It's not just the half-life of the material (short enough to make heat, long enough to have a slow changing energy curve) It's also the type of radiation.
For instance Cs137 is plentiful and easy to separate, and it's a beta emitter, which is shieldable. But its decay products are gamma emitters, which is not shieldable. So nope.
Pu238 is an alpha emitter, which is the easiest stuff to shield... and its decay products are (only) alpha emitters, or stable. It's the right stuff.
The problem is, you have to make it by exposing Neptunium-237 to neutron bombardment inside a reactor. Same thing you do to U238 to make Pu239 for weapons. And you get great gobs of Neptunium-237 when you run almost any reactor, and it's easy to separate if you're already running a PUREX line to extract plutonium-239 for nuclear weapons.
But there'd be no earthly reason to do that, unless you are breeding plutonium on purpose for weapons, and that requires the output of a special reactor capable of 90-day fuel changes. That is a short list consisting of Hanford style purpose-built reactors, CANDU, or RBMK.
Aha... suddenly you understand the importance of the rather oddball, silly and dangerous RBMK in Soviet nuclear strategy.
By the way, loading neptunium into a CANDU reactor for short term neutron exposure is exactly what Canada is doing for us, to help us manufacture some RTG fuel. The Russians, our friends in space exploration, could do the same with their RBMK reactors... and again I suspect that’s the reason Russia still keeps them. Not for Pu238 but Pu239.
I suppose you could also drop a Neptunium rod in a pure civilian BWR or PWR type, and stop it every 30-90 days to change fuel. But it's a huge production to change fuel: you must cool the reactor and remove the top of the pressure vessel, control rod drives and all. The question, is if you leave the Neptunium rod in there for the normal refueling interval of several years, will that overexpose it, have the lovely Pu238 capture another neutron or three, and yield the useless-to-an-RTG Pu239 or the makes-an-RTG-dangerous Pu240 or 241?
I don't know the answer to that question, but I can tell you it doesn't work for weapons Pu239 breeding. Pu240/241-contaminated Pu239 is useless for bomb making, because its spontaneous fission will create neutrons at inopportune times shall we say. That is why we can let countries like Iran have BWR/PWR type reactors; they would have to change fuel every 30-90 days to prevent accumulation of Pu240/241, and everyone with an IR satellite watches their reactors' cooling systems to make sure they are not doing that.
I’ve been asked for a sidebar on what’s bad about Pu240 and Pu241? 240 after all has a decay chain that is all alpha, so what’s the prob? The spontaneous fission releases gamma, that you must then shield, and also creates 2 daughter isotopes out of at least 8 possibilities, and those have their own decay chains, often releasing gamma or beta. It’s out of control at that point. Other than that, Mrs. Lincoln, if Pu240 chooses to alpha-decay instead of split, the subsequent decays are all alpha, which would be alright... but Pu241 can’t say the same. If you’re leaving the Neptunium-237 in the reactor long enough to get Pu240, you’re also getting Pu241.
Aside from the other way Pu240/1 affect bombs, the gamma emitters are problematic for both bombs and RTGs. Because humans have to handle those bombs, sailors have to sleep right next to them, and spacecraft need RTG shielding to be liftable. In fact the Navy uses special Pu239 in their bombs, for the sake of crews. Their U238 “ore” spent far less time in the reactor, so has far less Pu240/1 at the expense of far less Pu239 also, meaning lower yield “ore”.
And then, you have to set up a PUREX line to separate out both the neptunium and plutonium, and IAEA is just gonna love that.
By the way, the reason it's so important to make the correct plutonium isotope in the reactor is that separating plutonium chemically from other stuff is a straightforward chemical task; but separating plutonium isotopes from each other is effectively impossible. It's only possible with natural uranium because it's 3 units apart instead of 1 (235 vs 238), and even then it's ridiculously hard.