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.