A criticality accident (also sometimes referred to as an "excursion" or "power excursion") occurs when a nuclear chain reaction is accidentally allowed to occur in fissile material, such as enriched uranium or plutonium. This releases neutron radiation which is highly dangerous to surrounding personnel and which causes induced radioactivity in the surroundings. When such incidents occur outside reactor cores and test facilities where fission is intended to occur, they pose a high risk both of injury or death to surrounding workers and of release of radioactive material. While dangerous, the low densities involved in these accidents limit the chain reaction, preventing them from becoming a nuclear explosion.

Criticality can be achieved by metallic uranium or plutonium, and also by compounds and liquid solutions of these elements. The isotopic mix, the shape of the material, the chemical composition of solutions, compounds, alloys and composite materials, and the surrounding materials all influence whether the material will go critical, that is will sustain a chain reaction. The calculations can be complex, so installations both civil and military that handle fissile materials employ specially trained criticality officers to monitor operations and prevent criticality accidents.

Criticality accidents have occurred both in the context of nuclear weapons and nuclear reactors. In 1945, for example, Los Alamos scientist Harry K. Daghlian, Jr. 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 nine months 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 an intense pulse of X-rays and gamma rays. 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

• radiation poisoning
• critical mass
• nuclear fission
• List of nuclear accidents

External links

• Press release on a report on criticality accidents from Los Alamos National Laboratory (http://www.lanl.gov/worldview/news/releases/archive/00-099.shtml)