April 30, 1986

United States intelligence sources said today that the nuclear disaster in the Soviet Union started as long as four or five days ago and was continuing to spread radioactive material into the atmosphere. Most experts agreed that the graphite core of the Chernobyl reactor, at Pripyat in the Ukraine, had caught fire and was burning fiercely. Details of the accident remained scarce today, the day after the Russians announced that an accident had taken place at the reactor. Without such details, experts found it difficult to speculate about the short- and long-term dangers the disaster posed to health and the environment. Soviet Technology Faulted But they faulted Soviet technology, which uses graphite, a form of carbon, to moderate nuclear reactions. In the United States, water is used as a moderator.

New and unpublished test results show that a failure of a safety seal on the space shuttle Challenger was virtually inevitable because of a combination of cold weather on the morning of the launching and serious design flaws. The test results, conducted for the Presidential panel studying the accident and summarized for The New York Times, also determined that the joint would sometimes begin to fail at temperatures as high as 50 degrees Fahrenheit. In the past, officials of the National Aeronautics and Space Administration have testified that they felt confident the shuttle could be launched at far lower temperatures without undue risk to the crew. Failure Was Probable The Challenger was launched Jan. 28 in 36-degree weather, but investigators estimate that the temperature of the joint that contained the failed seal was about 28 degrees, a temperature at which failure is more likely than not, the tests show. The analysis of the accident, in which the seven crew members died, is expected to serve as the centerpiece of the Presidential commission's report, due in early June. On Monday panel members received a summary of the results, based on tests conducted in recent weeks primarily by NASA engineers and outside aerospace experts working for the commission's working group analyzing data and design.

What Happened In an unusual public admission, the Soviet state radio said a ''disaster'' had occurred at a nuclear power plant in the Ukraine. West German and Swedish officials said Soviet officials had asked for help in controlling a burning nuclear reactor. The nuclear accident, described by Swedish experts as potentially the worst ever at a power plant, sent a radioactive cloud across parts of the Soviet Union, Eastern Europe and Scandinavia. Moscow provided few details, but intelligence sources believe that the accident occurred last Thursday or Friday and that radiation was continuing to spew yesterday. Western experts say they believe graphite used to moderate the nuclear reaction in the plant caught fire. They disagree on whether the fire was associated with a meltdown, in which nuclear fuel rods burn out of control.

The Soviet Government was reported today to have asked West Germany and Sweden for assistance in handling a fire in a nuclear reactor core. The reports, from officials in those countries, came amid indications that a reactor accident reported Monday in the Ukraine, 70 miles north of Kiev, was a major disaster, perhaps the worst in the history of nuclear power. [ The United States formally offered humanitarian and technical assistance to the Soviet Union to help it deal with the accident. Page 6. ] 'Radiation Situation' Stable The developments came as the Soviet Government issued its second official statement on the accident in the Chernobyl nuclear power station at Pripyat, saying that the ''radiation situation has now been stabilized.''

Japan today seems to have it all - economic might, political stability, an increasingly affluent populace and companies moving into new markets around the world. The question that preoccupies the Japanese, however, is: What could go wrong? This is a nation obsessed with its own future, endlessly analyzing and anticipating what twists lie ahead. Government agencies churn out reports projecting the future course of the economy, and suggesting what steps Japan should take to adapt in time. Polls abound charting what people think may happen in the next decade or two.