Tissue-free water tritium and non-exchangeable organically bound tritium concentrations in fish near coastline during and after operation of Japan's first nuclear fuel reprocessing facility.

The risks from radioactive wastewater release from nuclear facilities into the ocean are a global concern. Radioactive contaminants, such as tritium (3H), in both forms of tissue free water tritium (TFWT) and non-exchangeable organically bound tritium (NE-OBT), can be incorporated into marine biota and cause radiation doses to biota and future consumers. However, no studies have been conducted to measure both forms of 3H in marine fish as well as evaluate the residence time in the vicinity of a nuclear fuel reprocessing facility. Here, fish from a brackish lake and from the Pacific Ocean coastline of Japan, which are near such a facility, were collected between 2006 and 2021. The reprocessing facility was operational between 2006 and 2009, during which time about 300 times more tritiated water was discharged per year into the ocean compared to the period when the facility was not operational. During operation the annual release was 30 times higher than the treated water released annually from Fukushima Daiichi. As expected, TFWT and NE-OBT concentrations increased in marine fish during operations and had peak values of 3.59 ± 0.03 and 0.56 ± 0.03 Bq/L, respectively. Total dose rates to the fish were 36,000 times lower than the 10 µGy h-1 benchmark. Concentrations gradually decreased to pre-operational levels as the facility was turned off with NE-OBT taking twice as long. Fish sampled from the brackish lake tended to have more incorporated TFWT and NE-OBT concentrations than ocean fish. This indicates that ocean tides might have contributed to the accumulation of discharged tritiated water in the lake via a narrow water channel, which highlights the importance of examining all marine ecosystems in future operations. In both marine environments, the estimated committed effective dose using the highest observed data through ingestion was well below public limits (91,000 times lower).

Tritium distribution in typical plants around tritium laboratory in south-west of China.

A low-temperature desorption method was developed to remove the tissue free water tritium (TFWT) in plants, which dramatically reduces the sample-preparation time from several days to only 2-3 h. Two factors, including spatial distribution and seasonal variation, that influence the tritium distribution and the organic bound tritium (OBT)/TFWT ratios in plants were investigated based on three different kinds of subtropical-basin plants located near the decommissioned nuclear facility in south-west of China. Under the same environmental condition, higher tritium concentration was seen in pine needle (PN) compared with flat bamboo-root (FBT) and wild cotton (WC), while these three plants show similar location- and season-dependent trend of tritium distribution. The location-dependent investigation shows that OBT and TFWT concentration are significantly higher in plants growing in the downwind direction of the tritium laboratory than that in plants growing in the upwind and cross direction. In different seasons, the TFWT in plants reached peak in winter and valley in summer, while the OBT increased continuously with the season changing from spring to winter. The ratios of OBT/TFWT were investigated to evaluate the ability of plants to concentrate tritium into organic matter, which were 0.97-2.72 for PN, 0.89-1.64 for FBR, and 0.80-1.62 for WC.

Assessment of the tritium distribution in the vegetation cover in the areas of underground nuclear explosions at the Semipalatinsk test site.

This paper provides results of assessment of the tritium distribution in the vegetation cover in the areas of underground nuclear explosions at the Semipalatinsk Test Site (STS). The research was conducted at the former "Degelen" test site along the streams that are one of the main channels of tritium migration from underground nuclear explosions epicenters. The dominant plant species Carex supine and Achnatherum splendens that belong to different ecological groups in relation to humidity were selected as representatives of the vegetation cover. The TFWT (tissue free water tritium) and OBT (organically bound tritium) activity concentrations in the vegetation cover were measurement. TFWT activity concentration in the samples of both plant species had high values with an average of up to 30 kBq kg-1. The OBT activity concentration was 1-2 orders of magnitude lower than the TFWT in all plant samples. The TFWT and OBT activity concentrations in vegetation samples are closely correlated (r = 0.75, p < 0.05). No statistically significant difference was found between the content of tritium in the samples of Carex supine and those of Achnatherum splendens taken at the same locations. OBT/HTO ratios for vegetation samples of both species were close to equilibrium ratio used in environmental transfer models. In some cases, OBT/HTO ratios were significantly lower than one, which indicates that simple environmental transfer models may not accurately predict the behavior of HTO and OBT in different environmental compartments. The average OBT/HTO ratio for soil samples (0.6 ± 0.1) close to the equilibrium value indicates the equilibrium condition at the research area. The obtained [OBT]plant/[OBT]soil ratios indicate that soil organic matter accumulates tritium from year to year. However, in some locations with high tritium contamination ratios [OBT]plant/[OBT]soil were more than one due to OBT activity in soils is almost the same as OBT activity in plants. It was found that the nature of the spatial distribution of tritium in the vegetation cover in the areas of underground nuclear explosions is complex, and obviously depends on the location of the tunnels in which nuclear tests were conducted, as well as on the peculiarities of the hydrological regime of underground and surface waters, which are the main channels of tritium migration in the research area. Thus, the vegetation cover reflects the spatial distribution of tritium contamination in the sites of underground nuclear explosions and can be used as an indicator of the radiation situation when monitoring radiation-hazardous areas.

