M.M. Ramirez, “Safeguards Analysis for Neptunium-237 in High-Level Waste Through Computational and Radiochemical Methods”, M.S. Thesis, Nuclear Engineering, Texas A&M University, College Station, TX (2021).
Used nuclear fuel disposition is a major nuclear waste management problem worldwide at the closing end of the nuclear fuel cycle since long-lived actinides can cause safety and criticality concerns. Effective separation of these nuclides can lead to safer storage practices and the establishment of more advanced nuclear material safeguards. In the case of ²³⁷Np, which is believed to be weapons useable, little is stated in the International Atomic Energy Agency (IAEA) safeguard protocols. Neptunium-237 has a fast neutron fission cross section comparable to that of ²³⁹Pu, and its production rate is roughly 0.1% of used nuclear fuel. The amount of ²³⁷Np produced is low; however, the growing trove of used nuclear fuel is a proliferation risk, especially if the separation of long-lived actinides becomes an industry standard. Production of ²³⁷Np was evaluated using ORIGEN2 to simulate one tonne of various fuels for varying reactor types. Burnup simulations comparisons were also made between data points to monitor the overall production for a given reactor. Based on the results, it was determined that a Pressurized Water Reactor (PWR) produced the most ²³⁷Np, respectively followed by Boiling Water Reactor (BWR), CANada Deuterium Uranium (CANDU) Reactor, and Fast Breeder Reactor (FBR). These results are further supported by the fact that PWRs and BWRs have a higher ²³⁵U content than a CANDU, which burns natural uranium, and FBRs, which burn depleted uranium mixed with plutonium. Comparisons were also made with unique irradiated uranium samples irradiated at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) and at the Missouri University Research Reactor (MURR). These samples were irradiated at low burnup conditions and experimentally designed to mimic the irradiation of an FBR and CANDU. Analyses of these samples were completed using inductively coupled plasma mass spectrometry (ICP-MS) to quantify the amount of ²³⁷Np in the irradiated samples and to draw conclusions about neptunium production in low-burnup fuels.