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A 'criticality accident' sometimes referred to as an 'excursion', or a 'power excursion' occurs when a nuclear chain reaction accidentally occurs in fissile material, such as enriched uranium or plutonium. This releases neutron radiation which is highly dangerous to surrounding personnel and causes induced radioactivity in the surroundings.
Nuclear fission is supposed to occur inside reactor cores and inside some test facilities, however, when criticality accidents occur, they pose a high risk of serious injury or death to workers up to tens of metres away. Although dangerous, the low densities of fissile material and the long insertion time involved in these events limit the fission yield and peak power, preventing them from becoming a large scale nuclear explosion.

Contents

Cause

Observed effects of Criticality accidents

Types of Criticality accidents

Records

See also

Motion pictures

External links

Cause

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

Observed effects of Criticality accidents

'Blue glow'
Many criticality accidents have been observed emitting a blue flash of light and the material heats up substantially.
This blue flash or "blue glow" is often incorrectly attributed to Cherenkov radiation, most likely due to the very similar color of the light emitted by both of these phenomena. This is merely a coincidence.
Instead, the blue glow of a criticality accident actually results from the spectral emission of the excited ionized atoms (or excited molecules) of air (mostly oxygen and nitrogen) falling back to unexcited states, which happens to produce an abundance of blue light. This is also the reason electrical sparks in air and lightning, appear blue. It is an interesting coincidence then, but nothing more, that the color of Cherenkov light and light emitted by ionized air are a very similar blue, despite their very different methods of production.
It has also been proposed by some, that the blue flash is produced when beta radiation from the criticality event enters the eye of the observer and causes the emission of Cherenkov radiation as it traverses the vitreous humor of the eye. Though this effect is possible and was in fact noted by Apollo astronauts during their trip to the moon when they closed their eyes, the effect observed by the Apollo astronauts was due to exposure to very high energy cosmic rays, not beta particles.
In addition, the flashes seen by the Apollo astronauts were almost always described as being white with only one event described as being "blue with a white cast, like a blue diamond" while descriptions of the blue light accompanying criticality events is almost universally described as being "a blue glow".
The only situation where Cherenkov light may contribute a significant amount of light to the blue flash is where the criticality occurs underwater or fully in solution (such as uranyl nitrate in a reprocessing plant) and this would only be visible if the container were open or transparent.
'Heat effects'
It has also been reported that witnesses close to a criticality event feel a "heat wave" when it occurs. It is not known though, whether it may be a psychosomatic reaction to the terrifying realization of what has just occurred, or if it is actually a physical effect of heating (or nonthermal stimulation of heat sensing nerves in the skin) due to energy emitted by the criticality event. For instance, while the accident which occurred to Louis Slotin (a yield excursion of around 3×10¹⁵ fissions) would have only deposited enough energy in the skin to raise its temperature by fractions of a degree, the energy instantly deposited in the plutonium sphere would have been around 80 kJ; sufficient to raise a 6.2 kg sphere of plutonium by around 100°C (specific heat of Pu being 0.13 J·g⁻¹·K⁻¹). The metal would therefore have reached sufficient temperature to have been detected a very short distance away by its emitted thermal radiation. This explanation thus appears inadequate as an explanation for the thermal effects described by victims of criticality accidents, since people standing several feet away from the sphere also reported feeling the heat. It is also possible that the sensation of heat is simply caused by the nonthermal damage done to tissue on the cellular level by the ionization and production of free radicals caused by exposure to intense ionizing radiation.

Types of Criticality accidents

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 deliberately achieved in a nuclear reactor used for physical experimentation, but then goes out of control for one reason or another.

Records

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

★ On 21 August 1945, Los Alamos scientist Harry K. Daghlian, Jr. suffered fatal radiation poisoning after dropping a tungsten carbide brick onto a sphere of plutonium. The brick acted as a neutron reflector, bringing the mass to criticality.

★ Nine months later, another scientist, Louis Slotin accidentally irradiated himself during a similar incident, when a critical mass experiment with the very same sphere of plutonium took a wrong turn. Immediately realizing what had happened he quickly disassembled the device, likely saving the lives of seven fellow scientists nearby. Slotin succumbed to radiation poisoning nine days later.

★ On 15 October 1958, a criticality excursion in the heavy water RB reactor at the Boris Kidrič Institute of Nuclear Sciences in Vinča, Yugoslavia killed one and injured five.

★ On 23 July 1964 – Wood River Junction facility in Charlestown, Rhode Island. A criticality accident occurred at the plant, designed to recover uranium from scrap material left over from fuel element production. An operator accidentally added a concentrated uranium solution to an agitated tank containing sodium carbonate, resulting in a critical nuclear reaction. This criticality exposed the operator to a fatal radiation dose of 10,000 rad (100 Gy). Ninety minutes later a second excursion happened, exposing two cleanup crews to doses of up to 100 rad (1 Gy) without ill effect.[1]

★ On the 10 December 1968 Russia, Mayak, a nuclear fuel processing center in central Russia was experimenting with plutonium purification techniques. Two operators were using an "unfavorable geometry vessel in an improvised and unapproved operation as a temporary vessel for storing plutonium organic solution." The operators were decanting plutonium solutions into the wrong type of vessel. After most of the solution had been poured out, there was a flash of light, and heat. Startled, the operator dropped the bottle, ran down the stairs, and from the room. This is also the subject of a Darwin Award.

★ On 23 September 1983, an operator at the RA-2 research reactor in Constituyentes, Argentina received a fatal radiation dose of 3700 rads (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured.

★ Between June 24 1990 and July 1 1990, about four years after the Chernobyl accident, signs of a sub-critical neutron multiplication event occurred inside room 304/3 at the damaged reactor (see Russian Research Centre Kurchatov Institute report). The neutron increase was by a factor of about 60, much less than the increase that would result from a criticality. A gadolinium solution was injected to absorb neutrons and the neutron level returned to the original level.

★ 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.

See also

★ Radiation poisoning

★ Critical mass

★ Nuclear fission

★ List of nuclear accidents

★ Nuclear Criticality Safety

Motion pictures

★ ''The Beginning or the End'', a 1947 MGM movie that was the first Hollywood film to depict a person (played by actor Robert Walker) killed in an accident similar to the real-life Slotin criticality event.

External links

★ Press release on a report on criticality accidents from Los Alamos National Laboratory

★ List of radiation accidents

★ "A Review of Criticality Accidents" by Los Alamos National Laboratory (Report LA-13638), May 2000. Coverage includes United States, Russia, United Kingdom, and Japan. Also available at this page, which also tries to track down documents referenced in the report.

★ U.S. report from 1971 on criticality accidents to date

★ The criticality accident in Sarov, IAEA, 2001 — well documented account of a criticality accident

★ Glowing Georji: A Darwin Award for a 1994 incident in Russia