The circumstances of the Japanese accident were out of the ordinary. Nevertheless, the production of fuel for electricity-generating reactors routinely causes environmental degradation and submits workers to some level of radiation. Furthermore, criticality accidents are among the types of accidents that can occur in plants handling low-enriched uranium. The amount of uranium that can safely be in a given place depends on such factors as the enrichment level of the uranium, the geometric shape of the uranium, and whether the uranium is confined and is in contact with a substance that slows down neutrons, in particular the hydrogen in water, organic solvents, and plastic materials. Any well regulated nuclear facility has rules specifying how much uranium can be in any given place within the facility, that handles or stores uranium.
Criticality incidents occur frequently in installations involved in the production of fuel.
At Portsmouth in the same period there were 183 reported events, of which 155 dealt with nuclear criticality safety. Such incidents, which may be as apparently ordinary as the 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 margin of error, ie workers are limited to putting an amount of uranium that is considerably less than the critical mass in a given place. Thus if a worker exceeds the rules by a small percentage, no accident occurs. The severe accident takes places when workers err on an unexpectedly large scale, as at Tokai, where about seven times the permitted amount of uranium was placed in a container.