Uranium oxide concentrate from mining is not significantly radioactive - barely more so than the granite used in buildings. It is refined to form "yellowcake" (U3O8), then converted to uranium hexafluoride gas (UF6). As a gas, it undergoes enrichment to increase the U-235 content from 0.7% to about 3.5%. It is then turned into a hard ceramic 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, either as UF6 or as U3O8. Some is used in applications where its extremely high density makes it valuable, such as the keels of yachts. It is also used (with recycled plutonium) for making mixed oxide fuel (see below) and to dilute highly-enriched uranium from weapons stockpiles which is now being redirected to become reactor fuel.
MANAGING HLW FROM SPENT FUEL
HLW from reprocessing UK, French, Japanese and German spent comprises 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 metres high and stored for eventual disposal deep underground.
But if spent reactor fuel is not reprocessed, it will still contain all the highly radioactive isotopes, and then the entire fuel assembly is treated as HLW. After 40-50 years the heat and radioactivity have fallen to one thousandth of the level at removal. This provides a technical incentive to delay disposal until a low level of about 0.1% of the original radioactivity is reached.
After storage for about 40 years the spent fuel assemblies are ready for encapsulation and permanent disposal underground. Direct disposal has been chosen by the US, Switzerland and Sweden, although in Sweden it will be recoverable if future generations come to see it as a resource.
Increasingly, reactors are using 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. This provides less incentive to reprocess.
Any spent fuel will still contain some of the original U-235 as well as various plutonium isotopes which have been formed inside the reactor core. In total these account for some 96% of the original uranium and over half of the original energy content (ignoring U-238). Reprocessing, undertaken in Europe and Russia, separates this uranium and plutonium from the wastes so that they can be recycled for re-use in a nuclear reactor as a mixed oxide (MOX) fuel. This is the "closed fuel cycle."
Plutonium arising from reprocessing comprises only about 1% of commercial spent fuel. It is recycled through a mixed oxide (MOX) fuel fabrication plant where it is mixed with depleted uranium oxide to make fresh fuel. European reactors currently use over 5 tonnes of plutonium a year in fresh MOX fuel, although all reactors routinely burn much of the plutonium which is continually formed in the core by neutron capture. The use of MOX simply means that some plutonium is incorporated into fresh fuel. (Plutonium arising from the civil nuclear fuel cycle is not suitable for bombs. It contains far too much of the Pu-240 isotope because of the length of time the fuel has spent in the reactor.)
Major commercial reprocessing plants operate in France and the UK, with a capacity of some 4,700 tonnes a year and cumulative civilian experience of 60,000 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 (as MOX) being returned to Japan to be recycled. Russia also reprocesses some of its spent fuel as well as fuel from Soviet-designed reactors in other countries.
COST OF RADIOACTIVE WASTE MANAGEMENT
Financial provisions are made for managing all kinds of civilian radioactive waste. The cost of managing and disposing of nuclear power plant wastes represents about 5% of the total cost of the electricity generated.
Many nuclear utilities are required by governments to put aside a levy (eg 0.1 cents per kilowatt hour in the USA) to provide for management and disposal of wastes. So far some US$ 18 billion had been committed to the US waste fund by electricity consumers.
DISPOSING OF HIGH-LEVEL WASTES
In 2001 there was about 250,000 tonnes of spent fuel in storage, much of it at reactors. Annual arisings of spent fuel are about 12,000 tonnes, and 3000 tonnes of this goes for reprocessing. Final disposal is therefore not urgent in any logistics sense.
France is furthest ahead with preparation for HLW disposal. In 1989 and 1992 it commissioned commercial plants to vitrify HLW left over from reprocessing oxide fuel, although there are adequate facilities elsewhere, notably in the UK and Belgium. The capacity of these western European plants is 2,500 canisters (1000 t) a year, and some have been operating for two decades.
Loading silos with canisters containing vitrified high-level waste in UK, each disc on the floor covers a silo holding ten canisters
The Australian Synroc (synthetic rock) is a more sophisticated way to immobilize such waste, and this process may eventually come into commercial use for civil wastes (it is curently being developed for US military wastes).
The process of selecting appropriate deep final repositories is now under way in several countries with the first expected to be commissioned some time after 2010. Sweden is well advanced with plans for direct disposal of spent fuel, since its Parliament decided that this is acceptably safe, using existing technology. The US has opted for a final repository in Nevada. There is also a proposal for an international HLW repository in optimum geology - Australia or Russia are possible locations.
To date there has been no practical need for final HLW repositories, as surface storage for 30-50 years is first required so that heat and radioactivity can dissipate to levels which make handling easier.
SOURCES: OECD NEA, 1996, Radioacvtive waste Management in Perspective IAEA ,1992, Radioactive Waste Management An IAEA Source Book, & IAEA Bulletin 40,1; 1998, ENS NucNet, UI, Uranium Information Centre, Ltd