[vipa123]

I am not an expert at this, but if I had to guess I would agree with your answer. Helium, even neutron enriched radioactive helium -if there is such a thing-, will evaporate into the air instantly, it will also rise to highest location being lighter than essentially every other atmospheric gas. This make it easy collect, contain, or cleanse, if performed. It will also rise above the atmosphere and get blown away by the solar winds should it escape the building. Additionally, helium is inert so it will not form other chemical compounds that will stay around, or enter the human body. Also because it is inert your body will not generally absorb it, certainly not like it would tritium water. Also comparing to water, helium will not dissolve other chemicals, radioactive or not, into it like water does. One last thing is the cooling potential of liquid helium is immense, being able to absorb massive amounts of heat during evaporation. Assuming they want to use a helium refrigeration cycle.

Fingers crossed I got some of this right...

[bogeholm]

I’m no nuclear physicist (only a climate/math physicist), so I had to look it up. If a helium-4 nucleus were to absorb a fission neutron, it looks like it would transmute to helium-5, which has a half-time of 602 ys (yoctoseconds), and decays by neutron emission[0]. So pretty much status quo.

For context, in 602 ys, light travels a distance equivalent to around a hundredth of an atom width, so it’s a rather short moment.

[0]: https://en.wikipedia.org/wiki/Isotopes_of_helium