High-level radioactive waste is lethal and it remains dangerous for thousands of years. If someone were to stand 30ft (9m) away from a small amount of fresh waste from a nuclear reactor for ten minutes, he would have only a 50 per cent chance of living. A nuclear reactor’s spent fuel contains a deadly cocktail of radioactive products, like plutonium, strontium and caesium.
Fortunately the volume of high-level nuclear waste is small. A typical plant, generating 1000 megawatts of electricity, produces about two and a half cubic yards (two cubic metres) of waste a year.
Storage methods vary. In the USA, some processed waste is stored in double-walled stainless-steel tanks surrounded by 3ft (1m) thick concrete cladding. But most is immersed in special pools near the nuclear plants, in the form of spent fuel rods still inside the original cladding. Unfortunately this is not a long-term solution.
In Britain the waste is stored as a liquid, the colour of strong tea, in steel tanks encased in concrete, similar to those used in America. The waste generates hear as the radioactive atoms decay, so the tanks have to be cooled to prevent the liquid boiling dry, which could eventually cause a radioactive leak. Cold water is pumped through coils inside the tanks.
However, although they have already been used for 40 years, tanks are also only a temporary storage solution.
Possibly the best answer at the moment is to fuse the waste into glass cylinders to be stored deep underground. A demonstration plant in Marcoule, France, has been carrying out this process since 1978.
The waste is dried and reduced to a solid residue by heating it inside a rotating drum. It is then mixed with silica and boron, and other glass-making materials, poured through a vertical chamber and heated to ( . A stream of molten glass emerges from the bottom, to be cast into stainless-steel containers about twice the size of an old-fashioned milk churn. A year’s output from a 1000 megawatt plant fills 15 of these canisters. After the glass has solidified, the lids are welded on.
The canisters are stored in special ‘pits’ in a neighbouring building at Marcoule. Each consider produces 1.5 kilowatts of heat and is cooled by air. The British and the Americans are also beginning to adopt this process. The waste is safe so long as it is monitored, but ultimately it should be put where it can remain without further human intervention.
One proposal is to surround the canisters with a jacket cast iron or copper, and then store them in underground caverns. The canisters would be placed in holes or trenches, then covered with concrete or a clay called bentonite, which absorbs escaping radioactive material.
The canisters should last up to 1000 years before they become corroded and let any radioactivity escape. After 500 years the radioactivity will have dropped to about the level of the original uranium ore. Experts believe that as long as the caverns are well suited and sufficiently deep – several hundred metres – it would take a million years before any material could seep to the surface, and by that time all but the tiniest traces of the radioactive waste would have decayed. The areas chosen for the ‘dumps’ should contain no valuable minerals; in case some future civilization should stumble across the waste while mining. Eventually the caverns could be sealed off and forgotten. The waste would be sealed behind so many barriers that escape in any imaginable time scale would be impossible.
The difficulty is finding sites where local people agree to have nuclear waste stored. Nobody relishes the idea of a nuclear dump close to their home. In the end, the nuclear waste authorities may well be forced to drill caverns beneath existing reprocessing facilities, or under the sea, rather than try to find new sites on land.
Picture Credit : Google