Development of a Comprehensive Fuel Assembly Criticality Safety Model that Groups Multiple Fuel Assembly Designs as One Content Type for Type AF Packages

Traditionally, fresh fuel assembly shipments in Type A, fissile packaging require analyzing and licensing specific individual fuel assembly designs as separate contents. This method is cumbersome and can reveal proprietary data. In addition, any minor change in the dimensions of a design could require a new analysis to demonstrate continued compliance. To mitigate this issue, a new criticality analysis method was developed to arrange fuel assembly designs with similar parameters into bounding groups that represent a single comprehensive fuel assembly (CFA). Fuel assembly designs are then grouped by the primary parameters that contribute a large part to the criticality of the system, which include lattice size (i.e. 17x17), fuel rod pattern (i.e. the number and location of fuel rods and non-fuel holes), and nominal fuel rod pitch. The bounding CFA is developed through analysis by modeling all combinations of the fuel designs’ most reactive secondary parameters, which consist of fuel pellet diameter and radial cladding dimensions. The CFA is then analyzed in the single package and package array analyses to demonstrate compliance with the transport regulations. For a fuel assembly to be shipped in the package, it must match the primary parameters of a CFA and be bounded by the secondary parameters specified for the CFA. This method has been licensed for a Type AF, PWR fresh fuel package. Application of the criticality method has reduced the number of fuel assemblies specified in the certificate of compliance by approximately half. In future analyses, this method may be improved further by collecting a larger number of fuel assemblies into a grouping with one representative CFA, for example, based on lattice size (i.e. 17x17).