Helium solubility in SON68 nuclear waste glass

Helium behavior in a sodium borosilicate glass (SON68) dedicated to the immobilization of high-level nuclear waste is examined. Two experimental approaches on nonradioactive glass specimens are implemented: pressurized helium infusion experiments and ³He ion implantation experiments. The temperature variation of helium solubility in SON68 glass was determined and analyzed with the harmonic oscillator model to determine values of the energy of interaction E(0) at the host sites (about -4000 J/mol), the vibration frequency (about 1.7 × 10¹¹ s^-1), and the density of solubility sites (2.2 × 10²¹ sites cm^-3). The implantation experiments show that a nondiffusive transport phenomenon (i.e., athermal diffusion) is involved in the material when the helium concentration exceeds 2.3 × 10²¹ He cm^-3, and thus probably as soon as it exceeds the density of solubility sites accessible to helium in the glass. We propose that this transport mechanism could be associated with the relaxation of the stress gradient induced by the implanted helium profile, which is favored by the glass damage. Microstructural characterization by TEM and ESEM of glass specimens implanted with high helium concentrations showed a homogeneous microstructure free of bubbles, pores, or cracking at a scale of 10 nm.