Criticality accident

A criticality accident is an uncontrolled nuclear chain reaction. It is sometimes referred to as a critical excursion or a critical power excursion and represents the unintentional assembly of a critical mass of a given fissile material, such as enriched uranium or plutonium, in an unprotected environment. A critical or supercritical fission reaction (one that is sustained in power or increasing in power) generally only occurs inside reactor cores and occasionally within test environments; a criticality accident occurs when the same reaction is achieved unintentionally and in an unsafe environment. Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed is still incapable of producing a nuclear detonation of the type seen in fission bombs, as the reaction lacks the many engineering elements that are necessary to induce explosive supercriticality. 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, 60 criticality accidents have occurred, including 22 in collections of fissile materials located in process environments outside of a nuclear reactor or critical experiments assembly. Although process accidents occurring outside of reactors are characterized by a large release of radiation, the release is localized and has caused fatal radiation exposure only to persons very near to the event (less than 1 m), resulting in 14 fatalities. No criticality accidents have resulted in nuclear explosions.

Cause

Criticality occurs when sufficient fissile material (a "critical mass") is in one place such that each fission of an atom of the material, on average, produces a neutron that in turn strikes another atom causing another fission; this causes the chain reaction to become self-sustaining within the mass of material. Criticality can be achieved by using metallic uranium or plutonium or by mixing compounds or liquid solutions of these elements. The chain reaction is influenced by parameters noted by the acronym MAGIC MERV - for Mass, Absorption, Geometry, Interaction, Concentration, Moderation, Enrichment, Reflection and Volume.

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. The calculations that predict the excursion characteristics can also be complex, as this requires knowledge of the likely process upset conditions.

The assembly of a critical mass establishes a nuclear chain reaction, resulting in an exponential rate of change in the neutron population over space and time leading to neutron radiation and a neutron flux. This radiation contains both a neutron and gamma ray component and is extremely dangerous to any unprotected nearby life-form. The rate of change of neutron population depends on the neutron generation time, which is characteristic of the neutron population, the state of "criticality", and the fissile medium.

A nuclear fission creates approximately 2.5 neutrons per fission event on average. For every 1000 neutrons born in fission, 7 are delayed neutrons which are emitted from the fission product precursors, called delayed neutron emitters. This delayed neutron fraction, on the order of 0.007 for uranium, is crucial for the control of the neutron chain reaction in reactors. It is called one dollar of reactivity. The lifetime of delayed neutrons ranges from fractions of seconds to almost 100 seconds after fission. The neutrons are usually classified in 6 delayed neutron groups. The average neutron lifetime considering delayed neutrons is approximately 0.1 sec, which makes the chain reaction relatively easy to control over time. The remaining 993 prompt neutrons are born very fast, approximately 1 μs after the fission event.

Nuclear reactors operate at exact criticality. When at least one dollar of reactivity is added above the exact critical point (the point where neutrons produced is balanced by neutrons lost per generation) then the chain reaction does not rely on delayed neutrons, and the rate of change of neutron population increases exponentially as the time constant is the prompt neutron lifetime. Thus there is a very large increase in neutron population over a very short time frame. Since each fission event contributes approximately 200 MeV per fission, this results in a very large energy burst as a "prompt critical spike". This spike can be easily detected by radiation dosimetry instrumentation and "criticality accident alarm system" detectors that are properly deployed.

Accident types

Criticality accidents are divided into one of two categories:

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

• 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

The prompt critical excursion is characterized by a power history with an initial prompt critical spike as previously noted, that either self terminates or continues for an extended period as a tail region that decreases over time. The former was only 1 of the 22 process accidents, the latter is noted for reactors and critical assemblies. The transient critical excursion is characterized by a continuing or repeating spike pattern after the initial prompt critical excursion. The longest of the 22 process accidents lasted 37 hours. The 1997 Tokai Mura criticality accident lasted 18 hours. The exponential excursion is characterized by a reactivity of less than one dollar added, where the neutron population rises as an exponential over time, but not reaching prompt critical. The exponential excursion can reach a peak power level, then decrease over time, or reach a steady state power level, where the critical state is exactly achieved for a "steady state" excursion.

The steady state excursion is also a state which the heat generated by fission is balanced by the heat losses to the ambient environment. This excursion has been characterized by the Oklo natural reactor that was naturally produced within uranium deposits in Gabon, Africa about 1.7 billion years ago.

Recorded incidents

At least sixty criticality accidents have been recorded since 1945. These have caused at least twenty-one 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, and the others from research reactor accidents.

