Lots of stuff can burn and produce heat, but you don't just put any old flammable thing into your car, you use a specific type of refined petroleum that has well known properties that you can design an engine around.

Likewise, while lots of radioactive material can put off heat that can be used to drive a turbine, by using a specific blend of U235 and U238 the whole system operates in a controlled and well understood way.

There is a program to blend in plutonium from decommissioned nuclear bombs as a fuel for reactors, but it is fairly complex and probably wouldn't be done except for security / political reasons.

There are also designs on the drawing board for something akin to a garbage incinerator, the primary purpose would be to eliminate waste, but much or all of the energy to do so would come from the waste products themselves.