Steven L. Krahn

Natural thorium resources and recovery: Options and impacts

Abstract This paper reviews the front end of the thorium fuel cycle, including the extent and variety of thorium deposits, the potential sources of thorium production, and the physical and chemical technologies required to isolate and purify thorium. Thorium is frequently found within rare earth element–bearing minerals that exist in diverse types of mineral deposits, often in conjunction with other minerals mined for their commercial value. It may be possible to recover substantial quantities of thorium as a by-product from active titanium, uranium, tin, iron, and rare earth mines. Incremental physical and chemical processing is required to obtain a purified thorium product from thorium minerals, but documented experience with these processes is extensive, and incorporating thorium recovery should not be overly challenging. The anticipated environmental impacts of by-product thorium recovery are small relative to those of uranium recovery since existing mining infrastructure utilization avoids the opening and operation of new mines and thorium recovery removes radionuclides from the mining tailings.

Assessment of the Potential of By-Product Recovery of Thorium to Satisfy Demands of a Future Thorium Fuel Cycle

A long-standing concern about the future implementation of thorium fuel cycles has been the availability of a thorium fuel cycle infrastructure, including thorium mineral recovery. Globally, while thorium is known to be a relatively abundant element, there is currently little commercial demand for thorium, leaving many of the world’s largest thorium deposits unexploited. However, adoption and subsequent expansion of the thorium fuel cycle may not require “thorium mines” because a number of mining operations (notably titanium and uranium) already extract considerable amounts of thorium, which is presently discarded. Nearly 100000 tonnes of thorium per year could be recovered from active mine sites, with most of this coming from titanium mining (˜80000 tonnes/yr of thorium) and uranium mining (˜9000 tonnes/yr of thorium). This output would be sufficient to satisfy even the most optimistic demand for thorium resources in the near future.

The Reemergence of the Thorium Fuel Cycle: A Special Issue of Nuclear Technology

Thorium was intensively studied from the 1960s to the 1980s in the United States and internationally as a potential basis for future nuclear fuel cycles. After demonstration of feasible thorium fuel cycle concepts, the United States decided instead to pursue liquid metal, fast breeder reactors using uranium and plutonium. Worldwide interest in thorium fuel cycle development continued at a reduced level, with India having invested the most resources. Recently, the thorium fuel cycle has been the subject of renewed interest, partly due to speculated growth in nuclear energy worldwide (hence putting potential strain on uranium reserves) and partly due to the pursuit of advanced reactor concepts designed to enhance safety and economics—which also have the potential to use thorium to further improve fuel cycle performance. This renewed interest often addresses new possibilities for using thorium in the modern era; however, it can be difficult to discern between actual, unique characteristics of the new thorium concepts and misconceptions disseminated by advocates and detractors. Therefore, right now is a good time to discuss experience with the thorium fuel cycle to date, provide an even-handed description of its inherent attributes, and identify some of the data gaps that have yet to be resolved. The renewed interest in thorium is supported, in part, by a resurgence of national and industrial programs focused on thorium-based nuclear fuel cycles. India has described plans for a three-stage nuclear energy strategy that integrates thorium-based fuels: Stage 1 involves natural uranium–burning pressurized heavy water reactors, to produce plutonium and stockpile it for future use; Stage 2 uses this stockpiled plutonium in fast breeder reactors with thorium blankets to produce 233U (and additional plutonium) and recycles plutonium back to the fast reactor; finally, Stage 3 uses recovered 233U (from Stage 2) in advanced heavy water–moderated, light water–cooled reactors. Currently, Stage 1 is operational, Stage 2 is in advanced testing, and Stage 3 is in advanced design.1 China is planning to build two experimental molten salt reactors: The first, which is to commence operation in 2017, will use low-enrichment uranium, spherical pebble fuel, and LiF-BeF2 molten salt as the coolant; this is intended to set the stage for a second molten salt reactor (scheduled to commence operation in 2020), which will use thorium-based fluid fuel and include fuel salt processing.2 China is also considering the use of Canadian-designed fuels in pressurized heavy water reactors, which have the potential to incorporate thorium.3 Also, Thor Energy is conducting experiments focused on demonstrating fuel manufacturing, materials, and nuclear performance of PuO2-ThO2 and UO2-ThO2 ceramic fuels. Test pins composed of thorium-uranium and thoriumplutonium oxide mixtures are currently being irradiated in the Halden test reactor, and additional testing of thoriumbased oxide fuel pins is planned.4 Differences in the major technical features of the thorium-233U fuel cycle and the currently implemented fuel cycle, based on 235U and plutonium, present implications for facility design and operation, along with waste disposal. To set the context for this special issue of Nuclear Technology, we will discuss some of the major technical features and characteristics that help to frame the dialogue regarding the use of thorium (a more comprehensive summary is found in Ref. 5). Thorium is fertile but does not contain natural fissile isotopes, so external fissile material is required to produce 233U at the onset of fuel cycle implementation. Thorium-based fuels offer the potential for higher conversion ratios than uranium-based fuels in thermal reactors, since (a) 233U has a relatively low neutron capture (nonfission) cross section compared NUCLEAR TECHNOLOGY · VOLUME 194 · iii–iv · MAY 2016

Comparative Assessment of Thorium Fuel Cycle Radiotoxicity

Abstract This paper compares the radiotoxicity of thorium-based and uranium-based spent nuclear fuels and reprocessing wastes to inform evaluation of whether thorium-based fuels are significantly less radiotoxic than uranium-based fuels, as has been claimed at times in the technical literature. A consistent approach for calculating the radiotoxicity is established for four oxide fuel types in a pressurized water reactor: low-enrichment uranium, uranium with plutonium fissile material, thorium with 233U fissile material, and thorium with plutonium fissile material. The results of the calculations are presented to display the radiotoxicity trends and are analyzed to determine (a) what underlies the indicated radiotoxicity trends for decay times from 1 year to 20 million years and (b) factors that may have led to erroneous conclusions concerning the comparative radiotoxicity of thorium- and uranium-based fuels. The overall conclusion is that the ingestion radiotoxicity of thorium-based fuels containing 233U or plutonium fissile materials is similar to the radiotoxicity of uranium-based fuels containing 235U or plutonium fissile materials but that within this overall similarity there are significant differences in radiotoxicity in specific eras during decay.

