The safety assessment of a geological repository for used nuclear fuel must ensure that future generations are shielded from radiation from fission products, in particular those released by re-criticality events. An investigation is required to understand whether or not criticality can actually be achieved. In fulfilling this end, this study assesses the uncertainty in the composition and total mass of precipitates forming in the far-field due to variation in transport parameters. The Latin Hypercube Sampling technique is employed to generate an accurate, random distribution of variables employed in the transport model and to assess the uncertainty of attaining a critical mass. The average characteristics of the damaged fuel from the Fukushima Daiichi reactor cores is used as the reference waste form. Results are compared to the minimum critical masses of previous studies to assess the criticality safety margin.