An experimental investigation of helium diffusion in a simulated zinc borosilicate nuclear waste glass is described. The helium inventory was generated by /sup 244/Cm doping and the influence of ..cap alpha.. damage demonstrated. Diffusion at short times and low fractional release values follows Arrehnius behavior and the diffusion coefficient is given as D/sub U/ = 2.1 x 10/sup -3/ exp (-15,000/RT). At longer times and higher fractional release values, trapping effects are observed, causing an apparent decrease in diffusivity by about one order of magnitude. Some estimates are presented with respect to canister pressurization as a function of storage time, temperature, and glass geometry.