DE-FOA-0000065
ARPA-E Control Number: 25A4106
An End-to-End Integrated Liquid-Salt Reactor System for Nuclear Waste Transmutation without Reprocessing and Long-term Storage G.I. Maldonado, L.F. Miller, A.E. Ruggles, K. Sorensen University of Tennessee-Knoxville J.C. Gehin, D.E. Holcomb, G.D. DelCul Oak Ridge National Laboratory ABSTRACT In light of the Yucca Mountain project’s termination, the United States must address the continued accumulation of spent nuclear fuel generated by light water reactors (LWRs) which, without further processing, requires the qualification of a large and costly repository for up to millions of years. We propose a novel concept for an energyproducing, simpler, safer, and more affordable end-to-end integrated system that can transmute long-lived LWR waste to a form requiring orders-of-magnitude less volume and storage time, and without dedicated reprocessing systems. This concept is based on a variant of the molten-salt reactor (MSR) technology successfully developed and tested at Oak Ridge National Laboratory in the 1950s-1970s. The new concept will include transformational features beyond those of current systems being pursued, such as; (1) an integrated waste-transmutation system that directly accepts LWR fuel and eliminates long-lived transuranic nuclides; (2) a liquid-salt homogeneous fuel form that requires no fuel fabrication; (3) a system that does not create pure streams of proliferation-sensitive materials, and (4) a path toward sustainable, greenhouse gas-free, and transuranic-free nuclear energy based on thorium, a cheaper and more abundant fuel. TECHNICAL SECTION ARPA-E has been chartered as an organization intended to make “transformative” investments in energy—investments that would be considered too large, too long-term, too risky for other organizations to consider. Our proposal constitutes one of those investments. Project Objectives In brief, we propose to; (1) provide a sound and practical technical path to solving the current problem of nuclear waste, and (2) jump-start a long-term approach to nuclear energy generation without long-lived waste. We propose that such a solution exists and; is technically feasible, significantly simpler and more affordable than other alternatives being considered, and can be achieved in a reasonable period of time. We estimate that the technology readiness level for this area of research to be currently a mixture of TRL-2 and TRL-3 and we project to lay out a near-term development path to raise the level or all areas to at least TRL-3, while defining the path to TRL-4 within the time and budget requested.
The University of Tennessee-Knoxville and Oak Ridge National Laboratory
DE-FOA-0000065
ARPA-E Control Number: 25A4106
Figure 1. ORNL Molten-Salt Reactor Experiment reactor room (left), MSRE graphite core (middle), and Aircraft Reactor Experiment (ARE) building that was retrofitted for the MSRE (right). Figure 1 shows a couple of photographs of actual facilities and experiments successfully designed and operated at Oak Ridge National Laboratory (ORNL) between the 1950s and 1970s. In early 1950, ORNL began examining reactor concepts based around liquidfluoride and chloride salts into which nuclear fuels like uranium and thorium would be dissolved. This interest was based around the remarkable properties of fluorine. Lacking only one electron to complete its outer electron shell, elemental fluorine is extremely reactive with nearly any other material. Once reacted, it becomes a negatively-charged ion (fluoride) and associates with positively-charged metal ions like lithium, sodium, magnesium, and beryllium in very chemically-stable salts. Fluorine is such a powerful oxidizer that it can even strip uranium and thorium oxides from their covalently-bound oxygen atoms, forming salts like uranium tetrafluoride (UF4) and thorium tetrafluoride (ThF4). UF4 and ThF4 are the valence equivalents of UO2 and ThO2, and are even more chemically stable. But UF4 and ThF4 have an important difference from UO2 and ThO2 in a reactor; because they are ionically-bonded rather than covalently-bonded and in a fluid form rather than a solid form, they can tolerate the punishing radiation environment of a nuclear reactor core. Ionic bonds are more like “associations� than bonds, so severe displacements have no effect on the overall material properties of the fuel. Contrast that with covalently-bonded, solid oxide fuel forms, where radiation fields lead to swelling, cracking, and failure of fuel integrity. Another remarkable property that fluorine possesses is that it consists of only a single natural isotope (19) and that it has a very low neutron absorption cross-section; in fact, one of the very lowest found in nature. This makes the fluoride form of nuclear fuel unique in that it is in a form that is suitable for both nuclear operation (burnup of nuclear waste), reprocessing operations (separation of fission products from fuel), and initial formulation of nuclear fuel from existing nuclear waste. We propose to fluorinate existing spent nuclear fuel with fluorine-bearing materials in order to convert it into this useful form.
