Minamisoma, Dose Reconstruction Exercises

The recent JAMA letter discussing radiation exposure among people in the city was quickly heralded by the western media as “proof” Fukushima exposures were low. We explained here how these scans were instead a very small slice of the total picture of exposure.

The data used in the study discussed in the JAMA letter is partially publicly available via the Minamisoma municipal website.

We looked at some of the body scans done for local residents. One that was in a higher range for children under high school age was used to do a hypothetical dose reconstruction. The common thought has been that people were exposed during the first days and weeks of the disaster. During this time reactors exploded and radioactive steam leaked continually. On March 14th unit 3 at Fukushima Daiichi exploded. The plume from this explosion eventually wafted over Minamisoma. We used March 14, 2011 as the exposure date for our hypothetical dose reconstruction. These are for cesium exposure only and do not include other exposures such as iodine 131, strontium, plutonium and many other isotopes people in the region are being exposed to.

Scans were done with a Whole Body Counter machine (WBC), read more about Whole Body Counters here.
Units are in becquerels, read more about becquerels here.
Biological half life is the time it takes for half of the isotope inside the body to be eliminated out of the body. This varies based on age and personal metabolism.

We used a pair of WBC scans from a child younger than high school age from Minamisoma. The original scan was done in September 2011.
9.8.2011
test data: Cesium134= 1230bq Cesium137= 2653bq
10.27.2011
test data: Cesium134= 1228 Cesium137= 1758bq

Hypothetical Dose Reconstruction #1:
Done as assuming an adult with a biological half life of 70 days.
intake date assumed: 3.14.2011
biological half life: 70 days
hypothetical dose reconstruction of: 16,000 bq Cesium 137 in March 2011
hypothetical dose reconstruction of: 7400 bq Cesium 134 in March 2011
total combined hypothetical dose reconstruction: 23,400bq combined cesium.
*the second scan for Cesium 137 on October 27 should be higher, about 1900 bq, not 1758bq
*the second reading for Cesium 134 on October 27 is too high, should be about 900bq, not 1228 bq

————————
Hypothetical Dose Reconstruction #2:
Done as assuming a child with a biological half life of 42 days.
intake date assumed: 3.14.2011
biological half life: 42 days
hypothetical dose reconstruction of: 42,448 bq Cesium 137 in March 2011
hypothetical dose reconstruction of: 20,000 bq Cesium 134 in March 2011
total combined hypothetical dose reconstruction: 62,448bq combined cesium.

——————

Hypothetical Dose Reconstruction #3:
Done as assuming a child with a biological half life of 140 days.
intake date assumed: 3.14.2011
biological half life: 140 days
hypothetical dose reconstruction of: 7,000 bq Cesium 137 in March 2011
hypothetical dose reconstruction of: 3,000 bq Cesium 134 in March 2011
total combined hypothetical dose reconstruction: 10,000bq combined cesium.
*this one is an outlier as 140 days is not assumed to be a very common biological half life yet.

Results:
These hypothetical dose reconstructions give a range of possible initial exposures of between 42,000 bq (children) and 7,000 bq (old adults) for Cesium 137. For Cesium 134 the range would be 20,000 bq (children)  to 3,000 bq (old adults). A combined cesium range of 62,448 bq (children) to 10,000 bq (old adults).

Conclusions:
The reconstructions above suggest that some residents’ initial internal contamination with cesium-134 and cesium-137 might have been greater than the results of Tsubokura and others (2012) may indicate, if the isotopes had been incorporated only briefly after the reactor explosions. By contrast, the greater than expected whole body count observed on Oct. 27 in our case implies that the incorporation of the isotopes may have continued over a lengthy period of time. For the estimation of total committed doses residents received, exposure to other incorporated radionuclides must be accounted for, and exposure doses from external radiation sources remain to be added. If the exposures in Minamisoma were not large initial exposures but ongoing exposure, the sources need to be identified and solved. Because so little is actually understood about the disaster & exposures, a conservative approach would consist of providing assistance to any resident who wishes to leave.

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