The badly damaged nuclear reactor at Three Mile Island was still being slowly cooled down today as technicians began to move onto the island from their trailer camp on the east bank of the Susquehanna River.

Two of the 40 to 50 trailers making up "Trailer City," housing the hundreds of nuclear engineers and technicians who converged here after the accident, made their way to the island, where there is more room, a better tie-in to telephone lines and more convenient sanitation. Other trailers are expected to follow, as chances of radiation exposure continue to diminish on the island.

Meanwhile, temperatures of the reactor's cooling water were being held at 250 degrees and pressures at 850 pounds per square inch while technicians siphoned off whatever hydrogen gas remained in the water, in an effort to further minimize the danger of a bubble forming inside the chamber. It was a large hydrogen bubble two weeks ago that threatened to worsen the peril of the Three Mile accident.

Water samples drawn out of the reactor by technicians wearing protective masks and clothing were sent to Idaho Falls, Idaho, to be analyzed for hydrogen content. Oficials of the federal Nuclear Regulatory Commission directing the cool-down want to be as sure as possible there is no more undissolved gas in the reactor chamber before moving to what they call "cold shutdown."

Despite their protective garb, the six technicians received a total radiantion dose of 800 millirems, including one dose of 270 millirems. While this was below allowable quarterly limits, these technicians will not be unduly exposed to radiation for at least another week.

One reason the reactor is not being rushed into cold shutdown is that four thermocouples inside the reactor core are stil registering temperatures higher than 300 degrees. One is reading 348 degrees near the center of the core, where NRC officials believe damage is so great that the fuel rods are twisted and bent into shapes that make it hard for cooling water to reach them.

"The core geometry in the upper regions, especially near the center," said an NRC bulletin today, "is believed to be severely distorted due to loss of fuel-cladding integrity in that region."

The NRC disclosed that there was a momentary "puff" of radioactive iodine into the air above the plant on Saturday, presumable while technicians were changing filters in the auxiliary building next to the reactor where waste water has been pumped.

Iodine-131 levels right above the island were measured at noon on Saturday at 119 picocuries per cubic meter of air, 10 times what was found there on Friday and slightly in excess of allowable limits.

The NCR said it was not seriously concerned with the radioiodine release but said it would try to prevent its happening again. The NRC said that nnone of the iodine escaped off the island.