H. Yamanishi

J. Shimada

K. Miyamoto

M. Andoh

S. Yokoyama

T. Takahashi

T. Uda

National Institute for Fusion Science (NIFS-Repository)

 The levels of tritium in the atmosphere are nowadays almost only of natural origin and of the same range as before the era of the nuclear tests. In order to appraise the influence of tritium released from nuclear facilities to the environment, it is necessary to confirm the effect of tritium appearing overlapped on background tritium levels.  The purpose of this work is to confirm the background tritium level in natural water samples at the NIFS site and to understand the tritium behavior which is necessary to evaluate the behavior of the tritium released from the NIFS to the environment. The environmental tritium concentration around the nuclear or fusion facilities must be monitored not only to ensure safety control but also to gain the consent of local residents. Tritium concentrations and stable isotopes of oxygen and hydrogen in rain water, stream water and groundwater at the NIFS (National Institute for Fusion Science, Gifu prefecture, Japan) site were analyzed to understand behavior of the natural tritium in coupling with rain event. Conductivity, temperature and flow rate of the stream were monitored continuously. Fig. 1 shows the tritium concentration in rain water, stream water, river water, and groundwater samples. The range of tritium concentrations in rain were 0.09-0.78 Bq/l (average 0.37±0.14 Bq/l). The tritium concentration is low in summer and autumn and is high in winter and spring. The average tritium concentrations of stream and river water (R-10, R-15 and Toki River) and groundwater were 0.32±0.04, 0.34±0.06, 0.34±0.03and 0.25±0.02Bq/l, respectively. A significant difference is not observed at the concentrations of the stream and the river waters. As for the tritium concentration of rain, the lower concentrations compared to that of the stream, the river and the groundwater were observed depending on the precipitation period and the rainfall intensity. The tritium concentration of the groundwater is constant approximately through the year regardless the sampling holes. As the average concentration of groundwater was lower than the average concentration of rain, the groundwater at this site may be sufficiently mixed with rain and would have the residence time of a several years.  The stable isotopes of D and 18O in water samples collected in this study were measured with a mass spectrometer (DeltaS, Thermo electron Inc.). A seasonal variation of the δ value which reflects the difference of rain origin in summer and winter was seen in the stable isotopic ratio change of rain in 2007-2009 collected at NIFS site. This is the same as the typical change of rain in Honshu Island, Japan. The δ value of the stream and river waters shows the value of mixed summer and winter precipitation, but with a characteristic distribution of the stable isotopic ratio which is close to the Meteoric Water Line in the summer, what reflects relatively high amount of rainfall in summer season in Japan. The variation range of the stable isotopic ratio in the stream and the river waters is less than 10 % compared with that of rain. This suggests an effective mixing in the ground aquifer at NIFS site.  The two component separation is carried out using the δ18O, the conductivity and the tritium concentration at the rain event (2009/11/10). The results using the δ18O of the groundwater (-8.1‰) and the rain (-4.2‰) and the conductivity of the groundwater (0.130 µS/cm) and the rain (0.010 µS/cm) make possible to estimate the degree of mixing of the old water and the new water. The mixing trends in δ18O and conductivity are similar. However, the tritium concentration did not give the similar result, because the tritium concentration of the groundwater was lower than that of the stream water in this rain event. The seasonal variation of tritium concentration in rain should be considered as another factor controlling the tritium concentration of groundwater at NIFS site. The tritium data gives variable information as far as groundwater age which is difficult to derive from δ18O and conductivity. This work is performed with the support and under the auspices of the NIFS Collaboration Research Program.（NIFS10KSBA001）  Fig. 1 Tritium Activity in rain, stream water (R-10, R-15), river water (Toki), and groundwater at NIFS site    To assess the impact on public health caused by nuclear fusion facilities, it is necessary to understand the behavior of tritium in terrestrial environment for estimation of internal dose by ingestion of tritium reasonably. Therefore, we are developing a dynamic compartment model to predict tritium behavior in terrestrial environment. In this study, we tried to develop a dynamic compartment model to predict the tritium behaviors in ‘atmosphere – soil – river and groundwater flow’ system at the site of NIFS.  In a dynamic compartment model, the environment for the assessment is described as an assembly of compartments. The environment is divided to some parts, appropriately. Every compartment in the model is corresponding to environmental part. Simultaneous ordinary differential equations are used to describe the change of inventories of tritium in each compartment. In this study, the migration prediction code, MOGRA1) was used for development of the dynamic compartment model and case studies.  The conceptual illustration of major transfer pathways of environmental tritium considered in this study is shown in Fig. 1. The area considered in the study is upper air and soil of the site of NIFS.  The structure of dynamic compartment model is shown Fig. 2. In this study, both of HT and HTO are considered in air and soil. The wind flow causes inflow and outflow of HT and HTO in air. The HT and HTO in air are deposited into soil by dry deposition or wet deposition mechanisms. The deposited HT from air into soil is oxidized to HTO by a microorganism in soil rapidly. A part of HTO in soil is reemitted into air and the remainder is eliminated from soil by surface flow or infiltration.  For an analysis by using dynamic model, it is necessary to determine the values of the transfer coefficients among compartments, inflow rate into each compartment and outflow rate from each compartment, appropriately. In general, these values were determined by using sub-models and parameters used in the sub-models. The sub-models describe the transfer mechanisms of tritium in the environment. For an estimation of parameter values used in the sub-models, we use some monitoring data and references. For example, it is assumed that the mean concentrations of HT and HTO in air are 8 mBq m-3-air and 4 mBq m-3-air by using recent monitoring data at the site, respectively. The values of some parameter such as deposition velocity of HT and THO were determined with reference to ACUTRI code2). The case studies were carried out in a steady-state condition for a validation of the model, sub-models and parameter values. We assumed both of rainy condition and unrainy condition. The amount of eliminated tritium from soil compartment was estimated at 4x105Bq h-1 and 3x104 Bq h-1 in rainy condition and unrainy condition, respectively. It was estimated that the mean tritium concentration in outflow water from the site was about 2 Bq L-1. This estimated concentration is several times higher than the measured concentration at the downstream of the site. This result suggests that the model, sub-models and parameters used in this study are appropriate for safety assessment of nuclear fusion facilities because internal dose by ingestion of drinking water is overestimated. However, it was suggested that more detail information and discussion are required for more realistic simulation of tritium behavior in a terrestrial environment. Especially, it is important to understand the behavior of HTO at rainy condition.  1) Amano, H. et al.: J. Nucl. Sci. Technol., 40, (2003) 975-979. 2) Yokoyama, S. et al: JAERI-Data/Code 2002-022 (2002). Fig. 2. The structure of dynamic compartment model developed in this study. Fig. 1. Conceptual illustration of major transfer pathways of environmental tritium. 337§11. Study on Development of Dynamic Compartment Model for Estimation of Environmental Tritium Behavior at the Site of NIFSTakahashi, T. (Kyoto Univ. Research Reactor Inst.),  Andoh, M. (Japan Atomic Energy Agency),  Shimada, J. (Kumamoto Univ.),  Miyamoto, K. (National Inst. of Radiological Sci.),  Yokoyama, S. (School of Health Sci., Fujita. Health Univ.),  Yamanishi, H. (Kinki Univ. Atomic Energy Inst.),  Uda, T.

Study on Development of Dynamic Compartment Model for Estimation of Environmental Tritium Behavior at the Site of NIFS

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Study on Development of Dynamic Compartment Model for Estimation of Environmental Tritium Behavior at the Site of NIFS

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