I've reposted this in the nuclear engineering forum, which seems more appropriate. I'd just delete it, but can't see the way to do so. I've been debating decay rates with another poster in another forum. Basically, the other poster has stated "The half-life/decay rate of the substances used in nuclear power generation have nothing to do with the generation of power. That they ARE radioactive is. Spent fuel has very little to do with the ratio of remaining parent atoms to inert daughter or non-inert daughter atoms. It has to do with neutron saturation in the reactor. The point that you keep missing is can be explained this way: If you started off with 100% U235 (half-life = 700,000,000+/- years), you would wind up with "spent" fuel rod packs with more than 95% of the U235 still present. It certainly doesn't seem like - with more than 95% fissionable material left - the half-life/decay rate has much to do with the fuel rod packs becoming spent. But you keep thinking it does." I've been trying unsuccessfully to locate something that says that decay rates must be considered during design and operation of a nuclear plant. Can anyone help me with an on-line, reputable source? Thanks.