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

Date	Location	Description	Injuries	Fatalities	Refs
1944	Los Alamos	Otto Frisch received a larger than intended dose of radiation when leaning over the original Lady Godiva device for a couple of seconds. He noticed that the red lamps (that normally would flicker intermittently when neutrons were being emitted) were 'glowing continuously'. Frisch's body had reflected some neutrons back to the device, causing it to go critical, and it was only by quickly leaning back and away from the device and removing a couple of the uranium blocks that Frisch escaped harm but, he said, "if I had hesitated for another two seconds before removing the material ... the dose would have been fatal". On 3 February 1954 and 12 February 1957, accidental criticality excursions occurred causing damage to the device, but fortunately only insignificant exposures to personnel. This original Godiva device was irreparable after the second accident and was replaced by the Godiva II.	0	0
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. When that happened, the water began to function as a highly effective moderator rather than just a neutron reflector. Three people received non-fatal doses of radiation.	3	0
21 August 1945	Los Alamos	Scientist Harry K. Daghlian, Jr. suffered fatal radiation poisoning and died 25 days later after accidentally dropping a tungsten carbide brick onto a sphere of plutonium, which was later (see next entry) 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.	0	1
21 May 1946	Los Alamos	Scientist Louis Slotin accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same "demon core" sphere of plutonium responsible for the Daghlian accident. Slotin surrounded the plutonium sphere with two 9-inch diameter hemispherical cups of the neutron-reflecting material beryllium; one above and 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. Slotin quickly disassembled the device, likely saving the lives of seven fellow scientists nearby. Slotin succumbed to radiation poisoning nine days later. The demon core powered the first peacetime atomic bomb test, 32 days after Slotin's death.	8	1
16 June 1958	Oak Ridge, Tennessee Y-12 incident	The first recorded uranium-processing–related criticality occurred at the Y-12 Plant. 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 but most subsequently contracted cancer.	8	0
15 October 1958	Vinča Nuclear Institute	A criticality excursion in the heavy water RB reactor at the Vinca Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe.	5	1
30 December 1958	Los Alamos	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.	0	1
3 January 1961	SL-1, 40 miles west of Idaho Falls	SL-1, a United States Army experimental nuclear power reactor underwent a steam explosion and meltdown due to improper withdrawal of the central control rod, killing its three operators.	0	3
24 July 1964	Wood River Junction	The facility in Richmond, Rhode Island was designed to recover uranium from scrap material left over from fuel element production. An operator, intending to add trichloroethane to a tank containing uranium-235 and sodium carbonate to remove organics, erroneously added uranium solution instead, producing a criticality excursion. The operator was exposed 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. The operator involved in the initial exposure died 49 hours after the incident.	0	1
10 December 1968	Mayak	The 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." 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; this was followed by a large nuclear reaction that irradiated the shift supervisor with a fatal dose of radiation, possibly due to an attempt by the supervisor to pour the solution down a floor drain.	0	1
23 September 1983	Centro Atomico Constituyentes	An operator at the RA-2 research reactor in Buenos Aires, Argentina received a fatal radiation dose of 3700 rad (37 Gy) while changing the fuel rod configuration with moderating water in the reactor. Two others were injured.	2	1
30 September 1999	Tōkai	At the Japanese uranium reprocessing facility in Ibaraki Prefecture, 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.	0	2

• 

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

• 

  A re-creation of the Slotin incident. The inside hemisphere with the thumb-hole next to the hand is beryllium (replacing the uranium tamper in a Fat Man bomb), with an external larger metal sphere under it, of aluminium. The 3.5-inch-diameter (89 mm) plutonium "demon core" (the same as in the Daghlian incident) was inside at the time of the accident, and would not be visible. However, its dimensions are comparable with the two small half-spheres shown resting nearby.

• 

  Image of the Lady Godiva assembly in the scrammed (safe) configuration.

• 

  Image of the Lady Godiva assembly, showing the damage caused to the supporting rods after the excursion of February 1954. Note the images are of different assemblies.

There was speculation although not confirmed within criticality accident experts, that Fukushima 3 suffered a criticality accident. Based on incomplete information about the 2011 Fukushima I nuclear accidents, Dr. Ferenc Dalnoki-Veress speculates that transient criticalities may have occurred there. 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.” 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. 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 radioactivity 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.

Observed effects

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 an ionized-air glow. Due to the similar mechanism of production, the blue glow is thought to resemble the "blue flash" seen by Harry Daghlian and other witnesses of criticality accidents.

Blue glow

See also: Ionized-air glow

Many criticality accidents have been observed to emit a blue flash of light.

The blue glow of a criticality accident results from the fluorescence of the excited ions and atoms (or excited molecules) of air (mostly oxygen and nitrogen) falling back to unexcited states, which produces an abundance of blue light. This is also the reason electrical sparks in air, including lightning, appear electric blue. The smell of ozone was said to be a sign of high ambient radioactivity by Chernobyl liquidators.

This blue flash or "blue glow" is often incorrectly attributed to Cherenkov radiation. It is a coincidence that the color of Cherenkov light and light emitted by ionized air are a very similar blue as their methods of production are different. Cherenkov radiation does occur in air for high energy particles (such as particle showers from cosmic rays) but not for the lower energy charged particles emitted from nuclear decay. In a nuclear setting, Cherenkov radiation is instead seen in dense media such as water or in a solution such as uranyl nitrate in a reprocessing plant. Cherenkov radiation could also be responsible for the "blue flash" experienced in an excursion due to the intersection of high-intensity gamma radiation with the vitreous body within the eyeballs of those in the presence of the criticality. This would also explain the absence of any record of blue light in video surveillance of the more recent incidents.

Heat effects

Some people reported feeling a "heat wave" during a criticality event. It is not known whether this may be a psychosomatic reaction to the terrifying realization of what has just occurred, or if it is a physical effect of heating (or nonthermal stimulation of heat sensing nerves in the skin) due to energy emitted by the criticality event.

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. In dense air, over 30% of the emissions lines from nitrogen and oxygen 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.

See also

Related terms and concepts

• Criticality (status)
• Nuclear and radiation accidents
• Nuclear criticality safety

In popular culture

• The Beginning or the End, a 1947 MGM movie that was the first Hollywood film to depict a person (played by actor Tom Drake) killed in an accident similar to the real-life Slotin criticality event.
• Edge of Darkness, a 1985 British television drama where a character dying from radiation poisoning deliberately induces a criticality event as proof that he is in possession of plutonium, and to expose an enemy to a fatal radiation dose.
• 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 during research into using the fictional element "Naquadriah" in atomic weapons.
• 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.
• 1000 Ways to Die, a program airing on Spike TV related a docufiction story titled "Gone Fission", depicting terrorists involved in a criticality event similar to the Daghlian accident.
• List of films about nuclear issues

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