Radiation-induced dimensional changes and creep in carbonaceous materials

Abstract

At high fast neutron exposures, the dependences of the irradiation-induced dimensional changes on the structure of turbostratic carbons depart from those established at lower exposures. After an initial densification which is first order with respect to the density defect, the densification ceases and the material expands. Concomitantly, the rate at which the material changes shape increases. It is thought that the acceleration of the rate of dimensional change is a result of the autocatalytic influence of radiation-generated defects and irradiation-induced increases in anisotropy. The new porosity responsible for the expansion at high doses is thought to be associated with large intercrystalline strains that are relaxed by radiation creep. Measurements of the macroscopic creep strains indicate that strain rates of at least about 2, 3 and 6% per 10 21 n/ cm 2 ( E >> 0.18 MeV) can be accommodated at 600, 900 and 1200 °C, respectively, and total strains of about 10% in tension are possible. These measurements, together with the dimensional change behavior of isotropic carbon coatings for nuclear fuel particles, show that for survival to high neutron exposures, the density must be chosen so that coatings will not fail early in life due to densification and so that intolerable anisotropic behavior will not be encountered late in life. The data suggest that, for the designs most commonly used, the optimum densities for isotropic carbons lie in the range 1.5 to 1.8 g/cm 3. Comparison of the data for turbostratic carbons and data for graphitic materials shows that at approximately 1200 °C, the dimensional change rate can vary by two orders of magnitude with crystallinity. The influence is considerably less at 600 °C.