CHERNOBYL, U.S.S.R. -- When Reactor No. 4 at the Chernobyl nuclear power station exploded at 1:23:58 a.m. on April 26, 1986, Yuri Alexeyev was fast asleep. But for him, and millions of other Soviets either living in the disaster area or drafted to help with the cleanup campaign, life changed with that moment.

"The first thing that happened to me was that I stopped going to weddings and started going to funerals," said the 40-year-old turbine engineer, who is convinced that radioactive fallout has caused the mysterious and frequently sudden deaths of many of his fellow Chernobyl workers. "People I worked with, people my age who outwardly seem perfectly healthy, are dying, and nobody is able to explain why."

It is only now, five years after the explosion, that Soviet leaders and ordinary people alike are beginning to grasp the full scale of the world's worst nuclear disaster. The fallout from Chernobyl -- the equivalent of 10 Hiroshima bombs -- has left a swath of agricultural land the size of Holland permanently poisoned. About 200,000 people have already been evacuated from their homes. Nearly 5 million people live in contaminated areas.

But the impact of Chernobyl goes far beyond the environmental and public health crises that are now unfolding in the western Soviet republics of the Ukraine and Byelorussia. The explosion of the supposedly fail-safe nuclear reactor is increasingly seen as the culminating moment in the collapse of a political and economic system that was both cruel and hopelessly inefficient.

"Chernobyl demonstrated the insanity of the command-administrative system," said Grigory Medvedev, a nuclear-safety expert who waged a four-year campaign to publish an exhaustive minute-by-minute account of the disaster. "It took place in an atmosphere of total secrecy. Information about previous nuclear accidents was hushed up, so the operators did not know what risks they were running. Our atomic bureaucrats convinced everybody that a major nuclear disaster in the Soviet Union was impossible."

If Chernobyl was a symbol of a system that failed, the way in which the Kremlin reacted to the disaster has reflected the successes and shortcomings of Mikhail Gorbachev's attempt to breathe new life into the 70-year-old socialist experiment. The stifling Communist Party monopoly over information has burst wide open. But it is now clear that numerous mistakes were committed during the course of the cleanup campaign, compounding the original tragedy.

Like Gorbachev's perestroika reform program, the Soviet response to Chernobyl has been contradictory, erratic and ultimately inadequate. Five years after the tragedy, there is still no comprehensive list of all the potential victims. People living in seriously contaminated areas complain of a lack of hospitals, doctors and basic medical supplies. Despite numerous improvements in safety procedures at nuclear power plants, 17 flawed Chernobyl-type reactors remain in service across the Soviet Union.

The Cost of Secrecy

Known locally as "the Zone," the area around Chernobyl is a nuclear wasteland cordoned off from the rest of the world by barbed-wire fences and signs reading "Danger -- Radioactivity -- Keep Out." Abandoned villages, neglected farms and roads overgrown with weeds make the Zone a place of sinister, almost oppressive stillness. It is so quiet that the squawking of a wild duck or wind whistling in the trees can be heard for miles around.

Entering the Zone is like going back in a time machine to a now-vanished world. Faded propaganda signs urge workers to "fulfill the decisions of the 27th Communist Party Congress," which was held in February 1986. Schools, hospitals and apartment blocks are boarded up. With the exception of the 20-story-high white sarcophagus that encases the devastated reactor, virtually everything has been left as it was on the morning of Saturday, April 26, 1986.

When the disaster struck, Gorbachev had been in office just over a year. Glasnost, or openness, was little more than a slogan. The immediate instinct of most Soviet officials was to pretend that nothing serious had happened.

"Panic is even worse than radiation," announced Boris Shcherbina, the head of the first government commission on Chernobyl, rejecting appeals by civil defense workers for an immediate evacuation of the entire area. The town of Pripyat, which is right next to the reactor, was not evacuated until 36 hours after the explosion. Officials waited until May 5 before evacuating the rest of the 18-mile exclusion zone around Chernobyl.

According to the account pieced together by Medvedev, the managers of the power plant refused to believe that the reactor had blown up even when the evidence was staring them in the face. Told that a geiger counter was showing readings of at least 250 roentgens an hour -- millions of times above normal levels -- plant director Viktor Bryukhanov described the instrument as defective and said it should be tossed in the garbage.

