ESTIMATION OF THE CRITICAL SIZE OF DETECTABLE FLAWS IN DUCTILE CAST IRON USING ULTRASONIC TESTING

Abstract To evaluate the critical size of detectable flaws in ductile cast iron (DCI) for storage casks of spent nuclear fuels with ultrasonic testing (UT), a round robin test was conducted. The test was performed under contract with nine companies. The main results are: (1) The average detection efficiency of 48 artificial defects in three test blocks was about 80%. (2) The detectability increased with the defect size. (3) As the nodularity of the test block became insufficient, the detectability decreased. (4) The average diameter of detected defect was larger than that of actual defects. (5) The deterministic critical size of detectable flaws in DCI was set as 6 mm dia, using normal beam UT. (6) The statistical critical size of detectable flaws also was estimated to be 13.8 mm. (7) The compensated statistical critical size of detectable flaws with angle beam UT was estimated to be 16.9 mm. This size has the safety factor of 2 or more for the allowable flaw size of DCI casks.