Properties of zirconium alloys and their applications in light water reactors (LWRs) - Materials' Ageing and Degradation in Light Water Reactors - page 117

20

10

1700-2200 ppm
2700-3100 ppm

0

0 100 200

Testing temperature (°C)

300 400

4.29 Elongation (%) as a function of the testing temperature for the
specimens hydrided at 700 ° C (Bai, 1996).

commonly on grain boundaries or intragranularly. For pure zirconium the
most common habit plane is near [101 ̅ 0] and for Zircaloy or alloys it is [101 ̅ 7],
which is ~15° from the basal plane. Intragranular precipitation is less com-
mon and is more likely to occur in materials with large grain size, at inter-
metallic particles, at dislocations or as a result of very rapid cooling rates.
Factors that determine the orientation of the precipitating hydrides in addi-
tion to grain size include stress, texture and cold work. As a result, the fab-
rication process will have a strong effect on the hydride orientation. Good
reviews of the various hydride precipitates are given by Ells (1968), Cox
and Rudling (2000) and Coleman (2003). Figure 4.37 illustrates hydrides in
3-dimensions after the metallographic etching process, which exaggerates
the actual length of the hydrides.
The orientation of the hydride platelets that form during normal reac-
tor operation, preferentially near the cooler cladding OD, usually have
axial-circumferential orientation in tubes (respectively axial-tangential ori-
entation in strips) and they remain so during wet storage of the spent fuel.
The hydrides can become oriented in the radial (through thickness) direc-
tion if they are precipitated during operation under high tensile stresses, for
example from fuel swelling, or precipitated from solid solution by cooling the
alloy from a higher temperature under a tensile hoop stress. Reorientation
could occur during reactor operation during cool-down or power cycling,
although it is generally unlikely. However, it can occur during dry storage if
the internally pressurized cladding is at a high temperature, holds suffi cient
hydrogen in solution and is then cooled under a suffi ciently high hoop stress.
The hydrides in solution will precipitate in the radial orientation, while the
hydrides that did not dissolve will remain in their original circumferential
orientation.
This is most likely to occur during rapid cool-down from high tempera-
tures when a cask drying or evacuation procedure is applied rather than
during storage.

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