  The effect of pressure on the room temperature solubility of hydrogen in
Zircaloy-4 was examined using synchrotron X-ray diffraction on small ground
flake samples in a diamond anvil cell at pressures up to 20.9 GPa. Different
combinations of hydrogen level/state in the sample and of pressure transmitting
medium were examined; in all three cases examined, it could be concluded that
pressure resulted in the dissolution of d hydrides and that interstitial
hydrogen retards the formation of w Zr. A pressure of around 9 GPa was required
to halve the hydride fraction. These results imply that the effect of pressure
is thermodynamically analogous to that of increasing temperature, but that the
effect is small. The results are consistent with the volume per Zr atom of the
a, d and w phases, with the bulk moduli of a and d, and with previous
measurements of the hydrogen site molar volumes in the a and d phases. The
results are interpreted in terms of their implication for our understanding of
the driving forces for hydride precipitation at crack tips, which are in a
region of hydrostatic tensile stress on the order of 1.5 GPa.
