August 15, 2011

expert reaction to paper reporting radioactive sulphur from Fukushima detected in California

Research published in Proceedings of the National Academy of Sciences reported measurements of radioactive sulphur from the Fukushima nuclear plant in California.

Prof Francis Livens, Professor of Radiochemistry, University of Manchester, said:

“The term ‘neutron leakage’ used by the authors of this study may be misinterpreted by the general reader. The sulphur-35 that was detected is likely to have been generated in seawater (used as emergency cooling water in the reactors) by neutrons from spontaneous nuclear fission and nuclear absorption of alpha-particles – continuing nuclear fission chain reactions (the normal source of energy in a reactor) are not necessary to produce this level of neutrons. The paper represents a useful tracer experiment, but the sulphur-35 levels are low and of no radiological significance.”

Prof Jim Smith, Professor in Environmental Physics, University of Portsmouth, said:

“The amounts of S-35 detected in the U.S. are tiny and are of no significance to human health. Even at Fukushima, the paper predicts only about 18 mBq of S-35 per cubic metre of air – many thousands of times lower than air concentrations of the much more important radiocaesium and radioiodine typically measured in the Fukushima region after the accident.”

Prof Paddy Regan, Professor of Nuclear Physics, University of Surrey, said:

“This is an interesting paper which basically shows that neutrons produced in the core of the Fukushima reactors following their emergency shutdown (likely mostly beta-delayed neutrons from the fission fragments and some of the natural fission ones) interacted with specific material in the seawater (a naturally occurring isotope – Chlorine-35) which was used to cool the reactor down in the absence the normal water cooling (since the pumps were knocked out). This created the radioactive form of sulphur, (sulphur-35 or 35S). It needs to be noted that this radioisotope is also continuously created naturally from cosmic ray bombardments on the atmosphere. It is a useful ‘tracer’ nucleus because of its relatively short half-life (just under three months) which is short enough to be able identify / deduce where any increase source of this material have come from.

“The fact that the increased amount of 35S was seen for a few days in California following the airborne releases from Fukushima can be used to show the wind currents and speeds across the pacific ocean. This is very interesting from a scientific point of view, but people should certainly NOT read into it any radiation health scare in California from Fukushima. The levels were certainly increased and the sensitivity of the equipment showed that increase very clearly. However, the overall levels of this type of radiation compared to natural background are tiny and they pose no health risk. Also, the date shows (as expected), a spike in the increased 35S level followed by the expected decrease to approximate background levels over a few days.”

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

“Seawater used to cool the cores will have been exposed to a flux of neutrons sufficient to produce S-35, which will have been released to the environment in the same way as other radionuclides that have contaminated the seawater. Neutrons have not ‘leaked from the core’ – they would be present in any event along with the gamma, beta and alpha radiation from the fuel elements (which is of course why the core is heavily shielded). Quite possibly, other radioactive isotopes may have been produced by neutron activation of stable elements in the seawater, but most will have short half-lives and will not have persisted for long enough to be detected in the environment.”

‘Measurement of radioactive 35S in California: Evidence of neutron leakage from the Fukushima nuclear plant’, published in Proceedings of the National Academy of Sciences on Monday 15 August.