Tritium and radiocarbon in the western North Pacific waters: post-Fukushima situation

Temporal variation of tritium concentration in monthly precipitation collected at a Difficult-to-Return Zone in Namie Town, Fukushima Prefecture, Japan

This article discusses tritium concentrations in monthly precipitation in part of the Difficult-to-Return Zone in Namie Town during 2012-2021. The tritium concentrations, which were measured with a low background liquid scintillation counter after carrying out an enrichment procedure, fluctuated seasonally from 0.10 ± 0.02 to 0.85 ± 0.02 Bq L-1. This range of concentrations is concluded to not be unusual based on comparisons with the concentrations at other sites and estimates of the past range of the concentrations. Moreover, no significant variations in observed tritium concentrations were observed due to decommissioning work at the Fukushima Dai-ichi Nuclear Power Plant. These results contribute to understanding the background level of tritium concentration in precipitation before the oceanic discharge of treated water from the Fukushima plant. In addition, this article evaluates the amount of tritium supplied to the ocean by terrestrial rainwater pouring into the Pacific Ocean via Ukedo River, which flows through Namie Town; this information will contribute to the discussion on the impact of the oceanic discharge of treated water.

A critical review and update of modelling of treated water discharging from Fukushima Daiichi NPP

Since the accident at the Fukushima Daiichi nuclear power plant (FDNPP) in March 2011 seawater is still needed to cool the reactor cores. This water, contaminated with radionuclides, has been collected in tanks and treated on the site of the FDNPP. In 2021, the Japanese government decided to gradually discharge treated water into the ocean, which started on the 24th of August 2023 and will continue for the next 30 years. This paper provides a critical analysis of the models that were used in the different radiological impact studies. Based on the analysis, a hydrodynamic and a compartment models with a harmonized setup were used to estimate the impact of the discharge on humans and biota. Doses obtained with these two models were within one order of magnitude for humans (<0.1 μSv/year) and for biota (<10-6 mGy/d) indicating that harmonization of the model parameters improved the reliability of the simulation results.

Impact of the Fukushima Accident on 3H and 14C Environmental Levels: A Review of Ten Years of Investigation

The investigation of the impact of the Fukushima accident is still going on although more than ten years have passed since the disaster. The main goal of this paper was to summarize the results of tritium and radiocarbon determinations in different environmental samples, possibly connected with the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. A document containing compiled data may serve as a solid basis for further research in the selected fields. To accomplish such effort, we went through dozens of relevant published papers, reporting 3H and 14C activity concentrations in precipitations, groundwater, seawater, river systems, tree rings, and, in some more extraordinary samples, such as herbaceous plants or debris from the damaged reactor buildings. As the referenced results would not be obtainable without adequate analytical techniques, the most common methods for routine measurement of tritium and radiocarbon concentrations are discussed as well. We believe that the correct identification of the affected environmental compartments could help quantify the released 3H and 14C activities and track their following fate, which could be especially important for plans to discharge contaminated water from the FDNPP in the upcoming years.

Tritium and radiocarbon in the water column of the Red Sea

Despite being the busiest transient sea in the world due to the Suez Canal, radionuclide distribution studies in seawater and sediment of the Red Sea remain rare. A sampling expedition in the Red Sea was conducted from June 9 to July 6, 2021, visiting a transect of several deep sampling stations located along the central axis of the basin from the Gulf of Aqaba to the southern Red Sea (near Farasan Island, Saudi Arabia). The collected seawater profile samples were analyzed for tritium, radiocarbon and oxygen-18. The observed tritium levels in surface waters of the Red Sea peaked at 0.3-0.4 TU, similar to the values observed in the western Arabian Sea (decay corrected). The values observed at waters below 150 m were around 0.2 TU, however, at depths of 450 and 750 m, tritium minima (<0.2 TU) were observed, which could be associated with a partial return flow of bottom waters from the southern to the northern Red Sea. At two stations at the depth of about 550 m, deep Δ14C minima were observed as well (-4‰ and -10‰), documenting ongoing transport of carbon in the water column, important for sink of anthropogenic carbon.

