Saying it has restarted a nuclear facility shut down since 2007, North Korea announced Tuesday that it has also upgraded its nuclear weapons program. The country has been working to restart the Yongbyon reactor since 2013.

The news about the reactor that produces weapons-grade plutonium was announced by the Korean Central News Agency, which says the Yongbyon 5MWe reactor and other facilities have "started normal operation."

Citing the director of North Korea's Atomic Energy Institute, the agency also issued a threat, saying that if the U.S. and other nations "behave mischievously," North Korea "is fully ready to cope with them with nuclear weapons any time."

The news agency also said that the country's nuclear scientists have made some innovations in their work at the reactor, which it says has been rearranged and adjusted.

The Yongbyon reactor is North Korea's main source for building up its plutonium stockpile, according to Karl Dewey, the proliferation editor at IHS Jane's Intelligence Review.

Despite the restart, Dewey says via email, a number of problems could affect the reactor. For one thing, its cooling tower was dismantled in 2008.

"The reactor itself is cooled with carbon dioxide gas that blows over the hot fuel rods," Dewey writes. "This gas also needs to be cooled, and with the destruction of the cooling tower, North Korean engineers have had to find an alternative way of completing this secondary cooling loop."

The reactor had been shut down as part of the negotiations between North Korea, the U.S. and other nations over its nuclear weapons program.

"But in recent years, under the rule of Kim Jong Un, Pyongyang has vowed never to give them up," Jason Strother reports for NPR's Newscast desk. "And now the North says it's ready to use these weapons to counter what it calls Washington's hostile policy."

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