Previous work on hydrogen absorption has been extended to cover the effect of radial hydrides and greater hydrogen concentrations, and the effect of neutron irradiation on Zircaloy-2 pressure tubes. Specimens with radial hydrides were produced by modification of tube-manufacturing route and by hydriding under pressure in lithiated water. Both series of tests showed that the effect of the hydrides was marked at 20 C but was small at 300 C. Thus, at design condition, the critical crack length is of the order of 3 to 4 in. Critical crack-length tests on irradiated tubes show that, up to doses of 10²¹ n/cm² (>1 MeV), there is no decrease in critical crack length on irradiation.

Results from small specimens and tube behavior have been correlated. This has permitted assessing the effect of a large number of material variables (for example, H₂ content, hydride orientation, neutron dose, etc.). For example, the effect of irradiation up to doses of 2 × 10²¹ n/cm² (Ni) has been assessed on the basis of both tension and notched specimens. These show that the effect on impact specimens saturates at 10²⁰ n/cm² but there is a progressive but small increase in tensile strength over the range of neutron dose examined.

This work shows that the critical crack length of through-thickness defects under reactor-design conditions is of the order of 3 to 4 in. Leakage can be expected to occur when the through-thickness crack is of the order of 0.5 in. and failure due to instability is not, therefore, considered feasible.