Irradiation-induced dimensional changes and creep of isotropic carbon

Abstract

This paper describes an investigation of the dimensional behavior of unrestrained and spherically restrained isotropic carbons deposited at both high temperatures (> 1500°C; HTI carbons) and low temperatures (< 1500°C; LTI carbons) during irradiation at 600°, 1000°, and 1250°C to fast-neutron fluences near 2 × 10 21 n/ cm 2( E > 0.18 MeV). The investigation covered, in particular, the effect of the initial density on the unrestrained dimensional changes and on the total strain accommodated without fracture in the spherically restrained specimens. For isotropic carbons (Bacon anisotropy factor < 1.1), the unrestrained dimensional changes were primarily those due to irradiation-induced densification, and the strain accommodated consisted mostly of irradiation-induced creep strain. The largest dimensional changes and highest total strains accommodated were in low-density carbons. At 1000°C the accommodated strain rates were 0.04 per 10 21 n/cm 2 and 0.05 per 10 21 n/cm 2 for LTI and HTI carbons, respectively. At 600°C the respective accommodated strain rates were 0.028 per 10 21 n/cm 2 and 0.025 per 10 21 n/cm 2. At 1250°C both an upper and a lower limit on the density of the restrained LTI carbons that survived irradiation were observed. The lower density limit apparently revealed the maximum densification strain rate that can be tolerated. The existence of the upper density limit is discussed.