EVALUATION OF VEGETABLES IN TSUKUBA FOR CONTAMINATION WITH RADIOACTIVE MATERIALS FROM THE ACCIDENT AT FUKUSHIMA DAIICHI NUCLEAR POWER PLANT

A large amount of radioactive material was released into the atmosphere after the accident of the Fukushima Daiichi Nuclear Power Plant following the Tohoku earthquake on 11 March 2011, and traces of these materials were detected in Tsukuba. Because radioactive materials can adhere to vegetables, the authors made a qualitative evaluation of vegetables in Tsukuba, estimated internal exposure dose based on quantitative measurement results, and investigated several decontamination methods. Qualitative analysis of vegetable contamination was done by autoradiography. Quantitative analysis was done using a high-purity germanium detector. To assess decontamination, two methods were tested: one with running water and the other with boiling water. In addition, boiled soup stock was measured. In the qualitative evaluation by autoradiography, radioactive materials were not uniformly distributed but adhered to vegetables in clumps and hot spots. In the quantitative evaluation to measure contamination of outer and inner leaves of sanchu lettuce, it was observed that the concentration of I-131 was 8,031.35 +/- 764.79 Bq kg(-1) in the outer leaves and 115.28 +/- 20.63 Bq kg(-1) in the inner leaves. In addition, the concentration of Cs-137 was 1,371.93 +/- 366.45 Bq kg(-1) in the outer leaves and 9.68 +/- 15.03 Bq kg(-1) in the inner leaves. This suggests that one can greatly reduce internal exposure dose by removing the outer leaves if one has to eat vegetables just after a nuclear accident. In the decontamination assessment, a decontamination efficiency of up to 70% was achieved by boiling vegetables for 20 min.