Q: (a) Define what is meant by a criticality accident in the context of fuel fabrication. (b) State three methods used to control criticality in a fuel fabrication plant handling low-enriched uranium (LEU). (c) At what enrichment level does criticality become a significant concern for UO in the presence of water moderation? (d) Why is criticality control more challenging for MOX fuel than for LEU UO?
A:
(a) A criticality accident is an unintended, self-sustaining nuclear chain reaction. In fuel fabrication, it could occur if sufficient fissile material accumulates in a favourable geometry with moderation.
(b) Three methods: (1) Mass control — limiting the mass of fissile material in any single operation or container; (2) Geometry control — using vessels of favourable geometry (narrow diameter tanks, thin slabs) that cannot sustain a chain reaction regardless of the amount of material; (3) Moderation control — controlling the amount of water or hydrogenous material that can come into contact with the fissile material.
(c) For UO with water moderation, criticality becomes a concern above approximately 1 wt% U-235 (the minimum critical enrichment for an infinite homogeneous system of UO and water is approximately 1%). Practical concern begins around 1—2% enrichment. For commercial LEU (3—5%), criticality controls are essential.
(d) MOX fuel contains plutonium, which has a much lower critical mass than U-235 (approximately 10 kg for Pu-239 vs ~52 kg for U-235 in similar geometry). This means tighter mass limits, stricter geometry controls, and additional safety margins are required.