Modeling technique for calculating plutonium inventory in spent MOX fuel

As part of a disposition strategy for excess defense plutonium, a MOX fuel fabrication facility is being constructed on the Savannah River Site. The facility will take weapons grade plutonium and blend it with depleted uranium to make MOX fuel that will then be irradiated in a reactor. The intention is to convert direct use material into a form more in line with the spent fuel standard. A model has been constructed to calculate the plutonium content of discharged MOX fuel of this type throughout its life time using Wolfram’s Mathematica. The model takes an initial composition for the spent MOX fuel from ORIGEN and accounts for production and loss terms for each isotope of plutonium. The composition of the spent fuel is then known at any future time. The predictions of this model have been compared with predictions for the decay of this fuel produced by ORIGEN and good agreement was found. The maximum difference between the model’s prediction and ORIGEN’s prediction was 6% and was usually a fraction of a percent for most plutonium isotopes. The plutonium isotopic content of discharged weapons grade MOX fuel is of interest for a number of reasons. Firstly, the isotopic content is relevant to the desirability of the spent fuel to potential proliferators. MOX fuel of this kind after irradiation has a much higher plutonium content than LWR fuel so its isotopic content is important in assessing its desirability. Secondly, a major issue in the storage of spent fuel is the heat generation of the fuel. Pu238 is one of the major contributors to the heat production in a discharged fuel assembly so it is desirable to know how the amount of Pu238 changes over time. Finally, the radiotoxicity of the spent fuel is important in rendering the fuel “self protecting”. Many of the actinides contribute to the radiotoxicity of the fuel.