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This is a detailed study on oxide (CO 2) and reduced (hydrocarbons, C n H m) 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 (CH 4) 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. • 14C release in oxide and reduced forms from Kaiga PHWR NPP, India were quantified. • Mean value of the normalized release rate was 1.08 ± 1.80 TBq GW(e)−1 a−1. • 14C release in reduced form was <1.27% of the total release. • Maximum excess 14C activity in ambient air in the vicinity of the NPP was 222 Bq kg−1C 97 pMC (980.7‰). [ABSTRACT FROM AUTHOR]