PLANS to decommission the wrecked reactor responsible for Britain鈥檚 worst
nuclear accident have been shelved because of fears that it could catch fire
again. The move has serious implications for the dismantling of other reactors,
such as Britain鈥檚 ageing Magnox plants and the damaged reactor at Chernobyl in
Ukraine.
The damaged British reactor is Windscale pile 1 at the Sellafield complex in
Cumbria. The problem is a type of energy called Wigner energy, which becomes
trapped in the reactor鈥檚 graphite moderator when neutrons dislodge carbon atoms
from their crystalline lattice. Wigner energy was the main cause of the accident
at Windscale in 1957.
Pile 1 started operation in 1950, making plutonium for Britain鈥檚 first
nuclear weapons. But after the accident, which spread radioactivity over
northern England, the reactor was shut down permanently.
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In 1997, the pile鈥檚 owners, the UK Atomic Energy Authority, hired a
consortium of British Nuclear Fuels (BNFL), Rolls-Royce and Nukem of Germany to
dismantle it by 2005 for a fee of 拢54 million. But BNFL engineers suspect
that some of the 15 tonnes of uranium fuel left in the pile have formed uranium
hydrides, which could ignite spontaneously in the presence of oxygen.
They say such a fire could cause a 鈥渞unaway release鈥� of the Wigner energy
trapped in the 2000 tonnes of graphite surrounding the core, which they say
would stoke the flames. The worst outcome could be a repeat of the 1957
conflagration in which temperatures soared to over 1200 掳C.
The consortium was planning to dismantle the reactor using remote
manipulators, while enveloping it in the inert gas argon to prevent the uranium
hydrides bursting into flames. But engineers are worried that pumping argon over
the reactor core could lead to an escape of radioactive gas because the pile鈥檚
concrete shield may not be airtight. Attempting to minimise leakage by reducing
argon pressure around the core would suck in oxygen and increase the fire risk.
The consortium has abandoned this plan for the time being and is considering
replacing the argon with water.
Barry Hickey, the UKAEA manager in charge of the decommissioning, is
disappointed at the delay. 鈥淲e recognise that the consortium has encountered
some difficulties in translating their original concept into a detailed design,鈥�
he says. 鈥淏ut there is no tearing hurry to get it done quickly. The important
thing is to get the best solution.鈥�
There could be similar risks, says industry newsletter Nucleonics
Week, when it comes to dismantling Chernobyl reactor 4, which was destroyed
by an explosion in 1986. The Ukrainian reactor also contains uranium compounds
and graphite. Britain鈥檚 first generation of Magnox nuclear power stations also
used graphite to moderate the nuclear reaction, so may also contain Wigner
energy. Four of them have already been closed, and BNFL announced last month
that the other seven would shut by 2021.
BNFL claims that there has not been a significant build-up of Wigner energy
in the Magnox reactors because their high operating temperatures return the
graphite crystal lattice to its original state through a process called
annealing. But John Large, an independent nuclear consultant, argues that
temperature variations within the reactors could have stored up enough Wigner
energy to release 21 megawatts of heat. 鈥淲igner energy is a problem,鈥� he told
快猫短视频.
