Criticality accident
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| Image:Godiva-before-scrammed.jpg | Image:Godiva-after-scrammed.jpg |
| 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.
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Cause
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.
Description
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 (and, in some unusual instances, beta particles). 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 then goes out of control for one reason or another.
Confusion with Cherenkov radiation and other effects
Very often, the "blue glow" which accompanies a criticality accident is wrongly attributed to Cherenkov radiation, most likely due to the very similar color of the light emitted by these phenomena. However, this is merely a coincidence. Cherenkov radiation is produced by charged particles travelling faster than the speed of light in a medium other than a vacuum. 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 (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 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 in "natural" nuclear decay rarely exceed energies of 20 MeV Cherenkov radiation produced in air is nearly impossible. 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. 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 simillar blue despite their very different methods of production.
It is 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 the Apollo astronauts when they closed their eyes, it was due to exposure to very high energy cosmic rays, not beta particles. Also it is thought that the flashes of light seen by the Apollo astronauts was due to a combination of the direct stimulation of the retina by the passing of the charged particle and of the Cherenkov radiation produced by the particle. This is a highly unlikely explanation for the mechanism of the criticality accident blue flash phenomenon for several reasons; one of which being that if the light were emanating from the eye itself, virtually all sensation of directionality of the light would be lost and the observer would see an even blue glow everywhere. In fact, the opposite is reported by witnesses to criticality events and the directionality of the blue flash is apparently readily detected (such as in the case of the security officer on duty during the accident involving Harry Daghlian). 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 practically universally described as being a "blue glow".
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 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 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·K)). 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 though, since people standing several feet away from the sphere also reported feeling the heat. It is also possible that the sensation of heat is merely 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.
Records
Criticality accidents have occurred both in the context of nuclear weapons and nuclear reactors.
In 1945, Los Alamos scientist Harry K. Daghlian, Jr. suffered fatal radiation poisoning after dropping a tungsten carbide brick onto a mass of plutonium. The brick acted as a neutron reflector, bringing the mass to criticality.
Nine months later, another scientist, Louis Slotin accidentally irradiated himself while performing a critical mass experiment with two half-spheres of plutonium, subsequently succumbing 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 Vinca, Yugoslavia killed one and injured five.
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.
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.
See also
External links
- Press release on a report on criticality accidents from Los Alamos National Laboratory
- List of radiation accidents
- This is the best review of real life criticality events
