Since "depleted" nuclear reactor fuel rods are still radioactive, why not try to collect the radiation energy in them the same way that a solar cell works? By surrounding a fuel rod with transparent material casing in a silicate substance impregnated with an electron collector array, it could be possible to collect that radiation energy. The substance would also be connected in series and parallels (to control voltage and wattage produced) using the same comb-type structure used on solar cells. Or it could even use a tighter configuration. Wouldn't the emitted energy be captured the same way that solar energy is collected? The energy from light and radiation seems to be related.
I used to operate a real-time x-ray machine to examine jet-engine parts but do not have that job any longer. All I would need to do is to x-ray a solar cell (or a stack of them) hooked up to a voltmeter to see if it will work. If this does work or even is remotely feasible, all of those fuel rods wasting away could be used to power many different things and in this configuration should not need a cooling system. Since there would only be one rod per "battery," the "battery" housing could be lead, tungsten, or some other material since the radiation from the rod is not too high.
Photo credit to Paul Stevenson.