Investigation of the effects of a variation of fuel assembly position on the ex-core neutron flux detection in a PWR

This work shows the impact of potential displacements of the fuel assembly positions in the reactor core on the signal values of the ex-core instrumentation of a KWU – type pressurized water reactor in order to understand in detail the impact on the calibration factor of excore detectors. This was done with a range of Monte Carlo calculations which simulated the detailed geometrical effect by stepwise changing the positions of fuel assemblies for selected, conservative scenarios. First, criticality calculations were carried out for the chosen core configurations and corresponding surface sources on the core barrel were determined. In these calculations, the distances were varied between the fuel assemblies which were in the line of sight of the ex-core instrumentation. A maximal change of the fluxes on the surface of the core barrel of 4%/mm could be calculated under conservative assumptions for the combination of displaced fuel assemblies. In addition, a dependence of this effect as a function of cycle burnup was analyzed. In a second step transport calculations for the ionization chambers were performed using the surface sources. An increase of the reaction rate at the chambers of up to 3%/mm has been calculated.