November 2, 2011

experts react to detection of radioactive xenon at Fukushima site

Tests indicated the release of radioactive xenon at the site of the Fukushima nuclear power station in Japan.

Prof Paddy Regan, Department of Physics, University of Surrey, said:

“A number of different Xenon isotopes are created following the fission of uranium. Of those reported by Tepco (133Xe and 135Xe), 133Xe has the longest half-life, but this is still only 5 days. I would therefore expect that so many months after the initial accident, any of these isotopes created around the time of the tsunami would have mostly all decayed away by now. Since Xe is a noble gas, it does not bind to other atoms and can escape from the main body of the reactor fuel.

“Hence this does not (again) look like a major (new) 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. The criticality is not present, but there is a natural fission decay from Uranium which would also produce some radioactive Xe – although much less than if the fuel assembly in the reactor were critical. As with other reports, this one does not appear to show any new radiological hazard from the disaster.”

Prof Steve Jones, independent nuclear and environmental consultant, said:

“The TEPCO press statement refers to the possible detection of Xe133 (half life 5 days) and Xe135 (half life 9.2 hours). If these really are present, they must have been produced by nuclear fission.

“However that doesn’t mean that a nuclear chain reaction is occurring. In the reactor core there will be nuclides present that undergo spontaneous fission and emit neutrons – Pu240 being a particular example. The xenon may be produced by the spontaneous fission itself or by other fissions prompted by the emitted neutrons. However, so long as the system is well below the critical level needed for a self-sustaining chain reaction, all will be well.

“If spontaneous fission is occurring other short lived nuclides, including I131, may be present at very low concentrations.”

“At present, assuming low concentrations (there seems some doubt whether it has been detected or not) health risks for all practical purposes are nil.

“The issue is of course whether there is any danger of the system going critical again – that’s why TEPCO injected boron. This won’t stop spontaneous fission but it will mop up any spare neutrons and reduce the risk of a self-sustaining chain reaction.”