Sunday, December 27, 2009

Wastes from the nuclear fuel cycle / Waste management

Radioactive wastes occur at all stages of the nuclear fuel cycle, the process of producing electricity from nuclear materials. The cycle comprises the mining and milling of the uranium ore, its processing and fabrication into nuclear fuel, its use in the reactor, the treatment of the spent fuel taken from the reactor after use and finally, disposal of the wastes.

The fuel cycle is often split into two parts - the "front end" which stretches from mining through to the use of uranium in the reactor - and the "back end" which covers the removal of spent fuel from the reactor and its subsequent treatment and disposal. This is where radioactive wastes are a major issue.

Residual materials from the "front end" of the fuel cycle

The annual fuel requirement for a l000 MWe light water reactor is about 25 tonnes of enriched uranium oxide. This requires the mining and milling of some 50,000 tonnes of ore to provide 200 tonnes of uranium oxide concentrate (U3O8) from the mine.

At uranium mines, dust is controlled to minimise inhalation of radioactive minerals, while radon gas concentrations are kept to a minimum by good ventilation and dispersion in large volumes of air. At the mill, dust is collected and fed back into the process, while radon gas is diluted and dispersed to the atmosphere in large volumes of air.

Residual wastes from the milling operation contain the remaining radioactive materials from the ore, such as radium. These wastes are discharged into tailings dams designed to retain the remaining solids and prevent any seepage of the liquid. Eventually the tailings may be put back into the mine or they may be covered with rock and clay, then revegetated.

The tailings are around ten times more radioactive than typical granites, such as used on city buildings. If someone were to live continuously on top of the Ranger tailings they would receive about double their normal radiation dose from the actual tailings (ie they would triple their received dose).

With in situ leach (ISL) mining, dissolved materials other than uranium are simply returned underground from where they came.

Uranium oxide (U3O8) produced from the mining and milling of uranium ore is only mildly radioactive - most of the radioactivity in the original ore remains at the mine site in the tailings.

Turning uranium oxide concentrate into a useable fuel has no effect on levels of radioactivity and does not produce significant waste. First, the uranium oxide is converted into a gas, uranium hexafluoride (UF6), as feedstock for the enrichment process.

Then, during enrichment, every tonne of uranium hexafluoride becomes separated into about 130 kg of enriched UF6 (about 3.5% U-235) and 870 kg of 'depleted' UF6 (mostly U-238). The enriched UF6 is finally converted into uranium dioxide (UO2) powder and pressed into fuel pellets which are encased in zirconium alloy tubes to form fuel rods.

Depleted uranium has few uses, though with a high density (specific gravity of 18.7) it has found uses in the keels of yachts, aircraft control surface counterweights, anti-tank ammunition and radiation shielding. It is also a potential energy source for particular (fast neutron) reactors.

Wastes from the "back end" of the fuel cycle

It is when uranium is used in the reactor that significant quantities of highly radioactive wastes are created. More than 99% of the radioactivity produced during the fission reaction is retained in the fuel rods. The balance is within the reactor structure.

About 25 tonnes of spent fuel is taken each year from the core of a l000 MWe nuclear reactor. The spent fuel can be regarded entirely as waste (as, for 40% of the world¹s output, in USA and Canada), or it can be reprocessed (as in Europe). Whichever option is chosen, the spent fuel is first stored for several years under water in large cooling ponds (see chapter 8) at the reactor site. The concrete ponds and the water in them provide radiation protection, while removing the heat generated during radioactive decay.

The costs of dealing with this high-level waste are built into electricity tariffs. For instance, in the USA, consumers pay 0.1 cents per kilowatt-hour, which utilities pay into a special fund. So far more than US$ 18 billion has been collected thus.

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