Constant stress irradiation creep experiments on graphite in BR-2

Abstract

The irradiation induced creep of near-isotropic polycrystalline graphites under constant tensile and compressive stresses in BR-2 is considered. It is shown that up to a fast neutron dose of 50 × 10 20 n.cm −2 there is no large effect of irradiation temperature on creep strain between 300° and 650°C, and the increase in creep strain with dose at constant stress is similar in compression and tension up to 1 per cent creep strain. Comparison of the results with those on anisotropic materials shows a correlation of creep and elastic strain, which suggests creep deformations are also controlled by crystal shear. The thermal expansion coefficient of graphite is significantly affected in opposite directions by compressive and tensile creep, but Young's modulus is not affected by creep strain. Changes in dimensions and thermal expansion coefficient of tensile and compressive creep specimens and their controls during thermal annealing experiments are described. These show a large apparent recovery in creep strain which is not observed in irradiation annealing experiments. Possible interpretations of the results are discussed.