Waste Management in the Nuclear Fuel Cycle

• Nuclear power is the only energy-producing technology which takes full responsibility for all its wastes and fully costs this into the product.

• The amount of those wastes is very small relative to the wastes produced by fossil fuel electricity generation.

• Two different strategies are utilised for managing high-level wastes.

• The radioactivity of all nuclear wastes diminishes with time.

• Safe methods for the final disposal of high-level waste are technically proven but have so far not been required.

Uranium mining generates fine sandy tailings, which contain virtually all the naturally occurring radioactive elements found in the uranium ore. These are emplaced in engineered tailings dams and finally covered with a layer of clay and rock to inhibit leakage of radon gas from them and to ensure their long term stability. In the short term, the tailings material is often covered with water.

At each stage of the fuel cycle there are proven technologies to safely dispose of the radioactive wastes but in some cases they have not have been implemented due to public acceptance problems or because they are not presently required.

The radioactivity of all nuclear waste decays with time. All radionuclides contained in nuclear waste have a half-life - a term which refers to the time it takes for any given radionuclide to lose half of its radioactivity. Thus radioactive waste eventually decays into non-radioactive elements. The more radioactive a particular isotope, the faster it decays.

The basic objective in the management and disposal of radioactive waste is the protection of people and the environment. This means achieving sufficient isolation or dilution of the waste so that any return of radionuclides to the biosphere is at a rate or concentration which is innocuous. Some wastes therefore need deep and secure burial.

In the European Community 160Ê000 tonnes of radioactive waste of all kinds is produced each year (about 100 container loads after treatment), compared with 20 million tonnes of toxic chemical waste (which does not become less hazardous with time).

TYPES OF RADIOACTIVE WASTE

Low-level Waste is generated from hospitals 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 does not require shielding during handling and transport and is suitable for shallow land burial. To reduce its volume, it is often compacted or incinerated before disposal.

Intermediate-level Waste contains higher amounts of radioactivity and some requires shielding. It typically comprises resins, chemical sludges and metal fuel cladding, as well as contaminated materials from reactor decommissioning. It may be solidified in concrete or bitumen for disposal. Generally short-lived waste (mainly from reactors) is buried in a shallow repository, but long-lived waste (from fuel reprocessing) will be disposed of deep underground.

High-level Waste arises from the use of uranium fuel in a nuclear reactor. It contains the fission products and transuranic elements generated in the reactor core. It is highly radioactive and hot. It is like the "ash" from "burning" uranium. The high-level waste accounts for over 95% of the total radioactivity produced in the process of nuclear electricity generation.

CONVERSION, ENRICHMENT AND FUEL FABRICATION

The uranium oxide concentrate from mining is not significantly radioactive, - barely more so than the granite of Australia's Parliament House. It is refined, then converted to uranium hexafluoride gas so that it can undergo enrichment of the U-235 content from 0.7% to about 3.5%. It is then turned into an oxide (UO2) for assembly as reactor fuel elements.

The main by-product of enrichment is depleted uranium, principally the U-238 isotope, which is stored. Some is used in applications where its extremely high density makes it valuable, eg the keels of yachts. It can also be used to dilute highly-enriched uranium from weapons stockpiles now being redirected to reactor fuel.

MANAGEMENT of HIGH-LEVEL WASTES from SPENT FUEL

Spent fuel gives rise to high-level waste which may be either:

• the spent fuel itself in fuel rods, or
  
• the principal waste arising from reprocessing this.

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

• Immobilise waste in an appropriate matrix such as borosilicate glass or synthetic rock (fuel pellets are already a very stable ceramic: UO2);
• Seal inside a corrosion-resistant container, such as stainless steel;
• Locate deep underground in a suitable and preferably dry repository with long-term geological stability; &
• Surround containers with an impermeable backfill such as bentonite clay.

The high-level waste from reprocessing UK, French, Japanese and German spent fuel is largely liquid. It consists of the highly-radioactive fission products and some transuranic elements with long-lived radioactivity. It generates a considerable amount of heat and requires cooling. This is vitrified into borosilicate (Pyrex) glass, encapsulated into heavy stainless steel cylinders about 1.3 m high and stored 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. After one year the heat and radioactivity drops to one tenth the level at removal, and after 40 years it has dropped by a further factor of 10, providing a technical incentive to delay disposal until this low level of about one percent of original radioactivity is reached.

After storage for about 40 years they are ready for encapsulation and permanent disposal underground. This direct disposal option is the US and Swedish policy, though in the latter case it will be recoverable if future generations come to see it as a resource.

Increasingly, reactors are starting off with fuel enriched to over 4% U-235 and burning it longer, to end up with less than 0.5% U-235 in the spent fuel, which provides less incentive to reprocess.

