Enhanced activities of organically bound tritium in biota samples

Assessing OBT formation and enrichment: ROS signaling is involved in the radiation hormesis induced by tritium exposure in algae

Tritium is the main component of radioactive wastewater from nuclear power plants and can be migrated into organisms to form organically bound tritium (OBT), which may pose a potential risk to aquatic ecosystem. Hence, it is essential to monitor OBT conversion in the presence of tritium exposure. In this study, the effects of pretreatment methods such as digestion on the recovery of tritium were discussed. It was found that microwave digestion pretreatment could improve the recovery of tritium by up to 90 % and allow OBT measurement with a small sample size equivalent to about 60 mg (dry weight). In addition, the efficiency of OBT transformation was different among biological samples, and the radiation hormesis phenomenon was induced by tritium exposure (3.7 × 10⁶ Bq/L) in microalgae Chlorella vulgaris（C. vulgaris）. The tritium exposure may induce radiation hormesis through the reactive oxygen species (ROS) signaling pathway, thus improving the photosynthetic capacity and metabolism level of C. vulgaris. Furthermore, enhancement of photorespiration metabolism and the antioxidation system may be important means for C. vulgaris to balance damage by tritium radiation. This study provides insights for further investigating OBT behavior, and will contribute to understanding the equilibrium damage mechanism of algae exposed to tritium.

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.

Experimental database on water equivalent factor (WEQp) and organically bound tritium activity for tropical monsoonal climate region of South West Coast of India

Tritium in the form of tritiated water is easily incorporated into terrestrial biota as tissue free water tritium (TFWT). A part of TFWT is converted into organically bound tritium (OBT) through metabolic processes. For the computation of NE-OBT activity (expressed as Bq L^-1 of combustion water) in terrestrial plants, knowledge on 'water equivalent factor (WEQ_p)', defined as the volume of water produced from the combustion of 1 kg of the dry sample, is essential. On a global scenario, experimental data are not available on this parameter. This paper presents (i) a method for determination of WEQ_p by combustion method using a tube furnace system, (ii) a large database (N = 294) on WEQ_p parameter for samples of tropical monsoonal climate region of the Indian subcontinent, and (iii) NE-OBT activity in terrestrial biota samples (N = 186) collected from the vicinity of a PHWR nuclear power plant of India. The data generated in this study on WEQ_p serves for the validation of the data compiled in IAEA (2009 and 2010), which are estimated based on the hydrogen content of protein, fat and carbohydrates, and the fractions of protein, fat and carbohydrates. The WEQ_p varied in the ranges of 0.492-0.678 L kg^-1 (GM = 0.569 Bq L^-1, GSD = 1.06), 0.520-0.630 L kg^-1 (GM = 0.557 Bq L^-1, GSD = 1.02) 0.473-0.633 L kg^-1 (GM = 0.562 Bq L^-1, GSD = 1.02) for non-leafy vegetables, leafy vegetables, and fruits, respectively. A comparison between the experimental WEQ_p data with those compiled in the IAEA report revealed that the maximum deviation between the two data sets is <10%. The NE-OBT activity in the food samples collected from 2.3 to 20 km zone around NPP had a geometric mean (GM) value of 25.4 Bq L^-1 (GSD = 1.6, N = 186). Variations in NE-OBT activity with different seasons of the year are discussed.

Distribution of organically bound tritium (OBT) activity concentrations in aquatic biota from eastern Canada

