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Some radioisotopes such as plutonium-238 rapidly produce many alpha particles. According to this, the alpha particles acquire electrons and become helium atoms.

1 mole (238g) of plutonium-238 will produce 0.5 moles (2g) of helium gas in 87.7 years. So what happens to all this gas? Does it gradually build up inside the metal, placing it under increasing pressure?

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    $\begingroup$ Helium bubble formation resulting in degraded mechanical properties is important in a number of material systems (e.g. sciencedirect.com/science/article/pii/S0168583X19302770 as one recent paper). Yes, you get bubble formation, mechanical degradation, bursts of helium released, etc. $\endgroup$
    – Jon Custer
    Commented Dec 16, 2019 at 22:07

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Yes, old plutonium pits accumulate helium. The alpha particle moves about 10 micrometers and then gets stuck. After 50 years of storage a typical 1kg plutonium sample contains 200 cm$^3$ of helium (which would correspond to three atmospheres of pressure in the same empty volume). There is a fair bit of other processes due to other decays and particles messing up the crystal lattice, producing bubbles and voids that make it swell. Apparently it is best to store the plutonium warm so it naturally anneals.

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