Localized neutron flux assessment and verification studies using MCNPX PWR full core model

Abstract

The existence of significant local intra-assembly and azimuthal neutron flux variations at or around PWR control rod fingers and which would eventually not be captured by lower order transport deterministic reactor analysis methods, was examined using simplified full-core Monte Carlo calculations. The locations of specific interest were the clad surfaces and absorber material zones around the tips of partially inserted control rods in controlled assemblies located at various core locations. A fixed-source option was adopted in order to reduce the modeling efforts and computational costs. However, the validity of the fixed-source approach for predictions of the local flux distributions was verified by comparisons against corresponding calculations performed in eigenmode and against deterministic CASMO/SIMULATE solutions. Noticeable azimuthal neutron flux variations up to ∼30% were observed but only for the thermal energy range. For the fast (>1MeV) neutron flux, the predicted azimuthal variations were found to be bounded by an upper value of about ∼10%. On the other hand, significant planar flux non-uniformities between individual control rods within a controlled assembly were seen for all energies. Hence, in addition of having allowed for a strengthened knowledge on the behavior of local flux variations around PWR control rods, these results also provide a first assessment of the likely uncertainty level in control rod tip fluence estimations based on lower resolution deterministic reactor analysis codes.