Criticality accident

A criticality accident is an accidental uncontrolled nuclear fission chain reaction. It is sometimes referred to as a critical excursion, critical power excursion, divergent chain reaction, or simply critical. Any such event involves the unintended accumulation or arrangement of a critical mass of fissile material, for example enriched uranium or plutonium. Criticality accidents can release potentially fatal radiation doses if they occur in an unprotected environment.

Under normal circumstances, a critical or supercritical fission reaction (one that is self-sustaining in power or increasing in power) should only occur inside a safely shielded location, such as a reactor core or a suitable test environment. A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance.

Though dangerous and frequently lethal to humans within the immediate area, the critical mass formed would not be capable of producing a massive nuclear explosion of the type that fission bombs are designed to produce. This is because all the design features needed to make a nuclear warhead cannot arise by chance. In some cases, the heat released by the chain reaction will cause the fissile (and other nearby) materials to expand. In such cases, the chain reaction can either settle into a low power steady state or may even become either temporarily or permanently shut down (subcritical).

In the history of atomic power development, at least 60 criticality accidents have occurred, including 22 in process environments, outside nuclear reactor cores or experimental assemblies, and 38 in small experimental reactors and other test assemblies. Although process accidents occurring outside reactors are characterized by large releases of radiation, the releases are localized. Nonetheless, fatal radiation exposures have occurred to persons close to these events, resulting in more than 20 fatalities. In a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions.

The table below gives a selection of well documented incidents.

{| class="wikitable sortable"

! Date !! Location !! Description !! Injuries !! Fatalities !! Refs

| || Los Alamos || Scientist John Bistline was conducting an experiment to determine the effect of surrounding a sub-critical mass (35.4 kg) of uranium enriched to an average of 79.2% U-235 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. An estimated 3-4×10<sup>16</sup> fissions occurred and the temperature of the metal may have risen to 200º Celsius. Three people (Bistline, J. Kupferberg, and H. Hammel) received non-fatal doses of radiation. A classified postwar report said that: "No ill effects were felt by the men involved, although one lost a little of the hair on his head. The material was so radioactive for several days that experiments planned for those days had to be postponed." || 3 || 0 ||

| || Los Alamos || Scientist Harry Daghlian 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 ||

| || 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 involved in 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 and the assembly thereby subcritical, contrary to normal protocols. When the screwdriver accidentally slipped, the cups closed around the plutonium, sending the assembly supercritical. Slotin quickly disassembled the device, likely sparing others in the room from lethal exposure, but Slotin himself died of radiation poisoning nine days later. The demon core was melted down and the material was reused in other bomb tests in subsequent years. || 8 || 1 ||

| || 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 flickered intermittently when neutrons were being emitted) were "glowing continuously". Frisch's body had reflected some neutrons back to the device, increasing its neutron multiplication, 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. Afterwards 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 only insignificant exposures to personnel. This original Godiva device was irreparable after the second accident and was replaced by the Godiva II.|| 0 || 0 ||

| || Vinča Nuclear Institute || A criticality excursion occurred in the heavy water RB reactor at the Boris Kidrič Nuclear Institute in Vinča, Yugoslavia, killing one person and injuring five. The initial survivors received the first bone marrow transplant in Europe. || 5 || 1 ||

| || 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 rad (36.385 to 45.715 Sv) according to later estimates. The other operators reported seeing a bright flash of blue light and found Kelley outside, saying "I'm burning up! I'm burning up!" He died 35 hours later. || 0 || 1 ||

| || SL-1, west of Idaho Falls||SL-1, a United States Army experimental nuclear power reactor underwent a steam explosion and core disassembly due to improper manual withdrawal of the central control rod, killing its three operators by explosion force and impaling. || 0 || 3 ||

|7 February 1965

|Severodvinsk

|Refueling of Soviet November-class submarine. Reactor lid needed repositioning, and had control rods attached. Lid and thus rods were withdrawn too far, causing reactor criticality. All personnel were withdrawn.