Ten years of investigations of Fukushima radionuclides in the environment: A review on process studies in environmental compartments

In March 2011, severe nuclear accident happened at the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) after the gigantic earthquake and following huge tsunami wave. A lot of investigations to assess environmental and radiological impacts of released radionuclides have been conducted by domestic and international organizations. Environmental radioactivity research related to the FDNPP accident has spread widely over different scientific fields due to specific features of the accident, and specifically its impact on the marine environment. The present paper summarizes major lessons learned from the environmental investigations of the FDNPP accident. Environmental radioactivity studies have typical interdisciplinary character; especially physics and chemistry are fundamental as a base of process studies in the environment. In this sight, we review chemical aspects regarding FDNPP-derived radiocesium transfer within and between compartments (atmosphere, ocean and land). We also discuss future trends in investigations of behavior of anthropogenic radionuclides in the environment, important not only for a better understanding of impacts of the FDNPP accident on the environment, but also for improving our general knowledge of the total environment in the Anthropocene era and its protection for the future.

A modeling approach to estimate 3H discharge from rivers: Comparison of discharge from the Fukushima Dai-ichi and inventory in seawater in the Fukushima coastal region

Estimation of ³H discharge from river catchments is important to evaluate the effect of Fukushima Dai-ichi discharge and future planned ³H release to the ocean on the coastal environment. Using a previously developed model based on the tank model and observed ³H concentration in river water, the ³H discharge from the Abukuma River and 13 other rivers in the Fukushima coastal region were estimated from June 2013 to March 2020. The ³H discharge from catchments of the Abukuma River and 13 other rivers in the Fukushima coastal region during 2014-2019 were estimated to be 1.2-4.0 TBq/y. These values were approximately 2-22 times larger than the annual ³H discharge from the Fukushima Dai-ichi after 2016, indicating the significance of ³H discharge from the catchments through the rivers. This estimation is expected to be useful to evaluate and predict ³H concentrations and inventories in the Fukushima coastal region for consideration of planned ³H release to the ocean.

Discharge of contaminated water from the Fukushima Daiichi Nuclear Power Plant Accident into the Northwest Pacific: What is known and what needs to be known

The Japanese government approved a plan to discharge Fukushima Daiichi Nuclear Power Plant Accident contaminated water (FDNPPACW) into the Pacific Ocean. It immediately caused a new wave of global concern and anxiety. To assess this matter, this work briefly reviewed the dispersion of FDNPPA-derived radionuclides in the Pacific Ocean in the past and the resulting impacts on marine biota. Combining the drafted plan of discharging FDNPPACW and the public's concerns, 5 points, including (1) the detailed plan of discharging FDNPPACW, (2) the isotopes left in the advanced liquid processing system (ALPS)-treated water and their amounts, (3) the stability of the Kuroshio Extension, (4) the fates and transports of the main radionuclides (left in the ALPS-treated water) in North Pacific seawater, (5) and bioaccumulations and the ecological half-lives of the main radionuclides (left in the ALPS-treated water) in marine biota in the North Pacific, remain to be known to understand the impacts of discharging FDNPPACW into the Pacific Ocean.

Transport and dispersion of tritium from the radioactive water of the Fukushima Daiichi nuclear plant

Japan recently announced plans to discharge over 1.2 million tons of radioactive water from the Fukushima Daiichi Nuclear Power Plant (FDNPP) into the Pacific Ocean. The contaminated water can poses a threat to marine ecosystems and human health. To estimate the impact of the plan, here, we developed a three-dimensional global model to track the transport and dispersion of tritium released from the radioactive water of the FDNPP. The pollution scenarios for four release durations (1 month, 1 year, 5 years, and 10 years) were simulated. The simulation results showed that for the release in short-duration scenarios (1 month and 1 year), the peak plume with high tritium concentration shifted with the currents and finally reached the northeastern Pacific. For the long-duration scenarios (5 years and 10 years), the peak plume of the contaminated water was confined to coastal regions east of Japan.

