Continuous energy coarse mesh transport (COMET) method

Abstract

The coarse mesh radiation transport method COMET developed for reactor physics calculations originally treated the phase space variable continuously except for the energy variable. In this work, the method is extended to also treat the energy variable continuously. COMET uses incident flux response expansion coefficients that are generated from local (oarse mesh) fixed source calculations for obtaining the transport solution to whole core problems. In the new method, the energy dependence of the angular flux (spectrum) is factorized into a product of two functions: a strongly varying asymptotic function and a slowly varying (smooth) shape function representing the deviation from the asymptotic function. The method uses spectra from a single assembly or color set calculation as the asymptotic functions and the shape function is expanded by the 0th order B-splines in a set of coarse energy bins. This new factorization allows the use of continuous energy stochastic method for generating the incident flux response expansion coefficients for solving whole-core problems. The new COMET method was tested in a PWR benchmark problem in whole core configurations using UO2 and MOX fuels. It was found that the COMET results agree very well with the corresponding continuous energy Monte Carlo reference solutions. The difference between the COMET and MCNP eigenvalues varied from 29 to 65 pcm, while the mean relative difference in the pin fission density distributions computed by COMET and MCNP was in the range of 0.25%–0.63% for three (namely, controlled, uncontrolled and some-rods-in) core configurations. This difference is less than 2 standard deviations of the MCNP uncertainty. The computational efficiency of the continuous energy COMET was found to be consistent with the multigroup COMET (i.e., more than 4 orders of magnitude faster than MNCP). Based on the benchmark results, it can be concluded that the new continuous energy expansion theory allows COMET to perform high fidelity and highly efficient whole-core transport calculations in heterogeneous reactors.