Criticality accident: Wikis

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The left image shows the Lady Godiva assembly in the scrammed (safe) configuration, while the right image shows the damage caused to the supporting rods after the excursion of February 1954. Note the images are of different assemblies.[1]

A criticality accident, sometimes referred to as an excursion or a power excursion, is an accidental increasing nuclear chain reaction in a fissile material, such as enriched uranium or plutonium. This releases a surge of neutron radiation which is highly dangerous to humans and causes induced radioactivity in the surroundings.

A critical or supercritical nuclear fission (one that is sustained in power or increasing in power) normally is supposed to occur only inside reactor cores and (very occasionally) inside some test facilities. 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.

In the history of atomic power development, less than a dozen criticality accidents have occured in collections of fissile materials outside nuclear reactors, but most of these have resulted in death, by radiation exposure, of the nearest person(s) to the event. However, none have resulted in explosions.

Contents

Cause

Image of a 60-inch cyclotron, circa 1939, showing an external beam of accelerated ions (perhaps protons or deuterons) ionizing the surrounding air and causing a blue glow. Due to the very similar mechanism of production, the blue glow is thought to resemble the "blue flash" seen by Harry Daghlian and other witnesses of criticality accidents. Though the effect is often mistaken for Cherenkov radiation, the two are distinct phenomena.

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:
  1. Prompt Criticality Excursion
  2. Transient Criticality Excursion
  3. Exponential Excursion
  4. Steady State Excursion

Incidents

The sphere of plutonium surrounded by neutron-reflecting tungsten carbide blocks in a re-enactment of Harry Daghlian's 1945 experiment.[2]

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

  • On 4 June 1945, an experiment at Los Alamos to determine the critical mass of enriched uranium became critical when water leaked into the polyethylene box holding the metal. Three people received non-fatal doses of radiation.[3]
A re-creation of the Slotin incident. The inside hemisphere next to the hand is beryllium, with an external larger tamper under it, of natural uranium. The 3.5 inch diameter plutonium "demon core" (the same as in the Daghlian incident) was inside, and is not seen.
  • 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.[5]
  • 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.[6]
  • On 30 December 1958, a Los Alamos chemical operator named Cecil Kelley working on plutonium purification switched on a stirrer on a large mixing tank, which created a vortex in the tank. The plutonium, dissolved in an organic solvent, flowed into the vortex. Due to a measurement error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,600 rads according to later estimates. The other operators reported seeing a flash of light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later.[7]
  • 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 dropped a concentrated uranium solution into 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 when a plant manager returned to the building and turned off the agitator, exposing himself and another administrator to doses of up to 100 rad (1 Gy) without ill effect.[8][9]
  • On 10 December 1968, Mayak (Russia), 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 container. 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."[10] 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.
  • 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.[11]
  • 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.[12]

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 23Na being the next highest energy beta emitter[13]) 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 electric 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.

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 people reported feeling a "heat wave" during a criticality event.[14][15] 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;[citation needed] 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.[citation needed] 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.

An alternative explanation of the heat wave observations can be derived from the discussions above regarding the blue glow phenomenon. A review of all of the criticality accidents with eyewitness accounts indicates that the heat waves were only observed when the fluorescent blue glow (the non-Cherenkov light) was also observed. This would suggest a possible relationship between the two, and indeed, one can be readily identified. When all of the emission lines from nitrogen and oxygen are tabulated and corrected for relative yield in dense air, one finds that over 30% of the emissions are in the ultraviolet range, and about 45% are in the infrared range. Only about 25% are in the visible range. Since the skin detects infrared light directly as heat, and ultraviolet light is the accepted cause of sunburn, it is likely that this phenomenon can explain the heat wave observations. [16]

See also

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Motion pictures and television

  • 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.
  • Edge of Darkness, a 1985 British television drama where a character deliberately induces a criticality event as proof that he is in possession of plutonium
  • Fat Man and Little Boy, a 1989 Paramount picture, portrays a fictional composite of Harry K. Daghlian and Louis Slotin who dies after 2 hours of exposer when two hemispheres, which are separated by a wedge, connect accidentally.
  • "Meridian," an episode of Stargate SG-1, where a criticality accident similar to the Slotin incident occurs.

Notes

  1. ^ McLaughlin et al. pages 81-82
  2. ^ McLaughlin et al. pages 74-75
  3. ^ McLaughlin et al. page 93, "In this excursion, three people received radiation doses in the amounts of 66, 66, and 7.4 rep.", LA Appendix A: "rep: An obsolete term for absorbed dose in human tissue, replaced by rad. Originally derived from roentgen equivalent, physical."
  4. ^ McLaughlin et al. pages 74-76, "His dose was estimated as 510 rem"
  5. ^ McLaughlin et al. pages 74-76, "The eight people in the room received doses of about 2100, 360, 250, 160, 110, 65, 47, and 37 rem."
  6. ^ McLaughlin et al. page 96, "Radiation doses were intense, being estimated at 205, 320, 410, 415, 422, and 433 rem.74 Of the six persons present, one died and the other five recovered after severe cases of radiation sickness."
  7. ^ The Cecil Kelley Criticality Accident
  8. ^ McLaughlin et al. pages 33-34
  9. ^ Johnstone
  10. ^ McLaughlin et al. pages 40-43
  11. ^ McLaughlin et al. page 103
  12. ^ McLaughlin et al. pages 53-56
  13. ^ Decay Radiation Search
  14. ^ McLaughlin et al. page 42, "the operator saw a flash of light and felt a pulse of heat."
  15. ^ McLaughlin et al. page 88, "There was a flash, a shock, a stream of heat in our faces."
  16. ^ Minnema, "Criticality Accidents and the Blue Glow," American Nuclear Society Winter Meeting, 2007.

References

External links


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