Ocean general circulation model simulations of anthropogenic tritium releases from the Fukushima Daiichi nuclear power plant site.

Distribution of oceanic 137Cs from the Fukushima Dai-ichi Nuclear Power Plant simulated numerically by a regional ocean model

Review of Characteristics of Post-Accident Waste Generated in Fukushima Daiichi Nuclear Power Plant Site and Issues to be Addressed in Processing and Disposal Stages The accident at Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Station has produced and will continue to produce various types and large amounts of waste contaminated by radionuclides. The literature and published internet information on possible types of waste produced are reviewed from the viewpoint of their characteristics. Issues associated with the waste were selected and analyzed for each stage of the future waste management, considering the characteristics and properties of the waste obtained so far. The stages considered are current (temporary) storage, processing including decontamination and solidification/packaging, storage up to disposal, transportation, and disposal.

Radioactive cesium (137Cs) is distributed in the global ocean due to global fallout from atmospheric nuclear tests, release from reprocessing plants in Europe, and supply to the ocean due to the Fukushima Daiichi Nuclear Power Plant accident. In order to detect future contamination by radionuclides, it is necessary to understand the global distribution of radionuclides such as 137Cs. For this purpose, the IAEA is compiling a database of observation results (MARIS). However, since the spatio-temporal densities of observed data vary widely, it is difficult to obtain a complete picture from the database alone. Comparative validation using ocean general circulation model (OGCM) simulations is useful in interpreting these observations, and global ocean general circulation model (CESM2, POP2) simulations were conducted to clarify the behavior of 137Cs in the ocean. The horizontal resolution is 1.125° longitude and 0.28° to 0.54° latitude. The minimum spacing near the sea surface is 10 m, and the spacing increases with depth to a maximum of 250 m with 60 vertical levels. Climatic values were used for driving force. As a source term for 137Cs to the ocean, atmospheric fallout from atmospheric nuclear tests was newly established based on rainfall data and other data, and was confirmed to be more reproducible than before. Furthermore, the release from reprocessing plants in Europe and the leakage due to the accident at the Fukushima Daiichi Nuclear Power Plant were taken into account. 2020 input conditions were assumed to continue after 2020, and calculations were performed from 1945 to 2030. The simulated 137Cs activities were found to be in good agreement, especially in the Atlantic and Pacific Oceans, where the observed densities are large. On the other hand, they were underestimated in the Southern Hemisphere, suggesting the need for further improvement of the fallout data. 137Cs concentrations from the Fukushima Daiichi Nuclear Power Plant accident in March 2011 were generally in good agreement, although the reproducibility remained somewhat problematic due to insufficient model resolution. In other basins, the concentration characteristics were able to be determined, although the observed values were insufficient. Radioactivity concentrations of atmospheric nuclear test-derived 137Cs may continue to be detected in the global ocean after 2030. The results of this simulation are useful for planning future observations to fill the gaps in the database.

Following the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011, large quantities of radioactive materials were released into the atmosphere and ocean. Since the FDNPP nuclear accident, Tokyo Electric Power Company (TEPCO) operators have been implementing measures to reduce groundwater inflow into the FDNPP damaged reactor buildings while pumping water to cool the nuclear reactors and fuel debris. The resulting huge water volume began the discharge into the ocean from August 2023, after being treated by an Advanced Liquid Processing System (ALPS) to remove radionuclides for acceptable discharge levels except tritium. Since then, tritium concentrations in seawater and aquatic ecosystems near the FDNPP site are continuously monitored and disseminated publicly. It is essential to assess the long-term safety threshold of ALPS-treated water discharge procedure in terms of tritium concentration in coastal areas of Japan and the Pacific Ocean. However, there is no global oceanic simulation with tritium concentration and, by extension, no projection of tritium concentration at Pacific Ocean scale.In this study, we used the TEPCO ALPS treated water release plan as an input to the ocean general circulation model (OGCM) COCO4.9, which is the ocean component of the Model for Interdisciplinary Research on Climate, version 6 (MIROC6 [1]). This approach allowed us to simulate the anthropogenic tritium concentration in the ocean due to ALPS treated water release in the forthcoming decades. The spatial distribution and temporal evolution of the projected tritium concentrations in different parts of the Pacific Ocean, as well as the impact of global warming on them, were analyzed. Moreover, the anthropogenic tritium concentration following the FDNPP accident was modeled to evaluate how large the tritium concentrations due to current treated water release are compared to the accidental one in 2011. Finally, given that oceanic tritium concentrations are mainly controlled by ocean mixing, our study represents a valuable opportunity to evaluate the impact of the Kuroshio current representation in COCO4.9 on tritium concentrations at non-eddy-resolving and eddy-resolving horizontal resolutions.[1] Tatebe et al., Geosci. Model Dev., 12, 2727–2765, doi:10.5194/gmd-12-2727-2019, 2019.

