The flashing lights, pressure gauges and temperature readings indicate something has gone wrong somewhere within the intricate innards of a powerful nuclear reactor.

The operators know they have seconds to identify the problem and react correctly -- or they might flunk the test they are taking in a simulated nuclear-reactor control room.

This mock emergency is played out over and over in 90 percent of the nuclear power plants in this country, according to Link Simulation Systems, a Silver Spring company that designs and assembles the simulated control rooms.

Link, a division of Singer Co. of Stamford, Conn., is one of hundreds of companies across the nation that are jostling for a share of the multibillion-dollar business of providing safety equipment and quality control services for nuclear reactors.

With construction of nuclear plants winding down, and with no orders for new plants since 1978, the companies and individuals with nuclear power expertise have turned to the business of maintaining, refueling and retooling existing plants here and abroad.

"Billions of dollars are spent in this country for servicing nuclear plants," said John W. Landis, senior vice president of Stone & Webster Engineering Corp. of Boston, which designs, constructs and services energy facilities. "Right now, it is the major business in the industry."

The nuclear service business is estimated to be worth about $2 billion a year, and the competitors include the big guns of nuclear construction -- General Electric Co. and Westinghouse Electric Corp. -- as well as small consulting and technology firms.

The companies involved in the business say it is hard to separate "safety related" services from other services because all aspects of a reactor's operation include safety concerns. Both safety and reliability depend on sound construction, engineering and maintenance as well as on the proper training of operators.

"You never stop worrying about safety or wanting to improve reliability," Landis said. "They are so intertwined."

Training Operators Is Important

Link officials agree, noting that their simulators are used for training reactor operators in routine procedures as well as testing their reactions to reactor malfunctions.

"If you want to talk about safety of nuclear reactors, there is nothing more important than the good training of operators," said Jay Whitney, vice president and chief administrative officer of General Physics Corp., of Columbia, Md., which provides training, operation and maintenance services to the nuclear industry.

Whitney noted that human error was blamed for the accidents at the Three Mile Island reactor and at the Chernobyl reactor in the Soviet Union.

General Physics, which attributes about half of its $66.5 million in revenue last year to nuclear power services, trains operators on simulators such as those provided by Link. About $20 million of last year's revenue reflected nuclear power plant training services.

Link says each of the 54 simulators it has built is unique because each is an exact replica of the client's real control room. "If their board has yellow lights, then ours has yellow lights -- if theirs has a sticker missing, then we take off the sticker," said Nancy Ryan, marketing and communications director of the company.

Selling for about $8 million to $12 million each, simulators use customized computer software to mimic the operation of each of the thousands of valves, pipes, wires, cables, pumps and other components of a nuclear reactor.

Operators in training learn on a simulator how to perform routine tasks such as starting and stopping the reactor, and might spend hundreds of hours on the simulator before taking their certification exams, said Philip J. Swanson, who ran such a program for a New England utility and is now Link's director of power plant operations. A certified operator typically might spend 20 hours a week, every six weeks, on a simulator to brush up on skills, practice unusual procedures or test reactions to crisis situations.

The simulator can mimic 250 to 350 different malfunctions in a variety of different situations -- such as a broken valve during start-up or a loss of coolant at full power.

Simulators Help Build Confidence

"Nothing ever happens the same way twice," said Swanson, who was an operator for 25 years. "Most of the enduring lessons you learn are those you learn the hard way, and it's better to learn those lessons on a simulator."

Swanson said the simulator is a "stress reducer" and "confidence builder" for operators. Ryan said the machines also sharpen skills that could become dulled by the monotony of a smoothly running reactor. "It's a very boring job, actually. The reactor is supposed to run smoothly, and it usually does."

Another area of safety services is quality control -- the system for ensuring that equipment is built and procedures are followed in accordance with certain standards.

The Nuclear Regulatory Commission sets requirements that must be met before a reactor can be licensed to load fuel or to operate. State utility regulators have their rules. The American National Standards Institute sets engineering standards for the industry. And after the Three Mile Island accident in 1979, the utilities that use nuclear reactors formed the Institute for Nuclear Power Operations to help plants meet the industry's own safety criteria.

Public debate continues over whether such standards are strict enough and if they are enforced properly.

Companies in the quality control business design and implement systems for checking to see if such standards are met.

International Energy Associates Ltd. (IEAL), based in the District, is completing a quality control assessment of the Comanche Peak Steam Electric Station, a nuclear power plant built by the Texas Utilities Generating Co. of Dallas.

In 1984, the NRC decided not to allow the plant to load fuel because of questions raised about the quality control and assurance program there. The Texas company hired IEAL to determine the extent and significance of alleged safety problems there.

Part of the problem was that "the paper trail had broken down," said J. Mark Elliott, president of IEAL. The utility had not documented that each step of the construction process had been performed properly. So IEAL designed a program to check quality control at the plant, reinspected certain aspects of construction and is analyzing the findings, Elliott said.

In this case, IEAL was not asked to make recommendations on how to fix the problems it might uncover, but it has been asked to do so for other clients. Other services might include checking to see if the plant is staffed properly or if the staff is trained properly.

IEAL also offers quality-control record management services to help utilities cope with the "tremendous amount of documentation required" to meet regulatory requirements, said Elliott, who is also president of the energy and environmental systems group of ERC International , the Vienna, Va., parent company of IEAL.

Some industry experts say most of the explicitly safety-related service work has been done since Three Mile Island, and that most of the future nuclear service business lies in the maintenance area.

A large part of quality assurance is focused on improving plant performance and reliability, Landis said. "You want to assure not only that it is safe . . . but also that it reliably delivers power when it is required to."

With 93 commercial plants operating in this country and another 19 under construction or ready to begin operation, the market for such work is large. But the field is crowded, experts said.

"The service business is very competitive," said Roger Mattson, vice president of technical services for IEAL. "So many people were involved in the designing and constructing of nuclear plants. Now they are no longer building them, so all those people have gone into consulting firms."