Muncipal Inspection Services for Radioactivity in Food For Self Consumption in Fukushima and Utilizaition of State-Of-The-Art Equipment for Testing

抄録

Since the accident at the Fukushima Daiichi NPS, food inspection for radioactivity in home-grown products and wild foods collected by residents for their own consumption has been conducted in Fukushima. To inspect these foods, 300 temporary inspection stations were set up in public halls or assembly halls in municipalities. Due to a decrease in the number of inspections required, some laboratories have been consolidated or closed, but inspections are still ongoing. The conventional gamma-ray spectrometry technique using a NaI (Tl) scintillation spectrometer was utilized for the testing. Since the scintillation detector is inferior to the Ge detector in energy resolution, a screening method was applied for the inspection. For conventional radioactivity measurement in foods, however, sample preparation procedures, such as cutting and machining techniques, are required to homogenize radioactivity. Therefore, it is difficult to use measured cut samples for self-consumption even if radioactivy are not detected. Under such situations, municipal laboratories were strongly requested by residents to return the measured samples for their own consumption. In response to such a demand, state-of-the-art device named “non-destructive radioactivity measurement device” was launched to measure radioactive cesium in whole samples contained in a plastic bag so that all products can be inspected without any sample preparation procedures (Ishii, 2015). The device was installed in municipal food inspection stations in 2015. A total of 83835 samples were tested in 2015, of which 68 % were measured with the non-destructive radioactivity measuring devices. Although the number of tests decreased year by year down to about 18000 samples in 2022, 84 % of all samples were tested with non-destructive devices. The test pass rate (<50 Bq kg^-1) for about 350000 samples since 2015 has exceeded 90%, indicating that efficient screening tests have helped the residents. In case of using such non-destructive measurement devices, it would be important to determine the approximate detection efficiency for samples of various sizes and shapes, within acceptable uncertainty, only from the information on mass and type of the sample. The performance evaluation of such devices had been conducted by Yamada et al. (2023). Furthermore another approach using artificial radioactive samples was adapted.to determine possible uncertainties arising from variation of sample shapes. As a preliminary result, 10 % or less standard uncertainty due to variation of sample shapes were observed.