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The energy that graphite released upon a cascade of defects by neutron radiation is 2.7 MJ/kg, however there is no data on how quick this energy is released. I am interested in this as this not mentioned anywhere on the Windscale disaster

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    $\begingroup$ For those of us who had never heard of the Windscale disaster: en.m.wikipedia.org/wiki/Windscale_fire. $\endgroup$
    – Ed V
    Commented Jun 15, 2021 at 23:47
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    $\begingroup$ No actual facts, but relaxation of an interstitial atom to an available hole is rather instantaneous. Thus the reaction is limited by the diffusion of holes. (diffusion of interstitials would be orders of magnitude slower). Diffusions of either kind is slow in graphite, as the recrystallization temperature indicates. Combine these opinions into; rather swiftly, then slowing down. Milliseconds to reach a thousand degrees, would be my guess. $\endgroup$
    – Stian
    Commented Jun 16, 2021 at 7:09

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Some considerations:

  • Every displaced atom kicks its peers somewhat in order to dissipate the extra energy from getting where it belongs.
  • The release is triggered by heating the irradiated material.
  • The process propagates by heating the adjacent regions.

Looks pretty much like a gunpowder deflagration, except for the much lower energy scale. The net result is few tens of Kelvin heating up.

The propagation timescale is likely to be something like the heat exchange in the solid (no agile hot gases produced to speed it up) and well below the speed of sound in the material.

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There are some papers in which Wigner effect was tested as e method of energy storage. A MORE interesting question is 'What OTHER materials can be used to store energy - how much and how fast can that energy be released?'.

The Russian patents on what is known as 'Red Mercury' are where the compound Hg2Sb2O7 is suggested and which I suspect MIGHT be based on theoretical work.

If you can produce a material that can store a LOT of Wigner energy and will also detonate would, I guess, allow all of the energy to be released VERY quickly indeed.

That said, I do not have the skillset to even estimate the veracity of the patents.

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    $\begingroup$ Wigner effect is unlikely to store more energy than the latent heat of melting for the substance in question, for obvious reasons. While a sudden self-heating by few tens of K could be an engineering pain, I fail to see this as a convinient energy storage. Hand warmers anyone? $\endgroup$
    – fraxinus
    Commented May 9 at 21:15

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