Abstract
During a severe accident of a light water reactor, the failure of piping of the reactor cooling system could occur due to a thermal load, resulted from the heat transfer from a high temperature gas generated in the reactor core and decay heat released from fission product deposits. It is considered that, under such a condition, the short-term creep at a high temperature causes the piping failure. The objective of the present study is to predict the piping failure quantitatively. For this purpose, the development of an analytical method for the accurate prediction of the creep deformation is required, in which a creep constitutive equation taking the creep damage into account should be used, in order to evaluate the structural integrity of the piping during the severe accident.
In this paper, creep constitutive equations considering the tertiary creep was fabricated for cold-drawn type 316 stainless steel (SUS316) based on the isotropic damage rule proposed by Kachanov-Rabotnov. In addition, creep analyses were performed for a pipe made of cold-drawn SUS316 under a condition that elevated temperature distribution was established in the pipe wall. The numerical results show that the damage of the pipe is quantitatively described by the damage variable introduced in the finite element analyses, and the failure characteristics are in reasonable agreement with those observed in a piping failure test. The failure time does not agree well with that the time of the piping failure test. It is, however, indicated that we can estimate the state of the failure of the coolant piping under severe accident by the accurate estimation of the temperature.
