Criticality accident

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 increase of nuclear chain reactions 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.

Critical or supercritical nuclear fission (one that is sustained in power or increasing in power) generally occurs inside reactor cores and occasionally within test environments. A criticality accident occurs when a critical reaction is achieved unintentionally. Although dangerous, typical criticality accidents cannot reproduce the design conditions of a fission bomb, so nuclear explosions do not occur. The heat released by the nuclear reaction will typically cause the fissile material to expand, so that the nuclear reaction becomes subcritical again within a few seconds.

In the history of atomic power development, sixty criticality accidents have occurred in collections of fissile materials outside nuclear reactors and some of these have resulted in death, by radiation exposure, of the nearest person(s) to the event. However, none has resulted in explosions.^[2]

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 as explained in the article.

Criticality occurs when too much fissile material is in one place. 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 personnel to monitor operations and prevent criticality accidents.

Accident types

Criticality accidents are divided into one of two categories:

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

and

• Reactor accidents, where deliberately achieved criticality in a nuclear reactor becomes uncontrollable. 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.^[3]

Since 1945 there have been at least 60 criticality accidents. These have caused at least 21 deaths: seven in the United States, ten in the Soviet Union, two in Japan, one in Argentina, and one in Yugoslavia. Nine have been due to process accidents, with the remaining from research reactor accidents.^[2]

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

• On 4 June 1945, Los Alamos scientist John Bistline was conducting an experiment to determine the effect of surrounding a sub-critical mass of enriched uranium with a water reflector. The experiment unexpectedly became critical when water leaked into the polyethylene box holding the metal. Three people received non-fatal doses of radiation.^[4]

• 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, which was later nicknamed the demon core. The brick acted as a neutron reflector, bringing the mass to criticality. This was the first known criticality accident causing a fatality.^[5]

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 (89 mm) plutonium "demon core" (the same as in the Daghlian incident) was inside, and is not visible.

• On 21 May 1946, another Los Alamos scientist, Louis Slotin, accidentally irradiated himself during a similar incident using the very same sphere of plutonium responsible for the Daghlian accident. Slotin surrounded the plutonium sphere with two hemispherical cups of neutron reflecting material; one above and a larger one below. He was using a screwdriver to keep the cups slightly apart which kept the assembly subcritical. When the screwdriver accidentally slipped, the cups closed completely around the plutonium sending the assembly supercritical. 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.^[6]

• On 16 June 1958, the first recorded uranium processing related criticality occurred at the Y-12 Plant in Oak Ridge, Tennessee. During a routine leak test a fissile solution was unknowingly allowed to collect in a 55 gallon drum. The excursion lasted for approximately 20 minutes and resulted in eight workers receiving significant exposure. There were no fatalities, though five were hospitalized for forty-four days. All eight workers eventually returned to work.^[7][8]

• On 15 October 1958, a criticality excursion in the heavy water RB reactor at the Vinca Nuclear Institute in Vinča, Yugoslavia, killed one and led to the deaths of an additional five.^[9] The initial survivors received the first ever bone marrow transplant in Europe, but they all died because of incompatibility rejection.^[10][11][12]

• On 30 December 1958, the Cecil Kelley criticality accident took place at the Los Alamos National Laboratory. Cecil Kelley, a chemical operator 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 center of the vortex. Due to a procedural error, the mixture contained 3.27 kg of plutonium, which reached criticality for about 200 microseconds. Kelley received 3,900 to 4,900 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.^[13]

• On 23 July 1964, a criticality accident occurred at the Wood River Junction facility in Charlestown, Rhode Island. The plant was 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.^[14][15][16]

• On 10 December 1968, 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 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."^[17] 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.^[18][19]

• On 30 September 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.^[20][21][22]

• Based on incomplete information about the 2011 Fukushima I nuclear accidents, Dr. Ferenc Dalnoki-Veress speculates that transient criticalities may have occurred there.^[23] Noting that limited, uncontrolled chain reactions might occur at Fukushima I, a spokesman for the International Atomic Energy Agency (IAEA) “emphasized that the nuclear reactors won’t explode.”^[24] By March 23, 2011, neutron beams had already been observed 13 times at the crippled Fukushima nuclear power plant. While a criticality accident was not believed to account for these beams, the beams could indicate nuclear fission is occurring.^[25] Additionally, on April 15, TEPCO reported that nuclear fuel had melted and fallen to the lower containment sections of three of the Fukushima I reactors, including reactor three. The melted material was not expected to breach one of the lower containers, which could cause a massive radiation release. Instead, the melted fuel is thought to have dispersed uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, making the resumption of the fission process, known as a "recriticality" most unlikely.^[26]

Observed effects

Blue glow

Main article: Ionized air glow

Many criticality accidents have been observed to emit a blue flash of light and to heat the material 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^[27]) 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 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. It is worth remarking that the ozone smell was said to be a sign of high radioactivity field through Chernobyl liquidators.

