In a prank, a couple of teenagers recently climbed through the unlocked window of a Moscow research institute and frolicked through a laboratory holding enriched uranium, without setting off any alarms, according to a researcher who read a secret report on the incident.

In another facility with radioactive material, the alarms did not work at all -- because the electricity was turned off after the bill went unpaid.

In Lithuania in February, an armed gang of seven was arrested and 220 pounds of uranium seized. In Kazakhstan, police last December stopped a car in which they found about nine pounds of uranium.

In the Urals, Russian security services recovered four 198-pound containers of radioactive cesium and arrested four men on suspicion of stealing them from an industrial plant. A similar container was planted in a Moscow park by Chechen separatists in a high-profile warning to Russian authorities that they would not hesitate to terrorize the capital.

These are just a few examples of unguarded, stolen and missing nuclear materials that have become a frightening yet still largely hidden byproduct of the collapse of the Soviet Union. Today, Russia is a giant, unstable nuclear heap, with hundreds of facilities harboring at least 200 tons of plutonium and 800 to 1,200 tons of highly enriched uranium spread across thousands of miles.

Although there has yet to be a serious act of nuclear terrorism, and many cases have not involved enough fissile material to make a bomb, Western specialists say that Russia has only begun to secure these far-flung facilities against theft, accident and sabotage.

This week, the highest-level meeting ever to take place around the problems of nuclear material security and nuclear power safety is to convene in Moscow, with participants to include President Clinton, the heads of the other leading Western industrial democracies and Russian President Boris Yeltsin. Despite the obvious symbolism of the summit meeting being held a decade after the Chernobyl nuclear power plant disaster, however, many diplomats, nuclear specialists and critics say the event has more to do with reelection campaigns than with reining in runaway nuclear materials.

For Yeltsin, the summit tableau apparently is intended to be a showcase of his support by the leaders of the richest Western countries amid his uphill campaign for reelection in June against Communist Party leader Gennady Zyuganov. Yeltsin invited the leaders to Moscow last year in hopes of demonstrating before the election that the Group of Seven largest industrial democracies would become a Group of Eight with Russia at the table.

Thus, Yeltsin has little interest in allowing the Western leaders to poke into Russia's nuclear junkyards or highlight its atomic power plant vulnerabilities. And since most of the Western leaders, especially Clinton and German Chancellor Helmut Kohl, are committed to backing Yeltsin's candidacy for a second term, they do not want to embarrass him with intrusive or unpleasant questions.

As a result, according to Russian and U.S. officials who have participated in summit preparations, the meeting will produce communiques, but little in the way of concrete action on the dual agenda of atomic reactor safety and controlling the spread of nuclear materials.

"The summit leaders would be better served by having a prayer breakfast," said Thomas B. Cochran, director of the nuclear program of the Natural Resources Defense Council. "The only thing that stands between us and another nuclear accident is the grace of God."

Cochran faulted the Western leaders for being unwilling to shoulder additional costs of nuclear safety and security. "There is a lack of imagination in Western governments, and a lack of will," he said, adding that Russia's election season, rising nationalism and resistance from the Ministry of Atomic Energy, or Minatom, also have been serious obstacles to improving nuclear safety.

"In an election year in Russia, in which all the participants are attempting to help a fellow struggling for his life when the apparent alternative is a Communist, there will be less truth told than usual," said Graham T. Allison, director of the Center for Science and International Affairs at Harvard's John F. Kennedy School. The summit "is not going to be pointing to the problems and failings of the parties gathered and the guy they are trying to help."

Ten years after Reactor No. 4 at Chernobyl erupted in the world's worst nuclear accident, spewing radioactive fallout over Eastern Europe, the Western leaders are expected to reaffirm their December agreement with Ukraine on providing loans and grants to help mothball Chernobyl. But when Ukrainian President Leonid Kuchma arrives in Moscow, he will not be offered further aid. "This is not a time to sort of ante up to the bar and put more money down," a U.S. official said. "This will not be a pledging session."

Minatom, which runs a massive nuclear industry with a million workers, has resisted the shutdown agreement, and the Western leaders are not expected to press for the closure of 15 operating Chernobyl-type reactors in Russia.

On the separate issue of nuclear materials leaking out of the former Soviet Union, Russian and Western experts agree that the problem is serious but often elusive. The CIA told the Senate Armed Services Committee three weeks ago that "the chilling reality" is that "nuclear materials and technologies are more accessible now than at any other time in history -- due primarily to the dissolution of the former Soviet Union and the region's worsening economic conditions."

Most experts fear leakage of nuclear material by theft or diversion at the hands of individuals, rather than rogue states grabbing a loose warhead. Especially vulnerable, they say, are the more obscure facilities in the Soviet nuclear archipelago, a huge complex of laboratories, institutes and weapons plants. The success of strategic arms control talks in requiring Russia to dismantle nuclear warheads has a flip side: Every year, 3,000 warheads are dismantled, producing the equivalent of 15 tons of plutonium and 45 tons of highly enriched uranium.

In the Soviet era, the facilities were subject to tight discipline, but today's economic turmoil has thrown that into doubt. "The situation with Russian nuclear security is poor, and Yeltsin knows that better than Clinton," said Vladimir Orlov, director of the Center for Policy Studies in Russia, who tracks nuclear security issues. He recalled that the canister of cesium that the Chechen rebels left in a Moscow park was more a symbolic threat than a real one, but "money can work, and if someone is too insistent and too creative, it will work."

The summit leaders are expected to call for closer international coordination against nuclear smuggling. Orlov recalled that for several years Russian officials accused the West of provoking smugglers and denied that a problem even existed. But the meeting could at least mark a turn toward a more cooperative Russian attitude, Orlov said. "Yeltsin will give a signal that he recognizes this problem," he said, "and it does exist." CAPTION: LOCATION OF RUSSIA'S NUCLEAR FACILITIES Murmansk: naval fuel storage Archangelsk: naval fuel storage St. Petersburg: research/naval fuel Moscow: 2 research reactor facilities, 1 fuel fabrication Podolsk: research/space reactors Obninsk: research reactors Arzamas: weapons lab/assembly Novosibirsk: fuel fabrication Dimitrovgrad: breeder research, MOX fuel Zlatoust: weapons assembly Sverdlovsk: uranium enrichment, weapons assembly Chelyabinsk: plutonium/tritium production, weapons lab Tomsk: plutonium production, uranium enrichment Novosibirsk: fuel fabrication Krasnoyarsk: plutonium production, uranium enrichment Angarsk: uranium enrichment Petropavlovsk: naval fuel storage Vladivostok: naval fuel storage SOURCE: "Avoiding Nuclear Anarchy," by Graham T. Allison CAPTION: Some analysts say the upcoming meeting on nuclear safety in Moscow is being used by Russian President Yeltsin to showcase his global support as he runs for reelection.