Publications
Helium nano-bubble evolution in aging metal tritides
A continuum-scale, evolutionary model of helium (He) nano-bubble nucleation, growth and He release for aging bulk metal tritides is presented which accounts for major features of the experimental database. Bubble nucleation, modeled as self-trapping of interstitially diffusing He atoms, is found to occur during the first few days following tritium introduction into the metal and is sensitive to the He diffusivity and pairing energy. An effective helium diffusivity of 0.3 x 10{sup -16} cm{sup 2}/s at 300 K is required to generate the average bubble density of 5x 1017 bubbles/cm3 observed by transmission electron microscopy (TEM). Early bubble growth by dislocation loop punching with a l/radius bubble pressure dependence produces good agreement with He atomic volumes and bubble pressures determined from swelling data, nuclear magnetic resonance (NMR) measurements, and hydride pressure-composition-temperature (PCT) shifts. The model predicts that later in life neighboring bubble interactions may first lower the loop punching pressure through cooperative stress effects, then raise the pressure by partial blocking of loops. It also accounts for the shape of the bubble spacing distribution obtained from NMR data. This distribution is found to remain fixed with age, justifying the separation of nucleation and growth phases, providing a sensitive test of the growth formulation, and indicating that further significant bubble nucleation does not occur throughout life. Helium generated within the escape depth of surfaces and surface-connected porosity produces the low-level early helium release. Accelerated or rapid release is modeled as inter-bubble fracture using an average ligament stress criterion. Good agreement is found between the predicted onset of fracture and the observed He-metal ratio (HeM) for rapid He release from bulk palladium tritide. An examination of how inter-bubble fracture varies over the bubble spacing distribution shows that the critical Hem will be lower for thin films or small particle material. It is concluded that control of He retention can be accomplished through control of bubble nucleation.