The Finns who operate the first nuclear power plant to be exported to the West by the Soviet Union jokingly refer to it as their "Eastinghouse" reactor. But they see the future for Soviet nuclear power as anything but laughable.

The Finns call their Soviet-built plant "Eastinghouse" because it so closely resembles the 38 nuclear electric plants built round the world by Westinghouse, the American company preeminent in the field.

Westinghouse tried to land this contract eight years ago but lost to the Soviets even though it bid 20 percent less than the $250 million the Finns paid the Soviets. Therein lies a story.

For the last two years, Westinghouse has told the White House and the Congress that stricter controls on nuclear exports will mean a staggering loss of sales. Insisting that nuclear energy is here to stay, Westinghouse has said that if American companies fail to build the world's nuclear power plants, the French and the West Germans will.

There is little doubt that American companies such as Westinghouse may lose some of their share of the nuclear market but there is doubt that the French and Germans will capture that loss. If the Finnish experience is a guide, it may be the Soviets who win a large share of the market.

A trip to this small town on the Gulf of Finland by touring American journalists offers evidence that this may turn out to be one of the most serious economic threats the Soviet Union has ever mounted on the West.

The Soviet-built plant is already in operation, a shiny, whirring machine that last week started its fourth straight month without missing a kilowatt. An almost identical second plant is nearly constructed, less than 100 yards from the first. More than 600 Soviet construction workers are still at work there.

Talks are taking place concerning a third plant, which at 1 million kilowatts will be more than twice the size of the 440,000-kilowatt plant now in service. The Finns say they don't have to decide on this plant for another two years but it is clear from conversations here with executives of the government-owned Imatran Voima Power Co. that a decision of sorts has already been reached - the plant will be built by the Soviets.

"Why change something when it works?" asked Kalveli Numminen, director of Imatran. Anders Palmgren, superintendent of the Loviisa power station added: "The plant we have here happens to be one of the best in the world. The fuel works well, the vibrations in the turbines are low and we have not had a single fuel leak in more than 10,000 hours of operation. We are delighted with this nuclear power plant."

The Finns talk bluntly about how they would like to become salesmen for Soviet nuclear power around the world. The Finns here think that the Soviet nuclear power design is as good as any in the West and want to share in the profits they see tumbling into Soviet coffers once others turn to Moscow.

Until the Finns bought their first Soviet plant, things did not go well with Soviet nuclear exports. The Soviets have exported nuclear plants to East Germany, Poland, Bulgaria and Hungary but in the last few years lost bidding battles to Westinghouse for a plant in Yugoslavia and to a Canadian firm for a plant in Romania.

None of these Communist countries thought the Soviet nuclear design was safe. Five years ago, the Soviets were trying to sell nuclear plants without the thick concrete containment buildings around them that are the American trademark.

The Soviets did not build redundant cooling systems favored by American companies to cool down the reactor if its nuclear fuel should accidentally overheat. The Soviet computer controls of the plant were also incomplete.

The Finns convinced the Soviets to change their ways. They bought not only the Soviet reactor but also-steam generators and turbines. Then they went to a West German firm for the controls, a Dutch company for the concrete containment and to Westinghouse for a $10 million system to flood the reactor with 1,000 tons of ice if its heat and pressure need to be subdued in a hurry. The Soviets got the message and will now supply these items.

Currently said to be negotiating to buy Soviet nuclear plants are Cuba, Libya and a third unnamed country. Visitors now flock to Loviisa to watch the plant work and to talk about it with the Finns.

The Loviisa plant could win the Good Housekeeping award for industrial cleanliness. It is nearly spotless, from its white turbines to the dark green of its cooling tanks along the walls of the main reactor building. The reactor itself is silent. The only noise is the high-pitched whine of the turbines.

The Finns say one reason they picked the Soviet plant over the Westinghouse design is the financing, which other countries might not get from the Soviets. They loaned the Finns money at low interest rates, gave them uranium fuel free of charge for the first year and let the Finns sub-contract almost half the construction work to Finnish firms.

The way the Finns tell it, the country needs nuclear energy. Finland has no oil, coal or natural gas and pays out more than $1 billion a year to import energy. The country is so flat it has only six dam sites left to exploit, all of them expensive. The only domestic fuel is peat.

Nuclear power presently supplies 3 percent of Finland's electricity, and that figure is likely to rise to 35 percent within a few years. That would make Finland the world's most nuclear-dependent nation, but the Finns hope that their experience with plants such as "Eastinghouse" will turn to profits when other energy-strapped countries are shopping for Soviet nuclear technology.