It was a typical moment in the harassed day of Boris A. Semyonov, Soviet nuclear troubleshooter. The normally pliant Finns were complaining about being left in the dark about the Chernobyl nuclear accident, and he had the task of placating them.

"We will give you all the information necessary, but first we have to compile it ourselves," Semyonov explained in fluent English to the Finnish delegate, who had cornered him in the halls of the International Atomic Energy Agency (IAEA) here. The Russian's ready smile, smooth manners and well-cut suit marked him as an accomplished exponent of what could be called the Mikhail S. Gorbachev style of Kremlin diplomacy.

The Finn seemed only partly convinced. Finnish public opinion, he insisted, was extremely concerned about the effects of radioactivity. The Chernobyl accident had left an enormous impression on Scandinavian countries -- and extra information was vitally important in calming the situation down.

The snippet of conversation between the Soviet and Finnish nuclear officials, which was overheard by a reporter, reflected the effort now being made by Moscow to recoup from the diplomatic setback it suffered as a result of the April 26 accident at Chernobyl. Most European countries, including neutral states such as Finland, were dismayed by the length of time it took the Soviet Union to disclose details.

As Moscow's representative on the IAEA's board of governors, Semyonov, 56, symbolizes the new phase of the Kremlin's nuclear diplomacy: the attempt to reassure a worried world that the Soviet Union is a responsible nuclear power. But his defensiveness on several issues, including the initial news blackout, is also a reminder of the limits to international cooperation on controlling atomic energy.

As the deputy chairman of the Soviet atomic energy commission, Semyonov was the author of an IAEA paper several years ago praising the safety features of the Chernobyl-type graphite-cooled reactor in contast with the water-cooled reactors common in the West. Reminded about this paper at a press conference today, he smiled wrily but made no comment.

Semyonov did, however, disclose some new details about the clean-up operation at Chernobyl, including construction of what he described as "a concrete fridge" 100 feet below the floor of the contaminated reactor. The new foundation, reached by a newly dug tunnel, is designed to prevent any radioactive leak downward into the water supply while also acting to cool the base of the overheated nuclear core.

The Soviet official also disclosed that the temperature in the reactor core had fallen to about 200 degrees centigrade from 450 degrees after the accident. He said that radiation levels 40 miles from Chernobyl were down to 170 microrems an hour, or roughly 20 times normal background radiation.

Today's two-hour Soviet press conference followed an IAEA meeting in which the Soviet Union joined western countries in promising to set up a compulsory "early notification system" in the event of serious nuclear accidents in the future. The Kremlin's support for such an arrangement was interpreted by some western officials here as an implicit acknowledgement that it had been deficient in its initial handling of the accident.

Asked if there had been a breakdown of communications within the Soviet Union in the first few days after the accident, Semyonov said only that there had been "an incorrect appreciation of what happened on the part of the local authorities."

Speaking at a separate press briefing, the director general of the IAEA, Hans Blix, said that a compulsory early notification system would have avoided some of the initial confusion over the nature of the Chernobyl accident. The IAEA was not told of the accident until almost three days after the explosion. Radioactivity had already been detected above Scandinavia.

Commenting that prompt notification by Moscow would have been "most desirable," Blix added: "I take it that if the Soviet Union enters into a multinational agreement, they will also have to set up the administrative procedures to live up to that agreement."

The cautious remarks reflected the sensitive position of the IAEA. As a United Nations agency, it cannot afford to upset any of its member states, particularly one as powerful as the Soviet Union. Blix, who visited the scene of the disaster by helicopter earlier this month, said there was no question of the IAEA being transformed into "a supranational body responsible for nuclear security all over the world."

The U.S. State Department's representative on the IAEA board, Richard T. Kennedy, said after the meeting that "everybody, including probably the Soviets, regretted the late notification" of the accident. But he praised the contribution made by the Soviet delegation to this week's agreement.