Continuous Energy COMET Solution of the VHTR Core

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. As a result, COMET now has continuous energy Monte Carlo accuracy but with formidable computational speed. In this paper, we present the continuous energy COMET solution to a stylized Very High Temperature Reactor (VHTR) benchmark problem in which all the heterogeneities 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 The eigenvalue and pin and fuel block averaged for all control rods inserted and fully withdrawn were compared to the corresponding values calculated by the continuous energy Monte Carlo code MCNP. It was found that the agreement between the two codes is excellent (0.8% 2.2%) except in the fuel pins next to the reflector blocks, where differences near 6% were observed. (author)