Internal contamination with radionuclide

Now, at more than 7 months after Fukushima Nuclear Power Plant Accident, public concern is on the internal radiation exposure (IE) as well as external one. Here explained are fundamentals for correctly understanding and assessing IE. IE dose in the unit Sv requires complex calculation on physical and biological dynamics of the radionuclide and its assessment is performed with the committed effective dose (CED). CED is the time-integrated dose rate (t=50 and 70 y in adult and child, respectively; the whole life) of total of each organ or tissue. Estimation of IE can be conducted through actual radioactivity measurement by smearing of nasal cavity, bioassay (measuring radioactivity in excreta) and external whole body counting. The first and second ones are used for measuring alpha- and beta-emitters. The whole body counter has a detector of NaI (Tl) scintillator or Ge-semiconductor to analyze the energy spectrum of X- or gamma-ray, necessary for radionuclide identification. Counting the radioactivity in the whole human body is done with the counter correctly calibrated using phantom. CED is calculated from the measured radioactivity (Bq) with consideration of its ingested time/route and effective half life of the nuclide with use of its coefficient given in International Commission of Radiological Protection (ICRP) publication 68. At Fukushima Accident, National Institute of Radiological Sciences (NIRS) and Japan Atomic Energy Agency (JAEA) conducted the assessment of CED in related residents. NIRS calculated CED assuming that IE had begun on Mar. 12, 2011 (the maximum IE assumption), and obtained the result that CED >1 mSv was in none of 109 people tested. In addition, analysis of urinary Cs-137 measured by NIRS also revealed that none had >1 mSv eq. IE of the nuclide. (T.T.)