The nuclear Submarine Force counts on reliable operation of our propulsion plants at all times, and we rely on the performance of the fuel elements in our reactors without question.But long before the reactor fueling occurs, there is significant science, engineering, and precision production work that must take place in order to produce reliable, safe fuel.The process of getting the uranium out of the ground, converting it into a form that can be enriched, processing it through the enrichment plant, and fabricating the enriched uranium into reactor fuel is unforgiving, time consuming, and expensive.This paper is a survey level summary of the process of producing reactor fuel, with emphasis on the most difficult phase of the process—enrichment by isotope separation.Note—this paper is applicable to the generic nuclear fuel cycle, and is not intended to represent any specifics that apply to defense purposes.
THE NUCLEAR FUEL CYCLE
The basic nuclear fuel cycle is depicted in Figure 1.A step-by-step discussion illustrates the unique nature of each basic process.The entire fuel cycle is heavily regulated by the Nuclear Regulatory Commission, the Environmental Protection Agency, the Department of Energy, and state and local authorities.
1.Uranium Mines and Mills–Uranium ore is mined in various locations around the world, with most recent mining activity concentrated in areas where the deposits are richest.Natural uranium ore occurs as U3O8and it has, by weight, 0.711 percent U235.The busiest mining currently underway is in Kazakhstan, Australia, and Canada, which account for about 64% of the uranium mined, with other nations mining at low rates.Raw ore is useless without processing.The milling operation takes care of this by grinding the uranium oxide into a powder, the familiar yellow cake, which is then shipped to the conversion facility.
2.U3O8Conversion to UF6–Early researchers had to find a chemical compound of uranium that would facilitate the enrichment process.Uranium Hexafluoride is such a chemical.At room temperature, UF6is a solid, but when heated under vacuum, it sublimates to gaseous state, a form that adapts well to the current enrichment processes.Figure 2illustrates the advantages of uranium hexafluoride.Conversion plants chemically convert the yellow cake uranium oxide into uranium hexafluoride.In the United States, the converter is Honeywell, at their Metropolis, Illinois plant.
3.U235235 from the natural 0.711 percent to up to approximately 4.95 percent for the commercial power industry (Low Enriched Uranium or LEU).The process starts with induction of cylinders of feed material, and ends with cylinders of Enriched Uranium Product (EUP) for the power industry at thecustomer-specified assay, and cylinders of tails, which are the by-product of the process.Tails can be thought of as stripped outuranium hexafluoride, typically with an assay of about 0.20 to 0.35 percent U235. The enrichment industry is migrating from the energy-intensive gaseous diffusion process to the centrifuge process, with potential laser isotope separation on the horizon.Commercial enrichers include USEC (United States), URENCO (Europe), Areva (France), and Tenex (Russia).All these enrichers sell to the commercial utilities worldwide.Additionally, several other nations have launched their own enrichment programs, mostly using de-rivatives of the gas centrifuge.Laser isotope separation is being developed by GE-Silex (GE-Silex uses Australian-developed technology).
4.Conversion to UO2and Fabrication of Fuel Assemblies
-the Fuel Fabricators receive the product cylinders from the enrichers, and convert the uranium hexafluoride into uranium dioxide (UO2). The UF6 gas is chemically processed to form uranium dioxide (UO2) powder, which is then pressed into pellets, sintered into ceramic form, loaded into Zircaloy cladding, and constructed into fuel assemblies. The fuel fabricator in the United States is Nuclear Fuel Services in Er-win, Tennessee.
5.Consumption by Power Plants–the power industry and the naval nuclear propulsion program handle the fuel assemblies during fueling and refueling operations, and when the fuel in the reactor is spent, the assemblies are removed and stored in specialized facilities. Techniques for core load, refueling, and spent fuel removal vary according to type of reactor and application.
6.Spent Fuel Storage–the storage of spent fuel is currently the subject of much discussion.Spent fuel is typically stored in cooling pools.For commercial power plants, the spent fuel pools are adjacent to the reactor for logistics and radio-logical controls purposes.Dry cask storage of spent fuel is an option, and eventual underground storage of the casks is contemplated, either at Yucca Mountain (should it be rein-stated and licensed) or an alternate location.The legacy and handling of spent fuel is beyond the scope of this paper.
ENRICHMENT PROCESS BASICS<.h4>
As discussed, the purpose of the enrichment process is to extract a sufficient quantity of fissionable U235such that the resulting fuel elements can sustain critical operations of the power plant.Commercial nuclear power plants typically require enrichment to approximately 4.95% U235, or Low Enriched Uranium (LEU).Enrichment for defense purposes may require Highly Enriched Uranium (HEU).HEU enrichment in the United States was suspended in the last century, when defense needs were fulfilled. However, the ongoing capability to domestically enrich uranium will be important many decades from now, when the Navy will require a source of enriched uranium to satisfy its enduring need for HEU.
