A New Method to Measure the 240Pueff Mass and to Verify the Pu Fissile Content in FBR MOX Fuel Assemblies

This paper presents three neutron methods to measure the plutonium content in mixed-oxide (MOX) fuel assemblies that verifies the neutron multiplication and also the plutonium grade. The 240Pueff content in MOX fuel assemblies has been measured for decades using neutron coincidence counting. The present paper uses neutron coincidence counting combined with Differential Die-Away Self-Interrogation (DDSI),1,2 and multiplicity counting. The paper will focus on fast breeder reactor (FBR) MOX assemblies; however, the method also applies to light water reactor (LWR) MOX fuel assemblies. The plutonium recovered from nuclear weapons dismantlement is being converted into MOX powder for fuel assembly fabrication, and it is important to establish that the plutonium in the fuel assembly is from weapons grade plutonium (WPu) versus reactor grade plutonium (RPu). A key isotopic indicator between the two types of plutonium is the 240Pueff fraction that is typically 4-7 percent in the weapons grade material and 15-40 percent in the reactor grade material. This paper presents a 3He tube-based detector design that is optimized for determining the 240Pueff fraction in the plutonium. The primary purpose of this paper is to evaluate three analysis enhancements to standard neutron coincidence counting for the verification of the mass of WPu in FBR fuel assemblies.