Multiphysics Pebble-Bed Reactor Control Rod Withdrawal Study

Abstract

This work studied the responses of both a generic gas-and a fluoride-cooled pebble-bed reactor (PBR) concept-the gPBR-200 and gFHR, respectively-during reactivity insertion accidents. Both models rely on 2-D axisymmetric simulations to solve the neutron flux distribution, nuclide concentrations, and temperature across the core-in addition to numerous representative pebble and tristructural isotropic (TRISO) particle simulations for determining fuel and moderator temperatures. This not only allows for computing maximum temperatures in the core-thus enabling estimation of how near the fuel is to peak operational and safety limits-as prescribed by specified acceptable fuel design limits, which are determined in such a way that fuel is not damaged during operational or anticipated abnormal occurrences-but also predicting how much of the core exceeds a given temperature limit, as well as determining the local energy deposition rate.These models consider both control rod withdrawal (CRW) and control rod ejection (CRE) events. The former introduces a great deal more reactivity, as all the control rods are withdrawn (as opposed to a single one in the latter case), though at a much slower pace. In addition, for the gPBR-200, two limiting cases were considered: one with the core starting under hot full-power conditions and one with it starting under cold zero-power conditions. While the amount of reactivity added in the latter case is much higher (due to the far lower temperatures and the lack of neutron poisons such as Xe-135), the margin to temperature limits is also much more significant.Overall, for the design considered, none of the accidents resulted in the maximum fuel temperature reaching values close to the TRISO limit. However, the methodology presented herein could be very relevant if some designs consider reduced margins (e.g., higher temperatures) to achieve enhanced economics.Further model improvement is needed to better capture control rod worth, both in iii terms of cusping effects (as the rods are slowly withdrawn) and differential worth, especially as the tips of the rods near the upper cavity.