The impact of 238U 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.