" Enriched uranium dissolved in water or an organic solvent will become an active nuclear reactor, increasing in power, if a specific “critical mass” is accumulated. The hydrogen in the water or the solvent acts as a moderator, slowing the fission neutrons to an advantageous speed, and even a fairly low U-235 enrichment level, like 3%, will overcome neutron losses by non-productive absorption in the moderator. This has been realized since the earliest days of reactor engineering, and those who work with uranium solutions are quite aware of the possibility. "

― James Mahaffey , Atomic Accidents: A History of Nuclear Meltdowns and Disasters: From the Ozark Mountains to Fukushima