North Korea may have temporarily shut down its plutonium reactor due to water supply problems that could pose risks of radiation exposure in case of a natural disaster or other major incidents, a U.S. think tank said Monday, adding to controversy over the nuclear complex’s safety.

Recent satellite imagery indicated that the Yongbyon site in the country’s northwest appeared to have been struggling to ensure stable water supplies to its cooling systems in the aftermath of torrential rain and subsequent floods last summer, according to the U.S.-Korea Institute at Johns Hopkins University.

The 5-megawatt graphite-moderated reactor is believed to have been put back in operation last year, ending its closure under a 2008 disarmament agreement with the U.S.

Despite recent fixes such as water channel and dam construction, concerns linger over supply difficulties over the long term because the new channels and dams are built with sand and thus could be washed away by floods, the Washington-based think tank said.

“If the reactor’s secondary cooling system were to fail, so would the entire cooling system. The result would be a fire in the graphite core and the release of radioactivity,” analyst Nick Hansen wrote on its website, 38 North.

“While North Korea’s experience operating this system would increase its chances of quickly shutting it down before a fire broke out, the reactor’s lack of airtight containment could lead to the escape of some radioactivity even in small accidents. … The rapid loss of water used to cool the reactor could result in a serious safety problem.”

But the institute argued that any nuclear accident in Yongbyon would not bring about greater damage than the 1986 Chernobyl disaster given the reactor’s small size, dismissing recent comments by President Park Geun-hye.

In her speech late last month at a nuclear security summit at The Hague, she said that a fire at Yongbyon may result in a disaster far worse than the Ukrainian one, which killed 30 people in an explosion and additional 2,500 in related illnesses, citing a report by a Seoul National University nuclear scientist.

But the report has been criticized by many other scholars as “nonsense,” as the level of power generated by the Yongbyon reactor is less than one-hundredth of Chernobyl’s.

“However, a radioactive release in the atmosphere or river would cause an expanded local area of contamination,” Hansen noted.

“Also, Pyongyang’s likely lack of transparency could create a regional crisis, panicking the public in surrounding countries and raising tensions with governments anxious for further information.”

By Shin Hyon-hee (heeshin@heraldcorp.com)