WASHINGTON (Yonhap) ― North Korea can significantly increase its nuclear stockpile if a light water reactor under construction at its Yongbyon nuclear complex goes into operation, a U.S. nuclear scientist warned, urging Washington to restart negotiations with Pyongyang.

North Korea has so far used a 5-megawatt reactor at Yongbyon to make plutonium for nuclear weapons, roughly at a speed of one bomb worth of plutonium a year. But since a few years ago, the North has been building a larger-scale light water nuclear reactor that experts say could give Pyongyang enough plutonium to make about five or six weapons a year.

“From the bigger light water reactor they’re making, what I’m arguing is that in principle, if that can operate effectively and at high power over every year, they could probably make 30-40 kilograms just from that reactor,” Charles Ferguson, president of the Federation of American Scientists, said in an interview with Yonhap News Agency.

About 6-7 kilograms of plutonium is necessary to make one nuclear bomb, according to experts.

Ferguson said the North could use its uranium enrichment facility to make low enriched uranium as fuel for the light water reactor, rather than directly producing weapons-grade highly enriched uranium, because it can make plutonium out of spent fuel from the light water reactor.

“It depends on how frequently they refuel the reactor. They can take spent, radiated fuel out of the reactor quickly within a couple of months, then the plutonium coming out is more weapons grade. That’s one option for them. They can use this kind of smaller light water reactor,” he said.

The North could double its plutonium stockpile within just one year of operating the reactor, he said.

“Within a few years, they could have, they could start getting to the level of a state like Pakistan or India in terms of their plutonium production,” he said. “That’s why it is so important not to neglect North Korea but to re-engage on the political problem or to see if we can head off this production of more and more plutonium.”

Six-nation negotiations to end the North’s nuclear program have been stalled since the last round of talks in late 2008. Since then, the North conducted two more nuclear tests, one in 2009 and the other in 2013, and restarted the 5-megawatt reactor that had been shut down in 2007.

South Korean officials have warned the North could carry out its fourth nuclear test at any time.

North Korea has called for the unconditional resumption of nuclear negotiations. But South Korea and the U.S. have demanded the North first demonstrate through action it stands by its own commitment to abandon its nuclear program before any negotiations reopen.

On civilian nuclear energy cooperation talks between the U.S. and South Korea, Ferguson said there are a few options the U.S. government can take with regard to Seoul‘s demand to use the so-called “pyroprocessing” technology, a reprocessing technology considered posing less proliferation risks because it leaves separated plutonium mixed with other elements.

Seoul wants Washington to allow it to use the technology because it can reduce the headache of disposing of nuclear waste in a nation with a small territory. But Washington has been reluctant to allow South Korea to do that due to proliferation concerns.

The 1974 nuclear cooperation pact, known as the 123 agreement, had been scheduled to expire in March. But the two countries extended it by two years to March 2016 as they failed to find a compromise. Negotiations to revise the pact have been under way, with the last round taking place in Washington last month.

In 2010, the two sides also launched a joint 10-year study to see if the pyroprocessing technology is feasible.

“If it works ― we don’t know if it works, we‘re testing ― if it works, it might allow the reduction of the volume of the waste to be stored and it can reduce the time required to store the waste,” Ferguson said of how effective the technology can be.

Ferguson said the U.S. could give Seoul temporary permission to use the technology pending on the results of the study or give permission to do certain types of activities based on what the sides have learned so far from the joint study.

“Another option could be we’ll give you advanced consent to do these activities,” Ferguson said. “You can do it for a period of time, 10 years, 20 years, 30 years. That‘s what the Korean negotiators want. That’s their preferred option ... and it‘s very similar to the agreement Japan, the U.S. agreed to in 1988.”