Did you have Heat dispersers that were a bit damaged? IF yes: Heat disperser will emit heat to the reactor mantle, if the mantel is cooled down, to balance the difference between them and the hull. The first fast consumation came from the hot surface, the slow one, when the heat dispersers gave out heat to the hull, what the ice, if it reached 200, cooled away.
That would be it, thanks for clearing up something I overlooked.