Here's an overly simplified explanation. Suppose you burn a big log in the fireplace. After the log produced light and heat from the burning, you decide to reprocess the burnt log. You take a tool (e.g. a chisel) and remove the charred outside. Voila! You can now burn the log again. That's basically reprocessing in a nutshell.
Nuclear Fuel Cycle
A key to understanding the uranium industry is to first review the nuclear fuel cycle. The nuclear fuel cycle essentially involves the conversion of uranium ore to electricity by processing uranium through various forms and increasing its concentration.
Uranium found in nature consists largely of two isotopes, U235 and U238. The production of energy in nuclear reactors is from the fission or splitting of the U235 atoms, a process which releases energy in the form of heat. Natural uranium contains 0.7% of the U235 isotope. The remaining 99.3% is primarily the U238 isotope that doesn't contribute directly to the fission process. U235 and U238 are chemically identical but differ in their mass. U238 has three additional neutrons. This difference in mass is significant because it allows the U235 and U238 isotopes to be separated and makes it possible to increase or enrich the percentage of U235.
The major stages in the production of nuclear fuel are uranium exploration, mining and milling, refining and conversion, enrichment and fuel fabrication. The diagram below depicts the nuclear fuel cycle.
According to the WNA, the proportion of the cost of nuclear fuel breaks down by stage of the nuclear fuel cycle as follows: (i) mining - 46%, (ii) conversion - 5%, (iii) enrichment - 36%, (iv) fuel fabrication - 13%. While uranium accounts for approximately 46% of the total cost of the fuel for nuclear generators, it accounts for approximately only 6.5% of the total cost of electricity charged to electricity consumers.
Mining
Before uranium can be turned into a useable fuel source, uranium ore must be mined in one of a variety of ways depending on the characteristics of the deposit. Uranium deposits close to the surface can be recovered using an open pit mining method. Higher-grade, deeper deposits can be mined using conventional underground mining methods. If ground conditions are appropriate, the ore can be mined via in situ leaching, whereby oxidizing agents dissolve the uranium contained within the ore body, and the resulting solution is pumped to the surface for uranium recovery. Historically, the price of uranium has been too low to justify its recovery from mineral processing wastes known as tailings. However, with the increased price of uranium in recent years, it has become economically feasible to process the contents of surface tailings dumps to recover any contained uranium. These dams can be mined with high-pressure water cannons, creating a slurry which is pumped to the processing plant for uranium recovery.
Once the uranium ore or solution has been extracted via one of the above mining methods, it is transferred to a mill for primary refining. Mined ore is ground up and leaching is used to extract the uranium. The uranium is then removed from the leach solution and precipitated, producing concentrates containing 80-90% uranium oxide (U3O8). Uranium oxide (which is also known as yellowcake) is the most commonly priced and sold form of uranium. One tonne of uranium contains 2,600 lbs of U3O8 .
Conversion
U3O8 is typically shipped from the mine site in drums to a conversion facility for refining into uranium trioxide (UO3). The UO3 can then be processed for use in either light water nuclear reactors (LWRs) or in heavy water nuclear reactors (HWRs). In both cases, the uranium must be converted but no enrichment is necessary for the HWRs. Since most of the world's nuclear reactors are currently LWRs and approximately 94% of mined uranium is used in LWRs, the remaining discussion will focus on the fuel cycle for LWRs. The UO3 is further purified and converted into a gaseous uranium hexafluoride commonly referred to as UF6 or ‘‘hex''. Conversion plants are operating commercially in the United States, Canada, France, the United Kingdom and Russia.
Enrichment
The UF6 is then fed into an enrichment facility which increases the proportion of U235 from 0.7% to approximately 3.5 to 5.0%, depending on the specifications of the nuclear reactor for which the uranium is destined. In the enrichment process approximately 85% of the natural uranium feed is rejected as ‘‘depleted uranium'' or ‘‘tails'' (consisting primarily of U238).
As depicted in the table below (based on 2003 OECD and WNA estimates), large commercial enrichment plants are in operation in France, Germany, Netherlands, the United Kingdom, the United States and Russia, with smaller plants elsewhere. The enrichment market is an oligopoly, with four principal companies - Techsnabexport/Rosatom (38%), USEC Inc. (22%), Eurodif/Areva SA (21%) and Urenco Group (14%) - controlling approximately 95% of the global uranium enrichment capacity.
