A GPU-based direct Monte Carlo simulation of time dependence in nuclear reactors

A novel 3D Monte Carlo (MC) neutron transport code, GUARDYAN, was developed to simulate direct time dependence in nuclear reactors. GUARDYAN (GpU Assisted Reactor DYnamic ANalysis) addresses the huge computational need by exploiting massive parallelism available on modern Graphics Processing Units (CPUs). While the code is still under development, transient analysis on large scale problems is already obtainable. The implementation is verified via comparison of differential and integral quantities to MCNP6 results, including several criticality safety benchmarks. Unlike most conventional MC codes GUARDYAN is intentionally designed for time-dependent calculations supporting parallel scalability on state-of-the-art high performance computing platforms. The methodology of transport simulation thus differs in many aspects: generation-by-generation tracking is replaced by a time step method; branching of neutron histories, neutron banking is eliminated by statistical weight manipulations; a robust delayed neutron treatment is implemented. These concepts, along with advanced acceleration techniques for improving the performance of point-in-cell search routine and the delta tracking method, resulted in an efficient MC tool that seems to outperform existing methods for kinetic MC simulation. Transient analysis was performed on an LWR core demonstrating that simulation of one second of a transient requires around 50 h on a single GeForce GTX 1080 GPU. The power evolution produced by GUARDYAN during this transient was also compared to experimental data; remarkably close agreement was found despite the uncertainties in the MC model. (C) 2019 Elsevier Ltd. All rights reserved.