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Sieverts say it all

December 28, 2011

Coming to your neighborhood

By David Ritchie

Before ending that recent visit to Japan, I made a point of visiting the beach at Fukuoka, near Japan’s southernmost tip.

Cleanest beach I’ve seen in a long time. Either the waters are free of Fukushima debris, or a great cleanup crew is at work.

And the Hii River mouth, a short walk away? About as clean as an urban waterway can be. I saw one floating bottle and a few scraps of plastic. Period.

In short, all seemed well. Once online back at the hotel, however, I checked Fukushima’s status at Energy News and found that not all was well there, over the northeast horizon.

It appears, for example, that radioactivity is turning up now in drinking water from underground sources:

Moreover, a hot spot near Fukushima is up to 333 microsieverts per hour:

What does “microsieverts” mean? In brief:

A sievert (Sv) is a unit of dose equivalent radiation. It measures the biological effect of ionizing radiation on the body. That is, it measures how much damage an exposure to radioactivity does to you.

A microsievert is a millionth of a sievert. At 333 microsieverts per hour, you would get eight millisieverts per day, or 8/1000 of a sievert.

That may not sound like much. But hours and millisieverts add up.

They lead to a cumulative annual dose — how much you would get in a year from such exposure.

To find the cumulative annual dose in this case, multiply 8 millisieverts per day by 365 days. The answer is 2.9 sieverts per year.Round it off to 3.

One sievert makes you sick. Much more than one can kill you.

After the Chernobyl meltdown in 1986, the exposure rate that meant you had to move (because the spot was uninhabitable) was 350 millisieverts — about one-third of a sievert — in a lifetime.

In other words, if you stood in that aforementioned hot spot near Fukushima, absorbing 333 microsieverts per hour, you would reach the Chernobyl cutoff point in 44 days, or about six weeks.

See what cumulative exposure can do? It makes even one microsievert per hour serious business.

“So what?” you may say. “We’re not in Fukushima!”

No, but if you’re in the northern hemisphere, Fukushima is coming your way, through radioactive debris, ocean currents, and airborne particles.

Beware especially of those particles in the air. When they fall in rain or snow, the runoff or snow melt will travel downhill and collect as water in ditches, creeks, brooks, streams, and rivers. In the water will be hot particles.

As the water gathers, radioactivity will get concentrated. A microsievert here, a microsievert there … and soon you’re talking hot spots.

And we’re not talking only about short-lived radionuclides, either. Some of this fallout has half-lives measured in centuries.

So, could portions of North America, over the next few years, reach 100 millisieverts per hour … or 200 … or even 350, the lifetime threshhold for evacuation at Chernobyl?

Alaska, western Canada, and the US west coast are getting a particularly heavy dose from the skies — worse than western Japan, if this report is accurate:

O hot new world, that has such places in it!

Before long, one of them may be your home. A little new Chernobyl, all your own.

And most likely, you will have to be your own cleanup crew.

That is, unless you can move to the southern hemisphere.

Happy new year.

ⓒDavid Ritchie 2011

(David Ritchie lives and works in Seoul, Korea. He welcomes correspondence and asks only that it be civil in tone. Contact: kwriter [at]


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