Tritium forms discrimination in ryegrass under constant tritium exposure: From seed germination to seedling autotrophy.

Uncertainties remain regarding the fate of atmospheric tritium after it has been assimilated in grasslands (ryegrass) in the form of TFWT (Tissue Free Water Tritium) or OBT (Organically Bound Tritium). One such uncertainty relates to the tritium forms discrimination during transfer from TFWT to OBT resulting from photosynthesis (OBTphoto), corresponding to the OBTphoto/TFWT ratio. In this study, the OBT/TFWT ratio is determined by experiments in the laboratory using a ryegrass model and hydroponic cultures, with constant activity of tritium in the form of tritiated water (denoted as HTO) in the "water" compartment (liquid HTO) and "air" compartment (HTO vapour in the air). The OBTphoto/TFWT ratio and the exchangeable OBT fraction are measured for three parts of the plant: the leaf, seed and root. Plant growth is modelled using dehydrated biomass measurements taken over time in the laboratory and integrating physiological functions of the plant during the first ten days after germination. The results suggest that there is no measurable discrimination of tritium in the plant organic matter produced by photosynthesis.

The VATO project: An original methodology to study the transfer of tritium as HT and HTO in grassland ecosystem.

Tritium (3H) is mainly released into the environment by nuclear power plants, military nuclear facilities and nuclear reprocessing plants. The construction of new nuclear facilities in the world as well as the evolution of nuclear fuel management might lead to an increase of 3H discharges from the nuclear industry. The VATO project was set up by IRSN (Institut de Radioprotection et de Sûreté Nucléaire) and EDF (Electricité de France) to reduce the uncertainties in the knowledge about transfers of 3H from an atmospheric source (currently releasing HT and HTO) to a grassland ecosystem. A fully instrumented technical platform with specifically designed materials was set up downwind of the AREVA NC La Hague reprocessing plant (Northwest of the France). This study, started in 2013, was conducted in four main steps to provide an hourly data set of 3H concentrations in the environment, adequate to develop and/or validate transfer models. It consisted first in characterizing the physico-chemical forms of 3H present in the air around the plant. Then, 3H transfer kinetics to grass were quantified regarding contributions from various compartments of the environment. For this purpose, an original experimental procedure was provided to take account for biases due to rehydration of freeze-dried samples for the determination of OBT activity concentrations in biological samples. In a third step, the 3H concentrations measured in the air and in rainwater were reconstructed at hourly intervals. Finally, a data processing technique was used to determine the biological half-lives of OBT in grass.

The VATO project: Development and validation of a dynamic transfer model of tritium in grassland ecosystem.

