Criticality Assessment of Fuel Assemblies with Missing Fuel Rods An intractable Problem?

In current certificates of package approval the arrangement of water and guide tubes within the array of fuel rods of a fuel assembly is specified in detail. Fuel assemblies with deviating water and guide tube arrangements or missing rods are not allowed to be loaded into the casks. The reason behind is that the reactivity of a standard fuel assembly increases if some rods are removed. For a certain number and arrangement of missing rods a maximum of reactivity is reached. Due to the missing fissile material the reactivity will decrease again if further rods are then removed. For the comprehensive assessment of the maximum of reactivity all possible configurations of fuel rods and missing rods have to be investigated. The paper describes the problem at hand in detail giving estimates for the complexity of the analysis. A complete analysis is only possible for cases with a very small number of fuel rods. Fuel assemblies which are used in reality can only be treated with approximation methods. The paper describes two such methods. With the first method up to 8 x 8 fuel assemblies can be analyzed in detail by using reasonable simplifications and assumptions. For fuel assemblies with a higher number of fuel rods a Monte Carlo approach is used. This second method can be applied to all fuel assemblies currently in use. The accuracy of both approximation methods is compared for a 8 x 8 fuel assembly. The paper describes the statistical uncertainties and the influence of the parameters used for the individual calculations. Results are given for some of the mostly used fuel assembly types. Finally, a procedure for the proof of criticality safety of fuel assemblies with missing fuel rods is defined. The application of this procedure will allow more flexibility in the definition of the licensed contents with respect to number and arrangement of missing rods in the fuel assemblies.