Chapter 9
Waste management

• Types of radioactive waste (radwaste)
• Waste Disposal methods
• Wastes from the nuclear fuel cycle
• Disposal stages of high level waste

The aims of waste management are to process the wastes in such a way as to make them suitable for storage and disposal, and to store or dispose of them so that there are no unacceptable risks to present and future generations. Here disposal implies simply that there is no intention to retrieve them, rather than that it would be impossible to do so.

We described the discharge of effluents from the nuclear fuel cycle, but there are also other radioactive wastes. These come not only from the various parts of the nuclear fuel cycle - from the mining and processing of uranium to the dismantling of old nuclear installations - but also from medical, industrial and research activities involving radioactive materials.

• Types of radioactive waste (radwaste)

Exempt waste contains such a low con­centration of activity that it does not need to be treated differently from ordinary non-radioactive waste;

Low-level Waste is generated from hospitals, laboratories and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters etc. which contain small amounts of mostly short-lived radioactivity. It is not dangerous to handle, but must be disposed of more carefully than normal garbage. Usually it is buried in shallow landfill sites. To reduce its volume, it is often compacted or incinerated (in a closed container) before disposal. Worldwide it comprises 90% of the volume but only 1% of the radioactivity of all radwaste.

Intermediate-level Waste contains higher amounts of radioactivity and may require special shielding. It typically comprises resins, chemical sludges and reactor components, as well as contaminated materials from reactor decommissioning. Worldwide it makes up 7% of the volume and has 4% of the radioactivity of all radwaste. It may be solidified in concrete or bitumen for disposal. Generally short-lived waste (mainly from reactors) is buried, but long-lived waste (from reprocessing nuclear fuel) will be disposed of deep underground.

Table (15): Radioactive waste classification

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• Waste Disposal methods:

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

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.

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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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• Wastes from the nuclear fuel cycle

Radioactive wastes occur at all stages of the nuclear fuel cycle, the process of producing electricity from nuclear materials. The cycle comprises the mining and milling of the uranium ore, its processing and fabrication into nuclear fuel, its use in the reactor, the treatment of the spent fuel taken from the reactor after use and finally, disposal of the wastes.

The fuel cycle is often split into two parts - the "front end" which stretches from mining through to the use of uranium in the reactor - and the "back end" which covers the removal of spent fuel from the reactor and its subsequent treatment and disposal. This is where radioactive wastes are a major issue.

Residual materials from the "front end" of the fuel cycle

The annual fuel requirement for a l000 MWe light water reactor is about 25 tonnes of enriched uranium oxide. This requires the mining and milling of some 50,000 tonnes of ore to provide 200 tonnes of uranium oxide concentrate (U₃O₈) from the mine.

At uranium mines, dust is controlled to minimise inhalation of radioactive minerals, while radon gas concentrations are kept to a minimum by good ventilation and dispersion in large volumes of air. At the mill, dust is collected and fed back into the process, while radon gas is diluted and dispersed to the atmosphere in large volumes of air.

Residual wastes from the milling operation contain the remaining radioactive materials from the ore, such as radium. These wastes are discharged into tailings dams designed to retain the remaining solids and prevent any seepage of the liquid. Eventually the tailings may be put back into the mine or they may be covered with rock and clay, then revegetated.

The tailings are around ten times more radioactive than typical granites, such as used on city buildings. If someone were to live continuously on top of the Ranger tailings they would receive about double their normal radiation dose from the actual tailings (ie they would triple their received dose).

With in situ leach (ISL) mining, dissolved materials other than uranium are simply returned underground from where they came.

Uranium oxide (U₃O₈) produced from the mining and milling of uranium ore is only mildly radioactive - most of the radioactivity in the original ore remains at the mine site in the tailings.

Turning uranium oxide concentrate into a useable fuel has no effect on levels of radioactivity and does not produce significant waste. First, the uranium oxide is converted into a gas, uranium hexafluoride (UF₆), as feedstock for the enrichment process.

Then, during enrichment, every tonne of uranium hexafluoride becomes separated into about 130 kg of enriched UF₆ (about 3.5% U-235) and 870 kg of 'depleted' UF₆ (mostly U-238). The enriched UF₆ is finally converted into uranium dioxide (UO₂) powder and pressed into fuel pellets which are encased in zirconium alloy tubes to form fuel rods.

Depleted uranium has few uses, though with a high density (specific gravity of 18.7) it has found uses in the keels of yachts, aircraft control surface counterweights, anti-tank ammunition and radiation shielding. It is also a potential energy source for particular (fast neutron) reactors.

Wastes from the "back end" of the fuel cycle

It is when uranium is used in the reactor that significant quantities of highly radioactive wastes are created. More than 99% of the radioactivity produced during the fission reaction is retained in the fuel rods. The balance is within the reactor structure.