The University of Tennessee-Knoxville and Oak Ridge National Laboratory
DE-FOA-0000065
ARPA-E Control Number: 25A4106
Uranium oxide, when contacted with anhydrous hydrofluoric acid, is transformed. Oxygen binds with hydrogen to form water; fluoride binds with uranium to form uranium tetrafluoride. Such a process is done worldwide today as one of the earliest steps in uranium enrichment and is well-understood. The salient difference between that and our work is that we would do this step on irradiated nuclear fuel. After the fuel is fluorinated, it contains an excess of uranium for our purposes. That uranium is then separated from the other fluorides by continued fluorination, this time with elemental gaseous fluorine. This converts uranium tetrafluoride into uranium hexafluoride, which is gaseous and easily separated from the remaining fluorides. The gaseous uranium hexafluoride is then condensed and can be shipped to an enrichment plant, since UF6 is precisely the form of uranium used in enrichment. A large amount of uranium (many thousands of tonnes with significant market value) could be recovered in this fashion. The remaining fluorides consist of the short-lived waste (fission products) and long-lived waste (transuranics). Depending on how the fluorination process was conducted there might also be a great deal of zirconium tetrafluoride in the mix (from the original Zircaloy fuel cladding). By balancing the composition of this mixture with natural sodium fluoride (commonly used in toothpaste) we would form a salt mixture of NaFZrF4-AnF3 (where An stands for the mixture of actinides remaining in the salt). This salt mixture is nearly identical to the NaF-ZrF4-UF4 salt mixture successfully used by ORNL in the Aircraft Reactor Experiment in 1954. The ARE was the first fluoride reactor and demonstrated the inherent safety, stable operation, and flexibility of this reactor concept. Our salt mixture would then be introduced into a reactor that would be a modern version of the ARE in which fission would be used to destroy the transuranic isotopes and to generate electricity (from fission heat) simultaneously. Thus the “problem” of nuclear waste is transformed into a profitable solution—uranium from spent fuel is recovered and “waste” is destroyed to make electricity. Initially, fluoride salts are preferred because of the considerable experience available with them. However, another variant of the fluid-fueled reactor could be one that uses chloride salts. In particular, if a thorium fuel cycle is considered, chloride salts could provide improved destruction of the transuranic isotopes, and an enhanced ability to produce uranium-233 from natural thorium. Uranium-233 could then be used to start liquid-fluoride thorium reactors which would thereafter operate indefinitely on natural thorium and not produce transuranic waste. To accomplish these goals we will take advantage of nearly six decades of ORNL research into the fluorination of spent nuclear fuel and fluid-fueled reactors. The proposed research will develop in detail how the fluorination of spent nuclear fuel can be done safely and efficiently, with the maximum recovery of useful products. In addition, the research will show how this fuel mixture might be used directly to destroy waste and make electricity. The research will also examine other methods where the waste could be
The University of Tennessee-Knoxville and Oak Ridge National Laboratory
DE-FOA-0000065
ARPA-E Control Number: 25A4106
destroyed while making a new and more efficient fuel that would solve the nuclear waste problem once and for all. MISSION IMPACT SECTION Light water reactors (LWRs) consuming low-enrichment uranium have come to dominate the commercial nuclear power market over the last fifty years. However, and unfortunately, these reactors produced long-lived nuclear waste in their fuel in the form of fission products and transuranic (TRU) isotopes of plutonium, americium, curium, and neptunium. The nature of the LWR fuel form (solid uranium oxide) and the physics of the nuclear reactions in conversion of U-238, which is approximately 95% of the heavy metal in the LWR fuel, to transuranic materials meant that these light-water reactors would always produce more of this long-lived TRU waste than they consumed. Note in Figure 2 the large fraction of the decay heat load contribution from some of these long-lived actinides in LWR spent fuel, which incidentally affects the sizing and timerelated constraints of a long term nuclear waste repository. Do note, however, that the majority of fission products decay away 1 to 2 orders of magnitude quicker than TRU isotopes 1000 Years
100 Years
Am-241 Total Pu-240
Figure 2. Decay Heat from PWR spent fuel after discharge (Wigeland, et al., 2004) These issues were not considered particularly troubling to the United States in the early days of nuclear development. The US government agreed to assume responsibility for “spent nuclear fuel� (SNF) after it had passed through the reactor, and disposal options for such spent fuel seemed tractable and straightforward. So the development of the light-water reactor with its rather significant consumption of uranium went forward well into the 1970s. Today, 104 commercial LWRs operate in the US producing roughly 20% of our electrical energy while more than 30 applications for new LWR plants have recently been filed or are in the process of being submitted to the US Nuclear Regulatory for review.
