Application of Ex-vessel Neutron Dosimetry Combined with In-core measurements for Correction of Neutron Source Used for RPV Fluence Calculations

The paper deals with calculational and semi-analytical evaluations of VVER-1000 reactor core neutron source distributions and their influence on measurements and calculations of the integral through-vessel neutron leakage. Neutron activation measurements analyzed in the paper were carried out in ex-vessel air cavity at different NPP units with VVER-1000 during different fuel cycles. Time-integrated neutron source distributions used for DORT calculations were prepared by two different approaches based on a) calculated fuel burn-up (standard routine procedure) and b) in-core measurements by means of SPD & TC (new approach). Taking into account that fuel burn-up distributions in operating VVER may be evaluated now by analytical methods (calculations) only, it is needed to develop new approaches for testing and correction of calculational evaluations of neutron source. Results presented in this paper allow to consider a reverse task of alternative estimation of fuel burnup distributions. The approach proposed is based on adjustment (fitting) of time-integrated neutron source distributions, and hence fuel burn-up patterns in some part of reactor core, on the base of ex-core neutron leakage measurement, neutron-physical calculation and in-core SPD & TC measurement data.