Dynamic behavior in fast burst reactor with three-dimensional coupled multiphysics method

In this paper, the neutronic and thermoelastic behaviors of prompt supercritical bursts in the fast burst reactor are numerically studied by developing a 3D coupled multiphysics FEM (finite element method) code FBR-MPC based on the neutron transport theory for neutronics, thermal conduction equation for temperature and thermoelastic equations for displacement/stress. Because the feedback effect of core expansion on the neutronics can be accurately modeled by reforming the discretized element and changing the material density, the developed code is expected to be applicable for simulating the dynamic behaviors of all the bursts under controlled condition. To validate the developed code and obtain precise pictures of the dynamic behaviors, the slow, medium and fast bursts in the Godiva I reactor are investigated. The results show that the developed code can provide accurate results for all the three selected bursts. For the medium and fast bursts where the inertia effect is generated, the fission rate oscillation caused by the core vibration, which cannot be reproduced with the simplified models in early works, is first observed by using the developed FBR-MPC code. Due to the small core expansion and short burst time, the point kinetic and adiabatic approximations are suitable for the neutronic and temperature calculations for all the three selected bursts. At the time of displacement wave crest, the stress concentration in the core central regions becomes more significant for faster bursts.