Root Cause Analysis and Structural Integrity Evaluation for a Crack in a Reactor Vessel Upper Head Penetration Nozzle

원자로 상부헤드 관통노즐 균열에 대한 원인분석 및 건전성 평가

This paper presents the results of integrity assessment for the cracks happened in reactor vessel upper head penetration nozzles. The crack morphology for a boat sample from crack area was analyzed through microscope. The stress condition including weld residual stress around crack was analyzed using finite element analysis. From the results of crack morphology and stress condition, the crack was concluded as primary water stress corrosion cracking. The integrity of the cracked nozzle was assessed by the methodology provided in ASME Section XI. According to the assessment results, the remaining life of the cracked nozzle was 1.43 yrs. and the plant decided to repair it.