TRAPPED inside the equipment at a Japanese nuclear-processing facility are about 150 pounds of plutonium that engineers and inspectors cannot quite put their hands on.
Not that anyone would want to handle plutonium, the deadly stuff from which weapons can be made, but in today's world of stringent nuclear controls, it is embarrassing for a country not to be able to account for every bit.
The plutonium has accumulated inside handling chambers during processing in such a way that engineers can take measurements to know it is there, but cannot easily remove it. The result is a discrepancy between the amount of radioactive material that goes into the plant and what comes out.
The buildup inside chambers at the Plutonium Nuclear Processing Facility at Tokai, Japan, has been a matter for years of quiet consultation between Japanese officials and the International Atomic Energy Agency (IAEA), which monitors the world's nuclear industries for the UN.
But on May 9, all discretion evaporated when a Washington-based nuclear-watchdog group made public a letter it wrote to United States Secretary of State Warren Christopher calling for the plant to be shut down until the plutonium can be cleaned out from the equipment and its exact quantity ascertained. The US has a stake in the matter, because its spent nuclear fuel is processed at the Tokai plant.
Although measuring equipment indicates that the plutonium has not gone anywhere, the technology has a margin of error. That means that it is at least conceivable that some plutonium, perhaps even as much as the 18 pounds needed for a nuclear weapon, could be missing.
Japanese officials scrambled on May 10 to dispel this idea. ``This plutonium is not `missing' at all,'' Foreign Ministry press secretary Terusuke Terada tartly asserted. ``It is there.''
Even prominent nuclear critics, such as Jinzaburo Takagi of the Citizens' Nuclear Information Center, doubt that the radioactive material has left the plant. But there is the matter of appearances.
``In Japan's case it is probably not a case of diversion, but if the IAEA cannot determine or identify the exact place of the plutonium, then it is possible that other countries could think that Japan has diverted plutonium,'' Dr. Takagi said.
In the world of nuclear-energy production, `diverted' is shorthand for `diverted to a nuclear-weapons program,' which is what the IAEA, Japan, and the US suspect has happened in nearby North Korea. Even a hint of ``missing'' plutonium here places Japan on uncomfortable footing in efforts to press North Korea to account for all of its radioactive material.
Furthermore, Japan's long-time commitment to plutonium as a renewable-energy resource has been controversial and costly. The government can little afford more indications of mishaps and imprecision with so dangerous a substance.
Inside the Tokai facility are 17 ``glove boxes,'' large glass-paneled chambers that allow technicians, reaching in with gloves, to handle nuclear materials safely.
When the plant was designed and built in the mid-1980s, officials estimated that the amount of radioactive material that could be ``held-up,'' or stuck, in the glove-boxes would be approximately 3.3 pounds per chamber.
Officials now acknowledge that three of the glove boxes contain just under 95 pounds of held-up plutonium. Another 65 pounds are stuck inside the remaining boxes.
The processing and experimentation done at Tokai ``turned out to produce more `hold-up' than we expected,'' said Yukiya Amano, director of the Foreign Ministry's science and nuclear energy division.
The chambers are cleaned twice a year, another official said, but so far that pace has been insufficient to remove the accumulated material. When all the boxes are dismantled and cleaned, engineers will ``theoretically'' be able to account for all the plutonium that was put in, the officials say.
Engineers and IAEA inspectors noticed the buildup soon after the plant became operational in October 1988. The US and Japan even developed a special measuring device to assess the amount of plutonium accumulating in the glove boxes.
Despite efforts to correct the problem, it has been getting worse. Hiroshi Ikukawa, a Science and Technology Agency official, says a 1990 inspection showed about 44 pounds of held-up plutonium, a number that has since almost quadrupled.
Officials stridently reject any comparison to the North Korean situation. ``Japanese facilities are under safeguards [imposed by the IAEA], where North Korea's are not,'' Mr. Amano says. ``All facts are brought to the attention of the IAEA. We didn't hide anything.'' He says that Japanese officials will present by the end of May a plan to the IAEA on how to cut down the amount of plutonium caught in the glove boxes.
At the same, officials acknowledge that they did not voluntarily bring the matter into the open. ``We didn't think that matter was ... suitable for announcement,'' one of the officials said.