The University of Tennessee-Knoxville and Oak Ridge National Laboratory
DE-FOA-0000065
ARPA-E Control Number: 25A4106
Mission Impact #1: Solving the Problem of Nuclear Waste With the decision to terminate the Yucca Mountain project, the United States needs to address the issue of more than 50,000 metric tons of spent nuclear fuel currently accumulated at nuclear power plants. This amount of spent fuel is likely to double as the current plants live out their operational lifetimes (assuming no new LWRs are built). Without further processing, spent nuclear fuel requires the qualification of a large repository for a period of hundreds of thousands, and perhaps millions of years. Even if Yucca Mountain is opened and without a method to minimize waste, the buildup of spent nuclear fuel from LWRs will require construction of additional repositories, or a major expansion of the amount of spent fuel that Yucca Mountain would have been authorized to accept. The need for expanded repository space would have been greatly compounded had the US decided to move to a greater reliance on nuclear power using existing light-water reactor technology and is thus a concern with the potential growth of nuclear power. To address climate change concerns, vast new sources of CO2-free energy will be needed in the future. For nuclear energy to be one of these options the waste problem must be solved, permanently and in a politically acceptable fashion. The proposal we set forth here is a concrete and realistic plan to accomplish this lofty goal. Through the fluorination of existing spent nuclear fuel, we can remove the isotopes that pose longterm concern and destroy them through fission. Mission Impact #2: Long-term Nuclear Energy Solution without Long-lived Waste In the above-noted fission process we can make electricity and potentially make a new nuclear fuel from thorium that will allow us to start nuclear reactors that don’t produce long-term waste. Additionally, these new reactors also provide a development path that will allow us to make much more electricity from thorium than we currently produce from uranium, but without large-scale mining operations, without the production of longlived waste, and without uranium enrichment facilities. Each of these would be transformational considered in isolation, but considered together they are revolutionary. Some historical background is necessary to appreciate the thorium reactor concept and its implications. Thorium is a naturally-occurring element roughly three to four times more common that natural uranium and nearly 500 times more abundant than uranium-235. Thorium consists entirely of the single isotope thorium-232, which is a fertile isotope, meaning that a single neutron converts it (through a pair of intermediate steps) to the fissile nucleus uranium-233. The fission of uranium-233 produces enough neutrons (about 2.5) that make it possible to sustain the “burning” of thorium indefinitely, provided that waste products are removed.
