Is there a real nuclear fuel cell in this world ?

There are bateries that use the heat produced by nuclear decay to generate electricity. They are very expensive because they use pure radioactive elements and therefore only used in Space missions (for example the new Mars rover has one). But another type of mobile nuclear power cells is not in development. Fusion based energy will not be usefull until at least 2050 and then only in big powerplants and fission based energy is to dangerous.