Dictionary Thesaurus Word Dynamo Quotes Reference Translator Spanish

Criticality accident

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 normally is supposed to occur inside reactor cores and inside some test facilities. However, if fission occurs due to an accidental cause, such as a criticality accident, the radiation emitted poses a high risk of serious injury or even death to workers up to at least 20 metres (66 feet) 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.

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.

Accident types

Criticality accidents are divided into one of two categories:

Process accidents, where controls are generally in place to prevent any criticality,

and

Reactor accidents, where criticality is deliberately achieved in a nuclear reactor but then goes out of control. Excursion types can be classified into four categories depicting the nature of the evolution over time:

Prompt Criticality Excursion
Transient Criticality Excursion
Exponential Excursion
Steady State Excursion

Incidents

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

On 4 June 1945, Los Alamos an experiment to determine the critical mass of enriched uranium became critical when water leaked into the polyethylene box holding the metal. The radiation gave three people non-fatal doses.

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. This was the first known criticality accident causing a fatality.

On 21 May 1946, another Los Alamos scientist, Louis Slotin, accidentally irradiated himself during a similar incident, when a critical mass experiment with the very same sphere of plutonium (see demon core) 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.

On 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." In other words, 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. After the complex had been evacuated, the shift supervisor and radiation control supervisor re-entered the building. The shift supervisor then deceived the radiation control supervisor and entered the room of the incident and possibly attempted to pour the solution down a floor drain, causing a large nuclear reaction that irradiated the shift supervisor with a fatal dose of radiation. The shift supervisor's actions are the subject of a Darwin Award.

On 23 September 1983, an operator at the RA-2 research reactor in Centro Atomico Constituyentes, Buenos Aires, 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.

Observed effects

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.

Cherenkov radiation is produced by charged particles which are travelling through a dielectric substance at a speed greater than the speed of light in that medium. The only types of charged particle radiation produced in the process of a criticality accident (fission reactions) are alpha particles, beta particles, positrons (which all come from the radioactive decay of unstable daughter products of the fission reaction) and energetic ions which are the daughter products themselves. Of these, only beta particles have sufficient penetrating power to travel more than a few centimeters in air. Since air is a very low density material, its index of refraction (around n=1.0002926) differs very little from that of a vacuum (n=1) and consequently the speed of light in air is only about 0.03% slower than its speed in a vacuum. Therefore, a beta particle emitted from decaying fission products would need to have a velocity greater than 99.97% c in order to produce Cherenkov radiation. Because the energy of beta particles produced during nuclear decay do not exceed energies of about 20 MeV (20.6 MeV for ¹⁴B is likely the most energetic with 17.9 MeV for ³²Na being the next highest energy beta emitter) and the energy needed for a beta particle to attain 99.97% c is 20.3 MeV, the possibility of Cherenkov radiation produced in air via a fission criticality is virtually eliminated.

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, including lightning, appear blue. It is a coincidence 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 is believed to have been due to exposure to very high energy cosmic rays, 1% of which consist of 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

Some persons reported feeling a "heat wave" during a criticality event. 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.

Notes

References

Johnstone. List of radiation accidents, Wm. Robert Johnston
Johnstone. Wood River criticality accident, 1964, Wm. Robert Johnston
McLaughlin et al. "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.