The Soviet Union is believed to have closed nuclear reactors of the same type as the one that exploded in Chernobyl, effectively shutting down half of the country's nuclear power supply, according to western diplomatic sources.

Meanwhile an official Soviet statement today said that 18 persons were in serious condition as a result of the Chernobyl accident, and that efforts were continuing to "deactivate" the area around the plant, about 60 miles north of Kiev, the Ukrainian capital.

The Soviets already had said that two persons died in the accident, which diplomats report occurred Saturday but some U.S. experts date to Friday. A statement yesterday said that 148 people were being treated in hospitals and that 49 others had been treated and released.

It was unclear how the western sources obtained their information on the reported shutdown of the other Chernobyl-type reactors, and there was no official confirmation here. If the report is accurate, the shutdown would cut Soviet electricity supply by almost 5 percent nationwide -- more in the European part of the country, which is more dependent on nuclear energy.

The reported cutback comes at a critical time for the Soviet economy, with industry gearing up for an intensive retooling and reconstruction program.

Nuclear energy now accounts for 10 percent of Soviet electricity, producing 170 billion kilowatt hours a year. The Soviet Union is the world's third-largest producer of nuclear energy, ranking behind the United States and France.

Of the Soviet Union's 39 operational reactors, 20 are of the so-called RBMK-1000 model, of which eight -- including the one in Chernobyl -- were constructed in the last five years. The diplomatic sources said the nationwide shutdown took place yesterday. The three undamaged units at Chernobyl had been closed down earlier.

Since the accident at Chernobyl was confirmed Monday, the Soviets have provided little information on its cause, so it is not known whether the problem was related to the design of the reactor, its construction or maintenance at the plant. The RBMK-1000 reactors are all water-cooled and moderated by graphite.

Some western experts speculate that a fire is still burning at the Chernobyl reactor, although others have accepted recent Soviet statements that the blaze has been put out. Reports yesterday that another reactor had caught fire have been discounted.

As the incident at Chernobyl goes into its sixth day, much about what is happening in the area is still unclear. The Soviets have kept a tight grip on information: Today, for instance, the official statement reported that radioactivity in the plant's vicinity had dropped 1.5 to 2 times but did not give the levels before or after this drop, making the information virtually meaningless.

A statement last night said that a chain reaction was not taking place in the nuclear fuel, but it was not clear whether that meant a meltdown had not taken place, or whether the process had been arrested.

One diplomat said today that close examination of the photograph of the reactor seen last night on Soviet television showed further evidence of an explosion from the inside -- broken windows, a hole in the roof and twisted girders. "It was a terrifying picture," said one western analyst.

According to one western scientific adviser, the reactor damaged at the Chernobyl plant was the newest of the station's four units -- opened in the last three years -- and the only one to have a containment vessel. The picture showed that the concrete shell had been severely damaged.

Today's government statement, issued by the official news agency Tass, said no foreigners had been injured in the accident.

More than 100 foreigners came to Moscow today from Kiev, including about 70 British and 7 American students. Their reports of normal conditions in Kiev contradicted fears of widespread casualties and contamination of the city's drinking supply. The arriving students received an emotional greeting from their friends in Moscow, who clustered around them to stave off reporters.

Twenty-seven other students, all British, came in today from Minsk, the capital of Byelorussia, which reportedly lay in the path of the radioactive cloud as it drifted toward Scandinavia.

Eight French students who had been in Kiev were flown back to Paris today at the French Embassy's expense, because of concern that a train trip would expose them to radiation that had drifted west to Poland.

Hank Birnbaum, an American who acted as the Kiev group's spokesman, said this morning that foreigners in the area had been caught "between two worlds," not knowing what to believe.

"On the one hand, the Soviets were saying nothing and on the other hand, from the West, there was a flood of information, some exaggerated," he said. "We were in the middle, and near the accident."

A number of the students had wanted to stay in Kiev, where they had arrived for a study semester only two weeks ago. "Some of them wanted concrete facts and we were not getting any," said Birnbaum. "But it was decided better to be safe than sorry."

The students, who flew out this evening to London, were tested for radiation at a Soviet hospital, at Soviet insistence, and declared "effectively healthy."

But a radiologist aboard the plane apparently found levels of radiation on the students' clothes to be high enough to ask them to change into specially provided running suits. A nurse was also aboard the plane.

The Soviet government has doled out information about the accident in a series of short statements, starting Monday night.

The halting and incomplete information has frustrated western embassies, a number of which are advising their citizens to leave or keep away from Kiev. Several embassy spokesmen said the travel advisories were caused more by the lack of information than because of hard facts about the dangers in Kiev.

Yesterday, apparently in reaction to demands for more complete information, the Foreign Ministry received heads of missions of five European embassies, representing Britain, Finland, the Netherlands, France and Austria. Of the countries on the list, all but the Netherlands were actively evacuating citizens from Kiev.

Dutch Ambassador Frans van Agt was received in the "early morning hours" today after the Netherlands, which now holds the presidency of the European Community, had asked to deliver an offer of assistance from the community.

Diplomatic sources said the Dutch ambassador had first been refused a meeting at the Foreign Ministry and directed to the Atomic Energy Committee. But the embassy was told yesterday that no one at the committee could receive the ambassador until after the May Day holidays, which end Sunday. Finally, the early morning meeting at the ministry was arranged.

Soviet leader Mikhail Gorbachev and other members of the Soviet leadership presided today over the May Day procession in Red Square. May Day celebrations around the Soviet Union dominated the evening news, with a special emphasis on the Ukraine. The only mention of the Chernobyl accident was a reading of the government statement issued earlier in the day.