The probability of formation and detection of large flaws in welds

This paper reviews recent data that have come available on flaws in nuclear components. It examines whether the conclusions in a previous paper [Bullough R, et al. The probability of “large” defects in thick section butt welds in nuclear components. Int J Pressure Vessels Piping 2001; 78: 553–65] still hold in the light of this new information. A new model for estimating the distribution of flaws greater than 15 mm in through-wall extent for submerged arc welds is also reviewed. In this model, four potential flaw formation mechanisms are considered: (i) lack of fusion, (ii) weld metal solidification cracking, (iii) hydrogen-induced cracking in the heat-affected zone, and (iv) hydrogen-induced cracking in the weld metal. The distribution of large flaws remaining after pre-service inspection is calculated for each mechanism as the product of the probability of forming the flaw and the probability of the flaw being undetected during pre-service radiography. The assumptions and general applicability of the model are discussed.