Irradiation creep properties of a graphite for HTGRs

Two irradiation creep tests on near isotropic graphite (SM1 - 24) for HTGRs were performed at around 900 C in the JMTR. Neutron fluences ranged from 5.50 x 1024 n/m2 (E > 29fJ) to 12.4 x 1024 n/m2 (E > 29fJ), depending on the position of the specimen. Irradiation creep strain (εc) was obtained from the equation, εc = (σ/E0) [1-exp(-bφ)] + Kσφ, by measuring dimensional changes in unloaded and loaded tensile specimens before and after irradiation, where E0 is Young's modulus before irradiation, K creep coefficient, and b a constant. The value of K was estimated assuming that 1 - exp(-bΦ) = 1 over a range of neutron fluence tested here. Mercury porosimetry was made to add consideration to the mechanism of irradiation creep using unloaded and loaded specimens. Irradiation creep strain is proportional to stress and to neutron fluence for larger fluences. Irradiation creep coefficient is in inverse proportion to Young's modulus before irradiation, KE0 = 0.247. From the values of the average Young's moduli before irradiation for two irradiation creep tests, the creep coefficients were estimated to be 3.03 x 10-29 (MPa/m2)-1 and 3.18 x 10-29 (MPa/m2)-1. Mercury pore diameter distribution changes by irradiation, that is, pores smaller than 10 μm disappear partly, the total porosity decreases, and the stress tends to facilitate disappearance of pores. Young's modulus increases by irradiation. Increase in Young's. (author)