Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

Hydrogen from coolant radiolysis

In addition to the corrosion-generated hydrogen, there is also the issue of hydrogen radicals being formed as a result of the radiolysis of water and the reactions of H 2 with the radiolysis products 15 (e.g. OH ⋅ + H2 → H ⋅ + H2O); hydrogen is used in the RCS to suppress radiolytic oxygen and hydrogen peroxide formation. More details of all these effects are discussed elsewhere 16 .

Earlier international observations

Finally, there are earlier observations of hydrogen-induced blister cracking that have been reported in nuclear structural materials 17 , and in the past there has been considerable debate about the issue. A very old, specific, example of failures attributed to hydrogen occurred in retaining rings used to connect inlet assemblies to the reactor process tubes in a Hanford, water-cooled production reactor. Failures occurred in carbon steels and Type 420 stainless steel. The reported hydrogen sources were the fabrication process, hydrogen generated during corrosion of the ring by the process water, and from galvanic coupling. 18 In view of all of the above, the “trapping” of cathodically-generated hydrogen (due to primary water corrosion reactions), or from radiolytic hydrogen, inside existing “hydrogen flakes” is not improbable. Moreover, the (original) flakes may act as stress raisers, which will enhance the diffusion of the hydrogen to the stressed areas in the metal, possibly also resulting in hydrogen stress cracking (HSC). Also, the additional effect of irradiation is still largely unknown. Traditionally, it has been assumed that hydrogen significantly affects the fracture properties of pressure vessel steel in both the unirradiated and irraditated states at hydrogen contents above 2 ppm. Some studies have measured the hydrogen content of the cladding and found it to be 3-4 ppm after prolonged irradiation in PWR water. 19 This can also be assumed to be the equilibrium content at the cladding/base material interface. Although hydrogen diffuses quickly in the RPV steel at high temperatures, the presence of efficient hydrogen traps, such as the “hydrogen flakes”, poses a severe threat. The effect of irradiation and the importance of hydrogen in some observed low fracture toughness values clearly requires further research. Aging risks and crack growth

More details will be discussed during the presentation.

CONCLUSIONS

In June-July 2012 “thousands” of fissures were discovered in the Reactor Pressure Vessel (RPV) of the Belgian nuclear reactor Doel 3 during an ultrasonic inspection (UST). In September 2012 similar defects, but fewer in number, were also found in the reactor Tihange 2. Both RPV forgings were produced by the same fabricator which also delivered some 10 vessels to US nuclear plants. “Hydrogen flakes” originating from the processing of the original RPV ingots were identified as the root cause of the problem. After an initial series of investigations, the reactors were authorized to re-operate, until a number of anomalous embrittlement results were found in irradiation experiments on similar materials. After the stop of the reactors, new UST inspections