When I was a kid I first learned that nuclear power plants produced electricity by using the nuclear energy inn the form of generated heat to heat up water for steam turbines – and I was dreadfully disappointed* I had imagined some more direct and magical process. Using nukes to heat water just seemed so — uninspired.
Of course, as a practical matter using nuclear fission (and, eventually, fusion) to generate lots of heat is actually the reasonable way to do things. The result of zillions of fission processes manifests itself as heat, not as voltage differences or showers of directed electrons or other particles. And using the heat difference in something like a turbine to extract power is thermodynsamically much more efficient than , say, trying to extract power with thermocouples.
Shy of something like the spherical beta source Chronos described – which rely upon a particular and reliable particle source at its heart, you’re not going to find anything that will somehow give you more direct output.
Most of the heat is actually the result of kinetic energy of the split nuclei – it’s already in the form of heat. A lot of the rest is in the form of gamma rays. Even if you could harness those gamma rays in some way to make it into electric power, it’d probably be lossy. The nuclear bomb-pumped gamma ray laser actually generated its gamma rays second-hand, using the nuclear explosion to turn wires into plasma and then pumping that plasma to high energy levels, where it generated directed beams via amplified spontaneous emission along the directions of the wires (no feedback mechanism, so it wasn’t even a true coherent laser). So even extracting your nuke power in the form of light energy isn’t really a possibility.
*I think it was at the Hall of Science in Queens, New York, at the site of the 1964-5 World’s Fair, but after the Fair had closed.