March 25, 2011

Fukushima workers’ exposure to radioactive nucleotides

Tokyo Electric Power Company released information on the levels of exposure to the workers at the stricken Fukushima nuclear power plant.

Rick Hallard, a radiation protection consultant with more than 30 years’ experience working in a UK nuclear facility, said:

“These isotopes do look like fission products. Co60 usually comes from steel, but this could be from the pressure vessel. They could therefore have come from inside the reactor.

“The UK limit for skin dose is 500mSv. You would normally only start to see skin reddening (erythema) at perhaps 2000mSv – this varies between individuals. At his level of dose, the effect would last for a small number of weeks, then the patient would recover. There would be some enhanced risk of getting skin cancer, but probably still not particularly high. It may be possible for the skin dose received in Reactor 3 to be significantly higher than the measured whole body dose sufficient to give some skin reddening, but information is very uncertain so this is unclear at present.”

Prof Malcolm Sperrin, Professor of Medical Physics, Royal Berkshire Hospital, said:

“The exposure to the workers from the ‘puddle’ is clearly a significant event and the dose of 170mSv to the workers’ legs is high enough to be of concern. Last week, TEPCO issued guidance on the acceptable workers’ dose limits in the current emergency, recommending a basic limit of 100mSv where necessary, or 250mSv where worker involvement in the high radiation area is critical for on-going safety. In the light of this, the 170mSv, whilst significant, still falls below the upper limit. Such a dose may, however, result in surface lesions where the radiation is particulate and hence the skin dose is high and such an event will be properly looked out for and managed in the hospital facilities where the workers have been taken.

“Natural background radiation is likely to be between 3 and 5mSv per annum. A 170mSv dose equates approximately to an increased risk of developing a lifetime fatal cancer of 1 in 100 as compared to a natural cancer risk of 1 in 3. There remains insufficient data to formulate a more definitive response.”

Dr Paul Norman, Senior Lecturer in Nuclear Physics at the University of Birmingham, said:

“The information released by TEPCO does indicate that fission products were present in the water that the workers were exposed to. Because the readings we have seen come from the turbine building rather than the reactor building, it is possible but not certain that there has been a breach of containment. The other possibility is that the cladding of the fuel rods was compromised but the reactor vessel/containment was not – in this case, the fission products may have been released during the explosive steam ventings.

“In the case of Three Mile Island, for example (where the containment was not breached), some radioactivity within the water also found its way via the ‘sump’ into an auxiliary building. In that case it was not the turbine building, as the reactor design differs slightly (in the BWR the steam from the reactor goes straight to the turbine area, which is the building we are talking about). But there are similarities in terms of loss of core cooling, thus potential loss of the cladding integrity due to fuel melting, and one could imagine something similar may have happened here in the steam venting – i.e. radioactivity getting into an auxiliary building but without the reactor itself necessarily being breached.”

Prof Barry Marsden, Professor of Nuclear Graphite Technology at the University of Manchester, said:

“I can confirm the list given on the TEPCO website includes fission products and must have been released from damaged fuel as they are not naturally occurring. The atomic weight of fissionable Uranium is 235 and you can see most of these products are roughly half this value, generated by splitting Uranium atoms. These products will be highly radioactive, giving off predominately gamma, and beta irradiation, some with short half life – for example Iodine 131 (Beta, half-life 8 days) – and some with medium and long half-life – for example 5.3 years for Cobalt 60 (gamma) and 211,000 years for Technetium99 (beta).

“Any acute exposure to a person greater that 250mSv from the International Commission of Radiation Protection (ICRP) may lead to long term effects. This will obviously make remediation work more difficult at the plant. Eventually the water will need cleaning using methods such as ion exchange. The Japanese scientists and engineers will be very aware of these difficulties and will no doubt be taking appropriate actions.”

Dr Mike Thorne, independent consultant in radiological and environmental science, said:

“Co-60 is an activation product typically present in steels within the reactor pressure vessel, but it is present at only at a low concentration in these results. Tc-99m is a short-lived (half life 6.02 hours) fission product. The iodine and caesium isotopes are fission products and would have arisen from damaged fuel elements. Both these elements are volatile and are present in the gap inventory of fuel pins and are readily released when the cladding is damaged. Ba-140 is an alkaline earth with chemically similar properties to calcium. La-140 and Ce-144 are lanthanides. All of these are fission products. The La-140 and Ce-144 data are very interesting, because lanthanide elements are not volatile. This implies that particulate material has been released from the fuel pellets and escaped into the water. If this is the case, then actinides (e.g. plutonium) which are very similar chemically to the lanthanides would be expected to have been released by the same route.”