Reliability of strength and long-term life for ceramics/metal jointing parts under neutron irradiation

The ceramics/metal joints in the airtight seal parts for a neutron detector are studied on the reliability for strength and long-term life. Both functions of airtight and insulation are required for the airtight seal parts, which suffer a very bad influence from a viewpoint of long-term reliability as well as strength, because the residual stress is generated in these joints due to the difference of the thermal expansion coefficient between ceramics and metal. The ceramics subjected to neutron irradiation in-serve occurs in swelling (volume expansion) and decreases in strength Moreover, the copper interlayer rises in yield stress and hardens with increasing the plastic deformation due to the swelling of ceramics, and so lowers in function for stress relaxation. As a result, not only the internal stress in ceramics increases with passing the time in service, but also the facture strength of ceramics lowers, and finally a delayed fracture occurs at the ceramic/metal joints in the airtight seal parts. Therefore, in our study, the residual stress has been analyzed by a simulation model using FEM, so that both mechanisms of the increment in internal stress as well as mechanical damage can be clarified for the ceramic/metal jointing parts subjected to the neutron irradiation over long-term. On the other hand, the strength verification against the ceramic/metal joints are performed through simulation model analyses as well as the virtual tensile tests for small mock up specimens. From a viewpoint of fracture mechanics, the reliability for fracture life as well as strength of the airtight seal parts is discussed statistically considering the scatter of strength for Si sub 3 N sub 4 .