When graphite is employed as a reactor moderator, the variations of temperature and neutron flux within the moderator bars cause internal stresses to be generated in the graphite. The effect of these stresses on the dimensional changes of the graphite are of immediate interest in the field of reactor design. Samples of five nuclear-grade graphites have been irradiated under constant compressive stresses of 150 and 300 psi. Length changes of the stressed samples are compared with length changes of unstressed control samples, irradiated in the same assembly. Results obtained at 625ยฐC and for exposures up to 6500 MWd/At in a Hanford irradiation facility are presented. Additional data on one nuclear-grade graphite, irradiated under 100 psi constant tensile stress, are included.
1. Introduction
THE dimensional changes produced in graphite, as a result of neutron radiation, have been studied extensively. These dimensional changes are related in a complex manner to both the fast-neutron exposure and the irradiation temperature. Gradients in temperature and fast-neutron flux within the moderator bars results in differences in rates of dimensional change at different positions within the bars. These differences in dimensional change produce stresses which could lead to a self-*Work performed for the Atomic Energy Commission under Contract AT(45-l)-1350.