Re: helium confinement
In a past life, i dealt extensively with helium. It is a little noble gas molecule, say 3 Angstrom or so in size, doesnt like making bonds with anything, including itself. There is a fascinating paper called "The Weakest Bond" about the tiny energy well of the bound state of the He dimer, two Helium-4 atoms in a mutual energy well, which can be seen in diverging cross section for scattering states in He-He collisions at loooooow energy. I actually saw this effect in the lab and the observations were puzzling until the theory was developed some years later.
But to address the question: He is a slippery little bugger, the lightest gas other than hydrogen, and hydrogen is far more reactive than He, it makes H2 molecules, while helium doesn't make molecules at ll if it can help itself. So He is monoatomic. As far as "size" or crossection goes, He is actually harder to confine than H2. (However, if it gets out, you dont have to worry about explosion with He like you got to do with H.)
Confining He at high temperatures,pressures and radioactive levels is not for the faint hearted. The highest temperatures I ever had to deal with was around the same as in a pebble bed reactor, I used stainless steel knife edge into opposing copper gasket flanges, but i didnt have to deal with radioactivity, and if i lost confinement i didnt have to worry about a radioactive fire. Radioactivity degrates both stainless and copper, so you got to watch your duty cycles and replacement schedules much more carefully.
Gas cooling is tricky. They did something called AGR with CO2 in the UK but not too many were built, too complicated.