"Japan is about to join several nuclear-weapon states as a producer of separated plutonium on an industrial scale," the experts said in a joint letter posted on the Web site of the Union of Concerned Scientists, a U.S. nonprofit organization focusing on environmental and nuclear safety issues.
The letter said the plant could produce up to 8 metric tons of plutonium a year - enough for 1,000 weapons - for its plutonium-based reactors. It said Japan had no need to increase its already large stockpile of plutonium.
"At a time when the nonproliferation regime is facing its greatest challenge, Japan should not proceed with its current plans for the start-up of the Rokkasho reprocessing plant," the letter said, adding that minimizing global stocks of bomb-grade uranium and plutonium should be a top priority.
Among those who signed the letter are Nobel Prize-winning physicists Sheldon Lee Glashow and Leon Lederman, former Secretary of Defense William Perry and nonproliferation expert and former U.S. Pentagon official Henry Sokolski.
The nuclear weapon states who signed the 1970 nuclear Nonproliferation Treaty - the United States, Russia, the United Kingdom, France, and China - have all halted their production of plutonium for warheads, and their production of high-enriched uranium, the letter said.
However, France, the United Kingdom, Russia, and India continue to extract plutonium on a large scale from spent civilian reactor fuel.
"There continues to be a steady increase in the world stockpile of... plutonium, which stood at 235 metric tons at the end of 2003. This... is enough to make 30,000 nuclear weapons, each with a destructive power comparable to that of the Hiroshima and Nagasaki bombs," the letter said.
"Japan has shown great wisdom in not joining the 'club' of nuclear-weapon states. We urge it to show equal leadership in deciding not to add to the accumulation of excess stocks of separated civilian plutonium," the letter said.
Although Japan has never actively pursued atomic weapons, some nuclear experts say it is fully capable of developing them if it ever saw the need - especially if Communist North Korea threatened to use its nuclear capability in the region.
Some experts have also said that Iran, which Washington accuses of pursuing nuclear weapons, is using Japan as a model to achieve a nuclear weapons capability. Tehran denies this and says it is only interested in civilian nuclear power.