> Similarly, uranium sitting in the ground at low density is largely inoffensive, but increase its density up to the critical point and you end up with an uncon

> Similarly, uranium sitting in the ground at low density is largely inoffensive, but increase its density up to the critical point and you end up with an uncontrolled chain reaction. This is not going to happen. What is going to happen is that you're going to accidentally add a neutron moderator to uranium. Because water is a neutron moderator. Any amount of uranium denser than 1.2% will suddenly fission. It won't be a runaway reaction, so no explosion BUT anyone standing nearby or unprotected by a quite a few feet of rocks will be irradiated by slow neutrons, the worst possible way to get irradiated, and die painfully. During the discovery of fission one of the fuckups was that it was just noticed that uranium heats up when you put several pieces next to eachother. When the experiment was done it was decided it is prudent to store the samples in water. Well, that turned out to be anything but prudent. The container (a big metal vat) was gone and replaced by a deep hole in the ground the next day. Luckily there was no-one present for more than a few minutes. Still that researcher died of cancer (decades later, so very low dose)