On-The-Fly multigroup cross section generation method with continuous energy accuracy

In this paper, an on-the-fly multigroup cross section generation method is developed and implemented into the coarse mesh transport code COMET to account for the effects of spectral change and surrounding environment in reactor core calculations. The method consists of (1) a continuous energy stochastic response reaction rate generation module and (2) a deterministic multigroup cross section updating module. The first module is developed by extending the continuous energy response function generator for COMET to solve a set of local fixed-source transport problems with incident flux boundary conditions (or surface sources) and to generate a library of various response reaction rates. The second module constructs the phase space (continuous in space, angle, and energy) flux distribution in the regions of interest as a superposition of elements from the pre-computed library and then generates the multigroup cross sections on-the-fly. The method is benchmarked against the multigroup cross sections directly by continuous energy Monte Carlo in a set of stylized Very High Temperature Reactor (VHTR) single fuel block and whole core benchmark problems. Benchmark results demonstrate that the new method can achieve accuracy close (i.e., within two standard deviation of stochastic uncertainty) to that of continuous energy Monte Carlo while having on-the-fly computational speed.