Number density of grain face bubbles versus the average projected area of the bubbles, observed in UO₂ fuel with pellet average burnups between 9 and 33 MWd(kgU)⁻¹ by White [62], Walker et al. [59] and Kashibe and Une [61]. The solid line is a plot of equation (12) over its range of application in the model. The dashed line represents the saturated grain face condition ϕ₂ = n_b Ā_p = 0.48.