Comparative analysis of WWER-1000 benchmark calculations and improvements in hexagonal-Z diffusion methodology

The WWER-lOOO in-core fuel management benchmark has been established for the core design code package validation. The measured data have been provided by the Kozloduy Unit 5 Nuclear Power Plant (NPP). Benchmark calculations for the first three cycles have been performed and comprehensive comparisons between the obtained results and experimental data have been carried out. The corresponding differences have been analysed statistically and examined against the values of experimental uncertainty. It is concluded that further improvements in the comparisons will depend on both reduced measured errors and netronic model enhancements. To address the last issue an approach to develop generalized expressions for radial correction factors in the framework of the non-linear finite-difference type nodal formulation to the global neutron balance solution in hexagonal-Z geometry is proposed. The derivation of these expressions is based on higher order local balance equations and allows a modeling flexibility by employing different representations of neutron flux distribution. Two linear combinations of functions have been studied and benchmark calculations in two-dimensional hexagonal geometry have provided encouraging results