Hybrid tallies to improve performance in depletion Monte Carlo simulations

In order to solve the transition matrix of the Bateman equations during transmutation analysis, the following reaction rates are usually needed: (n, fission), (n, 2n), (n, 3n), (n, 4n), (n, p), (n, α), and (n, γ). Tallying all rates through a Monte Carlo simulation can be quite expensive for realistic light water reactors. An alternative approach to compute these rates is to collapse a very fine-group flux together with a pregenerated multigroup cross section (constructed with the same energy grid). While this approach provides a 3x speedup in the active cycles performance, it introduces a considerable memory penalty. The issue is that thousands of groups are needed to accurately resolve the (n, γ) rates, most notably that of ²³⁸U. This study explores a hybrid approach in which threshold reaction rates are computed through the flux tally route while more relevant rates such as (n, γ) and (n, fission) are handled with a standard reaction rate tally. This option provides more flexibility in reducing the total number of groups because threshold reactions, in general, have smoother shapes. Multiple error analyses were carried out on the proposed hybrid method; the results show that even when going as low as 300 groups, the eigenvalue is still within 100 pcm of a traditional simulation.