We all know spent fuel rods taken out from a reactor core keeps generating tremendous amount of heat and needs to be kept cool by running cool water. It is also known that if cooling system fails water will evaporate quickly leading to a blast and meltdown
If the fuel can still produce so much heat, why isn't it used as heat source for some low temperature systems, e.g. power generation via thermocouples, or home heatings for entire blocks or cities?
Why is all that spent fuel kept as a waste?