Light On Dark Water

7 responses to “More Sense About Radiation”

1. Jesse Canterbury

   March 21, 2011

   Some comments about this:
   (1) I think some things in your first paragraph are off. The chart says the typical yearly background dose due to natural sources and medical scans is ~3.65 milliSieverts (mSv). The dose in one day at a site 50 kilometers — 30 miles — from Fukushima on March 17 was ~3.6 mSv according to the chart. That’s a roughly a factor of 365 times the typical background dose, at a distance of 30 miles from the accident. Of course this will vary from day to day, probably mostly due to winds, but still — it’s not a place one would want to be. Levels at the reactor are no doubt much more harmful, hence the headlines you see about workers being evacuated, then let back in, etc.
   Also, you seem to be saying that the March 17 dose 30 miles from Fukushima — 3.6 mSv according to the chart — is many millionths less than that required for severe illness or death — 2-8 Sv according to the chart. But milli (prefix m) means 1/1000, not 1/1000000.
   (2) As a consequence of (1): One must be careful to include the time of exposure in reporting the radiation levels, and this is why many sources report radiation levels in units of Sieverts/hour. I just read that exposure levels “at the site” (whatever that means — at the gate? at the reactor?) are currently 2 mSv per hour. Thus in 50 hours’ time — just over two full days or just over a week of 8-hour days — standing at the same site at Fukushima will get you a cumulative dose of 100 mSv — which is the dose definitively linked to increased cancer risk, according to the chart. Even though levels seem to be decreasing, the workers at the plant are and have been in pretty serious danger, there is no doubt about that — dosages at their worksites are undoubtedly higher than that 2 mSv figure.
   (3) About harm to the US. I agree that there’s been a lot of hysteria. There’s a difference, I believe, between “radiation” and “radioactive material.” Yes, we are a long way from Japan, but the relevant thing to consider for possible danger to the US is not radiation that originates at the site, but rather radiation emitted from radioactive material that has spread from the site. If a giant ball of radioactive smoke from Fukushima managed to make it over here intact, it would be just as dangerous as it had been over there. The thing keeping us safe from such a scenario isn’t the inverse-square law you described, but rather the large amount of turbulent mixing and diffusion that occurs in weather patterns between here and Japan.
   I think people over there are right to be very concerned, but I agree with you that the risk here is minimal, and the hysteria in the US has gotten sort of, well, hysterical.
   I think it has to do mainly with (a) what-ifs — people asking what if a similar thing happened here; and (b) the fact that anything with the words “nuclear” and “radiation” in it will always carry a shadow. (b) is undoubtedly the driving force behind much of the concern.

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2. Mac

   March 21, 2011

   Thanks for keeping me honest. 🙂
   (1) Yes, that’s true for the Fukushima number in the green chart, but I was referring to the one in the blue chart, which is microS, and fortuitously almost the same units (3.5 vs. 3.6). Now, why there would be a 1000x difference there I don’t know. The way the two numbers are described made me think the higher one was sort of flukey and the lower one more consistent. But that’s a bit shaky, I admit.
   (2) That’s true about the time of exposure, but I reasoned that the heavier emissions aren’t likely to continue for very long, certainly (well, we hope) not a year. I have been figuring weeks but of course that’s just a fairly uninformed guess. And if the blue number referenced above is more typical, then the yearly exposure would be way less than the normal yearly background dose (I think–just approximating it in my head). But I think the gap between the blue and green numbers is a flaw in the chart–you’re left to just pick one of them without really knowing which is more probable.
   But I certainly didn’t mean to imply that there’s no danger right there at the plant, and in surrounding areas. Probably some of those very brave and dedicated workers are going to get sick. My main point was that it’s probably not something that’s likely to impact people thousands of miles away at all.
   (3) Quite right, and I thought of that later but was too lazy and/or busy to go back and clarify it. The inverse-square point is not totally irrelevant, because news reports were leaving people to imagine that the “plume” would be like a more or less constant density fog, enveloping them with radioactive stuff, whereas most of it would likely be well up in the atmosphere and pretty widely and unevenly distributed. I mean, a radioactive smoke particle two miles up in the air is probably not going to hurt anybody. No single person would likely be very close to very much stuff.
   Your (b) in the last paragraph is for (b)ottom line. 🙂 That is the bottom line explanation for the hysteria–radiation is just really scary, and there are very good reasons for that reaction. But it’s annoyed me that the media focus has been on this extremely remote (to the U.S.) danger, while they’re still pulling bodies out of the mud in Japan.
   Also, I’m not really a proponent of nuclear power but I’m not an opponent, either, and the scariness of nuclear power tends to drive out the worse-in-practice hazards of other energy sources.