Landside tritium leakage over through years from Fukushima Dai-ichi nuclear plant and relationship between countermeasures and contaminated water

There has been tritium groundwater leakage to the land side of Fukushima Dai-ichi nuclear power plants since 2013. Groundwater was continuously collected from the end of 2013 to 2019, with an average tritium concentration of approximately 20 Bq/L. Based on tritium data published by Tokyo Electric Power Company Holdings (TEPCO) (17,000 points), the postulated source of the leakage was (1) leaks from a contaminated water tank that occurred from 2013 to 2014, or (2) a leak of tritium that had spread widely over an impermeable layer under the site. Based on our results, sea side and land side tritium leakage monitoring systems should be strengthened.

Gross alpha and gross beta activities in selected marine species in Vietnam

The measured gross alpha and gross beta activities in the edible muscle tissues of eleven selected marine species along the coast of North Vietnam varied from 10.2 ± 1.5 to 73.2 ± 8.1 Bq/kg (wwt) and from 10.6 ± 0.4 to 68.8 ± 2.8 Bq/kg (wwt), respectively. The lowest gross alpha activity was recorded for bigfin reef squid (Sepioteuthis lessoniana) as a result of its carnivorous diet, and the highest alpha activity was noted for blood cockle (Anadara granosa) as a result of its omnivorous diet. However, the gross beta activities in both carnivorous and omnivorous species were similar. The highest and lowest gross beta activities were observed for narrow-barred Spanish mackerel (Scomberomorus commerson) and for bigfin reef squid and squid (Teuthida), respectively. All three aforementioned species have carnivorous diets. The calculated annual committed effective dose resulting from the consumption of 25 kg of muscle tissue per year varied from 192 to 1375 μS with an average of 689 μS.

Temporal changes in tritium and radiocarbon concentrations in the western North Pacific Ocean (1993-2012)

The western North Pacific is one of the most studied oceanic basins due to its diverse structure and important role in connection with the adjacent reservoirs. Tritium (³H) and radiocarbon (¹⁴C) have been frequently exploited as oceanographic tracers due to their suitable properties; several extensive observation projects, such as GEOSECS, WOCE and WOMARS, used these two radionuclides to investigate different oceanographic processes, pathways, ocean currents and time scales of deep and bottom water formation. Here we evaluate temporal changes in ³H and ¹⁴C levels in seawater of the western North Pacific Ocean from 1993 to 2012. When compared to the background levels from 1993, the data from 2012 suggests significant impact of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident on surface and vertical ³H seawater profiles, increasing its water column inventories in the southern part of the 149°E meridian by a factor of 2-7. On the other hand, ¹⁴C content in surface seawater has been steadily decreasing from 1993, with the accelerated rate from 2005, probably due to downwelling of bomb-produced radiocarbon and its transport along isopycnal layers. The influence of the Oyashio current on ¹⁴C levels in the northern part of the investigated transect and formation of its intrusion was also clearly visible in the collected datasets. Regarding bomb-produced radiocarbon, its water column inventories decreased or remained same from 2005 to 2012 at all stations, except the ones located in the coastal areas of the New Guinea island (3.5°S).

Seasonal and Spatial Distribution of Atmospheric Tritiated Water Vapor in Mainland China

To reveal the distribution of atmospheric tritium water (HTO) vapor and provide a baseline for tritium pollution control, a subnational survey was conducted in mainland China. As the largest study on HTO vapor in China that has ever been formally reported, this study provides a macroimpression of the atmospheric HTO specific activity from March 2017 to March 2018. A total of 102 passive samplers were deployed at 34 sites in 30 provinces to determine the seasonal and spatial distributions of HTO vapor. In general, the HTO specific activity in the atmosphere ranged from lower than the minimum detectable activity (0.18 Bq·L^-1) to 5.5 Bq·L^-1. Spatially, the specific activity of HTO was positively correlated to the latitude and the distance to proximal coastline. Seasonally, significantly higher HTO specific activities were observed in spring and relatively lower in summer. Based on correlation analysis, the atmospheric HTO distributions were considered to be the consequence of combined factors of the stratospheric-tropospheric net mass flux, the distance from the tropopause to the ground, the fraction of air mass that originated from ocean re-evaporation and long-distance transport from high-latitude continents.

