Power-peaking factors in the McMaster Nuclear Reactor

A review and update of the Safety Analysis Report is a current, ongoing project at the McMaster Nuclear Reactor (MNR). Using modem codes and techniques the reactor physics characteristics of the MNR core are being detailed. This paper describes the reactor physics analysis of power peaking in MNR using the code package WIMS-AECL/3DDT. The results are to be used as input for analysis on thermalhydraulic safety margins. MNR is a 2 MWth, light-water-moderated, pool-type reactor, composed of rectangular parallelepiped plate fuel. At present MNR is in the process of switching from HEU (93%) to LEU (20%) fuel by systematically replacing spent HEU fuel assemblies with fresh LEU assemblies. As a result, there are currently 5 different types of fuel assemblies in the MNR core. This paper describes the modelling of these fuel assembly types for power-peaking factor determination. Fuel assemblies were examined in both a fuel lattice and moderating environment and the effect of the absorber rod bank position was studied. The overall power-peaking factors were determined as products of local, radial and axial contributions. It was determined that adjacent moderating material has a significant effect on local power peaking in a fuel assembly as does burnup, loading and geometry of the fuel assemblies. Overall power-peaking factors of 2.47 → 3.73 are typical for 18-plate fuel whereas a limiting overall power-peaking factor of 5.04 was found for 10-plate HEU fuel in a typical MNR core. These numbers are found to be more conservative than previously reported estimates but should not result in any new safety issues. (author)