The effect of residual strain on the thermal conductivity of nuclear graphite

This report describes an investigation of the effect of residual strain caused by compressive pre-stressing on the thermal conductivity of nuclear graphite. The structural changes caused by the applied stress were investigated by measuring the alteration in bulk and nitrogen displacement densities. In the range of small residual strains, the thermal conductivity increases as do the bulk and nitrogen displacement densities. For larger values of the residual strain (after stressing up to ~90% of the fracture stress) the thermal conductivity decreases approximately to its original value, while the bulk and nitrogen displacement densities continue to increase. These observations are compatible with some degree of crack formation; however, other data do not entirely support this simple picture.