Zirconium alloys are described more fully in Chapter 2.07, Zirconium Alloys: Properties and Character­istics. They are used in fusion reactors partly because of their corrosion resistance in aqueous environments and low neutron cross-sections.157 However, zirco­nium readily forms embrittling hydride precipitates. Zirconium alloys oxidize and the surface ZrO2 may be an effective permeation barrier’ preventing both hydrogen release and formation of detrimental
hydrides. Andrieu eta/.158 demonstrated that the rate of tritium release of zircaloy-4 (Zry4) decreased sub­stantially upon oxide formation in tritiated water.

Zirconium has multiple phases at temperatures of interest: for example, a-, p-, and g-Zr coexist in equilibrium at 833 K. Most solubility and diffusivity studies have been conducted on the single-phase a-Zr generally at 773 K and below (Figure 16). Above this temperature, zirconium alloys dissolve up to 50 at.% hydrogen and this solubility decreases rapidly with decreasing temperature, causing hydride precipitates within the alloys. The solubility has been found to vary slightly with the alloying content. Yamanaka et a/.159 note that the solubility in the p-phase decreases with alloying additions, while the solubility in the a-phase increases with alloying additions.

The solubility of hydrogen in ZrO2, regardless of the crystal structure (10-4 to 10-5mol hydrogen per mol oxide), is much lower than in the base metal and is even lower than that in Al2O3. a-ZrO2 exhibits a solubility almost an order of magnitude lower than p-ZrO2.160

Greger et a/.161 have reviewed hydrogen diffusion in zirconium. The diffusivities reported in studies they cite and in others is plotted in Figure 17. At 623 K, the diffusivity of hydrogen in zirconium is 10 m s, while the diffusivity in

-19 -20 2 -1 158,163,165

ZrO2 is only 10 to 10 m s. ’’ Austin

106

‘as

105

CO

E о E,

>.

104

о w

1000

Temperature, 1000/T (K-1)

et al.163 were able to measure the diffusivity in both a — and р-phases by measuring the activity, due to tritium, in tomographic slices of samples. The diffu — sivity values do not have a very strong dependence on crystallographic orientation or on alloy composition.

some of these phases. However, these quantities appear to be relatively large for the zirconium-matrix material. Further, autoradiography shows depletion in some iron-rich precipitates and at 623 K, the diffusivity in ZrFe2 is 2.5 x 10-11 m2 s-1, slower than in bare zirconium.168 The permeability values through hydrides might be larger because of the high solubil­ity of hydrogen isotopes in the hydride phase. How­ever, the volume fraction of hydrides tends to be small and the activation energy has been shown to be independent of the presence of the hydride.169

Zirconium alloys that lack an oxide layer are not useful in hydrogen environments that exceed the solubility of hydrogen in zirconium, because of hydride formation. At relatively low use temperatures (<623 K), and in aqueous or otherwise oxidizing environments, zirconium oxide is able to grow and is an effective barrier against the permeation of hydro­gen. Above this temperature, the integrity of the oxide layer cannot be maintained and the effective perme­ation of hydrogen isotopes is increased substantially.