Graphite, Wigner energy

During a planned release of Wigner energy from graphite in Windscale No. 1 Pile, it became overheated. Oxidation of the graphite raised the temperature further, despite attempts to restrict access of air, and part of the reactor core reached an estimated temperature of 1300°C (Penney, 1957). About 6 to 8 tonne of uranium melted, but, in contrast to the previous operational experience, there was remarkably little dissemination of particulate uranium oxide (Chamberlain Dun-ster, 1958 Chamberlain, 1981). The high temperature and restricted air flow probably caused a skin of sintered oxide to form on the uranium. 

Figures 2 and 3 illustrate the typical channel structure, showing the vertical block junctions which embrace the fuel channels, the horizontal block junctions, and the slot feature in the base of the channel in which the graphite fuel boats were located. Figure 3 shows an example of extruded material, possibly lead, which has reached the fuel channels at a number of locations from adjacent isotope channels either during normal operation or during Wigner-energy anneals.

In addition each fuel element is separately contained and supported within a graphite sleeve - a refinement which eliminates bowing and the sticking of the elements and gives protection against the build-up of Wigner energy. 

The Wigner Disease . Wigner had contributed to the quantum mechanics of solids with his calculations, together with Seitz, of the cohesive energy of crystals. It was therefore natural for him, especially with his background in chemistry, to worry about the effect of neutron bombardment on the properties of graphite. He, together with F. L. Friedman and F. Seitz, estimated the number of displacements that a moderator atom would suffer in the course of... 

The Wigner effect was once a potential problem in the Hanford reactors. However, since the graphite operating temperatures have been increased, the stored energy is released during the normal operating cycle and special annealing runs are not required. 

---