(Cesium metal, called seshiumu セシウム in Japanese. This piece of cesium is encased in mercury, suigin 水銀, inside of glass. Photo courtesy of Wikipedia)
Someone mentioned this article on the Japan Times, and I wanted to post it here. This is a project map of the cesium fallout caused by the Fukushima disaster, just after the Great Tohoku Earthquake on March 11th, 2011. Much of this fallout was spread by wind and water, can you see how the area immediately to the northwest is affected the most. The government is becoming now aware of the extent of the fallout, and it’s quite widespread, but before you panic, let’s look carefully.
I’m not an expert on the subject at all, but I have been reading up on it a lot over the last few months learning more about how these things are measured.
Most articles related to Fukushima tend to use two measurements in particular:
- Becquerels – A measurement of the quanity of a certain type of radioactive material is in a certain area. This does not necessary radioactively itself, but just how much of the substance there is.
- Sieverts – A measurement of biological exposure to radiation. Unlike becquerels, this measures a more direct threat to people.
As this helpful chart by XKCD shows, people are constantly exposed to radiation on a normal basis. Even a banana is radioactive, because of the potassium (potashiumu ポタシウム) in it! Also, as the article explains, in many of places with fallout, your total exposure to radiation would be around 4-5 millisieverts a year, while the XKCD chart shows that the danger doesn’t begin until you reach 100 millisieverts.
Even though the dosage and exposure of radiation is pretty safe for most of Japan, the spread of radioactive material is still a HUGE headache. Cesium fallout has essentially reached all of Eastern Japan, and causes it to turn up in strange places like gutters in Yokohama. The cleanup is so huge that the government is trying to mobilize anyone it can, while the IAEA is suggesting it might be too much time and cost to be worth cleaning up everywhere.
As the top article shows, Cesium-134 has a half-life of 2 years, so in 2 years, it’s radioactivity will decrease by 50% (and in 2 more years it will decrease by 50% of that amount, and so on), though Cesium-137 will stay around for 30 years unfortunately. Strontium-90, found at Yokohama, has a half-life of 28 years but the related Strontium-89 has only 50 days.
Again, the lesson from all this I think is that the risk of radiation exposure is pretty low and exaggerated, unless you’re very close to the Fukushima reactor, but the bigger problem is the long-term environmental poisoning caused by the spread of materials. As the original article shows, fallout already spread pretty far, and in 30 years it will continue to spread and circulate from wind and rain. Of all the radioactive materials, Cesium is the most persistent, and likely to have the most negative impact on the environment. Like a bad oil spill, the impact to the environment by the cesium fallout are unpredictable, subject to complex atmospheric and conditions that will only make sense after years of study.
In layman’s terms: cesium is real pain in the ass.
Update: Forgot to link the XKCD radiation chart. Added now.