Impact of Inspection Time on Pebble-Bed Reactor Operation

Pebble bed reactors are among the most promising advanced nuclear reactor concepts.Their numerical simulation is complicated by their specific operation scheme, but improvements in numerical computing capabilities now make it possible to study the isotopic composition of each pebble individually.This is the objective of the Hyper-Fidelity depletion (HxF) method used in this study, coupled with a simplified model for pebble motion.The precision and flexibility of this algorithm is used to assess the impact on reactor operation of the time spent by the pebbles outside the core between two irradiations, called inspection time.Simulations carried out on the gFHR (generic Fluoride-salt-cooled High-temperature Reactor) demonstrate a drop in the xenon concentration of reinserted pebbles when inspection time is greater than two days, resulting in a reactivity insertion at the bottom of the core.A static approach over the duration of a time step is used to better quantify these effects, despite the discretization of reactor simulation.This study reveals in particular the interest of implementing a predictor-corrector in the HxF algorithm.The static approach is also used to simulate reactor shutdown, starting from an equilibrium core.Inspection time is found to have only a limited effect on reactor restart after shutdown.