Sheltering is one of the countermeasures for protection against radiation exposures in nuclear accidents. The effectiveness of sheltering is often expressed by the reduction factor, that is the ratio of the indoor to the outdoor cumulative radioactivity concentrations or doses. The indoor concentration is mainly controlled by the air exchange rate, penetration factor, and indoor deposition rate. The penetration factor and indoor deposition rate depend on the surface and the materials and structure of windows and doors as it is these openings in the building envelope that control penetration. We investigated experimentally these parameters of I2 and particles. The experiment was performed in two apartment houses, three single-family houses, and chambers. The obtained penetration factor ranged from 0.3 to 1 for particles of 0.3–1 μm and 0.15–0.7 for I2 depending on the air exchange rate. The indoor deposition rate for a house room ranged from 0.007 to 0.2 h−1 for particles of 0.3–1 μm and 0.2–1.5 h−1 for I2.
A method to estimate the surface concentration of radon decay products 214 Pb and 214 Bi from gamma dose rate changes due to rainfall has been developed. The conversion from the dose rate to surface concentration was performed by a dose rate model considering geometric conditions such as buildings and ground undulations around each monitoring station (MS). This method of calculating dose rate increase (DRI) was successful in reproducing the difference in the shape of the DRI decrease due to differences in the initial 214 Bi/ 214 Pb. The estimated concentration exhibited differences among the MSs despite uniform depositions. This difference was considered to have been caused by surface run-off and infiltration of radionuclides.
Air concentrations of 133 Xe, 131 I, 132 I, 133 I, 132 Te, 134 Cs, 136 Cs, and 137 Cs in the early stage of the Fukushima Daiichi Nuclear Power Station (FNPS1) accident were estimated for six locations in Ibaraki Prefecture from pulse height distributions measured with NaI(Tl) scintillation detectors continuously operated as a part of monitoring station systems. Air concentration of 133 Xe in Ibaraki was estimated for the first time, which showed the maximum value of 150 kBq m −3 in the plume arrived at the southern part of Ibaraki around noon of 15 March 2011. The plume was found to consist mainly of noble gases without other nuclides at significant level and to correspond to the hydrogen explosion at Unit 3. Spatiotemporal distributions of 131 I and other nuclides were discussed for the plumes detected during the periods of 15-16 and 20-21 March. Variations in radionuclide composition among the plumes and within each plume were also discussed.
A method of estimating surface radioactivity concentrations of key anthropogenic radionuclides from NaI(Tl) pulse-height distribution observed at a monitoring station (MS) was discussed. In the estimation, a realistic assumption on geometric distribution of source and obstacles around the detector of the MS including the infiltration of radionuclides into the ground was used and the results were compared with ones with a commonly used assumption of a uniformly distributed plane source. The surface radioactivity concentration was determined by comparing the count rates at the full-energy peak ranges between observation and calculation with an electron-photon transport code EGS5. It was shown that the estimated absolute values of concentration differed by a factor of ∼1.5 depending on the assumption of infiltration depth. The estimated surface concentrations of (131)I, (134)Cs and (137)Cs were in good agreement with ones determined by the in situ measurements with an HPGe detector and the cumulative values of daily surface depositions.
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