sends this quote from the Bulletin of the Atomic Scientists:
Most people would agree that keeping track of dangerous material is generally a good idea. So it may come as a surprise to some that the arrangements that are supposed to account for weapon-grade fissile materials—plutonium and highly enriched uranium—are sketchy at best. The most recent example involves several hundreds kilograms of plutonium that appear to have fallen through the cracks in various reporting arrangements. ... [A Japanese researcher discovered] that the public record of Japan’s plutonium holdings failed to account for about 640 kilograms of the material. The error made its way to the annual plutonium management report that Japan voluntarily submits to the International Atomic Energy Agency ... This episode may have been a simple clerical error, but it was yet another reminder of the troubling fact that we know very little about the amounts of fissile material that are circulating around the globe. The only reason the discrepancy was discovered in this case was the fact that Japan has been unusually transparent about its plutonium stocks. ... No other country does this.