Stresses In Damaged Reactors Could Effect Decommissioning
Trouble in Store -- The Wigner Effect
Stresses lurking in damaged reactors could scupper plans to dismantle them

PLANS to decommission the wrecked reactor responsible for Britain's 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's aging 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's 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's first
nuclear weapons. But after the accident, which spread radioactivity over
northern England, the reactor was shut down permanently.

In 1997, the pile's 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 £54 million. But BNFL engineers suspect
that some of the 15 tons of uranium fuel left in the pile have formed
uranium hydrides, which could ignite spontaneously in the presence of

How is Wigner energy created?
· High-energy neutrons strike graphite moderator in reactor core
· Kinetic energy of neutrons is large compared to binding energy of atoms in
crystal alttice.
· Atoms set in motion by strong collisions.
· Lattice becomes heated and elastically deformed, storing Wigner energy.
· Energy stored in lattice can be released as heat as lattice returns to
original shape.

They say such a fire could cause a "runaway release" of the Wigner energy
trapped in the 2000 tons 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 °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's concrete shield may not be airtight. Attempting to
minimize 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. "We recognize that the consortium has encountered
some difficulties in translating their original concept into a detailed
design," he says. "But there is no tearing hurry to get it done quickly. The
important thing is to get the best solution."

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's 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. "Wigner energy is a problem,"
he told New Scientist.

Rob Edwards

From New Scientist magazine, 17 June 2000.