I can’t find yield data for the concurrent creation and decay of fission products in an operating reactor. All the references I can find on the net are for the decay of a fixed lump. In an operating reactor the lump is (for a while) growing at the same time it is decaying. So, I started my own model in a spreadsheet. For an isotope with a half-life of one year, and a reactor that operates for one year, I find that 72.1% of the isotope remains at shutdown time (ignoring any parent decays). After ten years of operation, this isotope’s population levels off at double that, 144% of its initial yield. Is this a universal figure? Is the equilibrium point for any isotope 44% more than its initial yield? The math seems fairly straight-forward, though I never got the hang of partial differential equations. Is there an exact solution? Also, there is a rule-of-thumb: "After ten years, the radioactivity drops to effectively zero." Is there a similar rule: "After ten half-lives in a reactor, an isotope is at equilibrium?"