By Mari Saito and Antoni Slodkowski

TOKYO (Reuters) - In the weeks after the Fukushima nuclear plant was destroyed by a triple meltdown in March 2011, the plant's owner turned to three of Japan's largest construction companies for a quick fix to store radiated water that was pooling in the disaster zone.

The result was a rush order for steel tanks supplied by Taisei Corp, Shimizu Corp and Hazama Ando that were relatively cheap and could be put together quickly, according to the utility and three people involved in the project.

The tanks, which stand as tall as a three-storey building, were shipped in pieces and bolted together as makeshift repository for the cascade of water being pumped through the reactors of Fukushima every day to keep fuel in the melted cores from overheating.

The bolted tanks were sealed with resin and designed to last until about 2016 - long enough to buy time for Tokyo Electric Power, or Tepco, to work out a more permanent solution. But at least one of the tanks has already failed, leaking 300 tons of highly radioactive water that may have seeped into a drainage ditch and into the Pacific Ocean.

The discovery of the leak - which Tepco said on Friday was the fifth from the same type of tank - prompted Japan's first declaration of a nuclear incident since a 9.0 magnitude earthquake and tsunami triggered reactor meltdowns and hydrogen explosions that spewed radiation around Fukushima in 2011.

It has also focused attention on the uncomfortable end-game for the radiated water collecting at Fukushima.

Some 330,000 tons of contaminated water - enough to fill more than 130 Olympic swimming pools - has been pumped into storage pits and above-ground tanks at the crippled facility.

The sheer scale of the build-up has prompted some experts and officials to warn that in order to focus on containing the most toxic waste, less contaminated water will have to be dumped into the sea.

"Think about it in simple terms. If you don't release the water, there's nowhere to store it. So we also think it may have to be released," said Shinichi Nakayama, deputy director of the Nuclear Safety Research Center at the Japan Atomic Energy Agency and a member of a regulatory panel on Fukushima's problems.

Before the latest leak, Toshimitsu Motegi, Japan's minister of trade and industry, and Shunichi Tanaka, the top nuclear regulator, both indicated support for releasing water with low levels of radiation from Fukushima. No one has given any timeframe for such a move.

NOT BUILT TO LAST

Officials say the immediate priority is to figure out why the bolted storage tank failed less than two years since it was installed. They are also looking at adjusting plans for the more than 400,000 tons of additional water storage Tepco plans to build by 2016.

When Tepco commissioned the first bolted tanks the advantage was the relative speed with which contractors could finish the job just a few hundred meters from the wrecked reactor building. "These could be quickly built," said Masayuki Ono, a manager at Topco's nuclear division.

Tepco spokeswoman Mayumi Yoshida said a joint venture of Taisei, Shimzu and Hazama Ando won the first contract to build storage tanks at Fukushima in April 2011. She declined to say whether the contractors built the tank that began to leak. Tepco has not identified the cause of the leak, and has consistently declined to give details on the value of contracts it has awarded or winning bidders, citing a need to protect "corporate secrets". The Fukushima decommissioning is projected to cost at least $11 billion and take at least 30 years to complete.

Taisei, which built the structure around Japan's newest reactor at Tomari in Hokkaido in 2009, was heavily involved in the construction of the Fukushima tanks, according to three people involved, who asked not to be named. Workers and engineers at Fukushima have been put on an "emergency" footing to work on the storage tanks this week, they said.

Shimizu, which also has experience in building nuclear plants in Japan, had technology needed to build the bolted tanks and brought in experts, one of the sources said.

Taisei said it could not comment on individual client projects. Shimizu and Hazama Ando declined to comment.

There are 350 of the bolted-style tanks in place at Fukushima, and another 710 welded tanks, a more expensive design that takes longer to assemble. Nuclear Regulation Authority Commissioner Toyoshi Fuketa said on Friday that regulators also needed to examine the environmental risks posed by any failures of those tanks, especially in cases where they have been lined up directly on the ground rather than a concrete foundation.

Tepco plans to more than double the current storage capacity by 2016, but doesn't have a plan beyond that point. The math is daunting. The utility has to find space for an additional 400 tons of radioactive water each day because of the need to keep the reactors cool for the next seven years.

CONTROVERSIAL

A radiation filtering machine known as ALPS was supplied by Toshiba Corp to scrub the water clean of most radioactive elements, including cesium and strontium. The system, which remains in testing and under review by nuclear regulators, would leave treated water with tritium, a radioactive element typically discharged in the coolant water of reactors and considered one of the least dangerous radioactive elements.

Japanese officials have indicated support for releasing water containing tritium into the sea to make room to store more dangerous radioactive materials. But that seems certain to be controversial at a time when Japanese utilities are applying to restart nuclear stations that have been idled and Prime Minister Shinzo Abe is on a drive to sell nuclear technology to countries like India and Vietnam.

The World Nuclear Association, an international organization that promotes nuclear power, endorses a limited discharge at Fukushima. "Tepco has been prevented from discharging any treated water due to political opposition," the organization said in response to questions from Reuters. "Permitting sea release of treated water would alleviate the much larger problem of a demand for massive volumes of water storage."

Tepco's already shaky credibility with regulators and the Japanese public has been further damaged by recent events.

After months of denials, Tepco admitted in July that radioactive groundwater is reaching the sea. The government estimates 300 tons of radiated water are leaking every day.

Kajima Corp, a construction and civil engineering company, has proposed freezing the ground around the Fukushima reactor to create a 1.4-kilometer ring of frozen earth intended to stop groundwater from seeping into the wrecked reactor buildings. Contractors will prepare a report assessing the feasibility of that project by December.

Yuzo Onishi, a Kansai University professor who heads the government panel that endorsed the strategy in May, said it was the only option to block water in a tight space without cutting through tangled piping underneath the plant buildings.

"The opinion among the groundwater specialists is that Tepco has no idea what it's doing," he said. "We have asked them to bring in specialists on the ground."

Seawater sampling beyond the port surrounding the plant has not yet shown a rise in cesium or strontium levels, suggesting the contamination remains contained. But that has not relieved pressure on the local fishing industry, which has had to scrap plans to resume test fishing next month because of the recent leaks at the plant.

After months of discussion, a union representing Fukushima fishermen has moved closer to endorsing a Tepco plan to divert water away from the reactor complex and pump it directly into the sea. But the industry remains staunchly against any attempt by the utility to dump radiated water into the Pacific.

"We have to go back to the drawing board to figure out what to do next. Right now we can't see the future. There is so much uncertainty," said Takayuki Yanai, a trustee of a local fishing union.

(Additional reporting by Kiyoshi Takenaka and Aaron Sheldrick; Editing by Kevin Krolicki and Ian Geoghegan)

 

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