On the morning after the explosion, phone lines were cut to prevent information about the disaster from reaching the outside world. Unaware of the danger of radiation, the people of Pripyat went about their daily business. Children walked to school. Housewives went shopping at the market. Town officials later boasted that 16 weddings were held that day, showing how "normal" everything was.

"If we had known what had happened, of course we would have remained indoors and taken precautions. God knows how much radiation we might have been spared," said Nadezhda Spachenko, a plant engineer whose children now suffer from chronic headaches, nosebleeds and general fatigue.

"We lost three days," said Anton Romanovski, director of health in Byelorussia's southern Gomel region, which has been declared a disaster area. Romanovski said he was not officially informed about the disaster until the following Monday, by which time a cloud of radioactivity had already been detected over southern Sweden.

In an ironic twist of fate, many of the senior plant managers and Communist Party officials who tried to hush up the disaster became victims of the criminally inefficient system. One government minister was surprised to discover that his exposure to radiation had increased dangerously as a result of a stay in a Moscow clinic. It turned out that he had occupied the same bed as one of the firemen fighting the blaze at Reactor No. 4 -- and nobody had bothered to change the sheets.

"Chernobyl was not only the end of the stagnation period, the inevitable result of irresponsibility and incompetence, it was also to a certain extent retribution to the old-style leaders," said Medvedev, a former chief engineer at the plant. "When Shcherbina and the other ministers arrived in Chernobyl, they turned out to be absolutely incompetent. They gave orders in the radiation zone and, of course, got irradiated themselves. I don't think they understood what was really happening."

Shcherbina, who personally ordered the construction of a new town for Chernobyl workers in a highly contaminated area, died last August at age 70 after what the Soviet press described as "a serious illness." It is impossible to tell to what extent his death was caused by radiation. Under a secret 1988 decree drafted by Shcherbina, which remained in force until very recently, Soviet doctors were forbidden from citing radiation as a cause of death or illness.

A Mounting Health Crisis

In the Byelorussian village of Dronki, 12 miles north of Chernobyl, there are many signs of a hurried departure by the 700 residents. In one single-story wooden house, clothes lie strewn on the bed. The floorboards have been ripped up, apparently by looters looking for hidden valuables. In the neighboring hut, a copy of the local paper is dated May 1, 1986, with the headline "Happy Holiday, Comrades." It makes no mention of the Chernobyl disaster.

"They lined up buses outside the village and gave us six hours to leave our homes," recalled Nadezhda Tishkhenko, a 65-year-old collective-farm worker. "They said we would be going just for a few days, but we are still not allowed back."

The Tishkhenkos and many of their neighbors were taken 20 miles down the road to the supposedly uncontaminated village of Dubrovitsa. They moved into new concrete houses hurriedly built by the army and began cultivating their plots. Five years later, health workers have declared Dubrovitsa to be in a polluted zone. A new evacuation is now underway.

"Like so much to do with perestroika, the original evacuation was not thought out properly. These people should never have been moved here in the first place," said Romanovski, the regional health chief.

The nearby town of Khoiniki has officially been declared safe, but its residents are not encouraged by the fact that 18 doctors and 74 nurses, including the chief doctor at the local hospital, left town last year. It has been practically impossible to find replacements, since no one wants to move into a high-radiation zone.

According to Khoiniki's new chief doctor, Viktor Kobilko, the past few months have seen a sharp rise in tumors and birth defects. "A baby was born with six fingers the other day. Another was born without an ear," he said.

Particularly alarming, say local doctors, is the rising cancer rate among children. Last year, there were 19 officially reported cases of cancer of the thyroid gland among children in Byelorussia and 21 in the Ukraine. Before the Chernobyl disaster, such cases were practically unheard of; those being reported now are almost certainly due to radiation exposure, in the opinion of most medical experts.

In the absence of any systematic medical study of the affected population, it is impossible to say how many people have already died as a result of Chernobyl -- and how many others are suffering from radiation-related diseases. The official death toll remains 31, which was the number of Chernobyl workers and firefighters killed in the immediate aftermath of the explosion, but unofficial estimates run between 5,000 and 10,000.

