Phenomenology of Hydrogen Flaking in Nuclear Reactor Pressure Vessels*

Abstract

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.

Initiation of hydrogen induced cracks at secondary phase particles

“…These materials exhibit large segregation zones, both macro-and micro-segregation, since they originate from large forgings. Such segregation zones are formed during the casting process and are further deformed by the subsequent manufacturing processes, such as forging [8]. Due to the long time required for solidification, large ingots tend to have a larger degree of segregation compared to smaller ingots.…”

“…HIC occurs, for instance, in oil and gas sour service pipelines where hydrogen ingress into steel from H 2 S occurs [7]. Also hydrogen flakes as found in reactor pressure vessels [8] are an example of HIC and could be artificially reproduced by intensive electrochemical hydrogen charging [9]. HIC occurs when the hydrogen concentration in the steel matrix exceeds a threshold hydrogen concentration.…”

Electrochemical hydrogen charging to simulate hydrogen flaking in pressure vessel steels

“…Hydrogen flakes are cracks that typically initiate in high strength steels during metal solidification of large forgings [1,2,3]. The primary source of hydrogen is water vapor which originates from the atmosphere, furnace charge materials, slag compounds and alloy additions, refractory linings, and ingot molds.…”

The role of titanium and vanadium based precipitates on hydrogen induced degradation of ferritic materials

“…Hydrogen induced cracking can occur in metals subjected to high fugacity hydrogen environments, such as high pressure hydrogen gas environments or under extreme cathodic charging conditions, even without the application of an external load [21,22]. Hydrogen flakes as found in reactor pressure vessels [23] are also an example of hydrogen induced cracking. The internal pressure theory [24][25][26] states that hydrogen induced cracking results from the formation of high pressure hydrogen gas bubbles in internal voids and microcracks.…”