IAEA Introduction to the Nuclear Fuel Cycle

Published Date: 31 May 2010

<p>The raw material for today’s nuclear fuel is uranium. It must be processed through a series of steps to produce an efficient fuel for generating electricity. Used fuel also needs to be taken care of for reuse and disposal.</p><p>The nuclear fuel cycle includes the ‘front end’, i.e. preparation of the fuel, the ‘service period’ in which fuel is used during reactor operation to generate electricity, and the ‘back end’, i.e. the safe management of spent nuclear fuel including reprocessing and reuse and disposal.</p><p>If spent fuel is not reprocessed, the fuel cycle is referred to as an ‘open’ or a ‘once-through’ fuel cycle; if spent fuel is reprocessed, and partly reused, it is referred to as a ‘closed’ fuel cycle.</p> © A. Diesner-Kuepfer / IAEA <p>URANIUM MINING</p><p>Uranium is about 500 times more abundant than gold and about as common as tin. It is present in most rocks and soils as well as in many rivers and in sea water.</p> <p>The largest known resources of uranium ore are in Australia, Canada and Kazakhstan. Like other minerals uranium is mined in open pit or underground mines. In some cases the uranium is leached directly from the ore without mining. The concentration of uranium in the ore could be from 0.03 up to 20%.</p> © PhotoDisk <p>URANIUM MILLING</p><p>Milling is generally carried out close to a uranium mine. Uranium ore is crushed and chemically treated to separate the uranium.</p><p>The result is ‘yellow cake’, a yellow powder of uranium oxide. In the yellow cake the uranium concentration has been raised to more than 80%. The uranium oxide concentrate is shipped from the mill to a conversion facility.</p> © WNA <p>CONVERSION</p><p>Natural uranium consists primarily of two isotopes, 99.3% is U-238 and 0.7% is U-235. The fission process, i.e. the process by which energy in the form of heat is released in a nuclear reactor, mainly takes place in U-235. Most nuclear power plants today therefore require fuel with U-235 enriched to 3-5%.</p><p>To increase the concentration of U-235, yellow cake is first converted to a gaseous form, uranium hexafluoride gas (UF6) at a conversion facility.</p><p>UF6 gas is filled into large cylinders where it solidifies. These strong metal containers are shipped to an enrichment plant.</p> © WNA <p>ENRICHMENT</p><p>The uranium is enriched in U-235 either by pumping UF6 gas through porous membranes that allow U-235 to pass through more easily than heavier isotopes, such as U-238, or by introducing the gas in fast-spinning cylinders (‘centrifuges’), where heavier isotopes are pushed out to the cylinder walls.</p><p>The uranium remaining after enrichment has a lower content of U-235 and is known as depleted uranium.</p> © WNA <p>FUEL FABRICATION</p><p>Pellets are formed from pressed uranium oxide which is sintered (baked) at a high temperature (over 1400°C) to achieve a high density and stability.</p><p>The pellets are cylindrical and typically 8-15 mm in diameter and 10-15 mm long.</p><p>They are packed in long metal tubes to form fuel rods, which are grouped in ‘fuel assemblies’ for introduction into a reactor.</p> © MELOX / Marcoule, France <p>ELECTRICITY GENERATION</p><p>Inside a nuclear reactor, controlled fission occurs. Fission means that the U-235 atoms are split. The splitting releases heat energy that is used to boil water and produce high pressure steam. The steam turns a turbine connected to a generator, which generates electricity.</p><p>Some of the U-238 in the fuel is converted into plutonium (e.g. Pu-239) in the reactor core. Plutonium is also fissile and can be split generating heat. About one third of the energy in a typical nuclear reactor comes from the plutonium.</p><p>The fuel stays in the reactor for 3-6 years and about once a year, 20 to 30% of the fuel is unloaded and replaced by fresh fuel.</p> © Xuxin Zou / IAEA <p>SPENT FUEL STORAGE — I</p><p>When used fuel assemblies are removed from the reactor, the used (spent) fuel is hot and radioactive. The spent fuel is therefore stored under water, which provides both cooling and radiation shielding.</p><p>Later, spent fuel can be stored dry in shielded buildings or casks. Storage can be either at the nuclear power plant or elsewhere. The heat and radioactivity decrease over time. For instance, after 40 years in storage, the fuel‘s radioactivity will be about a thousand times lower than when it was removed from the reactor. Shielding and cooling will be required for several hundred more years.</p> © KKG <p>SPENT FUEL STORAGE — II</p><p>In an open fuel cycle spent fuel is regarded as radioactive waste which must be managed and safely deposited. In a closed fuel cycle, spent fuel may be reprocessed to generate more power. Secondary waste from reprocessing, however, must also be managed and deposited.</p> © GNS <p>REPROCESSING</p><p>The spent fuel contains uranium (96%), plutonium (1%) and waste products (3%). The uranium, with less than 1% fissile U-235 and the plutonium can be reused as fuel.</p><p>Some countries, which have choosen the closed fuel cycle, chemically reprocess spent fuel to separate the useable material, i.e. uranium and plutonium, from the unusable waste. Reprocessing facilities are large industrial facilities.</p><p>Recovered uranium from reprocessing can be returned to the conversion plant for conversion to uranium hexafluoride and subsequent re-enrichment. Plutonium is mixed with uranium to form new fuel (MOX fuel).</p> © COGEMA / Areva <p>MOX FUEL FABRICATION</p><p>The plutonium recovered through reprocessing could be used in the manufacture of mixed oxide (MOX) nuclear fuel. MOX fuel is a mixture of about 5% plutonium oxide in natural uranium oxide.</p><p>A single recycle of Pu in the form of MOX fuel increases the energy derived from the original uranium by some 12% (WNA).</p> © MELOX / France <p>HIGH LEVEL WASTE DISPOSAL</p><p>Spent nuclear fuel or high level waste can be safely disposed of deep underground, in stable rock formations such as granite, eliminating the health risk to people and the environment. The first disposal facilities will be in operation around 2020.</p><p>Waste will be packed in long-lasting containers and buried deep in the geological formations chosen for their favorable stability and geochemistry, including limited water movement. These geological formations have proven stability over hundreds of millions of years, far longer than the waste is dangerous.</p> © SKB / Sweden THE IAEA SUPPORTS INTERESTED MEMBER STATES IN<ul><li>Improving the performance of nuclear power plants and the nuclear fuel cycle;</li><li>Catalyzing innovation in nuclear power and fuel cycle technologies;</li><li>Developing indigenous capabilities around the world for national energy planning;</li><li>Deploying new nuclear power plants;</li><li>Preserving and disseminating nuclear information and knowledge; and</li><li>Advancing science and industry through improved operation of research reactors.</li></ul> © PhotoDisk