Nuclear Fuel Cycle: Enrichment, Fabrication, and Waste Storage

Introduction to the Nuclear Fuel Cycle

The nuclear fuel cycle is a set of different processes that utilize nuclear materials and then return them to their initial state in a cyclical manner. It begins with the mining of naturally occurring nuclear materials from the environment and ends with the safe and proper disposal of nuclear waste products back into the environment. Production of energy from uranium requires several unique processes. One term used in this context is front end, referring to the entire set of processes involved in making nuclear energy from uranium ore. These processes are: (1) mining, (2) crushing, (3) processing, (4) enrichment, and (5) fuel fabrication. After being used to produce energy, the nuclear material is known as spent fuel. The spent fuel must be converted at a reprocessing or storage facility if the operator wishes to recycle it. Short- or long-term storage of spent fuel, or its reprocessing, are collectively known as the back end processes of the nuclear fuel cycle (IAEA, 2012).

Uranium Ore Sources and Geology

The cycle starts with the mining or extraction of uranium (U) mineral. This mineral ore contains uranium in the form of various complex oxides, which are then reduced to U₃O₈ oxide. This oxide is subsequently taken through a conversion process to form uranium hexafluoride (UF₆), the form of uranium required for enrichment at isotope separation plants (Lamarsh and Baratta, 2001). Uranium deposits are found in a variety of geological forms. The most important are: the Proterozoic deposits found mainly in Australia and Canada; the roll-front types in Mesozoic–Cenozoic deposits of the USA and Kazakhstan; and the Iron Oxide–Copper–Gold (IOCG) deposits, whereby uranium is extracted as a by-product of copper mining in Australia.

The mineralization process that causes uranium to appear in different forms is determined by two of its properties: its high solubility in the hexavalent (U⁺⁶) state and its low solubility in the tetravalent (U⁺⁴) state. The geochemical behavior of uranium is reflected in its widespread leaching by oxidized meteoric and formation waters, and its precipitation in the reduced form UO₂ during redox reactions. The loss of water from soil through evaporation and plant transpiration in arid regions can result in the enrichment of uranium near the surface in calcrete deposits, as seen in Australia and Namibia. Proterozoic quartz-pebble conglomerates are also known to contain low-grade uraninite, as found in Canadian and South African deposits. The confirmed uranium deposits globally are sufficient to sustain present and predicted nuclear energy production demand for the next one hundred years. More advanced technologies, such as fast neutron reactors, are expected to extend nuclear fuel lifetime to over a thousand years (Lehmann, 2008).

Mining Environmental and Safety Considerations

The most significant effect of any mining activity is its impact on the quality and flow of water in the surrounding area. The main considerations are: (1) whether both surface and groundwater resources will remain suitable for human consumption; and (2) whether the flow of surface waters will remain sufficient to support indigenous aquatic and terrestrial life after the mining activity. Emissions into the atmosphere occur throughout every phase of the mining cycle, including exploration, extraction, processing, and construction. Mining activities also generate large amounts of waste materials containing fine particles that can be dispersed by wind, creating health hazards (ELAW, 2015).

Employees working in uranium mines are protected by regulations that limit their exposure to dust, noise, chemicals, and radiation under the Mine Safety and Health Administration (MSHA) and the overall Occupational Safety and Health Administration (OSHA). Protection of uranium mine workers is specifically regulated by the Nuclear Regulatory Commission and is also subject to state legislation. States have full jurisdiction over mining activities on land that is not federally owned, meaning that such activities may not be subject to federal agency oversight. Additionally, different mining companies maintain their own policies and procedures for limiting worker exposure to various hazards. The legislation and policies that govern mining and processing of uranium are ultimately determined by both the type and location of the mining activity. Consequently, a given mining facility may be governed by a combination of federal, state, and/or local employee protection and environmental regulations (Carlsen et al., 2013).

Enrichment Processes and Technologies

The milling or crushing of uranium oxides is not sufficient to render the material a compatible nuclear fuel for use in a power plant; additional conversions are required. Of any amount of naturally occurring uranium, only a small fraction—0.7%—is capable of undergoing fission, the process by which power is produced in a nuclear reactor. The only isotope capable of undergoing fission is uranium-235 (U-235); the other common isotope is uranium-238 (U-238). Many reactor types require an increase in the concentration of U-235, typically to approximately 3.5–5%. This process is known as isotope separation, whereby one isotope is concentrated (enriched) relative to others. For enrichment to occur, the uranium must be in gaseous form, which necessitates conversion of uranium oxide into uranium hexafluoride (UF₆), which is gaseous at comparatively low temperatures.

For reactors that do not use enriched uranium, the uranium oxide need only be processed to uranium dioxide. For enrichment, however, the uranium dioxide must be further converted to UF₆. The primary risk at this phase of the nuclear fuel cycle involves the use of hydrogen fluoride. Once produced, uranium hexafluoride is pumped into large metal cylinders—each capable of holding 14 tonnes—where it is left to solidify before being transported to enrichment facilities. The separation process splits the UF₆ gas into two streams: one that is progressively enriched to the required concentration (low-enriched uranium) and one that is increasingly depleted of U-235 (the "tails"). The final phase of enrichment is laser enrichment, which yields enriched uranium hexafluoride that is then converted back to enriched uranium oxide (WNA, 2014).

