Estimation of aquatic dilution factor for liquid effluent discharges from Kaiga Generating Station using tritium as a tracer :TK Reji, RM Joshi, TL Ajith, Sanyam Jain, MS Vishnu, IV Saradhi, A Vinodkumar, Radiation Protection and Environment

ORIGINAL ARTICLE
Year : 2022 | Volume : 45 | Issue : 3 | Page : 127--130

Estimation of aquatic dilution factor for liquid effluent discharges from Kaiga Generating Station using tritium as a tracer

TK Reji¹, RM Joshi¹, TL Ajith¹, Sanyam Jain¹, MS Vishnu¹, IV Saradhi², A Vinodkumar²,
¹ Environmental Survey Laboratory, Environmental Studies Section, Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre, Kaiga, Karnataka, India
² Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India

Correspondence Address:
T K Reji
Environmental Survey Laboratory, Environmental Studies Section, Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre, Kaiga - 581 400, Karnataka
India

Abstract

Very low-level radioactive liquid effluents much below the approved discharge limits are discharged to Kadra reservoir after dilution with condenser cooling water at Kaiga Generating Station. In addition, further dilution occurs from the discharge point to the nearest public utilization point in Kadra reservoir (Hartuga). The dilution obtained in Kadra reservoir is estimated by simultaneous measurements of tritium at the discharge point and at Hartuga. The on-site measurement indicates a dilution factor of 12.2 ± 2.4 for tritium. A dilution factor of 13.3 ± 3.4 was obtained using aquatic dispersion model.

How to cite this article:
Reji T K, Joshi R M, Ajith T L, Jain S, Vishnu M S, Saradhi I V, Vinodkumar A. Estimation of aquatic dilution factor for liquid effluent discharges from Kaiga Generating Station using tritium as a tracer.Radiat Prot Environ 2022;45:127-130

How to cite this URL:
Reji T K, Joshi R M, Ajith T L, Jain S, Vishnu M S, Saradhi I V, Vinodkumar A. Estimation of aquatic dilution factor for liquid effluent discharges from Kaiga Generating Station using tritium as a tracer. Radiat Prot Environ [serial online] 2022 [cited 2023 May 30 ];45:127-130
Available from: https://www.rpe.org.in/text.asp?2022/45/3/127/377234

Full Text

Introduction

The basic criterion for evaluation of a site for the location of a nuclear facility is to ensure that the site–plant interaction will not result in an unacceptable radiological impact.[1],[2] The safety objectives include consideration of optimization of exposure to the population. It shall be determined whether the site should have favorable characteristics for the effective dilution of radioactive discharges from the facilities. This article presents the estimation of aquatic dilution factor from in situ measurements using tritium as a tracer and its comparison with aquatic dispersion models.[3]

Four units of 220 MWe nuclear power stations are operational at Kaiga site. Water required for condenser cooling (condenser cooling water [CCW]) is drawn from the reservoir through a concrete tunnel of 4 m diameter. Subsequently, the outfall water flows through an artificially made canal of width about 20 m at bed level.[4] The length of the discharge canal is about 1.6 km. The water from the outfall point flows into Kadra reservoir by gravity. In Kaiga Generating Station (KGS), once-through condenser cooling is employed. The maximum flow of CCW and the liquid waste injection rates are 2,39,340 m3.h−1 and 30 m3.h−1 respectively.[4] The design of the discharge canal is to facilitate maximum mixing.[5],[6] The flow rates of the liquid effluent discharge pumps are adjusted with the flow rate of the CCW so that, at any point of time, there is no violation of technical specification limits.

Materials and Methods

Study area

KGS site, with a latitude and longitude of 14.86° N and 74.44° E, respectively, is located 33 km (aerial distance) east of coastal town, Karwar. It is situated 16 km upstream of the Kadra dam of Kali river hydroelectric project complex on the left bank of Kadra reservoir. Kali river flows from north-east to western direction with respect to project site. Four units of 220 MWe pressurized heavy-water reactors are located on the south bank of Kadra reservoir formed in Kali river at a mean sea level of 40 m. [Figure 1] shows the schematic diagram of the Kadra reservoir system. It receives water from the Kodasalli reservoir situated in the upstream in Kali river.{Figure 1}

Methodology

In this article, the dilution factor of the liquid effluent discharged into the Kadra reservoir at the nearest public utility point, Hartuga, is estimated using measurement data. The obtained results are compared with that of the aquatic dispersion model for small lakes and reservoirs to validate the observations. Tritium present in the liquid effluent was used as a tracer in this study. The locations from which surface water samples were collected are shown in [Figure 1]. All water samples collected were brought to the laboratory and analyzed for tritium concentration using a liquid scintillation analyzer, model Hidex 300SL as per the standard procedure. The minimum detectable activity at a 3 σ confidence level for this counting system for a counting time of 300 min and for a sample (water) to scintillator combination of 6 mL + 8 mL was found to be 7 Bq.l−1.

