At Unit 1, immediately after the earthquake occurred, off-site power was lost and the main steam isolation valve (MSIV) fully closed. As MSIV closure made the reactor pressure rise, two Isolation Condenser (IC) systems started up automatically to control the reactor pressure and water level. Subsequently, reactor pressure decreased after IC operation, but in order to comply with the rate of reactor coolant temperature drop, the operators shut down both IC (closed the isolation valves on the outside of the primary containment vessel) in accordance with the operations manual. Thereafter, they controlled the reactor pressure by repeatedly opening and closing the IC valve on one system. This was a normal operation which was in accord with the operations manual, and was not due to any discharge to outside the system.
Following the IC shutdown (valve closing), the reactor pressure reversed and began to rise. No abnormalities were seen in temperature, pressure or other parameters inside the containment vessel nor were there any indications of a discharge from the reactor pressure vessel.
After the tsunami hit, because a station black out occurred and DC power was lost, the IC became inoperable as well. Subsequently, when DC power was temporarily restored and the IC isolation valve outside the containment vessel became operable, the valve was briefly opened. However, because a steam discharge from the IC could no longer be seen from the IC venting pipe immediately after, the operators performed an operation to close the valve for their concern of a possible loss of IC cooling water.
There is a high possibility that the IC isolation valves inside the containment vessel were hardly open due to the loss of DC power, and it has been inferred that IC function was lost early after the station black out.