4.10.5.1 Wigner Energy

The release ofWigner energy (named after the phys­icist who first postulated its existence) was histori­cally the first problem of radiation damage in graphite to manifest itself. The lattice displacement processes previously described can cause an excess of energy in the graphite crystallites. The damage may comprise Frankel pairs or at lower temperatures the sp3 type bond previously discussed and observed by Urita et a/.20 When an interstitial carbon atom and a lattice vacancy recombine, or interplanar bonds are broken, their excess energy is given up as ‘stored energy.’ If sufficient damage has accumulated in the graphite, the release of this stored energy can result in a rapid rise in temperature. Stored energy accu­mulation was found to be particularly problematic in the early graphite-moderated reactors, which operated at relatively low temperatures. Figure 12 shows the rate of release of stored energy with

temperature, as a function of temperature, for graph­ite samples irradiated at 30 °C to low doses in the Hanford K reactor.32 The release curves are character­ized by a peak occurring at ^200 °C. This temperature has subsequently been associated with annealing of interplanar bonding involving interstitial atoms.20

In Figure 12, the release rate exceeds the specific heat and therefore, under adiabatic conditions, the graphite would rise sharply in temperature. For am­bient temperature irradiations it was found9 that the stored energy could attain values up to 2720Jg which ifreleased adiabatically would cause a temper­ature rise of some 1300 °C. A simple experiment,8 in which samples irradiated at 30 °C were placed in a furnace at 200 °C and their temperature monitored, showed that when the samples attained a temperature of ^70 °C their temperature suddenly increased to a maximum of about 400 °C and then returned to 200 °C. In order to limit the total amount of stored energy in the early graphite reactors, it became nec­essary to periodically anneal the graphite. The gra­phite’s temperature was raised sufficiently, by nuclear heating or the use of inserted electrical heaters, to ‘trigger’ the release of stored energy. The release then self-propagated slowly through the core, raising the graphite moderator temperature and thereby par­tially annealing the graphite core. Indeed, Arnold33 reports that it was during such a reactor anneal that the Windscale (UK) reactor accident occurred in 1957. Rappeneau eta/.34 report a second release peak at very high temperatures (^1400 °C). They studied the energy release up to temperatures of 1800 °C of graphites irradiated in the reactors BR2 (Mol, Belgium) and HFR (Petten, Netherlands) at doses between 1000 and 4000 MWdT-1 and at tempera­tures between 70 and 250 °C. At these low irradiation temperatures, there is little or no vacancy mobility, so the resultant defect structures can only involve interstitials. On postirradiation annealing to high tem­peratures, the immobile single vacancies become increasingly mobile and perhaps their elimination and the thermal destruction of complex interstitial clusters or distorted and twisted basal planes contrib­ute to the high-temperature stored energy peak.

The accumulation of stored energy in graphite is both dose and irradiation temperature dependent. With increasingly higher irradiation temperatures, the total amount of stored energy and its peak rate of release diminish, such that above an irradiation temperature of ^300 °C stored energy ceases to be a problem. Accounts of stored energy in graphite can be found elsewhere.1,8,29,32