Radionuclides in surface waters around the damaged Fukushima Daiichi NPP one month after the accident: Evidence of significant tritium release into the environment

Following the Fukushima nuclear accident (2011), radionuclides mostly of volatile elements (e.g., ¹³¹I, ^134,137Cs, ¹³²Te) have been investigated frequently for their presence in the atmosphere, pedosphere, biosphere, and the Pacific Ocean. Smaller releases of radionuclides with intermediate volatility, (e.g., ⁹⁰Sr), have been reported for soil. However, few reports have been published which targeted the contamination of surface (fresh) waters in Japan soon after the accident. In the present study, 10 surface water samples (collected on April 10, 2011) have been screened for their radionuclide content (³H, ⁹⁰Sr, ¹²⁹I, ¹³⁴Cs, and ¹³⁷Cs), revealing partly unusually high contamination levels. Especially high tritium levels (184 ± 2 Bq·L^-1; the highest levels ever reported in scientific literature after Fukushima) were found in a puddle water sample from close to the Fukushima Daiichi nuclear power plant. The ratios between paddy/puddle water from one location only a few meters apart vary around 1% for ¹³⁴Cs, 12% for ¹²⁹I (¹³¹I), and around 40% for both ³H and ⁹⁰Sr. This illustrates the adsorption of radiocesium on natural minerals and radioiodine on organic substances (in the rice paddy), whereas the concentration differences of ³H and ⁹⁰Sr between the two waters are mainly dilution driven.

Groundwater age and mixing process for evaluation of radionuclide impact on water resources following the Fukushima Dai-ichi nuclear power plant accident

Radionuclide contamination of groundwater causes critical impacts on water resources, human lives, and ecosystems. The intrusion of radionuclides into the groundwater flow system in Fukushima, Japan, could be illuminated by determining groundwater age and mixing processes. To do this, periodical field surveys were conducted in catchments contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident. Sampling began in May 2011, which was 2 months after the disaster, and continued through June 2012. Chlorofluorocarbon (CFCs), tritium, and oxygen and hydrogen stable isotopes were used as environmental tracers. The observed tritium concentrations suggested that the water contained accident-derived radionuclides that exceeded the natural background baseline. Groundwater ages in the selected two headwater catchments were estimated to be between 10 and 26 years by combined use of multiple CFCs concentrations. In addition, the governing groundwater flow system was mostly approximated by a piston flow model; however, modern water fraction was also suggested based on the relationship between CFC-11 and CFC-12. The estimated water age and isotopic signals among stream water, spring water, and groundwater revealed that the intrusion of radionuclides into the groundwater was caused by the mixing between groundwater and modern water sources such as soil water and precipitation with relatively high radionuclide concentrations. This mixing was facilitated by a weathered and fractured granite bedrock and a thin unsaturated subsurface layer in the study area. Continued long-term monitoring of radionuclides in the groundwater will be necessary for water resources management in the future. CAPSULE: Radionuclide intrusion into the groundwater is related to the mixing between radionuclide-poor groundwater and modern water with relatively high radionuclide concentration.

New ultra-sensitive radioanalytical technologies for new science

Recent developments in radiometric and mass spectrometry technologies have been associated in the radiometric sector mainly with underground operations of large volume Ge detectors, while the mass-spectrometry sector, represented mainly by accelerator mass spectrometry and inductively coupled plasma mass spectrometry has become the most sensitive technique for ultra-low-level analyses of long-lived radionuclides. These new developments have had great impact on investigations of rare nuclear processes and applications of radionuclides in environmental, life and space sciences. New scientific investigations have been carried out therefore which have not been possible before either because of lack of sensitivity or required large sample size.