Radionuclide metrology methods and analysis of the Joint Danube Survey 4 sediment samples

This low-level radionuclide metrology research work was carried out within the Joint Danube Survey 4 (JDS4), coordinated in 2018-2020 by the International Commission for the Protection of the Danube River (ICPDR). The gamma-emitting radionuclides of the sediment samples were analysed by low-level gamma-ray spectrometry. The activity concentration of ⁹⁰Sr was determined by liquid scintillation counting (LSC) after isolating the radio-strontium using a new radiochemical separation method. The results of the radiometric analysis of ⁹⁰Sr, ¹³⁷Cs and naturally occurring radionuclides ⁴⁰K, ²¹⁰Pb, ²²⁶Ra, ²²⁸Ra in recent riverbed sediment are presented and discussed focused on public health.

Delivery mechanism of (134)Cs and (137)Cs in seawater off the Sanriku Coast, Japan, following the Fukushima Dai-ichi NPP accident

To assess the delivery mechanism of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we examined vertical profiles of (134)Cs, (137)Cs, and (228)Ra concentrations and the (228)Ra/(226)Ra ratio in the water columns off the Sanriku Coast in the northwestern Pacific Ocean, in July 2012, along with their surface lateral variations in July 2009. Radiocesium concentrations exhibited maximum peaks (3-5 mBq/L for (134)Cs) at depths of 100-200 m, accompanied by high (228)Ra concentrations (0.6-0.8 mBq/L) in comparison with shallower depths (∼0.4 mBq/L). Taking the circulation patterns of currents in the area into account, it was inferred that radioactive depositions were supplied to the (228)Ra-rich Tsugaru Warm Current Water (TWCW) in the offshore area of the Sanriku Coast following the FDNPP accident, and that after the spring of 2011, this water (∼26.5σθ) was covered by lower density surface water, which helped intrude its way to depths of 100-200 m.