The discrepancy between the projected amount of plutonium extracted at a nuclear fuel reprocessing plant in Tokai, Ibaraki Prefecture, and the actual amount was 59 kg, not 206 kg as initially reported, the government said Tuesday.
The science ministry said the discrepancy was the result of a computational error and ruled out fears of material being proliferated.
"There is no fear that plutonium was removed and taken outside; there is also no fear that plutonium was lost," the ministry said in presenting its latest findings to the Atomic Energy Commission.
Government scientists said in January there should have been an additional 206 kg of plutonium extracted at the Tokai plant, which has processed 1,003 tons of spent nuclear fuel since 1977, extracting 6.9 tons of plutonium.
The ministry pointed to technical difficulties in estimating the amount of plutonium that is generated after uranium is consumed.
It said the government has sent the revised figures to the International Atomic Energy Agency and that the Vienna-based U.N. group has endorsed its findings.
According to the ministry, 106 kg of the 206 kg of "missing" plutonium was later found to have been mixed with high-level radioactive liquid waste and was either stored inside the Tokai facility or stored after it was solidified into glass.
The ministry said 29 kg of the missing plutonium was made up of short-life isotopes and degraded during storage.
The report says another 12 kg of the plutonium was probably stuck in fuel tubes that were disposed of separately from nuclear waste liquid, leaving the amount of plutonium still unaccounted for at 59 kg.