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Criticality accident

The Godiva device before and after an accidental excursion in February 1954 showing damage to the device.

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.


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.


Criticality accidents have occurred both in the context of nuclear weapons and nuclear reactors.

In 1945, 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.

On 15 October 1958, a criticality excursion in the heavy water RB reactor at the Boris Kidrič Institute of Nuclear Sciences in Vinca, Yugoslavia kills one and injures five.

On 23 September 1983, an operator at the RA-2 research reactor in Constituyentes , Argentina recieved a fatal 3700 rad dose of radiation changing the fuel rod configuration while moderating water was in the reactor. One dead, two injured.

In a very different incident in 1999 at a Japanese uranium reprocessing facility in Tokai, Ibaraki, 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.

Since 1945 there have been at least 21 deaths from criticality accidents; 7 in the United States, 10 in the Soviet Union, 2 in Japan, 1 in Argentina, and 1 in Yugoslavia. 9 have been due to process accidents, with the remaining from research reactor accidents.

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Last updated: 10-25-2005 20:53:08
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