Experiences of Population Monitoring Using Whole-Body Counters in Response to the Fukushima Nuclear Accident

Abstract

Whole-body counter measurements of residents of Fukushima Prefecture have been extensively performed after the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011. These measurements have demonstrated that the levels of internal contamination with radioactive cesium (Cs and Cs) in the residents are very low. This article provides an overview of and lessons learned from these whole-body counter measurements with emphasis on the technical problems encountered, and it discusses the effective use of whole-body counters for assessing the internal thyroid doses of individuals when direct measurements of I in the thyroid are difficult or impossible to implement for the total affected population in a short time after a nuclear reactor accident. The application of this dose reconstruction method requires determining the intake ratio of I to cesium isotopes at appropriate times and considers the short biological half-lives of cesium isotopes, in particular for children.

Thyroid Doses in Children from Radioiodine following the Accident at the Fukushima Daiichi Nuclear Power Plant

“…In WBC measurements over the period between June 27 and July 28, 2011, the ratios of subjects with the presence of both 134 Cs and 137 Cs in adults and children were 28.8% and 4.1%, respectively [21]. An overestimation of the cesium intake is also likely to occur in children based on an acute intake scenario on potentially the earliest day [17]. On the other hand, it has been found that the cesium detection rate tends to be higher in late evacuees from areas near the FDNPP in the case of adult subjects, suggesting that a trace of the early intake would appear in the WBC measurements until about half a year after the accident [22,23].…”

“…These measurements have been continued to the present; the number of measurements has reached 341,430 as of September 2019 (http://www.pref.fukushima.lg.jp/site/portal/ps-wbc-kensa-kekka.html). Details on the WBC measurements conducted in the prefecture have been described elsewhere [17].…”

“…However, one thing for sure is, as experienced in the past nuclear accidents, that the number of people to be examined regarding their radiation exposure can be overwhelmingly large. It is thus crucial for each county to establish and maintain a steady and effective population monitoring system as a part of a national-level scheme for radiation emergency response [17].…”

Individual Doses to the Public after the Fukushima Nuclear Accident

“…Technical issues raised for internal dose assessment by WBC of Fukushima Prefecture residents were well summarized by Kurihara et al [67]. The issues can be categorized as follows: (1) setting a suitable intake scenario to estimate intake amount from measured 134 Cs and 137 Cs body content (acute or chronic, inhalation or ingestion); (2) avoiding interference by the natural radionuclide 214 Bi in 134 Cs detection (both radionuclides emit similar energies of gamma rays, which makes spectrum analysis difficult); (3) standardizing calibration and measurement procedures; (4) establishing a method for estimating 134 Cs and 137 Cs body contents of children (whole-body counters were originally designed for measuring adult radiation workers); and (5) avoiding effects on measurements due to body surface contamination.…”

“…Considering such con-servative assumptions, internal doses estimated by WBC could be more reliable. Although estimation from WBC has issues related to the intake scenarios, the acute and chronic intake scenarios were considered both rational and appropriate assumptions for estimating individual internal doses to large populations from a single WBC measurement result [67].…”