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.
Physical basis Criticality occurs when sufficient fissile material (a critical mass) accumulates in a small volume such that each fission, on average, produces one neutron that in turn strikes another fissile atom and causes another fission. This causes the fission chain reaction to become self-sustaining within the mass of material. In other words, in a critical mass, the number of neutrons emitted over time, exactly equals the number of neutrons captured by another nucleus or lost to the environment. If the mass is supercritical, the number of neutrons emitted per unit time exceeds those absorbed or lost, resulting in a cascade of nuclear fissions at increasing rate.
Criticality can be achieved by using metallic uranium or plutonium, liquid solutions, or powder slurries. The chain reaction is influenced by a range of parameters noted by the mnemonics MAGIC MERV (mass, absorption, geometry, interaction, concentration, moderation, enrichment, reflection, and volume) and MERMAIDS (mass, enrichment, reflection, moderation, absorption, interaction, density, and shape). Temperature is also a factor in criticality.
Calculations can be performed to determine the conditions needed for a critical state, e.g. mass, geometry, concentration etc. Where fissile materials are handled in civil and military installations, specially trained personnel are employed to carry out such calculations and ensure that all reasonably practicable measures are used to prevent criticality accidents, during both planned normal operations and any potential process upset conditions that cannot be dismissed on the basis of negligible likelihoods (reasonably foreseeable accidents).
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 an increase in neutron flux. This increased flux and attendant fission rate produces radiation that 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. Hence, to maintain a stable, exactly critical chain reaction, 1.5 neutrons per fission event must either leak from the system or be absorbed without causing further fissions.
For every 1,000 neutrons released by fission, a small number, typically no more than about 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.
In steady-state operation, nuclear reactors operate at exact criticality. When at least one dollar of reactivity is added above the exact critical point (where the neutron production rate balances the rate of neutron losses, from both absorption and leakage) then the chain reaction does not rely on delayed neutrons. In such cases, the neutron population can rapidly increase exponentially, with a very small time constant, known as 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, which occur due to operator errors or other unintended events (e.g., during maintenance or fuel loading) in locations intended to achieve or approach criticality, such as nuclear power plants, nuclear reactors, and nuclear experiments. The Chernobyl disaster was in fact a criticality accident of very huge power, but is normally listed under the Nuclear meltdown-accidents, because it was a power plant whose core melted completely after the criticality excursion.
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, which either self-terminates or continues with a tail region that decreases over an extended period of time. The transient critical excursion is characterized by a continuing or repeating spike pattern (sometimes known as "chugging") after the initial prompt-critical excursion. The longest of the 22 process accidents occurred at Hanford Works in 1962 and lasted for 37.5 hours. The 1999 Tokaimura nuclear accident remained critical for about 20 hours, until it was shut down by active intervention. The exponential excursion is characterized by a reactivity of less than one dollar added, where the neutron population rises as an exponential over time, until either feedback effects or intervention reduce the reactivity. 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 in 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.
Known accidents According to modern estimations, 67 known criticality accidents have occurred globally between 1945 and 1999, with none confirmed since. They have occurred during experimentation and production relating to nuclear weapon cores, research reactors, commercial reactors, and naval reactors.
A 2000 Los Alamos report recorded 60 criticality accidents between 1945 and 1999. These caused 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 to reactor and critical experiment accidents.
The Los Alamos also grouped reactor and critical experiment accidents by material: 5 in fissile solutions, 15 in bare and reflected metal systems, 13 in moderated metal and oxide systems, and 5 in miscellaneous systems.
A 2014 University of Nevada report identified seven further criticality accidents prior to 2000 that were not included in the Los Alamos report. Five occurred in the maintenance and refuelling of Soviet nuclear submarine reactors, and two occurred in Japanese commercial reactors during testing and were covered up until 2007.
The table below gives a selection of well documented incidents.