TOKYO, FEB. 12 -- An emergency pressure-release valve failed to function when technicians tried to shut down a reactor over the weekend in Japan's worst nuclear power plant accident, the Energy Agency said today.

The agency did not say why it withheld disclosure of the safety system malfunction, which occurred Saturday and was reportedly the first during a nuclear accident in Japan.

Although the accidental radiation leak was the largest ever recorded in Japan, it did not pose a threat to humans or to the environment, said the officials, who spoke on condition of anonymity.

Anti-nuclear groups responded to today's disclosure by calling for similar reactors to be shut down in Japan.

When the valve failed to work Saturday, technicians had to use a water sprinkler to reduce pressure in the reactor's primary cooling system after a leak was detected at the 500,000-kilowatt reactor at Mihama Nuclear Power Plant in Fukui, western Japan, agency officials said.

By reducing pressure in the primary cooling system after an accident, more radioactive water is prevented from escaping into the secondary cooling system, an official at the government agency said.

Saturday's accident occurred when abnormal levels of radioactivity were detected in the reactor's secondary cooling system, apparently because of cracks or holes in the cooling tubes linking it to the primary system, the officials said.

The reactor was eventually shut down by an automatic safety system about 50 minutes after the leak was detected, news reports said. They said technicians did not shut it down earlier because they thought the gauge measuring radioactivity was broken.

However, Energy Agency officials said the time lapse before the reactor was shut down occurred because technicians were sampling water radiation levels.

Japan's government has adopted an ambitious nuclear power expansion program in an attempt to reduce its dependence on imported oil and provide for future energy needs.

Forty nuclear plants now supply nearly 30 percent of Japan's power needs. Government experts predict the share will rise to 36 percent by 1995. Japan imports more than 99 percent of its oil.