Radiation damage in graphite

Abstract

During irradiation by neutrons, the atoms of carbon are displaced from their normal positions in the tridimensional structure, which causes the appearance of vacant sites and interstitial atoms. The carbon layers are forced apart and the increase of the separation between the layers, which can be measured with x rays, gives a good indication of the damage caused by the radiation. At 30 deg C this spacing can increase approximately 16% before saturation starts, but when the irradiation temperature is increased, the increase of c is much weaker and saturation begins much sooner, below 1% at 500 deg C. In connection with the increase of c, there are important modifications in the volume density, the stored energy, the thermal conductivity, and other physical properties. The damage can be removed by a thermal anneal outside the reactor or, more simply, by an irradiation anneal at a lower temperature than the other anneal. The precise nature of the faults produced by irradiation is not known, but it appears that according to the increase of the degree of damage, the types of faults are more and more complex. It is difficult to explain in detail the relation between the variation of the crystallographicmore » unit cell and the modifications of the dimensions of macroscopic blocks of graphite. At low temperatures there is a complete increase of the volume of the block with the irradiation, but, at much higher temperatures, there is a reduction of the volume. This perhaps corresponds to the fact that the radiations cause a diffusion to the edge of the crystallites allowing a reduction of the volume density. (tr-auth)« less