Kiyohiro Kawamata

Effects of burn-off on thermal shock resistances of nuclear carbon materials

Abstract The purpose of this study is to evaluate the effects of burn-off on thermal shock resistances and thermal shock fracture toughnesses of a carbon fiber reinforced carbon composite (C/C composite) and three fine-grained isotropic graphites. These thermal shock resistances and thermal shock fracture toughnesses were degraded slightly by air oxidation at 500 °C. The extents of degradations of the thermal shock parameters were less than those of the mechanical and fracture mechanics properties, however, they were larger than that of the thermal diffusivity. In observations of the microstructures of the fracture surfaces after oxidation of the graphites, the size and the number of pores were increased and the fracture surfaces were rough due to oxidation of boundaries of graphite particles. After oxidation of the C/C composite, there were preferential removal of the boundary layer between carbon fiber and pyrolytic carbon matrix and pull out of carbon fiber.

Fracture criteria of reactor graphite under multiaxial stesses

Abstract New fracture criteria for graphite under multiaxial stresses are presented for designing core and support materials of a high temperature gas cooled reactor. Different kinds of fracture strength tests are carried out for a near isotropic graphite IG-11. Results show that, under the stress state in which tensile stresses are predominant, the maximum principal stress theory is seen as applicable for brittle fracture. Under the stress state in which compressive stresses are predominant, there may be two fracture modes for brittle fracture, namely, slipping fracture and mode II fracture. For the former fracture mode the maximum shear stress criterion is suitable, but for the latter fracture mode the following mode II fracture criterion including a restraint effect for cracks is verified to be applicable, σ 3 ;= σ c σ t K Ic K IIc −1 σ 1 −σ c where δ1 and δ3 are the maximum and minimum principal stresses, δt and δc are the tensile and compressive strengths and KIc and KIIc are the mode I and II fracture toughness values, respectively. The above equation is similar in form to the Coulomb-Mohr criterion. Also a statistical correction for brittle fracture criteria under multiaxial stresses is discussed. By considering the allowable stress values for safe design, the specified minimum ultimate strengths corresponding to a survival probability of 99% at the 950 confidence level are presented.

Effects of argon ion irradiation on the microstructures and physical properties of carbon fibers

Abstract Eight kinds of carbon fibers with different microstructures have been exposed to argon ion implantation at 175 MeV – 1 μA for 399 min using AVF cyclotron, Takasaki Radiation Chemistry Research Establishment, JAERI. After ion irradiation changes in diameters and cross-sectional areas of carbon fibers were determined by scanning electron microscopy. Tensile properties were measured before and after ion irradiation. As a result, the diameter generally tended to decrease after ion irradiation, except for the carbon fiber with the dual microstructure that has two directions (radial and circumferential) of basal planes in the cross-section of the fiber. Tensile strength decreased after ion irradiation. The decrease in tensile strength suggests that changes in axial microstructures due to ion irradiation give an influence on the mechanical properties of the fibers.

Effects of prestresses on mechanical properties of isotropic graphite materials

Abstract Graphite materials which are used for plasma facing components and other components are subjected to stresses due to the high heat flux from the fusion plasma. Some mechanical properties of graphite materials can change due to the prestresses. The property changes should be considered for the design of the plasma facing components. The purpose of this study is to examine the effects of prestresses on the mechanical properties of isotropic graphite materials. Compressive prestresses were applied to two kinds of isotropic fine-grained graphites (IG-430 and IG-11) at 298 K (both), 1873 K (IG-11), 2273 K (IG-11) and 2283 K (IG-430). As a result, the decrease in Youngs modulus for IG-430 due to high-temperature prestressing was 56% which was much larger than the 6.4% that was due to prestressing at 298 K. The results for IG-11 were the same as those for IG-430 graphite. This finding was considered to be due primarily to a difference in degree of the preferred orientation of crystallites in the graphite on the basis of the Bacon anisotropy factor (BAF) obtained from X-ray diffraction measurement of the prestressed specimens. Furthermore, high-temperature compressive prestressing produced an increase in the strength of the isotropic graphite, although room temperature prestressing produced no such effect. The results obtained here suggest that the isotropic graphite which is subjected to high-temperature compressive stresses can become anisotropic in service.

Observation and X-ray analysis of the microstructures for a bonding material of C/C composite and copper

Abstract The purpose of this study is to examine the microstructure of a bonding material of C/C composite and copper before and after thermal shock tests. Optical and scanning electron microscopy, and energy dispersive X-ray analysis were used to study the microstructures before and after thermal shock tests. In this study, the specimen were given thermal shock without an active cooling, therefore, these tests are considered to simulate only heat load at the most severe condition as an accident. The bonding material was not fractured by the thermal shock tests, however, thermal cracks and delaminations were found in the bonding layers.

Fracture mechanical properties and neutron irradiation effects of fuel compacts for the HTTR

Abstract To simulate the nuclear fuel for High Temperature Engineering Testing Reactor (HTTR), fuel compact models using SiC-kernel coated particles instead of UO 2 -kernel coated particles were prepared under the same conditions as those for the real fuel compact. The mechanical and fracture mechanics properties were studied at room temperature. The thermal shock resistance and fracture toughness for thermal stresses of the fuel compact were experimentally assessed by means of arc discharge heating applied at a central area of the disk specimens. These model specimens were then neutron irradiated in the Japan Material Testing Reactor (JMTR) for fluences up to 1.7 × 10 21 n/cm 2 ( E ·> 29 fJ ) at 900° C ± 50° C . The effects of irradiation on a series of fracture mechanical properties were evaluated and compared with the cases of graphite IG-110 used as the core materials in the HTTR.

The thermal shock resistance of a joining material of C/C composite and copper

Abstract Plasma facing materials for the next fusion reactor devices will have severe problems such as thermal shock fracture, surface erosion and injection of sputtering particles to plasma caused by charged particle fluxes and very high heat shocks. A joining material, in which a carbon fiber reinforced carbon composite was joined to oxygen-free copper by inserting a molybdenum plate and some metallic films in the joining layers, was developed for a solution of those problems. The thermal shock resistance and the thermal shock fracture toughness were evaluated by an eccentric local heating method of arc discharge. The joining material did not fracture during severe thermal shock tests such as plasma disruption, however, thermal and delamination cracks were observed at the joining parts by scanning electron microscope (SEM). These results can be useful in contributing to the development and the safety design of plasma facing components for fusion reactor devices.

Thermal Shock Resistances of a Joining Material of C/C Composite and Copper

The purpose of this study is to contribute to the development and safety design of plasma facing components for fusion reactor devices. We evaluated the thermal shock resistance and the thermal shock fracture toughness of a joining material which was used to join a carbon fiber reinforced carbon composite (C/C composite) to oxygen-free copper. We also examined the microstructures of the joining layers using a scanning electron microscope before and after thermal shock tests. The joining material did not fracture during the thermal shock tests. However, thermal cracks and delamination cracks were observed in the joining layers.