Nuclear plant workers suffer internal radiation exposure after visiting Fukushima
The government has discovered thousands of cases of workers at nuclear power plants outside Fukushima Prefecture suffering from internal exposure to radiation after they visited the prefecture, the head of the Nuclear and Industrial Safety Agency said.
Most of the workers who had internal exposure to radiation visited Fukushima after the nuclear crisis broke out following the March 11 quake and tsunami, and apparently inhaled radioactive substances scattered by hydrogen explosions at the Fukushima No. 1 Nuclear Power Plant.
The revelation has prompted local municipalities in Fukushima to consider checking residents' internal exposure to radiation.
Nobuaki Terasaka, head of the Nuclear and Industrial Safety Agency, told the House of Representatives Budget Committee on May 16 that there were a total of 4,956 cases of workers suffering from internal exposure to radiation at nuclear power plants in the country excluding the Fukushima No. 1 Nuclear Power Plant, and 4,766 of them involved workers originally from Fukushima who had visited the prefecture after the nuclear crisis. Terasaka revealed the data in his response to a question from Mito Kakizawa, a lawmaker from Your Party.
The Nuclear and Industrial Safety Agency said it received the data from power companies across the country that measured the workers' internal exposure to radiation with "whole-body counters" and recorded levels of 1,500 counts per minute (cpm) or higher. In 1,193 cases, workers had internal exposure to radiation of more than 10,000 cpm. Those workers had apparently returned to their homes near the Fukushima No. 1 Nuclear Power Plant or had moved to other nuclear power plants from the Fukushima No. 1 and 2 nuclear power plants.
Now, what are 'counts per minute', and can we convert them to an absorbed dose? A counts per minute reading is a measure of radioactivity. It is the amount of photons a geiger counter is able to detect after entering the gm tube of the device (calibrated to Cs-137). Each photon is presumed to have arisen from the decay of a radioactive isotope.
Although the 'counts per minute' value tries hard to indicate the amount of disintegrations per minute coming from a radioactive source, the two terms should not be confused. Most geiger counters have limitations, and will not detect all disintegrations taking place from a source, but will give you a good idea of the radioactivity originating in air, or coming from an object.
Nuclear disintegrations taking place within an atom are tied to the definition of the Becquerel. One becquerel = one disintegration per second. 60 counts per minute, theoretically, indicates 1 Becquerel of activity.
The readings mentioned in the article above stated a range of 1,500 to 10,000 cpm. Thus, they are reading between 25 to 166 becquerel of activity (assuming whole body), per person. The exact isotopes that NISA equipment is sensitive to is not mentioned in the article, but is also probably rated for Cesium.
We have the inferred activity, but what doses are these people getting? Take a look at the specifications stated on the back of a Radalert geiger counter:
For Gamma, the device detects x-rays down to 10 keV through end window, or 40 keV through case. Calibration is 1000 CPM = 1 mR Cs-137 per hour (indicated in the picture above). So for example, if we were measuring cpm with this particular unit, we will be able to establish a range of absorbed dose readings based on what was mentioned in the article. Most geiger counters are calibrated similarly, thus these people are receiving:
1500 cpm = 1.5 milliREM per hour. That is 15 microSv/hr, or 131 milliSv/year.
10,000 cpm = 10 milliREM per hour. That equals 100 microSv/hr, or 876 milliSv/year.
Some of these people will break 1 Sievert a year if readings are calculated from just part of the body and/or do not take all isotopes into account.
How's that "nobody has exceeded 250 milliSievert" claim doing, Tepco? Many of these guys most assuredly have, or will exceed that dose very soon now.
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Recommended exposure limits from the NRC:
"The NRC adopted the 100 mrem per year dose limit from the 1990 Recommendations of the International Commission on Radiological Protection (ICRP). The ICRP is an organization of international radiation scientists who provide recommendations regarding radiation protection related activities, including dose limits. These dose limits are often implemented by governments worldwide as legally enforceable regulations. The basis of the ICRP recommendation of 100 mrem per year is that a lifetime of exposure at this limit would result in a very small health risk and is roughly equivalent to background radiation from natural sources (excluding radon) (ICRP, 1991).100 mrem/yr = 1 mSv/yr
Good story. Not a lot of people have picked up on this. One thing I would point out: the body eliminates radioactive elements according to those elements' biological half life. The biological half-life is different to the radiological half-life. For cesium, the biological half-life is 90 days, so after 90 days your body gets rid of half of the ingested cesium. Still, 10000cpm is a big dose. Those guys would not be able to work anywhere near a nuclear plant again for a long time.
