Saturday, April 9, 2011

Did the Graphite in the Windscape Reactor Burn?

It is not entirely clear what happened in the Windscale reactor fire. The Windscale reactor has not yet been entirely dismantled, so how much damaged was done to its core graphite is still open to question. There is little doubt that metals in the core - Aluminum and Uranium - burned. The hastily prepared 1957 Penney report stated that the Windscale core graphite caught fire, but did it? When The UK Nuclear Safety Advisory Committee (NuSAC) meet in 2009 to examine evidence from the Windscale reactor, it found,
• Inspections have shown that there was NOT a graphite fire: damage to graphite, caused by severely overheated fuel assemblies, was localised.
The NuSAC report added,
Some members of NuSAC may be aware that recent attempts to burn GLEEP graphite (block form) in a high temperature incinerator succeeded only in making it hot.
The findings also stated,
There is evidence in the literature that under certain extreme conditions reactor graphite dust can be made to explode. The Pile 1 project has completed research at
ANNEX A Leeds University which has concluded that although the conditions can be recreated in the laboratory they will not exist in the reactor and the dust explosion hazard can be discounted. As an added precaution the production of fine dusts during dismantling operations will be closely controlled.
It would appear then the NuSAC's findings are completely in support of the General Atomics position that Nuclear Graphite does not burn.

Given these facts the assertion that there was a core graphite fire at Chernobyl ought also to be revisited.


Cyril R said...

Nuclear graphite - very dense high quality carbon - apparently can sustain burning indirectly, when water is added to very hot graphite, the water goes into a gas shift reaction:

H2O + C = H2 + CO

Those gasses can burn quite well. This may have happened at Chernobyl, because there was a positive void coefficient, positive initial reactivity control rods, lots of water as coolant available and no decent containment so environmental water could get in. None of these are physically present in Western reactors, even the ones that use graphite make sure there are no strong positive void coefficients and no water coolant plus a decent containment. So graphite is safe in those reactors, for example the British reactors use CO2 cooling and graphite moderation and a decent containment and no strong positive void coefficients.

LFTRs with graphite are even safer, because with no driver to destroy containment, and no water anywhere near the reactor (at least 2 loops away) and an inert blanket gas in the reactor and containment, there is absolutely no possibility of a grapite fire.

jagdish said...

Neutron bombardment of graphite is different from plain heating. The neutron move the atoms from their perch in the crystals and move them to interstitial positions.
The carbon moves to a different structure from graphite and could become more fire prone.
Windscale and Chernobyl, though far from ordinary, are documented cases of fires in moderator graphite.

Charles Barton said...

jagdish I believe that the NRC has concluded that Wignerian energy cannot cause graphite fires in Reactor. An NRC report stated, "From analyses of existing information it is concluded that only stored energy accumulations and releases below the burning temperature (650/sup 0/C) are pertinent. After reviewing the existing knowledge on stored energy it is possible to show that stored energy releases do not occur spontaneously, and that the maximum stored energy that can be released from any reactor containing graphite is a very small fraction of the energy produced during the first few minutes of a burning incident. The conclusions from these analyses are that the potential to initiate or maintain a graphite burning incident is essentially independent of the stored energy in the graphite, and depends on other factors that are unique for these reactors, research reactors, and for Fort St. Vrain. In order to have self-sustained rapid graphite oxidation in any of these reactors, certain necessary conditions of geometry, temperature, oxygen supply, reaction product removal, and a favorable heat balance must be maintained. There is no new evidence associated with either the Windscale Accident or the Chernobyl Accident that indicates a credible potential for a graphite burning accident in any of the reactors considered in this review."

Existential_joy said...

The french molten salt work is very persuasive at making the case that you do not want any graphite in your molten salt reactor core regardless - The only downside of as fast a spectrum as at all possible is the higher enrichment required, and.. eh, who cares?

Charles Barton said...

Rhe French do not know anything about graphite in MSRs that was not known at ORNL around 1970. There are several graphite issues that bother the French. The most important is graphite swelling with high flux radiation, and the fact that graphite has a positive temperature coefficient of reactivity in MSRs. You can still have an overall negative temperature coefficient of reactivity for a graphite moderated MSR. and graphite can be replaced from time to time. In addition, even if you designed a MSR with an overall positive coefficient of reactivity, a freeze valve would prevent it from running away. There is a huge scalability advantage in graphite moderated MSRs, because the require a much smaller inventory of fissionable materials.


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