Proof of concept for hyper-fidelity depletion of full-scale pebble bed reactors

Pebble bed reactors, either gas-cooled or fluoride-cooled, are among the most promising advanced reactor concepts with active development worldwide. The relatively small size of the pebbles and the fact that they are continuously recirculated makes the task of simulating fuel burnup particularly challenging. Most solutions proposed thus far suggest dividing the core into spectral zones within which fuel composition is assumed uniform, ignoring differences between the different pebbles in the zone. This work suggests that it is possible with current computing tools and with relatively modest computing resources to perform depletion of pebble bed reactors by depleting each pebble individually. The feasibility of this approach, named hyper-fidelity depletion, is demonstrated using a simplified model that assumes static pebbles in a regular lattice and employing the Monte Carlo code Serpent 2 that provides multiple features suitable for hyper-fidelity depletion, among which the most important is domain decomposition. The obtained results show that the spectral zone methods compared to hyper-fidelity can predict average trends, but are not adequate to determine distribution for quantities of interest such as burnup, fission products, etc.