Apparently "useable" uranium from ores is a somewhat small proportion, and using "breeder reactors" are able to utilize unseable uranium as fuel while producing more usable material. But exactly how much? Say all useable uranium will be used up in 100 years, if we used breeder reactors, how much would this be extended by? 200 years? 1000 years? By so much we don't have to worry before the next ice age?