Organically bound tritium: optimization of measurements in environmental matrices by combustion method and liquid scintillation spectrometry

Determination of carbon-14 in marine biota using oxidation combustion and gel suspension liquid scintillation counting

In this study, we report a new analytical method for determination of 14C in marine biota utilizing oxidation combustion separation combined with CaCO3 suspension of precipitates for liquid scintillation counting (LSC). The main factors affecting the oxidation combustion efficiency of tube combustion furnace and the counting efficiency of the LSC were systematically investigated and optimized. Under the optimal combustion conditions, the combustion recovery of carbon ranged from 85.6 % to 92.4 % for five marine samples. And the method achieved a detection limit of 0.13 Bq/g for 14C and repeatability of 3.9-9.1 %. The analytical turnover time was 6 h, and up to six samples could be processed simultaneously. Assessment of uncertainty components showed that the uncertainty in counting was the largest contributor, followed by combustion recovery and counting efficiency. The developed method enables reliable measurement of 14C in marine biota, providing valuable analytical tool for risk assessment purposes.

Toward an efficient determination of tissue-free water tritium in food

Introduction

Tritium in the environment constitutes a radiological concern because it can become part of the hydrogen pool in environmental and biological reservoirs and thereby expose people to radiation.

Methods

Tissue-free water tritium (TFWT) analysis in food is an important subject of environmental radiation monitoring which plays an important role in the estimation of health risks from environmental tritium exposure. At present, tritium content in food is generally determined by liquid scintillation counter (LSC). To improve the analytical efficiency in tritium determination, we developed a novel method to treat TFWT in food using microporous membranes.

Results

The microporous membrane treatment method developed in this study has the following characteristics: It has a wide range of application and can process TFWT samples with conductivity below 5 μS/cm. Sample loss for the microporous membrane treatment is approximately 5%. The average treatment time is only 5 min, significantly shortened compared with the currently used atmospheric distillation treatment method (1.5 h). The results of the comparison and spike experiment show that the samples prepared by microporous membrane treatment provides equally satisfactory tritium measurement results as classic distillation method.

Discussion

The developed microporous membrane method is simple to operate, efficient, and environmentally friendly, and effectively improves the analysis efficiency of TFWT in food.

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.

Carbon-14 emission from the pressurized heavy water reactor nuclear power plant at Kaiga, India

This is a detailed study on oxide (CO2) and reduced (hydrocarbons, CnHm) forms of 14C releases through gaseous effluents from the Kaiga nuclear power plant (NPP), on the West Coast of India, where 4 × 220 MW(e) pressurized heavy water reactors (PHWRs) are operating. The gaseous effluent from the common stack of reactor units 3 and 4 (each of 220 MW(e)) was sampled from 2017 to 2020 for 14C activity monitoring and analysed for 14C activity by liquid scintillation counting. The normalized release rate corresponding to the four-year monitoring period had a geometric mean value of 0.12 TBq GW(e)-1 a-1 (geometric standard deviation = 7.4), and the arithmetic mean with associated standard deviation was 0.75 ± 1.47 TBq GW(e)-1 a-1. The relative percentage contribution of reduced form (CH4) of 14C species was less than 1.27% of the total release. The normalized release rate from Kaiga NPP was similar to those reported for the other PHWR NPPs of the world. The 14C specific activity in the ambient air in the vicinity of the NPP was monitored at four locations. The maximum excess 14C activity values in the ambient air in the vicinity of the NPP, evaluated by comparing the specific activity recorded for the clean air region at ∼300 km from the NPP, were 65.1 Bq kg-1C (28.76 pMC) and 222.4 Bq kg-1C (98.23 pMC) for the years 2019 and 2020 respectively. In addition, the release rates were calculated from the Gaussian plume model using site-specific atmospheric dilution factors and the excess 14C specific activity measured at four off-site monitoring stations. The calculated values of release rates were in agreement (within a factor of ∼3) with the measured values.