About 25 tonnes of spent fuel is taken each year from the core of a l000 MWe nuclear reactor. The spent fuel can be regarded entirely as waste (as, for 40% of the world¹s output, in USA and Canada), or it can be reprocessed  (as in Europe). Whichever option is chosen, the spent fuel is first stored for several years under water in large cooling ponds (see chapter 8) at the reactor site. The concrete ponds and the water in them provide radiation protection, while removing the heat generated during radioactive decay.

The costs of dealing with this high-level waste are built into electricity tariffs. For instance, in the USA, consumers pay 0.1 cents per kilowatt-hour, which utilities pay into a special fund. So far more than US$ 18 billion has been collected thus.

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• Disposal stages of high level waste

Thus, to ensure that no significant environmental releases occur over periods of tens of thousands of years after disposal, a 'multiple barrier' disposal concept is used to immobilise the radioactive elements in high-level (and some intermediate-level) wastes and to isolate them from the biosphere. The principal barriers are:

• Immobilise waste in an insoluble matrix, eg borosilicate glass, Synroc (or leave them as uranium oxide fuel pellets - a ceramic)

• Seal inside a corrosion-resistant container, eg stainless steel.

• In wet rock: surround containers with bentonite clay to inhibit groundwater movement.

• Locate deep underground in a stable rock structure.

• Site the repository in a remote location.

For any of the radioactivity to reach human populations or the environment, all of these barriers would need to be breached before the radioactivity decayed.

If the spent fuel is later reprocessed, it is dissolved and separated chemically into uranium, plutonium and high-level waste solutions. About 97% of the spent fuel can be recycled leaving only 3% as high-level waste. The recyclable portion is mostly uranium depleted to less than 1% U-235, with some plutonium, which is most valuable.

Arising from a year's operation of a typical l000 MWe nuclear reactor, about 230 kilograms of plutonium (1% of the spent fuel) is separated in reprocessing. This can be used in fresh mixed oxide (MOX) fuel (but not weapons, due its composition). MOX fuel fabrication occurs at 5 facilities in Europe, with some twenty years of operating experience.

The 3% of the spent fuel which is separated high-level wastes amounts to 700 kg per year and it needs to be isolated from the environment for a very long time. These liquid wastes are stored in stainless steel tanks inside concrete cells until they are solidified.

1- Immobilising high-level waste

Solidification processes have been developed in France, UK, US and Germany over the past 35 years. Liquid high-level wastes are evaporated, mixed with glass-forming materials, melted and poured into robust stainless steel canisters which are then sealed by welding.

Borosilicate glass from the first waste vitrification plant in UK in the 1960s. This block contains material chemically identical to high-level waste from reprocessing. A piece this size would contain the total high-level waste arising from nuclear electricity generation for one person throughout a normal lifetime.

The vitrified waste from the operation of a 1000 MWe reactor for one year would fill about twelve canisters, each 1.3m high and 0.4m diameter and holding 400 kg of glass. Commercial vitrification plants in France, UK and Belgium produce about 1000 tonnes per year of such vitrified waste (2500 canisters) and some have been operating for more than 16 years.

Loading silos with canisters containing vitrified high-level waste in UK, each disc on the floor covers a silo holding ten canisters. Negatively pressurized compaction room, where dismantled pieces are compacted and placed in Specialized containers, as shown in the figure.

A more sophisticated method of immobilising high-level radioactive wastes has been developed in Australia. Called 'SYNROC' (synthetic rock), the radioactive wastes are incorporated in the crystal lattices of the naturally-stable minerals in a synthetic rock. In other words, copying what happens in nature. This process is now being tested in USA.

2- Final disposal of high-level waste is delayed to allow its radioactivity to decay. Forty years after removal from the reactor less than one thousandth of its initial radioactivity remains, and it is much easier to handle. Hence canisters of vitrified waste, or spent fuel assemblies, are stored under water in special ponds, or in dry concrete structures or casks for at least this length of time. (see chapter 8)

The ultimate disposal of vitrified wastes, or of spent fuel assemblies without reprocessing, requires their isolation from the environment for long periods. The most favoured method is burial in dry, stable geological formations some 500 metres deep. Several countries are investigating sites that would be technically and publicly acceptable. The USA is pushing ahead with a repository site in Nevada for all the nation¹s spent fuel.

One purpose-built deep geological repository for long-lived nuclear waste is in operation in New Mexico, though this only takes defence wastes.

After being buried for about 1,000 years most of the radioactivity will have decayed. The amount of radioactivity then remaining would be similar to that of the naturally-occurring uranium ore from which the fuel originated, though it would be more concentrated.

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