The injection of water to cool one of the reactors that suffered meltdowns at the Fukushima No. 1 nuclear complex in 2011 was delayed because the rubber parts in valves used to reduce reactor pressure had possibly melted, the plant operator said Thursday.

According to Tokyo Electric Power Co., the component that may have melted is part of a device used to open the so-called safety relief valve when steam building up inside the reactor pressure vessel needs to be released in an emergency.

Tepco had been unable to explain why workers faced difficulty opening the safety relief valves of the No. 2 reactor, but the company now says "one of the reasons" may have been because the component melted and was not able to function properly.

The temperature limit for the component was about 170 degrees Celsius, but Tepco found it was able to withstand that level of temperature for only several hours.

The nuclear crisis began on March 11, 2011, when earthquake-triggered tidal waves hit the plant, flooding electrical equipment and leading to the loss of reactor cooling systems.

The system that had kept cooling the No. 2 reactor ceased on March 14. Workers sought to inject water by using fire trucks, but could not do so because the pressure inside the reactor was too high.

Tepco thus sought to open the eight safety relief valves by using battery power, but the operation did not go smoothly. After several attempts the valves were finally opened, enabling water to be poured inside the reactor.

For a safety relief valve to open, it needs to be supplied with nitrogen gas through another valve, to which the rubber component in question is attached. But nitrogen gas might have leaked when the component melted, Tepco said.