ROOT CAUSE ANALYSIS AND CHARACTERISATION OF THE DOEL 3 AND TIHANGE 2 REACTOR PRESSURE VESSEL FLAWS

摘要

This paper presents the root cause analysis and characterisation of the indications discovered in two Belgian pressurized water reactor (PWR) pressure vessels (Doel 3 and Tihange 2), during in service inspections performed in 2012. The only plausible mechanism at the origin of the detected indications is hydrogen flaking during fabrication. Flaking could occur because of the local combination of high hydrogen concentration, stresses and susceptible microstructure. The phenomenology study performed on several materials including flaked materials provided precious information for the Safety Case: it confirmed the cause of the indications and evidenced that flaking exclusively occurs in segregated zones and that the ligament between flakes is sound. The paper also briefly addresses the possibility of evolution of the indications during operation: low cycle fatigue during transients is identified as the only mechanism likely to induce flaw growth in operation. A particular attention has been paid to the potential hydrogen effects.