During the decommissioning of the TEPCO’s Fukushima Daiichi Nuclear Power Station (FDNPS), it is crucial to precisely understand the distribution of dose rates and radioactive materials (hereinafter referred to as radiation distribution) in the decommissioning work environment to reduce worker exposure and efficiently plan decontamination. In this study, we focused on utilizing plastic scintillation fibers (PSFs) to measure the radiation distribution in the FDNPS decommissioning work environment. We developed a "wavelength-resolving analysis method" that measures the radiation distribution in high-dose rate environments, such as the interior of FDNPS reactor building, by analyzing the wavelength (color) of the scintillation light generated by the interaction between the PSF and radiation. This method utilizes the wavelength dependency of light attenuation within the fiber for the radiation distribution measurement. Preliminary evaluations at gamma irradiation facilities have confirmed that the method can be applied at high-dose rates of up to 10 Sv/h, which is suitable for the decommissioning environment.
 Figure 1 shows a demonstration setup and results of the radiation distribution measurement inside the FDNPS Unit 2 reactor building. The PSF was laid in the high-dose rate area of the air-conditioning room of the Unit 2 reactor building. The wavelength spectrum was measured by a spectrometer placed in a low dose rate area outside the room. By analyzing this spectrum, we obtained the radiation distribution along the PSF. As shown in Fig. 1c, the reverse estimation result of radiation distribution agreed well with the actual dose rate distribution trend (with a maximum dose rate exceeding 100 mSv/h) obtained using a survey meter; the actual trend shows a sharp increase towards the end of the fiber. This agreement validates the effectiveness of this sensor in the FDNPS high-dose rate environment.

This work was supported by JSPS KAKENHI Grant Number JP22K18129.