Soviet Union hit by nuclear disaster. The apparent meltdown at Chernobyl is a blow to the world's most ambitious nuclear power program. The accident caused widespread contamination of Soviet and foreign territories. Moscow is seeking help from the West as it aids victims.

The Soviet Union has been hit by what is believed to be the most serious accident in the history of nuclear power production. The Chernobyl nuclear power plant apparently suffered a core meltdown, leading to widespread contamination of not only Soviet territory, but neighboring countries as well. Two deaths have been confirmed, and thousands of people have been evacuated from the area surrounding the reactor, according to the Soviet news agency Tass.

Tass has provided only scant information about the accident. It said that part of the ``structural elements of the building housing the reactor'' had been destroyed or damaged. Western experts here say that indicates that the radioactive core of the reactor melted down, leading to an uncontrolled nuclear reaction.

Radioactive gases escaped from the reactor, and a radioactive cloud contaminated parts of the Soviet Union and sharply increased radiation levels in neighboring Finland and Sweden.

The accident occurred at the Chernobyl nuclear plant, north of Kiev, the capital of the Ukraine. The plant has four reactors, which are cooled by light water, nuclear experts say. They use graphite in the core as a moderator to help sustain the reaction.

Tass said medical aid was being administered at the site. Radio Moscow referred to ``victims'' of the ``disaster,'' but provided no further details.

Although Tass said the ``radiation situation'' at the plant had ``stabilized,'' Soviet officials asked West German and Swedish experts for technical advice on fighting fires in the plant. [An official of the West German atomic energy lobbying group said a Soviet diplomat had asked that antiradiation experts and medicine be made available, the Associated Press reported.]

The United States offered technical and humanitarian assistance.

Meanwhile, the Kremlin is refusing to provide details of what is actually taking place at the reactor site.

``We've heard nothing from the Soviets yet,'' a Western diplomat said.

The official silence is ``unsatisfactory,'' said another, since the accident has already affected nearby countries.

In Dublin, Stanley Clinton Davies, the European Community commissioner in charge of nuclear safety, said yesterday that the Soviets may have breached international law by not reporting the accident sooner.

It is believed that 50,000 people live within a 18-mile radius of the plant. The plant is situated on a reservoir from which the city of Kiev draws its water supply. There are unconfirmed reports that the water has been contaminated by radiation and is unsafe to drink. The population of Kiev is 2.4 million.

The Soviet Union, alone among major industrialized powers, is still ambitiously expanding its nuclear construction program. It is believed the country has some 40 to 50 operating nuclear reactors. Last year, it opened two of them to public inspection for the first time. But most, including the Chernobyl plant, are off limits to visitors.

Indeed, the Soviets have determinedly kept the lid on information about the accident, to the irritation and frustration of Western diplomats here.

The Soviet Foreign Ministry has been unable to provide details about the accident, and has turned down Western journalists' requests to travel to Kiev.

``I don't know whether it's possible right now, because it may be dangerous,'' spokesman Alexander Sazonov was reported to have said yesterday.

Tass ran a two-page account of nuclear accidents at US nuclear plants, citing criticism of the US nuclear program by environmental groups. No antinuclear groups are permitted in the Soviet Union, cutting off another source of information about the country's nuclear program.

Radiation levels in Finland and Sweden have soared, but most experts say there is little danger to residents of those countries. Other analysts, however, believe that widespread health consequences in other European nations may be disclosed once the extent of the disaster becomes known.

Before the source of the radiation was known, Swedish officials had considered shutting down one of their own nuclear plants. As suspicions grew that the radioactive cloud was being borne by the wind from the Soviet Union, the Swedish Embassy in Moscow questioned Soviet officials. The Soviets finally confirmed the disaster on Monday, after initial denials of any incidents. The disaster was believed to have taken place some time late last week.

Radio Moscow, in a broadcast yesterday, said nuclear power was ``a vital necessity for humanity'' and that ``drastic measures'' had been taken to ensure power reactors' ``reliability and safety.''

``Nevertheless,'' Radio Moscow said, ``this accident, and many others at nuclear power reactors in Western countries, show that the application of nuclear power, even for peaceful purposes, can be dangerous.'' The AP reports:

Safety precautions at Soviet nuclear power plants are so strict that ``the odds of a meltdown are one in 10,000 years,'' said Vitaly Sklyarov, the minister of power and electrification in the Ukraine, in the February issue of Soviet Life. The English-language magazine is published by the Soviet Embassy and circulated in the United States under a reciprocal agreement between the two countries.

A short feature, titled ``Born of the Atom,'' described life in the town of Pripyat, which grew up around the nine-year-old Chernobyl nuclear power plant, site of the accident. The article did not give details about the population of Pripyat, but noted that it is ``made up mostly of young people'' with an average age of 26.

``Robots and computers have taken over a lot of operations,'' said Pyotr Bondarenko, a shift superintendent at Chernobyl. ``In order to hold a job here, you have to know industrial safety rules to perfection and pass an exam in them every year.''

Soviet Life said Pripyat residents can see its nuclear power units from their apartment windows.

Chernobyl's reactors are housed in concrete silos and have ``environmental protection systems,'' the magazine said.

``Even if the incredible should happen,'' it said, ``the automatic control and safety systems would shut down the reactor in a matter of seconds. The plant has emergency core cooling systems and many other technological safety designs and systems.''