They thought they had a breeder reactor I guess. Typical reactors burn mostly U235 and a bit of bred-up Pu239 for an average burnup of the mined uranium of roughly 10 MWd/kg. Breeder reactors, on the other hand (both Thorium thermal MSR breeders and Uranium-Plutonium fast breeders) can get up very near the theoretical maximum of ~930 MWd/kg. The latter figure really demonstrates the sheer magnitude of the nuclear force in atoms. What a wonder to behold!
