Irradiation-induced Dimensional Changes Research Articles

Radiation-induced dimensional changes and creep in carbonaceous materials

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.

Radiation-induced dimensional changes in pyrolytic carbons deposited in a fluidized bed

A range of pyrolytic carbon structures representative of those used as coatings for nuclear fuel particles was irradiated at temperatures between 540° and 1060°C to fast neutron exposures of 1.5 to 2.4 × 10 81 nvt ( E > 0.18 MeV). Changes in linear dimensions, density, and crystallite size were measured. The data show that irradiation-induced dimensional changes in pyrolytic carbons depend sensitively on their structure. At ~1000°C the dimensional changes depend primarily on the degree of preferred orientation with a secondary dependence on the density of the carbon. The changes calculated for the crystallites are independent of the crystallite height, L c , in the range 30 to 145 Å but the changes are less for larger L c values. At ~600°C the dimensional changes depend on the L c , the density, and the degree of preferred orientation. At this temperature the calculated crystallite changes are very sensitive to variations in L c in the range 30 to 200 Å. At all temperatures the irradiation causes the carbons to densify. The irradiation tends to increase the L c value of carbons with initially small L c values and decreases those of carbons with initially high L c values. The observed irradiation-induced changes are discussed in terms of current theories of irradiation damage in carbons and graphite.

Relationship between Preferred Orientation, Thermal Expansion, and Radiation-Induced Length Changes in Graphite

Expressions may be derived for the thermal expansion coefficient of polycrystalline graphite in terms of the single-crystal coefficients and the preferred orientation of the sample. Thermal expansion and preferred orientation measurements were performed on pyrolytic, molded, hot-worked, extruded-rod, and extruded-tube graphites in order to test the validity of this model. They showed that such expressions adequately represent the behavior of several different graphites over a range of temperatures, provided that the ``accommodation'' of a part of the c-direction crystallite expansion by microcracks is taken into account. The amount of c-direction accommodation is a function of preferred orientation and the direction of measurement. It is small for the principal c direction in a highly oriented body and large for a random aggregate of crystallites and is probably controlled by the number of microcracks which form on cooling from the graphitization temperature. A model similar to that used for thermal expansion may be used to express the irradiation-induced dimensional changes in polycrystalline graphite in terms of the preferred orientation of the sample, a contraction in the crystallite a direction, and an expansion in the crystallite c direction which is partially accommodated by microcracks. The model was found to be in reasonable agreement with published irradiation data for a number of graphites.