In unrelated news 2,725 Russians died in a bizarre radioactive tea poisoning incident. The poison would have been fatal had the fall from the open windows not happened.
Mystery? What mystery? Polonium keeps the samovar hot, everyone knows this. They were energy conscious patriots trying something new so the boys on the frontline could keep themselves warm using the wood and coal that otherwise would have burnt to warm their tea. Patriarch bless these fine folks who died in the pursuit of keeping our selfless heroes cozy in the trenches.
polonium would be actually a decent choice, if you picked some heavy beta or gamma emiter you'd get massive amounts of water radiolysis which means that over time tea would be bleached out and you end up with water again. another good isotope would be plutonium 238
and samovar probably needs to be made out of tungsten or kept in reinforced concrete vault to keep radiation in and not out
Plutonium 238 is renowned as the most controllable nuclear fuel for spontaneous fission, common element of RTGs and such. I'd certainly go with it
And yes, certainly the case should be tough - and there should be the ability to passively remove heat, especially when tea isn't there (which is bad, but hey, let's not increase the tragedy by having nuclear meltdown)
the heat comes from alpha decay, not from spontaneous fission. if that was the case, you'd have massive neutron flux to deal with, but it's only alphas which are very easy to stop, like with polonium
just add high temperature heat pipes. tea doubles as shielding