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Monday, 18 July 2011

How Nuclear Power Work


Mining

Uranium deposits are found in rocks around the world. The two largest producers of uranium are Canada and Australia. Uranium is recovered either by mining hard rock or by in situ leaching (ISL), where dissolved from the orebody in situ, the solution then being pumped back to the surface. If uranium is mined the rock is crushed and then leached to dissolve out the uranium, which is then precipitated out of solution as the uranium oxide U3O8, sometimes known as 'yellowcake'

Conversion, Enrichment and Fuel Fabrication

Most reactors use fuel enriched in the U-235 isotope. The solid uranium oxide from the mine is converted into the gas UF6, which is then enriched in the U-235 isotope by one of two physical methods of enrichment. Diffusion enrichment, works by exploiting the different speeds at which U-235 and U-238 pass through a membrane. Centrifuge enrichment, works by passing the gas through spinning cylinders, the centrifugal force moving the heavier U-238 to the outside of the cylinder, leaving a higher concentration of U-235 on the inside.


Uranium dioxide pellets are produced from the enriched UF6 gas. The pellets are then encased in long metal tubes, usually made of zirconium alloy (zircalloy) or stainless steel, to form fuel rods. The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor.

Electricity Generation

Nuclear reactors produce electricity by heating water to make steam. The steam is then used to drive turbines that generate electricity. In this sense  nuclear power  plants are similar to other thermal power stations, where the heat from burning coal or gas is used to produce stream. A key difference of nuclear reactors is that they don't emit carbon dioxide. A nuclear chain reaction is so-called because when a U-235 atom splits (or fissions) in the reactor's core the neutrons released cause other uranium atoms to also undergo fission. A moderator slows down the neutrons to achieve this. The nuclear reactor uses control rods to ensure that this chain reaction occurs at a controlled rate.

Used Fuel Management

Used fuel from a nuclear reactor is first stored to allow most of the radiaoactivity to decay. Then it can either be reprocessed to recover the reusable portion, or it may be disposed of directly as waste. In reprocessing, the used f fuel is dissolved and the uranium and plutonium in the used fuel are separated from the waste fission products. Plutonium can then be combined uranium to make Mixed Oxide Fuel (MOX), which can be used in many modern reactors. Reprocessed uranium can be used in new uranium oxide fuel.
No underground facilities have yet been built for used fuel, although several are planned.

More on used fuel management 

Waste Management

Nuclear wastes range from low level wastes, which contain barely radioactive materials through to high level wastes such as used nuclear fuel that is not destined for reprocessing. Low level waste facilities are already in operation, taking wastes not only from the power generation sector, but also those from nuclear medicine. High level waste is currently kept in storage facilities and will finally be put into specially engineered underground repositories.

Decommissioning

Nuclear facilities are decommissioned after the end of their operating lives. All nuclear materials, machinery and plant are removed and it is ensured that the site is returned to a state where it can be used for new purposes.

Transport

Nuclear materials, such as feedstock in the fuel manufacture process, new fuel, used fuel and wastes are transported between nuclear facilities by road, rail and sea in specially designed containers. Care is taken to ensure that this is safe even in accident situations.

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