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 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, resulting in 14 fatalities. No
criticality accidents have resulted in nuclear explosions.
Cause Criticality occurs when sufficient
fissile material 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 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: Prompt Criticality Excursion
Transient Criticality Excursion Exponential Excursion
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.
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
“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 = Blue 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 of air 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 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 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 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 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 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. 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, 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 Notes
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, 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