Various enrichment methods have been demonstrated historically; however, only two have been deployed on a large scale—the centrifuge process and the gaseous diffusion process. Both use UF₆ as feed and exploit the approximately 1% mass difference between U-235 and U-238 molecules. The split streams produce low-enriched uranium and depleted uranium (DU). The amount of DU is significant because it indicates the total work required to produce a given quantity of enriched uranium from a given feedstock. Raw uranium ore typically contains about 0.7% U-235, recycled uranium about 1%, and depleted uranium approximately 0.25%. The capacity of an enrichment facility is measured in Separative Work Units (SWU)—a composite unit that relates the energy expended to the quantity of feedstock processed, measured in kilograms (or the larger unit, tonnes SWU) (World Nuclear Association, 2015).

The four main enrichment processes are diffusion, centrifuge, electromagnetic, and laser. Each has distinct characteristics. The diffusion process is adaptable to fluctuations in demand and energy cost but is highly energy intensive. The centrifuge process allows easy capacity expansion through modular addition but is less adaptable. The laser process makes the most efficient use of depleted uranium and also supports modular expansion.

The centrifuge enrichment process was first used in the 1940s. It produces approximately 2 million SWU per year and is highly energy efficient compared to the diffusion process, requiring only 50–60 kilowatt-hours per SWU. Like the diffusion process, it uses UF₆ gas as feedstock and relies on the 1% mass difference between the two uranium isotopes. UF₆ is fed into vacuum cylinders, each containing a rotor 3–5 m tall and 0.2 m in diameter. The rotors spin at approximately 50,000–70,000 rpm, causing the heavier U-238 to concentrate at the outer wall and the lighter U-235 to concentrate toward the center. A thermal gradient induces a counter-current flow that allows enriched uranium to be extracted axially—lighter isotopes from one end and heavier ones from the other. The enriched fraction becomes feedstock for the next stage, while the tail UF₆ is returned to the previous stage. The outer wall of the gas mixture moves at rates exceeding 400 m/s, generating centrifugal acceleration of nearly one million times gravity. Although a single centrifuge has smaller capacity than a single diffusion stage, its enrichment factor per stage is higher. The centrifuge cascade typically comprises 10–20 stages, compared with up to 1,000 or more stages in a diffusion cascade (World Nuclear Association, 2015).

The laser enrichment process is a technologically advanced method that delivers a higher yield of enriched uranium with less depleted uranium and lower capital and feedstock costs, providing significant economic advantages. There are two forms: molecular and atomic. The atomic process is based on photo-ionization, in which a high-energy laser at a specific frequency ionizes U-235 atoms selectively, leaving U-238 atoms unaffected. The positively charged U-235 ions are then attracted to a negatively charged plate within the chamber and collected. This process can also be applied to the enrichment of plutonium isotopes. The molecular process operates on the principle of photo-dissociation, in which laser radiation breaks the molecular bonds of gaseous UF₆ containing U-235, converting it to solid UF₅ ions that can be separated from the remaining UF₆ gas containing U-238. Because this molecular variant uses UF₆ as its starting material, it integrates more readily into the conventional fuel cycle than the atomic variant (World Nuclear Association, 2015).

Commercial-scale uranium enrichment was first achieved via the gaseous diffusion process, initially in the USA and later in France, Russia, the United Kingdom, and China. It is highly energy intensive, consuming approximately 2,500 kilowatt-hours per SWU, and accounts for roughly one quarter of total world enrichment capacity. The process works by pumping UF₆ gas under high pressure through a sequence of porous membranes. Because U-235 molecules are approximately 1% lighter than U-238 molecules, they travel faster and pass through the membrane pores at a higher rate. The emerging gas is therefore more enriched in U-235 than the gas that did not pass through (World Nuclear Association, 2015).

This separation step is repeated through a cascade of approximately 1,400 diffusion stages, each consisting of a diffuser, a compressor, and a heat exchanger to remove compression heat. The enriched UF₆ is drawn off at one end of the cascade and depleted uranium at the other, ultimately producing uranium enriched to 3–4% U-235. Although the enrichment factor per stage is lower than in the centrifuge process, diffusion facilities are capable of handling very large gas volumes, making them reliable and long-lasting (World Nuclear Association, 2015).

The electromagnetic process was among the earliest attempts at isotope enrichment. The original method, Electromagnetic Isotope Separation (EMIS), used calutrons—mass spectrometers specifically designed for the separation of uranium isotopes. EMIS was employed in the Manhattan Project to produce the highly enriched uranium used in the Hiroshima atomic bomb. The process applies mass spectrometer principles at a commercial scale: ions of U-235 and U-238 are separated according to the different arcs they trace when passing through a magnetic field, a result of their differing atomic masses. The electromagnetic process is generally about ten times more energy intensive than the diffusion process (World Nuclear Association, 2015).