There are other challenges to restarting the reactor.
North Korean scientists need to clean, check for any leaks, test components and replace ones that no longer work, according to No Hee-cheon, a nuclear expert at the Korea Advanced Institute of Science and Technology in Daejeon, South Korea.
"Nuclear material can be very corrosive. Cleaning the chemical equipment for reprocessing plutonium can be an overwhelming task," No said.
North Korea isn't thought to have nuclear-armed missiles that can hit the United States and is extremely unlikely to launch a direct attack on Seoul or its U.S. ally, knowing that military retaliation would threaten the leadership's survival.
Experts estimate it has enough plutonium for between four to eight crude plutonium-based weapons. But North Korea has yet to show that it has mastered the technology needed to shrink down warheads so they can be placed on missiles, although Pyongyang has bragged — as recently as Thursday — that it has "smaller, lighter" nuclear weapons ready to strike the U.S.
To back up that boast, however, Pyongyang needs more tests, which would deplete its limited supply of nuclear fuel. This motivation may partially explain the vow to restart Nyongbyon.
Two other larger plutonium reactors had construction halted because of a past nuclear disarmament deal; Hecker said the North Koreans claim both are unsalvageable. North Korea is also thought to be making progress on building a small experimental light-water reactor.
The North also suggested this week that it was boosting uranium enrichment efforts.
North Korea's uranium program worries Washington because the centrifuges that enrich the fuel into bomb-grade material are much easier to conceal than bulky plutonium reactors, which produce large amounts of heat that can easily be seen by satellites. A crude uranium bomb is also easier to produce than one made with plutonium, and North Korea has large natural uranium deposits.