Study of crack detection probability for maintenance reliability of power plant (Estimation of crack detection probability on the damage probability)

Abstract

This paper discuss about suitable damage size that should be detected by ultrasonic testing (UT) for nuclear power plant piping when consider about propagation of IGSCC. The power plant piping loop is inspected using UT as an in-service inspection (ISI). The reliability of UT result is affected by examination personnel. In this paper, the reliability of UT is defined simplified POD (Probability of detection) curve. The curve is defined by only three parameters, minimum detectable crack depth (a₀), most detectable crack depth (a₁) and maximum POD(m). The reliability of plant piping with different diameters are estimated by using this POD curve. The Failure probabilities were calculated with Monte Carlo approach. In case of low carbon stainless steel with IGSCC, a₁ and m are less than 6 mm and 0.96 respectively, the piping loop failure probability is under 1x10^-7. In another case of sensitized stainless steel with IGSCC, a₁ and m are less than 6 mm and 0.96, the piping loop failure probability is under 1x10^-3. It was found that the parameters of a₁ and m do not affect plant reliability when a₁ is between 3 to 6mm, and m is between 1.00 to 0.96. This result can be used to decide the requirement for examination personnel reliability.