Unirradiated fuel does pose a criticality hazard. However, because fuel elements are solid, visible and easily tracked, few criticality incidents have occurred involving fuel elements. The accumulation of uranium in liquid solutions (liquors) is a much more common cause of criticality incidents.
Three independent control methods are applied in any fuel-fabrication or handling facility to keep the system sub-critical with adequate margin:
| Control method | Principle | Examples in fuel fabrication |
|---|---|---|
| Mass control | Limit the quantity of fissile material in any one location below the safe-mass limit (typically a fraction of the bare critical mass). | Batch sizes in powder hoppers, presses and sintering boats; site-wide fissile-material accountancy. |
| Geometry (shape) control | Use vessels whose geometry cannot achieve a critical configuration (slabs, annular tanks, thin cylinders) regardless of fill level. | Slab tanks for UO slurries; annular dissolvers; “safe by shape” hoppers. |
| Moderation control | Exclude or limit neutron moderators (especially water) to keep the effective neutron multiplication low. | ”Dry” handling areas; bunds and floor drainage to prevent water ingress into UO stores; nitrogen blanketing of powder lines. |
For low-enriched (LEU) UO powder under optimum moderation, the minimum critical enrichment is approximately 1% U — so any UO enriched above ~1% must be handled under one of the controls above. Bare critical masses are roughly ~52 kg of U metal and ~10 kg of Pu metal; under optimum aqueous moderation these fall by an order of magnitude (a few hundred grams of Pu-239 in solution), which is why fissile solutions present a much greater criticality hazard than fissile metals (see Chapter 5 Tutorial Question 9).