Nuclear Materials Attribution for Reprocessed Uranium

This paper describes some research activities related to special nuclear materials attribution and proliferation resistance assessment carried out by the UK National Nuclear Laboratory (NNL). Given that the UK currently operates two reprocessing plants, it is important that UK should fully understand the characterization of the plutonium and reprocessed uranium (Rep U) that has been separated from nuclear fuel. NNL has previously studied plutonium attribution and has developed and implemented a methodology that is able to attribute a plutonium sample based on its measured plutonium isotopic composition. Attribution can be made to a particular type of reactor and the method also indicates the initial enrichment and discharge burnup of the fuel from which it originated. Plutonium was prioritized because of its sensitivities as a special nuclear material. Rep U is perhaps less sensitive, but the amount recovered from reprocessing is almost two orders of magnitude larger and it is no less important to be able to characterize it as fully as possible. The measured isotopic composition of a Rep U sample does not readily allow attribution in the same way as plutonium. This paper describes a simple procedure that was developed to allow Rep U attribution. It relies on a simple estimate of the initial U-235 content of the fuel from which the sample originates, based on the measured U-235 and U-236 contents. The initial enrichment estimate is then refined using a set of tabulations from the FISPIN (Fission Product Inventory) program and the measured U-236/U-235 ratio used to infer the discharge burnup. Finally, FISPIN tabulations of U-234 versus burnup can be used to infer the cooling time since reprocessing. The paper describes the approach and illustrates its application to some test samples.