Recently, the problem of hydrogen flaking resurfaced, when internal defects were detected in the reactor pressure vessels of two Belgian nuclear power plants. These defects turned out to be hydrogen flakes formed during the fabrication of these pressure vessels. The goal of this publication is to provide important insights into the phenomenon of hydrogen flaking, the different parameters that play a role in the mechanism, as well as the typical morphology and location of these flakes. Therefore an extensive literature study was combined with a detailed metallurgical characterization of a significant number of flakes. Hydrogen flaking is a fabrication problem, which is strongly linked with segregation phenomena. A combination of a sufficient amount of hydrogen, stresses and a sensitive microstructure causes hydrogen flaking. For these reasons hydrogen flakes inside large reactor pressure vessels are found in the so-called ghost lines, which originate from segregation processes during casting.

Corrosion in amine treating plants is known to cause integrity failures, plant shutdown, costly repairs, and so forth. The use of an amine treatment system for postcombustion CO 2 capture brings additional challenges in terms of the flue gas quality, flue gas composition, operating conditions, scale of operation, and so forth. These differences are expected to have a significant impact on the overall well-being of the plant and the maintenance strategy over its lifetime. The degradation of solvent, by oxidative and thermal degradation pathways, leads to the formation of various degradation products which are known to be corrosive. The oxidative degradation of amine leads to the formation of ammonia which can be emitted to the atmosphere in the treated flue gas stream. This study aims at verifying the interrelation between (1) solvent degradation, (2) corrosion, and (3) NH 3 emissions, based on two test campaigns of over 1500 operating hours, in a CO 2 capture pilot plant. An online tool for real-time corrosion monitoring, SmartCET, is presented and compared with offline corrosion coupon. The different process and operating conditions such as the change in the flue gas composition, start-up and shutdown, and solvent replacement were correlated with the variations in the corrosion parameters (general corrosion rate, pitting factor, and corrosion mechanism indicator (CMI)), for both campaigns. The three parameters showed only a small increase for most part of both the campaigns (>75 days), and followed a similar trend. However, during both the campaigns, a rapid increase of all the three parameters was observed. In one of the campaigns, the three parameters increased by about 2 orders of magnitude in a time period of only about 20 days, which points toward an autocatalytic out of control solvent chemistry. The common characteristic before the onset of autocatalytic solvent chemistry, is the re-start-up of the pilot plant after an extended period of shutdown. On the basis of these tests, the use of an online corrosion monitoring tool will enable long-term steady state operation of a full scale postcombustion CO 2 capture plant.

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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.

During the 2012 outage at Doel 3 (D3) and Tihange 2 (T2) Nuclear Power Plants (NPP), a large number of nearly-laminar indications were detected mainly in the lower and upper core shells. The D3/T2 shells are made from solid casts that were pierced and forged. Restart authorization in 2013 was accompanied by a number of “mid-term” requirements, to be completed during the first operating cycle after the restart. One of these requirements was the mechanical testing of irradiated specimens containing hydrogen flakes. These tests showed unexpected results regarding the shift in the Reference Temperature for Nil Ductility Transition (RTNDT) of the flaked material VB395 (Steam Generator shell rejected because of flakes) after irradiation. This paper presents the root cause analysis of this unexpected behaviour and its transferability (or not) to the D3/T2 Reactor Pressure Vessels (RPVs). A mechanistic and a manufacturing based approach were used, aiming at identifying the microstructural mechanisms responsible for the atypical embrittlement of VB395 and evaluating the plausibility of these mechanisms in the D3/T2 RPVs. This work was based on expert’s opinions, literature data and test results. Both flaked and unflaked samples have been investigated in irradiated and non-irradiated condition. All hydrogen-related mechanisms were excluded as root cause of the unexpected behaviour of VB395. Two possible mechanisms at the basis of the atypical embrittlement of VB395 were identified, but are still open to discussion. These mechanisms could be linked to the specific manufacturing history of the rejected VB395 shell. Since the larger than predicted shift in transition temperature after irradiation of VB395 is not linked with the hydrogen flaking and since none of the specific manufacturing history features that are possible root causes are reported for the D3/T2 RPVs, the D3/T2 shells should not show the unexpected behaviour observed in VB395.