Ed Kerr remembers vividly what he calls "the longest five seconds of his life." As a nuclear plant inspector for the International Atomic Energy Agency several years ago, he was checking "warm" plates containing plutonium when he suddenly felt one that "went cold."

The absence of plutonium in one of the plates suggested a hidden diversion of the dangerous nuclear material had taken place, presumably for the purpose of making bombs. But a quick check revealed that a dummy testing plate inadvertently had been mixed with the sample. A recalculation showed that all plutonium supplies under review were present.

Kerr, who now oversees development of IAEA's technical systems to track fissionable material, admits he still feels a shiver of fear when one of the agency's inspectors turns up what is euphemistically known as "an anomaly." But even though some 400 irregularities are reported every year, the agency always has found reasonable explanations and never has encountered a case confirming the misuse of nuclear fuel.

The IAEA inspectors rely on the cooperation of participating states and thus only can sound an alarm rather than prevent misuse of nuclear fuel. But some IAEA officials fear that the rapid growth of world plutonium supplies, which may expand even faster if more countries turn to breeder reactors for nuclear power, could overwhelm their ability to keep track of sensitive nuclear material in the future.

The onerous job of certifying that nations are not diverting nuclear materials for clandestine bombs is now being carried out by 270 IAEA inspectors from 63 countries.

During their annual on-site checks at nearly 900 nuclear facilities around the world, the inspectors employ a variety of scientific tools and surveillance cameras to observe power plants and track sources of plutonium and enriched uranium to ensure that the volatile material is not being channeled into building weapons.

Besides verifying that all radioactive material has been properly stored or accounted as waste, the inspectors also examine equipment and review countless spools of film looking for any unusual activity that might indicate illicit behavior.

The use of such "containment" devices as identifiable seals and cameras is vital to provide continuous supervision of uranium enrichment plants and spent-fuel storage. Two cameras monitor the nuclear fuel assemblies, taking pictures every 20 minutes.

The safeguards system has been praised widely and often cited as a possible precursor for on-site inspection of military weapons installations in the event of future arms control agreements.

Participating countries have agreed that the inconvenience of opening up their nuclear technology to outside inspectors is worth the assurance that other states are being subjected to the same rigorous controls.

Even so, the occurrence of frequent anomalies underscores the imperfect nature of any inspection regime. "Human error remains a problem no matter how good the methods and equipment," observed Kerr. "Sometimes our cameras run out of film, batteries are put in backward or somebody turns the lights out. But we're confident that in all the years we've been doing this, nobody has fooled us by diverting nuclear fuel."