Master of Science
Mechanical and Nuclear Engineering
Thorium has been researched for many decades as a possible alternative to uranium nuclear fuel. Thorium can be implemented in many different reactor designs including pressurized water reactors (PWRs), pressurized heavy water reactors (PHWRs), and molten salt reactors (MSRs). Its abundance in the earth, decreased long-lived transuranic waste, and claims that there are fewer proliferation concerns contribute to the attractiveness of using thorium as alternative nuclear fuel. However, possible proliferation pathways have been noted and must be investigated, particularly the potential diversion of 233Pa which can then decay to 233U – special fissionable material that should be under International Atomic Energy Agency (IAEA) safeguards.
To better understand the concern of this potential proliferation challenge of thorium, different nuclear material accountancy techniques were reviewed for their viability to quantify 233Pa if extracted from irradiated thorium fuel. Quantifying and tracking 233Pa is important for safeguards because 233Pa is a precursor for 233U. Without monitoring 233Pa, it is possible to produce high purity 233U outside of the safeguards monitoring system. Characteristics of interest of different material accountancy techniques included technology maturity, cost, precision, and time taken to acquire results. Some technologies, like hybrid K-edge densitometry (HKED) and passive gamma spectroscopy, appear to be viable techniques based on current literature. Due to the limited scope of this project, only passive gamma spectroscopy was further investigated.
Three different reactor types (PWR, CANDU, MSR) were modeled with mixed thorium-uranium oxide fuels that were burned until the fuel was spent. The protactinium in the used fuel was extracted at the time of shutdown and the change in isotopic content of the protactinium quantified. Gamma spectroscopy simulations were performed for the protactinium isotopes and their decay products at various decay times to understand protactinium generation within the reactor cores. Given the simplicity of the models and large assumptions made (e.g. no background, no shielding, no self-attenuation), the initial results indicate that though 233Pa is detectable for each reactor fuel types modeled at all decay times (0 to 300 days), more work should be completed with higher fidelity models.
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