Criticality accident
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.
A criticality accident occurs if the same reaction is achieved unintentionally, for example in an unsafe environment or during reactor maintenance.
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 a few reactor and critical experiment assembly accidents, the energy released has caused significant mechanical damage or steam explosions.
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.
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)[2] and MERMAIDS (mass, enrichment, reflection, moderation, absorption, interaction, density, and shape).
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).
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.
This spike can be easily detected by radiation dosimetry instrumentation and "criticality accident alarm system" detectors that are properly deployed.
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: 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.
These caused 21 deaths: seven in the United States, ten in the Soviet Union, two in Japan, one in Argentina, and one in Yugoslavia.
[49] The blue glow of a criticality accident results from the fluorescence of the excited ions, atoms and molecules of the surrounding medium falling back to unexcited states.
[53][54] 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.
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.