NE-OBT and TFWT activity concentrations in wild plants in the vicinity of the PHWR nuclear power plant and control regions of the tropical monsoonal climatic region of the Indian subcontinent

The results of the first detailed study, involving a large number of samples, on water equivalent factor (WEQ_p), non-exchangeable organically bound tritium (NE-OBT) and tissue free water tritium (TFWT) activity concentrations in predominant plant species of the tropical monsoonal climatic region, are presented. A total of 369 samples from the vicinity of the PHWR nuclear power plant (NPP) at Kaiga, West Coast of India, and 47 samples of the control region (region not affected by local anthropogenic sources) were analysed. The WEQ_p varied in the range of 0.347-0.666 L kg^-1 with an overall mean value of 0.540 ± 0.045 L kg^-1. The NE-OBT activity concentration varied in the range of <9.8-60.9 Bq L^-1 of combustion water (mean = 24.6 ± 11.5 Bq L^-1) and that of TFWT in the range of 9.2-60.5 Bq L^-1 (mean = 30.7 ± 10.9 Bq L^-1) in the vicinity of the NPP. Rigorous statistical analysis of the data confirmed that (i) the activity concentrations of both forms of tritium decreased with the increase in the distance between the sampling location and NPP, and beyond 10 km, it was similar to that of the control region, (ii) the incorporation of tritium released from the NPP into wild plant leaves is not species-dependent, (iii) the NE-OBT activity concentration in the 5-10 km zone exhibited a dependence on the prevailing wind regime with respect to the NPP, but not in the 2.3-5 km zone which suggests that the transport of tritium, released into the atmosphere as the gaseous effluent, through diffusion is a dominating factor governing its activity concentration in the 2.3-5 km zone. The NE-OBT to TFWT specific activity concentration ratio (R-value) had a mean value of 0.82 ± 0.27 (range: 0.38-1.64) for samples collected from the vicinity of the NPP and 1.93 ± 0.50 (range: 1.35-3.19) for the control region. Recording higher NE-OBT activity concentration and R-value at the control region highlights the necessity of detailed studies to understand the mechanism of NE-OBT partitioning in the terrestrial environment.

Optimisation of CO2 absorption and liquid scintillation counting method for carbon-14 specific activity measurement in atmospheric air

A method for the determination of ¹⁴C activity in the ambient air was optimised with the development of a simple setup for the regeneration of CO₂ from carbonate sample and saturating the absorber in <45 min for direct determination of activity by liquid scintillation counting (LSC). Atmospheric CO₂ was trapped in NaOH solution and precipitated as BaCO₃ by adding BaCl₂. The carbonate sample was taken in a newly designed regeneration system, subjected to acid hydrolysis, and the absorber (CarboSorb-E) was saturated with the CO₂ regenerated from carbonate sample. This allowed optimisation of CO₂ absorption by the absorber (up to ~ 2.3941 g of CO₂/10 mL with an average of 2.1688 g) and a minimum detectable activity value of 14 Bq kg^-1C for a counting time of 300 min (8 Bq kg^-1C for 1000 min) was achieved with Quantulus - 1220 LSC system. The necessity of (i) the measurement of the total volume of air sampled, (ii) the determination of trapping efficiency for CO₂ in the NaOH, recovery of ¹⁴C in chemical processing of BaCO₃, and subsequent regeneration and absorption processes, and (iii) independent determination of carbon content in the air for expressing the results in terms of ¹⁴C specific activity (Bq kg^-1C), are avoided in this method. The method is capable of yielding accurate results, in a considerably shorter time when compared to previously reported methods, with a deviation of <2.2% from the target value (with a relative standard deviation of 1.1%, and a relative error of 0.53%) when ambient air samples from clean air region (region not affected by local anthropogenic sources of ¹⁴C) are analysed. Validation of the method was performed by (i) analysing BaCO₃ sample derived from ambient air by accelerator mass spectrometry, and (ii) analysing the CO₂ produced from the combustion of IAEA C3 reference material. Upon validation, the suitability of the method for determining small excess ¹⁴C specific activity in the vicinity of a nuclear power plant was demonstrated.

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.