RECYCLING FUEL

Any spent fuel still contains some of the original U-235 as well as much of the plutonium which has been formed in the reactor; in total some 96% of the original uranium and over half of the original energy content (ignoring U-238). Reprocessing, such as undertaken in Europe, separates this uranium and plutonium from the actual wastes so that they can be recycled for use in a nuclear reactor as a mixed oxide fuel. This is the Òclosed fuel cycleÓ.

(This is very much what is to happen with the tiny quantities of spent fuel from the Australian research reactor at Lucas Heights near Sydney. Some of this spent fuel has been returned to UK for reprocessing, and the small amount of separated waste will eventually be returned to Australia for disposal as intermediate-level waste.)

The plutonium arising from reprocessing commercial fuel, though only about one percent of the spent fuel, is recycled through a mixed oxide (MOX) fuel fabrication plant where it is mixed with uranium oxide in fresh fuel. European reactors currently use over five tonnes of plutonium per year in fresh MOX fuel, though all nuclear power reactors routinely burn much of the plutonium which is continually formed within them by neutron capture. The use of MOX simply means that some plutonium is incorporated as part of the fresh fuel. (Plutonium arising from the civil nuclear fuel cycle is not suitable for bombs because it contains far too much of the Pu-240 isotope, due to the length of time the fuel has been in the reactor.)

Major commercial reprocessing plants are operating in France and UK, with capacity of almost 4700 tonnes per year and cumulative civilian experience of 55,000 tonnes over 40 years. These also undertake reprocessing for utilities in other countries, notably Japan, which has made over 140 shipments of spent fuel to Europe since 1979. At present most Japanese spent fuel is reprocessed in Europe, with the vitrified waste and the recovered U and Pu being returned to Japan to be recycled as fuel. In future the plutonium will be returned as mixed oxide (MOX) fuel elements.

DISPOSING OF HIGH-LEVEL WASTES

France is furthest ahead with preparation for disposal of high-level waste. In 1989 and 1992 it commissioned commercial plants to vitrify high-level waste following reprocessing of oxide fuel, though facilities exist elsewhere, notably UK and Belgium. Capacity of these western European plants is 2500 canisters (1000 t) per year and some have been operating for 16 years.

The Australian Synroc (synthetic rock) is a more sophisticated way to immobilize such waste, and this process may eventually come into commercial use.

Sweden is well advanced with plans for direct disposal of spent fuel, its Parliament having decided that this, using existing technology, is acceptably safe. The US has opted for an interim central storage facility and a final repository in Nevada.

The process of selecting appropriate deep final repositories is now underway in several countries with the first expected to be commissioned in the first decades of the next century. To date there has been no practical need for final repositories for high-level waste, as surface storage for 30-50 yrs is first required for heat and radioactivity to dissipate to facilitate final disposal.

For further information see Waste item on UI site.

The following table indicates the measures that various countries have in place or planned to store, reprocess and dispose of nuclear wastes. It is not comprehensive.

COST OF RADIOACTIVE WASTE MANAGEMENT

Financial provisions are made for the management of civilian radioactive waste of all kinds. The costs of managing and disposing of wastes from nuclear power plants represent about 5% of the total costs of electricity generated.

Many nuclear utilities are required by governments to put aside a levy (eg 0.1 cents per kilowatt hour in USA) to provide for management and disposal of wastes.

NATURE'S OWN NUCLEAR WASTE REPOSITORY

Geological disposal of radioactive materials is congruent with natural processes. Nature has already proven that geological isolation is possible through several natural examples (or "analogues").

The most significant case occurred almost 2 billion years ago at Oklo in what is now Gabon in West Africa, where six spontaneous nuclear reactions occurred within a rich vein of uranium ore. (At that time the concentration of U-235 in all natural uranium was about 3%.) These natural nuclear reactors continued for about 500 000 years before dying away. They produced all the radionuclides found in high-level waste, including over 5 tonnes of fission products and 1.5 tonnes of plutonium.

The radionuclides remained at the site and eventually decayed into non-radioactive elements. The study of such natural phenomena is an important component in the assessment of geologic repositories and is the subject of several international research projects. However, it must be noted that the Oklo reactions proceeded because groundwater was present as a moderator in the "enriched" and permeable uranium ore. An important criterion for artificial repositories is that they are likely to stay dry.

SOURCES:
OECD NEA, 1996, Radioacvtive waste Management in Perspective

IAEA ,1992, Radioactive Waste Management An IAEA Source Book

Nukem ,1994, Status of National Reprocessing and Waste Management Programs 1993, Report 2/1994.

Uranium Institute ,1991, The Management of Radioactive Waste.

Uranium Institute ,1992, Radioactive Waste and the Nuclear Fuel Cycle.