Tritium in water bodies around the Kaiga generating station

Quantification of excess Carbon-14 specific activity in terrestrial biota in the off-site locations of the PHWR nuclear power plant at Kaiga, India

This is the first detailed study on ¹⁴C activity in the environment surrounding a nuclear facility in India. Samples of food matrices and wild plants from the off-site locations of the PHWR nuclear power plant (NPP) at Kaiga were analysed by liquid scintillation spectrometry, results were validated by accelerator mass spectrometry, and an extensive database (N = 142) was established. The stable isotope ratio of carbon (δ¹³C) in terrestrial plants varied from -33.5 to -23.3 ‰. The maximum excess ¹⁴C activity recorded in terrestrial biota was 44 Bq kg^-1C (19 pMC). About 75 % of the samples exhibited specific activity in the range 228-249 Bq kg^-1C (101-110 pMC). Statistical tests on the ¹⁴C specific activity dataset for 2.3-5, 5-10, and 10-20 km radial zones confirmed that the impact of the operation of the NPP on the environment beyond 5 km is minimal. The study suggests that the ¹⁴C activity released through gaseous effluents from Kaiga NPP is transported to greater distances along the axis of the valley than that predicted by the Gaussian plume model and those reported for other NPP sites worldwide. This is due to the unique topography of the Kaiga valley in which wind flow channelling, strong winds in the valley mouth, and calm wind within the valley due to the blocking effect by hills for the south-westerly wind regime play dominant roles in the transport of gaseous effluents. The ¹⁴C specific activity values at upwind monitoring stations located at >5 km distance from the NPP during the south-westerly wind regime were higher than those observed during the north-easterly wind regime when the same monitoring stations were located on the downwind side. The ingestion dose to the population in the 2.3-5 km radius zone, attributable to the release of ¹⁴C from the NPP, was 0.75 μSv y^-1. This is a negligibly small fraction of the ICRP recommended dose limit of 1000 μSv y^-1 for the public from other than natural sources. The dose due to the natural ¹⁴C activity in the Kaiga region was 12 µSv y^-1, corresponding to the ambient natural activity of 230 Bq kg^-1 C.

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.

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.

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.