|Unknown

|18 January 1970

|Krasnoye Sormovo Factory No. 112, Nizhny Novgorod

|Construction of a Soviet submarine K-320. Weakly fixed provisional control rods lifted by high velocity hydraulic test cooling water. Radioactive water released in factory hall. Western claim and Russian denial of a factory fire.

|Unknown

| || Chazhma Bay, Vladivostok || The reactor tank lid of the nuclear powered Soviet submarine K-431 was being replaced, after it had been refuelled. The lid was laid incorrectly and had to be lifted again with the control rods attached. A beam was supposed to prevent the lid from being lifted too far, but this beam was positioned incorrectly, and the lid with control rods was lifted up too far. At 10:55 AM the starboard reactor became prompt critical, resulting in a criticality excursion of about 5·10<sup>18</sup> fissions and a thermal/steam explosion. The explosion expelled the new load of fuel, destroyed the machine enclosures, ruptured the submarine's pressure hull and aft bulkhead, and partially destroyed the fuelling shack, with the shack's roof falling 70 metres away in the water. A fire followed, which was extinguished after 4 hours, after which assessment of the radioactive contamination began. There were ten fatalities and 49 other people suffered radiation injuries, and a large area northwest across the Dunay Peninsula was severely contaminated.|| 49 || 10 ||

|18 June 1999

|Shika Nuclear Power Plant

|Rod withdrawal during a test on Unit 1 caused unintended criticality. Test measures prevented immediate reinsertion of control rods. Rods reinserted and criticality ended after 15 minutes. 6 workers in radiation controlled area. Gamma pocket dosimeters, film badges, exhaust pipe monitors, and site boundary monitoring posts showed no radiation change. Shift staff decided not to report the accident to prevent construction delays to Shika Unit 2. Covered up until 2007.

File:Partially-reflected-plutonium-sphere.jpeg|The sphere of plutonium surrounded by neutron-reflecting tungsten carbide blocks in a re-enactment of Harry Daghlian's 1945 experiment There have been suspected criticalities involved in the 2011 Fukushima nuclear accident and 2019 Nyonoksa radiation accident.

Additionally, the US State Department alleged in 2020 that Russia and possibly China have since 1996 up to 2019 carried out secret underground experiments involving supercriticality and thus a violation of the zero-yield standard, the Comprehensive Nuclear-Test-Ban Treaty, and possibly the Threshold Test Ban Treaty. Such experiments may have led to accidents similar to at Nyonoksa.

{| class="wikitable sortable"

! Date !! Location !! Description !! Injuries !! Fatalities !! Refs

| March 2011 || Fukushima Daiichi Nuclear Power Plant || 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 23 March 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 15 April, 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. || 24 || 1 ||

|8 August 2019

|Nyonoksa

|The Nyonoksa explosion and radiation accident killed five military and civilian specialists off the coast of Nyonoksa, in the White Sea. Russia claimed the accident was related to an "isotope power source for a liquid-fuelled rocket engine." A US delegate told the United Nations General Assembly First Committee that a nuclear reaction occurred. According to CNBC and Reuters, it occurred during recovery of a previously tested 9M730 Burevestnik nuclear-powered cruise missile left on the seabed to cool the fission core's decay heat.

Observed effects

thumb|right|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

It has been observed that many criticality accidents emit a blue flash of light. This is also the reason electric 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" can also be attributed to Cherenkov radiation, if either water is involved in the critical system or when the blue flash is experienced by the human eye. but not for the lower energy charged particles emitted from nuclear decay.

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 realization of what has just occurred (i.e. the high probability of inevitable impending death from a fatal radiation dose), or if it is a physical effect of heating (or non-thermal stimulation of heat sensing nerves in the skin) due to radiation 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 potentially identified. In dense air, over 30% of the emission 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 light (visible or otherwise) through its heating of the skin surface, it is possible that this phenomenon can explain the heat wave perceptions. However, this explanation has not been confirmed and may be inconsistent with the intensity of light reported by witnesses compared to the intensity of heat perceived. Further research is hindered by the small amount of data available from the few instances where humans have witnessed these incidents and survived long enough to provide a detailed account of their experiences and observations.

Notes

</references>

References

Johnston, Wm. Robert. List of radiation accidents
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 , 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