Corrosion in pressurized water reactors (PWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 6

Environmentally assisted fatigue occurs under the combined actions of
low frequency cyclic loading and oxidation. Therefore, ingredients are very
similar to those involved in SCC mechanisms in the sense that a synergy
operates locally between oxidation and mechanics. The major difference
with SCC is the nature of the loading: cyclic loading strongly promotes strain
localization in shear bands. The movement of dislocations (defects allowing
the non-reversible deformation of the metal) enhances oxide ingress and
failure, especially when shear bands emerge at the surface, breaking the pas-
sive layer. Therefore, strain rate is one of the key controlling parameters of
the mechanism.
Last, it should be noted that one of the corrosion products is able to
embrittle metals. Indeed, as reported before, the cathodic reaction (reduc-
tion of water) produces hydrogen which partly enters into the metal and
interacts with the microstructure. The entry of hydrogen is limited by
the growth of the passive layer; also its transport and interactions with
the metal strongly depend on temperature. Hydrogen embrittlement is the
process by which a localized accumulation of a suffi cient level of hydrogen
can eventually lead the metal to fracture under residual or applied stress.
Therefore, situations limiting the transport of hydrogen (low temperature,
presence of traps) promote such embrittlement. Other mechanisms of
introducing hydrogen into metals exist, such as manufacturing (welding)
and irradiation.

2.3 Major components experiencing corrosion
We continue the chapter by describing the major components within the
reactor which are subject to corrosive damage.

2.3.1 Reactor pressure vessel (RPV)

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Reactor vessel heads (RVH) can experience different types of corrosion. In
2002 boric acid crystal deposits and iron oxide were found to have fl owed
out from several openings in the lower service structure support skirt after
removal of insulation from the Davis-Besse RVH, after an accumulated
≈ 16 effective full power years (EFPYs) of operation. A large corrosion cav-

ity was found on the downhill side of the low-alloy steel RVH. 1 Boric acid
corrosion wastage occurred on the RPV head surface and lead to a total
low-alloy steel loss of ~4.3 cm 3 . Boric acid corrosion was not the only mech-
anism involved in the degradation: it was supposed that erosion-corrosion
may have played a role in the initial cavity formation; galvanic corrosion
between the low-alloy steel and the stainless steel occurred around the
perimeter of the exposed cladding; and axial stress corrosion cracks were
observed in fi ve control rod drive mechanism (CRDM) nozzles adjacent