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Report No.

Development of direct-type alpha dust monitor for in-situ measurement of airborne concentration during fuel debris retrieval and decommissioning of nuclear fuel cycle facilities

Tsubota, Yoichi   ; Honda, Fumiya; Nakagawa, Takahiro ; Kawasaki, Takashi ; Ikeda, Atsushi  ; Tamakuma, Yuki*; Tokonami, Shinji*; Momose, Takumaro  

A large number of radioactive aerosols, especially alpha particles, are expected to be generated during fuel debris retrieval on Fukushima - Daiichi (1F) site and decommissioning of nuclear fuel cycle facilities. For conventional alpha dust monitor, 1-channel of silicon surface barrier detector (SSBD) is often used as a detector which has not so high limit of count rates. Moreover, since radioactive aerosol collected on a filter paper are measured by a detector, it is difficult to calculate an airborne concentration at the sampling site because a conventional dust monitor measures an integrated radioactivity on a filter paper. In this work, we developed a prototype of alpha dust monitor aiming at an in-situ measurement of high concentration airborne alpha particles. The efficiency for U$$_{3}$$O$$_{8}$$ checking source was 80.8%. For high count rate test using 3 MBq of $$^{241}$$Am checking source, 2.1 $$times$$ 10$$^{7}$$ cpm was achieved which was more than 10 times higher compared to an conventional alpha dust monitor using SSBD. For follow-up test to the airborne concentration of alpha particles using radon chamber, counting rate of prototype alpha dust monitor was smoothly followed with respect to the radon concentration in a radon chamber.



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