綜觀核廢料管理 Waste Management: Overview編輯
- Like all industries, the thermal generation of electricity produces wastes. Whatever fuel is used, these wastes must be managed in ways which safeguard human health and minimise their impact on the environment.
- 核能發電是能源產業中，唯一完全負起處理廢棄物的責任，並且 costs this into the product
- Nuclear power is the only energy industry which takes full responsibility for all its wastes, and costs this into the product.
Nuclear power is characterised by the very large amount of energy available from a very small amount of fuel. The amount of waste is correspondingly very small. However, much of the waste is radioactive and therefore must be carefully managed as hazardous waste.
Since the radioactive wastes are essentially created in a nuclear power reactor, it is accepted that they are the responsibility of the country which uses uranium to generate power. There is no moral or legal basis for the responsibility to be elsewhere.
Radioactive wastes comprise a variety of materials requiring different types of management to protect people and the environment. They are normally classified as low-level, medium-level or high-level wastes, according to the amount and types of radioactivity in them.
Another factor in managing wastes is the time that they are likely to remain hazardous. This depends on the kinds of radioactive isotopes in them, and particularly the half-lives characteristic of each of those isotopes. (The half-life is the time it takes for a given radioactive isotope to lose half of its radioactivity. After four half lives the level of radioactivity is 1/16th of the original and after eight half lives 1/256th, and so on.)
The various radioactive isotopes have half-lives ranging from fractions of a second to minutes, hours or days, through to billions of years. Radioactivity decreases with time as these isotopes decay into stable, non-radioactive ones.
The rate of decay of an isotope is inversely proportional to its half-life; a short half life means that it decays rapidly. Hence, for each kind of radiation, the higher the intensity of radioactivity in a given amount of material, the shorter the half lives involved.
Three general principles are employed in the management of radioactive wastes:
The first two are also used in the management of non-radioactive wastes. The waste is either concentrated and then isolated, or it is diluted to acceptable levels and then discharged to the environment. Delay-and-decay however is unique to radioactive waste management; it means that the waste is stored and its radioactivity is allowed to decrease naturally through decay of the radioisotopes in it.
Radioactivity arises naturally from the decay of particular forms of some elements, called isotopes. Some isotopes are radioactive, most are not, though here the focus is on the former.There are three kinds of radiation to consider: alpha, beta and gamma. A fourth kind, neutron radiation, generally only occurs inside a nuclear reactor.
Different types of radiation require different forms of protection:
- Alpha radiation cannot penetrate the skin and can be blocked out by a sheet of paper, but is dangerous in the lung.
- Beta radiation can penetrate into the body surface but can be blocked out by a sheet of aluminium foil.
- Gamma radiation can go deeply into the body and requires several centimetres of lead or concrete, or a metre or so of water, to block it.
All of these kinds of radiation are, at low levels, naturally part of our environment, where we are all naturally exposed to them at low levels. Any or all of them may be present in any classification of radioactive waste
核廢料的種類 Types of radioactive waste (radwaste)編輯
Low-level Waste is generated from hospitals, laboratories and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters etc. which contain small amounts of mostly short-lived radioactivity. It is not dangerous to handle, but must be disposed of more carefully than normal garbage. Usually it is buried in shallow landfill sites. To reduce its volume, it is often compacted or incinerated (in a closed container) before disposal. Worldwide it comprises 90% of the volume but only 1% of the radioactivity of all radwaste.
Intermediate-level Waste contains higher amounts of radioactivity and may require special shielding. It typically comprises resins, chemical sludges and reactor components, as well as contaminated materials from reactor decommissioning. Worldwide it makes up 7% of the volume and has 4% of the radioactivity of all radwaste. It may be solidified in concrete or bitumen for disposal. Generally short-lived waste (mainly from reactors) is buried, but long-lived waste (from reprocessing nuclear fuel) is disposed of deep underground.
