Tokyo Electric Power Co. acknowledged it delayed announcing a pump failure at the No. 5 reactor of the Fukushima No. 1 nuclear power plant, and that its backup plan did not run smoothly.
Tokyo Electric Power Co. acknowledged it delayed announcing a pump failure at the No. 5 reactor of the Fukushima No. 1 nuclear power plant, and that its backup plan did not run smoothly.
The utility learned of the failure at 9 p.m. on May 28, but it did not disclose the problem to the public until 9 a.m. the following day, during which time water in the reactor neared the boiling point.
The Fukushima prefectural government issued a warning to TEPCO to immediately disclose information concerning the crippled nuclear power plant, the company said.
The failure was caused by a defective electric circuit in a makeshift pump that injects seawater used to remove heat from the spent fuel rod storage pool and the reactor itself, TEPCO said.
The company said it delayed its announcement of the problem because it had to immediately connect the system to a backup pump.
TEPCO also said it had another emergency water-injection means on hand if the situation deteriorated further.
Work to connect the cooling system to a backup pump started at 8 a.m. on May 29, 11 hours after the problem was detected.
The backup system started working at 12:50 p.m., the company said.
However, the connecting work took more than four hours to complete, longer than the three hours TEPCO initially expected.
During the connection work, water temperature at the reactor rose to 94.8 degrees, compared with 60.8 degrees at 5 p.m. on May 28.
The utility said it managed to keep the temperature below 100 degrees, allowing the reactor to remain in a "cold shutdown" state.
The company said it would have adopted other emergency water-injection measures if the temperature had risen above 100 degrees.
Junichi Matsumoto, a senior TEPCO official, said the company should have announced the failure of the cooling system earlier, considering the connecting work for the backup pump did not proceed smoothly.
TEPCO informed the Nuclear and Industrial Safety Agency and municipal governments in the area of the failure on the night of May 28. But NISA gave TEPCO the OK to not promptly release the information.
Hidehiko Nishiyama, a NISA spokesman, said there were sufficient backup measures to keep the reactor safe, and the agency thought TEPCO was handling the situation appropriately.
When the Great East Japan Earthquake and tsunami hit northeastern Japan on March 11, the No. 5 reactor was offline for a routine check, but it still contained fuel rods.
After the power and seawater pump failed, the fuel rods raised the water temperature inside the reactor.
TEPCO set up a makeshift seawater pump and brought the reactor to a cold shutdown state on March 20.
(This article was written by Jin Nishikawa and Tatsuyuki Kobori.)