Nuclear reactors continue to emit heat from the core even after the control rods have been inserted stopping the continuous fission reaction involving uranium. The heat that continues to be generated by the core after shutdown is referred to as Decay Heat and is the result of the further fission of the large number of unstable fission products created during the normal operation of the reactor. This heat is only a small fraction of that generated in the reactor normally and falls off quickly in an exponential fashion, but it is the cause of most major nuclear energy accidents that have occurred. This makes it the primary concern of operators of these facilities.
Nuclear Meltdown is designed around the operation and shutdown of a Pressurized Water Nuclear Reactor and allows the user to try their hand at stopping accidents like those that occurred at Three Mile Island and Fukushima. The app also allows the user to create their own accident scenario and see how they would fair at solving it.
This is a very simplified simulation of what can occur when one tries to shutdown one of these reactors and is meant only as a crude educational tool for the average person. IT SHOULD NEVER BE USED FOR ANYTHING OTHER THAN HELPING THE AVERAGE PERSON UNDERSTAND WHAT CAN POSSIBLY GO WRONG WHEN SHUTTING DOWN SUCH A SYSTEM.