umm, lets say fins on the cruiser are for heat rejection off the reactor, yeah, that’s it. Umm, yeah, well now I have to explain heat transfer into a vacuum, oh wait, that violates laws of physics. Umm, ok, I will think of something.
Ok I've got it; the "fins" are loaded with thousands of cascaded thermo-couples which utilize the temperature difference between the heat associated with the reactor and the cold of space to create a voltage potential. The resulting electrical energy is used to help power the less-essential ship-board operations. It is an attempt to recover some of the entropic energy lost due to reactor inefficiencies. Similar technology is currently being used on modern satellights in conjunction with solar cells to recharge power cells.
Are you sorry you asked yet?