Experimental Comparisons of 3D Reconstructed Pin-Power Distributions in Full-Scale BWR Fuel Assemblies

In this paper, reconstructed local fission rates obtained with the two-group nodal diffusion program PRESTO-2, used at the Leibstadt Nuclear Power Plant (KKL) in Switzerland, are compared with experimental results and MCNPX calculations. The experimental facility consists of a test zone, where the measurements are made, surrounded by a buffer zone and two driver zones that render the system critical and also contain the control rods. The test zone consists of a tank that contains a 3×3 array of BWR fuel assemblies of type SVEA-96+. Four cases are considered, all corresponding to a 1.23 m high active zone moderated with light water at room temperature: 1) axially uniform enrichment and gadolinium content, 2) like case 1 but with a L-shaped control blade completely inserted, 3) enrichment and gadolinium content change at the core mid-plane, 4) like case 2 but with the control blade partially inserted. The comparisons give insight into the accuracy of the pin power reconstruction methodology. The axially uniform case without control blade shows a good radial agreement and a well predicted axial curvature of the flux. On the other hand, systematic deviations are observed in the radial direction for the controlled cases, with the axial heterogeneities causing deviations around the discontinuity and also in the axial curvature of the flux.