The Nuclear Fuel Cycle
The long series of industrial steps from mining to power generation and disposal of waste is referred to as the nuclear fuel cycle. Following mining, uranium undergoes a series of steps – the front end of the nuclear fuel cycle – that prepare it for use in creating energy that we can use. The energy stored in the nucleus of an atom is used to create heat; that heat is used to create high-pressured steam; that steam is used to move blades on a turbine; that turbine rotates to generate electricity used by society. A given quantity of uranium will spend about three years in a reactor, after which it enters the back end of the nuclear fuel cycle which means the storage and disposal of nuclear waste.
There are two types of uranium mining: conventional mining and In-Situ Recovery, or ISR. Conventional mining involves the removal of ore (mineralized rock) from the ground followed by extraction of uranium from the rock at a mill. ISR methods do not require the removal of rock and sediment/soil from the ground, and thus its impact on the environment is far less. Oxygenated water is pumped into the ore zone where it mobilized uranium and is then pumped back to the surface for uranium extraction and refining.
Uranium is removed from the ore material typically by leaching in a conventional mining operation. With ISR, the leaching is done in the subsurface. In either ISR or conventional mining, uranium is then removed from the fluid by ion exchange, followed by a series of tightly controlled chemical treatments leading to the precipitation of yellowcake (uranium oxide, U3O8). Once the uranium is removed from the fluid by ion exchange, the water is then oxidized and re-injected into the ore deposit for ongoing ISR.
Conversion facilities produce a solid form of the gas uranium hexafluoride (UF6) from yellowcake by chemical processes. The UF6 gas is then pressurized and cooled, eventually forming a solid after several days. The solid UF6 is then shipped to an enrichment facility.
Naturally occurring uranium is made up of different isotopes, or atoms of the same element with differing numbers of protons and neutrons in it nucleus. Naturally occurring uranium is about 99.22% 238U, (the most stable form of uranium) and about 0.78% 235U, a less stable isotope which undergoes spontaneous fission, the spontaneous splitting of uranium atoms that provides the energy for nuclear reactors. There are two main types of uranium enrichment processes in use today, gaseous diffusion and gas centrifugation.
Once concentrated to the appropriate level of 235U, the UF6 is converted back into reduced uranium oxide UO2 or to a mixed oxide (MOX), depending on the type of reactor in which it will be used. The fuel is usually formed into ceramic pellets and encased in special metal alloy tubes (about 1 cm in diameter), bundles of which comprise a fuel rod assembly. The assembly is placed into the reactor to heat water. Depending on the particular reactor design, there may be up to 264 rods in a fuel assembly and up to 193 assemblies in in the reactor core.
There are 104 operational nuclear power generating stations in the U.S. (Figure 17). Of these, 69 are Pressurized Water Reactors (PWRs) and 35 are Boiling Water Reactors (BWRs). In both reactor types, water surrounds the core and is heated by a controlled chain reaction. The steam is then cooled, condensed back into water, and cycled back through the system.
Spent Fuel & Waste
Some spent fuel can be reprocessed to remove fissile material and reused in nuclear power generation, although this is not done in the United States. Several countries in Europe and Asia reprocess used fuel, as it reduces the amount of material that requires long-term disposal. Reprocessing contributes up to 25 percent more energy from the original uranium. The material that is not reprocessed, or that remains after reprocessing, is then ready for storage and disposal. Radioactive wastes are divided into three classes: low-, intermediate- and high-level waste on the basis of their level of radioactivity.
Low-level waste (LLW) includes materials such as paper, rags, tools, clothing, etc., that contain small amounts of short-lived radioactivity. All stages of the nuclear fuel cycle produce LLW. This type of waste accounts for about 90 percent of the total volume of all radioactive waste and only about 1 percent of the radioactivity in all waste. Most of this type of waste can be compacted or incinerated prior to being discarded safely in specially certified shallow landfills.
Intermediate-level waste (ILW) contains higher levels of radioactivity and requires shielding when handled. The half-life of most of this material is relatively short, so it can be discarded along with the LLW. By volume, ILW makes up about 7 percent of all radioactive waste and about 4 percent of radioactivity. Some ILW may ultimately end up in deep disposal sites.
High-level waste (HLW) is essentially spent reactor fuel and its fission products, which is both highly radioactive and remains very hot, and thus requires cooling and shielding. HLW accounts for the remaining 95 percent of radioactivity and 3 percent of the volume of all radioactive waste. In a process called vitrification, the hazardous material is mixed with a type of glass that incorporates the radioactive material into its structure. HLW requires a significant amount of time to cool and thus is stored on-site at the reactor for about fifty years. After about forty years, the material has only approximately 0.01 percent (one one-thousandth) of its original level of radioactivity and heat.
The final step is the actual permanent disposal of the hazardous material. There are currently no disposal facilities for HLW in the world, although some countries are working toward that objective. The total amount of spent nuclear fuel from U.S. reactors is about 62,500 metric tons. In terms of volume, that material would cover a football field to a depth of about 21 feet (Nuclear Energy Institute, May, 2010 data).