Computation of ex-core detector weighting functions for VVER-440 using MCNP5

The paper deals with the problem of weighting factor calculation and the determination of spatial weighting functions of ex-core detectors for VVER-440 using the Monte Carlo method. The computational results were obtained by the well-known code MCNP5 allowing high performance three-dimensional modeling of complex geometry of the in-vessel and ex-vessel reactor parts. Despite the fact that adjoint methods dominate in practice, forward mode of code running was chosen and applied to provide more accurate results contrary to the adjoint one. The calculation was performed for a boron lined proportional counter CPNB44 installed at the 3rd unit of NPP Jaslovské Bohunice. The base is the calculation of ex-core detector reaction rate induced by a neutron generated in a given volume element of a fuel pin. All the geometrical details and arising space heterogeneities were taken into account with the highest accuracy in the complex reactor model. Having obtained computational results, the weighted least square method was used to fit axial weighting functions. With respect to horizontal direction, the polyhedral approximation of closed Jordan surfaces was used to find the proper shape of horizontal weighting factor distribution. Sensitivity and parametric analysis was performed to evaluate the influence of various reactor operational parameters as well as the ex-core detector positioning on the weighting function values.

► Pin-wise spatial weight functions were determined for ex-core detectors using MCNP
► Ex-core detector response is mainly determined by a few peripheral fuel assemblies
► The weigh factor values vary significantly within a given fuel assembly
► Eight-fold decrease of weight factor values is observed in peripheral core region.