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Conference Article: The description of Wigner energy and its release from Windscale pile graphite for application to waste packaging and disposal

Abstract

.The graphite core of Windscale Pile 1 is to be dismantled and packaged for interim storage, together with graphite items, such as dowels and boats from both Pile 1 and 2. These retain substantial amounts of Wigner energy and thus to support their packaging, storage and ultimate disposal, it is necessary to understand the release of the Wigner energy in slow, low temperature cycles, driven by the exothermic curing of the grout. The basic theories of the accumulation of Wigner energy and its release are reviewed. These show that the lattice defects introduced by neutron irradiation form a population of sites of different energies which is independent of the graphite type. The release of energy from each type of site can be described by a rate expression which is first order in the stored energy of the site, S, and has an associated activation energy, E, of decay. The overall rate of release, S, is given by a sum over the population of sites and has an effective lower limit determined by irradiation temperatures or post-irradiation conditions. In order to predict the release of Wigner energy activation energy spectra are derived from differential scanning calorimetry of samples of Windscale graphite. It is shown, by numerical solution of the release equation, that the derived spectra predict well both the experimental data and the release of energy from specimens derived from the same sample of Windscale graphite but subjected to different temperature cycles. Simulation of stored energy release in anticipated storage conditions, where the graphite is subjected to a slow temperature cycle driven by the exothermic curing of the grout, shows that only a small portion of the total stored energy is released. The slower the cycle, the greater the proportion of energy release, but the rate of release is slower.

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Reference:

IAEA Technical Committee Meeting on "Nuclear Graphite Waste Management", held from 18-20 October 1999 in Manchester, United Kingdom

International Atomic Energy Agency, Vienna (Austria)

TCM-Manchester99, pp:47-64