Reevaluation of Spatial Weighting Factors for Ex-Core Neutron Detectors

The ex-core detector response in pressurized water reactors (PWRs) depends on not only power level but also core power distribution. Therefore, it is important to precisely calculate the assembly-wise spatial weighting factors for the ex-core detectors. Usually these factors are calculated with the one-dimensional transport code and point kernel calculational method, in which the neutron scattering effect outside of reactor vessel is neglected. But when the scattering effect is estimated to be rather big, we calculate the assembly-wise spatial weighting factors using the two-dimensional transport code, which includes the scattering effect. Consequently, we found that the weighting factors of peripheral assemblies that are remote from the detector but close to reactor vessel are rather big in comparison with the previous results. When we calculate the detector response during one control rod insertion test of three-loop PWR core using these weighting factors, the agreement between calculation and measurement is very good. A simple point kernel calculational method developed instead of the two-dimensional transport calculation that consumes much computer time.