A hot particle is a small, highly radioactive object, with significant content of radionuclides. Because radioactivity can be inherent to a substance or induced, and there are many initial sources of radioactivity, hot particles can originate from a multitude of sources.
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Hot particles contained in far traveled nuclear fallout range in size from 10 nanometers to 20 micrometers, whereas particles present in local fallout are significantly larger (100 micrometers to several millimeters). Hot particles can be identified by a Geiger counter, or by autoradiography i.e. fogging X-Ray film. Their age and origin can be determined by their isotopic signature.
Due to their small size, hot particles maybe swallowed, inhaled or enter the body by other means. Significantly, due to close proximity with cells and chronic nature of exposure, the intensity and duration of radiological effects per particle are increased. It has been likened to the difference between having an x-ray, or swallowing the x-ray machine, while it continues delivering x-rays until expelled (Halving the distance between radiation and its source increases the intensity of the radiation by a factor of 4). Thus ingesting a hot particle may be a million times more dangerous than passing the same particle in the street. This is a long standing concern.[1][2][3][4]
However, scientific consensus studies (e.g. CERRIE) conclude that the current ICRP risk model, despite being largely derived from studies of survivors of external radiation, adequately estimate the risk of hot particles i.e. internal radiation is no more dangerous than an equal amount of externally delivered radiation.[5][6] However, two members of CERRIE disagreed, notably Chris Busby who advocates several controversial physico-biological mechanisms that can vastly enhance the danger ingested particles e.g. Second Event Theory and Photoelectric Effect Theory.
Radiation can spread from a more radioactive substance to a less radioactive one by the processes of neutron activation and photodisintegration; this induced radioactivity increases the potential number of hot particle sources.
Hot particles released into the environment may originate in nuclear reactors. The Chernobyl disaster was a major source of hot particles, as the core of the reactor was breached, but hot particles are found near undamaged nuclear reactors as well.[7]
They also are a component of the black rain or other nuclear fallout resulting from detonations of a nuclear weapon, including the more than 2000 nuclear weapons tests in the mid-20th century.[8]
Atomic testing included safety trials of the devices using radioactive material which was not detonated; fissile material was sometimes dispersed, including plutonium vapor, plutonium aerosols of various sizes, plutonium oxide particulates, plutonium-coated particles, and sizeable lumps of plutonium-contaminated structural material.[8]
Accidents involving the nuclear engines, themselves usually called nuclear reactors, used in submarines, satellites, and other devices, usually crashes or malfunctions of other systems, can be a source. The crash of the nuclear reactor of the Kosmos 954 satellite, which failed to separate from the rest of the craft and entered a decaying orbit,[9][10][11][12] released hot particles.[8]
Accidents during transportation of nuclear weapons or nuclear waste is a potential source. A Boeing B-52 Stratofortress nuclear-armed bomber crashed in the area of the northwest Greenland town of Thule (since renamed to Qaanaaq),[13] releasing hot particles.[8]
The Committee Examining Radiation Risks of Internal Emitters (CERRIE), that was established by the UK Government, carried out a 3-year long independent expert review into the health risks of internal emitters (e.g. hot particles) and published its findings in 2003. CERRIE concluded there is no convincing evidence that the risks of internally delivered radiation differ from the risk projections derived from externally delivered radiation e.g. Japanese Atom Bomb survivors, and any differences between internal and external radiation are adequately accommodated by the established appropriate parameters (relative biological effectiveness, kinetic factors) in physiological models.[6] However, two members of CERRIE disagreed, notably Chris Busby who advocates several controversial physico-biological mechanisms that can vastly enhance the danger ingested particles e.g. Second Event Theory and Photoelectric Effect Theory.