Study on unstable ductile fracture of reactor primary piping, (1)

Small cracks have been found in the austenitic stainless steel pipings for BWRs, and it was judged that these were originated from the stress corrosion cracking in welding heat-affected zones. These cracks were detected before the fracture of pipings occurs. In order to confirm that leak before break always materializes, it is necessary to analyze the behavior of crack development in piping materials, especially its stability. The 304 stainless steel used for the cooling pipes does not cause brittle fracture, and the development of cracks in it is stable, but there is some possibility to become unstable because the piping systems themselves for nuclear reactors have compliance. In this study, a plate with a center crack was analyzed before treating the problem of pipings. On the basis of the result of low compliance tensile test on the test pieces with center penetrating slit made of 304 stainless steel, the J-R curve was made. Next, the starting point of unstable development by high compliance test was estimated according to the tearing instability criterion. The basic concept of the analysis of unstable ductile fracture, the outline of the experiment, the analysis and their comparison are described. (Kako, I.)