What caused radiation spike in north west China: North Korea, Europe or something else?
Levels of iodine-129 in capital of Shaanxi province peaked two days after hydrogen bomb test 2,000km away
Radiation levels in a Chinese city nearly 2,000km from a North Korean nuclear test site spiked following Pyongyang’s latest and most powerful nuclear weapons test in September, according to Chinese scientists.
However, the spike in iodine-129 levels Xian was probably not related to the detonation of a 100-kilotonne hydrogen bomb in a tunnel at the Punggye-ri nuclear test site on September 3 and was more likely to have originated in Europe, they said.
The spike was recently declassified by the Chinese Academy of Sciences, prompting heated discussion among researchers about its possible cause, with some disputing the Europe theory.
From September 3 to 11, levels of iodine-129 in Xian, capital of Shaanxi province in northwestern China, jumped to at least 4.5 times the average, according to readings picked up by instruments at the academy’s Institute of Earth Environment, which is based in the city.
Iodine-129 is an isotope of the element iodine that rarely occurs in nature. It is mostly produced by man-made fission and is closely monitored around the world as evidence of nuclear weapons tests or nuclear accidents.
The levels in Xian, nearly 2,000km west of Punggye-ri, peaked between September 5 and 6, when they were nine times as high as the day before the test.
Zhang Luyuan, a physicist at the institute who is leading the investigation of the incident, said she had goosebumps when she first saw the spikes on the chart.
“We thought we’d nailed it. The timing was almost perfect,” she told the South China Morning Post. “It could have been the first time fallout was recorded outside North Korea.”
But the matter turned out to be much more complicated than the researchers thought.
Zhang and her colleagues checked the data collected by devices set up along the Chinese-Korean border due to concerns in Beijing that the Punggye-ri test site, under a mountain near the border, might collapse and release a large amount of radioactive pollutants.
While some stations reported an increase in overall radioactivity, they did not detect trace elements such as iodine-129.
Zhang said the researchers pondered whether the radioactive particles might have been blown towards Xian but discovered winds had been blowing towards the east for most of the time in question.
The team also calculated that in order to generate enough fallout to boost the amount of iodine-129 in Xian by so much, the bomb detonated in North Korea would have had to have been “many, many times” larger than reported estimates, Zhang said.
The team now suspectedthe fallout might have come from western Europe, because two
of the world’s largest spent nuclear fuel recycling plants, in France and Britain, had released more than six tonnes of iodine-129 into the environment since the 1960s, more than 100 times the amount produced by all the nuclear weapons tests conducted in the atmosphere.
But that suggestion came under fire from many people in the research community, who pointed out that Xian was more than 8,000km from France and Britain.
Professor Guo Qiuju, a nuclear physicist leading the research programme on nuclear hazard monitoring at Peking University, said that if Europe was to blame, there must have been a very large, very serious accident that had not been disclosed.
“Europe has established maybe the world’s best network to monitor radioactivity in the environment,” she said. “If there was a cloud coming from there, it must have triggered alerts all along the way.”
But Guo, also a member of an expert panel that advises the Chinese government on dealing with North Korea’s nuclear threat, said the incident was unlikely to have been caused by the bomb test either.
“If a leak has indeed occurred, the stations on the high mountains at the border should have recorded similar or stronger signs,” she said. “The data is transparent. There is no cover-up.”
A nuclear safety expert who requested anonymity said Xian was home to a major research centre for China’s nuclear weapons programme.
The Northwest Nuclear Technology Research Institute, run by the People’s Liberation Army’s Equipment Development Department, operated a wide range of radioactive equipment in the city including a pulse reactor and powerful accelerators, the researcher said, adding “the possibility of a local accident cannot be ruled out.”
Zhang admitted the need to avoid public panic was one reason the information had been kept from the public until the end of November.
“Our investigation was not completed then,” she said.