The oxidation of zirconium by water releases hydrogen gas, which partly diffuses into the alloy. As this occurs, an increase of hydrogen in solid solution results in a lowering of the α/β transus temperatures and, if the solubility limit is exceeded, also leads to the formation of zirconium hydrides. These hydrides are less dense and are weaker mechanically than the alloy and so their formation results in blistering and cracking of the cladding – a phenomenon known as hydrogen embrittlement.
Thermo-Calc can be used to predict the sensitivity of different alloy chemistries to hydrogen, as can be seen in the figure here, which shows the calculated H solubility in HCP_Zr compared with published experimental data from Zuzek et al. (Bulletin of Alloy Phase Diagrams, 1990).