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High radiation levels outside Fukushima evacuation zone

Japan-evacuation-zones.jpgAs more radiation monitoring equipment arrives in Fukushima prefecture, we’re starting to get a sense of just how far the radioactive material from the stricken Fukushima Daiichi nuclear plant is travelling. Surprisingly high doses have been seen outside the evacuation zone set up by the government.

Today NHK, the Japanese broadcaster reported .17 millisieverts per hour (mSv/hr) 30 kilometres northwest of the reactor and 10 km outside the government’s evacuation zone around the plant (see map). There are also reports of .012 mSv/hr in Fukushima City, 60 km away from the plant. The United States is recommending that citizens stay at least 80 km from the plant, as is the British Foreign and Commonwealth Office. The conflicting messages have generated confusion. So what is safe?

Calculating the effects of these doses is difficult even for health physicists, but in terms of long-term health effects, it’s generally acceptable to make a worst-case estimate by multiplying the dose by hours in the day and days in the year. Doing so yields a rate of roughly 1500 mSv/yr at the station 30 km away and 100 mSv/yr 60 km away in Fukushima City. An exposure rate of 100 mSv/yr is considered the threshold at which cancer rates begin to increase, and 1500 mSv/yr is certainly dangerous.

It surprised one physicist I spoke to today to hear these dose rates. Radiation should decrease as the inverse square of distance (1/[4 π r2]). Thus, being 1 kilometre from the plant will decrease your radiation exposure by roughly a factor of ten and being 10 km from the plant will decrease it by a factor of a thousand. Based on the numbers being reported at the plant boundary, the numbers reported on NHK are way too high.


fukushima11.jpgThat’s the theory, but in practice radioactive materials often travel in a concentrated plume blown by the wind. In this way, high doses of radiation can pass over cities many kilometres from the site. The good news is that the doses won’t stick around for long, so the back-of-the-envelope calculations of 1500 mSv/yr or 100 mSv/yr aren’t particularly meaningful.

Because these plumes are only temporary, and can travel far without dispersing, the government might feel that the benefit of expanding the evacuation zone may be minimal. Moreover, stretching the zone even another 10 km would likely involve moving tens, and possibly hundreds, of thousands of people on top of the roughly 200,000 people that have already been asked to leave. At the moment Fukushima prefecture is beset by fuel shortages and fear, so even if the government wanted to move people out, they probably wouldn’t be able to.

There is one thing the government could do is release more data about the radiation blowing out across the prefecture, says Malcolm Sperrin, a medical physicist at Royal Berkshire Hospital in Reading, UK. Knowing the radiation type, how long it’s sticking around, and whether it is travelling as dust or an aerosol would allow experts to better advise people on protective measures. “We need to know what’s in that radiation,” he says.

UPDATE 18 March 09:30 UTC: The Japanese science ministry, MEXT, has stepped up its reporting of radiation levels throughout the region (It has also provided the location of monitoring stations, right). See Hideaki Shiraishi’s comment below for more information. A quick review of some of the graphs seems to support the view above that plumes of debris are responsible for temproary spikes in radiation.

More on radiation exposure:

Radiation exposure, beyond the numbers

Confusing radiation numbers swirl around Fukushima

For full coverage of the Fukushima disaster, go to Nature’s news special.

Credit: F. Bale/Nature/MEXT

Comments

  1. Report this comment

    Nicolau Werneck said:

    So you believe the government has data about the radiation that is being kept away from experts to analyse and advise about the best way to action?

    Are you putting in question the existence of such experts inside the government, or just their competence?

    Perhaps it’s not so easy to make data available when you are in the middle of such a crisis. And maybe it’s not very helpful to have experts from the other side of the world second-guessing all your decisions.

  2. Report this comment

    detritus said:

    It’s not only radiation which is travelling, but also radioactive particles which produce radiation.

    These can be blown away by the wind and of course can later be found several miles away. But direct radiation should decreas in intensity by the factor mentioned in the text.

  3. Report this comment

    Kiumars Lalezarzadeh, Ph.D. said:

    Kiumars Lalezarzadeh, Ph.D.

    March 16, 2011

    There may be available alternatives to cool or control the radiations.

    a) Use blue cool lighting all around

    b) Drop dry ice or ice on plant

    c) Use snow machines to shoot snow over / into the plants

    d) Deploy ships that would pump and shoot water on / into the plants

    e) Use sub-water pumps and pipes off shore to shoot water from the ocean on to / into the nuclear plants

    f) Place aluminum / lead walls all around the perimeter as close as possible

    g) Place rods and generate snow, rain and/or hail over / about the plants

    h) Create clouds of rain / hail / snow

    i) Place cooling walls all around as close as possible

    j) Create heaps of snow all around the area

    k) Reel down the water / ice / dry ice into the plant

    l) Reel down a robotic arm to move the element out

  4. Report this comment

    Kiumars Lalezarzadeh, Ph.D. said:

    m) Use deflectors, refractors, ray frequency mixers, microwave door like films all around the plant

  5. Report this comment

    Hideaki Shiraishi said:

    The government has been releasing the data of radiation levels around Fukushima Prefecture and in other prefectures twice a day. You can download them at the webpage (http://eq.wide.ad.jp/index_en.html) that you mentioned in your post on March 16. Many of them are written in Japanese, but, if you are interested, they are worth reading consulting someone who reads Japanese. There are more data available in the MEXT’s Japanese-language webpage concerning the earthquake. I think the government has been releasing to the public almost all the data that they have.

  6. Report this comment

    Ed said:

    Typo: It’s Reading, not Redding.

    Fixed Ed, thanks very much.

    —Geoff

  7. Report this comment

    Hideaki Shiraishi said:

    MEXT has changed the URLs for the English versions after I posted the above comment. You can find them here.

  8. Report this comment

    D said:

    “The good news is that the doses won’t stick around for long, so the back-of-the-envelope calculations of 1500 mSv/yr or 100 mSv/yr aren’t particularly meaningful.”

    Are you thinking about some of the short lived radioactive elements boiled out of the reactors?

    For example, the radiation from Cesium 137, a radioactive element related to the potassium our body needs, decays by half in 30 years. By 300 years the radiation level will drop to less than 1%.

    Just as rain falls in different amounts in different areas, the amount of fallout from the plant will differ in each location. The radioactive dust will settle on exposed surfaces.

  9. Report this comment

    e Devlet said:

    “The good news is that the doses won’t stick around for long, so the back-of-the-envelope calculations of 1500 mSv/yr or 100 mSv/yr aren’t particularly meaningful.”

    Are you thinking about some of the short lived radioactive elements boiled out of the reactors?

    For example, the radiation from Cesium 137, a radioactive element related to the potassium our body needs, decays by half in 30 years. By 300 years the radiation level will drop to less than 1%.

    Just as rain falls in different amounts in different areas, the amount of fallout from the plant will differ in each location. The radioactive dust will settle on exposed surfaces.

  10. Report this comment

    Dog Life Jacket said:

    NEXT has started to provide up-to-date monitoring data in English. See the bottom half of the following pages.

    Thank

  11. Report this comment

    Dog Life Jacket said:

    NEXT has started to provide up-to-date monitoring data in English. See the bottom half of the following pages.

    Thank