Tensile Strength Change of Silicon Nitride/Stainless Steel Joints during Neutron Irradiation

Active metal brazing of ceramic to metal has been applied to the fabrication of advanced airtight seals for LPRMs(Local Power Range Monitors) used in BWRs(Boiling Water Reactors) to improve their life and reliability. Silicon nitride was selected as the ceramic part of the joint because of its superior swelling resistance. A cone-shaped silicon nitride and a copper interlayer were employed to join them to stainless steel parts. The effect of the copper-interlayer shape on the residual stress was studied. The relation between mechanical properties of the silicon nitride and tensile strength of the joints was also evaluated. It turned out that the copper interlayer design had significant effects on the residual stress of the joints. It was also found that the tensile strength of the joints was remarkably dependent on fracture toughness of the silicon nitride. Based on these results, the tensile strength change of the joints during neutron irradiation was studied and a life of the seal parts was discussed. Fifty years are obtained for the initial fracture toughness 5.4 MPa·m1/2 silicon nitride and 25 years for that of 4.4 MPa·m1/2 silicon nitride. Therefore the life period of the seal parts using silicon nitride that has initial fracture toughness 4.6-5.9 MPa·m1/2 is predicted to be between 25 and 50 years. It is more reliable seal parts comparison to the conventional seal parts used currently applying alumina ceramics as the ceramic part of the joint.