For the purposes of this Act, "spent nuclear fuel" means the irradiated energy source that is permanently removed from the core of a nuclear fission reactor either because it can no longer effectively sustain a chain reaction due to the depletion of fissionable isotopes and the build up of by-products that interferes with the chain reaction or because safety or operating concerns dictate such removal.

(from Ch. 111 1/2, par. 230.21)

(Source: P.A. 81-1516.)