More than a month after a mishap during an underground nuclear test at the Nevada Test Site, radiation in a tunnel containing test equipment is still so high that the tunnel cannot be reentered for "several weeks," a Department of Energy spokesman said.

Monitors in the tunnel are registering "about 25 rads per hour," he said. Under current standards involving measurements of radiation exposure, 450 rads would kill half of those exposed, while five rads is considered a safe level over a year.

The radiation is being contained in the tunnel by a cement door 8 to 10 feet thick. But DOE officials reported yesterday that two of about 60 employes working in protective clothing to clear the area outside the test tunnel have shown evidence of exposure to higher than normal radiation levels.

The workers have cut through two similar cement doors put in place as safety measures to keep radiation from escaping.

One worker was found last Friday to have absorbed 200 millirems of radioactive iodine-131 in the thyroid. Occupational safety guidelines permit 5,000 millirems in any three-month period.

After three weeks of ventilating the tunnel, DOE officials said minute traces of radioactive xenon gas, too low to have health effects, are entering the air.

This was the third time in 10 years that such an accident has occurred, a Defense Nuclear Agency official said yesterday.

The April 10 test, nicknamed Mighty Oak, was designed to determine radiation effects of a nuclear explosion on different types of U.S. weapons, including the warhead for the MX missile, the warhead and other equipment associated with the new Trident II submarine-launched missile and equipment associated with the proposed Midgetman mobile intercontinental ballistic missile.

A low-yield nuclear device was exploded in a chamber, and the radiation output was designed to go down a tapered horizontal pipe, about one inch wide at one end and 25 feet wide at the other.

The pipe, one-third of a mile long, was located in a tunnel cut more than one mile into the Ranier Mesa. The nuclear device was at the narrow end of the pipe, and military hardware at the wide end.

The pipe was sealed before the test and the air pumped out to create a vacuum. Two steel doors inside the pipe, actuated by explosives, were to shut within tens of milliseconds after detonation, permitting only fast-moving radiation and X-rays created by the explosion to pass and hit the military targets.

Like X-rays administered at a doctor's office, they were to leave no trace of radioactivity on the targets.

The slower-moving rocks, sand and gravel made radioactive by the blast along with other debris and hot gases were to be stopped by the doors and prevented from continuing down the pipe.

In Mighty Oak, however, one or both steel doors apparently did not close. Instead, tons of irradiated material went down the pipe and "probably" contaminated and perhaps destroyed some of the targets and the approximately $15 million worth of test equipment in the pipe.

"We have no idea of the sense of the damage," a DOE official said yesterday. But, he added, "most of the data" from the $70 million test had been transmitted by cable to devices outside the pipe before the radioactive debris struck.