Supercriticality refers to exponential growth in the rate of fission (each fission event in turn causing more than one additional fission event), but unless oth

Supercriticality refers to exponential growth in the rate of fission (each fission event in turn causing more than one additional fission event), but unless other conditions are met then going supercritical does not mean you actually get a nuclear bomb style explosion. There is also the concept of prompt and delayed neutrons. Prompt neutrons are products of the fission of the fuel, while delayed neutrons are products of other fission products and are released shortly after the fission of fuel. Normally nuclear reactors run prompt-subcritical, delayed critical (meaning that each fission event causes on average less than one other fission event with prompt neutrons, but when you account for delayed neutrons then the average becomes one for one. The lag incurred by relying on delayed neutrons to push you up to critical allows the system to be controlled). During both the SL-1 accident and the second Demon Core accident, for a very brief moment fission events were causing more than one additional fission events with prompt neutron products. In other words they both went prompt-critical, which is a type of supercritical. Thankfully SL-1 disassembled itself, the Demon Core rapidly heated and expanded, and neither were brought into a supercritical configuration quickly enough. This is the fundamental difference between what happened when the demon core went supercritical when Louis Slotin fucked up and when it went supercritical when the they detonated the bomb made with it over the Pacific. "one of the design problems to overcome in constructing a bomb is to contract the fissile materials and achieve prompt criticality before the chain reaction has a chance to force the core to expand. A good bomb design must therefore win the race to a dense, prompt critical core before a less-powerful chain reaction (known as a fizzle) disassembles the core without allowing a significant amount of fuel to fission. " http://en.wikipedia.org/wiki/Prompt_critical#Nuclear_weapons http://en.wikipedia.org/wiki/Prompt_critical#Nuclear_weapons The second demon core accident was a fizzle. It went supercritical but without involving a significant amount of the fuel.