The liquid effluents from KGS are discharged into Kadra reservoir in batches and one batch is discharged on January 1, 2020, from 10:40 h to 15:40 h with a total release of 359 GBq tritium activity. The tritium activity of the liquid effluent was 6.14 k Bq.ml−1. Liquid effluent discharge started at 10:40 h and the flow rate was 8 m3.h−1. Subsequently, the flow rate was maintained at 12 m3.h−1 from 12:45 h till the discharge stopped at 15:40 h. Water samples from the discharge canal and reservoir water at Hartuga were collected simultaneously before, during, and after the release of the effluents from the waste management facility. The tritium activity of the samples was analysed using M/s Hidex make liquid scintillation analyzer as per the standard procedure.[7] The dilution factor was determined from the ratio of the activity of tritium in the discharge canal and Hartuga.

The results were compared with that of the activity estimated using the aquatic dispersion model, International Atomic Energy Agency, a generic model for the dispersion of liquid effluents discharged into small lakes and reservoirs.[3] A schematic diagram showing effluents released into the reservoir is shown in [Figure 2]. In this study, discharge water body, Kadra reservoir, is considered a small lake/reservoir. For a small lake or reservoir, the radionuclide concentration is assumed to be uniform within the entire impoundment. A box-type model has been used for small lakes and reservoirs. For a small lake or reservoir, the radionuclide concentration is assumed to be uniform within the entire impoundment. The steady-state concentration of tritium, which is used as a tracer, in the reservoir is given by Eq. 1.[3]{Figure 2}

C = Q/(qr + λ V) (1)

where

C = Steady-state tritium concentration in the reservoir (Bq.l−1)Q = Annual average tritium discharge rate (Bq.s−1)λ = Decay constant of tritium (1.78E-09 s−1)qr = River flow rate (m3.s−1),V = Volume of the reservoir[6] (2.84E + 08 m3).

Results and Discussion

The activity concentration of tritium in water samples collected on January 1, 2020, at the discharge canal and reservoir is presented in [Table 1]. On January 1, 2020, the inflow and outflow to Kadra reservoir were 236.8 m3.s−1 and the reservoir level was 30.4 m. The tritium activity in the discharge canal varied from below detectable level (7 Bq.l−1) to 355.5 Bq.l−1. The tritium levels at Hartuga are in the range of 19.3–31.1 Bq.l−1 with an average value of 25.7 Bq.l−1. This indicates the attainment of equilibrium value in the Kadra reservoir. Kadra hydroelectric power plant is a peak load station and operates during night time (18:00–06:00 h). Liquid effluent discharge from KGS is carried out during the daytime. During the effluent discharge period, the outflow will be 0 and hence dilution will be maximum. The effluent is further diluted by the inflow of water from the upstream hydropower station. In this process, tritium activity in the reservoir attains an equilibrium concentration. This validates the observation of equal concentration of tritium activity at Hartuga before and after the effluent discharge. The estimated dilution factor for tritium measurements carried out is in the range of 8.3–14.7 with an average value of 12.2.{Table 1}

[Table 2], column A gives the annual average tritium concentration measured in the discharge canal during the period 2010–2020. Liquid effluents from the Kaiga site are discharged to the receiving water body after diluting by CCW.[7] Annual tritium discharge from KGS through the liquid route is converted to release rate, considering effluent discharge of 5 h/day for 285 days in each calendar year. Tritium concentration at the discharge canal is estimated considering the dilution offered from the maximum flow rate of CCW. The range of tritium activity in the discharge canal was 73.1–148.0 Bq.l−1 with a mean and standard deviation of 106.5 ± 25.1 Bq.l−1. [Table 2], column B gives the annual average estimated tritium concentration in the Hartuga reservoir water during the period 2010–2020 using the values of the parameters in Eq. 1. During the study period annual average tritium activity in the reservoir water varied from 4.4 to 16.4 Bq.l-1 with mean and standard deviation of 8.5 Bq.l-1 and 4.0 Bq.l-1 respectively. The corresponding dilution factor estimated using the model ranged from 7.1 to 19.2 with an average value 13.3. The volume of water in Kadra reservoir varies on a day-to-day basis which depends on discharge from the upstream hydel project Kodasalli, discharge from Kadra reservoir, and input due to rainfall. Kadra power station operates regularly during the monsoon season but irregularly during summer and winter. When there is no net outflow, the reservoir can be considered lake. In lakes, complete mixing of the effluents takes place and there is no removal mechanism from the consideration of rate addition of the radionuclide. Since tritium is long-lived radionuclide, the concentration will raise at a rate proportional to the rate of addition of the nuclide.[8],[9] Hence, annual average activity levels will yield results close to the actual situations.{Table 2}

The dilution factor obtained by the model is very close to the experimental result and the data used for estimating liquid effluent release limit by regulatory authority.[10]

Conclusions

The dilution factor of liquid effluent at the nearest utility point of the public was estimated by the tracer method. The aquatic dispersion model was used to validate the results. The determined results are in close agreement with 12.2 and 13.3, respectively. The quantum of effluents discharged and the levels of tritium at public utility points are much less than the regulatory limits.

Acknowledgments

The authors would like to thank Site Director, Kaiga Site, Station Director 1&2 and Station Director 3&4 for their keen interest and encouragement. The assistance and support rendered by ESL staff are thankfully acknowledged.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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