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

Tritium: Its relevance, sources and impacts on non-human biota

Tritium (³H) is a radioactive isotope of hydrogen that is abundantly released from nuclear industries. It is extremely mobile in the environment and in all biological systems, representing an increasing concern for the health of both humans and non-human biota (NHB). The present review examines the sources and characteristics of tritium in the environment, and evaluates available information pertaining to its biological effects at different levels of biological organisation in NHB. Despite an increasing number of publications in the tritium radiobiology field, there exists a significant disparity between data available for the different taxonomic groups and species, and observations are heavily biased towards marine bivalves, fish and mammals (rodents). Further limitations relate to the scarcity of information in the field relative to the laboratory, and lack of studies that employ forms of tritium other than tritiated water (HTO). Within these constraints, different responses to HTO exposure, from molecular to behavioural, have been reported during early life stages, but the potential transgenerational effects are unclear. The application of rapidly developing "omics" techniques could help to fill these knowledge gaps and further elucidate the relationships between molecular and organismal level responses through the development of radiation specific adverse outcome pathways (AOPs). The use of a greater diversity of keystone species and exposures to multiple stressors, elucidating other novel effects (e.g., by-stander, germ-line, transgenerational and epigenetic effects) offers opportunities to improve environmental risk assessments for the radionuclide. These could be combined with artificial intelligence (AI) including machine learning (ML) and ecosystem-based approaches.

Tritium: Doses and Responses of Aquatic Living Organisms (Model Experiments)

Tritium is a byproduct of many radiochemical reactions in the nuclear industry, and its effects on aquatic organisms, particularly low-dose effects, deserve special attention. The low-dose effects of tritium on aquatic microbiota have been intensively studied using luminous marine bacteria as model microorganisms. Low-dose physiological activation has been demonstrated and explained by the signaling role of reactive oxygen species through the “bystander effect” in bacterial suspensions. The activation of microbial functions in natural reservoirs by low tritium concentrations can cause unpredictable changes in food chains and imbalances in the natural equilibrium. The incorporation of tritium from the free form into organically bound compounds mainly occurs in the dark and at a temperature of 25 °C. When tritium is ingested by marine animals, up to 56% of tritium is accumulated in the muscle tissue and up to 36% in the liver. About 50% of tritium in the liver is bound in non-exchangeable forms. Human ingestion of water and food products contaminated with background levels of tritium does not significantly contribute to the total dose load on the human body.

Distribution of tritium concentration in the 0-25 cm surface soil of cultivated and uncultivated soil around the Qinshan nuclear power plant in China

In this study, tritiated water (HTO) and organically bound tritium (OBT) activity concentration at different depth soil layers (0-5 cm, 5-10 cm, 10-15 cm, 15-20 cm, and 20-25 cm) were measured in uncultivated and cultivated soil samples collected in the vicinity of the Qinshan nuclear power plant (QNPP) in July, September, and December 2018. The concentration difference, the spatial and temporal distribution, the seasonal variation, and the OBT/HTO ratios were investigated. The average ratios of HTO concentration between uncultivated and cultivated soil moisture were 1.20 ± 0.24, 1.39 ± 0.46 and 0.95 ± 0.14 in July, September and December, respectively, the corresponding values for OBT were 1.17 ± 0.31, 1.22 ± 0.49 and 1.08 ± 0.28. In generally, the highest HTO concentration in uncultivated soil was found for the topsoil (0-5 cm) in July and September and for the deeper soil layer (20-25 cm) in December, while for cultivated soil, the highest levels were found for the middle layer soil in July, for the topsoil (0-5 cm) in September, and for the deeper layer soil (20-25 cm) in December. Both soils, the vertical profile distribution of OBT concentration showed no consistent tendency, and there were no significant differences in the HTO and OBT concentrations between different soil layers, except for the highest concentration. Whether uncultivated soil or cultivated soil, HTO activity concentrations showed an apparent spatial distribution and seasonal variability, decreasing with the distance to the release sources and with sampling time, while OBT concentrations showed lower spatial and seasonal variability than HTO. In most cases, the OBT/HTO ratios were less than 1, with average values of 1.01 ± 0.48 and 1.06 ± 0.86 for cultivated soil and cultivated soil samples, respectively. The results of this work suggest that farming may affect tritium behavior in soil, while the spatial and temporal distribution of tritium is only slightly impacted.