High-level Waste may be the used fuel itself, or the principal waste separated from reprocessing this. While only 3% of the volume of all radwaste, it holds 95% of the radioactivity. It contains the highly-radioactive fission products and some heavy elements with long-lived radioactivity. It generates a considerable amount of heat and requires cooling, as well as special shielding during handling and transport. If the used fuel is reprocessed, the separated waste is vitrified by incorporating it into borosilicate (Pyrex) glass which is sealed inside stainless steel canisters for eventual disposal deep underground.
On the other hand, if used reactor fuel is not reprocessed, all the highly-radioactive isotopes remain in it, and so the whole fuel assemblies are treated as high-level waste. This used fuel takes up about nine times the volume of equivalent vitrified high-level waste which is separated in reprocessing. Used fuel treated as waste must be encapsulated ready for disposal.
Both high-level waste and used fuel are very radioactive and people handling them must be shielded from their radiation. Such materials are shipped in special containers which shield the radiation and which will not rupture in an accident.
Whether used fuel is reprocessed or not, the volume of high-level waste is modest, - about 3 cubic metres per year of vitrified waste, or 25-30 tonnes of used fuel for a typical large nuclear reactor. The relatively small amount involved allows it to be effectively and economically isolated.
自然環境中的輻射 Radioactive materials in the natural environment編輯
Naturally-occurring radioactive materials are widespread throughout the environment, although concentrations are very low and they are not normally harmful. However, human activity may concentrate these so that they need careful handling- eg in coal ash and gas well residues.
Soil naturally contains a variety of radioactive materials - uranium, thorium, radium and the radioactive gas radon which is continually escaping to the atmosphere. Many parts of the Earth's crust are more radioactive than the low-level waste described above. Radiation is not something which arises just from using uranium to produce electricity, although the mining and milling of uranium and some other ores brings these radioactive materials into closer contact with people, and in the case of radon and its daughter products, speeds up their release to the atmosphere. (See also Radiation and Life.)
核燃料週期產生的核廢料 Wastes from the nuclear fuel cycle編輯
Radioactive wastes occur at all stages of the nuclear fuel cycle - the process of producing electricity from nuclear materials. The fuel 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 used fuel taken from the reactor after use, and finally, disposal of the wastes.
The fuel cycle is often considered as 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 used 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 27 tonnes of enriched uranium oxide. This requires the mining and milling of tens of thousands of tonnes of ore to provide about 200 tonnes of uranium oxide concentrate (U3O8) from the mine.
At uranium mines, dust is controlled to minimise inhalation of radioactive minerals, while concentrations of radon gas (seeping out of the rocks) 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.
At the mine, residual ground rock from the milling operation contain most of the radioactive materials from the ore, such as radium. This material is discharged into tailings dams which retain the remaining solids and prevent any seepage of the liquid. The tailings contain about 70% of the radioactivity in the original ore.
Eventually these tailings may be put back into the mine or they may be covered with rock and clay, then revegetated. In this case considerable care is taken to ensure their long-term stability and to avoid any environmental impact (which would be more from acid leaching or dust than from radioactivity as such).
The tailings are usually 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 mine 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, as the water is recirculated.
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編輯
It is when uranium is used in the reactor that significant quantities of highly radioactive wastes are created. When the uranium-235 atom is split it forms fission products, which are very radioactive and make up the main portion of nuclear wastes retained in the fuel rods.
About 27 tonnes of used fuel is taken each year from the core of a l000 MWe nuclear reactor. This fuel can be regarded entirely as waste (as, for 40% of the world's output, in USA and Canada), or it can be reprocessed (as in Europe and Japan). Whichever option is chosen, the used fuel is first stored for several years under water in cooling ponds at the reactor site. The concrete ponds and the water covering the fuel assemblies provide radiation protection, while removing the heat generated during radioactive decay.
Storage pond for spent fuel at UK reprocessing plant
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$ 32 billion has been collected thus.
There is also a relatively small amount of radioactivity induced in the reactor components by neutron irradiation. When the reactor is retired and dismantled these materials become wastes.
