Yasunori Mahara

Plutonium released by the Nagasaki A-bomb: Mobility in the environment

Abstract The first and second environmental releases of man-made 239 + 240 Pu came from nuclear explosions at Alamagordo and Nagasaki in 1945. The release at Nagasaki was more serious than at Alamagordo, because it happened in an area with a high population density. Unfissioned 239 + 240 Pu and various fission products (e.g. 90 Sr and 137 Cs) have been interacting here with various environmental materials (soils, sediments, and plants) under wet and temperate conditions for more than 45 years. To assess the environmental mobility of 239 + 240 Pu, the distributions of radionuclides from this release were investigated at Nishiyama (3 km east of the hypocenter) where heavy black rain containing unfissioned plutonium and fission products fell 30 minutes after the nuclear explosion. The vertical distributions of 90 Sr, 137 Cs and 239 + 240 Pu were determined in unsaturated soil cores up to 450 cm deep. Most radionuclides were found in the soil column 30 cm from the ground surface (95% of 90 Sr, 99% of 137 Cs and 97% of 239 + 240 Pu). However, 90 Sr and 239 + 240 Pu were detected in the groundwater as well below a depth of 200 cm. No 137 Cs was found below 40 cm from the ground surface or in groundwater. These observations reveal that about 3% of the total 239 + 240 Pu has been migrating in the soil at a faster rate than the remaining 239 + 240 Pu. Sharp peak of 137 Cs and 239 + 240 Pu, indicating heavy deposition from the Nagasaki local fallout of 1945, were found in sediment cores collected from the Nishiyama reservoir. On the other hand, since 90 Sr is mobile in fresh water sediments, there was no 1945 90 Sr peak in the sediment cores. 239 + 240 Pu peaks were unexpectedly discovered in pre-1945 sediment core sections. Although 90 Sr was found in these sections, no 137 Cs was found. By contrast to the distribution in sediment cores, 137 Cs in tree rings had spread by diffusion from the bark to the center of the tree without holding a fallout deposition record. Most of the 239 + 240 Pu was distributed in the tree rings following a similar deposition record to that found in sediment cores. Furthermore, a very small amount of 239 + 240 Pu (about 1%) was found unexpectedly in pre-1945 tree rings. The only reasonable explanation for these unexpected discoveries is the existence of mobile 239 + 240 Pu in the environment.

Dynamic changes in hydrogeochemical conditions caused by tunnel excavation at the Aspo Hard Rock Laboratory (HRL), Sweden

Abstract Hydraulic changes caused by tunneling at the Aspo Hard Rock Laboratory (HRL) in Sweden have been investigated over a period of 2a using different hydrochemical approaches, i.e. noble gas content, isotopic measurements and major ion concentrations. The dissolved noble gases (4He and Ne contents, and the ratio of 3He/4He, 40Ar/36Ar), stable isotopes, chemical concentrations of major ions, and 36Cl/Cl ratios, were determined in groundwater samples collected in the tunnel from borehole sections isolated by inflated packers. Groundwater was categorized into 3 groups based on 4He and Cl− contents: undisturbed groundwater (i.e. prior to tunnel construction) with high 4He and Cl− contents, groundwater that has been gradually changed by mixing with Baltic seawater and whose 4He and Cl− contents have gradually increased with increasing depth, and groundwater that has been totally changed due to a rapid mixing of Baltic seawater and/or shallow groundwater and whose 4He and Cl− contents are extremely low compared with other samples collected at the same surrounding depth. The oldest groundwater with a high salinity of more than 14,000xa0mg l−1 of Cl− is estimated to be more than 1.8 Ma old. The groundwater residence time ranges from 0.9 to 900 Ka in the mixing-zone. Groundwater in the disturbed zone where rapid mixing has occurred is hard to date reliably and its primary hydrochemical character has already been lost.

Plutonium mass balance released from the Nagasaki A-Bomb and the applicability for future environmental research

