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.
(Read more: Japan prime minister to call for stronger response to Fukushima water crisis: Nikkei)
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 Tepco'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.
(Read more: Japan nuclear body says radioactive water at Fukushima an 'emergency')
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.