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Encyclopedia > Criticality accident
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. Wikipedia does not have an article with this exact name. ... Wikipedia does not have an article with this exact name. ... Albert Einsteins letter to President Roosevelt in 1939 about his concern, about (Nuclear chain reactions) Click for closeup of letter A nuclear chain reaction occurs when on average more than one nuclear reaction is caused by another nuclear reaction, thus leading to an exponential increase in the number of... This article or section should include material from Fissile material In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of nuclear fission. ... // Enriched uranium is uranium whose uranium-235 content has been increased through the process of isotope separation. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery white Atomic mass (244) g/mol Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ... Neutron radiation consists of free neutrons. ... Radioactivity may mean: Look up radioactivity in Wiktionary, the free dictionary. ...


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 workers up to a few tens of meters away and of release of radioactive material in the immediate vicinity. While 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 nuclear explosion. The term insertion time is used to describe the length of time which is required to rearrange a subcritical mass of fissile material into a critical mass. ... It has been suggested that Nuclear explosive be merged into this article or section. ...

Contents

Cause

Image of a 60-inch cyclotron, circa 1939, showing 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
Image of a 60-inch cyclotron, circa 1939, showing 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 metallic uranium or plutonium or 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, i.e., sustain a chain reaction. The calculations 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. 60-inch cyclotron, circa 1939, showing beam of accelerated ions (perhaps protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. ... 60-inch cyclotron, circa 1939, showing beam of accelerated ions (perhaps protons or deuterons) escaping the accelerator and ionizing the surrounding air causing a blue glow. ... A pair of Dee electrodes with loops of coolant pipes on their surface at the Lawrence Hall of Science. ... ... For alternative meanings see proton (disambiguation). ... ... Harry K. Daghlian, Jr. ... Cherenkov radiation glowing in the core of a TRIGA reactor Cherenkov radiation (also spelled Cerenkov or sometimes ÄŒerenkov) is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in the medium. ... A sphere of plutonium surrounded by neutron-reflecting blocks of tungsten carbide. ... An isotope is any of several different forms of an element each having different atomic mass. ...


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 (or, in some unusual instances such as when underwater, 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 deliberately achieved in a nuclear reactor used for physical experimentation, but then goes out of control for one reason or another. In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... This article is about electromagnetic radiation. ... Beta particles are high-energy electrons emitted by certain types of radioactive nuclei such as potassium-40. ... Core of a small nuclear reactor used for research. ...


Confusion with Cherenkov radiation and other effects

The "blue glow" which accompanies a criticality accident is often wrongly 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 glowing in the core of a TRIGA reactor Cherenkov radiation (also spelled Cerenkov or sometimes ÄŒerenkov) is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in the medium. ...


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 [1]) 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. 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. Cherenkov radiation glowing in the core of a TRIGA reactor Cherenkov radiation (also spelled Cerenkov or sometimes Čerenkov) is electromagnetic radiation emitted when a charged particle passes through an insulator at a speed greater than the speed of light in the medium. ... A dielectric, or electrical insulator, is a substance that is highly resistant to electric current. ... The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness. In metric units, c is exactly 299,792,458 metres per second (1,079,252,848. ... For the generation of electrical power by fission, see Nuclear power plant An induced nuclear fission event. ... An alpha particle is deflected by a magnetic field Alpha particles or alpha rays are a form of particle radiation which are highly ionizing and have low penetration. ... Beta particles are high-energy electrons emitted by certain types of radioactive nuclei such as potassium-40. ... A positron is the antiparticle of the electron. ... In nuclear physics, a decay product, also known as a daughter product, is a nuclide resulting from the radioactive decay of a parent or precursor nuclide. ... AIR is a three-letter abbreviation with multiple meanings, as described below: The Annals of Improbable Research, a monthly magazine devoted to scientific humour All India Radio - Indias Government Radio service AIR, a popular electronica band from France. ... Density (symbol: ρ - Greek: rho) is a measure of mass per volume. ... The refractive index of a material is the factor by which electromagnetic radiation is slowed down (relative to vacuum) when it travels inside the material. ... The speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning swiftness. In metric units, c is exactly 299,792,458 metres per second (1,079,252,848. ... An electronvolt (symbol: eV) is the amount of energy gained by a single unbound electron when it falls through an electrostatic potential difference of one volt. ... It has been suggested that Boriding be merged into this article or section. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... Dissolving table salt in water In chemistry, a solution is a homogeneous mixture composed of one or more substances, known as solutes, dissolved in another substance, known as a solvent. ... Uranyl nitrate (UO2(NO3)2) is a water soluble yellow uranium salt. ...


