Speaker
Description
Reliable gamma-ray spectrometry is essential for radiation work management and radioactive waste characterization at the Fukushima Daiichi Nuclear Power Station (FDNPS). However, the measurement requirements vary significantly depending on the target scenario. In high-dose-rate areas inside reactor buildings, portable spectrometers must maintain spectral performance under severe count-rate conditions. In contrast, for collected samples such as smear paper samples, rapid and precise quantification of radioactivity is required. Therefore, gamma-ray spectrometric assessment at FDNPS should be considered not as the application of a single detector, but as the integrated management of multiple spectrometric approaches according to measurement purpose and environment.
To address the in-situ measurement challenge, we have developed a portable high-dose-rate gamma-ray spectrometer using a small 3 mm cubic CeBr3 scintillator combined with a 1 Gsps digital signal processor. The system was designed for use in reactor building working environments where conventional analog processing suffers from pulse pile-up and degradation of spectral information. Irradiation tests were conducted to evaluate the spectral response of the developed spectrometer under high-dose-rate conditions in a 137Cs irradiation field. The results showed that, by adopting the small-volume scintillator, the developed system achieved a dead time of only 2% even at 100 mSv/h, demonstrating that useful gamma-ray spectral information can be retained in dose-rate regions where portable gamma-ray spectroscopy becomes difficult with conventional systems.
As a complementary example of ex-situ gamma-ray assessment, rapid analysis of smear paper samples collected from the FDNPS Unit 5 reactor building drywell was performed using a large-volume LaBr3 spectrometer with a 3-inch diameter scintillator. In non-accident units such as Unit 5, it is important to quantify 60Co that has existed since before the accident, in contrast to accident-derived radionuclides that dominate in damaged units. Although the detector itself is commercially available, the measurement provided a practical case of rapid nuclide evaluation for real samples from the FDNPS decommissioning field. The results showed that, for 60Co in smear samples, a detection limit below 4 Bq/cm2, which is the criterion for material carry-out, was achieved by employing the large-volume scintillator. In the presentation, we will discuss the practical workflow for decommissioning-related gamma-ray assessment.