Seismic Fluid-Structure-Interaction Analysis of NHR200-II Fuel Assembly

Abstract The fuel assembly (FA) stands as a pivotal component within the reactor core, impacting safety, reliability, and economic viability of the entire facility during the lifetime. As a newly-designed nuclear heating reactor, the FA of the 200 MW nuclear heating reactor (NHR200-II) embodies a novel configuration, characterized by a 9 × 9 fuel bundle arrangement inside, which are clamped by three spacer girds along the height direction, and fully enclosed by a square zirconium channel outside for improving effective coolant flow rate. Under the earthquake event, possible deformations of fuel cladding or outer channel, buckling instability of spacer grids, even damage of the structures and control rod insertion failures can occur and further pose unaffordable safety risks. Therefore, it is necessary to implement a comprehensive seismic fluid-structure-interaction (FSI) analysis with regard to the NHR200-II fuel assembly to evaluate the structural integrity. In this paper, a particle finite element method (PFEM) based partitioned two-way FSI scheme is used to capture the seismic responses of NHR200-II FA, that is, implicit finite element method for structure dynamics, PFEM for fluid motion. To accurately solve the FSI phenomenon, it is essential to couple the fluid solver and structural solver advancing with a same time step, i.e., strongly-coupled on the FSI interfaces. The specific fluid solver and structural solver are combined together in the commercial finite element code LS-DYNA to carry out the two-way FSI simulation, which greatly improves computational efficiency. This finite-element based numerical framework has been validated in detail in the previous work on the axial flow-induced vibration analysis of a single NHR200-II fuel rod. At present, the seismic excitations applied to a fuel assembly level are not available, which are usually generated from several analyses of a site-specific earthquake, including sequential analyses of soil-structure-interaction model, the reactor pressure vessel (RPV) internals and FA array models. Thereby the computation employed a time-compressed El-Centro N-S earthquake (1.5 times amplification, ∼0.36 g peak acceleration) for the preliminary evaluation of the seismic FSI responses of NHR200-II FA. The results present stress distribution of each component (including the top & bottom nozzle, fuel cladding, zirconium channel, spacer grids), channel-spacer grid interactions (transverse impact force), dynamic clamping of the spacer girds, acceleration responses, and response spectra of the FA at typical positions. The results would confirm the reasonable structural design and robust seismic performance of NHR200-II FA and also are broadly applicable to the seismic response of fully submerged fuel assembly with square-type channel outside.