A survey of eastern Canadian biota was conducted to determine the distribution of activity concentrations of organically bound tritium (OBT). Fish samples were collected from Lake Ontario and the St. Lawrence River in areas continuously receiving inputs of tritiated water (HTO) from operating nuclear power plants, and from Lake Nipissing, a background area. Components of their aquatic environments were sampled. The data collected also provides some insight on how tritium activity concentrations in ambient water influence tissue free water tritium (TFWT) and OBT activity concentrations in biological tissues. Using an ALOKA liquid scintillation system, fish TFWT and OBT were quantified. Fish TFWT averaged 1.6 ± 0.1 Bq/L in Lake Nipissing and 3.1 ± 0.3 Bq/L in Lake Ontario. In contrast, TFWT ranged from 11.1 to 80.8 Bq/L in the St. Lawrence River near the Gentilly-2 Canada Deuterium Uranium (CANDU) site. Fish tissue OBT levels were 4.0 ± 0.4 Bq/L and 5.3 ± 0.2 Bq/L for Lake Nipissing and Lake Ontario, respectively, and between 18.1 and 134.2 Bq/L for CANDU sites. The activity concentrations of TFWT and OBT were reviewed for algae, freshwater mussel and various fish samples collected near Gentilly-2, Pickering and Darlington Nuclear Power Generating Stations. TFWT in aquatic biota was found to correlate with the tritium activity concentrations measured in waters at the time of sampling (TFWT/HTO of ambient water was 0.3-4.3). The OBT concentration factors (OBT/HTO of ambient water) were found to be higher in freshwater mussels (between 17 and 47) compared to algae and fish (0.3-10). These results point to a heterogeneous distribution of biota OBT content in these aquatic ecosystems. It was also noted that all fish and algae samples were found to be within the range of tritium activity concentrations that has been historically measured in the same waters. Values in freshwater mussels were above this range.

Assimilation and transport of organic bound tritium in an irrigated pine forest

The speciation of radioactive tritium (T) in a naturally-established subtropical loblolly pine forest that has been irrigated with highly-contaminated pond water for the last 20 years is reported. This irrigation project was created to limit the underground transport of a tritium-rich plume which also contains low levels of toxic organics, metals and radionuclides such as carbon-14 (14C) from a nearby low-level waste burial ground. The levels of tritiated water (HTO) in the wood cores were not influenced by recent irrigation activities. However, the tritium levels in the last 20 years of tree growth were more than 3-fold higher than that of tritium in the older growth. This was due to recent irrigation with organic-bound tritium (OBT)-rich water and subsequent accumulation of high levels tritium as OBT relative to tissue HTO. High levels of pond irrigation water OBT resulted from biogenic processes that converted HTO to OBT. Data for 14C that were acquired for some of the forest materials indicated that the processes controlling the movement and accumulation of 14C in this system are somewhat different than that of tritium. Spectroscopic characterization of tree core tissue of <20 years in age found no explanation for the unusually wide dark growth rings. It was concluded that the trees were over-irrigated based on results from other published studies with wood from severely-flooded areas. Although HTO is indeed toxic to biota, OBT represents a relatively greater hazard to biota because it can be bioaccumulated and retained for long periods of time in living tissues.

An updated review on tritium in the environment

Various studies indicated more or less recently that organically bound tritium (OBT) formed from gaseous or liquid tritium releases into the environment potentially accumulates in organisms contradicting hypotheses associated to methods used to assess the biological impact of tritium on humans (ASN, 2010). Increasing research works were then performed during the last decade in order to gain knowledge on this radionuclide expected to be increasingly released by nuclear installations in the near future within the environment. This review focusses on publications of the last decade. New unpublished observations revealing the presence of technogenic tritium in a sedimentary archive collected in the upper reaches of the Rhône river and findings from the Northwestern Mediterranean revealing in all likelihood the impact of terrigenous tritium inputs on OBT levels recorded in living organisms are also presented. Identifying and understanding the physicochemical forms of tritium and the processes leading to its persistence in environmental compartments would explain most observations regarding OBT concentrations in organisms and definitively excludes that tritium would "bio accumulate" within living organisms.

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.

Changes in HTO and OBT activity concentrations in the Perch Lake aquatic ecosystem

Perch Lake, a small shallow shield lake located on the Chalk River Laboratories (CRL) site, contains elevated levels of tritium due to inputs from a nearby nuclear waste management area. The releases have been going on for many years but tritium levels in Perch Lake have been gradually decreasing since about year 2000. Lake water, sediments, aquatic plants, clams and fish were collected during the summer and fall of 2003 and 2013 at three locations in the lake. HTO activity concentrations were measured in all samples and OBT activity concentrations were measured in sediments, plants, clams and fish. In 2003, 2013, HTO activity concentrations in lake water were roughly uniform in time and space, except close to the shoreline where concentrations were fluctuating according to stream water and groundwater tritium levels in streams entering the lake. HTO activity concentrations of biota were similar to concentrations in lake water at the site where they were collected. OBT activity concentrations in biota were not always correlating with the lake water HTO levels. OBT to HTO ratios were found to be less than 1 for aquatic plants, around 1 for clams and fish and above 1 for birds reared on the shore of the lake.