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Hint of new fission at Fukushima reactor no cause for alarm

Scientists have downplayed the significance of the detection of radioactive xenon at the Fukushima nuclear power plant in Japan, announced today.

As xenon radionuclides have short half-lives of up to just a few days, the detection, if confirmed, would mean that nuclear fission reactions of some form had restarted. But experts say that small amounts of fission in the reactor core would not be that surprising, and there seems no danger of either a self-sustaining critical chain reaction or significant release of fission products into the environment. Nature has also learned that ultrasensitive radionuclide detectors in the region, part of a global system to detect minute radionuclide signatures from atomic bomb tests, have so far not detected a trace of any newly-released xenon.

Tepco, the company that operates the Fukushima power plant, announced earlier today that nuclide analyses of exhaust gas from the reactor containment vessel of reactor 2 yesterday found a possible detection of xenon-133 (half life of just over 5 days) and xenon-135 (half-life of 9.14 hours). It noted, however, that there was no change in the temperature or pressure of the reactor, and no increase in radiation levels around the plant. Nonetheless, as a precaution it today injected into the reactor a solution of boric acid which strongly absorbs neutrons and so dampens fission reactions.

The xenon could result from fission of radionuclides in the core, but if it is at a low level as seems the case, there is probably little to worry about.

“This does not look like a major release of radiation from the plant, but it is worth noting that even if the fuel is cooled, there is still a small amount of residual natural fission of the large amount of uranium fuel in the core,” said Paddy Regan, a nuclear expert at the University of Surrey in the United Kingdom, adding that the amounts released would be far less than were the fuel to go critical. The detected xenon, he said, “does not appear to show any new radiological hazard from the disaster.”

Gerhard Wotawa, a researcher at Austria’s weather service, the Central Institute for Meteorology and Geodynamics in Vienna, says data from the global network of detectors operated by the Vienna-based Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) have seen no uptick in atmospheric xenon levels over the past few days. The CTBTO detector network proved critical in monitoring environmental release from the accident itself – see some of my earlier pieces on this: Radiation data from Japanese disaster starts to filter out and Exclusive: Nuclear test ban agency has valuable radiation monitoring data from Japan nuclear accident – but can’t share them.

Xenon itself is harmless as it’s not absorbed by organisms or by the environment, and is dispersed in the atmosphere. During the major early radioactive emissions after the Fukushima accident, more xenon was released than any other element, with recent estimates putting the total amount of xenon-133 released at 1.7 × 10 to the power of 19 Bq, greater than the total radioactive release of 1.4 × 10 to the power of 19 Bq from Chernobyl – see Fallout forensics hike radiation toll.

In other news, Japan on Tuesday restarted a reactor at the Genkai Nuclear Power Plant – see the New York Times story here – the first reactor to restart since the Fukushima catastrophe.

For full coverage of the Fukushima disaster, go to Nature’s news special.


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