A loss of coolant accident (LOCA) occurs when a main pipe bursts resulting in reactor coolant entering the containment building. This break must be large enough that emergency systems cannot make up the lost coolant. While in the event of a LOCA emergency systems will react to shut down the fission reaction the residual heat takes time to drop to safe levels. At the same time the LOCA has caused the coolant to depressurize and stagnate. This both limits the flow of water around the fuel, minimizing cooling but also causes boiling which insolates the fuel allowing fuel temperature to rise. In some reactors such as CANDU this boiling has the effect of increasing reactivity which can pose serious dangers to the reactor. If cooling of the fuel is not resumed the residual heat from the fuel can cause the remaining coolant to boil off known as ‘dry out’ which leaves the fuel entirely uncooled which can lead to fuel melting.