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Year : 2022  |  Volume : 45  |  Issue : 2  |  Page : 65-70  

Dilution factor and atmospheric dispersion pattern for Kalpakkam site

Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre, Environmental Survey Laboratory, Kalpakkam, Tamil Nadu, India

Date of Submission21-Oct-2022
Date of Decision01-Nov-2022
Date of Acceptance02-Nov-2022
Date of Web Publication20-Dec-2022

Correspondence Address:
T Jesan
Environmental Monitoring and Assessment Division, Bhabha Atomic Research Centre, Environmental Survey Laboratory, Kalpakkam - 603 102, Tamil Nadu
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/rpe.rpe_27_22

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Assessment of radiological impact and for planning and preparedness programs, monthly atmospheric dispersion patterns at Kalpakkam have been studied using observed surface meteorological data during 2015–2020. The influence of meteorological factors such as wind speed, wind directions, percent of calm, atmospheric stability, and rainfall on dispersion patterns has been evaluated. The higher wind speed range (>4 m/s) is observed from 11:00 h to 19:00 h with an average wind speed of 4.17 m/s and the highest value observed in the month of May. Extremely stable F category and percent of calm observed to be lowest in the month of June. This study reveals that the high concentration area is toward seaside sectors during the southwest monsoon season from June to September and the remaining months, January to May and October to December, the most probable dispersion toward southwest, South, and North covering land sectors. Wet deposition due to maximum rainfall and more rainy days in the month of November during the northeast monsoon. The study provides site-specific information on dispersion patterns, an essential tool and crucial support for risk management with respect to radiological impact, monitoring and assessment in normal and emergency scenarios of a nuclear facility, and guiding resources for siting and design of the new facility in and around Kalpakkam.

Keywords: Calm, dispersion, model, plume

How to cite this article:
Jesan T, Manonmani C, Ramkumar S, Saradhi I V, Kumar A V. Dilution factor and atmospheric dispersion pattern for Kalpakkam site. Radiat Prot Environ 2022;45:65-70

How to cite this URL:
Jesan T, Manonmani C, Ramkumar S, Saradhi I V, Kumar A V. Dilution factor and atmospheric dispersion pattern for Kalpakkam site. Radiat Prot Environ [serial online] 2022 [cited 2023 Jan 28];45:65-70. Available from: https://www.rpe.org.in/text.asp?2022/45/2/65/364559

  Introduction Top

The protection of workers and members of the public, both during routine releases and accident situations, is the foremost concern during the operation of a nuclear power plant (NPP). The atmosphere is a major exposure pathway for radioactive materials from an NPP to reach the public through dispersion in the environment.[1] Dispersion is a combination of advection and diffusion due to the wind motion and turbulence of air near the earth's surface, respectively. The concentration of contaminant released into the air at a particular location is estimated through the atmospheric dispersion model by mathematical simulation of the advection–diffusion equation.[2] Atmospheric dispersion models play a vital role in risk assessment and management to take the necessary action by decision-makers during both normal and emergency releases and it was highly realized during the Fukushima NPP accident.[3] For the local scale spread of contaminants Gaussian plume model (GPM), which is a core of the regulatory model, can be used for concentration evaluation due to continuous atmospheric releases with reasonable accuracy.[1]

  Materials and Methods Top

Kalpakkam is a tropical coastal site located (12°33'N and 80°11'E) on the east coast about 75 km South of Chennai, India. The shoreline of Kalpakkam is oriented about NNE–SSW direction and is approximately linear. Kalpakkam site consists of Madras Atomic Power Station (MAPS) with two units of pressurized heavy-water reactors, a fast breeder test reactor, a reprocessing plant and a Prototype Fast Breeder Reactor (1 × 500 MWe) is under construction.

