With North Korea pledging to close down its nuclear test site as a part of its own denuclearization efforts, the focus is on how the shutdown process will unfold.

During his summit with South Korea’s President Moon Jae-in last month, The North Korean leader Kim Jong-un said he would invite experts and journalists from South Korea and the US to watch the dismantling of the Punggye-ri facility in North Hamgyong Province.

But the condition of the site makes the shutdown a complicated task, analysts said.

One of the methods being considered is using explosives to destroy the underground tunnels at Punggye-ri site, but with the mountain above thought to be badly weakened, there are fears of a massive leak of radioactive material. 

“Blowing up the entre nuclear facility at Mount Mantap is a really dangerous idea,” said Kim Dong-yub, a professor at Kyungnam University’s Far East Institute. “It could result in a massive leak of radioactive material.”

North Korea has conducted all of its six nuclear tests at the Punggye-ri site, shutting down the site’s East Portal after the first nuclear test in 2006. And analysts have suggested that the North Portal, at which all of the other five tests took place, might no longer be fit for tests.

According to South China Morning Post, a group of Chinese geologists concluded the powerful sixth nuclear test had turned the mountain “into fragile fragments,” which could allow radioactive dust to escape through cracks or holes.

“It is necessary to continue monitoring possible leaks of radioactive materials caused by the collapse incident,” Wen Lianxing, a geologist with the University of Science and Technology of China, who led the Chinese research team, told the Post.

During the inter-Korean summit last month, North Korean leader Kim Jong-un was reported to have said there were two tunnels at Punggyeri site in good condition, playing down international speculation that could no longer be used.

US-based North Korean analysis website 38 North asserted that the two portals remain “viable” for nuclear tests, corroborating Kim’s assertion.

“Two mountainous areas accessible by the South and West Portals remain viable, and could support future underground nuclear testing if there were to be a political decision to do so,” 38 North said in its analysis.

Most recently, there has been a series of reports suggesting that North Korea has been taking initial steps to close down the Punggye-ri site. US broadcaster CBS reported that the North Koreans have started pulling cables from the tunnels at the test site.

Although North Korea has yet to confirm the reports and there is no specific information about where the removal work took place, intelligence sources here suspect it to be the South Portal.

“There is an indication that we haven’t noticed before at the South Portal,” said an anonymous intelligence source, according to Yonhap News Agency. “We believe it is likely to be a part of the North’s effort to shut down the nuclear site.”

The southern portal is expected to be easier to shut down than the nuclear-damaged North Portal.

But with Mount Mantap weakened, and surrounding rock increasingly fractured due to the series of nuclear tests, the danger of using explosives would still be risky.

Given the risks involved in detonating explosives to shut down the nuclear site, some analysts have proposed a “burial,” using a mixture of lime and sand to seal every tunnel well below the ground.

But North Korea and international inspectors might not agree on this option, because the process would require longer period of work and the presence of numerous experts.

Another challenge is getting rid of residual nuclear material -- such as plutonium and enriched uranium -- which might well remain at the nuclear site even after the facility is buried.

“Plutonium and enriched uranium can be excavated easily. With reprocessing works, they can be used for nuclear warheads. … They need to be transformed completely through chemical processes,” Seo Gyun-ryul, a nuclear engineer and analyst at Seoul National University, said in a media interview. 

By Yeo Jun-suk (jasonyeo@heraldcorp.com)