Soviet authorities told a group of western diplomats today that no more radioactive emissions are crossing Soviet borders.

The briefing was the Soviet Union's first official response to requests for specific information submitted by western embassies here after the accident at the Chernobyl nuclear power plant.

Today's evening news, meanwhile, announced that Soviet leader Mikhail Gorbachev will speak to the nation on television Wednesday night. Although the subject of the speech was not announced, diplomats speculated that it will be about the accident at Chernobyl.

Gorbachev has not made any public comments about the accident since it occurred April 26. The announcement of his address was rare advance notice for a leader's speech here and was seen as a sign of its significance.

In the past week, the Soviets have moved to provide more information on the accident, apparently in reaction to the strong criticism that they withheld information during the first week.

Today's Communist Party newspaper Pravda said the situation at the plant had "become a little easier." In an interview, a deputy prime minister, Ivan Silayev, said that the reactor is "still not harmless" and that there are still radioactive materials on the territory of the station and the evacuated zone within an 18-mile radius of the plant.

Silayev indicated that "deactivization" of the zone will be a long process that could take months.

In the same article, Evgeny Velikhov, vice president of the Soviet Academy of Sciences and head of the cleanup operations in Chernobyl, described how specialists moved to prevent the core of the reactor from melting through the concrete floor and into the ground and underground waterways.

Press accounts here have said 5,000 tons of sand, boron and other materials were dumped on the site to control the fire. Velikhov today said the dumping put extra pressure on the reactor structure.

The biggest concern was for water that lay beneath the reactor, which experts feared could be contaminated by "a heated crystal" from the reactor.

"How would a heated crystal from the reactor behave itself and could we hold it back if it plunged into the earth," he said. "Never had anyone in the world found themselves in such a complex situation: we had to evaluate the situation very accurately, and not make a single mistake."

The course chosen by the specialists turned out to be the right one, Pravda concluded. "Water was pumped from underneath the reactor, holes drilled, a 'cooling zone' created to absorb the heat of the reactor. The process of burying it began," it said.

The Soviets told visiting international experts last week that they were going to "entomb" the reactor in concrete.

Silayev said crews would tunnel under the damaged reactor and pump in liquid nitrogen to form a cushion under it.

In an interview today with selected western news agencies, Ivan Yemilianov, deputy chief of the state atomic construction firm, said the plant would be sealed in concrete for "hundreds of years."

Tonight's Izvestia, the government newspaper, further described measures taken to contain the radioactive contamination in the region around the plant.

Silayev told Izvestia that about 300,000 square yards of contaminated surfaces are being covered each day with a neutralizing membrane or film, especially developed for the disaster.

"The film reliably covers the surface, hence radioactive dust and particles cannot . . . get into the soil and water," Silayev said.

He also told how a "method of water-glass flooding" is being used to coat roofs and structures. He said work to ring the region's sewage system had been completed, thus "completely eliminating the possibility of rainwater getting from the station into the Pripyat River," which empties into the Dnieper River, the chief water source for Kiev, 80 miles from Chernobyl.

Helicopters are flying over the site, dropping lead pellets "to isolate ultimately all openings through which the release of radioactivity into the atmosphere is possible," Tass reported.

Deputy Prime Minister Boris Shcherbina, head of a special government commission probing the incident, told the envoys from 15 western countries during a two-hour session that the original explosion had been caused by hydrogen produced by a mix of zirconium and water, diplomats said.

The data given today about the explosion "in essence confirms speculation that superheated water at the bottom of the reactor core mixed with zirconium, which holds the radioactive elements," said one western specialist. The explanation, however, provided no clue as to why the water overheated, he added.

A full account of the accident and its causes is expected to be ready by about July, the diplomats were told today.

Milk from the contaminated area will not be sold for three months, but if after that it is tested and found safe, it will be reprocessed and marketed as cheese and butter, the diplomats were told.

Soviet officials have reacted vehemently against bans adopted by the European Community yesterday on agricultural products from Eastern Europe and the Soviet Union.

A Foreign Ministry spokesman said yesterday that the only logical restrictions would be on products from the region around the stricken plant. In Moscow, goods prepared for export, including vodka, are now being checked for radiation, and shipped out with special certificates.

The Soviet Union exports little agricultural produce, but the ban has been publicly interpreted here as a political act, and part of the "anti-Soviet hysteria" that Soviets say is being created around the Chernobyl accident.

Soviet officials also told the foreign diplomats that tests have shown radiation levels in Moscow to be normal, while in Kiev, capital of the Ukraine, today's level was registered as a relatively harmless 0.22 milliroentgens an hour.

Today, a second group of foreign journalists was given a tour of Kiev. The group included representatives from Communist Party papers. A group of western reporters toured the area late last week.

Of the border monitoring points agreed on with the International Atomic Energy Agency, only two in Moldavia, southwest of Chernobyl, showed "above background" levels, the diplomats were told.

Tass tonight said 100,000 people from the area had been checked by doctors for radiation. There was no elaboration today on casualty figures of yesterday that said six people died of burns or radiation in the accident, and that 35 are in "grave" condition. It is unclear whether the six deaths included the two originally reported.