Even though the chain reaction ceases, radioactive decay in the fuel means that heat is still produced. Immediately after stopping (1 second), output is about 14% of maximum. After one day it falls to about 0.6%, and it continues to fall but more slowly. Even 0.6% of full output is more than enough to vaporize all the water in the tank and melt the fuel. A shut-down reactor is therefore cooled by means of a special cooling system known as the residual heat removal system, RHRS.
The fig. Residual heat 1 shows the approximate progress of residual heat as a function of time. The curve appears as the sum of a number of exponential decays with different half-lives (and weights). The first minutes are dominated by continued fissions, because the so-called delayed neutrons imply that neutron density in the core falls only slowly towards zero (T½ ~ 1 minute).