Re: Chapelcross reactor-further info

[julian.ginniver@ukaea.org.uk]

Dear all,

     Chapelcross and Calder Hall were the first Magnox reactors in the UK.

Their purpose was to supplement the production of Plutonium undertaken at

the Windscale Piles at Sellafield.  The Windscale piles were air cooled,

and because of the short time given for design and construction the heat

produced from the nuclear reaction was simply rejected to the atmosphere.

The Windscale piles were closed following the fire which occurred during

the annealing of one of the piles to release Wigner energy that had built

up during operations.  The purpose of the Calder Hall and Chapelcross

reactors was to assist in the production of plutonium, but also to

demonstrate the feasibility of Nuclear Power.  As has already been

identified the true series of Power reactors began with Berkeley power

station in Gloustershire.  The reasons, as I understand them, for the

closure of the 4 of the Commercial Magnox reactors are as follows.



Berkeley was closed because the UK Nuclear Installations Inspectorate NII

(UK version of the NRC) required improvements to be carried out to the fuel

route at the plant.  The cost of these improvements was estimated to be

greater than the potential return from operating the reactor for it's

remaining lifespan.



Trawsfynydd, situated on an inland man made lake in North Wales was shut

down for a routine outage/overhaul in 1991.  During the outage steel

samples were recovered form the reactor to assess the effects of neutron

irradiation on the steel pressure vessel.  The results of the analysis

indicated higher than expected levels of neutron embrittlement of the

steel.  the reactor remained shutdown for an extended period while the

operator undertook modifications to the boilers/steam generators to

increase the temperature of the gas returning to the reactor.  This would

have increased the temperature around the welds where the duct joined the

penetration and would have restored the ductile properties of the steel.

It was the intended to submit a safety case to the NII for restart.  At

this time the decision was taken that the station would be uneconomic to

run over its remaining operational life.  The modifications to the boilers

resulted in a significant loss of electrical output.



Hunterston A, located on the west coast of Scotland was owned and operated

by the South of Scotland Electricity Board SSEB.  I'm not sure of the

reason for the closure of this station but belive that it was due to excess

generating capacity within Scotland.  SSEB also operated two large Advanced

Gas Reactor Stations (AGRs) and these are able to supply approximately 40%

of the generating needs of Scotland.



In addition to those stations identified already, a  fourth Magnox Reactor

was closed in 2001.  This was Hinkley Point A in Somerset.  This reactor

was shut down for a routine outage.  All reactors in the UK need approval

from the NII before they can recommence operation.  Before approving the

restart of this reactor the NII required further work to be undertaken to

demonstrate the safety of the reactors (each commercial Magnox site having

two reactors).  Given the uncertainty over the length of time it would take

to gain approval from the NII a decision was taken that this would be too

great a commercial risk and the operator BNFL Magnox Generation announced

that it would be closed.



At this time BNFL Magnox announced the closure dates for the remaining

reactors.  There remains some uncertainty around the closure dates for

Oldbury and Wylfa, because these have concrete pressure vessels, unlike the

first seven plants which had steel pressure vessels.  For commercial plants

the life limiting factor was the steel pressure vessel.  The concrete

pressure vessel stations should be able to operate for longer periods,

however the Magnox fuel cannot be stored for extended period, either in

water filled ponds, or in on site dry fuel stores.  The fuel has to be

reprocessed at Sellafield.  However the reprocessing plant is due to be

shut down sometime around 2010 and there are other issues at the Sellafield

site that will prevent the reprocessing of Magnox fuel around this time.

The operator was intending to operate the last two reactors with a modified

fuel of slightly enriched uranium with a different fuel clad material.

This would then have been reprocessed through THORP.  Once again however,

commercial aspects have stopped this.  It was felt that the commercial and

financial risk that this would entail would be greater than the potential

return.  The long term future of these two reactors continues to remain in

doubt.



And finally, as I understand it,  the reason why the Calder Hall and

Chapelcross reactors have been given approval for 50 years of operation

(compared to 40 for the majority of the commercial stations) is that as the

earliest of this design, they are relatively low power, low efficiency

units and this has meant that many of the life limiting factors that

applied to the commercial plants have not been a problem.



Julian Ginniver



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