The institute has not ruled out that the shutdown may be related to refueling or repairs.

As late as last month, satellite photos analyzed by the International Atomic Energy Agency (IAEA) had shown that the reactor was probably operational. IAEA inspectors have had to rely on satellite imagery and other secondary information after losing access to the country in 2009.

The 5-megawatt facility had been powered down between 2007 and April 2013 under an aid-for disarmament accord, but went back online as tensions increased again last year.

Yongbyon is North Korea’s only major nuclear power generating facility. Its first, small scale experimental reactor went online in 1986, and was followed by several more by the early 1990s. It was shut down in 1994 in accordance with the US-North Korea Agreed Framework. The Agreed Framework had pledged to create light water reactors to replace the country’s graphite-based, weapons-grade plutonium-generating reactors at Yongbyon. However, the framework was abandoned in 2003 as distrust between the United States and North Korea grew.

In 2006, 2009 and 2013, the country conducted nuclear tests with plutonium generated at the facility, which is estimated to create 6 kilograms of the material per year. This is enough to allow the country to produce about one nuclear weapon annually. North Korea’s third and most recent nuclear test in 2013 produced an explosion which was estimated to have had a yield of 6-7 kilotons, according to the Russian Defense Ministry. The atomic bomb detonated by the United States over Hiroshima in 1945 yielded 16 kilotons. The biggest nuclear weapon ever detonated was the Soviet Union’s Tsar Bomb, which yielded 50-58 megatons when it was tested over Severny Island in 1961, 7,000-10,000 times the yield of the North Korean bomb.

UN Security Council resolutions have repeatedly demanded that North Korea stop its nuclear and ballistic missile testing activities, but it’s considered unlikely that the country will give up its nuclear ambitions, given the nuclear program’s status as its “treasured sword” against a potential foreign intervention.

Earlier this week, So Se Pyong, North Korea’s Ambassador to the United Nations in Geneva noted that Pyongyang was set to resume the six-party talks with the participation of North Korea, South Korea, Russia, the United States, China and Japan, the Guardian reported.

Russian Foreign Minister Sergei Lavrov, speaking with his North Korean counterpart RI Su Yong in Moscow on Wednesday, confirmed that the resumption of the talks was possible, despite difficulties.