The only situation where Cherenkov light may contribute a significant amount of light to the blue flash is where the criticality occurs underwater or in solution (such as uranyl nitrate in a reprocessing plant) and this would be visible only if the container were open or transparent.

Heat effects

Some people reported feeling a "heat wave" during a criticality event.^[28][29] It is not known, however, whether this 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, see above) 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 feels infrared light directly as heat, and ultraviolet light is a cause of sunburn, it is likely that this phenomenon can explain the heat wave observations.^[30]

See also

• Nuclear and radiation accidents
• Nuclear criticality safety

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 of exposure 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.
• Infinity, a 1996 story of Richard Feynman played and directed by Matthew Broderick. There was a sub story of a death due to a criticality accident.
• Day One, (TV 1989) A history of the A-bomb development.
• List of films about nuclear issues

Notes

1. ^ McLaughlin et al. pages 81-82
2. ^ ^{a b} "Criticality accidents report issued". Los Alamos National Laboratory (LANL). 2000-07-19. http://www.lanl.gov/news/index.php/fuseaction/home.story/story_id/1054. Retrieved 2011-04-01. 
3. ^ McLaughlin et al. pages 74-75
4. ^ 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."
5. ^ McLaughlin et al. pages 74-76, "His dose was estimated as 510 rem"
6. ^ 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."
7. ^ Y-12’s 1958 nuclear criticality accident and increased safety
8. ^ Criticality accident at the Y-12 plant. Diagnosis and treatment of acute radiation injury, 1961, Geneva, World Health Organization, pp. 27-48.
9. ^ 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 shortly afterward, and the other five recovered after severe cases of radiation sickness."
10. ^ "1958-01-01". http://www.johnstonsarchive.net/nuclear/radevents/1958YUG1.html. Retrieved 2011-01-02. 
11. ^ Vinca reactor accident, 1958, compiled by Wm. Robert Johnston
12. ^ Nuove esplosioni a Fukushima: danni al nocciolo. Ue: “In Giappone l’apocalisse”, 14 marzo 2011
13. ^ The Cecil Kelley Criticality Accident
14. ^ McLaughlin et al. pages 33-34
15. ^ Johnstone
16. ^ Database of radiological incidents and related events—Johnston's Archive: Wood River criticality accident, 1964
17. ^ McLaughlin et al. pages 40-43
18. ^ McLaughlin et al. page 103
19. ^ http://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1983/in83066s1.html
20. ^ McLaughlin et al. pages 53-56
21. ^ http://www.nrc.gov/reading-rm/doc-collections/commission/secys/2000/secy2000-0085/2000-0085scy.pdf
22. ^ http://www.nrc.gov/reading-rm/doc-collections/commission/secys/2000/secy2000-0085/attachment3.pdf
23. ^ "Has Fukushima’s Reactor No. 1 Gone Critical?". Ecocentric - TIME.com. 2011-03-30. http://ecocentric.blogs.time.com/2011/03/30/has-fukushimas-reactor-no-1-gone-critical/. Retrieved 2011-04-01. 
24. ^ Fukushima Workers Threatened by Heat Bursts; Sea Radiation Rises By Jonathan Tirone, Sachiko Sakamaki and Yuriy Humber Mar/31/2011 http://www.bloomberg.com/news/2011-03-30/record-high-levels-of-radiation-found-in-sea-near-crippled-nuclear-reactor.html
25. ^ Neutron beam observed 13 times at crippled Fukushima nuke plant TOKYO, March 23, Kyodo News http://english.kyodonews.jp/news/2011/03/80539.html
26. ^ Japan Plant Fuel Melted Partway Through Reactors: Report Friday, April 15, 2011 http://www.globalsecuritynewswire.org/gsn/nw_20110415_5020.php
27. ^ Decay Radiation Search
28. ^ McLaughlin et al. page 42, "the operator saw a flash of light and felt a pulse of heat."
29. ^ McLaughlin et al. page 88, "There was a flash, a shock, a stream of heat in our faces."
30. ^ Minnema, "Criticality Accidents and the Blue Glow," American Nuclear Society Winter Meeting, 2007.

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.

External links

• Press release on a report on criticality accidents from Los Alamos National Laboratory
• U.S. report from 1971 on criticality accidents to date
• The criticality accident in Sarov, IAEA, 2001 — well documented account of a criticality accident