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millisieverts


Japan’s nuclear plant problems and what they mean – a little context

Right now we’re seeing all sorts of reports come out of Japan as to what is happening at the Fukushima nuclear plants.   All of them are tinged with sensationalism, and many of them contain no context to enable the reader to understand what is being reported in terms of the severity of the problem.  For instance:

Readings reported on Tuesday showed a spike of radioactivity around the plant that made the leakage categorically worse than in had been, with radiation levels measured at one point as high as 400 millisieverts an hour. Even 7 minutes of exposure at that level will reach the maximum annual dose that a worker at an American nuclear plant is allowed. And exposure for 75 minutes would likely lead to acute radiation sickness.

Yes, but what does that mean outside the plant?  And, how many millisieverts an hour do we naturally absorb just going about our daily lives.  Both of those answers would help the reader assess the real danger of such radiation levels.

What you’ll find is that if you take an airplane and fly from say Atlanta to Chicago at 39,000 feet, you can expect to absorb 2 millirems of radiation.

So how does that convert to millisieverts?  You math whiz types can figure it out here with these conversion factors:

  • 1 rem = 10-2 sievert (Sv)
  • 1 millirem (mrem) = 10-5 sievert (Sv)
  • 1 millisievert (mSv) = 10-3 sievert (Sv)
  • 1 millisievert (mSv) = 0.1 rem

To help others, 1 millisieverts equals 100 millirems.  And 1 Sievert equals 1000 millisieverts.  To give you an idea of what the number above means in millisieverts (mSv), we typically absorb 6.2 mSv per year in the US.

Now that number has some context and you can relate it to the danger outlined above.

As to the effect.   Here’s a good table outlining the effects of different levels of absorption:

  • 0–0.25 Sv: None
  • 0.25–1 Sv: Some people feel nausea and loss of appetite; bone marrow, lymph nodes, spleen damaged.
  • 1–3 Sv: Mild to severe nausea, loss of appetite, infection; more severe bone marrow, lymph node, spleen damage; recovery probable, not assured.
  • 3–6 Sv: Severe nausea, loss of appetite; hemorrhaging, infection, diarrhea, skin peels, sterility; death if untreated.
  • 6–10 Sv: Above symptoms plus central nervous system impairment; death expected.
  • Above 10 Sv: Incapacitation and death.

So given the information above, 3 hours at 400 mSv is equivalent to 1.2 Sv.    It’s recoverable but with damage.

As for exposure outside the plant – the levels of radiation drop sharply away from the plant.   So those in the most danger, obviously, are those within the plant trying to contain the problem.  Reports say that most of the plant workers have been evacuated and about 50 continue to battle the problems in the reactors.  Where the problem for the public may occur is if there is a release of radioactive clouds of steam, or through explosions that eject material (think dirty bomb).  And naturally much of the impact would be determined by wind direction.   If it is blowing directly east over the ocean, the cloud would do much less harm than if it blew west over  populated areas of Japan.  Additionally, the materials effect would dissipate as the cloud expanded and traveled.  The possibility of any significant amount of radiation reaching the US, for instance, is not particularly high.

Finally, this article by the NYT is actually a good one for background about the problems the Japanese face and the possible outcomes.   For once, they attempt to keep the reporting less sensational and more focused on relating facts.

~McQ

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