IMPACT OF HYDROGENOUS MATERIAL MODELING IN CRITICALITY SAFETY ANALYSES OF TYPE A, FISSILE SHIPPING PACKAGES

A large criticality uncertainty evaluated in type A, fissile transport packages is the effect of hydrogenous packing materials that may be a more effective moderator than water in the void space of the packaging. These material effects in conjunction with the geometric representations and material and fabrication tolerances compose the uncertainty of a package criticality safety analysis. When modeling hypothetical accident conditions (HAC), it is reasonable to assume that the same HAC event that might produce fuel lattice expansion would also tend to concentrate hydrogenous packing material in that same fuel region. The hydrogenous materials combined with other HAC effects may result in an increase in the package criticality uncertainty. During transport, hydrogenous packing materials such as polyethylene cluster separators, bags, and cushioning are used to support the fuel assembly integrity, but also represent a potential for increased criticality in the system during HAC due to the repositioning of Hydrogen content. For example, a result of the fire test of the RAJ-II BWR fissile package was the melting of the fuel assembly packing materials within the inner container, which showed melted polyethylene parts and attachment of the molten polyethylene on the dummy fuel rods. Using the SCALE 6 code, a criticality evaluation was conducted to combine the implications of polyethylene redistribution with the HAC of transport. The criticality analysis models are established to follow the melting progress of the polyethylene parts in accordance with the fire test conditions. The models incorporate optimized water moderation and lattice expansion for an arrayed system of damaged packages. For HAC, the polyethylene materials are represented by a mixture of the density weighted packing components. Prior analyses showed the maximum increase in criticality uncertainty from modeling the polyethylene was as a uniform distribution on the fuel rod surface regardless of the condition of transport. Results of the polyethylene redistribution analysis showed that a combination of the drop, thermal, and optimized water moderation effects result in the largest contributor to criticality uncertainty; a 3% increase caused by all polyethylene packing materials melted together in the lattice expanded region at the bottom of a vertical positioned package.