Nuclear power stations represent an efficient means of producing electricity, and they produce no greenhouse gases. But toxic radioactive waste is left over from the process of nuclear fission, and this presents a problem. In the fifty years or so since we started using nuclear fission reactors, a lot of dangerous waste material has built up.
Why is There So Much Nuclear Waste?
Not all radioactive waste comes from nuclear reactors. Some is produced by hospitals, where various procedures involving radiation are carried out; and in some countries, there is a legacy of waste from nuclear weapons programmes. But most of the waste is the result of nuclear power generation. In fact, nuclear power stations do not produce a tremendous amount of waste material in comparison to fossil fuel power stations; but because of the difficulties inherent in disposing of this material, it has accumulated.
Nuclear reactors are powered by bundles of fuel rods containing pellets of uranium. Not all the uranium will take part in the reaction. Only the isotope uranium-235 is fissile, or burnable, and this isotope accounts for a small proportion of the uranium in the fuel rods; most of it is uranium-238. The fusion reaction will only continue for as long as the fuel contains a certain minimum level of uranium-235. Over time, the level drops.
When the fuel is ‘spent’, the fuel rods are taken out and replaced. But although these spent fuel rods no longer contain enough uranium-235 to fuel fission, most of the uranium-238 and a small proportion of the uranium-235 still remain, and the spent fuel is highly radioactive.
It is estimated that in the US, somewhere in the region of 60,000 metric tons of spent nuclear fuel has been produced over the years.
Disposal of Nuclear Waste
This material will remain radioactive for literally thousands of years.
Quite apart from the dangers of contamination, another issue is that nuclear waste could be used to produce nuclear weapons. There are therefore very real security risks involved.
One approach to disposing of nuclear waste is to bury it permanently. This is sometimes called ‘geological disposal’. It has to be buried in a purpose-built repository at a depth of between two hundred and a thousand metres, somewhere where there is no danger of it being disturbed by human or natural intervention – either by terrorists looking for weapon material, or by earthquakes or flooding. Finding a suitable site to bury nuclear waste is not easy.
Reprocessing and Recycling of Nuclear Waste
It is possible, however, to reprocess nuclear waste and separate out uranium and plutonium, which can be recycled.
France is generally regarded as the leader in nuclear waste reprocessing. It is the only country that has an established, large-scale, successful nuclear waste reprocessing facility, although other countries do reprocess to some extent. The process used at La Hague, in Normandy, is an advanced version of the Purex (‘plutonium-uranium extraction’) process developed in the 1950s. The waste is broken down, and ends up as a supply of two separate re-usable elements, plus sealed canisters of toxic material.
The temperature of the spent fuel bundles arriving at the reprocessing plant is around 300 degrees Celsius, and they are left to cool for up to five years before being chemically processed. Processing extracts uranium, which is stored because at present it is not considered economic to re-use it. A small quantity of plutonium is also extracted; this is sold, for re-use in blending nuclear fuel.
The chemicals and solutions used in the Purex process also have to be disposed of, and these are very dangerous as they consist mainly of acids and extremely radioactive spent fuel residues. This mixture is vitrified – which means that it is encapsulated inside borosilic glass, a strong and non-fragile substance (like Pyrex) – and sealed in a metal canister. In this way the toxic material is effectively isolated from the environment and relatively safe to handle. Each canister is less than one-and-a-half metres high, with a diameter of less than half a metre, and a large reactor would produce around 20 of these canisters a year.
The arguments in favour of reprocessing nuclear waste are that the quantity of toxic, unusable material is greatly reduced; it is released in a manageable package that is convenient to transport; and the intrinsic value of those elements of spent fuel that can be re-cycled is recognised. On the other hand, reprocessing poses certain dangers in that it involves more transportation and handling of the waste material, and there are concerns that plutonium, once separated out, could be hijacked for terrorist activities. So opinions are divided; but it is clear that we need a strategy for dealing with nuclear waste, especially if more nuclear power stations are to be built in the future.