The first thing visitors notice about the largest nuclear power complex in the Soviet Union is a containment dome taking its final shape around the fifth nuclear power reactor to be built here, 300 miles south of Moscow on the banks of the River Don.

Made of steel and concrete and standing almost 250 feet high, the containment dome will be completed next month in time for the December commissioning ceremonies for the 1 million kilowatt nuclear plant it encloses.

That will make it the largest nuclear power plant of its kind in the Soviet Union. But what really makes it unique is that it is the first to be built with containment to isolate any dangerous radioactivity that might escape from the reactor in the event of a serious accident.

Never before in the 24 years the Soviet Union has been building nuclear power stations did they put a containment dome around a single one of the 29 previous reactors.

Almost all the 151 nuclear stations operating in 20 countries around the world operate inside containments. All 71 of the nuclear plants in the United States were built with containments. Without them, they could not have obtained a license to operate from the Nuclear Regulatory Commission.

If nothing else, the lack of containments illustrates the Soviet approach to nuclear safety which might best be described as "different".

None of the workers in Soviet nuclear plants wear the dosimeters so familiar elsewhere in the world to measure accidental exposure to radiation.Visitors are routinely brought into the rooms housing Soviet reactors. A touring group of American journalists last week was taken to the top of a small reactor in Moscow's Kurchatov Institute, where the only thing that stood between them and the eerily beautiful blue glow of the reactor's burning uranium was 15 feet of water.

The tour was arranged by the Atomic Institute Forum, a Washington trade association made up of major nuclear suppliers including Westinghouse and General Electric.

The worst possible accident the Soviets equip their nuclear plants to expect is a single break in the largest pipe carrying cooling water to the reactor. U.S. plants are built with complete emergency cooling systems to handle simultaneous breaks at both ends of the same pipe, a remote accident, to be sure, but one that could result in such a sudden loss of cooling water that the uranium fuel might overheat and even melt its way through the floor of the reactor.

A melting of the core is considered "impossible" in the Soviet Union, where the approach to safety is to take the utmost care in construction and to back up pumps and valves with redundant spares and emergency generators to supply power if the main electrical system fails.

The Soviets are proud of their nuclear safety record. They have run four small nuclear plants under difficult conditions without an accident at Bilibinska, in the permafrost region of Magadan. Two months ago, they kept running a much larger plant in Armenia without damage during an earthquake. Soviet safety experts claim they now have the equivalent of 2,000 years of nuclear operating experience without a "single major failure."

When they built the reactor for the first nuclear icebreaker Lenin, Soviet engineers tested it out in the shipyard where the icebreaker was being built to convince the workers and crew-members of its safety.

"How safe was it?" asked chief nuclear safety engineer Yuri Sivintsev at the Kurchatov Institute in Moscow. "In 1969, the Lenin collided at full speed with an iceberg that tore a whole in the ship one meter wide.Plates and dishes broke and I suffered a bad bruise on my head but the reactor went right on working."

Here at Novovoronezh the other day, plant director Vitaly Sedow told the American journalist that the plant was that day celebrating the 14th anniversary of its first reactor, a 210,000-kilowatt unit started up in 1964. Later reactors were built in 1969, 1971 and 1972 that together generate another 1,240,000 kilowatts. Sedov said there has not been a single fuel leak in any of the four reactors.

"The worst experience we have had here happened to a turbine, not the nuclear reactor," Sedov said. "We had to shut down one of the units for one month but it was to fix the turbine, not the reactor."

Nevertheless, the Soviets are feeling a little sensitive about not having containments anywhere else but the fifth reactor at Novovoronezh. Plant director Sedov led the visiting journalists on a tour of the turbines in the older parts of the plant but did not offer them entrance to the reactor room right next door.

Why are the Soviets building a containment around the fifth reactor at Novovoronezh? Why did they add $30 million and six months construction time to a project that has already taken six years? Sedov and construction director Dmitri Vprosorovski say simply that the order to add a containment to the fifth reactor came from Moscow.

American observers think the containment was built at the fifth Novovoronezh unit because of its size. A 1 million kilowatts, it will be the biggest pressurized water reactor in the Soviet Union and the prototype for many plants still to be built. The turbines, the pumps, the valves, the steam generators and the cooling pipes are all bigger than the machinery built for smaller plants.

Another reason, according to Western experts, is that the Soviets want a larger share of the export market. They have exported plants to Finland and most of the East Bloc countries but lost bids in Yugoslavia and Romania because those two countries would only buy plants with containments.

How do the Soviets site their nuclear plants? A lot closer to population centers than the United States does.

The Novovoronezh complex lies less than two kilometers (1.2 miles) from the town of Novovoronezh, whose population is now 25,000 and on its way to doubling. There is a story told here that the mayor of Belyoyarsk in the Ural Mountains asked the authorities to locate their first nuclear plant further out of town than they planned and they obliged. The plant is four kilometers from town instead of two.

Soviet fishermen represent the only group in the country that has successfully put pressure on nuclear planners. The first two plants built at Novovoronezh took their cooling water right from the river Don but a regulation was immediately made that the discharge water could heat up the river by no more than 9 degrees Fahrenheit at the point of discharge and no more than 6 degrees half a kilometer downstream to protect the fish in the river.

The next three plants were built with enormous and expensive cooling towers, that recycle their own water, which they get from wells. The fifth and largest plant nearing completion is being built with its own artificial lake, whose surface measures five square kilometers (two square miles).

The Soviets do not think the disposal of atomic wastes is anywhere near as serious a problem as it is made out to be in the United States, though they appear no closer to a permanent solution.

They refuel their nuclear plants once a year, then place the removed spent fuel in "swimming pools" filled with water where they are kept for three years to cool off. The spent fuel is then placed in steel casks and taken on railroad cars to what is a apparently an away-from-reactor storage site at an unidentified location.

At Armyansk and Dimitrovgrad, liquid wastes arebbeing pumped into layers of clay 1,500 to 6,000 feet in the ground. The Soviets say there is no water in clay to leach the wastes away from their burial sites but also admit the wastes they are pumping into the earth are not the "hot" wastes containing the most dangerous or longlasting levels of radioactivity.

Where and how they will dispose of the hot wastes is still under discussion at places like the Kurchalov Institute, where most questions of nuclear safety are resolved. Just as in Europe, America and Japan, abandoned coal and salt mines are among the prime candidates for the permanent burial of waste. The Soviets insist they see no inheret danger in the burial of nuclear waste if it is done right.

"There is a uranium lode in Gabon in Africa that has conducted its own natural chain reaction for almost 2 million years," Sovintev tokd the visiting journalists. "None of the radioactive wastes from that natural reactor has harmed anybody in that time."