Ahot particle is a microscopic piece of radioactive material that can become lodged in living tissue and deliver a concentrated dose of radiation to a small area. A generally accepted theory proposes that hot particles within the body are vastly more dangerous than external emitters delivering the same dose of radiation in a diffused manner.[1][2][3][4] Other researchers claim that there is little or no difference in risk between internal and external emitters, maintaining that individuals will likely continue to accumulate radiation dose from internal sources even after being removed from the original hazard and properly decontaminated, regardless of the relative danger from an internally sourced radiation dose compared to an equivalent externally sourced radiation dose.
The theory has gained most prominence in debates over the health effects ofnuclear accidents,dirty bombs or fallout fromnuclear weapons, all of which can spread hot particles through the environment. The currentICRP risk model for radiation exposure is derived from studies of victims of external radiation, and detractors claim it does not adequately estimate the risk of hot particles.
Hot particles contained in far-travelednuclear fallout range in size from 10 nanometers to 20 micrometers, whereas those present in local fallout may be much larger (100 micrometers to several millimeters).Hot particles can be identified by aGeiger counter, or byautoradiography,i.e., fogging X-Ray film. Their age and origin can be determined by theirisotopic signature.
Due to their small size, hot particles may be swallowed, inhaled or enter the body by other means. Once lodged in the body, cells very near the hot particle may absorb much of its radiation, and be bombarded in a very sustained and concentrated fashion. By contrast, an external radioactive source delivering the same total amount of radiation over the whole body would give a relatively minute dose to any one cell.[5][6][7][8]
TheCommittee Examining Radiation Risks of Internal Emitters (CERRIE), established by the UK Government, carried out a 3-year-long independent expert review into the health risks of internal emitters (i.e., hot particles) and published its findings in 2003. The study failed to reach consensus, but the conclusion of the majority of its members was that the currentICRP risk model, despite being largely derived from studies of survivors of external radiation, adequately estimates the risk of hot particles, and that any differences between internal and external radiation are adequately accommodated by the established parameters in physiological models (relative biological effectiveness, kinetic factors);i.e., that internal radiation does not seem to be significantly more dangerous than an equal amount of externally delivered radiation. However, they noted significant levels of uncertainty regarding dose estimates for internal emitters, especially regarding less common radionuclides such as239Pu and241Am, and even more common ones such as90Sr.[9] Two of the twelve members disagreed with the overall findings, notablyChristopher Busby who advocates controversial physico-biological mechanisms such asSecond Event Theory and Photoelectric Effect Theory, by which he believes the danger of ingested particles could be greatly enhanced.
Another study largely corroborates the CERRIE findings, though emphasizing the paucity of useful data, substantial uncertainties over accuracy, and the existence of evidence for at least some modest "enhanced cell transformation for hot-particle exposures".[10]
Hot particles released into the environment may originate innuclear reactors ornuclear explosions. TheChernobyl disaster was a major source of hot particles, as thecore of the reactor was breached, but they have also been released into the environment through illegal dumping of low-level waste atDounreay.[11] They are also a component of theblack rain or othernuclear fallout resulting from detonations of anuclear weapon, including the more than 2000nuclear weapons tests in the mid-20th century.[12] Nonradioactive substances can be turned radioactive primarily throughneutron activation, though other reactions are also possibilities; thisinduced radioactivity can be dispersed in hot particles.
Cold War nuclear tests included safety trials in whichfissile material was not detonated, but 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.[12]
Accidents involving satellites and other devices are another source. The crash of theKosmos 954 satellite released hot particles from its onboardBES-5 nuclear powerplant.[12]
Accidents during transportation of nuclear weapons or nuclear waste are another potential source. ABoeing B-52 Stratofortress nuclear-armed bomber crashed in the area of the northwest Greenland town of Thule (since renamed toQaanaaq),[13] releasing hot particles.[12]
Common failure ofnuclear fuel may result infuel fleas, which can be found in some facilities that processspent nuclear fuel.