I've been reading about nuclear reactors lately, and I wondered how scientists in the 50s, without modern computing equipment, or maybe even now could predict the thermal power outputs of reactors (I'm guessing primarily determined by the number of fissions per second after the short lived decay heat generating fission products build up to an equilibrium) under various conditions. And then I thought that maybe with a lot of moderated reactors, as long as the reactors being compared are built with similar geometry, materials, fuel enrichment, etc., the thermal power density maybe won't vary as much with the reactor volume (for volumes much larger than what is needed to maintain criticality with similar reactor design), so they could make predictions based on experiments with smaller reactors before building something with 1 GW or more thermal power output. Also, I've read that its impossible or nearly impossible to produce a uranium bomb with less than ~90% U-235. So, I'm wondering if there is a theoretical limit to the number of fissions occurring per time per unit volume that could be imposed solely by the volumetric fissile isotope density w/o any non-fissile neutron absorbing isotopes, moderator, or coolant present (below a certain maximum fissile isotope density that may vary for different isotopes), or, if this is not the case, if such a theoretical limit (on volumetric fission rate density) imposed solely by volumetric fissile isotope and non-fissile neutron absorbing isotope density could be reached practically and safely in a reactor. Also, I was wondering how long a neutron might last on average (mean) after a fission event before it gets absorbed, causes another fission, or decays in both fast and moderated reactors (I know with only the latter, spontaneous decay, they last about 10 minutes, but I'm not sure how long they do with the previous 2). I know moderated neutrons' "targets" have higher absorption and fission cross-sections, but I suspect that thermal neutrons, since they travel much slower, last longer before getting absorbed or inducing fission, but don't cover as much distance before such an event happens as fast neutrons.