Isn't not 'burnt' by any reasonable interpretation of the word, what happens is that the fuel becomes too hot to handle in any of today's working reactors because of the buildup of radioactive fission products as well as transuranic species. If the fuel rods were left in the reactor, they'd probably swell and eventually block circulation of the primary coolant, and the high radioactive flux would have other negative effects on the reactor core material itself (as well as on any workers that had to fix the mess).
This is well known and many proposals have been made to recover this energy from the 'spent' 'burnt' fuel rods, but it's mostly been an expensive mess. Reprocessing to recover the transuranics for use in MOX (plutonium-based fuel, basically) is really expensive as well as generating large amounts of highly toxic waste as well as raising the plutonium-nuclear-weapon proliferation issue.
Instead, this 2000 tons per year represents an ever-increasing long-term headache that generally has to be stored onsight for about a decade at the nuclear power plant before it can be transferred to long-term dry cask storage, all at considerable expense and with high security costs.
With costs for solar/wind/storage dropping every year, isn't it time to give up on this pipe dream?