High-level radioaztive waste How do we store it; how do we dispose of it? Disposal of high-level radioactive waste is a topic that arouses great concern among government officials as well as the public. There is no question that extensive exposure to radiation can damage human tissues and cause genetic changes. So a main objective of either storing or disposing of high-level radioactive waste is to isolate it from the general public. 1; principle, there is a difference between disposal and storage. Disposal implies that no future action is contemplated with the exception of environmental monitoring or restrictions on future use of the disposal site. Storage, on the other hand, implies an intention to take further action at a later date to retrieve, treat, examine, or dispose of the waste.
Management of waste There are several types of high-level radioactive waste. Radioactive aqueous waste results from the solvent extraction cycle for the reprocessing of spent fuel rods. In the past 40 years of nuclear energy and nuclear weapons production in the US.,some 17.6 million gallons of highly radioactive liquid wastes have accumulated (ES&T, May 1982, p. 271A). In some countries any waste with levels of radioactivity intense enough to generate sienificant auantilies of hear bv decav i s h o classi’fied as high-level waste. 1; addition, in countries where reprocessing is not envisioned, the spent fuel from the reactor is classified as highlevel waste. These details are described in a publication of the World Health Organization (WHO) entitled “Nuclear Power: Management of High-Level Radioactive Waste.” This publication, also identified as WHO Regional Publications, European Series No. 13, stems from a 1980 Working Group on Health Implications of High-Level Radioactive Waste Disposal, which was held in Belgium. As with most 0013-936X183/0916-0413A$01.50/0
public health problems, an exact evaluation of the health aspects of the disposal of such waste is not possible. This European Region has 33 active member states and is unique in that many of them are industrialized countries with highly advanced medical services. In brief. there are three main methods for the management of hieh-level radioactive waste resultine from the fuel cycle: the so-callA stow-away cycle, the throw-away cycle, and the reprocessing cycle. However, there are problems with and criticisms of these options. The stowaway cycle defers the problem of treatment and ultimate disposal; it does not offer a final waste management solution. At first sight, the throw-away cycle appears simple because it has fewer processing stages, but its drawback is that it does not recover uranium or plutonium for future energy generation. On the other hand, reprocessing with vitrification-the changing of the waste into a glass or a
@ 1983 American Chemical Society
glassy substance by heat and fusion-does meet many of the requirements of modern waste management; it ensures that the waste form is suitable for disposal, that the amount of plutonium disposed of in the environment is minimized, and that energy conservation is achieved. Methods for the management and interim storage of high-level radioactive waste are in use and well proven. Disposal methods have not yet been selected, but placement of these wastes in vitrified form in suitable geological formations has received the most attention. Such wastes can be stored in wateror air-cooled facilities. This option has been explored in Canada and more recently in the US. This storing of spent fuel rods without reprocessing is the stow-away fuel cycle. It avoids the problem of plutonium proliferation since the plutonium is locked in the highly radioactive fuel rod, its extraction for military purposes would require sophisticated and elaborate equipment, which is unlikely to be widely available. In reprocessing, uranium and plutonium are separated from the fission products by putting a nitric acid solution of the fuel in contact with an immiscible solvent such as tributyl phosphate in an organic diluent. Referred to as the raffinate, this aqueous solution is highly radioactive; it is concentrated by evaporation and then stored in specially designed stainless steel tanks. Stainless steel is chosen as the canister material because it resists the chemical and radiation damage of the casting operation and subsequently of the hot, intensely radioactive waste in storage. This raffinate consists primarily of an aqueous solution of the nitrates of the fission products and actinides. About 10 m3/yof concentrated waste is produced for each gigawatt of electricity. Most of the high-level waste that has been produced by the reproEnviron. Sci. Technol.. VoI. 17, NO.9. 1983
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cessing of nuclear fuel in various countries is now stored either as liquid or as salt cake in underground tanks. This storage of high-level radioactive liquid waste is now a routine operation: it has been practiced for about 35 years, and according to the WHO report there has been no recorded leak into the ground, with the exception of the “first” storage tanks at Hanford.
Solidification Vitrification, a solidification route that has been developed for immobilization of high-levelradioactive waste, converts the liquid waste into a block
Chromatography provides selective, accurate analysis of anions such as chloride, phosphate, nitrate and sulfate, from ppb to high pprn concentrations. And it does it in as little as 10 minutes, with minimal or no sample preparation. For precisely these same reasons, Dionex systems are also used in over 150 power plant facilities to monitor anions, cations, and organic acids in process steam and plant feedwater in order to prevent stress cracking, corrosion and equipment failure. Find out more. Circle the reader service so.’. NO; number and we’ll send you our application notes, Deterrnination of Anions in Acid Rain and 0 4 8 /on ChromatqgMinutes mphv in Energy and Power Production.
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is cheaper than glass but has similar properties. Ceramics are more expensive than the other forms but can incorporate 70% waste, whereas glass can only handle up to 30% waste. So the cost of the ceramic disposal system might be less than the glass system considering its greater capacity for waste. The most straightforward solidification process for high-level radioactive liquid waste, requiring no additive, is to heat the waste liquid, driving off the water and the nitric acid, and to calcine the nitrates into oxides, producing a brown, powdered calcine.
Methods for the management and interim storage of high-level radioactive waste are in use and well proven. Disposal methods have not yet been selected of alkali borosilicate glass that is cast into a stainless steel container. In the U.S., zinc borosilicate glasses have been developed. Other vitrification processes are being investigated in the U S . but are at an early stage of development. The only vitrification process currently in industrial operation is the French AVM process. According to the WHO report, the process produces a block of borosilicate glass weighing about 360 kg, which is cast into a stainless steel canister 0.5 m in diameter and 1 m high. The canister is sealed with a welded lid, decontaminated, and stored in steel-capped cylindrical cells in a concrete vault that has a concrete base. Typically, four steps are involved in the conversion of liquid waste to glass. These include: evaporation of the water and nitric acid, calcination of the nitrates to oxides, reaction of the oxides with added chemicals to form glass, and casting of the molten glass into a container. Critics of the use of glass as an immobilization medium point out that glass is in a metastable thermodynamic state, so that devitrification is always possible and could lead to a deterioration of the properties of the solidified waste. Other immobilization media that have been investigated but now abandoned in the U S . include the use of ceramic and concrete. Concrete
This technique has been used on an industrial scale at Idaho Falls in the U S . , where a fluid-bed calcination plant has been in operation since 1963. Several other processes for the development of improved immobilized waste forms are in progress. These alternatives also use calcining as the basis for the process. For example, in the U S . the “Supercalcine” process produces a calcine made by heating to 1100 OC a mixture made by adding 23% of an additional constituent such as lime. Other immobilization processes are being investigated, but are technically more complex, requiring higher manufacturing temperatures and extra mechanical operations. Many of the alternative immobilized waste “glass” forms are said to be superior to borosilicate glass for disposal because they are more durable, but they have not yet been thoroughly tested. At present the most advanced concept for the disposal of these wastes is isolation in mined geological repositories, but other alternatives are being investigated. These include subseabed disposal ( E S & T , January 1982, p. 28A) and delayed disposal by which spent fuel rods or canisters of solidified high-level waste are kept in pools or air-cooled vaults for a century or more while scientists decide on a permanent solution to the problem of high-level radioactive waste disposal. -Stanton Miller
