Another North Korean nuclear test could result in radioactive materials being released into the atmosphere and reaching Japan’s main northern island of Hokkaido, South Korean officials warned.

Pyongyang’s six nuclear tests at its Punggyeri site have weakened the mountain in the area to such an extent that radioactive substances could spew out from the next test.

In addition, there are indications that North Korean soldiers are being treated at a hospital for radiation exposure from the tests, according to a source knowledgeable about North Korean affairs.

“There are holes between 60 and 100 meters (in length) around the area of Mount Mantap of Punggyeri,” Nam Jae-cheol, the South Korean administrator of the Korea Meteorological Administration, said at the National Assembly on Oct. 30. “If additional nuclear tests are conducted, there is the possibility (of radioactive materials leaking out).”

Nam also pointed out that radioactive materials could spew from the site if it were to collapse in an earthquake.

A landslide occurred on part of Mount Mantap after the sixth nuclear test in September. Several minor temblors have also been recorded since that nuclear test.

Officials of the Korea Institute of Ocean Science and Technology (KIOST) on Oct. 30 reported on the results of a computer simulation of possible leaks of radioactive materials after the September test to the National Assembly’s committee on agriculture, food and marine products.

According to sources, the study, based on meteorological conditions at the time of the test, showed that radioactive materials would have been carried into the atmosphere in a northeasterly direction from the Korean Peninsula and could have reached a wide area touching Hokkaido, the Chishima island chain and even part of the Russian Far East.

A North Korean military hospital in Chunghwa county in North Hwanghae province was treating soldiers exposed to radiation from the nuclear tests, according to the source on North Korean affairs. The county borders Pyongyang.

The treatment is for soldiers who work at the Punggyeri nuclear test site and their families.

The site is designated an off-limits area, but more than a thousand members of a single regiment of the North Korean military have been deployed there to dig tunnels and patrol the surroundings.