The enrichment process follows a similar set of steps, regard-less of the isotope separation technology or final level of enrichment (assay).These steps are as follows:
- Liquid sampling of the incoming feed cylinders –it is vitally important to ascertain the starting composition and assay of incoming feed, for accountability of nuclear material, quality of product, and to properly assess the work input that will be required for isotope separation to the exact customer-specified assay.
- Feeding the enrichment cascade –heating the cylinders and transferring their contents into the enrichment process via sublimation and mixing.
- Product Withdrawal –the desired assay product is with-drawn from the top of the cascade, cooled, and stored for sampling.
- Sampling and transfer to customer cylinders –the customers require certification of both the assay, and the work amount that goes into the isotope separation for their product.This is accomplished via precision weighing of the product cylinders after the end product has been liquefied and allowed to homogenize in the cylinder.
- Tails storage in tails cylinders –the tails cylinders are cooled and stored for future use as low-assay feed material or nuclear waste.
The key to the enrichment process is the operation of the cascade.In a cascade setup, the uranium hexafluoride undergoes isotopic separation in a series of stages.Each stage produces an enriched product and a depleted product.The enriched product is fed to the next higher stage in the cascade.The depleted productis fed to the next lower stage in the cascade.The desired customer product is withdrawn from the top of the cascade.The tails are withdrawn from the bottom of the cascade.In order to minimize losses from inter-stage mixing, the feed cylinders are fed into a stage that contains an assay that is approximately the same as the feed assay.Figure 3 illustrates a notional cascade.Figure 3 is valid either for gaseous diffusion or centrifuge operations.
ENRICHMENT NUCLEAR SAFETY CULTURE AND TECHNOLOGY
Nuclear Safety Culture. All current enrichment plants operate with the majority of plant systems at an elevated temperature and high vacuum.This ensures that the uranium hexafluoride stays in the gaseous state throughout the process.Therefore, the plant systems are carefully monitored for vacuum leaks, for any potential escape of uranium hexafluoride gas, and for temperatures and pressures at all key stages.The enrichment process embraces a nuclear safety culture which encompasses unique practices:
- Critical attention to cleanliness—uranium hexafluoride is an extremely reactive chemical.It combines readily with almost any foreign material, resulting in degradation of the enrichment process.
- Nuclear material control and accountability—because enrichment facilities use appreciable quantities of special nuclear materials, they are accountable for precise inventory control, and periodic audit and high-level security of their operations.
- Chemical safety and vigilance—the escape of uranium hexafluoride gas in an enrichment plant is typically not a threat to public safety or security. However, since UF6readily reacts with many other chemical compounds, leak prevention and detection is a must.The most critical concern is combination with atmospheric moisture to form Hydrogen Fluoride gas, which can be lethal if inhaled.
- Nuclear criticality safety—all enrichment plants must adhere to rigid standards that govern the proximity and assay of nuclear materials throughout the process, and must monitor the plant for potential criticality accidents when handling uranium hexafluoride.
Gaseous Diffusion Technology utilizes a compressor/converter assembly to produce isotope separation.The compressor pushes the heavy gas molecules through a diffusion membrane in the convertor.The smaller U235F6molecules diffuse through the membrane, and the larger U238F6molecules do not.As described above, the enriched stream is fed to the next higher stage of compressor/converters, and the depleted stream goes to the next lower stage of compressor/converters.See Figure 4for a typical gaseous diffusion arrangement.An interesting sidelight in gaseous diffusion is that the coolant used for the modern U.S. gaseous diffusion plants was R-114, which is widely used in submarine air conditioning plants.
Gas Centrifuge Technology utilizes centripetal acceleration of the uranium hexafluoride gas molecules to separate the heavier U238F6molecules from the lighter U235F6molecules at the wall of the centrifuge rotor.The layering by molecular weight, assisted by flow patterns inside the centrifuge, facilitates isotope separation via a product scoop at the top of the centrifuge machine, and a tails scoop at the bottom of the centrifuge machine.A notional gas centrifuge is illustrated in fig 5.
The nuclear fuel cycle is an important underlying process that supports the nuclear power industry.There is an ongoing need for all steps of the fuel cycle to support the approximately 400 commercial power reactors worldwide.The technological sophistication, engineering discipline, careful attention to detail, accountability, and operation in a highly-regulated environment of fuel cycle facilities is consistent with the overall nuclear safety culture of the nuclear power utilities. The United States needs to maintain proper focus on technology development and facilities necessary to meet our country’s and world’s demands for decades to come.