Dimensional Changes and Radiation Creep in Graphite at Very High Neutron Doses

Abstract

RADIATION creep1–4 is the deformation produced by the combined effect of stress and irradiation in a nuclear reactor. In graphite, radiation creep consists of a permanent set, transient creep, and steady-state creep. The permanent set and transient creep are comparable with the elastic strain and it is normally assumed that the steady-state creep is proportional to the stress and the total neutron dose. It is of some interest to know whether the steady-state creep can become large compared with the elastic strain. The initial creep rate is such that this requires a fast neutron dose of ∼ 1022 n cm−2. Such neutron doses can be obtained in the course of a few months in the Dounreay Fast Reactor, at the centre of which the neutron flux is ∼ 2.5 × 1015 n cm2. Graphite samples have been irradiated in the fast reactor under stress and creep strains of several per cent have been obtained. The elastic strain at breaking in tension is ∼ 0.1 per cent.