Using Reactor Models to Predict Physical Material Content of Spent Fuels: Applications to Nuclear Materials Oversight

Because interest is high in recycling neptunium (Np) as well as americium (Am) back into the fuel cycle to decrease the radiotoxicity of disposed waste, considerable research has gone into techniques for separating these actinides. This has resulted in the description of a large number of separation techniques in the literature. Recently, the International Atomic Energy Agency Board of Governors tasked the Secretariat with maintaining oversight of these Np and Am nuclides, or alternate nuclear materials (ANMs). Calculations of the production of ANMs using reactor analysis codes may provide a cost-effective route to determine levels of these materials in spent fuel. Several reactor analysis codes are examined for their ability to accurately predict the amounts of Np and Am isotopes (specifically, 237Np, 241Am, and 243Am) present in spent fuels from different reactor types; however, before employing calculational methods for oversight, the accuracy of the codes must first be assessed. Code accuracy in determining all three isotopes is evaluated with respect to initial fuel composition and reactor type, including pressurized water reactors (PWRs), boiling water reactors (BWRs), Russian water-cooled, water-moderated reactors (VVERs), pressurized heavy water reactors (PHWR), high temperature gas reactors (HTGRs), and fast breeder reactors (FBRs). Radiochemical measurements of the higher actinides from the literature were used to judge code accuracy. For some reactor types, modification of existing data libraries may be necessary for cost-effective oversight of ANMs using calculational methods. From a nuclear materials oversight standpoint, knowledge of the amounts of these nuclides present in spent fuel is vital for determining appropriate measures.