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 con- templated with the exception of envi- ronmental 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 aque- ous waste results from the solvent ex- traction 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 ra-

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 medi- cal services.

In brief. there are three main dioactivity intense enough to generate sienificant auantilies of hear bv decav

methods for the management of hieh-level radioactive waste resultine

i s h o classi’fied as high-level waste. 1; addition, in countries where repro- cessing is not envisioned, the spent fuel from the reactor is classified as high- level waste.

These details are described in a publication of the World Health Or- ganization (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

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 stow- away cycle defers the problem of treatment and ultimate disposal; it does not offer a final waste manage- ment solution. At first sight, the throw-away cycle appears simple be- cause it has fewer processing stages, but its drawback is that it does not re- cover uranium or plutonium for future energy generation. On the other hand, reprocessing with vitrification-the changing of the waste into a glass or a

glassy substance by heat and fu- sion-does meet many of the require- ments of modern waste management; it ensures that the waste form is suit- able for disposal, that the amount of plutonium disposed of in the environ- ment 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 place- ment of these wastes in vitrified form in suitable geological formations has received the most attention.

Such wastes can be stored in water- or 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 extrac- tion for military purposes would re- quire sophisticated and elaborate equipment, which is unlikely to be widely available.

In reprocessing, uranium and plu- tonium are separated from the fission products by putting a nitric acid solu- tion of the fuel in contact with an im- miscible solvent such as tributyl phosphate in an organic diluent. Re- ferred 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 elec- tricity. Most of the high-level waste that has been produced by the repro-

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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 re- port 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 immobil- ization of high-level radioactive waste, converts the liquid waste into a block

is cheaper than glass but has similar properties. Ceramics are more expen- sive than the other forms but can in- corporate 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 solidifi- cation process for high-level radioac- tive 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, pro- ducing 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 de- velopment. The only vitrification pro- cess currently in industrial operation is the French AVM process. According to the WHO report, the process pro- duces 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, decontami- nated, and stored in steel-capped cy- lindrical 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 ni- tric acid,

calcination of the nitrates to ox- ides,

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 im- mobilization medium point out that glass is in a metastable thermody- namic 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 de- velopment of improved immobilized waste forms are in progress. These al- ternatives 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 man- ufacturing temperatures and extra mechanical operations. Many of the alternative immobilized waste “glass” forms are said to be superior to boro- silicate glass for disposal because they are more durable, but they have not yet been thoroughly tested.

At present the most advanced con- cept for the disposal of these wastes is isolation in mined geological reposi- tories, but other alternatives are being investigated. These include subseabed disposal (ES&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