Lights in three colors blinked and flashed on the control panel, the lines on the graphs moved slowly right and then held steady, the hands of the dials edged upward and stayed there. "That's it!" someone shouted.

A cheer went up from the engineers and technicians jammed between the computer consoles and the walls of lights and buttons. It was 7:16 a.m. last Wednesday, and the nuclear reaction in the core of the North Anna 1 nuclear power plant had become self-sustaining for the first time.

The moment of criticality, as the event is called, meant several things:

The nation's 69th electricity-producing nuclear reactor was working and was ready to be brought gradually to its full capacity;

The North Anna Environmental Coalition has lost another round in its five-year battle against the facility;

The Virginia Electric and Power Co., the nation's eighth largest electric utility, would soon ask another $82 million from its customers.

The North Anna nuclear power station is Vepco's second nuclear plant, and, when complete in about 1984, will have cost more than $2.5 billion and have the capacity to produce half the total amount of electricity Vepco expects to need.

From the North Anna Visitors' Center on a nearby wooded ridge, the two gray concrete domed cylinders of North Anna units 1 and 2 (two more will be built later) resemble huge, snub-nosed bullets partly buried in the earth. The green power generation building looms behind like a giant aircraft hangar, two football fields in length and only half finished, while the rusty frames of units 3 and 4 dominate the foreground and Lake Anna shimmers behind it all.

Swarms of construction workers employed by the general contractor, Stone and Webster, move between the piles of pipes and cables and odd-shaped bundles of machinery that will be the rest of the facility.

Nearly 96,000 people have come through the rolling farmland and trailer campgrounds of Spotsylvania and Louisa counties, 90 miles southwest of Washington, to view this sight during the 4 1/2 years that Unit 1 has been under construction. "People are really curious about nuclear power," said visitors center supervisor Edna Musser.

Not permitted to get any closer to the complex, visitors peer at it through telescopes and punch buttons in the centers to get answers to questions like, "Can the urnaium in a power station explode like an atomic bomb?" The answer: "No. There is no more similarity between the two than between the nitrogen in lawn fertilizer and the nitrogen in dynamite."

The visitors are told that North Anna 1 has just begun to produce electricity. By midweek it should be putting out more than the 40 megawatts used by its own lights, air conditioners, computers and other equipment: by the end of May in should be in commercial operation, pumping its full 934 megawatts into the Vepco system that serves 1.2 million customers in Virginia, northeastern North Carolina and a corner of West Virginia.

One megawatt lights 10,000 bulbs of 100-watt strenght for one hour. North Anna 2, about a year behind, will be equal in size to Unit 1, while units 3 and 4 are planned to produce 938 megawatts each by 1984. "Each unit is basically a self-contained power station." explained Peter Beament, Vepco executive administrator for power, as he showed a visitor around the site.

Guarded concrete block walls topped with barbed wire divide the green hangar down the mddle, the humming generators of Unit 1 on one side, the sphaghetti of cables and piles of pipe for Unit 2 on the other.

"It's not as though you could turn a switch and the power comes on, though," noted Douglas Cochran, one of Vepco's spokesmen. "There are thousands of small and different systems that each operate independently."

The control room complexity reflects that. Knobs and buttons bear cryptic labels: "exciter volts," "stripper trim," and the provocative, "excess letdown," among many others. Nearly 1,000 miles of electric cable and 100 miles of pipes are involved in units 1 and 2, much of it for backrups to backup safety equipment. The earthquake detection equipment, Beament said, "is so sensitive it picks up the lunch break here because the heavy equipment stops moving."

The nuclear reaction itself takes place in a reactor vessel, a capsule that is 42 feet long and 16 feet across, with walls up to one foot thick. The two reactor vessels for units 3 and 4 lie in a seemingly causal fashion on their sides in a fenced storage yard near the visitors center entrance, their pipe openings covered with red seals until their installation. "Weather won't hurt them," said Beament, smiling.

He patted the nuclear fuel assembly on the wall of the visitors center, the silvery collection of fuel-stuffed zinc alloy rods that form the heart of the entire complex within the reactor vessel. It looked like a long, rectangular bird cage with more rods and grids where the birds should be.

"If you live in a brick house, you'll get more radiation from that than you get from this when it's delivered here, Beament said. Inside each rod are cylindrical pellets of uranium oxide (3 percent uranium).A total of 36,000 rods are grouped into 157 assemblies and interspersed with "control rods" - solid bars of silver, cadmium and indium - that catch as much radiation as necessary to keep the process at the right level.

North Anna was "started" six months ago when a special rod of beryllium and polonium went, along with the fuel rods, into the reactor vessel that was by then filled with water, and began emitting particles called neutrons. The neutrons collided with the atoms of uranium oxide fuel and caused them to emit more neutrons, other particulars and heat.

As the neutrons kept colliding with more atoms of fuel, a controlled chain reaction built up to the point last Wednesday where the process was self-sustaining and needed no further help from the starter rod: This is criticality. After that point is reached, the heat can be drawn off to boil water into steam, which drives the huge blades of a turbine, which turns a generator to make electricity.

"I stayed up all night waiting for criticality, but it was kind of anticlimatic, really," said Don Hopper, supervisor of the health physics department, apologizing for his bleary eyes. Hopper had put new office traffic rules into effect Wednesday morning to make sure that water or other samples of possibly "dirty" radioactive material being tested in one laboratory would not be tracked into a "clean" room, but two employes walked casually through the wrong way. "I'll have to see about that," he said. "We're not used to it yet."

Oscar the Phantom is a silent partner in Hopper's job. Legless, armless and covered with a pink plastic blanket, the plastic, water-filled Oscar has plastic, water-filled organs that can be loaded with various radioactive dosages. He spends his days riding slowly through a steel-walled machine that gives radiation readings for each organ, which are then compared with real people's readings.

The dosage counters are so sensitive that the steel of Oscar's machine and in most of the laboratory structures. Hopper said was cut from tanks and ships and other excess military equipment made before World War II, because nuclear bomb blasts have put man-made radiation into the atmosphere since then. "Everything built since then is contaminated," he said. "and these machines are so delicate they pick it up."

No North Anna workers have yet received any kind of radiation dosage, he said, "but when they come in here from other places we can tell by which organs are 'hot' just about what kind of job they had in the past," he said.

Safety from radiation remains the great worry of North Anna's critics. "When you see all that array of machinery it takes a real cynic not to say, 'Isn't this marvelous?" but that should be balanced by some time reading about the malfunctions in plants all over the country," said June Allen, head of the North Anna Environmental Coalition.

The group led a five-year battle against the North Anna nuclear plants and raised safety questions that delayed the operating license for months and caused Vepco to be fined $32,000. "Given the suspect equipment and the history of sloppy operations here, there's no reason to believe North Anna is going to be a safe unit," she said.

Other Vepco critics jammed a hearing in Richmond last week to protest Vepco's request that part of North Anna's costs. $82 million, be added to the charges passed on to customers in their basic electricity bills. Under Virginia law, rate payers begin helping to pay for a new facility when it begins producing electricity.

Although consumers think their electricity bills are high enough already, Vepco officials have said North Anna I will save customers $50 million its first year alone by using cheap nuclear fuel instead of oil or gas. The remaining $32 million, they say, will be recovered quickly as future oil and gas costs rise.