Fuel fleas

Fuel fleas are microscopic hot particles of new or spent nuclear fuel. While small, they tend to be intensely radioactive.

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The fuel particles, typically sized about 10 micrometers, are a strong source of beta and gamma radiation and a weaker source of alpha radiation. The disparity between alpha and beta radiation (alpha activity is typically 100-1000 times weaker than beta, so the particle loses much more negative-charged particles than positive-charged ones) leads to buildup of positive electrostatic charge on the particle, causing the particle to "jump" from surface to surface and easily become airborne.

Fuel fleas are typically rich in uranium 238, and contain an abundance of insoluble fission products. Due to their high beta activity, they can be detected by a Geiger counter. Their gamma output can allow analysis of their isotope composition (and therefore their age and origin) by a gamma ray spectrometer.

Fuel fleas, and hot particles in general, are very dangerous when ingested.

When the fuel pellets are not thoroughly dried during manufacturing, the excess moisture reacts with the hot metal and releases hydrogen, which enters the lattice of the zirconium metal of the cladding of the fuel rod. The resulting hydrogen embrittlement leads to formation of microscopic holes in the cladding, through which the fuel particles can escape and through which the cooling water can enter the fuel rod, further accelerating the process.