The effort to vent radioactive krypton gas from the crippled Three Mile Island nuclear plant began this morning, and, like so much else involving this reactor, it immediatley went wrong.

At 8:04 a.m., four minutes after the venting began, alarms flashed, valves slammed shut and everything stopped.

But unlike the last time alarms sounded here, at the beginning of the accident that closed this power plant more than a year ago, these alarms proved false. And the venting, which had begun in a festive atmosphere before a horde of politicians, regulators, Metropolitan Edison Co. utility officials and reporters, resumed again in the afternoon and continued on a restricted basis into the evening.

The alarms went off when monitors picked up indications that high levels of radioactive particles were coming out of the 160-foot stack along with the krypton gas, and the operation was shut down.

It turned out that, as the Nuclear Regulatory Commission's Harold Denton put it, "The monitor has seen krypton and thought it was particulates." Another alarm system checking the radiation level of gas releases was silent, indicating no problem.

The utility, with NRC aproval, shut down until 5 p.m. and then cautiously resumed venting on a low-volume test basis. But even the weather seemed to be malevolent. A fierce thunderstorm ripped through the area, causing the test venting to halt from 7:08 until 8:13 p.m.

Full scale venting, at a planned rate of 120 cubit feet of air per minute, on Sunday afternoon is to await further confirmation that no particles are in the gases.

"It just seems like everything that's done here is a special operation and doesn't go right the first time," Denton told the 100 or so reporters gathered across the Susquehanna River from the plant."I told my wife this morning I gave it only a 50-50 chance of working out today."

He described what happened as "overshooting" in which the monitor reacted to the surge of radioactive gases heavily. The readings, he said, shot past the true level into the alarm range and would have led to automatic shutdown had operators not turned the vents off first.

The idea of the low-volume test is to hold monitor readings below the official danger level. Even though the monitors are confused, Denton continued, "I just don't want to abandon the regulatory commission limits without more verification that there are no particles there."

Barring further hitches, Met Ed plans to get rid of all 57,000 curies of gaseous krypton-85 inside the reactor building within the next three weeks, diluting it with pumped in air before letting it go to radiation levels less than 1/1000th of what they are now inside the building.

High radiation levels have prevented anyone from entering the reactor building to do maintenance work since a series of errors and equipment failures shut down the plant and loosed radiation over the countryside here on March 28, 1979.

NRC commissioner Joseph M. Hendrie, who was chairman then, began pushing for venting six months ago on grounds that more equipment failures were inevitable without maintenance. He was here today to watch it happen.

"I think we're finally going to make it," he said, "This marks a truly significant step in getting on with the clean up."

Nuclear critics went to great lengths to stop the venting, but the U.S. Court of Appeals in Washington ruled Friday that it could go ahead.

Officials took pains today to stress the low level of the radiation coming out with the krypton. Standing at the observation center for the entire three-week venting period, they said, would result in as much extra radiation as half an hour of suntanning, or spending 28 minutes in an airplane, or sleeping with someone every night for eight months: about 0.1 millirem. Natural background radiation nationwide is about 100 millirem per year.

But some people weren't convinced. Nancy McKay of Etters, Pa., who is expecting a baby in six weeks, said last week that she and her two-year-old son would visit relatives this weekend in Dillsburg, 15 miles away. "At least I won't be two miles away," she said. "I don't know anybody that lives any further."

Andrea Hand, also pregnant, took her son and daughter to Connecticut. "It's fear," she told reporters before she left. "I'm pregnant and I don't trust the authorities."

The antinuclear group, Three Mile Island Alert, which did not oppose the venting, reported that 43 of 91 persons who had called in the past two weeks said they would leave during the venting, but Met Ed said only four of its 360 callers were packing up.

Local officials had speculated that as many as one in five of the 135,000 area residents might flee. But State Police Lt. Frank Lewski, commander of the York station, said today that there were no signs of anything other than normal beach or mountain-bound weekend traffic.

For the majority of people here, nothing much changed. A two-month-old group called "Friends and Family of TMI," composed of 200 relatives of Met Ed workers and other pro-nuclear people, passed out doughnuts and coffee to reporters, chatting easily about how they had conquered their depression in the wake of last year's accident by learning more about nuclear power.

"We were frustrated and worried and very depressed back then," said founder Kathy Heilman, whose husband is a control room operator at the plant. The group, she said, has helped boost morale in the plant and in this shady little town too, where pro-nuclear signs and TMI T-shirt entrepreneurs abound.