The wind speed and wind direction at Kalpakkam site are measured by anemometer and wind vane sensors installed on a 50-m height meteorological tower at three different levels (10 m, 30 m, and 50 m) above the ground. The tower is located at Anupuram DAE Township, Kalpakkam, situated 5.6 km radial distance away from MAPS. In addition, observations of atmospheric temperature at 2 m and 6 m, atmospheric pressure at 2 m, relative humidity at 2 m, solar radiation at 6 m, and rainfall at 1 m are recorded with corresponding sensors. Ten-min data are collected using SymphoniePRO data logger placed at Micrometeorological Laboratory. SymphoniePRO desktop application software is used for retrieving the data.

The categorization of wind speed class is done with the letters P, Q, R, S, T, U, and V designated to represent 3–5, 6–11, 12–19, 20–29, 30–38, 39–50, and 51–61 km-h−1, respectively. Wind speed <3 km-h−1 are considered calm hours. The complete rotation of 360° is divided into 16 compass directions, with the center line of the sector coinciding with the direction ray. The wind data are presented as the wind rose, a graphical tool in a circular format used to give a brief view of how wind speed and direction are distributed at the location. The diffusion parameters, a function of atmospheric stability, are parameterized in terms of discrete stability classes. These classes range from A, most unstable, to F, most stable. Class D represents the neutral stability class. The hourly wind speed is used to classify stability classes into unstable (A, B, and C), neutral (D), and stable (E and F) classes based on the standard deviation of the wind direction in combination with the scalar mean wind speed at 10-m height. Extrapolated wind speed (U100 m) at stack height (Z100 m) of 100 m was obtained using empirical power law profiles from the 50 m height (Z50 m) measurement (U50 m) and stability-dependent coefficient p.[4] The power law of wind profile is expressed as:

Where P = 0.111 for unstable categories, P = 0.142 for neutral categories, and P = 0.33 for stable categories.[4] For long-term (month or year) dose calculation, the sector-averaged Gaussian plume model is used for finding the concentration (Bq-m−3) distribution.[1] For a given source term rate Q (Bq-sec−1) corrected with decay and deposition, the plume sector average concentration of radionuclide in a GPM is given by:

in which U is the mean wind speed, σz is the vertical dispersion coefficient at receptor distance xr from ground level for stack of height H, and θ is the angular width of the sector usually π/8 radians. Dilution factor (s-m−3) for the 16 compass directions of 22.5° angular width for various cardinal distances can be estimated for unit release (Q = 1Bq-s−1) using GPM.

  Results and Discussion Top

As Kalpakkam is a coastal site, it has a moderate atmospheric temperature range due to occurrences of sea breezes, which has an impact on wind speed and direction and dispersion of releases due to the formation of sea breeze fumigation (SBF).[1] [Figure 1] shows average wind rose for 50-m height measured data from 2015 to 2020. As shown the predominant wind directions for Kalpakkam site are South–southeast (9.96%), South (8.65%), and East-North-East (8.38%), and the predominant speed class is 12–19 km-h−1 (R-52.5%). The monthly mean wind speed at 50-m height and percent of calm for each month using hourly meteorological observed data from 2015 to 2020 at Kalpakkam is presented in [Figure 2]. The average wind speed was observed to be 4.17 m/s (blue dotted line) and 1.13% (red dotted line) calm conditions are observed for wind speed <0.83 m/s. Percent of calm was observed to be lowest in the month of June and >2% in the months of March, September, and October. Month of May shows the highest average wind speed during the 6 years following April month with a relatively lower value of standard deviation. Atmospheric dispersion at a site depends on wind speed as the higher the wind speed more will be the dilution due to the advection of pollutants to a longer distance in the given period.[5] [Figure 3] shows the diurnal hourly distribution of the monthly mean wind speed from 2015 to 2020 measured at 50-m height. In all months, the higher wind speed range (>4 m/s) is observed from 11:00 h to 19:00 h due to favorable unstable conditions for the sea breeze and also during April to August from 11:00 h to 23:00 h, due to favorable sea and land breeze conditions.
Figure 1: Wind rose

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Figure 2: Monthly mean wind speed and percent of calm. The error bar indicates the standard deviation over 2015 to 2020

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Figure 3: Hourly distribution of monthly mean wind speed

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During the time of 00:00 h to 07:00 h, lower wind speed range (<3.5 m/s) is observed in the months of January to March and September to October in comparison to favorable land breeze conditions months of April to August.

