tepco claimed that there was a small but non-zero probability that the exposed fuel assemblies in the unit 4 reactor could reach criticality. the bbc commented that criticality would never mean a nuclear explosion, but could cause a sustained release of radioactive materials. criticality is usually considered highly unlikely, owing to the low "enrichment level" used in light water reactors. american nuclear engineer arnold gundersen, noting the much greater power and vertical debris ejection compared to the unit 1 hydrogen blast, has theorized that the unit 3 explosion involved a prompt criticality in the spent fuel pool material, triggered by the mechanical disruption of an initial, smaller hydrogen gas explosion in the building.