Like all industries, electricity, produces waste heat. Whatever the fuel, the waste must be managed to protect human health and minimize their environmental impact.
Nuclear energy is only the energy industry, which takes full responsibility for all waste and product costs.
Radioactivity arises naturally from the decomposition of various forms of elements, called isotopes. Some isotopes are radioactive, most are not, although in this publication "Focus on the former.
There are three types of radiation to be considered: alpha, beta and gamma. The fourth type, neutron radiation, usually occurs within the nuclear reactor. Different types of radiation require different forms of protection:
Alpha radiation can not penetrate the skin and can be blocked by a sheet of paper, but it is dangerous to the lungs.
Beta radiation can penetrate the body, but can be blocked by a sheet of aluminum foil.
Gamma rays can penetrate the body and requires several centimeters of lead or concrete, or water meters to lock.
All these types of radiation, at low levels, of course, part of our environment. Or they may all be present in the waste classification. Radioactive waste contains a variety of materials requiring different types of management to protect humans and the environment. They are generally classified as low, medium or high level waste, depending on the quantity and type of radiation in them.
Another factor in the management of waste is the time when they are likely to remain dangerous. It depends on the type of radioactive isotopes in them, and in particular its lifetime characteristic of each of these isotopes. Half-life is the time taken by the radioactive isotope to lose half its radioactivity. After four half-life of the level of radioactivity is 1/16th of the original and after eight half-lives 1/256th.
Different radioactive isotopes, which have half-lives ranging from fractions of seconds to minutes, hours or days, until billions of years. Radioactivity decreases with time as these isotopes decay into stable, these non-radioactive. The decay rate of isotopes is inversely proportional to its half-life, short average half-life that decays rapidly. Thus, for each type of radiation, the greater the intensity of radiation in a given amount of material, more than half the life of the parties.
Three main principles are used in the management of radioactive waste:
Concentration and memory
Dilute-and-dispersion
Delay and decay.
The first two are also used in the management of waste other than radioactive waste. Waste, or concentrate, then single or dilution to an acceptable level, then discharged into the environment. Delay and distribution, however, is unique to the management of radioactive waste, which means that waste is stored and can reduce the radioactivity by decay of naturally radioactive isotopes it contains.
Types of radioactive waste (radioactive waste)
The low-level waste is generated from hospitals, laboratories and industry, and the cycle of nuclear fuel. It consists of paper, rags, tools, clothing, filters etc. which contain small amounts of radiation of relatively short duration. It is safe to use, but must be removed more carefully than normal garbage. It is usually buried in shallow landfills. Reduce its volume, it is often compacted or incinerated (in a closed container) before disposal. Around the world, covers 90% of the volume, but only 1% of the radioactivity of all radioactive waste.
medium-level waste contains large amounts of radiation and may require special shielding. is generally composed of resins, chemical sludge and reactor components and contaminated materials from reactor decommissioning. He made the world a volume of 7% and 4% of the radioactivity of all radioactive waste. It can be solidified in concrete or bitumen for disposal. Usually short-lived waste (mainly from reactors) is buried, but long-lived waste (from reprocessing nuclear fuel) will be removed from deep underground.
High-level waste can be used the same fuel, or waste his principal reprocessing. And only 3% by volume of all radioactive waste, it has 95% of the radioactivity. It contains highly radioactive fission products of heavy elements, and a little long-term radioactivity. Generates considerable amounts of heat and do not require refrigeration, as well as special shielding during handling and transport. If you use the fuel is processed, the vitrified waste separated by integrating it into borosilicate (Pyrex) glass which is encased in stainless steel containers for permanent storage deep underground.
On the other hand, if reactor fuel is not reprocessed, all remain highly radioactive isotopes, and so all the fuel elements are considered HLW. The fuel used to take about nine times the size of equivalent vitrified high activity, which results from reprocessing, and seems ready to retire.
The high level waste and the fuel is highly radioactive and managing people must be protected against radiation. These materials are shipped in special containers which prevent leakage and radiation, which is no fracture in the accident.
Is the fuel is used for reprocessing or not, the volume of HLW is low - about 3 cubic meters per year from 1925 to 1930 tons of vitrified waste or fuel for a typical large nuclear reactor. A relatively small amount can effectively and economically isolated.
Radioactive materials in the environment
Naturally occurring radioactive materials are ubiquitous in the environment, even if the concentrations are very low and they are generally not harmful.
Soil naturally contains many radioactive materials - uranium, thorium, radium and radon, a radioactive gas, which escapes into the atmosphere continuously. Many parts of the crust are more radioactive than the waste with low levels described above. Radiation is not something which arises simply using uranium to generate electricity, although the mining and milling of uranium ore and some other makes such radioactive materials into closer contact with people, in the case of radon and its decay products, accelerates their release into the atmosphere. (See also radiation, and life in this series.)
