The quantity of radioactive cesium 137 that has escaped Japan's crippled Fukushima Daiichi atomic facility could exceed by more than twofold the level determined in an official calculation, specialists said in a report quoted by Bloomberg on Thursday (see GSN, Oct. 27).
The six-reactor power plant was damaged by the March 11 earthquake and tsunami that left more than 20,000 people missing or dead in Japan. Radiation releases on a level not seen since the 1986 Chernobyl disaster forced the evacuation of about 80,000 residents from a 12-mile ring exclusion zone surrounding the site in Fukushima prefecture. Unlivable conditions could persist in the plant's vicinity for 20 years or longer, Tokyo indicated in August.
Measurements collected globally suggest the events resulted in the release of 35,800 terabecquerels of cesium into the environment, 42 percent of the amount to have escaped in the Chernobyl incident, U.S. and European experts said in a report published in Atmospheric Chemistry and Physics. The Japanese government in June said only 15,000 terabecquerels of the isotope, which breaks down at a rate of 50 percent every three decades, had moved out of the plant.
Roughly 20 percent of the cesium landed on Japanese territory, while air currents carried the other material out to sea, the document indicates (Tsuyoshi Inajima, Bloomberg, Oct. 27).
The assessment suggests the disaster's effect on the Japanese capital was much smaller in scope than it might have been, Nature quoted one expert as saying.
"There was a period when quite a high concentration went over Tokyo, but it didn't rain," said Andreas Stohl, an atmospheric researcher at the Norwegian Institute for Air Research. "It could have been much worse" (Geoff Brumfiel, Nature, Oct. 27).
No adjustments have been made to plant contaminant calculations issued by the Japanese Nuclear and Industrial Safety Agency and Tokyo Electric Power, the Fukushima site's operator. The quantity of contaminants moving out of the plant has fallen to one eight-millionth of the maximum level recorded in the crisis, the power company indicated last week.
Japanese Environment Minister Goshi Hosono in August said Tokyo would amend its computation.
“We don’t need to add much to what was emitted in the early days,” NISA atomic crisis official Yasuo Kosaku told Bloomberg. However, “the June estimate may have to be revised," he added.
Stohl said the expert assessment draws on a broader range of measurements than the Japanese government, which focused largely on information gathered within the country, Nature reported. The difference might help to explain the divergence in the two estimates, he said.
Personnel behind the government's figure "wanted to get something out quickly," said Stohl, who suggested their effort was understandable.
"Taking account of the radiation that has drifted out to the Pacific is essential for getting a real picture of the size and character of the accident," added Tomoya Yamauchi, a Kobe University radiation scientist who has been taking ground contamination readings in Fukushima prefecture.
The official and independent estimates are notably comparable when ambiguities in calculation procedures are taken into account, said Gunma University volcano specialist Yukio Hayakawa, who has also carried out computations related to the crisis (Brumfiel, Nature).
When the plant operator began firing water on the No. 4 reactor's spent fuel cooling pond as a temperature control measure, the release of cesium 137 "suddenly dropped," Bloomberg quoted the report's authors as saying. “This indicates that emissions were not only coming from the damaged reactor cores, but also from the spent fuel pool of unit 4,” the document states (Inajima, Bloomberg).
The pond was apparently not harmed significantly, according to Japanese officials who discounted the area as a major cause of radioactive material emissions. "The release from unit 4 is not important," Masamichi Chino, a Japanese Atomic Energy Authority expert who aided in the preparation of the government calculation, said in remarks reported by Nature.
Still, the reading of the cooling pond's significance "looks convincing," said Lars-Erik De Geer, an atmospheric expert with the Swedish Defense Research Agency in Stockholm (Brumfiel, Nature).
Damage to the facility also led to the escape of 16.7 million terabecquerels of xenon 133, Bloomberg quoted the expert paper as saying. The radioactive isotope breaks down at a speed of 50 percent over 5.2 days and poses little threat, Tetsuo Ito, who heads the Atomic Energy Research Institute at Kinki University, said in a telephone discussion.
Separately, contaminants might have begun drifting from the site before the March 11 tsunami hit, 45 minutes following the earthquake, according to the report. “This early onset of emissions is interesting and may indicate some structural damage to the reactor units during the earthquake,” it states.
The Japanese atomic agency stands by its view that the facility sustained no major harm during the earthquake, spokesman Tadashige Koitabashi said in a telephone interview. The official did not address the new assessment directly.
The tsunami compromised the plant's auxiliary power systems, halting its temperature mitigation mechanisms and prompting overheating in three reactors, according to the atomic agency and Tokyo Electric Power. Radioactive materials passed out of containment structures damaged by blasts at the facility (Inajima, Bloomberg).