Evaluation of public dose from FHR tritium release with consideration of meteorological uncertainties

Tritium management is a potentially significant issue in fluoride-salt-cooled high-temperature reactors (FHRs), as these reactors can produce tritium at high rates. Potential impact of the tritium released into the environment needs to be investigated to help determine the maximum-allowable tritium-release rate from an FHR plant. In this study, a dose assessment on the public resulting from FHR tritium release was performed via computational modeling. Three potential locations for FHR construction, i.e., the Hanford site, Idaho Falls in Idaho, and Oak Ridge in Tennessee, were selected. Atmospheric tritium dispersion was modeled using computer code family GENII and a parametric study of key meteorological variables was carried out. An uncertainty analysis was performed to examine the reliability of the prediction of dose for the year 2020. It is discovered that conditions in favor of lower public dose level from FHR tritium release include low atmosphere temperature, high wind speed, high relative humidity, and high tritium release point. It is also discovered that for different geological locations, the dominance of meteorological parameters differs significantly. Among the three locations modeled, although the Hanford site might be the most suitable location for FHR construction in the past, in the near future, Oak Ridge would possess advantages in the dose assessment aspect over the other two. We assumed that the tritium release rate from an FHR plant is given at 18.5 TBq/day and compared the probability of the maximum individual dose exceeding the regulatory limit (0.4 mSv/y). According to the prediction of dose for the year 2020, this probability is extremely low. While for Idaho Falls, it is 91.62% and for the Hanford site, 44.27%. The results indicate that effective measures should be taken for tritium control in FHR.

Tritium in precipitation on 5 sites in North-West France during the 2017-2019 period

Between October 2017 and May 2019, measurements of tritium in rainwater were carried out at several sites in north-west France. Tritium is an important tracer for hydroclimatic studies and this work provided up-to-date data that we compared with Global Network for Isotopes in Precipitation (GNIP) measurements. Of the various sites studied, some could potentially be affected by atmospheric gaseous emissions from the nuclear industries in the region (reprocessing plant, nuclear power plant). On our reference site, the activities measured in rainwater are often below the decision threshold (<0.15 Bq.L^-1). Two other sites with little impact from nuclear industries have mean activities of less than 0.7 Bq.L^-1. At the two Cherbourg sites closer to the nuclear industries, the activities in rainwater are slightly higher on average, though still close to 1 Bq.L^-1, but the activities are more variable when the rainfall accompanies an air mass from the Orano La Hague nuclear site. Using existing GNIP data and a simple model to simulate predicted data up to 2019, it is shown that all our measured data are comparable with the predicted activities for GNIP stations with a marine influence, in the case of the reference site and the sites with little impact from nuclear industries, and for GNIP stations with a continental influence, in the case of the other sites. Seasonal variation in activities was detected, with greater activities in the spring-summer period corresponding to the well known 'spring leak' phenomenon. This study also reveals significant differences between the activities measured on the western side of France (influenced by the Atlantic Ocean) and those measured in a continental zone. The mean levels of tritium in rainwater in France, excluding any nuclear influence, can be estimated on average at less than 0.3 Bq.L^-1 in the western marine zone, and at around 1 Bq.L^-1 in the continental zones.

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.

Multi-channel plastic-scintillator-based detection system for monitoring tritium in air

To overcome the limitations of the ionization chamber-based tritium monitor, a design for a multichannel plastic scintillator-based detection chamber for monitoring tritium in air is proposed. The performance of the chamber was characterized by Monte Carlo-based calculations with various design parameters such as thickness of the plastic scintillator (t) and number of channels (n). We considered the volume and detection efficiency of the chamber to evaluate the performance of the detector. The expected counting rate curve was in good agreement with the expected simplified physical model. The minimum detectable activity (MDA) was shown at t = 0.5 mm and n = 48 and estimated to be 29.9 kBq/m³. Compared with the experimentally estimated MDA, the relative difference was approximately 24%. However, this difference is understandable considering the different lower level of discrimination and light loss during transportation assumed in the model. The proposed tritium detection chamber is expected to be useful for environmental monitoring at some level as well as for monitoring of tritium leaks from fusion or CANada Deuterium Uranium (CANDU)-type reactors.