In the atmospheric dispersion studies, diffusion part is generally characterized by turbulence stability categories for regulatory purposes. [Figure 4] presents the average monthly variation of each stability category from 2015 to 2020 at Kalpakkam. The stability distribution study reveals that the neutral category D is most predominant at Kalpakkam due to moderately high winds (R speed class 3.3–5.3 m/s) at the site. Most of the daytime hours are predominantly unstable category (A, B, and C) with few hours of the neutral category (D) as expected due to tropical regional weather conditions at Kalpakkam, and the majority of nighttime hours are shared between stable (E and F) and neutral categories. Extremely stable F category is lowest during the month of June, followed by July and August. Stable category is relatively lower for number of night hours during southwest monsoon comparing to northeast monsoon.
Figure 4: Monthly stability class distribution: Monthly unstable (A, B, and C), neutral (D), and stable (E and F) stability classes distribution for each month using hourly meteorological observed data from 2015 to 2020 at Kalpakkam

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[Figure 5] shows the average monthly rainfall and rainy days variation over the period from 2015 to 2020 at Kalpakkam and the error bar indicates the standard deviation. Maximum rainfall and rainy days occurred during the month of November during the northeast monsoon period and the second greater number of rainy days in the month of August (southwest monsoon).

Higher value of standard deviation occurs in the month of November for rainfall and rainy days. The lowest rainfall was 567.1 mm in the year 1968, and the highest was 2440.3 mm during 2015.[6] Maximum daily rainfall of 317 mm occurred on December 2, 2015. The estimated 1000 years return period of rainfall corresponds to maximum daily rainfall, maximum monthly rainfall, and annual rainfall is 491.7 mm, 1618.2 mm, and 3473.0 mm, respectively for Kalpakkam site.[6]
Figure 5: Monthly rainfall and rainy days distribution: Monthly average rainfall and rainy days observed data from 2015 to 2020 at Kalpakkam

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The atmospheric processes of decreasing the concentration (dilution) of gaseous releases are indicative of the dispersal effect at a site. Quantitative estimation of the atmosphere's dilution and dispersal effect on releases is χ, concentration C divided by source strength Q at a given distance and direction with respect to the source. The χ also known as atmospheric dispersion or dilution factor represents the atmosphere's capability to dilute and disperse releases over the averaging period at a given site. The dispersion patterns based on observed surface meteorological data (2015–2020) at Kalpakkam were simulated using GPM for each month with a constant unit (Bq/s) radioactive release rate. [Figure 6] shows the spatial distribution of the average monthly dilution factor for 6-year period from 2015 to 2020 for 100-m height releases.
Figure 6: Average monthly dilution factor distribution patterns at Kalpakkam from 2015 to 2020 for a 100 m height releases