This paper describes an experimental investigation to explore the effects of inlet steam quality on Terry steam turbine performance. The Terry steam turbine is a single-stage, compound-velocity impulse turbine known for its low or atmospheric operating pressure, resilient performance under off-normal conditions, and minimal maintenance needs. Contrasting with a bladed steam turbine, upon entering through the steam inlet nozzle, the steam flow is redirected from being coaxial to the turbine wheel to being tangential to the wheel “buckets,” machined semi-circular volumes that are shaped into the body of the wheel. This flow path maximizes the steam impulse. Terry turbines are employed in the reactor core isolation cooling (RCIC) system in over 20 U.S. boiling water reactors with a Mark I containment design and also at the Fukushima Daiichi Nuclear Power Plant site. The RCIC system provides cooling to the reactor pressure vessel by utilizing decay heat steam to power a Terry turbine, which in turn provides power to a RCIC pump. During the Fukushima Daiichi Nuclear Power Plant accidents in 2011, the RCIC system in Units 2 and 3 functioned far beyond the RCIC system’s expected capabilities. The Terry steam turbines successfully operated under two-phase flow conditions for up to 70 h. This unexpected performance implies that the Terry turbines may have safety capabilities hitherto unknown. Given the possibility of expanded safety capabilities, the current study investigates the Terry turbine performance during two-phase steam-water ingestion. These tests serve to expand the knowledge base for the model development, validation, and verification of Terry turbine behavior in response to a beyond design-basis event, in a cost-effective manner without putting at risk a nuclear-grade full-scale Terry turbine at a nuclear power plant. Terry turbine performance is investigated experimentally over a range of turbine inlet steam qualities from 1.0 (dry steam) to 0.05 and inlet pressures of 30 to 60 psia. Turbine power output and turbine efficiency were found to be higher at higher inlet steam qualities and higher pressure. Another key finding was that the Terry turbine can continue to perform even with a very low inlet steam quality due to the bucket design.

Pu isotopes in the seawater off Fukushima Daiichi Nuclear Power Plant site within two months after the severe nuclear accident

A dynamic compartment model was developed to estimate 137Cs concentrations in both seawater and sediment that had been released in the TEPCO’s Fukushima Daiichi Nuclear Power Plant 2011 accident. The model parameters were set from the best fitting of the results observed at the two closest sampling stations to the FDNPP site, then applied these parameters to three other stations located about 15–43 km away from the FDNPP. The 137Cs concentrations in seawater and sediment estimated by the developed model agreed well with observation results for these stations.

The deposition densities of radiocesium and the air dose rates in undisturbed fields around the Fukushima Dai-ichi nuclear power plant; their temporal changes for five years after the accident