Evaluating the Collective Radiation Dose to Workers from the U.S. Once-Through Nuclear Fuel Cycle

The Electric Power Research Institute (EPRI) is sponsoring the development of tools to support long-term strategic planning for research, development, and demonstration and for evaluation of advanced nuclear fuel cycles (NFCs). The EPRI-sponsored work under way at Vanderbilt University (VU) is developing a new, comparative risk assessment tool for NFCs. In the course of conducting a demonstration application of the assessment tool, it was observed that the relative contribution of NFC operations to radiological worker impacts estimated by the assessment tool was substantially different from widely used historical data and conventional wisdom. This paper analyzes these differences by first describing the NFC and the nature of radiological worker impacts. Then, the assessment tool developed by VU is described, along with assessment results; historical data relevant to radiological worker impacts are then summarized, and key differences between assessment results and previously reported impacts are identified. This comparison is followed by an analysis of the major factors causing the differences and an assessment of their validity. Finally, several implications of the differences are discussed.

Environmental Impacts, Health and Safety Impacts, and Financial Costs of the Front End of the Nuclear Fuel Cycle

FEFC processes, unlike many of the proposed fuel cycles and technologies under consideration, involve mature operational processes presently in use at a number of facilities worldwide. This report identifies significant impacts resulting from these current FEFC processes and activities. Impacts considered to be significant are those that may be helpful in differentiating between fuel cycle performance and for which the FEFC impact is not negligible relative to those from the remainder of the full fuel cycle. This report: • Defines ‘representative’ processes that typify impacts associated with each step of the FEFC, • Establishes a framework and architecture for rolling up impacts into normalized measures that can be scaled to quantify their contribution to the total impacts associated with various fuel cycles, and • Develops and documents the bases for estimates of the impacts and costs associated with each of the representative FEFC processes.

Advancing Risk-Informed Decision Making in Managing Defense Nuclear Waste in the United States: Opportunities and Challenges for Risk Analysis: Managing Defense Nuclear Waste

An omnibus spending bill in 2014 directed the Department of Energy to analyze how effectively Department of Energy (DOE) identifies, programs, and executes its plans to address public health and safety risks that remain as part of DOEs remaining environmental cleanup liabilities. A committee identified two dozen issues and associated recommendations for the DOE, other federal agencies, and the U.S. Congress to consider, as well as other stakeholders such as states and tribal nations. In regard to risk assessment, the committee described a risk review process that uses available data, expert experience, identifies major data gaps, permits input from key stakeholders, and creates an ordered set of risks based on what is known. Probabilistic risk assessments could be a follow-up from these risk reviews. In regard to risk management, the states, in particular, have become major drivers of how resources are driven. States use different laws, different priorities, and challenge DOEs policies in different ways. Land use decisions vary, technology choices are different, and other notable variations are apparent. The cost differences associated with these differences are marked. The net result is that resources do not necessarily go to the most prominent human health and safety risks, as seen from the national level.

Applications for Thorium in Multistage Fuel Cycles with Heavy Water Reactors

Abstract Certain characteristics of heavy water reactors (HWRs), such as a more flexible neutron economy compared to light water (due to reduced absorptions in hydrogen), online refueling capability, and having a thermal neutron spectrum, make them potentially attractive for use with a thorium fuel cycle. Three options that combine HWRs with thorium-based fuels are considered in this paper: a Near-Term option with minimal advanced technology requirements, an Actinide Management option that incorporates the recycle of minor actinides (MAs), and a Thorium-Only option that uses two reactor stages to breed and consume 233U, respectively. Simplified, steady-state simulations and corresponding material flow analyses are used to elucidate the properties of these fuel cycle options. The Near-Term option begins with a low-enriched uranium oxide pressurized water reactor (PWR) that discharges spent nuclear fuel, from which uranium and plutonium are recovered to fabricate the driver fuel for an HWR that uses thorium oxide as a blanket fuel. This option uses 28% less natural uranium (NU) and sends 33% less plutonium to disposal than the conventional once-through uranium fuel cycle on an energy-normalized basis. The Actinide Management option also uses spent nuclear fuel from a PWR using enriched uranium oxide fuel (both a low- and high-enrichment variant are considered), but the uranium is recycled for reuse in the PWR while the plutonium and MAs are recycled and used in conjunction with thorium in an HWR with full recycle. Both enrichment variants of this option achieve a more than 95% reduction in transuranic actinide disposal rates compared to the once-through option and a more than 60% reduction compared to closed transuranic recycle in a uranium-plutonium–fueled sodium fast reactor. The Thorium-Only option breeds a surplus of 233U in a thorium-based HWR to supply fissile material to a high-temperature gas-cooled reactor, both of which recycle uranium and thorium. This option requires no NU and produces few transuranic actinides at steady state, although it would require a greater technology maturation effort than the other options studied. Collectively, the options considered in this study are intended to illustrate the range of operational missions that could be supported by fleets that integrate thorium and HWRs.