Nuclear Fuel Cycle

  Nuclear Fuel Cycle


  Fuel Processing


The nuclear fuel cycle is the series of industrial processes which involve the production of electricity from uranium in nuclear power reactors

Uranium is a relatively common element that is found throughout the world. It is mined in a number of countries and must be processed before it can be used as fuel for a nuclear reactor
Fuel removed from a reactor, after it has reached the end of its useful life, can be reprocessed to produce new fuel

To prepare uranium for use in a nuclear reactor, it undergoes the steps. These steps make up the 'front end' of the nuclear fuel cycle. (front end means before energy production from fuel)

  • mining and milling,
  • conversion,
  • enrichment and
  • fuel fabrication

After uranium has spent about three years in a reactor to produce electricity, the used fuel may undergo a further series of steps including temporary storage, reprocessing, and recycling before wastes are disposed. Collectively these steps are known as the 'back end' of the fuel cycle. (back end means after energy production from fuel)  

 

  Nuclear Fuel : Uranium


Uranium is a slightly radioactive metal that occurs throughout the Earth's crust. It 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. It is, for example, found in concentrations of about four parts per million (ppm) in granite, which makes up 60% of the Earth's crust.

In fertilisers, uranium concentration can be as high as 400 ppm (0.04%), and some coal deposits contain uranium at concentrations greater than 100 ppm (0.01%). Most of the radioactivity associated with uranium in nature is in fact due to other minerals derived from it by radioactive decay processes, and which are left behind in mining and milling

There are a number of areas around the world where the concentration of uranium in the ground is sufficiently high that extraction of it for use as nuclear fuel is economically feasible

  Uranium Mining & Uranium Milling


Uranium Mining

Both excavation and in situ techniques are used to recover uranium ore.

Excavation may be underground (120 m deep) and open pit mining 

Open pit mines require large holes on the surface, larger than the size of the ore deposit, since the walls of the pit must be sloped to prevent collapse. Underground mines have relatively small surface disturbance and the quantity of material that must be removed to access the ore is considerably less than in the case of an open pit mine. Special precautions, consisting primarily of increased ventilation, are required in underground mines to protect against airborne radiation exposure

In situ leach (ISL) mining, where oxygenated groundwater is circulated through a very porous ore body to dissolve the uranium oxide and bring it to the surface. ISL may be with slightly acid or with alkaline solutions to keep the uranium in solution. The uranium oxide is then recovered from the solution as in a conventional mill

Uranium Milling

Milling, extracts the uranium from the ore (or ISL leachate). Most mining facilities include a mill, although where mines are close together, one mill may process the ore from several mines. Milling produces a uranium oxide concentrate which is shipped from the mill. It is sometimes referred to as 'yellowcake' and generally contains more than 80% uranium. The original ore may contain as little as 0.1% uranium, or even less.

In a mill, the ore is crushed and ground to a fine slurry which is leached in sulfuric acid (or sometimes a strong alkaline solution) to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U3O8) concentrate. After drying and usually heating it is packed in 200-litre drums

About 200 tonnes is required to keep a large (1000 MW) nuclear power reactor generating electricity for one year

The remainder of the ore, containing most of the radioactivity and nearly all the rock material, becomes tailings, which are emplaced in engineered facilities near the mine (often in a mined out pit). Tailings need to be isolated from the environment because they contain long-lived radioactive materials in low concentrations and maybe also toxic materials such as heavy metals.  However, the total quantity of radioactive elements is less than in the original ore, and their collective radioactivity will be much shorter-lived.

  Conversion and Enrichment


The uranium oxide product of a uranium mill is not directly usable as a fuel for a nuclear reactor and additional processing is required. Only 0.7% of natural uranium is 'fissile', or capable of undergoing fission, the process by which energy is produced in a nuclear reactor. The form, or isotope, of uranium which is fissile is the uranium-235 (U-235) isotope. The remainder is uranium-238 (U-238).

Isotope separation is a physical process to concentrate (‘enrich’) one isotope relative to others. The enrichment process requires the uranium to be in a gaseous form. The uranium oxide concentrate is therefore first converted to uranium hexafluoride, which is a gas at relatively low temperatures

Uranium oxide is first refined to uranium dioxide, is then converted into uranium hexafluoride, ready for the enrichment plant. The main hazard of this stage of the fuel cycle is the use of hydrogen fluoride. The uranium hexafluoride is then drained into 14-tonne cylinders where it solidifies. These strong metal containers are shipped to the enrichment plant

The enrichment process separates gaseous uranium hexafluoride into two streams, one being enriched to the required level and known as low-enriched uranium; the other stream is progressively depleted in U-235 and is called 'tails', or simply depleted uranium

The main enrichment process in commercial plants uses centrifuges, with thousands of rapidly-spinning vertical tubes. As they spin, the physical properties of molecules, specifically the 1% mass difference between the two uranium isotopes, separates them. A laser enrichment process is in the final stage of development.

The product of this stage of the nuclear fuel cycle is enriched uranium hexafluoride, which is reconverted to produce enriched uranium oxide.

  Fuel Fabrication


Reactor fuel is generally in the form of ceramic pellets. These are formed from pressed uranium oxide (UO2) which is sintered (baked) at a high temperature (over 1400°C) . The pellets are then encased in metal tubes to form fuel rods, which are arranged into a fuel assembly ready for introduction into a reactor

27 tonnes of fresh enriched fuel is required each year by a 1000 MWe reactor

  Power Generation and Burn-up


Several hundred fuel assemblies make up the core of a reactor.

Typically, some 44 million kilowatt-hours of electricity are produced from one tonne of natural uranium. The production of this amount of electrical power from fossil fuels would require the burning of over 20,000 tonnes of black coal or 8.5 million cubic metres of gas.

As with as a coal-fired power station about two thirds of the heat is dumped from nuclear power plant, either to a large volume of water (from the sea or large river, heating it a few degrees) or to a relatively smaller volume of water in cooling towers

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