A nuclear reactor was shut down Saturday after a defective steam generator leaked large quantities of highly radioactive water into a cooling chamber, government and power company officials said yesterday. The accident caused the emergency core cooling system (ECCS) to pour a huge amount of water into the reactor's core to prevent fuel rods from melting down, the officials said.

It was the first time an emergency device shut down a troubled nuclear reactor in Japan, which has 38 nuclear power plants.

The 500,000-kilowatt pressurized water reactor at Fukui, 220 miles west of Tokyo, is owned and operated by Kansai Electric Power Co.

Small pipes in the steam generator developed cracks or holes, allowing radioactive coolant water to flow from the main cooling system into the subsystem that converts steam into water. Government and power company officials played down the accident, saying the emergency system performed successfully.

Antinuclear activists expressed serious concern over the effects of possible radioactive leaks on the environment.

``If the ECCS failed to operate, the accident could have caused a meltdown of the core, the magnitude of which would equal the accident at Three Mile Island,'' the Kyodo news agency quoted nuclear expert Nisaburo Takagi as saying.

``There is little to worry about, because the radiation discharged to the atmosphere has been kept to an absolute minimum by a special filter,'' a power company spokesman said.

Nuclear issues are a sensitive matter in Japan, the only country ever attacked by atomic weapons. The accident in Fukui follows a gubernatorial election in Aomori prefecture last week, where nuclear power was a central issue. Despite heavy campaigning by a antinuclear candidate, the pro-nuclear incumbent won re-election by a wide margin.

The country plans to build 40 more nuclear power plants over the next 20 years and almost double the country's reliance on nuclear power to 16.7 percent of total primary energy supplies.

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