

The central control room of Unit 3 is pictured after lights went on at the Fukushima Daiichi nuclear power plant on Tuesday. Associated Press

However, they chose to ignore the vulnerability at existing reactors and instead focused on fixing the issue in future ones, government and corporate documents show. There was no serious discussion of retrofitting older plants with the alternative technology, known as "isolation condensers," government advisers said.

Fukushima Daiichi, the plant at the heart of Japan's crisis, relied mainly on electrical systems to power the emergency cooling of its reactors—a design that failed in the March 11 earthquake and tsunami. When the main power went down and backup generators failed, cooling water couldn't get to the nuclear fuel. Overheating then led to explosions, fires and significant release of radiation in the early days after the quake. Isolation condensers, by contrast, don't require electric power.

Efforts to spray water into the No. 4 reactor at the Fukushima Daiichi plant on Tuesday. Reuters

"There has been little to no talk about the need to retrofit existing reactors" with additional safety systems, said Muneo Morokuzu, a former Toshiba Corp. reactor designer who now studies industry policy at the University of Tokyo. "Mostly people thought there was no need to go that far."

Japanese officials declined to say why retrofitting older plants wasn't discussed, while the plant's operator, Tokyo Electric Power Co.,or Tepco, said it would look into the matter. Experts said doing so was likely deemed too costly and cumbersome given what was seen as a small risk of total power outage. Even if a refit had been ordered in recent months, it wouldn't have been complete when the quake struck.

On Tuesday, Tepco took another step toward recovery by reconnecting power to all six reactors and turning on the lights in reactor No. 3, the most visibly damaged of the six after a series of explosions. Tepco still needs to turn on cooling systems. Workers resumed spraying water on storage pools where radioactive rods are stowed, an effort to prevent overheating.

Last October, the Nuclear Safety Commission of Japan, one of the nation's two main safety regulators, met to set its long-term agenda. Takanori Tanaka, head of the partly government-supported Nuclear Power Engineering Center, showed the commissioners a PowerPoint slide advocating new technologies that would reduce "residual risk concerning earthquakes and tsunamis," according to documents attached to the meeting minutes.

WSJ's Peter Landers reports a plan to revamp efforts to cool nuclear power plants did not include the cooling of existing plants.

His presentation focused on improving backup systems for future reactors, as part of a broader pitch to regulators for a new generation of safer plants.

"The Nuclear Safety Commission was just starting a basic discussion of the need to install more diverse safety cooling systems in future reactors," said Mr. Tanaka in a telephone interview Tuesday. A spokesman for the commission declined to comment.

In January Hitachi Ltd., a top nuclear-plant maker, described the advantages of reactors with a backup cooling system not reliant on electricity. In a company journal, Hitachi said it was "gaining the cooperation of power companies and the government" in building a next-generation reactor that would "enable a response to long-term loss of electric power."

Hitachi spokesman Yuichi Izumisawa said Tuesday: "We don't think there are safety problems" with reactors in use now, but the company is developing "something even better."

The Hitachi article referred specifically to an old idea that has gained new attention in recent years, a clever but low-tech device called an isolation condenser.

A condenser works this way: If the reactor core heats up, the condenser receives naturally rising steam and circulates it through a tank of cool water, with no need for electric power. The device relieves pressure in the core, although it works only for a few days because its pool heats up.

Mr. Morokuzu, the former Toshiba reactor designer, said the condenser "is a way to fill the gap in an emergency, when the plant's external electricity gets knocked out somehow, either by an earthquake or for some other reason."

Of Fukushima Daiichi's six reactors, only No. 1—the first reactor to be built, in 1971—had an isolation condenser. Tepco said the condenser, which dated from the plant's construction, worked after the quake but eventually stopped. A Tepco spokesman said he didn't have information about why it stopped working.

Reactor Monitor

Some experts say heat inside No. 1, which is smaller than modern reactors, may have overwhelmed the condenser's capacity. Design changes to new reactors with condensers may address this.

Engineers say the field has seen philosophical shifts over the years. Early reactors, like No. 1 at Fukushima Daiichi, had an isolation condenser. It is known as a "passive" system: It works on its own, without requiring outside power. Later reactors adopted "active" systems that relied on electrical pumps and the like, often with redundant layers that engineers believed would provide more protection against overheating or breakdown.

Today, passive systems are again in vogue. General Electric Co. and Hitachi, which have a global alliance in reactors, use isolation condensers in their latest designs. The condenser is part of GE's Economic Simplified Boiling Water Reactor, for which GE is trying to win certification at the U.S. Nuclear Regulatory Commission.

Under the Japanese plans being discussed in recent months, new reactors wouldn't have both an active and a passive backup-cooling system, but rather just passive systems. According to the Hitachi article, the condenser would be used in place of the other backup method. The article says eliminating the active backup saves money and makes the reactor easier to maintain.

On March 12, the day after the quake, the crisis worsened at reactor No. 1 as rising pressure and heat forced Tepco to open a vent and release radioactive steam. Later that day, there was an explosion at reactor No. 1.

The other five reactors at Fukushima Daiichi, completed later in the 1970s, relied on a form of backup cooling that used electrical generators, which were knocked out by the tsunami.

All six of the reactors at the Fukushima Daiichi plant were designed by GE and were built by GE, Hitachi and Toshiba Corp.

A GE spokesman said GE opted for the active backup-cooling system in Fukushima Daiichi units 2 to 6, and in other reactors, because it believed the system was safer than earlier condensers and was better suited to bigger reactors. Some new reactors that now use condensers have other design changes that make the condenser more appropriate, he said. Toshiba declined to comment.

Akira Omoto, a member of a government advisory body helping to tame Fukushima Daiichi, said Friday the quake exposed a "lack of diversity" in emergency cooling mechanisms. The plant's reactors have two to three backup generators each, he said, but the tsunami knocked them out. "The problem was Fukushima Daiichi reactors had only one emergency way to cool their fuel vessels"—either the electrical generators, or in the case of reactor No. 1, the condenser.

—Rebecca Smith contributed to this article.