ReplyDeleteThe CPM depnds on the surface of measument : 10,000 CPM on a surface of 1m2 is the same as 1 CPM on a surface of 1 cm2.
ReplyDeleteValues on workers are "whole body counts" so the measurement area is huge compared the small area used in a cheap portable sensor. So using the 1500 cpm = 1.5 mRem/h conversion factor is irrelevant.
To give you some perspective, eating a banana and you ingest 20 Bq, or 1200 cpm. A scintigraphy will get you more than 10 million Bq, or 600 million cpm.
Natural radioactivity of anybody is from 5000 to 20,000 Bq, or 300,000 to 1.2 million cpm, essentially from radioactive potassium.
So the Fukushima Dai ichi workers' 10,000 cpm is definitely not a health concern.
Yes, determining doses are not simply reliant upon calculating isotope decay. The biological half lives of various isotopes are also a factor and would contribute to the decline in total dose. I've posted about this before (http://falloutphilippines.blogspot.com/2011/04/breast-milk-from-woman-in-chiba.html), but i'm confident that these doses are still in the ballpark. These people were subjected to ingested isotopes of unknown initial activity that are now entrenched systemically within the body. The radioactivity is exposing critical organs directly, as opposed to an external dose where skin and muscle would attenuate much of the radiation. What is most perturbing, however, is what was stated in the article here - "In 1,193 cases, workers had internal exposure to radiation of more than 10,000 cpm". What's the upper limit? It could be 10,010cpm or 100,000. All bets are off calculating doses in these cases because we just don't know the high end.
ReplyDeleteIn any case it's difficult to calculate an accurate dose value without understanding exactly which isotopes they were exposed to. The article is claiming that these figures are whole body, but it could very well be a reading from just a part of the body. We are not in a position to quality control their equipment or methodology so just have to take their word for it.
They could get an accurate thyroid reading and work HL's retroactively to help ascertain the initial I-131 exposure. That would help determine the Iodine contribution to the equation and work out a total anticipated thyroid dose, which i would imagine in at least some of the cases to be pretty significant.
Jean, the daily yomiuri, in a similar story, states that "normal internal radiation level would range from several hundred cpm to 1,000 cpm" (http://www.yomiuri.co.jp/dy/national/T110518006065.htm).
ReplyDeleteSince we're discussing bioassays here, please read what the NRRPT guys (who reference the ICRP) have to say:
"Co-60 and Cs-137 are two nuclides that make up a large portion of most radioactive profiles. These isotopes also put out gamma radiation that is easily seen by the whole body counter.
We need to scale in hard to detects for a couple of reasons.
The “hard to detects” (alpha, beta, low activity gamma) cannot be seen by the whole body counter because it can’t penetrate outside the body or it’s just too low activity that it’s washed out by background, other nuclides, or noise.
The “hard-to-detects” are the major dose contributor. A small fraction of alpha emitters may give 50% of the exposure." Also prvided was this conversion - 810 Bq = 4860 dpm
200 mSv = 2 rem
(http://www.nrrpt.org/File/Practical_Whole_body_counting_and_Internal_dosimetry(1).ppt)
We know alpha is not a factor here, but most wbd's are detecting photons from 200kv on up, eliminating a massive range of energies contributing to exposure.
And indeed, Cs 137's beta, largely undetectable by whole body counter devices, is probably the biggest dose contributor of the lot.
I am all for others helping to get to the bottom of this and appreciate your input, but citing sources would help.
Nice one. That newspaper redacted the story. Click on the link and you can see that it is nothing like the original version.
ReplyDeleteThe "banana" equivalent dose comparisons have been thoroughly debunked as bad science used by the nuclear industry to mislead. Maybe you are just repeating what you learned, but it is medically incorrect. The potassium isotopes in a banana make up a small portion of the total (non-radioactive) potassium in the banana. This total potassium, in turn, makes up a tiny portion of your body's own potassium reserves. The ratio of isotopic potassium to non-isotopic potassium is the same in the banana as it is in your body, which maintains homeostatic levels of total potassium. Therefore, eating a million bananas will not increase your body load of radiation one little bit. With cesium 137, you should not be eating or inhaling it, and it increases the ratio of radioisotopes to the natural potassium that it mimics in the body. That may seem confusing, but it is 100% true.
ReplyDeleteUnfortunately, the Fukushima incident slowly became so much like the Chernobyl incident. I just hope that they would be able to resolve the Fukushima leakage already.
ReplyDeleteThis is a new idea for me, I have not careful solar radiation until an upcoming evaporation project we are installing a net pyrometer to estimate evaporation. dosimeter badges.I would like to potentially use this data for other purposes and will lookthrough some of these papers. It is interesting, thankyou.
ReplyDelete