Reply #24: no, spent rods really are spent.

[Sirveri]

Bottom line is that U238 is not a thermal fuel while U235 and Pu239 is. After the third pass through a modern reactor those rods are primarily burning Pu239 instead of the 3-5% U235 they were initially loaded with. But the generation rate of Pu239 from the activated U238 doesn't keep up with demand.

Basically core geometry says you're wrong, which is why it takes seven spent fuel rod assemblies to create a single PuMOX assembly. Unless we want to go with fast fission reactors like the old Soviet RBMK design or the Canadian CANDU design. I suggest staying away from RBMK though due to its positive thermal coefficient of reactivity.

Also refuel cycle is every 18 months (+/- 2 months), however the rods stay in for about 5 years, give or take a few months.