Interim Result of a Detailed Investigation on a Cracked Pipe Weld in the Pressurizer Spray Line of a PWR in Japan

Abstract

A circumferential crack was found at the pipe weld in the pressurizer spray line of a Japanese PWR in 2020. The depth of the crack was 4.4 mm, and the crack was propagated along the grain boundary. The investigation conducted just after detecting the crack revealed the specific high hardness of the heat affected zone (HAZ) around the crack. To obtain more information on the cracked material and to develop the technical basis for structural integrity assessment, the Ministry of Economy, Trade and Industry (METI) project launched in 2021. SEM/EBSD observation of cross sections around cracked pipe weld, SEM/EDS analysis of the cracked surface near the inner surface, and micro-Vickers hardness test were conducted in the METI project. The SEM/EBSD analysis revealed relatively high plastic strains in the vicinity of grain boundaries and the specific large grains in the HAZ around the crack. The cross- sectional SEM observation also revealed some microstructurally small cracks (MSCs) in the cracked HAZ and the uncracked HAZ. The depths of MSCs were relatively equal to a crystal grain size. This paper summarizes the results of the observation in the METI project in 2022.