Recently, it been discovered that significant levels of hydrogen can be stored in bubbles and voids in both stainless steels and pure nickel when the hydrogen is cogenerated with helium, especially in light water spectra where there are also environmental sources of hydrogen.73-75 It was shown in these studies that this phenomenon is a direct result of the 59Ni nuclear reactions. Previously, it was a long-standing percep­tion that such storage could not occur at reactor­relevant temperatures.

The retained hydrogen levels are in significant excess of the levels predicted by Sievert’s Law and appear to be increasing with both cavity volume and neutron fluence. Since these gases are known to assist in nucleation and stabilization of cavities, it is expected that the nonlinear 59Ni reactions discussed earlier may lead to a rapidly developing, nonlinear, cavity-dominated microstructure in stainless steels irradiated at temperatures characteristic of pressur­ized water reactors.

Figure 17 presents such a microstructure observed in a PWR flux thimble tube (cold-worked 316 stain­less steel) at ^70 dpa and 330 °C.76 There is a very high density (>1017cm~3) of nanocavities with dia­meters <3 nm in both the alloy matrix and especially on grain boundaries. The measured concentrations of 600 appm He and 2500 appm H in this specimen are thought to reside primarily within the cavities. Most importantly, these cavities are essentially invisible
under well-focused imaging conditions and can only be imaged using very large levels of under-focus. This implies that previous studies on similar mate­rials may have overlooked such cavity-dominated structures.

When this specimen and near-identical specimens were subjected to slow strain rate testing after irradi­ation, the fracture surface was indicative of ^100% intergranular stress corrosion cracking (IGSCC), with lower doses and gas levels producing propor­tionally less IGSCC.77 As hydrogen is known to be a contributor to grain boundary cracking, it appears plausible that hydrogen storage may accelerate the cracking process and that higher exposure will lead to an increasing susceptibility to cracking. This issue may therefore become increasingly important as PWRs previously licensed for 40 years are being considered for life extension to 60 and possibly 80 years.