When a material goes critical, what happens, and why is it that with something like Plutonium that it releases so much energy and radiation that it's able to kill someone with a relatively short exposure? It just sounds incredibly scary after reading about the criticality accidents from the 40s and 50s, how a seemingly stable piece of Plutonium (well, RELATIVELY lol) can trigger a critical state with just having something dropped on it and causing deaths.
First, a critical mass is the smallest amount of fissile material needed in order to achieve a self-sustaining chain reaction. The critical mass of a material depends on a number of different things: its nuclear fission cross-section, density, shape, purity, enrichment, temperature, and its surroundings.
When a material is said to have achieved criticality, this simply means it’s sustaining its own chain reaction. Typically, criticality is a good thing and occurs within a nuclear reactor or, occasionally, within a test environment. Criticality that occurs unintentionally and in an unsafe environment is referred to as a criticality accident and is frequently lethal to humans because they’re not shielded from radiation as they would be if it were achieved intentionally in a nuclear reactor.
For instance, Harry K. Daghlian, Jr. and Louis Slotin both accidentally achieved criticality with the same sphere of plutonium on two separate occasions (presumably these are the criticality accidents you’re referring to). When Daghlian dropped a tungsten-carbide brick on the assembly, it effectively acted as a neutron reflector, creating an environment in which the plutonium could sustain a chain reaction (because enough neutrons were being reflected to cause fissions which produced more neutrons to further fission the material and so on). He quickly removed the brick, again changing the environment so the mass could no longer sustain its own chain reaction.
A large release of radiation is characterized by these accidents and creates a localized event from a very high radiation exposure to persons nearest to the mass and (I believe) only 14 people in ~22 criticality accidents outside nuclear reactors have died from the effects of such a localized and high dose of radiation exposure. There’s also a significant release of energy and persons surviving criticality accidents often have reported feeling a wave of heat during the event but it’s unknown if this is a psychosomatic reaction or if it’s a physical effect of heating due to the energy released.
I hope that answers your question!