On the Mechanism of the Disproportionation of ZrCo Hydrides

Among the various getter materials the interalloy ZrCo has been selected by the ITER team as the reference material for the storage of hydrogen isotopes at the tritium plant because of its excellent getter properties, which are comparable to those of uranium. Only certain conditions, such as the presence of high partial pressure of H₂ at relatively low temperatures (350°C to 400°C), or during repeated hydrogen absorption-desorption heat cycles, have been a matter of concern, because under these conditions ZrCo can lose its gettering properties. Indeed, under repetitive loading/deloading cycling, the getter hydride (ZrCoH₃) tends to disproportionate, i.e., to convert into ZrH₂ and ZrCo₂ and thus show a significant performance degradation of its gettering properties. Disproportionation is a major drawback as it fixes almost irreversibly part of the hydrogen (hence, tritium) into a ZrH₂ form.

To understand the underlying mechanism leading to the disproportionation, a detailed investigation has been undertaken. Using thermal analytical methods and based on crystallographic considerations, we came to the conclusion that the driving force for such disproportionation has to be attributed to the hydrogen occupation (taking place during the hydridation) of the various crystallographic sites available to it. During the hydridation process [approximately]4% of hydrogen goes into the less-stable 8f₂ and 8e sites, where the Zr-H distance is shorter than the ZrH₂ distance. Therefore, during the dehydridation process these sites are not releasing the hydrogen, but rather they are generating the very stable ZrH₂, thus leading to the partial disproportionation of the material.

Therefore, we may conclude that ZrCo it is not adequate for the storage of tritium and other hydrogen isotopes within the tritium plant of ITER, and consequently, we would not recommend it for such use.