South Korean scientists are warning that radioactive fallout from another nuclear test by North Korea could reach Japan’s northern island of Hokkaido based on computer simulations of a possible leak.

The Asahi Shimbun is reporting that the six nuke tests Pyongyang has conducted so far at its Punggye-ri facility have weakened a surrounding mountain so much that fallout could escape in the next test and be blown by prevailing winds towards Japan. The facility at Mt. Mantap is also about 80 km from the Chinese border.

The Japanese newspaper also quoted a source knowledgeable about North Korean affairs as saying that some North Korean soldiers are being treated at a hospital for radiation exposure from the tests.

“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 October 30. “If additional nuclear tests are conducted, there is the possibility [of radioactive materials leaking out].”

Nam also noted the danger posed by nuclear fallout if it spewed out in an earthquake triggered by the deep underground blasts.

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

The Korea Institute of Ocean Science and Technology (KIOST) reportedly passed on the results of a computer simulation of the possible radioactive leaks to the South Korean National Assembly’s committee on agriculture, food and marine products on October 30.

According to sources, the study, based on meteorological conditions at the time of the test, showed that radioactive materials from a leak would be carried in the atmosphere in a northeasterly direction from Korea and would cover a wide swathe that includes Hokkaido, the Chishima island chain and even part of the Russian Far East.

Asahi’s TV channel, TV Asahi, reported earlier this week that a tunnel collapse at Punggye-ri killed more than 200 North Korean workers in September. The station cited North Korean sources as saying that the deadly incident took place in October during the construction of a new underground tunnel. The work suggests to analysts that Pyongyang has no intention of slowing its nuke program by abandoning the site.