Criticality incidents are among the types of accidents that can occur when handling low-enriched uranium. According to news reports, the Tokai-mura workers were handling uranium enriched to around 18 percent uranium 235, in preparation for fabricating mixed oxide fuel for the Joyo research reactor. Electricity-generating reactors usually use uranium enriched to around 4 percent. The amount of uranium that can be stored safely in a given place depends on several factors. Any well-regulated nuclear facility has rules specifying how much uranium can be in any given place.
Criticality incidents occur frequently in nuclear fuel plants.
Such incidents, which may involve nothing more than the careless juxtaposition of two containers of uranium, are accidents in the making. The only reason that they are not full-blown accidents is that criticality regulations include a significant margin of error designed to limit the amount of uranium workers handle to concentrations that are considerably less than critical mass. If a worker exceeds the rules by a small percentage, no accident occurs. Severe accidents happen when workers err on an unexpectedly large scale, as happened at Tokai, when workers poured about seven times the permitted amount of uranium into a container. Critical A criticality accident would not usually spread radioactivity far outside a plant as apparently happened at Tokai.