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The No. 1 reactor's emergency cooling system at the Fukushima No. 1 Nuclear Power Plant was not functioning even before tsunami triggered by the March 11 earthquake struck the facility, according to initial data released on May 16 by Tokyo Electric Power Co. (TEPCO).

The emergency cooling system had previously been believed to be operating until the tsunami hit the No. 1 reactor. TEPCO had based the results of its analysis of how the cooling system stopped functioning -- which were released on May 15 -- on the assumption that the system was damaged by the tsunami.

TEPCO had also said prior to the release of analytical results on May 15 that the cooling system became inoperative due to the tsunami.

"Pressure inside the reactor suddenly dropped due to the cooling system. It probably was manually halted," plant operator TEPCO explained. As the tsunami neared the power plant, workers repeatedly tried to halt the cooling system, resulting in the loss of the system's capabilities.

The revelation is expected to figure importantly in an upcoming governmental probe of the cause of the accident.

TEPCO's newly released data, totaling about 2,900 pages, included various records from the No. 1 reactor, a log of warnings, the handover diaries by operators in the central control room and reports on electricity restoration work.

According to the data, all control rods were put in a pressure vessel immediately after the earthquake occurred at 2:46 p.m. on March 11, and the reactor came to an emergency halt. Emergency diesel generators reactivated normally.

The No. 1 reactor's emergency condenser to cool the reactor automatically activated, but about 10 minutes later pressure inside the reactor suddenly dropped. The emergency cooling system is believed to have been manually halted at around 3 p.m.

The pressure vessel's data points to the possibility that plant workers started and stopped the cooling system several times before the tsunami hit the plant at around 3:30 p.m.

A TEPCO official said the workers, following an operating manual, probably tried to prevent the inside of the reactor from getting too cold. It is not clear if the condenser was operating by 6:10 p.m. during which time there were records of manually activated operations.

The workers also started venting steam to release radioactive condensation at the No. 1 reactor from 9:15 a.m. on March 12 to protect the containment vessel from a rupture.

At the No. 2 reactor, workers tried to vent steam twice between March 13 and 15, but TEPCO could not confirm a fall in pressure inside the containment vessel. At the No. 3 reactor, workers made several attempts to vent steam after March 13.