Radioactive waste management
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Radioactive waste management
The treatment and containment of radioactive wastes. These wastes originate almost exclusively in the nuclear fuel cycle and in the nuclear weapons program. Their toxicity requires careful isolation from the biosphere. Their radioactivity is commonly measured in curies (Ci). Considering its toxicity, the curie is a rather large unit of activity. A more appropriate unit is the microcurie (1 μCi = 10-6 Ci), but the nanocurie (1 nCi = 10-9 Ci) and picocurie (1 pCi = 10-12 Ci) are also frequently used.
Radioactive wastes are classified in four major categories: spent fuel elements and high-level waste (HLW), transuranic (TRU) waste, low-level waste (LLW), and uranium mill tailings. Examples of minor waste categories include radioactive gases produced during reactor operation, radioactive emissions resulting from the burning of uranium-containing coal, or contaminated uranium mine water.
Spent fuel elements arise when uranium is fissioned in a reactor to generate energy. Most of the existing radioactivity is contained in spent nuclear fuel and high-level waste. For the first 100 years, the toxicity is dominated by the beta- and gamma-emitting fission products [such as strontium-90 (90Sr) and cesium-137 (137137), with half-lives of approximately 30 years]; thereafter, the long-lived, alpha-emitting transuranium elements [for example, plutonium-239 (239Pu), with a half-life of 24,000 years] and their radioactive decay daughters [for example, americium-241 (241Am), with a half-life of 432 years, a daughter of plutonium-241 (241Pu), with a half-life of 13 years] are important. Burial in geologic formations at a depth of 500–1000 m (1600–3200 ft) appears at present the most practical and attractive disposal method. See Nuclear fuel cycle, Nuclear fuels reprocessing
However, geology as a predictive science is still in its infancy, and many of the parameters entering into model calculations of the long-term retention of the waste in geologic media are questionable. The major single problem is the heating of the waste and its surrounding rock by the radioactive decay heat. This heating can accelerate the penetration of groundwater into the repository, the dissolution of the waste, and its transport to the biosphere. Much effort has been devoted to the development of canisters to encapsulate the spent fuel elements or the glass blocks containing high-level waste, and of improved waste forms and overpacks that promise better resistance to attack by groundwater.
Although the radioactivity of the transuranic wastes is considerably smaller than that of high-level waste or spent fuel, the high radiotoxicity and long lifetime of these wastes also require disposal in a geologic repository. Waste with less than 100 nCi/g (3.78 Bq/kg) of transuranic elements will be treated as low-level waste.
Uranium is naturally radioactive, decaying in a series of steps to stable lead. It is currently a rare element, averaging between 0.1 and 0.2% in the mined ore. At the mill, the rock is crushed to fine sand, and the uranium is chemically extracted. The residues are discharged to the tailings pile. The tailings contain the radioactive daughters of the uranium. The long-lived isotope thorium-230 (230Th, half-life 80,000 years) decays into radium-226 (226Ra, half-life 1600 years), which in turn decays to radon-222 (222Rn, half-life 3.8 days). Radium and radon are known to cause cancer, the former by ingestion, the latter by inhalation. Radon is an inert gas and thus can diffuse out of the mill tailings pile and into the air. Ground-water pollution by radium that has leached from the pile has also been observed around tailings piles, but its health effects are more difficult to estimate, since the migration in the ground water is difficult to assess and also highly site-specific.
Although the radioactivity contained in the mill tailings is very small relative to that of the high-level waste and spent fuel, it is comparable to that of the transuranic waste. It is mainly the dilution of the thorium and its daughters in the large volume of the mill tailings that reduces the health risks to individuals relative to those posed by the transuranium elements in the transuranic wastes. However, this advantage is offset by the great mobility of the chemically inert radon gas, which emanates into the atmosphere from the unprotected tailings. New mill tailings piles will be built with liners to protect the ground water, and will be covered with earth and rock to reduce atmospheric release of the radon gas.
By definition, practically everything that does not belong to one of the three categories discussed above is considered low-level waste. This name is misleading because some wastes, though low in transuranic content, may contain very high beta and gamma activity. The current method of low-level waste disposal is shallow-land burial, which is relatively inexpensive but provides less protection than a geologic repository.
At the end of their lifetime, nuclear facilities have to be dismantled (decommissioned) and the accumulated radioactivity disposed of. Nuclear power plants represent the most important category of nuclear facilities, containing the largest amounts of radioactive wastes, which can be grouped in three classes: neutron-activated wastes, surface-contaminated wastes, and miscellaneous wastes. See Nuclear power, Nuclear reactor
The neutron-activated wastes are mainly confined to the reactor pressure vessel and its internal components, which have been exposed to large neutron fluences during reactor operation. These components contain significant amounts of long-lived nontransuranic radioactive isotopes such as niobium-94 (94Nb, an impurity in the stainless steel), which emits highly penetrating gamma rays and has a half-life of 20,000 years. These wastes are unacceptable for shallow-land disposal as low-level wastes. Disposal in a geologic repository is envisioned.