Abstract The existence of plutonium was publicly revealed on 9 August 1945 by the nuclear explosion of the Nagasaki A-Bomb whose Pu critical mass has been considered to be between 10–15 kg (still classified). Approximately 1.2 kg was fissioned during the detonation and the remaining mass was released into the environment. The height of the explosion was 503 m and the conditions were cloudy with a humidity of 71%. The ground temperature was 28.8°C with a light west-south-west wind of 3.7 m/s. The amount of both local and global fallout was investigated by measuring a fission product, 137Cs, and unfissioned Pu. Deposition rates were determined for surface soils, reservoir sediments, and the concentration in vegetation, fish and other living materials in the region, up to 100 km from the hypocentre. Only 0.0375 kg of Pu (or 0.3–0.4% of the total Pu in the bomb) was deposited as local fallout from the release. The highest concentration (64.5 mBq/g or 199 mBq/cm2 of 239+240Pu in the surface soils was found at 2.8 km east from the hypocentre where “black rain” precipitated 25 min after the detonation. The concentration rapidly decreased in both directions from this point. At 100 km east from the hypocentre, Pu fallout values were at background levels while only background levels were observed west of the centre. Average global fallout from weapon tests was 5.9 mBq/cm2 in this region, 32.5° latitude and 130° longitude. The reservoir sediments contained a Pu concentration of 142 mBq/cm2. This average was obtained by integrating 90 cm of the core layers (1910–1981 AD). Biological materials contained insignificant amount of Pu because their mass was small compared to the surface soils and the sediments. Only background levels of Pu were found on the west side of the hypocentre. The ratio of unfissioned 239+240Pu and fission product 137Cs can supply valuable information to aid in understanding the mechanism of Pu interaction with the environment. The ratio was as high as 45% at the centre of the “black rain” compared to the northern hemisphere average of 2.3%, as of 1990. At the time of “black rain” droplet formation 137Cs was not fully formed from the mass 137 fission chain, 137 Te → 137 I → 137 Xe → 137 Cs . Calculations showed that it would take nearly 1 h to form the total cumulative yield for 137Cs. The time of “black rain” formation was estimated to be 195 s for a 15 kg Pu bomb and 406 s for a 10 kg Pu-bomb. These results suggested that 13.8 kg or 92% (or 0.0064 mBq/cm2) of Pu was deposited throughout the northern hemisphere as global fallout.

Atmospheric Direct Uptake and Long-term Fate of Radiocesium in Trees after the Fukushima Nuclear Accident

Large areas of forests were radioactively contaminated by the Fukushima nuclear accident of 2011, and forest decontamination is now an important problem in Japan. However, whether trees absorb radioactive fallout from soil via the roots or directly from the atmosphere through the bark and leaves is unclear. We measured the uptake of radiocesium by trees in forests heavily contaminated by the Fukushima nuclear accident. The radiocesium concentrations in sapwood of two tree species, the deciduous broadleaved konara (Quercus serrata) and the evergreen coniferous sugi (Cryptomeria japonica), were higher than that in heartwood. The concentration profiles showed anomalous directionality in konara and non-directionality in sugi, indicating that most radiocesium in the tree rings was directly absorbed from the atmosphere via bark and leaves rather than via roots. Numerical modelling shows that the maximum 137Cs concentration in the xylem of konara will be achieved 28 years after the accident. Conversely, the values for sugi will monotonously decrease because of the small transfer factor in this species. Overall, xylem 137Cs concentrations will not be affected by root uptake if active root systems occur 10u2005cm below the soil.

Plutonium in tree rings from France and Japan

Abstract Plutonium, along with other radionuclide concentrations, was measured in evergreen tree rings from two different locations. This was used as an information source for the past two centuries. Tree rings are a product of annual layers and thus chronological information is clearly visible. Three trees were harvested in 1988–1990: a French white fir (137 years old) and a spruce tree (177 years old) from the France-Germany border near Nancy, France and a sugi (78 years old) from Nagasaki, Japan. The uniform branchless part of the trunks from the harvested trees were immediately separated into a set of tree ring samples each of which contained 3–20 years of growth. The separated samples were mechanically powdered, dried at 105°C to obtain the dry weight, ashed at 350°C to measure 40 K, 134 Cs and 137 Cs and ashed again at 600°C to determine 239+240 Pu. The highest 239+240 Pu concentration of 30.0 mBq/kg of dry wood was obtained from the tree rings from Nagasaki, located at the centre of the local fallout of the Pu A-bomb detonated in 1945. This concentration peak was, however, observed in tree rings of 1965–1967. The concentration was only 2.9 mBq/kg for the tree rings of 1944–1946. The contribution of the local fallout on the surface soils from the A-bomb was 181 mBq/cm 2 at the harvested area of the tree, while the contribution of global fallout by many weapons testing was 5.9 mBq/cm 2 (or 3.3% total fallout in the region). The reason for the over 20 year time lag of 239+240 Pu uptake by the tree rings is unknown because many factors influence the routes of Pu into the tree rings. Also the chemical form of Pu in surface soils may have been changed by the surrounding environment. The highest concentration in the tree rings from France was 9.4 mBq/kg which is about 31% of Nagasaki 239 + 240 Pu concentration. The harvested area did not have any recorded Pu sources other than global fallout. An interesting result was that that distribution of 134 Cs and 137 Cs concentrations in the French white fir was different from Nagasaki. Data suggested that these new radionuclide inputs were from the Chernobyl accident. The mobility (or diffusion coefficient) of cesium is 2–8 cm 2 /yr in the portion of heart-wood tree rings (1870–1955). Although tree rings can record chronological inputs of various trace elements, some elements cannot be used. These exceptions would be elements that: (1) are mobile within tree rings; (2) have little understood entry routes to the tree rings (via roots, leaves or barks); and (3) have unknown biogeochemical behaviour in the surrounding environment. Further investigation is warranted to use tree rings as a tool to record past environmental history.