<p>Radioactive cesium (<sup>137</sup>Cs) is distributed in the global ocean due to global fallout by atmospheric nuclear weapons tests, releases from reprocessing plants in Europe, and supplied to the ocean by the Fukushima Daiichi Nuclear Power Plant (1F NPP) accident. In order to detect future contamination by radionuclides, it is necessary to understand the global distribution of radionuclides such as <sup>137</sup>Cs. For this purpose, observed data have been summarized in a historical database (MARIS) by IAEA. The spatio-temporal density of the observations varies widely, therefore simulation by an ocean general circulation model (OGCM) can be helpful in the interpretation of these observations.</p><p>In order to clarify the behavior of <sup>137</sup>Cs in the global ocean, OGSM simulations were conducted. Parallel Ocean Program version 2 (POP2) of the Community Earth System Model version 2 (CESM2) is employed. The horizontal resolution is 1.125 degree of longitude, and from 0.28 degree to 0.54 degree of latitude. There are 60 vertical levels with a minimum spacing of 10 m near the ocean surface, and increased spacing with depth to a maximum of 250 m. The simulated period was from 1945 to 2030 with the circulation forced by repeating (“Normal Year”) atmospheric conditions. As input sources of <sup>137</sup>Cs to the model, global fallout from atmospheric nuclear tests, releases from reprocessing plants in Europe, and input from the 1F NPP accident were considered. It was assumed that the input conditions in 2020 would continue after 2020.</p><p>The simulated <sup>137</sup>Cs activity agrees well with the observed data in the database, especially in the Atlantic and Pacific Oceans where the observation density is large. Since <sup>137</sup>Cs undergoes radioactive decay with a half-life of 30 years, the inventory for each basin is the difference between the decay corrected cumulative input and flux. In the North Pacific, the inventory reached its maximum in 1966 due to the global fallout by atmospheric nuclear weapons tests. Fluxes from the North Pacific to the Indian Ocean, Arctic Ocean, and Central Pacific were positive, and the North Pacific was a source of supply for other ocean basins. The 1F NPP accident caused a 20% increase in the inventory in 2011. In the North Atlantic, the inventory reaches its maximum in the late 1970s, due to the releases from the reprocessing plant. The outflow flux from the North Atlantic to the Greenland Sea is larger than the other fluxes and is a source of supply to other ocean basins. After 2000, the inflow flux to the North Pacific from the Labrador Sea and the South Atlantic is larger than the outflow flux.</p><p>The time series of <sup>137</sup>Cs inventory in each ocean basin and the fluxes among ocean basins were quantitatively analyzed by OGCM simulations, and the predictions for the next 10 years were made.  The <sup>137</sup>Cs activity concentrations by global fallout can be detected in the global ocean after 2030. The OGCM simulations will be useful in planning future observations to fill the gaps in the database.</p>

Technical Aspects of the Release of “Contaminated” Water into the Sea from the Fukushima Daiichi Nuclear Power Plant Site

ABSTRACTAn improved light-weight Compton camera exhibiting low power consumption was developed to be mountable on an unmanned helicopter to detect cesium radiation hot spots and confirm the decontamination effect of cesium-affected areas. An increase in the Ce:Gd3(Al,Ga)5O12 scintillator array from 4 × 4 to 8 × 8 and expansion of the interlayer distance enhanced the detection efficiency and angular resolution, respectively. Measurements were performed over the Ukedo riverbed in Namie, Fukushima Prefecture (Japan). The helicopter's flight path and speed were pre-programmed to lines interspaced by 5 and 10 m intervals and 1 m/s, respectively, facilitating measurements over areas of 65 × 60 m2 and 65 × 180 m2 at a height of 10 m for approximately 20 and 30 min, respectively. Results provided accurate ambient dose equivalent rate maps at a height of 1 m with an angular resolution corresponding to a position resolution of approximately 10 m from a height of 10 m. Hovering flights were executed over hot-spot areas for 10–20 min at a height of 5–20 m. Gamma-ray images of these hot spots were obtained using a reconstruction software. Comparison between position-shifted measurement results showed that the angular resolution coincided with that evaluated in the laboratory (approximately 10°).

Following the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011, large quantities of radioactive materials were released into the atmosphere and ocean. Since the FDNPP nuclear accident, Tokyo Electric Power Company (TEPCO) operators have been implementing measures to reduce groundwater inflow into the FDNPP damaged reactor buildings while pumping water to cool the nuclear reactors and fuel debris. The resulting huge water volume began the discharge into the ocean from August 2023, after being treated by an Advanced Liquid Processing System (ALPS) to remove radionuclides for acceptable discharge levels except tritium. Tritium releases from the FDNPP accident and the ALPS treated water raise questions about the impact on tritium in precipitation in Japan, the removal time of anthropogenic tritium in groundwater and the oceanic transport of tritium from released ALPS treated water. In this two-part study, we present (1) the modeling of tritium in precipitation during the FDNPP accident using an atmosphere general circulation model (AGCM), and (2) a sensitivity simulation of tritium concentration in the ocean due to planned ALPS treated water release in the next decades by TEPCO using an ocean general circulation model (OGCM). For the atmospheric part, we used the isotope-enabled AGCM MIROC5-iso, in which tritium has been implemented [1], and adapted an estimated atmospheric release of iodine-131 [2] for the anthropogenic tritium source. We found good agreement with the tritium in precipitation observations in Japan for 2011 and subsequent years, despite MIROC5-iso’s rather coarse horizontal resolution (approximately 2.8°). Together with measured tritium data in Japan, our modeled results can be used to interpret mean transit times of Fukushima surface and groundwater systems and in other Asian systems (see abstract of Gusyev et al. in the same session).For the oceanic part, we used the OGCM COCO4.9, which is the ocean component of the Model for Interdisciplinary Research on Climate, version 6 (MIROC6 [3]), and the tritium discharge scenario from TEPCO. Tritium concentration at the ocean surface reaches approximately 3 Bq/m3 near the release site and varies between 0.01 and 0.25 Bq/m3 in the North Pacific Ocean, well below the natural tritium level (approximately 50 Bq/m3 [4]). For this kind of projection simulation, the use of a fully coupled atmosphere-ocean model would make it possible to model tritium concentration in both the atmosphere and the ocean, as well as the dynamics of exchanges within and between these water cycle reservoirs. [1] Cauquoin et al.: Modeling natural tritium in precipitation and its dependence on decadal variations of solar activity using the atmospheric general circulation model MIROC5-iso, J. Geophys. Res. Atmos., in review (minor revisions).[2] Katata et al., Atmos. Chem. Phys., 15, 1029–1070, https://doi.org/10.5194/acp-15-1029-2015, 2015.[3] Tatebe et al., Geosci. Model Dev., 12, 2727–2765, https://doi.org/10.5194/gmd-12-2727-2019, 2019.[4] Jenkins et al., Earth Syst. Sci. Data, 11, 441–454, https://doi.org/10.5194/essd-11-441-2019, 2019.

