Comparison of nested geometry treatments within GPU-based Monte Carlo neutron transport simulations of fission reactors

TL;DR

This paper compares three GPU-based neutron tracking strategies in Monte Carlo simulations of fission reactors, demonstrating their efficiency and flexibility, with the single tracker method showing notable speedup over CPU implementations.

Contribution

It introduces and evaluates three GPU tracking strategies for Monte Carlo neutron transport, highlighting the effectiveness of a flexible, single tracker approach across different reactor geometries.

Findings

All three GPU methods achieve over 77% of specialized tracker speed.

The single tracker method attains a 2.19× speedup over CPU execution.

Methods are suitable for typical reactor problems where tracking isn't dominant.

Abstract

Monte Carlo (MC) neutron transport provides detailed estimates of radiological quantities within fission reactors. This involves tracking individual neutrons through a computational geometry. CPU-based MC codes use multiple polymorphic tracker types with different tracking algorithms to exploit the repeated configurations of reactors, but virtual function calls have high overhead on the GPU. The Shift MC code was modified to support GPU-based tracking with three strategies: dynamic polymorphism with virtual functions, static polymorphism, and a single tracker type with tree-based acceleration. On the Frontier supercomputer these methods achieve 77.8%, 91.2%, and 83.4%, respectively, of the tracking rate obtained using a specialized tracker optimized for rectilinear-grid-based reactors. This indicates that all three methods are suitable for typical reactor problems in which tracking does not dominate runtime. The flexibility of the single tracker method is highlighted with a hexagonal-grid microreactor problem, performed without hexagonal-grid-specific tracking routines, providing a 2.19$\times$ speedup over CPU execution.