Where does the 1500MW of heat the reactor is producing go, immediately after the generators are isolated from the grid?
It seems like you might really be asking about a meltdown scenario. The reaction can be slowed with control rods, which stops/minimizes the heat from being generated. The previously generated heat still needs to be handled, however (by evaporation). https://en.wikipedia.org/wiki/Nuclear_reactor A meltdown occurs when the reaction can't be slowed down through normal means because the safety systems fail. For example,…
From the RISKS archives:
"A fail-safe system fails by failing to fail safe."