Tamotsu Saito

Irradiation Behaviors of Nuclear Grade Graphite in Commercial Reactor, (III) Mechanical Properties

Bending and compressive strengths, and Youngs modulus were measured for Pechiney nuclear grade graphite irradiated in the temperature range 220∼400°C in the environment of CO2 in a commercial reactor, up to the neutron fluence 6.2 × 1019 and 2.2 × 1020n/cm2 (E>0.85 MeV), respectively. All of them increased owing to neutron irradiation, and the changes in both strengths were almost similar in the whole range of irradiation temperature, however the changes in Youngs modulus depended on irradiation temperature. It was clarified in the present experiment that both strengths were related with Youngs modulus and the relation could be expressed by the formula σ=kE n, where σ and E are strength and Youngs modulus, respectively, and n is constant which has different value for bending or compressive strength and also for their measured direction.

Irradiation Behaviors of Nuclear Grade Graphite in Commercial Reactor, (II) Thermal and Physical Properties

Thermal conductivity, electrical resistivity and stored energy were measured for Pechiney nuclear grade graphite irradiated in the temperature range 220∼400°C up to the maximum neutron fluence 2.2 × 1020 n/cm2(E>0.85 MeV) in the environment of a carbon dioxide in a commercial reactor. Thermal conductivity decreased, electrical resistivity and stored energy increased owing to neutron irradiation and their changes were larger for the samples irradiated at lower temperatures. A linear relation between stored energy and fractional change in thermal resistivity was obtained for the irradiated samples and it was found that its proportional constant is about two times of that reported previously. The relation between thermal conductivity and electrical resistivity is discussed for irradiated samples as well.

Irradiation Behaviors of Nuclear Grade Graphite in Commercial Reactor, (I) Dimensional Change and Thermal Expansion

Dimensional changes and thermal expansion coefficient were measured for Pechiney nuclear grade graphite irradiated in the temperature range 220∼400°C up to the maximum neutron fluence 2.2 × 1020 n/cm2 (E>0.85 MeV) in the environment of CO2 in a commercial reactor. Dimensional shrinkage was observed for whole samples except for those which showed a slight expansion in the early stage of irradiation. Thermal expansion coefficient was almost constant up to about 7.0 × 1019 n/cm2. The measured dimensional changes were compared with the predicted ones obtained from a bromination method and it was confirmed that the actual dimensional change of graphite bricks in the reactor did not exceed the predicted ones. Dimensional changes were also discussed in relation to the change of crystallite parameters due to irradiation. Distortion of the reactor core was discussed by using the experimental results as well.

Effect of high-temperature neutron irradiation on the crystallites of graphite materials

Abstract Changes in crystal lattice parameter, c-axis lattice strain and apparent crystallite height induced by neutron irradiation at 720–1350°C were investigated by means of X-ray diffraction for fourteen specimens which included five bands of nuclear graphite and two kinds of pyrolytic graphite. All irradiated materials show a marked increase in c-axis lattice strain, while changes in the lattice parameters are very small. The relation between lattice strain and layer spacing was analyzed for pre- and post-irradiated graphites, and it is inferred that, whereas the predominant constituent of strain in the unirradiated graphite is the statistical fluctuation in distribution of layer spacings, for the irradiated material it is the distortion of layer planes. On the basis of the relation between c-axis lattice strain and apparent crystallite height, the stacking height of graphite layer planes is estimated to be about 70 A for pre-irradiated graphite.

Effects of high temperature neutron irradiation on dimension and thermal expansion coefficient of nuclear grade graphites.

This paper describes the experimental results and discussions on the changes in dimension, volume and thermal expansion coefficient of five nuclear grade graphites which were made from petroleum coke or gilsonite coke and irradiated in the JMTR at 670–1,260°C up to the maximum neutron fiuence 3.3×1021 n/cm2 (E>0.18 MeV). Dimensional or volume shrinkages for all samples were observed after irradiation and their changes depended on the kind of graphite studied. The changes in thermal expansion coefficient were also observed for three of five samples. The relations between the changes in dimension or volume and thermal expansion coefficient were obtained, and it was clarified that both the rates of irradiation shrinkage in dimension and volume of the sample having larger thermal expansion coefficient were smaller than for the sample having smaller thermal expansion coefficient.

Magnetic Susceptibility of Uranium Carbonitride and Oxycarbide

The magnetic susceptibility of solid solutions of U(C, N) (up to 50 mol/0 UN) and U(C,O) (up to 25 mol/0 UO) was studied in the temperature range of 77°300°K. These solid solutions all exhibit paramagnetism. With U(C, N), the susceptibility rises with increasing content. With U(C,O), the susceptibility scarcely changes up to approximately 5 mol/0 UO, beyond which point it increases with oxygen content. The temperature dependence of the susceptibility, though very weak, increases with oxygen content. The density of stale curve is considered on the assumption that the variation of susceptibility with composition is due to Pauli paramagnetism, and the magnetic behavior of U(C, N) and U(C, O) is explained by considering the shift of the Fermi level with progressive substitution of carbon by nitrogen or oxygen.