One of the issues with using a fission "pile" is that it pretty much only can generate heat.
If in some quantity, enough to need cooling off just while "running."
The trick is in converting that heat energy into mechanical energy. Converting water to steam is pretty typical, if as much for having had a couple of centuries to perfect steam mechanical plants.
But, that's not exactly an elegant answer for powering tanks. Whic his why the notion never really caught on.