COMET neutronics solutions to the prismatic VHTR benchmark problem

Abstract

Recently, the hybrid stochastic deterministic coarse mesh radiation transport method COMET for whole core criticality calculations was extended to treat the energy variable continuously consistent with its treatment of the rest of the phase space variables. In this paper, a surface-dependent asymptotic spectrum expansion scheme was used by the continuous-energy COMET to obtain the transport solutions to a stylized Very High Temperature Reactor (VHTR) benchmark problem in which all levels of heterogeneity that are of reactor physics importance are preserved. Because of its complex geometry and material distribution, this benchmark problem is challenging to solve with deterministic neutronic methods, even those based on transport theory. The eigenvalue and the fuel pin fission density distribution for all control rods fully inserted and fully withdrawn configurations were compared to the corresponding values calculated by the continuous-energy Monte Carlo code MCNP. The agreement between the two codes was found to be excellent. The eigenvalue difference was 66 pcm for the uncontrolled configuration and 69 pcm for the controlled configuration. The corresponding average pin fission density differences were 0.36% and 0.53% for the uncontrolled and controlled configurations, respectively. It was also found that COMET is 4–5 orders of magnitude faster than MCNP. This indicates that COMET has continuous-energy Monte Carlo like accuracy but with formidable computational speed for solving problems with multiple levels of heterogeneity such as the VHTR problem.