Critically , they were powering Supply the pumps that circulated coolant through the reactors ' cores to remove Removing decay heat , which continues after fission has ceased Process_end .

The spent fuel pool of previously shutdown Reactor 4 increased Cause_change_of_position_on_a_scale in temperature on 15 March due to decay heat from newly-added spent fuel rods ; but did not boil down Change sufficiently to expose Reveal_secret the fuel .