Validation of axial pin power distributions in a 10×10 BWR assembly across an enrichment boundary under full-density water moderation conditions

Critical experiments involving fuel pin gamma-scans have been performed to characterise total fission reaction rate axial distributions in a Westinghouse SVEA-96+ assembly under full-density water moderation [these investigations are part of the co-operation between PSI and Unterausschuss Kernenergie (UAK) or association of the Swiss nuclear operators, viz. Kernkraftwerk Leibstadt AG, Kernkraftwerk Gösgen-Däniken AG, BKW FMB Energie AG and Nordostschweizerische Kraftwerke AG]. The measurements have covered an 80-cm long region of the SVEA-96+ assembly involving two important axial heterogeneities: (a) the enrichment boundary separating the lower axial region (average 235U enrichment of 4.02 wt.%, 14 burnable absorber fuel pins) from the upper axial region (average 235U enrichment of 3.70 wt.%, 16 burnable absorber fuel pins), and (b) the presence of spacers at about 12 cm below this enrichment boundary. The experimental results have been compared to HELIOS/PRESTO-2 calculated values at three different levels, by means of radial point-wise, axial point-wise and axial node-wise comparisons. The results show good agreement (typically within 2%) between HELIOS/PRESTO-2 nodal pin powers and corresponding experimental values, with the largest discrepancy of 6-9% being highly localised in a pin subject to strong flux gradients and spectral interactions. Further improvements would require explicit three-dimensional lattice calculations and the development of three-dimensional pin power reconstruction methods.