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Windscale Pile problems

Nuclear Monitor Issue: 
#532
5188
27/06/2000
Article

(June 27, 2000) Decommissioning work on UK's Windscale Pile No 1, subject of one of the biggest nuclear disasters in the world (the 1957 fire), has been halted for a re-appraisal to be made as to how best remove around 15 tonnes of fire damaged fuel from the pile.

(532.5188) CORE - The Anglo-German consortium, headed by British Nuclear Fuels (BNFL), who were awarded the BP54 million (US$81 million) contract in 1997, intend to dismantle the reactor using remote manipulators, but cannot decide whether to use argon gas or water to prevent fire breaking out in the old fuel, much of which is suspected of being in uranium hydride form as a result of the 1957 fire.

Windscale Piles 1 & 2 were built in the late 1940's to produce military plutonium. Each with a thermal power rating of 180 MW, the piles were graphite moderated and air-cooled. No 1 Pile went critical in 1950 and operated until 1957 until its core overheated, causing the fire which damaged around a quarter of the core and contaminated areas of the UK and Europe. Fire temperatures reached over 1,200 degrees C. Both Piles were immediately closed down and decommissioning work has been ongoing since the mid 1980's.

WIGNER ENERGY
Concern about Wigner Energy goes almost all the way back to the beginning of the nuclear industry in Britain. To sustain nuclear fission in a reactor the neutrons which emerge at a speed of about 6000 miles per second are slowed down or 'moderated' to about one mile per second. This greatly increases the probability that they will produce a fission in a further Uranium-235 nucleus. In Britain the decision was made very early on to use graphite as the moderator. When fast neutrons collide with the carbon atoms of the graphite two things happen; firstly, they are slowed down by transferring their energy to the carbon atoms of the moderator, and secondly, part of this transferred energy is released as heat and part of it causes a displacement of the carbon atoms in the graphite crystal lattice. The graphite changes its shape and 'locks up' some of the collision energy; this is Wigner Energy.
In 1952, a spontaneous release of Wigner energy occurred whilst Windscale Pile No.1 was shut down. This alarmed the operators who then drew up a programme for the controlled release of this energy at regular periods. It was the misreading of thermocouples during one such 'controlled' release that precipitated the Windscale fire in 1957. Initially it was thought that the graphite moderated Magnox reactors would also require the periodic controlled release of Wigner energy. It was stated in 1962 that Calder Hall and the other Magnox reactors "would require a Wigner release in five years". In 1967 a reactor at Chapelcross experienced a meltdown in circumstances which have never been fully explained. The accident occurred at the time of restarting. It was later claimed that Magnox reactors operated at temperatures high enough to obviate the need for such measures. There is however, a suspicion that the 'convection cooling test' that the Central Electricity Generating Board (CEGB) wanted to carry out at Trawsfynydd in 1988 was linked to the need to release the Wigner energy that had built up. That test was abandoned as it bore too many similarities to the experiment that destroyed Chernobyl two years earlier.
WANA News, June 2000

The consortium has recently reviewed its plan to use an argon 'blanket' to smother pyrophoric materials in an effort to prevent their ignition and the possible run-away release of Wigner energy--the cause of the 1957 fire. Wigner energy, trapped in the reactor's graphite moderator when neutrons dislodge carbon atoms from their crystalline lattice, can later be released as heat. The core of Pile No 1 is estimated to contain 1300 cubic metres (approx 2,000 tonnes) of graphite.

Admitting that several problems on the use of argon had materialised, including technical, financial and supply constraints, the consortium is now considering the use of water instead of argon. At a recent Lake District conference, a spokesman for BNFL Engineering Ltd said "operating the core under a head of argon could lead to gas leaks from the core (because the pile's concrete shield may not be airtight), but an in-core depression (to reduce leakage) would increase the risk of core fire."

He admitted that there had been no means of determining the core's condition since the 1957 fire and that the full extent of the damaged zone was unknown. With the possibility that many fuel channels became sealed during the fire, thus preventing exposure of uranium hydride to air, he added "so if core dismantling is carried out in an air environment, ignition of the uranium hydride could occur and the ignition could then ignite a uranium metal fire. If that were to propogate to other flammable materials in the core, it is possible that the bulk temperature of the surrounding graphite could be raised sufficiently to cause a runaway Wigner energy release."

Other industry sources have said that the idea of water flooding has the advantage of ensuring temparatures were kept down to ambient, thus preventing a Wigner release. Borated water would provide the additional benefit of preventing criticality. Prior to water use however, a comprehensive programme of sealing cracks and openings in the bio-shield would have to be completed, the openings ranging from charge holes and control and shutdown rod penetrations to equipment access hatches and a void where the reactor was linked with the stack.

Dismantling of the pile was originally scheduled for completion in 2005. Currently an air-extraction system is operating on Pile No 1 to remove fuel decay heat via a two-stage filtering process, while maintaining the core at sub-atmospheric pressure. The consortium includes BNFL, Rolls Royce Nuclear Engineering Services and Nukem Nuklear GmbH and the project is funded by the Ministry of Defence and the DTI. International decommissioing experts view the project as second only in complexity to that of dismantling Chernobyl 4, also a graphite moderated reactor.

Sources:

  • New Scientist, 17 June 2000
  • Nucleonics Week, 18 May 2000

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