A tally of plutonium extracted at a nuclear fuel reprocessing plant in Tokai, Ibaraki Prefecture, since it began operating has come up 206 kg short, the government said Tuesday.
The Education, Culture, Sports, Science and Technology Ministry reported the finding to the government’s Atomic Energy Commission on Tuesday, and has also reported the case to the International Atomic Energy Agency.
It denied the possibility, however, that any plutonium had been illicitly removed from the facility.
Since operations started in 1977, the Tokai reprocessing facility, run by the governmental Japan Nuclear Cycle Development Institute, has extracted 6,890 kg of plutonium from spent nuclear fuel — falling 206 kg short of initial projections.
It takes between 5 kg and 8 kg of plutonium to produce an atomic bomb.
Ministry officials said part of the discrepancy may have been caused by a dilution of plutonium into waste water and other factors, including errors in computation.
Of the plutonium in question, government scientists believe 72 kg was mixed with highly radioactive waste water during the reprocessing stage and is now stored inside the reprocessing plant. Some 29 kg consisted of short-life isotopes, which were damaged in storage, apparently causing them to turn into a different type of element, they said.
The remaining 105 kg can be explained away as errors in estimating how much plutonium is produced when uranium is burned in the reactor, the government officials claimed.
The officials said more plutonium than originally projected had apparently stuck to fuel tubes, which were treated separately from nuclear waste water.
Some plutonium was solidified in glass after being mixed with highly radioactive waste water and was later disposed of, they said.
The officials said it is difficult to make an accurate calculation of the exact amount of plutonium to be extracted, because comparable data from reprocessing plants in the United States and Europe are not available.
The officials, concerned that similar discrepancies could occur at the Rokkasho reprocessing plant under construction in Aomori Prefecture, said they are having discussions with the IAEA to improve the measuring equipment and evaluating methods of plutonium.
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