In the heady days of the late 1960's - Early 1970's, NASA was already envisioning a robust, reusable space transportation infrastructure for transferring crew and cargo to and from the Moon, and later building to a crewed mission to Mars. The Space Transportation System was centered around the NERVA nuclear thermal rocket engine, by that point nearing flight-ready status. More about the nuclear shuttle can be read here: https://www.wired.com/2012/09/nuclear-flight-system-definition-studies-1971/
I added a pair of solar panels, which were not included in any of the original design sketches. Likely the original concept would have used fuel cells, much like on Apollo and the shuttle orbiter. PV cells weren't quite developed enough to see mass use in space at that time. There are nuclear rockets that let you "tap" the core for power when the engine was idle; basically you close off the propellant loop and have the vapor drive a turbine. However, you also need a lot of active cooling to keep the core cool. When operating as a rocket the exhaust plume does the cooling. These are so-called "bimodal" nuclear rockets. NERVA, however, was not such a rocket.