system
Given the following post, respond as if you're commenting on Reddit:
human
What happens if an uncontrolled nuclear chain reaction were to continue uninterrupted? Specifically in the examples of criticality accidents.

For example, in Louis Slotin's criticality accident. Viewers present saw a flash of blue light and Slotin disassembled the experiment.

What would happen if say Slotin ran away instead of disassembling it?
gpt
When your mass goes supercritical, its neutron flux will multiply continuously until something returns it to subcritical.

In a nuclear power reactor, the fuel heating up (doppler effect) initially returns the fuel to a critical condition for smaller excursions or reactor startup. Moderator/core heatup and/or voiding is used for stabilizing reactivity changes at higher power levels. For example, in a boiling water reactor at power conditions, when reactivity increases and the core goes into a super critical state, there will be an increase in boiling which will return it to a purely critical state. The final reactor power level will be higher than the initial power level, but the core will have returned to critical.

For any critical mass, if sufficient reactivity is injected such that the neutron flux will multiply faster than the doppler effect and other reactivity feedback mechanisms can stop it, then the flux will increase (increasing radiation dose, and also increasing heat), until something stops it. For a slow flux increase, this could be stopped by melting of the fuel (geometry change). For a rapid flux increase, this could be stopped by the fuel breaking apart (also a geometry change). 

Nuclear power reactors have built in neutron flux monitoring systems which will scram the reactor to halt any flux increases which have the potential to damage the reactor core. This is in addition to the limitations that the reactor is operated under to ensure core damage does not occur, along with the specific design and placement of nuclear fuel in the vessel to preclude an uncontrollable reactivity excursion.

Hope this helps. Also good question!