Mutsuo Inoue

134Cs and 137Cs activities in coastal seawater along Northern Sanriku and Tsugaru Strait, northeastern Japan, after Fukushima Dai-ichi Nuclear Power Plant accident

A total of 37 seawater samples were collected at 10 sites along the coastline of the Northern Sanriku and Tsugaru Strait, 250-450 km north of the Fukushima Dai-ichi Nuclear Power Plant in April-December 2009 and May-June 2011, and analyzed for (134)Cs and (137)Cs activities using low-background γ-spectrometry. The (134)Cs and (137)Cs activities measured in these samples in May 2011 were found to be 2-3 mBq/L and 2.5-4 mBq/L, respectively. By June, these values had decreased by 25-45%/month and 5-30%/month, respectively. These results can be plausibly explained by surface infusion of these isotopes into the sea by atmospheric transport from Fukushima and their subsequent reduction by water migration to off-shore and deeper regions.

Lateral variation of 134Cs and 137Cs concentrations in surface seawater in and around the Japan Sea after the Fukushima Dai-ichi Nuclear Power Plant accident.

A total of 82 surface seawater samples was collected in the Japan Sea and the southwestern Okhotsk Sea before and after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Analysis of (134)Cs and (137)Cs concentrations using low-background γ-spectrometry revealed that the (137)Cs concentration of the samples collected in June 2011 was 1.5-2.8mBq/L, which is approximately 1-2 times higher than the pre-accident (137)Cs level, while the (134)Cs concentration was less than detectable to 1mBq/L. In addition to (134)Cs being clearly detected (∼1mBq/L), (137)Cs concentration in water samples from the northeastern Japan Sea (2-2.8mBq/L) was also higher than that from the coast in the southwestern Japan Sea (∼1.5mBq/L). These higher concentrations in the northeastern Japan Sea could be ascribed to the atmospheric transport of nuclides from the FDNPP as aerosols and subsequent transport and dilution after delivery to the sea surface.

Spatial variations of low levels of 134Cs and 137Cs in seawaters within the Sea of Japan after the Fukushima Dai-ichi Nuclear Power Plant accident

Our method based on low background γ-spectrometry enabled the measurement of low radiocesium concentrations in only 20 L of seawater. In May 2011 after deposition of radiocesium, (134)Cs concentration in surface water within the Sea of Japan was confirmed to be significantly small (<0.1-1 mBq/L) by the method. The concentration was not detected (<0.1 mBq/L) below 50 m depth. The Fukushima-derived radiocesium migrated from the surface water of the Sea of Japan without advection to below the thermocline.

228Ra/226Ra ratio and 7Be concentration in the Sea of Japan as indicators for water transport: comparison with migration pattern of Fukushima Dai-ichi NPP-derived 134Cs and 137Cs

To assess the migration patterns of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we analyzed (228)Ra/(226)Ra ratios and (7)Be concentrations and compared them with (134)Cs and (137)Cs concentrations in seawater samples collected within the Sea of Japan before and after the FDNPP accident (i.e., during the period 2007-2012) using low-background γ-spectrometry. The (228)Ra/(226)Ra ratios in surface waters exhibited lateral and seasonal variations, reflecting the flow patterns of surface water. This indicates the transport patterns of the FDNPP-derived radiocesium by surface water. Cosmogenic (7)Be (half-life: 53.3 d) exhibited markedly high concentrations (5-10 mBq/L) at depths shallower than 50 m, with concentrations decreasing steeply (0.2-2 mBq/L) at depths of 50-250 m. The distribution of (7)Be concentrations suggests that the downward delivery of the FDNPP-derived radiocesium to below 50 m depth was negligible for a few months prior to its removal from the Sea of Japan.

Spatial variations of 226Ra, 228Ra, 137Cs, and 228Th activities in the southwestern Okhotsk Sea

We collected 14 water column seawater samples in the southwestern Okhotsk Sea and 7 surface samples around the northern area of Hokkaido Island, Northern Japan, and employed low-background γ-spectrometry with convenient minimal radiochemical processing to determine the activities of (226)Ra (half-life t(1/2)=1600 y), (228)Ra (5.75 y), (137)Cs (30.2 y), and (228)Th (1.91 y) in the samples. Activities of (226)Ra (~2.3 mBq/L), (228)Ra (~0.7 mBq/L), and (137)Cs (~1 mBq/L) of surface waters on the Okhotsk Sea side show notable differences from those on the Japan Sea side (Soya Warm Current Water; SWCW) (~1.5 mBq/L; 1.5-2 mBq/L; 1.4-1.6 mBq/L), indicating their different origins and lateral mixing patterns. All of these nuclides exhibit unique vertical profiles; activities of soluble (226)Ra, (228)Ra, (137)Cs, and reactive (228)Th exhibit small variations from 50 to 500 m depth ((226)Ra, ~2.2 mBq/L; (228)Ra, ~0.4 mBq/L; (137)Cs, ~1 mBq/L; (228)Th, ~0.13 mBq/L). These profiles can be explained by the convective mixing of surface water such as the East Sakhalin Current Water (ESCW) to this layer.

