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How
They Store Deadly Nuclear Waste
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High-level
radioactive waste is lethal and it remains dangerous
for the 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 percent chance of living. A nuclear
reactor’s spend 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 meters) 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 is 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 heat as the radioactiveatoms decay,
so the tanks have to be cooled to prevent the liquid
boiling dry, which could eventually caused a radioactive
lea.
Cold water is pumped through coils inside the tanks.
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However, although they have
already been used for 40 years, tanks are also only a
temporary storage solution.
Possibly the best answer 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 2100oF (1500oC). 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. |
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The canisters are stored
in special ‘pits’ in a neighbouring building
at Marcoule. Each canister produces 1.5kilowatts
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 of 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 upto 1000 years before
they become corroded and let and 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 sited and sufficiently deep, several hundreds
meters, 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. |
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