Oops yes :) That's what I meant of course. But my point was, even the fuel tubes themselves will have more than adequate shielding against the alpha particles

Oops yes :) That's what I meant of course. But my point was, even the fuel tubes themselves will have more than adequate shielding against the alpha particles emitted. So it's not even that dangerous that the Sherpas were using them to heat their beds :) By the way I have since looked it up and Plutonium 238 decays to Uranium 234 which is also an alpha emitter but with a huge half-life (235.000 years). So it looks lke it does not actually produce more penetrating radiation as it decays. The uranium will degrade to an unstable thorium isotope eventually too but this will be quite minimal due to the long half-life.