Modeling ²³³Pa Generation in Thorium-fueled Reactors for Safeguards

Technical Report · Wed Dec 01 04:00:00 EST 2021

DOI:https://doi.org/10.2172/1880332· OSTI ID:1880332

Davis, Victoria ^[1]; Goddard, Braden ^[2]; Hitt, George W. ^[3]; Richards, Scott ^[4]; Gariazzo, Claudio ^[4]

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1. Argonne National Lab. (ANL), Argonne, IL (United States); Virginia Commonwealth Univ., Richmond, VA (United States)
2. Virginia Commonwealth Univ., Richmond, VA (United States)
3. Coastal Carolina Univ., Conway, SC (United States)
4. Argonne National Lab. (ANL), Argonne, IL (United States)

Thorium has been considered as a possible alternative to uranium for nuclear fuel for many decades. It is three to four times more abundant in the earth than uranium and produces significantly less long-lived transuranic nuclear waste. Some claim thorium poses fewer proliferation concerns than other fuel types largely due to ²³²U buildup (and associated high energy gamma-emitting decay products) in the irradiated thorium fuel. However, to fully explore potential proliferation concerns, generation and subsequent decay of ²³³Pa produced in the reactor core still must be studied. With its half-life of 27 days, ²³³Pa decays to ²³³U, which is an International Atomic Energy Agency (IAEA) defined special fissionable material that can be used for nuclear weapons production. With more research being dedicated to thorium-fueled reactors, and several of these reactor designs possessing online fuel processing (allowing for on-site protactinium separation), it is important to understand this potential proliferation pathway. In particular, it is theoretically possible to extract protactinium from the irradiated fuel salt before it decays into ²³³U. This hypothetical potential diversion can become an even greater proliferation concern if the extracted protactinium is purified through a second separation of protactinium approximately ten days later to remove the short half-life decay products of ²³²Pa and ²³⁴Pa, thus resulting in a higher concentration of the ²³³Pa isotope, which decays into weapons usable ²³³U with hardly any ²³²U or ²³⁴U in it. To estimate 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 used thorium fuel. Characteristics of interest included technology maturity, cost, precision, and time taken to acquire results. Some technologies, like hybrid K-edge densitometry 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. 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 detectible for all the reactor types modeled at all decay times (0 to 300 days), more work should be done with higher fidelity models.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA), Office of Defense Nuclear Nonproliferation
• DOE Contract Number: AC02-06CH11357
• OSTI ID: 1880332
• Report Number(s): ANL/SSS-21/16; 172611
• Country of Publication: United States
• Language: English

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