Waste nuclear fuel cycle
Radioactive waste at all stages of the nuclear fuel cycle - the process of producing electricity from nuclear material. fuel cycle includes mining and milling of uranium ore, processing and production of nuclear fuel, its use in the reactor, the reprocessing of spent fuel from the reactor after use and finally disposal of waste.
fuel cycle, is often treated as two parts - the front part, which extends from mining, through the use of uranium in the reactor - and the end "back", which involves removing fuel used in the reactor and its treatment, and disposal. This is where the radioactive waste are major problems.
Residues from the front "of the fuel cycle
The annual demand for fuel for light water reactor l000 MWe is about 25 tons of enriched uranium oxide. This requires the extraction and processing of 50,000 tonnes of ore could provide about 200 tons of uranium oxide concentrate (U3O8) from the mine.
In uranium mines, dust is controlled to minimize inhalation of radioactive minerals and radon levels are minimized by good ventilation and dispersion of air in large quantities. The mill, dust collects and reintroduced into the process, while radon gas is diluted and dispersed into the atmosphere large quantities of air.
In the mine tailings in the bedrock milling operations include most of the radioactive material from the ore, such as councils. This material is discharged into tailings dams which retain the remaining solids and prevent leakage. Residue contains about 70% of the radioactivity of the ore origin.
At the end of the residues can be moved to the mine or may be covered with rock and clay, then covered with vegetation. In this case, great care is taken to ensure their long term stability and avoid the environmental impact (which would be more acid leaching or dust that radioactivity as such).
The releases are usually about ten times more radioactive than typical granites, such as those used in municipal buildings. If someone to live permanently in the top residues Ranger will receive approximately twice the normal dose of radiation from the tailings real (ie they have received the triple dose).
Preview situ (ISL) mining, soluble materials other than uranium are simply returned underground, where they come from, and the water is recycled.
uranium oxide (U3O8) produced from mining and milling of uranium ore is only slightly radioactive - most of the radioactivity of the original ore remains in the tailings.
Turning uranium oxide concentrate to useful fuel has no impact on the level of radioactivity of the waste and not significant.
First, the uranium oxide is converted into gas, uranium hexafluoride (UF6) as feedstock for enrichment.
Then, during the enrichment for each ton of uranium hexafluoride is divided into about 130 kg of enriched UF6 (3.5% U-235) and 870 kg "depleted uranium" UF6 (especially U- 238). The enriched UF6 is converted into uranium dioxide (UO2) powder and pressed into pellets of fuel, which are enclosed in zirconium alloy tubes to form fuel rods.
Depleted uranium has several uses, although the high density (density 18.7) must be applied in the keels of yachts, aircraft control surface counterweights, anti-tank ammunition and protection against radiation. It is also a potential source of energy for each reactor (SWIFT).
Waste from the back end of fuel cycle
In other words, when uranium is used in a reactor that significant quantities of highly radioactive waste are created. When an atom of uranium-235 is divided creates products of fission are highly radioactive and form a large part of nuclear waste stored in the fuel rods. There is also a relatively small amount of radioactivity in part caused by the radiation from neutron reactor.
About 25 tonnes of fuel used each year, taking deep l000 MWe nuclear reactor. This fuel can be regarded as entirely as waste (as in 40% of world production in the U.S. and Canada) or can be reprocessed (as in Europe and Japan). Whatever your choice, the fuel is stored after the first years under water in the reactor cooling ponds. concrete ponds and water, including fuel assemblies provide protection against radiation, while removing the heat produced by radioactive decay.
Pond to store spent fuel in the United Kingdom to the processing plant
The costs of proceedings in the high-level waste are included in electricity rates. For example, the U.S., consumers pay 0.1 cents per kilowatt hour, what tools to pay the special fund. To date, over 18 billion dollars raised this way.
Treatment
If the fuel is used for further transformation, it is dissolved and chemically separated uranium, plutonium and waste of high-level solutions. Approximately 97% of fuel used can be recycled leaving only 3% of high level waste. The recycling of depleted uranium is used primarily for less than 1% U-235, with some plutonium, which is most precious.
From the operation of a typical year l000 MWe nuclear reactor, about 230 kilograms of plutonium (1% of spent fuel) is separated in the reprocessing. This can be used fresh mixed oxide (MOX) (but not weapons, because of its composition). manufacture of MOX fuel produced in Europe, with approximately 25 years of operational experience. The main factory is located in France and launched in 1995. Japan is slightly smaller plant to begin in 2012. Across Europe, the permit more than 35 reactors load 20-50% of cores with MOX fuel.
Separated high-level waste - about 3% of reactor fuel typically used - up to 700 kg per year and must be isolated from the environment for a very long time.
Commercial reprocessing plants operate in France and Britain, with a capacity of 5,000 tons of spent fuel per year - the equivalent of at least one third of world production. In total, more than 90,000 tons of spent fuel has been treated more than 40 years.
immobilization of high level waste
The solidification
have been developed in several countries over the last fifty years. HLW liquid is evaporated to dryness, mixed with glass forming materials, melted and poured into a robust stainless steel containers, which are then sealed by welding.
borosilicate glass factory waste vitrification first in the UK in 1960. This block contains waste chemically identical to high from reprocessing. A piece of this size will contain all the high-level waste from nuclear power plants generating electricity for a person in a normal life.