In this paper, a dynamic compartment model with a high temporal resolution has been investigated to describe tritium transfer in grassland ecosystems exposed to atmospheric 3H releases from nuclear facilities under normal operating or accidental conditions. TOCATTA-χ model belongs to the larger framework of the SYMBIOSE modelling and simulation platform that aims to assess the fate and transport of a wide range of radionuclides in various environmental systems. In this context, the conceptual and mathematical models of TOCATTA-χ have been designed to be relatively simple, minimizing the number of compartments and input parameters required. In the same time, the model achieves a good compromise between easy-to-use (as it is to be used in an operational mode), explicative power and predictive accuracy in various experimental conditions. In the framework of the VATO project, the model has been tested against two-year-long in situ measurements of 3H activity concentration monitored by IRSN in air, groundwater and grass, together with meteorological parameters, on a grass field plot located 2 km downwind of the AREVA NC La Hague nuclear reprocessing plant, as was done in the past for the evaluation of transfer of 14C in grass. By considering fast exchanges at the vegetation-air canopy interface, the model correctly reproduces the observed variability in TFWT activity concentration in grass, which evolves in accordance with spikes in atmospheric HTO activity concentration over the previous 24 h. The average OBT activity concentration in grass is also correctly reproduced. However, the model has to be improved in order to reproduce punctual high concentration of OBT activity, as observed in December 2013. The introduction of another compartment with a fast kinetic (like TFWT) - although outside the model scope - improves the predictions by increasing the correlation coefficient from 0.29 up to 0.56 when it includes this particular point. Further experimental investigation will be undertaken by IRSN and EDF next year to better evaluate (and properly model) other aspects of tritium transfer where knowledge gaps have been identified in both experimental and modelling areas.

Transfer of tritium into laying hen's meat and eggs at prolonged intake with atmospheric air, water and grass meal.

Following a continuous intake of tritium (T) by laying hens' over a 55 day period, an increase of НТО concentration both in eggs and meat was observed over the first 2 weeks for intakes via inhalation and ingestion of water and grass meal. After this time, equilibrium of the T in these products occurred. It was found that when the intake of HTO is from water, air and grass meal, the ratio of its activity concentration in muscular tissue to that in eggs does not exceed 1, 4, and 6 respectively. The ratio of ОBТ concentration to that of НТО in the meat of hens (ОBТ/HTO) when intakes were from water, air and grass meal were 0.08, 0.09 and 0.7, respectively.

The valley system of the Jihlava river and Mohelno reservoir with enhanced tritium activities.

The Dukovany nuclear power plant (NPP Dukovany) releases liquid effluents, including HTO, to the Mohelno reservoir, located in a deep valley. Significantly enhanced tritium activities were observed in the form of non-exchangeable organically bound tritium in the surrounding biota which lacks direct contact with the water body. This indicates a tritium uptake by plants from air moisture and haze, which is, besides the uptake by roots from soil, one of the most important mechanisms of tritium transfer from environment to plants. Results of a pilot study based on four sampling campaigns in 2011-2015 are presented and discussed, with the aim to provide new information on tritium transport in the Mohelno reservoir - Jihlava River - plants ecosystems.

Determination of changes to TFWT and OBT concentrations in potatoes and Swiss chard as a result of preparation for human consumption.

Ingestion is one of the most important pathways to consider for calculating tritium dose to human beings. The objective of this study is to determine changes to TFWT and OBT concentrations in food as a result of its preparation for consumption. The contribution of OBT to the total tritium dose can be reduced by the oxidation of OBT during food preparation. The results show that preparation for consumption can result in reductions of up to 46% in TFWT concentration and 54% in OBT concentration in potato, and 22% in TFWT concentration and 57% in OBT concentration in Swiss chard.

TOCATTA: a dynamic transfer model of (3)H from the atmosphere to soil-plant systems.

This paper describes a dynamic compartment model (TOCATTA) that simulates tritium transfer in agricultural plants of several categories including vegetables, pasture and annual crops, exposed to time-varying HTO concentrations of water vapour in the air and possibly in irrigation and rainwater. Consideration is also given to the transfer pathways of HTO in soil. Though the transfer of tritium is quite complex, from its release into the environment to its absorption and its incorporation within the organic material of living organisms, the TOCATTA model is relatively simple, with a limited number of compartments and input parameters appropriate to its use in an operational mode. In this paper, we took the opportunity to have data obtained on an ornamental plant - an indoor palm tree - within an industrial building where tritium was released accidentally over several weeks (or months). More specifically, the model's ability to provide hindsight on the chronology of the release scenario is discussed by comparing model predictions of TFWT and OBT activity concentrations in the plant leaves with measurements performed on three different leaves characterized by different developmental stages. The data-model comparison shows some limitations, mainly because of a lack of knowledge about the initial conditions of the accident and when it actually started and about the processes involved in the transfer of tritium. Efforts are needed in both experimental and modelling areas for future evaluation of tritium behaviour in agricultural soil and plants exposed to gaseous HTO releases and/or to irrigation with contaminated water.