The vertical structure of the variability in the equatorial Pacific in a high-resolution ocean general circulation model (OGCM) simulation for 1985–94 is investigated. Near the equator the linear vertical modes are estimated at each grid point and time step of the OGCM simulation. The characteristics of the vertical modes are found to vary more in space than in time. The contribution of baroclinic modes to surface zonal current and sea level anomalies is analyzed. The first two modes contribute with comparable amplitude but with different spatial distribution in the equatorial waveguide. The third and fourth modes exhibit peaks in variability in the east and in the westernmost part of the basin where the largest zonal gradients in the density field and in the vertical mode characteristics are found. Higher-order mode (sum of third to the eighth mode) variability is the largest near the date line close to the maximum in zonal wind stress variability. Kelvin and first-meridional Rossby components are derived for each of the first three baroclinic contributions by projection onto the associated meridional structures. They are compared to equivalent ones in multimode linear simulations done with the projection coefficients and phase speeds derived from the OGCM simulation. This suggests that in addition to the first-mode-forced equatorial Kelvin and Rossby waves earlier found in the data, forced waves of higher vertical modes should also be observable. For the first two vertical modes, the anomalies in the linear and the OGCM simulations have a similar magnitude and usually present similar propagation characteristics. Phase speed characteristics are however different in the eastern Pacific with larger values for the OGCM. The effect of zonal changes in the stratification is tested in the linear model for a stratification change located either in the eastern or in the western Pacific. This results in a significant redistribution of energy to higher modes via modal dispersion. In particular the third mode increases to a magnitude closer to the one in the OGCM simulation. Gravest modes are also affected. This suggests that modal dispersion plays an important role and should be considered when interpreting data as a combination of linear long equatorial waves.

Dispersion and removal characteristics of tritium originated from nuclear power plants in the atmosphere

The role of the Yangtze River discharge in changing sea‐surface salinity (SSS) in the East China Sea during the 1997–2001 periods is investigated by using (1) in situ SSS measurements along a merchant ship track running from Taiwan strait to Korea strait, (2) ocean general circulation model (OGCM) simulations and specific sensitivity studies, and (3) in land precipitation data to infer estimates of the river discharge in absence of gauged station data. The river effects on 1997–2001 sea surface temperature (SST) and surface currents are also examined. Both in situ SSS‐SST measurements and OGCM simulations clearly indicate that the integration of the 1997 drought signal and 1998–1999 flooding signal over the catchment area of the Yangtze River are manifest in the ocean where local precipitation played a rather modest role. Sensitivity studies with the OGCM with and without the river discharge further reveal that these signals can extend over several hundred kilometers off the river mouth, consistent with the along track in situ measurements. In particular, SSS along the ship track can drop from about 34 psu in August 1997 to less than 26 psu in July 1998 (almost the same for 1999) at the latitude of the river mouth although located 500 km to the west of the ship track. Model results indicate that the dynamic and thermodynamic effects of the freshwater input from the river may be phase lagged by up to a season with maximum surface warming occurring in the summer months when the river discharge is maximum whereas the largest differences in mixed layers with and without the river input occur during later winter and late autumn. The possible positive feedback between the increase of river runoff yielding warm SST anomaly off the river mouth and so enhanced atmospheric convection and precipitation is discussed.