I don’t disagree that in comparison to many other nasty byproducts of the U235 chain, most Uranium isotopes are not that “hot” but the danger of HEU comes partl

I don’t disagree that in comparison to many other nasty byproducts of the U235 chain, most Uranium isotopes are not that “hot” but the danger of HEU comes partly from the increased presence of the U234 isotope, not just U235. So that isotope solely is not the right half life to do math around. While U235 is about as stable as U238, most purification techniques result in selecting the lower weight isotopes, which selects the hotter U234 as well. And while there is less of it, if I recall correctly eventually U234 dominates in terms of radiation output.