Safety and Regulatory Issues of the Thorium Fuel Cycle (NUREG/CR-7176)

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Publication Information

Manuscript Completed: December 2013
Date Published: February 2014

Prepared by:
Brian Ade
Andrew Worrall
Jeffrey Powers
Steve Bowman
George Flanagan
Jess Gehin

Oak Ridge National Laboratory
Managed by UT-Battelle, LLC
Oak Ridge, TN 37831-6170

M. Aissa, NRC Project Manager

Prepared for:
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

NRC Job Code V6299

Availability Notice

Abstract

Thorium has been widely considered an alternative to uranium fuel because of its relatively large natural abundance and its ability to breed fissile fuel (²³³U) from natural thorium (²³²Th). Possible scenarios for using thorium in the nuclear fuel cycle include use in different nuclear reactor types (light water, high temperature gas cooled, fast spectrum sodium, molten salt, etc.), advanced accelerator-driven systems, or even fission-fusion hybrid systems. The most likely near-term application of thorium in the United States is in currently operating light water reactors (LWRs). This use is primarily based on concepts that mix thorium with uranium (UO² + ThO²), add fertile thorium (ThO²) fuel pins to LWR fuel assemblies, or use mixed plutonium and thorium (PuO² + ThO²) fuel assemblies.

The addition of thorium to currently operating LWRs would result in a number of different phenomenological impacts on the nuclear fuel. Thorium and its irradiation products have nuclear characteristics that are different from those of uranium. In addition, ThO2, alone or mixed with UO2 fuel, leads to different chemical and physical properties of the fuel. These aspects are key to reactor safety-related issues.

The primary objectives of this report are to summarize historical, current, and proposed uses of thorium in nuclear reactors; provide some important properties of thorium fuel; perform qualitative and quantitative evaluations of both in-reactor and out-of-reactor safety issues and requirements specific to a thorium-based fuel cycle for current LWR reactor designs; and identify key knowledge gaps and technical issues that need to be addressed for the licensing of thorium LWR fuel in the United States.

An evaluation of in-reactor safety issues was performed based on nuclear physics fundamentals and available experimental data and study results. The Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition (NUREG-0800) has been reviewed to identify specific items that would be impacted by changing the fuel to a form that contains thorium. Quantitative analyses were performed using the SCALE code system to compare key performance parameters of both (Th,U)O2 and (Th,Pu)O2 fuels against UO2 and MOX fuels in LWRs. The reactivity coefficients, assembly power (between surrounding UO2 assemblies and the assembly of interest), and single-assembly controlled lattice reactivities are compared for beginning, middle, and end of life.

The SCALE fuel assembly models from the in-reactor analyses were also used in ORIGEN calculations for low, normal, and high discharge burnup values to evaluate out-of-reactor characteristics of spent thorium fuel. Calculations were performed for all four fuel types to compare the depleted fuel isotopics, decay heat, radiological source terms, and gamma spectra.

Based on these evaluations, potential impacts on safety requirements and identification of knowledge gaps with regard to once-through LWR thorium fuel cycles were identified. Recommendations for additional analysis or research to develop a technical basis for the licensing of thorium fuel are summarized in phenomena identification and ranking tables (PIRTs). The PIRTs provide an assessment of how the changes in the phenomena could be addressed (e.g., additional analysis, new data required, experimental validation, etc.).