Burnup Verification Measurements for Spent Nuclear Fuel

The U.S. Department of Energy is presently scheduled to begin transport of spent fuel from utility reactor sites to a federal storage facility in 1998, using casks certified by the U.S. Nuclear Regulatory Commission. Maximizing the capacity of transport casks now being designed is essential to reduce costs and to maintain public and occupational risks \"as low as reasonably achievable.\" The spent fuel to be transported in the immecliate future has been cooled for a decade or more, and its racliation output is greatly reduced due to natural raclioactive decay. The reduced requirements for heat dissipation and racliation shielcling allow more spent fuel to be loaded into a cask of a fixed gross weight. Conceptually, cask capacity can be increased to the point where, under certain conditions, nuclear criticality safety must be considered in the design of transport casks (Sanders and Westfall, 1990). The reduced reactivity of the \"burned up\" fuel permits about twice as many spent assemblies to be safely transported in each cask as could be accommodated if the assemblies were fresh, unburned fuel. The loaded casks are to be transported in a dry condition. Nuclear criticality becomes possible during the transport of spent fuel only if: (1) the cask is involved in an accident, (2) the accident is severe enough to breach the cask, (3) the cask is flooded with water that contains a low level of neutron absorbers, and (4) the fuel has unacceptably high reactivity. The criticality of the loaded, flooded cask can be calculated from three parameters which are cataloged at the reactor for each assembly. The three factors are: initial enrichment, usually expressed as weight percent U235; bum up (gigawatt days per metric tonne of U metal); and the cooling time (years) (Brady and Sanders, 1991). Casks designed taking advantage of the reduced reactivity of the burned fuel to calculate criticality are called \"burn up creclit\" casks. The characteristics of fuel acceptable for loading into a burn up creclit cask can be specified by a loading curve as shown in the example of Figure 1. Acceptable assemblies are configured in the cask so that, under flooded conclitions, the system is less than 95% of critical. The curve delineates the minimum burn up creclit required for a particular initial enrichment. The use of bum up creclit in cask design raises the possibility that a cask could be misloaded with unacceptable fuel, if such assemblies are present. Racliation measurements can be used to help prevent misloading of a cask by verifying that each assembly has the appropriate characteristics.