This mod adds "Plutonium" as a resource to the game, which is used by RTGs. Plutonium is self contained within an RTG so there is no need to place a tank or anything to get Plutonium. The Mk.1 RTG will last approximately 10 Kerbal years before the module is rendered useless. The upgraded and slightly bigger Mk.2 model will last moderately longer (1.25x the size of the Mk.1), but do to the faster fission reaction, the Plutonium is used slightly faster in it.
RTGs cannot be refilled.
Simply extract the BetterRTGs folder to your Gamedata along with the module manager.
Pretty much like every other mod.