A (likely fatal) mishap with a nuclear reactor. On some nuclear-powered submarines, a last resort in the case of a loss of coolant emergency is to flood the core with seawater to draw off the excess heat. While this ruins a very expensive reactor, it theoretically saves the rest of the submarine from the meltdown.

A cold water accident occurs when the seawater admitted is, as seawater usually is, cold. While cold water is fine for absorbing excess heat, it also traps more neutrons than standard coolant or even warmer water. When the neutrons get trapped, they trigger more fission reactions, generating more heat, in a chain that eventually generates temperatures that no amount of coolant can absorb.

Eventually, a cold water accident causes the opposite of what was intended: rather than cooling the reactor, it causes it to approach meltdown.