dimensional (3-D) two-phase transient computer simulations of the injection of a jet of gaseous fission products into a partially blocked sodium fast reactor (SFR) coolant channel following localized cladding overheat and breach. The phenomena governing accident progression have been resolved at two different spatial and temporal scales by the intercommunicating computational multiphase fluid dynamics codes PHASTA (at direct numerical simulation level) and NPHASE-CMFD (at Reynolds-averaged Navier-Stokes level). The issues discussed in the paper include an overview of the proposed 3-D two-phase-flow models of the interrelated phenomena that occur as a result of cladding failure and the subsequent injection of a jet of gaseous fission products into partially blocked SFR coolant channels and gas-molten-sodium transport along the channels. An analysis is presented on the consistency and accuracy of the models used in the simulations, and the results are shown of the predictions of gas discharge and gas-liquid-metal two-phase flow in a multichannel fuel assembly. Also, a discussion is given of the major novel aspects of the overall work.