RE: Graphite Quiz: Re: Chapelcross -(3)

[julian.ginniver@ukaea.org.uk]

While there is no argument that during the accidents at Windscale and

Chernobyl there was combustion of  graphite material, I would draw

attention to the comment made by Paul Shafer.  "It requires very high

graphite internal heat".  In both cases there were much higher temperatures

present in the reactors than would have existed within the normal operating

regime.  While there is still some debate about the actual cause of the

fire at Windscale, it either began with the overheating of the fuel or

possible the overheating of a lithium cartridge which was also present in

the reactor it was not the graphite that began to burn first.  The cause of

the additional heat in Chernobyl is well known.  My understanding is that

the graphite temperatures in Magnox reactors can be in the order of 900 to

1000 deg C and in AGRs they are higher at 1100 to 1200 Deg. C.  The only

operational issues with operating graphite at these temperatures is the

reaction of the graphite with the CO2 and some loss of moderator to the gas

which then plates out at a cooler part of the circuit (i.e. on the

Boiler/Steam generator tubes).  This problem manly affects the AGRs as the

rate of the reaction is only significant at the higher temperature.  The

problem was identified before the construction of the AGRs began, and it

has been resolved by the addition of controlled amounts of methane to the

reactor gas.  It has for a long time been considered a possible life

limiting factor for these reactors, and the loss of heat transfer in the

boilers has also had an impact on the generating capacity of some stations.

There is a need for very high purity graphite for use in reactors, and this

was an area where much work took place as part of the power reactor

programme.  Too many impurities in the graphite can prevent a reactor from

achieving criticality.



Without wishing to descend into a situation of "our are better than yours"

I would just like to point out that when any reactors go outside their

operating regime there are many issue which could lead to a release of

fission products to the environment. E.g.  I believe that the zircalloy

cladding of PWR fuel will react with water at high temperatures leading to

the production of hydrogen and the potential (see TMI) of a "hydrogen burn"

and pressure transient within the containment building.



What should be remembered is that regardless of what is taking place within

the fuel and reactor, to reach the environment there must be a release

path.  In both the Windscale and Chernobyl accidents there was a direct

path to the environment once the accident had taken place.  Although the

presence of filters "Cockroft's follies" on the stacks at Windscale did

reduce the magnitude of the release.  The only release from TMI being of

short duration during the planned venting of the containment.  The presence

of robust containment being the key to preventing a release.



Apologise once again for the length of this, but being new to the radsafe

list I don't seem to have mastered the ability to keep my ramblings short.



Regards

     Julian Ginniver



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