Hideto Matsuo

The effect of porosity on the thermal conductivity of nuclear graphite

Abstract During service in a high-temperature reactor, graphite will be oxidized by impurities such as water vapour present in the helium coolant. Oxidation will affect the thermal conductivity of the graphite and hence the fuel temperature. This report describes experiments on the effect of oxidation at 1000°C by water vapour of a semi-isotropic moulded graphite. The value of thermal conductivity at room temperature decreases with increasing weight loss, but not linearly, the decreases being most rapid at low weight losses. The percentage change in thermal conductivity is approximately linear with the increase in open porosity.

Capability of energy selective neutron irradiation test facility (ESNIT) for fusion reactor materials testing and the status of ESNIT program

Abstract The concept of a high energy neutron irradiation facility named Energy Selective Neutron Irradiation Test Facility (ESNIT) was conceived at Japan Atomic Energy Research Institute (JAERI) as a facility for materials research in generic nuclear applications for which major potential research incentives were originated from nuclear fusion application. Technological survey and feasibility studies of ESNIT have continued since 1988 and two international workshops have been held in 1989 and 1991. In this paper, the capability of ESNIT for fusion reactor materials testing and the status of technological survey activities including deuteron accelerator system, target system and experimental system are summarized.

Characteristics of an energy selective neutron irradiation test facility (ESNIT) for material irradiation studies

Abstract The concept of an energy selective neutron irradiation test facility (ESNIT) was generated at the Japan Atomic Energy Research Institute, and a program for the preliminary technical survey was initiated in 1988. The potential benefit of such a facility is that it makes possible that neutron-irradiation experiments on materials can be carried out with well-defined energy spectra up to reasonably high flux and fluence levels and with high manipulability. A significant fraction of its applications will be focused on fundamental aspects of radiation damage in fusion reactor materials.

Effect of oxidation on physical properties of nuclear grade graphites

Abstract Changes in thermal conductivity, coefficient of thermal expansion (CTE). electrical resistivity, and Youngs modulus were measured for the two nuclear grade graphites IG-11 and H451 oxidized thermally in air at 723 K and by water vapor at 1123 K. Thermal conductivity, CTE and Youngs modulus decreased and electrical resistivity increased owing to the oxidation. The changes were dependent on the graphites as well as the oxidation condition. The results were analyzed and discussed. Furthermore the changes in thermal shock resistance were discussed for the thermally oxidized nuclear grade graphites.

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.

Thermal expansion of some pre-stressed nuclear grade graphites

Abstract Thermal expansion coefficients (CTE) of some nuclear grade polycrystalline graphites were measured in the range from room temperature to 1173 K after pre-stressing to levels comparable with their fracture strength. The CTEs has been found to increase with residual strain independently of the samples as well as their manufacturing processes. Analytical discussions are given on the effects of both pre-stressing and/or thermal annealing. Phenomenological formulae appropriately expressing CTEs in heating and cooling processes have been derived.

Present status of ESNIT (energy selective neutron irradiation test facility) program

Abstract The present status of technical studies of a high energy neutron irradiation facility, ESNIT (energy selective neutron irradiation test facility), is summarized. Technological survey and feasibility studies of ESNIT have continued since 1988. The results of technical studies of the accelerator, the target and the experimental systems in ESNIT program were reviewed by an International Advisory Committee in February 1993. Recommendations for future R&D on ESNIT program are also summarized in this paper.

Irradiation-induced property changes of thermally oxidized nuclear graphites

Abstract Effect of irradiation on the changes in properties such as length, volume, bulk density, thermal conductivity, thermal expansivity, electrical resistivity, and Youngs modulus were studied for two kinds of nuclear graphites which were thermally oxidized by water vapor prior to irradiation. The samples were irradiated at 800–1020 °C to the neutron fluence 6.6 × 10 24 n/m 2 (E > 29 fJ) . The experimental results have shown that the irradiation-induced percentage changes of the properties were the same for both thermally oxidized and unoxidized samples.