Fukushima was not prepared for 15 m tsunami - Tepco report

The crippled Fukushima Daiichi nuclear power plant's No.4, No.3, No.2 and No.1 (R-L) reactor buildings are seen from bus windows in Fukushima prefecture November 12, 2011.

TOKYO (Reuters) - The tsunami that struck Fukushima nuclear plant in March was far larger than what the facility was equipped to handle, the operator of the plant said on Friday in its first official assessment of the worst nuclear disaster in 25 years.

In March the huge tsunami unleashed by a 9.0-magnitude quake overcame the walls protecting the Daiichi plant on the Pacific coast 240 kilometres (150 miles) north of Tokyo, knocked out its cooling systems and triggered a nuclear meltdown.

"The size of the tsunami we came up against was far beyond our expectations," Tokyo Electric Power (Tepco) (9501.T) executive vice president Masao Yamazaki told reporters as the operator presented an interim assessment report detailing what happened at the plant and steps taken to cope with the disaster.

Tepco, repeating its initial assessment, said the tsunami that hit Daiichi exceeded 15 metres in some areas, overcoming the 10 metre sea walls.

"As stated in the assessment report, the precautionary measures we have been preparing did not suffice and radiation was allowed to leak out as a result. For causing this grave accident we apologise deeply," Yamazaki said.

Some Tepco shareholders sued the utility in November, accusing it for failing to heighten the plant's sea walls although in 2008 it simulated a tsunami exceeding 15 metres hitting the plant, domestic media have reported.

A third-party committee that inspected Tepco's interim assessment said the scale of the disasters in March was beyond what Tepco and even the government had anticipated.

"It can be said that earthquakes and tsunamis needed to have been given even more serious thought and the government and experts in the field also need thorough self examination," the committee stated, adding that Tepco could have been complacent in assuming that they had all sufficient safeguards in place.

MELTED FUEL

Earlier this week, Tepco offered its latest analysis of what could have taken place in Daiichi's three damaged reactors, citing its simulation that suggested fuel in one of them could have melted through the pressure vessel that encased it.

That would be a more grave scenario than previously thought, though Tepco said the melted fuel was likely to have been contained by an outer layer called the containment vessel made of concrete and steel.

Cleanup efforts by Tepco have since curbed the amount of radiation the crippled plant is emitting and temperatures of reactors that contained melted fuel have been brought well below boiling point after Tepco installed new cooling systems.

Prime Minister Yoshihiko Noda may declare a cold shutdown on December 16 as Tepco's assessment showed that temperatures for the nuclear fuel lying at the bottom of the containment vessel have stabilised, the Yomiuri newspaper reported on Friday.

A cold shutdown is one of the conditions that must be met before the government considers lifting its entry ban within a 20 km (12 miles) no-entry zone around the Daiichi that has about 80,000 residents to evacuate.

(Editing by Tomasz Janowski)