The impact of U resonance elastic scattering approximations on thermal reactor Doppler reactivity

The effects of accurate modeling of neutron scattering in ²³⁸U resonances are analyzed for typical light water reactor (LWR) and next generation nuclear plant (NGNP) lattices. An exact scattering kernel is formulated and implemented in a newly developed Monte Carlo code, MCSD (Monte Carlo slowing down), which solves a neutron slowing down in an infinite homogeneous medium and is used to generate resonance integral data used in the CASMO-5 lattice physics code. It is shown that the exact scattering kernel increases LWR Doppler coefficients by ∼10% relative to the traditional assumption of asymptotic elastic downscatter for ²³⁸U resonances. These resonance modeling improvements are shown to decrease hot full power eigenvalues by ∼200 pcm for LWRs and ∼450 pcm for NGNPs.