Measurement of anthropogenic radionuclides in post-Fukushima Pacific sea water samples

Following the accident at the Dai-ichi Fukushima nuclear power plant in 2011, a vast number of Pacifîc sea water samples from many locations far from Fukushima have been collected by Japanese investigators. Due to dilution, the activities of radionuclides from North Pacifîc sea water samples are very low, which calls for extraordinary measures when being measured. This paper focusses on the metrological aspects of the gamma-ray spectrometry measurements performed on such samples in two underground laboratories; at HADES (by JRC-IRMM in Belgium), and at Ogoya (by Kanazawa University in Japan). Due to many samples and long measurement times, all available HPGe-detectors needed to be employed. In addition to single coaxial detectors, this involved multi-detector systems and well-detectors. Optimization of detection limits for different radionuclides and detectors were performed using Monte Carlo simulations.JRC.D.4-Standards for Nuclear Safety, Security and Safeguard