Rossi-alpha Benchmark Validation of a Static Alpha Eigenvalue Capability in OpenMC

Abstract

A static alpha eigenvalue capability was implemented in a modified version of the open-source Monte Carlo radiation transport code OpenMC and validated against Rossi-alpha measurements from 21 delayed-critical benchmark experiments and 33 subcritical configurations spanning fast, intermediate, and thermal systems with U-233, HEU, IEU, LEU, and plutonium fuels. The effective delayed neutron fraction was calculated using the k-prompt method, and the prompt neutron lifetime was calculated using the iterated fission probability method, both evaluated within the standard k-eigenvalue power iteration. The delayed-critical alpha eigenvalue was calculated from these quantities using the point kinetics equation alpha_dc = -beta_eff / ell_p. Agreement was generally within 10% for fast metal systems and within 5% for thermal solution systems. Subcritical extrapolation studies derived from the SHE-8 and STACY benchmark families show that alpha_dc remains stable as the system is driven subcritical.