In that case (back to my musing)....Why don't they just put an "antenna" around a radioactive source and pull current out of it, instead of messing around with an archaic boiler and steam-punk engine?
ie convert radiation directly to current.
They kinda do. However what is the wavelength of 10 exahertz? It is 1.18 microinches. That's twice the length of the antenna that you need. Care to build one or two for me, along with rectifier diodes? :-)
This is subatomic scale, and this is why the radiation can affect atoms. Electrons themselves are antennas for the energy. A single atom of Hydrogen (one proton, one electron) has diameter of 2 microinches. The smallest atom is larger than the wavelength of gamma radiation!
You still can convert energy of gamma radiation into thermal energy. But it's about the same as boiling a cup of tea by harvesting kinetic energy from a hail of bullets on a battlefield. Each gamma photon is so destructive that you will be awash in radioactive products of collisions. Sure, eventually they all decay into something more or less stable, and thermal energy will be released. But it will be a dirty business, not unlike what happens inside of a nuclear reactor.
Much of the energy released with nuclear fission manifests itself as kinetic energy. As the particles are slowed by bouncing off of other particles in the reactor, the KE is converted into heat. Steam turbines are a relatively efficient way of converting the heat energy (lower thermodynamic value) into electricity (high thermodynamic value).
I recommend some Thermodynamics courses, including some graduate level Statistical Thermodynamics, to understand this better. Maxwell is not going to help you here.