Location of Enrichment Facility Enrichment Process
Capacity
(1000kg SWU/annum)
Russia Centrifuge 20,000
France Diffusion 10,800
United States Diffusion 8,000
Germany-Netherlands-UK Centrifuge 5,850
China Mostly Centrifuge 1,300
Japan Centrifuge 900
The capacity of enrichment plants is measured in terms of ‘‘separate work units'' or SWUs. The SWU is a complex unit which is a function of the amount of uranium processed and the degree to which it is enriched and the level of depletion of the remainder. Enrichment accounts for approximately 36% of the cost of nuclear fuel and approximately 5% of the total cost of the electricity generated by a nuclear reactor. Enrichment services are sold in SWUs. Where the price of uranium is relatively low, a customer (such as a utility company) may request that the enrichment facility use more uranium and less SWUs in order to enrich the UF6. Conversely, as the price of uranium rises, SWUs become relatively cheaper and the customer may specify that more SWUs be used and less uranium.
Two main enrichment processes are used on a commercial scale, the gaseous diffusion process and the centrifuge process. At present, the gaseous diffusion process accounts for about 40% of the global uranium enrichment capacity. The diffusion process involves forcing UF6 under pressure through a series of porous membranes or diaphragms. As U235 molecules are lighter than the U238 molecules, they move faster and have a slightly better chance of passing through the pores in the membrane. The UF6 that diffuses through the membrane is thus slightly enriched, while the gas which did not pass through is depleted in U235. This process is repeated many times in a series of diffusion stages called a cascade. The gas must be processed through approximately 1,400 stages in order to obtain a product with a concentration of 3-4% U235.
The centrifuge process is economic at a smaller scale as compared to the diffusion process. It involves the feeding of UF6 gas into a series of vacuum tubes each containing a rotor one to two metres in length and 15-20 cm in diameter. When the rotors are spun rapidly, at 50,0000 to 70,000 rpm, the heavier molecules with U238 increase in concentration towards the cylinder's outer edge. There is a corresponding increase in the concentration of U235 molecules near the centre. These concentration changes are enhanced by inducing gas to circulate axially within the cylinder. The enriched gas forms part of the feed for the next stages while the depleted UF6 gas goes back to the previous stage. Eventually enriched and depleted uranium are drawn from the cascade at the desired assays.
Although the capacity of a single centrifuge is much smaller than that of a single diffusion stage, its capability to separate isotopes is much greater. Centrifuge stages normally consist of a large number of centrifuges in parallel. Such stages are then arranged in cascade similarly to those for diffusion. In the centrifuge process however, the number of stages may be only 10 to 20 instead of a thousand or more for diffusion.
The trend in the enrichment industry is to retire obsolete diffusion plants. As set out in the September 2006 Nuclear Issues Briefing Paper 33 prepared by the Uranium Information Centre, it is estimated that centrifuge enrichment plants will account for approximately 65% of uranium enrichment in 2007 and 96% by 2017.
After Enrichment
The enriched uranium is finally converted by a fabricator and made into fuel pellets (ultimately a sintered ceramic), which are encased in metal tubes to form fuel rods, typically up to four metres in length. A number of fuel rods compose a fuel assembly that is loaded into the nuclear reactor. The complete cycle from exploration for uranium to production of electricity is referred to as the front-end of the nuclear fuel cycle.
Utilities may also purchase uranium through spot and near-term purchases from traders as well as producers. Spot market buying usually calls for delivery within one year rather than multiple year delivery dates. In this regard, traders generally purchase uranium through organizations, such as utilities, that hold excess inventory.
It is important to understand the way in which utilities with nuclear power plants buy their fuel. Instead of buying fuel bundles from the fabricator, the usual approach is for utilities to enter into contracts with various suppliers at each stage of the uranium processing stages. Utilities may purchase a combination of U3O8, UF6, enriched uranium and fabricated fuel pellets. Sellers consist of suppliers at each of the four stages of uranium processing as well as brokers and traders. Depending on the stage at which the uranium product is purchased, the purchasing utility will be responsible for any remaining processing of the uranium required in order to generate the appropriate fuel for its nuclear plant. Although uranium prices have increased considerably during the last few years, many uranium producers are still parties to legacy contracts with purchasers at lower historical prices.
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我是100%的中国人。
I can read that. I am also 100% Han.