The University of Tennessee-Knoxville and Oak Ridge National Laboratory
DE-FOA-0000065
ARPA-E Control Number: 25A4106
Consumption of thorium resources opens the door to truly staggering amounts of future potential energy sources. Since thorium is not particularly scarce in the Earth’s crust, early pioneers like Dr. Alvin Weinberg foresaw a future where mankind “burned the rocks” for energy—in other words, that the small amounts of thorium present in average crustal material could be profitably mined for energy because of the vast energy density inherent in these natural fuels. Each cubic meter of typical crust contains enough thorium (in a volume of about four sugar cubes) to provide for the electrical needs of an average American for a decade. Brief Historical Account of the Technological Legacy Available The technology to realize the potential of thorium came about in an indirect fashion. In the late 1940s, the US Air Force began investigating a nuclear-powered bomber capable of flying to the Soviet Union and returning without refueling. To accomplish this difficult task, a nuclear reactor was needed that was very small, highly-efficient, selfcontrolling, and immune to disturbances that plague other reactors. They knew from the outset that in order to provide the heat necessary to drive a turbojet engine (using nuclear heat in place of heat from fuel combustion) they would need a reactor whose fuel was stable at very high temperatures. This, in turn, implied a fuel form that had exceptional chemical stability. The Air Force quickly discarded any possibility of using water-cooled reactors to reach such temperatures. They were fundamentally incapable of doing this because of basic limitations in the properties of water. They were looking for a chemical fuel form that had even greater chemical stability than the oxide fuel used in typical reactors—and there was only one—liquid fluoride salt. The Air Force funded the Oak Ridge National Lab to develop a reactor based on fluoride salts in order to determine whether such a reactor would be stable in actual operation. The Aircraft Reactor Experiment became operational in November 1954 and demonstrated that operation of a reactor in this advanced configuration was feasible. The ARE was based on uranium fluoride salt dissolved in a mixture of sodium and zirconium fluoride salts. As the flowing salt passed through a “core” where neutrons were sloweddown by collisions in beryllium oxide, fission heat was generated. As the salt passed out of the core to an external heat exchanger, fission ceased and heat was transferred at high temperature (1600F) to a coolant fluid. Within several years the fluoride reactor program had transitioned from aircraft reactors to reactors designed to “burn” thorium. Another test reactor was built, the Molten-Salt Reactor Experiment, and it operated for five years, demonstrating that this technology was amenable to operation on a variety of nuclear fuels and under a variety of conditions. Some of these facilities are shown in Figure 1. Despite its successful demonstration, the fluoride reactor based on thorium threatened the established AEC direction of the liquid-metal fast breeder reactor. Unlike the LMFBR, the fluoride reactor could not generate weapons-grade plutonium because it operated with slowed-down neutrons instead of fast neutrons. Furthermore, the chemical “trick” it used to separate uranium from thorium would not work when trying to separate plutonium
The University of Tennessee-Knoxville and Oak Ridge National Laboratory
DE-FOA-0000065
ARPA-E Control Number: 25A4106
from uranium. The AEC moved to shut research on the thorium reactor down and had its chief proponent, ORNL lab director Alvin Weinberg, fired from his position in 1970. Nevertheless, interest in thorium-fueled fluoride reactors has persisted in the decades since the shutdown of research at ORNL, sustained by the inherent merit of the concept and its compelling possibilities. A modern concept is called a liquid-fluoride thorium reactor (LFTR, pronounced LIF-ter) and would utilize uranium-233 tetrafluoride dissolved in a mixture of lithium-7 fluoride and beryllium-fluoride salts. This “core” salt would be surrounded by a “blanket” of LiF-BeF2 salt, this time containing thorium tetrafluoride instead of uranium. Fission of U-233 would provide the neutrons necessary to sustain the conversion of thorium into U-233 and then into energy through fission. U233 formed in the blanket would be easily removed by fluorination to UF6 and then introduced into the core salt by reduction back to UF4. The heat of fission would be carried by the salt to an external heat exchanger to a coolant salt. The coolant salt in turn would transfer heat to a gaseous working fluid (probably helium, CO2, or nitrogen) which would drive a closed-cycle gas turbine system. The high temperatures that could be reached by the fluoride salt would enable conversion efficiencies in the gas turbine of roughly 50%, which is a marked improvement on the current efficiency levels of 35% reached by light water reactors. Furthermore the heat rejection properties of the gas turbine would enable desalination and air-cooling possibilities for the waste heat that are currently not practical for steam turbines and their condensers used in existing reactors. To start the LFTR most efficiently, a modest amount of uranium-233 is needed. If we could produce this valuable substance even as we destroy today’s nuclear waste, we would open the door to a transformative energy future!
The University of Tennessee-Knoxville and Oak Ridge National Laboratory