Evaluation of the Isotopic Composition of wg-MOX During Long Term Storage

As part of the plutonium disposition and management agreement (PDMA), the U.S. and Russia declared a total of 68 MT (each State declared 34 MT) of weapon origin plutonium as excess material. To minimize the proliferation risk of separated, weapon origin plutonium (wPu), the U.S. is planning to process it in a mixed oxide (MOX) fuel fabrication facility being constructed in Savannah River Site (SRS) near Aiken, South Carolina. The facility will take excess weapon-grade plutonium, remove impurities, and mix it with uranium oxide to form MOX fuel (95% 238UOx, 5% 239PuO2) pellets for reactor fuel assemblies. These assemblies will be eventually irradiated in commercial light water reactors (LWR’s). This approach aim to removes the separated excess wPu from the inventory and decreases the proliferation risk in its management. This paper provides an evaluation of the characteristics of irradiated wg-MOX fuel and its isotopic inventory as it evolves with time, taking into consideration the decay heat, fission products (FP) and actinides inventory, and Pu content. The ORIGEN code was used to model and determine the isotopic inventory of wg-MOX after discharge. The resulting composition was modeled for very long time (> 100 years), calculating the time-dependent Pu isotopic inventory, activity and thermal output of the discharged fuel. These values were compared to typical UOx spent fuel composition. Preliminary calculations show that discharged wg-MOX fuel maintains a much larger plutonium inventory throughout its lifetime. In addition, the actinide activity of the discharged wg-MOX was significantly higher than the UO2 after discharge. The initial thermal output of the discharged fuels was found to be similar, with the MOX fuel being significantly higher after the first decade. The activity and thermal output are fundamentally important for the management of the discharged fuel both for transportation and very long term storage. The plutonium content and plutonium isotopics are also of great interest from a safety, security and safeguards standpoint.