Vitreous-life 1000 years MWe reactor is expected to take approximately twelve containers, each 1.3m high and 0.4 m in diameter and holding 400 kg of glass. Europe vitrification plants produce about 1,000 commercial tons per year of such vitrified waste (2500 cartridges), and some have been operating for more than 20 years.
Loading silos with canisters containing vitrified high level waste in the UK, each disc is on the operating floor elevator ten boxes
More sophisticated methods of immobilization of highly radioactive waste has been developed in Australia. The name "SYNROC (synthetic rock), radioactive waste is incorporated into the mineral crystals naturally stable in the rock synthesis. In other words, copying what happens in nature. This process is being tested in the States USA.
Waste disposal
waste disposal of high level is delayed for 40-50 years to allow its radioactivity to decay, after which less than one thousandth of its initial radiation remains, and it is much easier to use. Therefore, the combustion products of vitrified waste canisters are used or stored in water in special ponds or in dry concrete structures or casks for at least the same period.
Vitreous final sale or use of fuel elements, without treatment, they must be isolated from the environment for a long period. The methods most favored burial in a dry, stable geological approximately 500 meters. Many countries consider the parties would be technically and social acceptance. United States continued to guard the site for all people of the fuel used in Nevada.
A guard has been built deep geological radioactive waste in the long term (although defense applications only) is already in use in New Mexico.
Once buried in about 1000 years, most of the radioactivity will be broken. The level of radioactivity, and the rest will be similar to natural uranium ore from which it originates, although it is more concentrated.
Layers of protection
To ensure there are no significant releases of Perio environment long after the elimination of multiple barrier "concept is used to remove the immobilization of radioactive elements in the high (and intermediate) waste and isolate it from the biosphere. The main obstacles are the following:
Immobilise waste in an insoluble matrix, eg borosilicate glass, SYNROC (or leave them in the form of pellets of uranium oxide fuel - ceramic).
Wrap it in stainless steel resistant to corrosion, for example.
containers with bentonite clay surround prevents any movement of groundwater, if the repository can be wet.
Located deep in stable rock.
For each of radioactivity to reach human populations or the environment, all these obstacles must not be breached before the radioactivity decayed.
What happens in the U.S. and Europe?
United States high-level civil wastes all remain stored in the form of fuel used in the reactor sites. It is intended to symbolize the fuel elements and remove them in the archives of underground engineering for Yucca Mountain in Nevada. It is a program that is funded by electricity consumers to U.S. $ 26000000000 to date (ie@0.1 cents per kWh), about U.S. $ 6 billion have been issued.
In Europe, part of the fuel is stored at reactor sites, even awaiting disposal. However, much of the energy in Europe, the past is sent for processing at Sellafield in the United Kingdom and La Hague in France. The recovered uranium and plutonium are then returned to their owners (the MOX fuel fabrication plant) and sorted waste (about 3% of spent fuel) are vitrified, sealed in stainless steel containers, and either stored or returned . Ultimately, they will go to geological disposal.
Sweden is a big difference. At no centralized storage of fuel used in CLAB near Oskarshamn, and symbolize the fuel used when the geological storage of about 2015. Finland is to establish a disposal facility at Olkiluoto. European funds are at a level similar to the U.S. per kWh.
Previous Natural
We have an example in nature that the final disposal of high level waste is safely underground. Two billion years in the Oklo in Gabon, West Africa, the chain reaction started spontaneously in the deposits of uranium ore concentrate. natural nuclear reactors in operation continues to create hundreds of thousand years of plutonium and highly radioactive waste all day in a nuclear power reactor. Despite the existence at the time of large quantities of water in the area of these materials remained, which were created and, possibly, elements other than radioactive decayed. The proof is there.
Alternatives to Nuclear Energy
No technology is completely safe and without consequences for the environment. It is therefore appropriate to compare the production of electricity from nuclear energy with other options available to us. (See also: Energy for the world: Why uranium? In this series) combustion of coal in power plants is still the main source of electricity in the world, and hydroelectric power, gas and uranium.
1000 MWe light water reactor uses about 25 tons of enriched uranium per year, which requires the extraction of approximately 50,000 tons of uranium ore. For comparison, a 1000 MW coal power requires the extraction, transport, storage and incineration of approximately 3.2 million tons of coal per year. It is approximately 7,000,000 tonnes of carbon dioxide, not to mention sulfur dioxide, according to the type of coal. Solid wastes from coal-fired power and can cause significant damage to the environment and health. (See also uranium, electricity and the greenhouse effect in this series)
Many people are concerned about possible global warming by enhancing the greenhouse effect. Much of this is the result of a steady increase in carbon dioxide in the atmosphere over the last 150 years. combustion of fossil fuels, especially coal, electricity contributes about 10 billion tons of carbon dioxide into the atmosphere each year.
via world-nuclear
