South Korea says its scientists have detected high levels of radioactive gas after the North claimed it had successfully staged a nuclear fusion reaction last month. <br/><br/>The officials in Seoul have now revealed that South Korean scientists detected extremely high levels of xenon gas near the border between the two Koreas. Xenon is produced during the nuclear fission process. <br/><br/>North Korea announced last month it had conducted a nuclear fusion reaction, a claim met with much scepticism in the South because of the technical difficulties of staging such an experiment. Nuclear fusion promises limitless energy but can also be used to make hydrogen bombs.