Environmental Engineering Reference
In-Depth Information
Corrosion of zirconium alloys is an electrochemically-driven process
affected by the microstructure and microchemistry of the alloy surface, the
nature of the oxide layer that forms, the temperature at the metal/oxide
interface, the chemistry and thermohydraulics of the corrodent water, the
effects of irradiation and the effects of time. Table 4.7 gives information
on the various types of commercial power reactor systems currently being
used throughout the world. In comparing BWRs with PWRs, with corrosion
mechanisms in mind, the main features are:
BWR coolant boils; PWR coolant does not. This has an important effect
at the oxide/water interface.
PWR coolant contains a high concentration of hydrogen; BWR coolant
does not. Complementarily, BWR coolant contains a high concentration
of oxygen, PWR coolant does not. This has an important effect on cor-
rosion processes.
PWR components generally operate at higher temperatures than BWR
components. Corrosion processes are temperature dependent.
Both reactor types employ chemical additions to the coolant which may
affect corrosion and buildup of deposits on fuel rods.
It also should be noted that BWR zirconium alloys continue to be primarily
Zircaloy-2 or slight variants of Zircaloy-2. PWR zirconium alloys no longer
tend to be Zircaloy-4, for reasons of insuffi cient corrosion resistance (and
hydriding resistance) at high burnup, but have moved toward zirconium
alloys with Nb additions.
The type of oxides which form during corrosion in reactor water can be
classifi ed into several categories. The two most basic are uniform and nodu-
lar corrosion. The 'uniform' category has an extension - 'patch' or acceler-
ated uniform. The fourth category is 'shadow corrosion', which can look like
thick uniform corrosion but has some characteristics of nodular corrosion.
The fi fth category is crud-related corrosion, which is a temperature driven
process induced by poor heat transfer in crud-impregnated corrosion layers.
These categories will be discussed later, but are introduced here. Table 4.8
(Garzarolli in Adamson
et al
., 2002) gives a useful summary of characteris-
tics of various corrosion types.
Uniform corrosion occurs in both PWRs and BWRs. The oxide itself
is uniform in thickness and consists of several different layers. For either
in or out of reactor, the initial shape of the corrosion-versus-time curve is
as shown in Fig. 4.41 in the pre-transition region. The fi rst transition point
occurs at around 2 µm oxide thickness in PWRs. The shape of the post-tran-
sition curve in PWRs depends on several variables: initial SPP size, irradia-
tion, amount of cold work, specifi c alloy, water chemistry, temperature, local
thermohydraulics and hydride concentration. For Zircaloy-4 the corrosion
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