[neutrino]

there could still come a time when such an 'investment' in energy is still feasable.

Sure. There are lots of possibilities, if (a very big if) we develop technologies that generate a large net energy surplus. Nuclear reactors (the fission variety) may not offer a very big payoff; the energy invested to build the facility is large, and so is the energy investment to cart off and store the waste.

Fusion - if achieved - might offer a way out.

[Wonder Warthog]

"Nuclear reactors (the fission variety) may not offer a very big payoff; the energy invested to build the facility is large, and so is the energy investment to cart off and store the waste."

By comparison to the energy investment necessary to extract, refine, and use petroleum, the comparative energy cost of fission is miniscule.