By Do Je-hae
South Korea and the United States have not found any radioactive residue from North Korea's May 25 nuclear test, a report said Monday.
Yonhap News Agency quoted an unnamed government official as saying the blast at Punggye-ri in Gilju, North Hamgyong Province, had yet to show any signs of radioactivity.
A U.S. Air Force WC-135 aircraft has not detected any suspect elements from recent air samples taken above the East Sea, the official said.
``This leads us to believe that contrary to general expectations, North Korea has succeeded in sealing the nuclear test site,'' he said.
Following a nuclear test, radioactive material such as Krypton-85 and Xenon-135 are normally detected in air samples, according to researchers.
``Our radioactivity detectors are functioning properly,'' the official said.
However, the North's nuclear test site is in a distant location from South Korea and it is an underground facility, making it difficult for the South to measure any radioactivity, he said.
The official also raised the possibility of 2 to 3 additional nuclear test sites in the vicinity of Punggye-ri.
``However, there are few signs indicating that North Korea is bracing for a third nuclear test at the moment,'' he said.