Simulation of dynamic characteristics of NHR200-II fuel assembly

Abstract

NHR200-Ⅱ is a 200 MW nuclear heating reactor. The integrity of the fuel assembly should be ensured during the normal operations and design basis accidents. In the latter cases (e.g. earthquake), its deformation or damage should not prevent the insertion of control rods to shut down the reactor. Meanwhile, during the operating time, there are two aspects that attention should be paid, (i) the mechanical performance of internal core materials might be worsened by the increase of burnup and (ii) dynamic loads involving hydraulic erosion, flow-induced vibration might damage the assemblies. Therefore, the investigation of the dynamic characteristics of the fuel assembly with consideration of burnup effects is necessary. The fuel rod with pellets and cladding was approximated by the beam elements with a circular cross-section applied with calibrated material properties. Connector elements were used to replace the three-arc springs and dimples on the grid and thus simplify the geometry of the spacer grid. The agreements between the detailed and simplified models indicate the equivalence of these two methods. Then two simulations were performed to evaluate the dynamic characteristics of NHR200-II fuel assembly at the beginning of life. The modal simulation of the fuel bundle and channel suggests that the channel could be assumed stationary to the fuel bundle during the dynamic analysis. The seismic simulation conducted in the specific boundary conditions indicates that the integrity of spacer grids can be guaranteed and there was no buckling of the gird. No permanent deformation of the channel was observed, and its relative displacements were sufficiently small to ensure the insertion of control rods. Finally, burnup effects on the structural behavior of the fuel rod are discussed. The initial conditions of NHR200-II fuel assembly and its material properties at different burnup times are provided by referring to two irradiated tests. Various sensitivity analyses had been conducted to evaluate the effects of material properties and pellet-cladding interaction on the resulting failure limit and stiffness of the fuel rod.