Spent nuclear fuel in most older reactors retains more than 90% of its potential energy.
In the form of U-238? Perhaps too, but look near enrichment plants and you will see yards full of steel cylinders filled with depleted Uranium hexafluoride.
https://www.google.co.uk/maps/@53.2635448,-2.9592278,327m/da...
So the spent fuel does not contain valuable energy. Better off removing the requirement to be able to retrieve it and just get on with disposing of it.