Criticality accident
A criticality accident is when nuclear fission is unexpectedly and accidentally triggered. The term refers to the achievement of criticality in fissile material, such as uranium, plutonium, or thorium, which releases an often fatal amount of beta radiation (neutrons).
Criticality accidents have occurred both in the context of nuclear weapons and nuclear reactors. In 1945, for example, Los Alamos scientist Harry Daghlian accidentally irradiated himself while performing a critical mass experiment with two half-spheres of plutonium, disfiguring his hand and eventually dying of radiation poisoning a month later (another scientist, Louis Slotin performed the same experiment a year later and also had a similar fatal accident). In a very different incident in 1999 at a Japanese uranium reprocessing facility in Tokaimura, workers put a mixture of uranyl nitrate solution into a precipitation tank which was not designed to dissolve this type of solution and caused an eventual critical mass to be formed, and resulted in the death of two workers from radiation poisoning.
Most criticality accidents result in what is called a "blue flash," when surrounding air is ionized by a neutron blast (Cherenkov effect). Criticality accidents can be generally divided into one of two categories: process accidents, where controls are generally in place to prevent any criticality, and research reactor accidents, where criticality is purposely achieved in a nuclear reactor used for physical experimentation, but for one reason or another goes out of control.
Since 1945 there have been at least 21 deaths from criticality accidents; seven in the United States, ten in the Soviet Union, two in Japan, one in Argentina, and one in Yugoslavia. Nine have been due to process accidents, with the remaining from research reactor accidents.
See also
