Irradiation creep properties of a near-isotropic graphite

Abstract

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 × 10 24 n/m 2 (E> 29 fJ) to 12.4 × 10 24 n/m 2 (E> 29 fJ) , depending on the position of the specimen. Irradiation creep strain (ϵ 0) was obtained from the equation ϵ c = (σ/E 0)[1-exp(−bΦ)] + KσΦ , by measuring dimensional changes in unloaded and loaded tensile specimens before and after irradiation, where E 0 is the Young's modulus before irradiation, K the creep coefficient, and b a constant. The value of K was estimated assuming that 1-exp(−bΦ) ∼-1 over the range of neutron fluence tested here. Mercury porosimetry was employed to add consideration to the mechanism of irradiation creep using unloaded and loaded specimens. The irradiation creep strain is proportional to stress and to neutron fluence for larger fluences. The irradiation creep coefficient is in inverse proportion to Young's modulus before irradiation, KE 0 = 0.247 . From the values of the average Young's moduli before irradiation for two irradiation creep tests, the creep coefficient was estimated to be 3.03 × 10 −29 (MPa/m 2) −1 and 3.18 × 10 −29(MPa/m 2) −1 , respectively. The mercury pore diameter distribution changes upon irradiation, that is pores smaller than 10 μm disappear partly, the total porosity decreases, and the stress tends to facilitate disappearance of the pores. The Young's modulus increases as a result of irradiation. The increase in Young's modulus after a creep tests is smaller than that after irradiation only. The experimental result obtained here is consistent with the explanation for the mechanism of irradiation creep in which two to six interstitial clusters as a pinning point to basal slip disappear during the irradiation creep test.