Opening of the closed water area and consequent changes of 228Ra/226Ra activity ratios in coastal lagoon Nakaumi, southwest Japan

In Lake Nakaumi, the second largest coastal lagoon in Japan, artificially closed (Honjyo) area, which was left untouched for 28 years, was partly opened in May, 2009. (228)Ra/(226)Ra ratio of waters in Honjyo area and Lake Nakaumi showed a well-tuned seasonal variation exhibiting high value in summer. After the opening event, however, the (228)Ra/(226)Ra ratios in the Honjyo water showed an unclear seasonal variation in both surface and deep water. This opening event caused the change of active movement of lake and marine water.

Application of low background gamma-ray spectrometry to environmental monitoring samples: Water leaching treatment for 40K-removal

In the usual measurements of radionuclides in the environmental samples by g-ray spectrometry, Compton scattering of 1461 keV g-rays from 40K severely interferes with the detection of artificial radionuclides in marine and agricultural products. In order to eliminate the interference of 40K, we have developed a simple and convenient “water leaching treatment” method applicable to seaweed (sargasso) samples. By this treatment, over 98% of the potassium in seaweed samples is removed without notable losses of artificial and/or natural radionuclides. In combination with the low-level g-ray counting in the Ogoya underground laboratory, the detection limit could be improved by ~1 order of magnitude.

Spatial variations of 226 Ra, 228 Ra, 134 Cs, and 137 Cs concentrations in western and southern waters off the Korean Peninsula in July 2014

We examined the spatial distributions of 226Ra, 228Ra, 134Cs, and 137Cs concentrations (activities) in seawater off the western and southern Korean Peninsula in July 2014. Radium-228 (and 226Ra) concentrations in water samples varied widely from 5 to 14 mBq/L (2-4 mBq/L), showing a negative correlation with salinity, particularly at the surface off the western Korean Peninsula. This indicates that the seawaters in this area are fundamentally comprised of 228Ra-poor and high-saline Kuroshio Current water and 228Ra-rich and low-saline water (e.g., continental shelf water), with various mixing ratios. Although Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived 134Cs was below the detection limit (<0.08 mBq/L) in waters off the western Korean Peninsula, low level 134Cs (0.1-0.2 mBq/L) was detected in waters off the southern Korean Peninsula accompanied by higher 137Cs concentrations (1.6-1.9 mBq/L) relative to that off the western Korean Peninsula. Combined with the lower radium concentrations, the detection of 134Cs is explained by mixing of FDNPP-derived radiocesium-contaminated Kuroshio Current water.

Vertical profiles of Fukushima Dai-ichi NPP-derived radiocesium concentrations in the waters of the southwestern Okhotsk Sea (2011–2017)

We examined the vertical 134Cs and 137Cs concentration profiles in the southwestern Okhotsk Sea in 2011, 2013, and 2017. In June 2011, atmospheric deposition-derived 134Cs from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) was detected at depths of 0-200 m (0.06-0.6 mBq/L). In July 2013, 134Cs detected at depths of 100-200 m (∼0.05 mBq/L) was ascribed to the transport of low-level 134Cs-contaminated water and/or the convection of radioactive depositions (<0.03 mBq/L at depths of 0-50 m). In July 2017, 134Cs was detected in water samples at depths above 300 m (0.03-0.05 mBq/L), and the inventory, decay-corrected to the FDNPP accident date, exhibited its maximum value (85 Bq/m2) during this period. Combining temperature-salinity data with the concentrations of global fallout-derived 137Cs led to a plausible explanation for this observation, which is a consequence of re-entry of FDNPP-derived radiocesium through the Kuril Strait from the northwestern North Pacific Ocean to the Okhotsk Sea and subsequent mixing with the south Okhotsk subsurface layer until 2017.

Radiocesium in the swash zones off the coast of the Japan Sea

Radiocesium concentrations were measured in seawater and sediment samples collected in the swash zones in Ishikawa and Niigata prefectures, off the coast of Japan Sea opposite to the side where TEPCO Fukushima dai-ichi Nuclear Power Plant (FDNPP) is located in September 2016 and August 2017, five to six years after the accident. Cs-134 in the seawater samples was detected, suggesting the intrusion of FDNPP-derived radiocesium in both swash zones. FDNPP-derived radiocesium was appeared to be transported by the Tsushima Warm Current. In the surface sediments only 137Cs was detected during the sampling period. We could not find out the presence of the FDNPP-derived radiocesium in the corresponding sediment on the swash zones; however, detected radiocesium in those sediments was assumed to be influenced by 137Cs of FDNPP-derived radiocesium little for Ishikawa area and some for Niigata area.