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.
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