Uranium Processing Overview

The nuclear fuel cycle is the series of industrial processes which involve the generation of energy from uranium in nuclear reactors. Uranium must be processed before it can be used as fuel for a reactor. The fuel cycle starts with the mining of uranium and ends with the disposal and reprocessing of nuclear waste. Although enriched uranium is a critical component for both nuclear power generation and nuclear weapons, the kind of uranium and plutonium made for nuclear weapons is different from that of nuclear fuel for nuclear power plants. Bomb-grade uranium is highly enriched, with over 90% of the isotope Uranium-235 instead of up to 5% in commercial reactors. Bomb-grade plutonium is mostly pure Plutonium-239, meaning over 90% compared to about 60% in reactor-grade fuel, and is made in special reactors. Specific infrastructures and processes were developed for the refining and enrichment of uranium and plutonium for the U.S. atomic weapons program commonly known as the Manhattan Project. Even so, it is useful to consider the overall fuel cycle that starts with the mining of uranium and ends with the disposal and reprocessing of nuclear waste, while also considering the specific materials, processes, and compounding problem of waste born from the U.S. nuclear weapons complex.

World Nuclear Association, "Nuclear Fuel Cycle Overview," May 2020 [last updated], https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview.aspx.To prepare the element for use in a nuclear reactor, it must undergo the processes of mining and milling, conversion, enrichment, and fuel fabrication. This is referred to as the “front end” of the nuclear fuel cycle. After the fuel has been used to produce electricity, the fuel may go through reprocessing and recycling before being disposed—the “back end” of the fuel cycle.

Excavation and in situ techniques are used to recover uranium ore. Canadian Nuclear Association, "Uranium Processing," accessed August 1, 2020, https://cna.ca/technology/energy/uranium-processing/.After the uranium ore is mined, it is transported to a nearby mill for processing. Milling is the process of extracting uranium from the ore: the ore is crushed with water and treated with acid, and then the uranium is separated, purified, and dried. The result is a uranium oxide concentrate that is sometimes referred to as “yellowcake” (U₃O₈ ). The remainder, which contains most of the radioactivity, becomes tailings. These need to be stored in a secure location because their long-lived radioactive materials can affect the environment and nearby populations.

The material produced from the mill is not directly usable as fuel and must be refined at a conversion facility. Most is then transported to an enrichment plant (only a small number of reactors do not require uranium to be enriched). World Nuclear Association, "Uranium Enrichment," May 2020 [last updated], https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/conversion-enrichment-and-fabrication/uranium-enrichment.aspx.Uranium found in nature consists largely of two isotopes, U-235 and U-238; U-235 is the main fissile isotope of uranium. Most reactors (light water reactors) require uranium to be enriched from .7% to 3-5% U-235 in their fuel.

World Nuclear Association, "Nuclear Fuel Cycle Overview," May 2020 [last updated], https://www.world-nuclear.org/information-library/nuclear-fuel-cycle/introduction/nuclear-fuel-cycle-overview.aspx.Usually reactor fuel is in the form of ceramic pellets. Fuel manufacturing begins by pressing either the natural or enriched powder into small cylindrical shapes that are then baked at a high temperature. The pellets are then encased in metal tubes, creating fuel rods. The surface areas are then ground and finished to prepare them for insertion into fuel assemblies.

Historically the research and development undertaking of nuclear weapons known as the Manhattan Project used enriched uranium in high concentration (i.e., “weapons-grade” uranium) to create the gun-type fission weapon called Little Boy deployed in World War II. Fissile U-235 makes up less than 1% of naturally-occurring uranium, and is chemically identical to the more common U-238, and with nearly the same mass. Their separation, therefore, proved difficult: chemical techniques normally employed to purify substances could not be used to separate them. Methods had to be developed to enrich uranium in ways that could exploit tiny differences in mass between the heavier, more abundant U-238 isotope and the lighter fissile U-235.

