NPP Life Management_vs02

(b)

Figure 7: Typical hydrogen induced cracks (source: MTI Atlas of Corrosion and Related Materials Failures – electronic ed.)

Hydrogen blistering or cracking is controlled by minimizing corrosion and is normally not a problem in neutral or alkaline environments and with high-quality steels that have low impurity and inclusion levels. Nevertheless also under the primary water chemistry conditions of the reactor coolant system (RCS) of PWRs, with a typical pH T of approx. 6.9 to 7.4 (corresponding to a room temperature pH around 10; cf. Figure 8) the primary cathodic corrosion reaction will be:

- → H + OH -

H 2 O + e

Even for very low corrosion rates of the stainless steel cladding (e.g. 0.1 to 1 micron/yr) this will result in significant quantities of corrosion-generated hydrogen atoms that may enter into the base metal (Cf. Section 3.4).

Hydrogen and NPP Life Management: Doel 3 and Tihange 2

Figure 8: pH control in PWR primary coolant by adjusting the lithium concentration as the boron is consumed during fuel burn-up. The trajectory commonly employed over a typical fuel cycle is marked by the dark path (EPRI PWR Primary Water Chemistry Guidelines TR-105714-V1R4).

10

The internal austenitic stainless steel cladding of an RPV is sometimes considered to prevent hydrogen diffusion and potential hydrogen-induced cracking problems in the pressure vessels.