Effects of applying the doppler broadened rejection correction method for LEU and MOX pin cell depletion calculations

Abstract

The classical treatment for determining the scattering neutron energy and angle given epithermal scattering from heavy nuclides is to sample the target nuclei velocity using the Maxwellian ideal gas law while assuming a constant scattering cross section. This treatment has been shown to be inadequate to capture the upscattering effects from certain heavy nuclides with strong epithermal scattering resonances, e.g., 238U. A method to correctly account for the resonance scattering effects has been implemented in the MC21 Monte Carlo code. The method is based on the Doppler Broadening Rejection Correction (DBRC) method and also allows for the use of weight adjustment in lieu of rejection sampling. Whereas previous work has focused on applying the resonance scattering correction to 238U only, the MC21 implementation applies the rejection correction more broadly, including all uranium and plutonium isotopes in the models analyzed. This work demonstrates the effect of applying the DBRC to both LEU and MOX pin cell depletions on eigenvalue and nuclide inventories as a function of burn-up. As compared to a reference case lacking any resonance scattering correction, the effect on uranium and plutonium inventories is significant when applying the DBRC. In the case of the MOX fuel system, significant differences in the plutonium concentrations are also seen when DBRC is limited to just the 238U, suggesting that the DBRC method should be applied more broadly.