Environmental Engineering Reference
In-Depth Information
A more crucial parameter than metal coolant temperature is the metal
oxide interface temperature which is diffi cult to measure but may be calcu-
lated with large uncertainty. A further complexity arises as the thermal con-
ductivity reduces with burnup (due to penetration by the coolant into the
porosity, cracking and spalling of the oxide fi lms and crud deposition). The
measurement of the thermal conductivity of the loose or non-adherent crud
layers, which modifi es the metal oxide interface temperature, is extremely
diffi cult as the properties of subsequent layers deposited may not be the
same. It is known that the thermal conductivity of the crud is higher than the
zirconia layer or water or steam 50 which in a way increases the heat transfer
characteristics.
Uniform corrosion occurs in both PWRs and BWRs. The oxide that forms
is uniform in thickness, consists of several different layers and depends on
many factors such as initial SPP size, extent of cold work and irradiation,
alloy and water chemistries, temperature, local thermohydraulics, etc. The
microstructure of the Zircaloys used in BWRs is continuously evolving,
leading to dissolution of the SPP and formation of small and thin patches
of white oxide on the otherwise black uniform oxide layer, which thicken
at an accelerated rate. The sensitivity of nodular corrosion can be related
to the second phase particles present in the alloy, though the number of
nodules may not bear a one-to-one relation with the number of particles.
Nodular corrosion is encountered in BWRs and starts appearing after a few
to 100 days from the start of operation and usually saturates at higher expo-
sure times. Nodules, in general, do not form in Zircaloys with small SPP
sizes (<0.1
m) but initiate early in materials with large SPPs and grow at a
decreasing rate with fl uence. Figure 1.29 51 shows the appearance of nodular
corrosion on the fuel clad of a BWR fuel pin. The shape of nodular corro-
sion can be lenticular or spherical and growth in Zircaloy-2 decreases at high
burnups. 52 The nodular corrosion problem can be eliminated (or delayed)
by judiciously controlling the second phase particle sizes through appro-
priate
μ
￿ ￿ ￿ ￿ ￿ ￿
quench treatment although this may enhance uniform corrosion. 53
Nodules, whose thickness greatly exceeds the uniformly growing fi lm, are
prone to spalling and promote hydrogen pick-up. They can also be a cause
of introducing zirconia particles to the coolant. Though PWR and WWER
structures are not prone to the nodular corrosion attack, nodular corrosion
can be a problem if steam forms at the oxide-coolant interface. 54
β
1.29 Nodular corrosion on the fuel clad of a BWR fuel pin. 51
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