Criticality accident

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 released by 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 released very quickly, 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:

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 Tokaimura nuclear 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 (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.

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, atoms and 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 particles with the vitreous humour 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.