"The most frequent cause of death is sudden heart attack, which is itself caused by fear and great psychological stress," said Alexeyev, who heads the Ukrainian branch of the Chernobyl Union, which was set up to protect the interests of the 600,000 "liquidators," or cleanup workers. "I still think about going to work at the plant in an armored car and watching the needle on the dosimeter go up to 200 roentgens an hour. The leaves in the forest were yellow from radiation."

Soviet health officials dispute the claims of thousands of deaths, arguing that the death rate among the liquidators has not been significantly higher than among the general population. The Chernobyl Union maintains that the victims are predominantly young men, aged 30 to 40, who should otherwise be perfectly healthy.

Power plant workers who received high doses of radiation immediately after the explosion appear to have little difficulty getting qualified medical attention. But the Soviet medical system seems unable to cope with the army of soldiers, Chernobyl residents and liquidators who received smaller doses and now complain of various aches and pains. For these unsung veterans of the cleanup campaign, it can take 18 months to get an appointment at a radiation center.

"Immediately after the disaster, the people who took part in the cleanup job were described as heroes. Now they are being treated as shirkers," said Medvedev. "The Chernobyl catastrophe showed that our health system was absolutely unable to take care of these people, or even show elementary charity toward them. The system has reacted to the liquidators' pleas by saying 'go to hell.' "

'A Nation of Mutants'

The longtime consequences of the explosion at Chernobyl's Reactor No. 4, which was caused by a combination of operator error and faulty design, stagger the mind. They range from great issues of state -- such as the future of the Communist system and the safety of nuclear power -- to intensely personal moral dilemmas.

"Those of us who received large radiation doses at Chernobyl have been deprived of the moral right to bear children," said Vladimir Lisitsyn, who helped with the cleanup operation as a radiation specialist. "Even our children, who were evacuated immediately after the disaster, will have to think very carefully about the risks of having children."

In an interview with a Soviet magazine last year, the disgraced director of the Chernobyl plant recalled that the first concern of the government commission seemed to be to get the devastated reactor back into operation as rapidly as possible. Five years later, Soviet officials are having a hard job persuading a skeptical public of the need to keep any nuclear power stations open.

In the five years since Chernobyl, not a single new nuclear reactor has been commissioned in the Soviet Union. Anti-nuclear activists have succeeded in halting construction work on 39 nuclear reactors, including several that were practically ready to go on line. Earlier this year, the Ukrainian legislature voted to close the remaining three reactors at Chernobyl by 1995 and switch to alternative sources of energy.

In demanding the complete closing of Chernobyl, the anti-nuclear campaigners say there is a now a major threat to the water supply of Kiev, the Ukrainian capital. The contaminated cooling pond for the Chernobyl nuclear reactors lies right next to the Pripyat River, which feeds into the major water reservoir for the city.

"Radioactive dirt is already contaminating our water supply. The whole south of the Ukraine is threatened. A colossal genetic crisis awaits newborn children. We could become a nation of mutants," said Vladimir Yavorivskii, a leading Ukrainian writer and opposition legislator.

Chernobyl managers insist that such fears are exaggerated, while conceding that there could be a danger of radioactivity seeping into the water supply if measures are not taken. The solution, they say, is to build a permanent sarcophagus for Reactor No. 4 and leakproof walls between the cooling pond and the Pripyat River.

"It's simply impossible to close the plant immediately," said Chernobyl's new director, Mikhail Umanets. "We supply the Ukraine with 8 percent of its electricity. You can't just make that up overnight. It would also lead to mass unemployment."

The Soviet Union's deputy power minister, Yevgeny Petryayev, described the mass closing of nuclear power stations as "a {land} mine under the foundation of our economy." With oil production in sharp decline and the future of nuclear power in doubt, the country now faces an energy crunch that is likely to add a serious complication to Soviet economic reform efforts.

But the most lasting impact of Chernobyl may be in the effect it had on the minds of the survivors. Until recently, the Ukraine and Byelorussia had the reputation of being the most conservative republics in the Soviet Union. But the disaster, and the subsequent cover-up, radicalized public opinion and helped create a sense of aggrieved nationalism.

"Chernobyl woke us up," said Yavorivskii, the Ukrainian legislator and writer. "It made us understand that we were not masters of our own land. The union and republic leadership treated the Ukraine like a colony that could be exploited as mercilessly as possible. What happened here was an indictment of the imperialist system of government."