The Manhattan Project at first relied on electromagnetic isotope separation, but this method was abandoned after the war owing to its high costs and poor efficiency (but it played an important role in the development of the field of nuclear medicine). Throughout the Cold War, gaseous diffusion served as the dominant uranium enrichment technology, which also required vast amounts of electricity and immense facilities. Clinton Engineer Works at Oak Ridge, Tennessee performed much of this work, Wikipedia, "Enriched Uranium," accessed June 25, 2021, https://en.wikipedia.org/wiki/Enriched_uranium#Codename.with the codename “oralloy”—a shortened version of Oak Ridge alloy—given to weapons-grade highly enriched uranium during the Manhattan Project, after the location of the complex. The gaseous diffusion plant (K-25 / K-27) at Oak Ridge became the prototype for a new generation of plants, and operated along with facilities in Portsmouth, Ohio and Paducah, Kentucky to enrich uranium for military reactors and nuclear weapons. The technology was deemed obsolete in 2011, steadily replaced by later generations of technology, including gas centrifuge enrichment and relatively new laser excitation.

The second atomic bomb used in World War II was a plutonium implosion-type weapon known as Fat Man. Efforts to produce plutonium worked in parallel to uranium enrichment. First discovered by researchers at the University of California-Berkeley, then produced by the world’s first artificial nuclear reactor—the Chicago Pile-1—at the University of Chicago’s Metallurgical Laboratory, plutonium production came online at Hanford site reactors in Washington and the X-10 Graphite Reactor at Oak Ridge, where uranium was irradiated and transmuted into plutonium. Uranium and plutonium were then chemically separated, using a bismuth phosphate process.

The Cold War demand for fissile materials to make nuclear weapons for the arms race necessitated the reprocessing of spent nuclear fuel—the general name of an extensive set of processes, such as the removal of metal pilings from spent nuclear fuel assemblies, the dissolving of uranium fuel in boiling concentrated nitric acid, chemically separating out the uranium and plutonium isotopes, and storing the leftover dissolved fission materials in large storage tanks. On the one hand, nuclear fuel recycling suggests a straightforward process of converting spent plutonium and uranium into a “mixed oxide” that can be reused, for example, to produce electricity in nuclear power plants, and to reduce the volume of high-level radioactive waste. Since the 1990s and disarmament efforts, military uranium has become available for electricity production wherein it is mixed with depleted uranium from the enrichment process before its use in power generation. For over two decades, one-tenth of U.S. electricity was made this way from reprocessed Russian weapons uranium.

On the other hand, national energy security and non-proliferation concerns have led to vehement criticisms of reprocessing efforts and technologies. Moreover, the problem of waste besets not only the “closure of the fuel cycle” pursued by regulatory bodies today but raises questions about the cyclical metaphor/framework for understanding nuclear fuel and energy production. Radioactive and other wastes are produced along every step of the fuel cycle: from the ore spoils of uranium mining to the wastes generated from refining the ore, enriching the uranium and turning it into fuel, transporting radioactive materials, reprocessing used fuel, removing radioactive byproducts, and so forth. Essentially everything in contact with radioactive materials, including the people who handle the materials, the containers in which they are stored and moved, the buildings, repositories, and landscapes in which they are placed, become assimilated into the waste stream, contaminated with radioactivity or potentially affected by radiation.

Sources

Campaign for Nuclear Disarmament. “The Links between Nuclear Power and Nuclear Weapons.” Accessed June 25, 2021.

Canadian Nuclear Association. "Uranium Processing." Accessed January 13, 2023.

NTI Education Tutorials. “Nuclear101, Module 2: Uranium Enrichment.” Accessed June 25, 2021.

U.S. Energy Information Administration. "Nuclear Fuel Cycle." July 12, 2022 [last updated]. Accessed January 13, 2023.

U.S. Nuclear Regulatory Commission. “Uranium Enrichment.” December 2, 2020 [last updated]. Accessed January 13, 2023.

Wikipedia. “Clinton Engineer Works.” Accessed June 25, 2021.

World Nuclear Association. “Conversion and Deconversion.” January 2022 [last updated]. Accessed January 13, 2023.

World Nuclear Association. "Nuclear Fuel Cycle Overview." April 2021 [last updated]. Accessed January 13, 2023.

World Nuclear Association. “Processing of Used Nuclear Fuel.” December 2020 [last updated]. Accessed January 13, 2023.

World Nuclear Association. "Uranium Enrichment." October 2022 [last updated]. Accessed January 13, 2023.