Bolt Breakage Incident for Low-Level Radioactive Waste Container（2）― Quantitative Assessments on Delayed Fracture Sensitivity of Lid Bolts for Low Level Radioactive Waste Container by Slow Strain Rate Test (SSRT)

We found a total of five fractured lid bolts on low-level radioactive waste (ìLLWî) containers in storage from February to June 2015. We identified the cause to be delayed fracture, which can occur in high-tensile strength bolts. Therefore, to prevent any such recurrences, we decided to use material with a lower tensile strength to improve the susceptibility to delayed fracture. Lowering tensile strength is valid for preventing the recurrence of a delayed fracture, as indicated by reference publication1). On the other hand, we conducted a slow strain rate test (SSRT) to quantitatively assess the effectiveness of the recurrence prevention measure. As a result of SSRT, we confirmed that the delayed fracture susceptibility of 900MPa class tensile strength bolts, which have a reduced tensile strength, was a lower than that of 1200MPa class tensile strength bolts, which experienced delayed fracture. There was no significant difference in susceptibility to delayed fracture between 900MPa class tensile strength bolts and those having a tensile strength of 1000MPa for which we have never experienced a delayed fracture in over 20 years. Consequently, we confirmed that using 900MPa class tensile strength bolts is valid as a  measure for preventing recurrence of delayed fracture.