When a mass of Uranium or Plutonium goes critical outside of an experiment or a reactor vessel, it is called a "Criticality Accident", or sometimes an "Unshielded Criticality". Not surprisingly, most of these events have happened in fuel processing facilities, where nuclear fuel is dissolved in a solution. Some criticality accidents have happened in research laboratory settings though.
Remember that an unreflected Plutonium sphere weighing just 24 lbs can become supercritical? That sphere would be just 4.5 inches across. That's how much material it takes to make a Plutonium weapon. Suppose we had far less Plutonium, and it was dissolved in a liquid that could moderate neutrons, allowing far easier neutron capture by the Pu-239 nucleus... It wouldn't be enough to create a weapon, but it is would be enough to start a nuclear chain reaction. In this case you would only need about 1.5 lbs of Plutonium. That isn't a lot, especially if a facility is processing a batch containing several pounds of material in an aqueous solution.
It's happened. Below is a partial list of Criticality Accidents from Wikipedia detailing a few of the major events.
- 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. 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.
- On 21 August 1945, Los Alamos scientist Harry K. Daghlian, Jr. suffered fatal radiation poisoning after accidentally 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.
- On 21 May 1946, another Los Alamos scientist, Louis Slotin, accidentally irradiated himself during a similar incident (called the "Pajarito accident" at the time) using the same sphere of plutonium responsible for the Daghlian accident. Slotin surrounded the plutonium sphere with two 9-inch diameter hemispherical cups of 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. 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.
- Otto Frisch received a larger than intended dose of radiation in 1954, when leaning over the Lady Godiva device for a couple of seconds. He noticed that the red lamps that flickered 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.
- 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.
- On 15 October 1958, a criticality excursion in the heavy water RB reactor at the Vinca Nuclear Institute in Vinča, Yugoslavia, killed one and injured five. The initial survivors received the first bone marrow transplant in Europe.
- 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.
- On 23 July 1964, a criticality accident occurred at the Wood River Junction facility in Richmond, Rhode Island. The plant 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.
- 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." 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.
- 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.
- 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.
The bad thing about being near an unshielded nuclear reaction is the massive quantity of radiation received by your entire body. It's like a shotgun blast, only at the molecular level. The situation is similar to having a major sunburn, only it is not limited to the skin - it is throughout the body. Your cells are instantly damaged and/or killed in numbers far beyond the ability of the body to cope, and you pretty much start to die and decay at the cellular level. With a large acute radiation exposure, there is little that can be done no matter what medical treatment is available. Nasty.
Another interesting thing about these criticality accidents is that everyone sees a "blue flash", yet CCTV cameras do not record this flash at all... To understand why people see a blue flash but cameras do not, it is necessary to know about Cerenkov Radiation. Below is a look down at the bottom of a pool of water, at some very radioactive fuel rods that are stored in a grid. Pretty cool, right? They are making their own light underwater just by being radioactive.
Here is how Cerenkov radiation works: The speed of light in water is only about 80% the speed of light in air. Fuel that has recently been removed from a nuclear reactor is highly radioactive. Much of the decay results in beta particles (electrons) being ejected from the nucleus of the decaying atoms at very high energy, and as we know, this energy is really speed. In fact the beta particles can be ejected at greater than the speed of light in water. This light is the result of beta particles traveling faster than 80% of light through water.
Back now to the Blue Flash that criticality accident victims see, but that cameras do not: It is believed what people are seeing is Cerenkov radiation being generated in the liquid inside their eyeballs and corneas by the pulse of neutrons, beta particles and gamma radiation. Since Cameras have no water in front of their detectors, Cerenkov radiation would not be produced.
If you find this sort of thing as fascinating as I do, here is a link to a .pdf that is very comprehensive, describing all known accidents, how they occurred, and the outcome of each. Frequently, those people nearest the criticality lost their lives, limbs, or eyesight shortly after the event.
Although I have worked around reactors and radioactive materials quite a bit, and am very comfortable in that setting, I don't think I would be comfortable working in a nuclear fuel processing facility, or anywhere near Plutonium. ...not that anyone is going to come looking for me to perform that work, of course!!!