Police cordoned off several miles of roads and about 200 workers were evacuated from the Fort St. Vrain nuclear power plant 40 miles north of Denver yesterday after radioactive helium leaked for two hours from the plant's nuclear reactor cooling system.

Colorado health department officials said there were no serious contamination problems or injuries from the leak, which was contained shortly before noon yesterday (EST). State officials also discounted any possibility that winds blowing from the plant in the general direction of Denver were carrying unusual amounts of radiation.

Dr. Anthony Robbins, head of the state health department, said he had ordered filters on air monitors around Denver changed after the leak in order to check for any signs of radioactivity in the area.

But Robbins and other state and federal officials at the plant site said they were unable to detect any significant amounts of radiation from the leak outside a one-mile radius of the plant boundaries. None of the residents of farms surrounding the plant were evacuated after the leak, officials said.

A spokesman for the Nuclear Regulatory Commission, which has oversight responsibility for the plant, said yesterday it was still to early to tell whether the Fort St. Vrain leak was the most serious radiation release incident from a U.S. nuclear plant.

Al Hazle, a radiation expert for the state health department, said that while workers at the plant received moderate amounts of radiation exposure from the leaking helium none received enough to required them to be quarantined. Hazle said the maximum exposure level for the workers was one to five roentgents, slightly above the fallout levels around the world during atmospheric nuclear testing.

The leak was caused, Hazle said, by a low-pressure valve giving way in a water buffer surrounding the reactor's helium cooling system.

Hazle said tne helium in the cooling system became radioactive iodine from the reactor which passed in a gaseous form through the ceramic fuel containers inside the reactor. When the buffer gave way, he said, the helium escaped into the building.

The force of the rapidly escaping helium overcame the building's "negative pressure" safety system that is designed to force any loose gas into the ventilation system rather than to allow it to escape. The leaking radioactive gas did escape, mainly through the building's chimney, Hazle said.

A spokesman for the Public Service Co. of Colorado, which owns the nuclear plant, said that, at the peak of the leak, monitors on the chimney were measuring between 20 and 30 times the normal radiation levels.

The $1 billion plant was still in the process of undergoing startup tests when yesterday's accident occurred. A Public Service Co. spokesman said the plant was operating at about 68 per cent of its 300,000 kilowatt capacity but was shut down to allow the nuclear fuel to cool after the leak.

Both federal and state nuclear experts said it was not clear whether the nuclear fuel inside the reactor had heated up to dangerous levels after the coolant started to leak.

State officials ordered an emergency reaction plan into operation after the leak was discovered. The plan, which is called a "category two' level operation, is designed for moderates radiation releases, officials said.

After the leak, all local roads within five miles of the plant were sealed off by the Colorado state police and the local sheriff's officers. Workers at the plant were evacuated to Johnstown, Colo., 10 miles away, and local hospitals were placed on alert status.

State health teams were sent to the plant to monitor radiation leakage, and a federal investigation team was sent to the site from Dallas, Tex., by the Nuclear Regulatory Commission.

State officials said late yesterday that they were still unclear about what triggered the leak or the exact amount of radiation that made its way out of the plant.