URANIUM AS NUCLEAR FUEL SOURCE

The fuel most widely used by nuclear plants for nuclear fission. In nuclear fission atoms are split apart to form smaller atoms, releasing energy. Nuclear power plants use the heat from nuclear fission to produce electricity.

Uranium Is Abundantly Found in Nature but Must be Processed into Fuel.

Uranium is nonrenewable, though it is a common metal found in rocks all over the world. Uranium occurs in nature in combination with small amounts of other elements.

Nuclear plants use a certain kind of uranium, U-235, as fuel because its atoms are easily split apart. Though uranium is quite common, about 100 times more common than silver, U-235 is relatively rare.

Economically recoverable uranium deposits have been discovered principally in the western United States, Australia, Canada, Africa, and South America. Once uranium is mined, the U-235 must be extracted and processed before it can be used as a fuel. Mined uranium ore typically yields one to four pounds of uranium concentrate (U3O8 or "yellowcake") per ton, or 0.05% to 0.20% U3O8.

Typical Conventional Uranium Mill

Most of Our Uranium Is Imported

Owners and operators of U.S. civilian nuclear power reactors purchased the equivalent of 53 million pounds of uranium during 2008. Uranium delivered to U.S. reactors in 2008 came from six continents:

• 14% of delivered uranium came from the United States
• 86% of delivered uranium was of foreign-origin:
  • 42% was from Australia and Canada
  • 33% originated in Kazakhstan, Russia and Uzbekistan
  • 11% came from Brazil, Czech Republic, Namibia, Niger, South Africa, and the United Kingdom

Enrichment

Because less than one percent of uranium ore contains uranium-235, the form used for energy production, uranium must be processed to increase the concentration of uranium-235. This process—called enrichment—increases the percentage of uranium-235 from one to five percent.

It typically takes place at a gaseous diffusion plant where the uranium hexafluoride is pumped through filters that contain very tiny holes. Because uranium-235 has three fewer neutrons and is one percent lighter than uranium-238, it moves through the holes more easily than uranium-238. This method increases the percentage of uranium-235 as the gas passes through thousands of filters.

Fuel Fabrication

The enriched uranium is taken to a fuel fabrication plant where it is prepared for the nuclear reactor. Here, the uranium is made into a solid ceramic material and formed into small barrel-shaped pellets. These ceramic fuel pellets can withstand very high temperatures, just like the ceramic tiles on the space shuttle. Fuel pellets are about the size of your fingertip, yet each one can produce as much energy as 150 gallons of oil. The pellets are sealed in 12-foot metal tubes called fuel rods. Finally, the fuel rods are bundled into groups called fuel assemblies.






Uranium Fuel Cycle:




Nuclear Reactor


The uranium fuel is now ready for use in a nuclear reactor. Fissioning takes place in the reactor core. Surrounding the core of the reactor is a shell called the reactor pressure vessel. To prevent heat or radiation leaks, the reactor core and the vessel are housed in an airtight containment building made of steel and concrete several feet thick. The reactor core houses about 200 fuel assemblies. Spaced between the fuel assemblies are movable control rods. Control rods absorb neutrons and slow down the nuclear reaction. Water also flows through the fuel assemblies and control rods to remove some of the heat from the chain reaction. The nuclear reaction generates heat energy just as burning coal or oil generates heat energy. Likewise, the heat is used to boil water into steam that turns a turbine generator to produce electricity. Afterward, the steam is condensed back into water and cooled. Some plants use a local body of water for cooling; others use a structure at the power plant called cooling towers.

Used Fuel Storage

Like most industries, nuclear power plants produce waste. One of the main concerns about nuclear power plants is not the amount of waste created, which is quite small compared to other industries, but the radioactivity of some of that waste. The fission process creates radioactive waste products. After about three cycles, these waste products build up in the fuel rods, making the chain reaction more difficult. Utility companies generally replace one-third of the fuel rods every 12 to 18 months to keep power plants in continuous operation. The fuel that is taken out of the reactor is called used fuel. The used fuel contains both radioactive waste products and unused fuel. The used fuel is usually stored near the reactor in a deep pool of water called the used fuel pool. The used fuel cools and loses most of its radioactivity through radioactive decay. In three months, the used fuel will lose 50 percent of its radiation; in one year, 80 percent; in 10 years, 90 percent. The used fuel pool was intended as a temporary method for storing used nuclear fuel.

However, there is no permanent storage solution yet for used nuclear fuel, and fuel pools space is running out. The nuclear industry has designed dry cask storage as another temporary solution. Now, the used fuel stays in the pool for five to seven years. Then, it is moved elsewhere on the nuclear power plant site to be stored in vaults or dry casks. Each of these methods for managing used nuclear fuel puts the fuel into airtight, steel and concrete structures. The U.S. Nuclear Regulatory Commission has stated that it is safe to store used fuel on site for at least 120 years. Eventually, the used fuel will be reprocessed and/or transported to a permanent federal disposal site.

Image of an used fuel storage pool




Reprocessing
Used fuel contains both radioactive waste products and unused nuclear fuel. In fact, about one-third of the nuclear fuel remains unused when the fuel rod must be replaced. Reprocessing separates the unused nuclear fuel from the waste products so that it can be used in a reactor again.Currently, American nuclear power plants store the used fuel in used fuel pools—without reprocessing. Reprocessing is more expensive than making new fuel from uranium ore. If uranium prices rise significantly or storage becomes a bigger problem, reprocessing may gain favor.