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Worker doses at Fukushima

Dear Reader,

Some very serious claims have been made that TEPCO has employed homeless men and other unsuitable workers. This also includes other people who were open to explotation. It is also claimed that these workers were given more dangerous duties than a normal TEPCO worker. These are serious charges, sadly in the past homeless men have been exploited to do dangerous work. For example several cases have occured in the US where dangerous asbestos stripping work was given to homeless men. You will be glad to know that the guilty have been punished, but I suspect that the punishment does not reflect the gravity of the crime.

But lets get back to the TEPCO case, while I suspect that if a company was to illegally hire ordinary workers to do radiation or nuclear work that the company would not wish to disclose this fact to the public, but I would like to know if any of the people making these serious allegations have any hard evidence to support these claims. To make a false accusation of a serious crime is a malicious falsehood and may well also be a crime itself (wasting police time), while many people may be unhappy about the way in which TEPCO have behaved it does not give the unhappy people a license to lie about TEPCO.

I would like to point out that TEPCO have released data for radiation doses to workers on the Fukushima site, this data suggests to me that in the first year the work with the highest exposure was done by TEPCO workers rather than subcontractors. This may well be due to the fact that the “Fukushima 50” include people who have had large doses of radiation which they got during the worst days of the crisis.

On average a TEPCO worker at Fukushima got a dose of 24.68 mSv in the first year at the site, the number of workers was 3422 people so this means that TEPCO workers have had 84.45 man Sv. Using the assumption that a 1 Sv dose gives you a 5% chance of cancer and assuming that the LNT (Linear No Threshold) rule is true then I have calculated that extra 4.2 cancer cases will occur amoung the TEPCO staff as a result of the accident.

The average dose to a subcontractor was 9.59 mSv, and as there were 17600 subcontractors then the subcontractors got a collective dose of 168.784 man Sv which I estimate will lead to 8.4 extra cancer cases. Below is shown a bar chart of the radiation doses for both TEPCO staff and subcontractors. If we assume that all temps are subcontractors then this data suggests that the worst of the work was undertaken by regular TEPCO staff.

Bar chart of worker doses in the first year at the Fukushima nuclear power plant

If we look at monthly data for external exposure during much of 2011 we will see an interesting trend, the average exposure level for TEPCO workers and contractors did go down in the first half of the time, originally TEPCO staff were getting higher doses than the subcontractors but then the average dose to the two groups of workers becomes roughly the same.

Average worker dose per month at the Fukushima nuclear power plant site

The problem is that unless TEPCO release the exposure data for every worker then it is not possible to estimate the ESD and work out if the slightly higher exposure which the subcontractors got is a real difference or not. I always warn people to ask what the ESD is for data when they want to know if one data set is different to another.

If we assume that TEPCO is being honest about radiation exposure on site then I see no strong evidence to suggest that the average subcontractor is being exposed to more radiation than the average TEPCO worker. I think with the eyes of the world on them they would be very foolish to be dishonest and I imagine that they are telling the truth about worker doses.

But if we look at the external dose that the most exposed person gets each month then we see a different story until we consider the ESD. It might look like the workers with the highest exposure are subcontractors, but if we make an estimate of the ESD based on the last six months of the data then the difference betweeen the two groups becomes rather small when compared with the ESD, I think that a clear failure for the most exposed subcontractor to be exposed to a dose which is at least the sum of the two ESDs higher than the highest exposed TEPCO worker suggests that the difference in the following graph might not be significant.

The dose of the most exposed worker each month at the Fukushima site

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