Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 132

Tube surface with nodules

Metallographic cross
section

Uniform oxide

Nodular oxide

Zircaloy

20 μ m

ZrO 2

(a) Normal

uniform

Zry

Nodular

(b)

Increased
uniform

(c)

4.45 Corrosion morphology for Zircaloy in BWRs (Adamson et al .,
2007).

alloys such as stainless steel or Inconel. The oxide thickness is unusually
large and often appears to be particularly dense and uncracked. For exam-
ple shadow corrosion oxide induced by a stainless steel control blade bun-
dle is shown in Fig. 4.46. Shadow corrosion has 'always' been present in
BWRs, but not in PWRs primarily related to the high PWR hydrogen con-
centration which reduces or eliminates galvanic potentials between dissim-
ilar alloy components. In BWRs shadow corrosion caused no performance
issues until recently when at one reactor fuel failures were induced by
unusually severe 'enhanced spacer shadow corrosion' (Zwicky et al ., 2000 ).
More recently, shadow corrosion has been alleged to be involved in BWR
channel bow problems (Mahmood et al ., 2010). Both issues are addressed
later in this chapter.

￿ ￿ ￿ ￿ ￿ ￿

Accelerated uniform corrosion

The observation that uniform corrosion of BWR materials may increase
at high fl uence (burnup) has been introduced. The main factor driving this
increase is connected to the initial size distribution of the SPPs and their