By bringing the temperature differential down, we could use a fairly weak heat source to charge up a device.
Would we be able to use a tiny amount of shielded radioactive material to provide a constant heat source and use that heat for the temperature differential?
A radioactive device that gives off that much heat wouldn't make sense for something like your TV remote control. However, maybe it could help power your off-the-grid house.
Does a radioactive material exist that could give off a few Celsius degrees of heat for years?
Would it be feasible to have homes use that material (properly shielded) without being a threat to national security and safety?
(Technically,) Yes.
> without being a threat to national security and safety?
No
No - the "hotness" is pretty much intrinsically unsafe, people can break into the shielding, and if you limit it to easily containable alpha emission it has a short life.