The tsunami that triggered the nuclear disaster at Japan's Fukushima Daiichi nuclear power station March 11 also tore into its neighbor, the Fukushima Daini plant 6 miles to the south, flooding all but one of eight heat exchange systems that provide cooling to the plant's four reactors.

But unlike the older Daiichi station, the Daini units withstood the blow through a combination of circumstances, including sheer luck: Unlike at the Daiichi plant, which lost all outside power, one of the transmission lines providing power to the Daini reactor safety systems survived the earthquake. That permitted emergency cooling measures to operate and prevent meltdown of three of four reactor cores. One Daini unit never lost power.

A new review of impact of the earthquake and tsunami on the Daini plant has been published by the Electric Power Research Institute, which sent a team of experts to the site in May at the request of Tokyo Electric Power Co. (TEPCO), operator of the two Fukushima plants.

Japan, alone among the world's nuclear nations, requires automatic reactor shutdown controls that are activated by a severe earthquake, and these systems worked as intended at the four Daini 1,100-megawatt plants, which were all in operation when the earthquake struck. The reactors scrammed, and over three days of crisis response, TEPCO crews were able to lower reactor temperatures and bring the units into a cold shutdown state, their condition today.

The main focus of the EPRI team, headed by technical executive Ken Huffman, was providing advice on how to maintain the Daini plant safely while TEPCO determines whether and how to try to bring the units back online. But the EPRI report also contains insights for U.S. plant operators and the Nuclear Regulatory Commission staff as they consider how reactors in this country should be strengthened to withstand earthquakes and severe flooding, says David Modeen, director of external affairs for EPRI's nuclear power sector, in Charlotte, N.C.

The EPRI report documents the damage caused by the tsunami, whose power broke through doors and penetrated air intake openings, flooding and disabling recirculating pumps that transfer heat from reactors to the sea. The emergency diesel generator in one unit was also knocked out by flooding.

'Think a little harder' about flooding

"While the flooding that ensued from the tsunami was damaging, the physical momentum of the tsunami wave also damaged structures in its path including railings, cranes, vertical equipment doors, trucks, cars, and non-seismic structures such as warehouses and office buildings," the EPRI report says.

EPRI recommended to TEPCO that hatches in the heat exchanger buildings "be relocated or replaced with watertight closures and that the air intakes for the emergency diesel generators in the reactor buildings be relocated. If these major water entry paths were addressed, significant flooding would not be expected from a future large tsunami. Even if a watertight condition could not be achieved, the extent of in-leakage would be manageable."

"It doesn't have to be a 15-meter tsunami that might cause you trouble," said Modeen. "Our plants do have design requirements for internal and external flooding. As we're looking at our plants, we are asking, even if you don't think there is an outside [flooding] challenge, is the door watertight? If not, why not? Isn't this good insurance? You need to factor in that sort of thinking. ... This event has shown us the need to think a little harder."

The EPRI report does not document the differences in the March 11 impacts on the Daini and Daiichi plants, but it notes that the newer Mark II model reactors at Daini, whose containment structures are larger than the Daiichi Mark I units, did not have to resort to venting containment structures to prevent dangerously high pressures. Failures in the Daiichi venting systems led to hydrogen explosions there.

The EPRI account describes TEPCO operators' search for portable electricity supplies and replacement pumps in the midst of the catastrophe, a subject of the "lessons learned" review by the NRC and the U.S. nuclear industry.

Better instrumentation needed

"Within hours of the event, the [Daini] station initiated delivery of vehicles with self contained electrical generators. During the first day following the event, cables and replacement motors were located at the TEPCO Kashiwazaki Kariwa Nuclear Power Station and at equipment manufacturers.

"On the second day, cables were delivered by helicopter to the station and motors were delivered by helicopter to the Fukushima airport followed by overland truck to the station. ... On the third day, the motors were installed and about 9 kilometers of temporary power supply cables were run from the station's radiological waste building to and through the heat exchanger buildings."

Plant operators need to be sure that redundant emergency equipment is available and can be connected and operated "without any heroic measures," Modeen said. The crisis at Fukushima underscores the need for a hard new look at how plants can be affected by large emergencies that affect multiple units, he added.

Another NRC review issue -- the case for better instrumentation to monitor reactor conditions during and after emergencies -- also gets attention in the EPRI report. There are no readings of the height of the tsunami at Daini because the measuring gauge was swept away.

Earthquake measurements were truncated by a software recording error. EPRI recommended that TEPCO provide instrumentation to directly measure the depth and temperature of cooling water in the units' spent fuel pools -- a source of major uncertainty and confusion during the peak of the Daiichi crisis.

The survival of one power line to the outside grid, while good fortune, nonetheless exposed a vulnerability, EPRI said. A critical transformer serving the circuit was slightly damaged by the earthquake. Had the damage been worse, that link to the outside might have been broken, too. TEPCO should consider hardening transformers against earthquake threats, and creating more redundant sources of electric power to the station. The two major 500-kilovolt power lines to the plant use common transmission towers and the same substation. One of the two 500-kilovolt lines failed, but the towers survived.

EPRI suggested providing an additional independent source of emergency alternating-current power to Daini, such as a buried submarine cable transmission line with thick insulation and steel reinforcement that could be connected to another generating plant south of Daini.