Optimisation of a batch thermal combustion method using a tube furnace oxidation system (pyrolyser) and LSC for carbon-14 determination in environmental matrices

Accelerator mass spectrometry and benzene synthesis coupled with liquid scintillation spectrometry are often used for accurate measurements of ¹⁴C activity in the environmental matrices. Thermal oxidation is one of the methods employed for ¹⁴C determination in environmental matrices. In this method, the sample is oxidised at high temperature (600-900 °C) to convert carbon species to CO₂ and trapped in an amine-based absorber for determining the activity in a liquid scintillation counting (LSC) system. In this study, the performance of a commercially available tube furnace system (pyrolyser), for batch combustion of samples, was evaluated for the determination of ¹⁴C specific activity in terrestrial biota samples. Significant improvements over the manufacturer specified method, which is primarily designed for analysis of samples with activity well above the environmental background level, was implemented to achieve accurate determination of ¹⁴C specific activity at ambient background level. In the improved method, the CO₂ produced from the combustion of the sample was isolated from the combustion products through cryogenic trapping and then absorbed in the absorber (Carbo-Sorb E) through a simple off-line transfer process. This allowed (i) optimisation of CO₂ absorption by the absorber (2.2477 g of CO₂/10 mL), (ii) achieving good accuracy and precision in the measurements, and a minimum detectable activity value of 13 Bq kg^-1C for a counting time of 300 min (7 Bq kg^-1C for 1000 min), (iii) avoiding uncertainty associated with the determination of recovery of ¹⁴C in the combustion and trapping process, and (iv) elimination of the need for an independent determination of carbon content (%) for expressing the results in terms of ¹⁴C specific activity. The method is capable of yielding accurate results with a deviation of <2.4% from the target value for IAEA C3 quality assurance reference material (with a relative standard deviation of 1.40%, and relative error of 0.34%). The combined uncertainty (1σ) associated with the measurements was computed to be 3.4%. Upon optimisation, the suitability of the method for the determination of ¹⁴C specific activity in typical terrestrial biota samples of clean air region (region not affected by local anthropogenic sources) and for the quantification of a small increase in the ¹⁴C activity above ambient levels in the vicinity of a nuclear power plant is demonstrated.

Determination of Organically Bound Tritium (OBT) Concentration in Fish by Thermal Oxidation and Liquid Scintillation Counting Method

Effluents containing tritium (H) dispersed into the fresh water or marine environment from nuclear facilities can be taken up by biota. Aquatic and marine organisms are among the important pathways through which tritium can enter into the human body, and hence, assessment of the extent of pollution of these ecosystems is very important for radiation dose assessments. Tritium present in environmental matrices can be classified as tissue-free water tritium (TFWT) and organically bound tritium (OBT). Optimization of a method for the determination of OBT in fish, based on thermal oxidation of the sample, is discussed. Samples were subjected to thermal oxidation in a pyrolyser system, and the water produced from the combustion was analyzed by liquid scintillation spectrometry. Results show that a maximum of ~2 g of processed fish sample can be combusted efficiently in the pyrolyser. Using this method, a recovery of 84% was achieved, and minimum detectable activity (MDA) for the method was determined to be 8.5 Bq kg (sample weight = 2 g, counting time = 30,000 s, and detection efficiency = 20%).

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.

Search for tritium in air in a room equipped with 14 MeV neutron generator with tritiated targets

Eight documented tritiated targets were stored, as well as some very old targets with unknown activity, in a room equipped with an ING-114 14 MeV fast neutron generator. When the neutron generator was running, the tritiated targets were irradiated with a deuterium beam. The aim of this work is to determine the tritium content in the room's atmosphere, as well as the radiation exposure of workers in the room. In this study, isotopic exchange was assumed. This means that tritium from the targets diffused into the air, where it reacted immediately with oxygen particles to form vapour. These vapour molecules diffused into open vessels containing deionized water (50 ml in 120 ml plastic containers). Fifty vessels were arranged along the length (every 0.50 m) and width (every 1 m) of the room. Additionally, there were three vessels placed in the room for shorter periods (5, 7, and 12 days) together with a vessel that was exposed to the tritium for the full duration of the experiment (18 days) to determine the saturation curve. Based on the measured tritium contents, a map of the spatial distribution of tritium in the room was created. The results were used to calculate the radiation dose for a person working in the room and showed no significant contribution to the approved average annual dose for workers.