Changes in isotope ratio and content of dissolved helium through groundwater evolution

Abstract The dissolved He content and He isotope ratio are proxy indicators of groundwater evolution in the Shimokita peninsula. The record of 3H and excess bomb tritiogenic 3He reveals the intrusion depth of shallow and young groundwater into deep groundwater. The record of tritiogenic 3He suggests that prior to the period of nuclear testing, the natural production level of 3H irradiated by cosmic rays was probably 6 TU. Helium isotope ratios in the groundwater converge to that of the regional crustal He with increasing depth and dissolved He content. The regional degassed He has a 3He/4He (R) ratio of 7.24 × 10−7 which consists of 6% mantle He (with R=1. 1 × 10−5) and 94% radiogenic He (with R=1 ×10−8). The magnitude of degassing He flux is 5×10−9 m3/m2 a. Based on the accumulation of He, and taking into consideration the degassing He flux, groundwater at depths greater than 300 m below sea level is estimated to be stagnant, exhibiting residence times in excess of 102 Ka.

Mobile and immobile plutonium in a groundwater environment

Abstract The mobility of plutonium was measured by laboratory experiments conducted under conditions simulating the groundwater environment. The results thus obtained are discussed in comparison with those obtained with an in situ study on fallout plutonium migration. Plutonium (IV) was found in a series of laboratory experiments to have large distribution coefficients ( K d ). On the other hand, the K d values of the fallout plutonium were about one-tenth as large as those of Pu (IV). This might be ascribed to either the different oxidation states or the difference in the chemical forms. Furthermore, a trace amount of mobile plutonium was confirmed to be produced even in the simulated groundwater environment. The production rate of mobile plutonium and the degree of change in chemical forms of plutonium during underground migration were found to be strongly influenced by the amount of suspended solids in the groundwater.

Fixation and mobilization of 60Co on sediments in coastal environments.

Abstract All Japanese nuclear power generating stations, placed on coastal areas, have produced an enormous amount of radioactive wastes in the past 15 yr. Surprisingly, the produced radioactive wastes have not as yet been disposed of by any means, but have been piled up at storage facilities in sites facing the sea. Radioactive cobalt (60C), a hazardous radionuclide of concern in radiation protection, has survived as one of the major components after short-term storage (e.g. two decades). The purpose of this study is to elucidate the underground migration of radioactive cobalt from the radioactive wastes that may be disposed of at burial sites in coastal regions in the future. The following two subjects are emphasized: (a) the mechanisms of fixation and mobilization of radioactive cobalt on sediments in coastal environments and (b) possible chemical forms of 60Co remaining in the water phase after interaction and desorption. Under aneraobic conditions, 30% of 60Co added to a sand sediment and freshwater system was potentially mobile. A contrast, more than 98% of the 60Co under aerobic conditions was permanently fixed on the sediment in the described system. Meanwhile, most of the mobile 60Co, produced under anaerobic conditions in seawater, consisted of non-ionic cobalt associated with low molecular weight materials. In conclusion, 60Co released in aquatic environments was categorized as mobile, exchangeable or irreversible, depending on the magnitude of its mobility. In particular, redox conditions in surroundings play a significant role in the distribution of 60Co between the aforementioned three categories.

Probability of Production of Mobile Plutonium in Environments of Soil and Sediment

Mobile plutonium was found in the bottom sediment in the Nishiyama reservoir in Nagasaki after more than 40 years from deposition of local fallout released in the explosion of the ABomb in 1945. Less than 10% of total deposited plutonium had turned into a mobile form in the bottom environment of the reservoir. The environmental conditions at bottom sediment is expected to be rich organic materials and high bacterial population under anaerobic conditions. Anaerobic bacteria have a high ability to uptake plutonium into cell during their growth. The Kd of plutonium to living bacteria is 20 times greater than the dead bacteria under anaerobic conditions. The results of field observations combined with empirical laboratory tests indicate that mobile plutonium in soil and sediment may be affected not only by binding with dissolved natural organic materials but also by the number of living anaerobic bacteria.

Plutonium mobility and its fate in soil and sediment environments

Abstract Mobile plutonium was found in waterlogged soil and reservoir sediment from the Nishiyama district of Nagasaki, accumulating from the local fallout released in the explosion of the A-bomb in 1945. Less than 10% of the total deposited plutonium had turned into a mobile form in the bottom sediment of the reservoir. The environmental conditions in the soft and sediment are expected to be rich in organic materials and to have high bacterial activities under anaerobic conditions. During growth, anaerobic bacteria have a strong ability to take up and retain plutonium. The K d of plutonium for living bacteria is 20 times higher than for dead bacteria under anaerobic conditions. The results of field observations combined with empirical laboratory tests indicate that mobile plutonium in soil and sediment may be affected not only by the binding to natural organic materials but also by activities of living bacteria. Furthermore, the intensity of the reducing environment dominates the degree of mobility of plutonium in the soil and sedimentary environments.