Wednesday 31 August 2011

Radiation (3)

I'm still struggling with the terminology and the science, trying to understand, in the absence of any official guidance, how we should live our daily lives here in Fukushima.

Today I'm going to look at the thing that's been worrying us for a while: internal exposure (for those of you who want the Japanese that's  naibu hibaku 内部被爆). Normally about 10% of the radiation you receive is internal, from the radon gas you breathe in and from food.

At the end of June, 122 people from Namie, Iitate and Kawauchi, within 20 km of the reactor, who had been evacuated to Fukushima city were tested at the National Institute of Radiological Sciences in Chiba for internal exposure. They left with much fanfare and I'd been keeping an eye out for the results. But I couldn't find anything in the Institute's website, only a brief one page press release on the METI website (28 July), and an article at the bottom of an inside page in the local paper. Good news not headline news?

None of those tested had any trace of iodine. But Caesium 134 was found in half of them and Caesium 137 in a third of them.  The readings were given in becquerels (highest 3,800).

Becquerels? Becquerel was the person who discovered radioactivity - before the Curies. It's a measure of the rate at which energy is emitted by a radioactive source. I read somewhere that if you think of a lightbulb, becquerels measure the light shining from the bulb, a gray is when the light hits the skin, and a sievert is how you would measure the heat absorbed by the body. The trouble with becquerels is that they come in frighteningly huge numbers. Fukushima Daiichi is currently said to be emitting 20 billion becquerels of radioactivity per day. And that is a fifth of what it was a month ago! The government's limit (controversial) for food safety is 500 becquerels per kilogram of food. The beef during the scare (ban lifted last week) was showing two or three times that level.

There's a complicated formula for measuring the accumulated dose (50 years for adults, 70 years for children) as these isotopes have long lives But an expert said that even the highest level in the people tested was the equivalent of under 1 mSv.

Another test done by Minamisoma on about 900 residents showed low levels of internal radiation exposure and no one required immediate treatment. 'When converted to exposure over the next 50 years, one resident measured just above 1 millisievert of radioactive caesium, while tests on most of the other residents revealed exposure of 0.1 millisievert or less'.

So all very reassuring. Then I came across this video of one Professor Kodama of Tokyo University testifying in the Diet. There's no date on it but it's probably a while back (June?).  He says, (and you're not going to like this, Mum!):
1. Fukushima Daiichi has put out 20 more times radioactivity than the Hiroshima bomb.
2. The government have been negligent in not giving  out information sooner especially to pregnant women and infants, and not tackling the clean-up
3. Measures of internal exposure by whole body counters are useless. It depends where in the body the isotopes collect and what trigger they get later in life.
If you press the cc button at bottom right of the picture (next to 360p) you get English sub-titles. There's a Part Two as well.

To be fair, the clean-up has now started. The government has started a test, a model area, in Fukushima City where they're going to clean the whole area, including woods and fields. Local authorities continue with the clean up of school playgrounds. Schools have re-routed the routes for children walking to school  to avoid 'hot spots'.

But it's hard to understand. I wish we could have more practical advice. All we're told is that everything is OK and there's nothing to worry about.

Anyway, if those people in Minami Soma, much nearer the reactor than I am, only had internal exposure of 0.1 to 1 mSv, I think I'll take one tenth of my exposure to the air, which I estimated as 2.1 mSv to March next year, so 0.21 mSv. That brings my running total to 4.725 mSv for the year. Next time I'll look at food.
And then I'll stop. This is getting depressing.

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