One of the most striking aspects of plants for producing heavy water or 235 U is the long time they must be operated when first started before it is possible to withdraw enriched material of specified product composition from them. This is because the amount of desired isotope held up in the plant may represent many days or even months of normal production, and at start-up the plant must be run without product withdrawal for a time sufficient to produce the plant’s working inventory of desired isotope. The purpose of this section is to derive approximate relations that may be used to estimate the so-called equilibrium, or start-up, time of an isotope