Washington (Platts)--7Sep2012/452 am EDT/852 GMT

Key details of how the accident at Japan's Fukushima I nuclear plant played out have yet to be determined and may not be known for five years or more, when important parts of the plant are safer to enter, officials with the Japanese and US nuclear industries told a US National Academies review committee Thursday.

A committee of the National Academies is conducting the study, which is mandated by Congress, on behalf of the US Nuclear Regulatory Commission. The committee held its second data-gathering session Thursday in Washington. The panel was established in March and information-gathering will continue through 2013, committee chairman Norman Neureiter said at the meeting.

A report to NRC and Congress is expected to be delivered in April 2014, Neureiter said.

Officials with Tokyo Electric Power Co., owner and operator of the Fukushima I plant, discussed the accident with committee members, saying that in hindsight, tsunami protection measures at the site were inadequate. The protection level was set at 3.1 meters (10.2 feet) above sea level when the plant was originally permitted in 1966, but that was raised to 5.7 meters in 2002, Tepco said in a presentation.

However, the tsunami on March 11, 2011 reached an estimated height of more than 14 meters, overwhelming the station's electrical distribution system and leading to a complete loss of power at four of the six nuclear units. Three of those units experienced melting of all or most of their nuclear fuel, resulting in explosions and radiation releases in the days following the tsunami.

A Japanese civil engineering society had discounted the possibility of a large earthquake and tsunami off the coast of Fukushima prefecture because of a lack of evidence of such events since the 17th century, Tepco manager Shin Takizawa said. A study in 2002 that suggested such an earthquake was possible and historical reports of a large tsunami in the area in the ninth century were under review at the time of the 2011 earthquake, but had not been confirmed, he said.

"In hindsight, if we were able to return to 2002 ... we would have taken different actions," Takizawa said.

An official with the Institute for Nuclear Power Operations, a US industry group set up to promote nuclear safety after the partial meltdown of a unit at Three Mile Island in 1979, said while the sequence of events at Fukushima following the earthquake is becoming increasingly clear, some details will take several years to learn. INPO produced a report in November providing a minute-by-minute description of events at the Fukushima units.

Steve Meng, manager of emergency preparedness for INPO, said that study showed the difficult circumstances under which Fukushima workers found themselves, without emergency lighting, critical instrument readings and power to operate valves, along with increasing radiation levels and a series of explosions as time went on. The first explosion, which damaged equipment that was being installed to provide emergency power, was "one of the key turning points for the event," Meng said.

The nuclear industry expects to learn "quite a bit more" about the accident in the next five or more years, INPO senior vice president Bill Webster told the committee. Information about the position of valves in a key cooling system and other details from within the reactor building will be discovered when those areas are safe to enter, he said.

Yasunori Yamanaka, manager of Tepco's nuclear safety engineering group, said following the meeting that one of the key pieces of information that will be learned in the coming years is the location and condition of the core of nuclear fuel in the three reactors that experienced meltdowns. Tepco believes the uranium fuel of unit 1 at Fukushima I almost entirely melted its way through the bottom of the thick steel reactor vessel and poured to the floor of the containment structure, eating through a portion of the concrete floor of that area.

The company plans to use cameras and other surveillance equipment to determine how much of the core of units 2 and 3 remain in the reactor, Yamanaka said. The company believes almost half of the fuel in those units, which maintained core cooling longer, melted, he said. It may take more than five years before Tepco can determine whether the fuel in those units remains in the bottom of the reactor vessel or also fell to the containment floor, Yamanaka said.

--William Freebairn, william_freebairn@platts.com --Edited by Lisa Miller, lisa_miller@platts.com

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