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 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 similar blue, despite their very different methods of production. The noun spectrum (plural: spectra) has a variety of meanings. ... It has been suggested that this article or section be merged with quantum state. ... AIR is a three-letter abbreviation with multiple meanings, as described below: The Annals of Improbable Research, a monthly magazine devoted to scientific humour All India Radio - Indias Government Radio service AIR, a popular electronica band from France. ... General Name, Symbol, Number oxygen, O, 8 Chemical series Nonmetals, chalcogens Group, Period, Block 16, 2, p Appearance colorless (gas) very pale blue (liquid) Atomic mass 15. ... General Name, Symbol, Number nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15, 2, p Appearance colorless Atomic mass 14. ... The term Blue may refer any of a number of similar colors. ... // During early investigations into electricity via Leyden jars and other instruments, a number of people (D. William Wall (1708), Stephen Gray (1735), and Abbé Nollet) proposed that small-scale sparks shared some similarity with lightning. ...


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, 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 were 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 inside 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 almost universally described as being "a blue glow". Vitreous humour is the clear gel that fills the eyeball, lying between the lens and the retina in the eye. ... Description Role: Earth and Lunar Orbit Crew: 3; CDR, CM pilot, LM pilot Dimensions Height: 36. ... Cosmic rays can loosely be defined as energetic particles originating outside of the Earth. ... Harry K. Daghlian, Jr. ...


Heat effects

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 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·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, 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. A psychosomatic illness is one with physical manifestations and supposed psychological cause, often diagnosed when any known or identifiable physical cause was excluded by medical examination. ... A sketch used by doctors to determine the amount of radiation to which each person in the room had been exposed during the excursion. ... General Name, Symbol, Number plutonium, Pu, 94 Chemical series actinides Group, Period, Block n/a, 7, f Appearance silvery white Atomic mass (244) g/mol Electron configuration [Rn] 5f6 7s2 Electrons per shell 2, 8, 18, 32, 24, 8, 2 Physical properties Phase solid Density (near r. ... A kilojoule (abbreviation: kJ) is a unit of energy equal to 1000 joules. ... Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the objects temperature. ... For other uses, see radical. ... ...


Records

Criticality accidents have occurred both in the context of nuclear weapons and nuclear reactors. The mushroom cloud of the atomic bombing of Nagasaki, Japan, 1945, rose some 18 kilometers (11 mi) above the hypocenter. ... Core of a small nuclear reactor used for research. ...

  • Nine months later, another scientist, Louis Slotin accidentally irradiated himself during a similar incident, when a critical mass experiment with two half-spheres of plutonium took a wrong turn. Immediately realizing what had happened he quickly disassembled the device, likely saving the lives of seven fellow scientists nearby. Slotin subsequently 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 1964Wood 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 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. August 21 is the 233rd day of the year (234th in leap years) in the Gregorian Calendar. ... 1945 (MCMVL) was a common year starting on Monday (the link is to a full 1945 calendar). ... Los Alamos National Laboratory, aerial view from 1995. ... Harry K. Daghlian, Jr. ... Tungsten carbide, WC, or W2C, is a chemical compound containing tungsten and carbon, similar to titanium carbide. ... Nuclear weapon designs are often divided into two classes, based on the dominant source of the nuclear weapons energy. ... A sketch used by doctors to determine the amount of radiation to which each person in the room had been exposed during the excursion. ... Radiation hazard symbol. ... October 15 is the 288th day of the year (289th in leap years). ... 1958 (MCMLVIII) was a common year starting on Wednesday of the Gregorian calendar. ... Yugoslavia (Jugoslavija in all South Slavic languages, Југославија in Serbian and Macedonian Cyrillic) is a term used for the three separate political entities that existed during most of the 20th century on the Balkan Peninsula in Europe. ... July 23 is the 204th day (205th in leap years) of the year in the Gregorian Calendar, with 161 days remaining. ... 1964 (MCMLXIV) was a leap year starting on Wednesday (the link is to a full 1964 calendar). ... Wood River Junction, Rhode Island is a small village located in the town of Richmond. ... September 23 is the 266th day of the year (267th in leap years). ... 1983 (MCMLXXXIII) was a common year starting on Saturday of the Gregorian calendar. ... The rad is a unit of radiation dose, with symbol rad. ... The gray (symbol: Gy) is the SI unit of absorbed dose. ... April 26 is the 116th day of the year in the Gregorian Calendar (117th in leap years). ... 1986 (MCMLXXXVI) was a common year starting on Wednesday of the Gregorian calendar. ... The nuclear power plant at Chernobyl prior to the completion of the sarcophagus. ... 1999 (MCMXCIX) was a common year starting on Friday, and was designated the International Year of Older Persons by the United Nations. ... Tōkai (東海村; -mura) is a village located in Naka District, Ibaraki, Japan. ... Uranyl nitrate (UO2(NO3)2) is a water soluble yellow uranium salt. ... Yugoslavia (Jugoslavija in all South Slavic languages, Југославија in Serbian and Macedonian Cyrillic) is a term used for the three separate political entities that existed during most of the 20th century on the Balkan Peninsula in Europe. ...


See also

Radiation hazard symbol. ... A sphere of plutonium surrounded by neutron-reflecting blocks of tungsten carbide. ... For the generation of electrical power by fission, see Nuclear power plant An induced nuclear fission event. ... Pathways from airborne radioactive contamination to man This article covers notable accidents involving nuclear material. ...

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