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Lateral and temporal profiles for concentrations of Cs and Cs were examined in marine sediments obtained from coastal and offshore areas on the Sea of Japan-side of central Honshu. In 2013, concentrations declined sharply from an estuarine (30 Bq/kg-dry for Cs) to offshore areas. In the offshore Sado Basin, the concentration of short-lived Cs, which was not detected (<0.3 Bq/kg-dry) in 2010, was 1-15 Bq/kg-dry in 2011-2013. Discharged riverine materials affected by radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant have been delivered to this offshore basin. [ABSTRACT FROM AUTHOR]

The concentrations of Pu, Pu and Am in the sediments at the coast of Japanese islands from 2008 to 2011 varied widely from one sampling site to another and were generally lower in sandy sediments at shallower sites, and higher in clayey sediments at deeper sites. In contrast, there seemed to be no temporal variation in the concentrations during the survey period. The Am/Pu activity ratio was relatively higher than the global fallout presumably owing to the ingrowth of Am from the ancestor, Pu originating mainly from the hydrogen bomb explosion tests in the Marshall Islands in the 1950s. [ABSTRACT FROM AUTHOR]

Spatiotemporal distributions of anthropogenic radionuclides in surface sediments off Miyagi, Fukushima, and Ibaraki Prefectures were analyzed on the basis of data collected during the monitoring program launched by the Japanese Ministry of Education, Sports, Science and Technology in 2011 right after the Fukushima Nuclear Power Plant accident. Concentrations of 137Cs in the surface sediments varied spatially by two orders of magnitude from 1.7 to 580 Bq kg-dry-1, and there was no obvious correlation between 137Cs concentration and the proximity of the sampling location to the site of the accident. The total inventory of 137Cs accumulated in the upper 3 cm of surface sediment in the monitoring area was estimated to be 3.78x1013 Bq, that is 0.1-2% of the total 137Cs flux from the plant to the ocean as a result of the accident (the percentage depends on the model used to estimate the total flux). The spatial variations of 137Cs concentration and inventory depended on two main factors: the 137Cs concentration in the overlying water during the first several months after the accident and the physical characteristics of the sediment (water content and bulk density). The tem-poral variations of the concentrations of other anthropogenic radionuclides (90Sr, 95Nb, 110mAg, 125Sb, 129Te, and 129mTe) in the sediments were also investigated. The temporal variations of the activity ratios of these nuclides to 137Cs suggest that before the Fukushima-derived nuclides became homogeneous in seawater, they were removed from the water to the sediment. [ABSTRACT FROM AUTHOR]

Abstract: To assess the migration patterns of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we analyzed 228Ra/226Ra ratios and 7Be concentrations and compared them with 134Cs and 137Cs 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 228Ra/226Ra 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 7Be (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 7Be 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. [Copyright &y& Elsevier]

Concentrations of 129 I and 127 I in kelps (Saccharina) collected from coastal waters off northern Japan were monitored from 2007 to 2019. During the 2007-2008 test operation of the Rokkasho nuclear fuel reprocessing plant, 129 I discharge from the plant increased, and the 129 I concentration and 129 I/ 127 I atom ratio in the kelps reached maxima of 42 μBq/g-dry and 264 × 10 -11 , respectively. By 2009, both had decreased by one order of magnitude. After the Fukushima Dai-ichi Nuclear Power Plant accident in 2011, the 129 I concentration and 129 I/ 127 I atom ratio in the kelps increased to 2.24 μBq/g-dry and 11.6 × 10 -11 , respectively. After 2012, the ratio in kelps decreased to (2.1-8.9) × 10 -11 , which is almost the same as the seawater value off Aomori Prefecture before the test operation. The 129 I/ 127 I atom ratio in kelps thus represents the ambient seawater ratio during the growth period of the kelps., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The historic spatiotemporal distribution of 137Cs in the seawaters and sea-floor sediments adjacent to nuclear power plants in Japan are summarized, using data obtained over a period of time more than 20 years prior to the disaster at the Fukushima Daiichi Nuclear Power Plant in 2011. Relatively uniform distributions of 137Cs were observed both in the surface seawaters (1 m in depth) and in deeper seawaters (10 to 30 m above the seabed and ranging from tens to hundreds of meters in depth) independent of the geographical position, although lower concentrations were observed in significantly deeper bottom seawaters. Conversely, there were wide variations in 137Cs levels between sediments, such that higher 137Cs concentrations were observed in the deeper sampling locations. A mathematical model describing the successive transfer of 137Cs from surface waters through deeper waters to sediments suggested that the transfer rate of 137Cs from deep water to the sediments, and the loss rate from bottom sediments, were both greater than the transfer rate from surface water to deeper water. It was found that the calculated regression lines for 137Cs depletion rates over time for surface waters, deeper waters, and sediments were approximately parallel when plotted on a semi-logarithmic coordinate system, regardless of the sampling location. A radionuclide depletion half-life was calculated to be 4 months to 16 years with the geometric mean of 2.22 y for the sediments in the Fukushima region, suggesting that nuclear contamination will be remediated over time through sediment redistribution processes such as remobilization, bioturbation, and migration due to sea currents., (Copyright © 2013 Elsevier B.V. All rights reserved.)

A radioactivity survey was launched in 1991 to determine the background levels of ²³⁹+²⁴⁰Pu in the marine environment off a commercial spent nuclear fuel reprocessing plant before full operation of the facility. Particular attention was focused on the ²⁴⁰Pu/²³⁹Pu atom ratio in seawater and bottom sediment to identify the origins of Pu isotopes. The concentration of ²³⁹+²⁴⁰Pu was almost uniform in surface water, decreasing slowly over time. Conversely, the ²³⁹+²⁴⁰Pu concentration varied markedly in the bottom water and was dependent upon the sampling point, with higher concentrations of ²³⁹+²⁴⁰Pu observed in the bottom water sample at sampling points having greater depth. The ²⁴⁰Pu/²³⁹Pu atom ratio in the seawater and sediment samples was higher than that of global fallout Pu, and comparable with the data in the other sea area around Japan which has likely been affected by close-in fallout Pu originating from the Pacific Proving Grounds. The ²⁴⁰Pu/²³⁹Pu atom ratio in bottom sediment samples decreased with sea depth. The land-originated Pu is not considered as the reason of the increasing ²³⁹+²⁴⁰Pu concentration and also decreasing the ²⁴⁰Pu/²³⁹Pu atom ratio with sea depth, and further study is required to clarify it., (Copyright © 2010 Elsevier Ltd. All rights reserved.)