Accumulation and diffusion of radiogenic helium in beryllium

Abstract

Accumulation of radiogenic helium isotope 4He, its diffusion characteristics, and microstructure of beryllium have been studied after irradiation at 70 and 200°C by neutrons with an energy of E > 0.1 MeV to fluences of (1.0–13.3) × 1022 cm−2. Experimental results concerning the concentration of radiogenic helium in beryllium after irradiation and subsequent isothermal annealings for 1 h in a temperature range of 700–1200°C have been obtained. The accumulation of helium corresponds to a linear dependence up to a neutron fluence of ∼6 × 1022 cm−2 (E > 0.1 MeV); at fluences from 6 × 1022 to 13 × 1022 cm−2 (E > 0.1 MeV), a deviation from the linear dependence toward the decrease in the rate of accumulation is observed. The diffusion of radiogenic helium in beryllium at annealing temperatures of 70–200°C is low; intense diffusion and degassing of helium from beryllium start at an annealing temperature of 700°C. At annealing temperatures of 1100–1200°C, virtually all helium escapes from the irradiated beryllium.