Fuel fleas are microscopic

hot particle A hot 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 controversial theory proposes that hot particles within the body are vastly more ...

s of new or spent

nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...

. While small, they tend to be intensely

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

. The fuel particles, the size about 10 micrometers, are a strong source of

beta Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labiod ...

and

gamma radiation A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically s ...

and a weaker source of

alpha radiation Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus) and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an at ...

. The disparity between alpha and beta radiation (alpha activity is typically 100–1000 times weaker than beta, so the particle loses much more negatively charged particles than positively charged ones) leads to buildup of positive

electrostatic charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respectiv ...

on the particle, causing the particle to "jump" from surface to surface and easily become airborne. Fuel fleas are typically rich in

uranium Uranium is a chemical element with the symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Uranium is weak ...

-238 and contain an abundance of insoluble

fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...

s. Due to their high beta activity, they can be detected by a

Geiger counter A Geiger counter (also known as a Geiger–Müller counter) is an electronic instrument used for detecting and measuring ionizing radiation. It is widely used in applications such as radiation dosimetry, radiological protection, experimental ph ...

. Their gamma output can allow analysis of their

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

composition (and therefore their age and origin) by a

gamma-ray spectrometer A gamma-ray spectrometer (GRS) is an instrument for measuring the distribution (or spectrum—see figure) of the intensity of gamma radiation versus the energy of each photon. The study and analysis of gamma-ray spectra for scientific and techni ...

. Fuel fleas can be very dangerous if they become embedded within a person's body, but are generally not considered more dangerous than an equal amount of radioactive material evenly distributed throughout the body. An exception would be if the flea was embedded in a particularly vulnerable organ such as the cornea of the eye or inhaled into the lungs.Some paper suggests a 1 in 10 to 1 in 2 chance for a lesion per hot particle inhaled – http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/28/061/28061202.pdf The most likely cause of fuel fleas is when the cladding surrounding the nuclear fuel becomes ruptured or cracked (known as "fuel pin failure"), allowing the fuel particles to escape and allowing the coolant to enter the fuel rod, further accelerating the process. In water-cooled reactors, this can be due to the reaction of the

zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...

alloy cladding with the cooling water, which produces

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

. The hydrogen can be absorbed into the cladding material, resulting in

hydrogen embrittlement Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can permeate solid metals. Once absorbed ...

. Embrittled cladding is less

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

and more susceptible to cracking. This process is avoided in modern reactors by carefully monitoring the fuel assemblies, limiting operating lifetime of the fuel, and by using alloys developed to resist hydride formation.

References

Radioactive waste {{nuclear-stub