核廢料再處理 Reprocessing 編輯
If the used fuel is later reprocessed, it is dissolved and separated chemically into uranium, plutonium and high-level waste solutions. About 97% of the used fuel can be recycled leaving only 3% as high-level waste. The recyclable portion is mostly uranium depleted to less than 1% U-235, with some plutonium, which is most valuable.
Arising from a year's operation of a typical l000 MWe nuclear reactor, about 230 kilograms of plutonium (1% of the spent fuel) is separated in for recycle. This can be used in fresh mixed oxide (MOX) fuel (but not weapons, due its composition).
The separated high-level wastes - about 3% of the typical reactor's used fuel - amounts to 700 kg per year and it needs to be isolated from the environment for a very long time.
Immobilising separated high-level waste
Solidification processes have been developed in several countries over the past fifty years. Liquid high-level wastes are evaporated to solids, mixed with glass-forming materials, melted and poured into robust stainless steel canisters which are then sealed by welding.
Borosilicate glass from the first waste vitrification plant in UK in the 1960s. This block contains material chemically identical to high-level waste from reprocessing. A piece this size would contain the total high-level waste arising from nuclear electricity generation for one person throughout a normal lifetime.
The vitrified waste from the operation of a 1000 MWe reactor for one year would fill about twelve canisters, each 1.3m high and 0.4m diameter and holding 400 kg of glass.
Loading silos with canisters containing vitrified high-level waste in UK, each disc on the floor covers a silo holding ten canisters
A more sophisticated method of immobilising high-level radioactive wastes has been developed. Called 'SYNROC' (synthetic rock), the radioactive wastes are incorporated in the crystal lattices of the naturally-stable minerals in a synthetic rock. In other words, copying what happens in nature. This process is still being developed for specialist application.
核廢料棄置 Waste disposal編輯
Final disposal of high-level waste is delayed for 40-50 years to allow its radioactivity to decay, after which less than one thousandth of its initial radioactivity remains, and it is much easier to handle. Hence canisters of vitrified waste, or used fuel assemblies, are stored under water in special ponds, or in dry concrete structures or casks, for at least this length of time.
The ultimate disposal of vitrified wastes, or of used fuel assemblies without reprocessing, requires their isolation from the environment for a long time. The most favoured method is burial in stable geological formations some 500 metres deep. Several countries are investigating sites that would be technically and publicly acceptable, and in Sweden and Finland construction is proceeding in 1.9 billion year-old granites.
One purpose-built deep geological repository for long-lived nuclear waste (though only from defence applications) is already operating in New Mexico, in a salt formation.
After being buried for about 1000 years most of the radioactivity will have decayed. The amount of radioactivity then remaining would be similar to that of the corresponding amount of naturally-occurring uranium ore from which it originated, though it would be more concentrated.
Layers of protection after disposal編輯
To ensure that no significant environmental releases occur over a long period after disposal, a 'multiple barrier' disposal concept is used. The radioactive elements in high-level (and some intermediate-level) wastes are immobilised and securely isolated from the biosphere. The principal barriers are:
- Immobilise waste in an insoluble matrix, eg borosilicate glass (or leave them as uranium oxide fuel pellets - a ceramic).
- Seal inside a corrosion-resistant container, eg stainless steel.
- Surround containers with bentonite clay to inhibit any groundwater movement if the repository is likely to be wet.
- Locate deep underground in a stable rock structure.
For any of the radioactivity to reach human populations or the environment, all of these barriers would need to be breached, and this would need to happen before the radioactivity decayed to innocuous levels.
A natural precedent編輯
One particular example in nature provides strong reassurance concerning final disposal of high-level wastes underground. Two billion years ago at Oklo in Gabon, West Africa, chain reactions started spontaneously in concentrated deposits of uranium ore. These natural nuclear reactors continued operating for hundreds of thousands of years forming plutonium and all the highly radioactive waste products created today from exactly the same processes in a nuclear power reactor. Despite the existence at that time of large quantities of water in the area, these materials stayed where they were formed and eventually decayed into non-radioactive elements. The evidence remains there.