ESTIMATION OF THE CRITICAL DETECTABLE SIZE OF NATURAL 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 using ultrasonic testing (UT), round robin tests have been conducted. One test was to detect artificial defects using the normal and angle beam UT and a second test was to detect natural defects using UT. This paper describes the results of the second test and discussions including the previous test results.

The main results are:

(1) The detected echo level of natural defects was lower than the artificial defects of a 6 mm diameter by about 10 dB or more.

(2) The detectability increased with the defect size.

(3) The estimated size of detected defects was larger than that of actual natural defects.

(4) The statistical critical detectable size of natural flaws was estimated once again to be 16.9 mm, which was the same as the previous paper concerning artificial defects.

(5) This size has the safety factor of 2 or more for the allowable flaw size of DCI casks.

KEY WORDS:

• Ultrasonic testing
• round robin test
• ductile cast iron
• natural flaw
• critical size
• storage cask