North Korea is believed to have restarted a small research reactor at its Yongbyon nuclear complex in a possible attempt to make tritium, a key ingredient for hydrogen bombs, a U.S. research institute claimed Wednesday.

The Institute for Science and International Security cited two unidentified sources as saying that the Soviet-built IRT-2000 reactor is believed to be running on indigenously produced highly enriched uranium as fuel.

The reactor, which first went into operation in 1965, is different from the North's main five-megawatt reactor that has long been the focus of international attention and scrutiny as the source of weapons-grade plutonium for the communist nation.

In the past, the North used the IRT reactor to produce medical isotopes, but now the regime "may also use it to make tritium for its nuclear weapons program," the institute said in a report by nuclear experts David Albright and Serena Kelleher Vergantini.

Tritium is one of the ingredients for hydrogen bombs, a more powerful and sophisticated nuclear weapon than conventional atomic weapons. The North carried out its fourth nuclear test on Jan. 6, claiming that it tested a hydrogen bomb. Experts have cast doubt over the claim, pointing out that the yield was too low.

According to ISIS, the North had operated the IRT reactor using highly enriched uranium imported from Russia. Since the early 1990s, the North has been unable to import HEU and therefore operated the reactor intermittently using its existing HEU inventory.

That fuel was completely exhausted by 2011, the institute said.

"However, two independent sources have stated that North Korea may be operating this reactor using indigenously produced fuel," it said. "Enriched uranium could be produced indigenously either at the Yongbyon centrifuge plant or at another unknown location."

The institute said, however, that the North has reportedly experienced problems making the new fuel.

"We have received a report that two of the new fuel elements failed and melted sometime between 2012 and 2013, requiring a complicated cleanup," the institute said. "The report of fuel melting raises serious reactor safety concerns and should stimulate a conversation possibly initiated by South Korea about nuclear safety cooperation with North Korea." (Yonhap)