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3. Jesse Canterbury

   March 22, 2011

   So going back to (1)…IMO it’s better to work with the larger number because it represents a greater threat to human safety, which is what this hubbub is all about. An hourly dose that’s that much greater than normal is good reason to evacuate the area. Even if other areas aren’t seeing that same level, there’s no guarantee that they won’t, given the proximity to the plant and shifting winds/weather/etc. (The factor of 1000 is due mainly to the units of microsieverts [uSv, hard to type a mu] in the blue part and millisieverts [mSv] in the green, which differ by a factor of 1000.)
   It’s true about the chart — it contains enough flaws so as to necessitate fairly careful examination.
   On to (2): as described in the chart, an ill-defined location at Chernobyl still emits a few mSv/hour, a pretty dangerous level — 25 years after the event. Fukushima is not near as bad as Chernobyl was, but they’re going to be dealing with this for a long time, depending on the extent of damage.
   Speaking of Chernobyl — there is still a lot of uncertainty about what exactly the long term effects are. There are studies saying that cancers linked directly to the Chernobyl event were limited to those directly exposed, while others claim many many more cases due to food & water contamination that is still being found over there. So those “safe” limits on the chart, talking about maximum permissible yearly dose and so forth — I don’t really know what that’s based on. This uncertainty may be present in the fact that only at high levels (100 mSv/year) is radiation exposure definitely linked to cancer, at least as described by that chart. In the face of a lot of uncertainty about something that is dangerous, a high level of caution — but yes, not panic — is warranted.
   I agree about nuclear power, mostly. It stacks up well against other common energy sources in terms of cleanliness, effect on the environment, etc. But the fuel & waste are still pretty nasty things that I don’t think we’ve learned how to do with effectively yet.

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4. Mac

   March 22, 2011

   I don’t feel like I know enough to have a very definite view about (1), because there’s no indication of the duration of the higher level, whether it was associated with some specific transient event, etc.
   There is some interesting stuff in the comments on that chart. And a link to this article at New Scientist. Reassuring to see that the most dangerous isotope has a pretty short half-life (a week, it seems to be saying).
   I know about micro- and milli-, btw. I used to have a job where I actually had to be concerned about how many measured-in-micros (later nano-) instructions I was using. For some reason a fragment of a conversation sticks in my mind–something or other was discovered to be taking a millisecond and someone said “A millisecond is forever.”

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5. Mac

   March 22, 2011

   I mean: when I say I don’t have a definite view on (1), I mean I don’t take a stand on which of those numbers is more likely to be the one that matters most in this particular situation. Certainly if you’re there and making the decision, you want to be very cautious. The important thing about the chart is to establish some perspective.

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6. Jesse Canterbury

   March 23, 2011

   In my mind, using the lower number to say, in effect, “hey look, it’s not that bad” would be a bit irresponsible when much higher levels have already been measured. It’s like saying during a thunderstorm, “sure g’head and run across that open field — chances are you won’t get struck by lightning.”
   Fortunately, the decision-makers are not making their decisions based on a chart put together by a comic-strip writer, however well-educated in science he might be. (We hope that’s the case, anyway.) Presumably those people giving orders for things like evacuations, food quarantines, and the delineation of the various zones, have much more comprehensive data from which to draw conclusions.

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7. Mac

   March 24, 2011

   Of course–I think we are sort of talking crossways here, as I certainly didn’t mean to imply otherwise. This is aimed at the hysteria taking place thousands of miles away. I took that to be xkcd’s point, too. All I meant about (1) was that I don’t know which is the more reliable/typical figure. Or was, I should say, since it’s changing constantly, to wit.

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