Concerns of fission at unit 2

02 November 2011

The possible detection of xenon at Fukushima Daiichi 2 has led to questions over ongoing fission, but detection is uncertain and a translation error may have exacerbated anxiety.


Temperatures and pressures at all the damaged reactors at Fukushima have been stable and declining for several months, and all are now far below the target temperature of 100ºC: units 1, 2 and 3 are at 59.4ºC, 76.3ºC and 71ºC respectively. Airborne radioactive emissions from the site have dropped to within normal operating limits.


Tokyo Electric Power Company (Tepco) continues to focus on repairing and remediating the site to bring it to a proper level of safety and ultimately decommission it. As part of this work, the company restarted a gas management system in unit 2's containment vessel on 28 October. It took samples through that system and announced three days later: "It was found out that there was a possibility that a short-half-life radionuclide (xenon-133 and 135) was detected."


These are the kinds of isotopes left after a uranium atom fissions and their short half-life of less than six days means their presence would indicate fission had occurred fairly recently. Furthermore, the fact that the sample was taken for the first time also indicates that fission could have been ongoing for some time at a low level.



Undeniably confusing


The possible discovery of xenon at unit 2 has been lent extra drama by a translation error in Tepco's press release. After describing the uncertain potential detection of xenon, the absence of other fission products and stability of the reactor the company then contradicted all that by stating "it was undeniable that fission had occurred" based on the apparent presence of xenon.


In Japanese, an assertion is called undeniable if it cannot be proven either way. In this case the release implied fission was an 'undeniable' fact, when it perhaps ought to have put it as an 'undeniable' possibility.

However, fission would also result in other products such as isotopes of caesium, iodine and molybdenum, which were not reported from the same samples or detected at all at the site boundary. Tepco also noted that there had been no recent fluctuation of the reactor's temperature or pressure.



The company said it has begun to inject boric acid into the reactor vessel as one measure to ensure fission does not recur.
Researched and written
by World Nuclear News