Trouble in Store -- The Wigner Effect
Stresses lurking in damaged reactors could
scupper plans to dismantle themPLANS 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 oxygen..
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.
http://www.newscientist.com/news/news_224334.html
Comments:Wigner energy effects also played a dominate role in Chernobyl as well. All forms of radiation can induce Wigner energy into materials. Wigner effects cause temperature release avalanches in materials. Graphite just happens to be one of the worst effects as the temp excursion in the presence of oxygen can set the graphite on fire like a very hot explosive charcoal grill and with that boils off all the fuel products rich in Sr-90, Cs-137, and others.
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