ZAPS: Zone-Averaged Accelerated Pebble Bed Simulator

Abstract

This work presents zone-averaged accelerated pebble bed simulator (ZAPS), a fast and flexible Monte Carlo tool for pebble bed reactor analysis. Built on the widely used Serpent2 transport code and implemented through the modular Cerberus Python interface, ZAPS couples explicit Monte Carlo physics with an efficient zone-based treatment of pebble motion and fuel depletion. The framework’s originality lies in its dual-loop architecture: an outer transport loop that updates flux distributions and an inner recirculation loop that simulates pebble movement and burnup under a fixed flux. This strategy drastically reduces the number of costly transport calculations, enabling equilibrium-state determination within only a few hours on a standard workstation. When benchmarked against the reference hyper-fidelity framework providing pebble-resolved data, using the generic fluoride-salt-cooled high-temperature reactor model, ZAPS reproduces average isotopic compositions and discharge burnup with excellent agreement while achieving a reduction in runtime of more than an order of magnitude—up to 100× less CPU time to reach equilibrium. These results demonstrate that ZAPS currently provides one of the fastest pebble bed reactor simulation capabilities available, supporting detailed parametric studies and advanced applications such as fuel-cycle investigation and optimization.

The authors' abstract, as published at the source. Nuclear Science and Engineering, 2026 · DOI ↗