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).