The entire primary loop of the nuclear reactor - the pressure vessel, pipes, and pumps that contain the coolant (water) - are housed in the containment structure.

At unit three the plant managers decided to flood only the pressure vessel hanging in the neck of the containment vessel, not the whole of primary containment.

Now this containment obviously must be robust and must be designed such that whatever internal pressure is released from the primary system can be managed by the containment.