Sunday, December 27, 2009

Waste Disposal methods / Waste management

In many countries, short lived waste is disposed of in near surface repositories, which are normally either lined trenches several metres deep or concrete 'vaults' constructed on or just below the ground surface. The disposed waste is covered with a few metres of earth, and often a clay cap to keep water out. A similar disposal method is used in some countries for the disposal of large amounts of NORM waste, such as tailings from the mining and milling of uranium. For example, Sweden operates a repository under the bed of the Baltic Sea at Forsmark for its more active (but mostly short lived) low/intermediate level waste.

Many low/intermediate level wastes do not occur in a form that is immediately suitable for disposal; they have to be mixed into an inert material such as concrete, bitumen or resin. In the past, some countries disposed of these wastes into the ocean, but since that has been prohibited by the London Convention, these wastes are normally stored awaiting decisions on the method of disposal. Among the most likely options is a reposi­tory deep underground in good geological conditions. Although many countries have plans for geological repositories of this type, only the USA is currently operating one, the Waste Isola­tion Pilot Plant (WIPP) in New Mexico, for wastes containing actinides.

Where the intention is to dispose of spent nuclear fuel directly rather than reprocess it, the spent fuel is stored, either at reactor sites or in special central facilities. This is partly to allow the fuel to cool, but clearly it must continue until a disposal facility is available. High level liquid waste from reprocessing operations is normally kept in special cooled tanks, but facilities to solidify it by incorporation in vitreous material are being built. The glass blocks will be stored for several decades to allow them to cool before eventual disposal, probably deep underground.


Decommissioning is the process that takes place at the end of the working life of a nuclear facility (or part of a facility), or any other place where radioactive materials were used, to bring about a safe long term solution. This might include decontaminating equipment or buildings, dismantling facilities or structures, and removing or immobiliz­ing remaining radioactive materials. In many cases, the ultimate objective is to clear the site of all significant radioactive residues, but this is not always possible or necessary.

To date, relatively few full scale commercial nuclear facilities have been completely decommissioned. However a great deal of experience has been gained from the decommissioning of a wide variety of facilities, including a few nuclear power plants, several prototype and research reactors, and many laboratories, workshops, etc. The fact that many nuclear reactors around the world are approaching the end of useful life has focused attention on the issues associated with decommissioning.

Decommissioning requires strict control of operations to optimize­ the protection of workers and the public. For dealing with the most radioactive parts of facilities, particularly reactor cores, remote handling techniques have been developed. Dismantling of large facilities also generates large volumes of 'waste'. Some of this will be Low/intermediate level radioactive waste and needs to be managed accordingly. However, there may also be large amounts of structural materials - such as steel and concrete - that are not significantly radioactive. Special procedures may be needed to 'clear' such materials as exempt, meaning that they do not have to be treated as radioactive waste.

Disposal criteria

There has been considerable discussion of the criteria to be used in judging the acceptability of waste disposal methods both from a radiological protection point of view and from the wider social perspective.

The first criterion would seem to be that people in future generations should be protected to the same degree as they would be at present. However, it is difficult to translate this requirement into practical standards of radiological protection. For example, activity may only emerge from a deep repository many thousands of years later, and we have no idea what the habits or ways of life of our descendants will be so far into the future.

A second requirement is to apply the principle that all exposures should be as low as reasonably achievable (ALAR) once economic and social factors have been taken into account. This means that the various options for managing a particular type of waste – including treatment, immobilization, packaging and disposal - should be compared on the basis of the associated risks, costs and other less quantifiable, but no less important factors. Some of this comparison will be within the scope of radiological protection, but other influences could determine the eventual decision.

The third criterion about waste disposal is what weight to give now to a mathematical probability of harmful effects for society in the distant future. This problem is not unique to waste disposal nor to radiological protection, although it is particularly pointed here. The most ethical answer may be to assume that present conditions persist and that harm to future generations is of equal importance as harm to this generation. This response must of course be tempered by the uncertainties of making predictions of potential effects centuries and millennia from now.

NORM (naturally occurring radioactive materials) waste consists of often very large amounts of waste containing fairly low concentrations of naturally occurring radionuclides (though these concentra­tions are often higher than those found in nature). This type of waste is generated in the mining and processing of uranium and other minerals, such as phosphates used in fertilizers; NORM waste is produced in mining and fertilizer processing

Alpha waste (or transuranic waste) - waste containing alpha emitting radionuclides such as isotopes of pluto­nium - is treated as a separate category in some countries; and High level waste refers only to spent fuel from a reactor (in countries where this is regarded as a waste) or to the highly active liquid produced when spent fuel is reprocessed. The volume of this type of waste is very low, but its activity is so high that it generates considerable heat.

Different countries classify wastes in different ways. but a number of general categories can be identified.

High-level Waste may be the spent fuel itself, or the principal waste 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 spent fuel is reprocessed, the separated waste is vitrified by incorporating it into borosilicate (Pyrex) glass which is sealed inside stainless steel canisters (as shown in figure) for eventual disposal deep underground.

On the other hand, if spent 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 spent fuel takes up about nine times the volume of equivalent vitrified high-level waste which results from reprocessing and which is encapsulated ready for disposal.

Both high-level waste and spent fuel are very radioactive and people handling them must be shielded from their radiation. Such materials are shipped in special containers which prevent the radiation leaking out and which will not rupture in an accident.

Whether 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 spent fuel for a typical large nuclear reactor. The relatively small amount involved allows it to be effectively and economically isolated.

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