IMPARTING REALISM TO THE CRITICALITY EVALUATION OF A BWR FUEL ASSEMBLY PACKAGE - Presentation

The criticality evaluation is a demonstration of the most reactive configuration of the individual package in isolation, arrays of undamaged packages, and arrays of damaged packages. The most reactive configuration for the fuel assembly contents in a BWR package must take into consideration a number of parameters that include partial length fuel rods, neutron absorbing burnable absorber rods in the fuel bundle, rearrangement of the fuel bundle during accident transport conditions in the form of lattice expansion, and partial loadings of fuel rods. Packaging material composition and arrangement of packaging materials are also important to consider in the demonstration of maximum reactivity. Values must be assigned for these parameters that may not be known with a high degree of certainty, such as burnable absorber rod distribution, lattice expansion, and packaging material composition during a fire. Imparting realism to the criticality evaluation requires a thorough understanding of the effect that impact, fire and water immersion may have on the package configuration and material properties. Evaluating the sensitivity of neutron multiplication to intrinsic material property uncertainties can be accomplished by applying perturbation methods. However, evaluation of sensitivity to other package configuration uncertainties is a more heuristic process that requires a detailed understanding of the fuel assembly design and package performance during accident transport conditions. There is no guarantee for a particular sequence of impacts or complete progression of a fire during a transport accident, yet intermediate conditions that result in the maximum neutron multiplication are often overlooked. A criticality evaluation of a BWR package has been done to demonstrate a realistic maximum neutron multiplication using values for parameters that takes into consideration credible intermediate transport conditions. Values for parameters used in the criticality evaluation are assigned in a manner consistent with constraints imposed by the fuel assembly design and performance of the contents and packaging materials during the sequence of mechanical, thermal and water immersion tests.