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The average monthly plume direction statistics and structure for the whole 6-year period indicates that the high concentration area is located in east direction sectors from the NPP during 4-month period of southwest monsoon season from June to September. During this season, the air travels from the NPP toward the Bay of Bengal Sea and the smallest amount of spread occurs over the landside sectors. It can be clearly seen that the release spread is practically restricted toward the mainland of Kalpakkam during the southwest monsoon. During the remaining 8 months, January to May and October to December, the most probable dispersion toward southwest, South, and North covering land sectors with a significant dilution of releases due to prevailing meteorological conditions at Kalpakkam. Dilution in the month of October seems to spread almost all the sectors, which could be due to the high percent of calm conditions and the transition period from southwest monsoon to northeast monsoon. Dry and wet depositions are critical routes to change the atmospheric dispersion patterns and concentration of releases. During northeast monsoons, high probable wet deposition of landside South and southwest, land sectors due to maximum rainfall and more rainy days in the month of November. The inducing ground deposition effect of rainfall and rainy days on dispersed releases can be approximated using empirical relations with the type of releases, deposition velocity of radionuclides, and intensity of rainfall. [Figure 7] presents the monthly average dilution factor distribution patterns during 2015–2020 for ground release cases. It shows the same pattern (covering almost the same sectors) with flattened increased dose distribution in comparison to relatively sharp edge sectors dose distribution of 100-m height releases [Figure 6]. The results of all the atmospheric dispersion patterns observed are useful for scaling to any specific release scenarios by renormalization of the overall concentration amount.[3],[4] The continuous constant releases during the assessment periods taken in this study can be considered reference source term pattern for all other release categories.[7] SBF occurrences at Kalpakkam are about 10% in a year, which will affect concentration pattern during elevated releases and can be assessed with correction factor.[1],[8]
Figure 7: Average monthly dilution factor distribution patterns at Kalpakkam from 2015 to 2020 due to ground releases

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Results indicate that the atmospheric dispersion directions are mainly determined by the large-scale meteorological conditions and local-scale diurnal air circulation in most of the period during a month at Kalpakkam site. The results also indicate that local scale dispersion of atmospheric releases spread toward the Bay of Bengal seaside during the southwest monsoon (June to September) which significantly decreases the exposure of radionuclide's to the public residing in landside sectors. In the remaining 8-month period of the year, the dispersion toward the land sectors has a significant dilution of releases due to prevailing meteorological conditions. The reliable and site-specific information on dispersion patterns of releases generated provides essential information and valuable resources to decision-makers for prompt impact assessment and management during normal and emergency conditions. The findings of the dispersion patterns also provide crucial support and guiding resources for siting and design of new facilities in and around Kalpakkam.


We thank Shri Sudhir Babanrao Shelke, Station Director, MAPS, Shri M Chenthamarakshan, Chief Superintendent, MAPS, and Shri S.Ravi, Superintendent QA, MAPS for extending the requisite facilities and continuous support.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Jesan T, Anand S, Rajaram S, Ravi PM, Tripathi RM. Computation of Gamma dose Due to Atmospheric Releases with Sea Breeze Fumigation, 20th National Symposium on Radiation Measurements (NSRP-20) October 28-30, 2015 Mangalore University; 2015.  Back to cited text no. 1
Stockie J. The mathematics of atmospheric dispersion modelling. SIAM Rev 2012;53:349-72.  Back to cited text no. 2
Yoshikane T, Yoshimura K. Dispersion characteristics of radioactive materials estimated by wind patterns. Sci Rep 2018;8:9926.  Back to cited text no. 3
Hukkoo RK, Bapat VN, Shirvaikar VV. BARC-1412 “Manual of Dose Evaluation from Atmospheric Releases”. Mumbai, BARC;1988.  Back to cited text no. 4
Leelossy A, Mészáros R, Lagzi I. Short and long term dispersion patterns of radionuclides in the atmosphere around the Fukushima Nuclear Power Plant. J Environ Radioact 2011;102:1117-21.  Back to cited text no. 5
Jesan T, Manonmani C, Rajaram S, Ravi PM, Tripathi RM, Pradeepkumar KS. “Statistical Analysis of Rainfall Events at Kalpakkam”, 3rd National Conference on Reliability and Safety Engineering (NCRS-2016), SSN College of Engineering, Chennai; December 1-3, 2016.  Back to cited text no. 6
Bilgic E, Gunduz O. Atmospheric dispersion patterns of radionuclides originating from nuclear power plant accidents under various release types. Int J Energy Prod Manage 2019;4:75-85.  Back to cited text no. 7
Jesan T, Anand S, Manonmani C, Ravi PM, Tripathi RM. “Identification of Sea Breeze at Kalpakkam Site” 20th National Symposium on Environment (NSE – 20) 13-15 December, 2018, IIT, Gandhi Nagar; 2018.  Back to cited text no. 8


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]


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