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 Table of Contents 
ORIGINAL ARTICLE
Year : 2021  |  Volume : 44  |  Issue : 2  |  Page : 79-91  

Long-term trends in gamma radiation monitoring at the multi-facility nuclear site, Kalpakkam, South-India


1 Safety Quality and Resource Management Group, Indira Gandhi Center for Atomic Research, Kalpakkam, Tamil Nadu, India
2 Safety Quality and Resource Management Group, Indira Gandhi Center for Atomic Research, Kalpakkam, Tamil Nadu; Homi Bhabha National Institute, Mumbai, Maharashtra, India

Date of Submission31-May-2021
Date of Decision15-Jun-2021
Date of Acceptance08-Jul-2021
Date of Web Publication23-Oct-2021

Correspondence Address:
Deepu Radhakrishnan
Environmental Assessment Division, Safety Quality and Resource Management Group, Indira Gandhi Center for Atomic Research, Kalpakkam - 603 102, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.rpe_18_21

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  Abstract 


In this work, the analysis of the long-term environmental radiation monitoring data collected within the Department of Atomic Energy site, Kalpakkam using gamma monitors such as GammaTRACERS (GTs) from 2013 to 2018 and Autonomous Gamma Dose Loggers (AGDLs) located in the site boundary at distances from 1.25 to 2.5 km from 2016 to 2018 are presented with respect to dose variation in different locations/wind sectors/seasons. The average background dose rates are in the range of 140–220 nGy/h except at a location (GT2) where a dose rate of 400 ± 20 nGy/h is found. It is observed that the detectors generally measure the normal background doses and at times slightly higher doses (above background) due to exposure to 41Ar plume during normal operations from Madras Atomic Power Stations. The monitors were categorized into four groups: Group 1 to Group 4. Dose rates higher than normal were observed in Group 1 detectors during winter and North-east monsoon seasons and in Group 3 and Group 4 detectors during summer and South-west monsoon seasons and during January to March months in Group 2 detectors. The gamma dose rates during 41Ar plume transit vary in the range of 600–900 nGy/h. The cumulative annual effective doses at the site boundary, analyzed from 2016 to 2018, due to normal operational releases varied from 11 μSv to 114 μSv in different sectors which is significantly lower compared to annual dose limit (1000 μSv) for public.

Keywords: Autonomous Gamma Dose Loggers, environmental radiation monitoring, fission product noble gases, gamma radiation, GammaTRACERS, inverse source term, nuclear power plants


How to cite this article:
Radhakrishnan D, Boopathy M, Gopalakrishnan V, Rakesh P T, Chandrasekaran S, Srinivas C V, Venkatesan R, Venkatraman B. Long-term trends in gamma radiation monitoring at the multi-facility nuclear site, Kalpakkam, South-India. Radiat Prot Environ 2021;44:79-91

How to cite this URL:
Radhakrishnan D, Boopathy M, Gopalakrishnan V, Rakesh P T, Chandrasekaran S, Srinivas C V, Venkatesan R, Venkatraman B. Long-term trends in gamma radiation monitoring at the multi-facility nuclear site, Kalpakkam, South-India. Radiat Prot Environ [serial online] 2021 [cited 2021 Dec 6];44:79-91. Available from: https://www.rpe.org.in/text.asp?2021/44/2/79/329133




  Introduction Top


Nuclear energy is one of the potential sources of clean energies in demand today. The operation of nuclear facilities such as power reactors and fuel cycle facilities necessitate continuous surveillance of radiation to detect abnormal radiation levels in the event of uncontrolled releases which may be harmful to population. To improve the environmental radiological safety around the nuclear facilities radiological networks are set up by nuclear establishments and there are many studies on the utility of such monitoring.[1],[2],[3] Besides catering to the needs of continuous radiation surveillance, these radiological networks also measure the ambient radiation levels at the site and thus offer potential application to assess the characteristics of radiation levels due to natural and manmade activities such as normal operational releases of radionuclides from nuclear facilities.

The Kalpakkam site of Department of Atomic Energy (DAE) on the South-east coast of India is a multi-facility nuclear complex with operational power reactors, test reactors, reprocessing facilities, and waste management facilities. At present, the site houses two units of 220 MW Pressurized Heavy Water Reactor (PHWR),[4] a 40 MW Fast Breeder Test Reactor,[5] Kalpakkam Reprocessing Plant, Compact Reprocessing Facility for Advanced Fuels (CORAL) in Lead Shielded Facility, and experimental reactor, KAMINI.[6]

In view of multiple nuclear facilities at this site and taking into account of the experiences of some of the past nuclear accidents that happened in the world, there has been increasing demand for rapid real-time radioactivity measurements.[7] Hence, environmental radiation monitoring of the site has been considered highly essential for various objectives such as continuous radiation surveillance[8] and to maintain baseline data against which emergency or accident releases can be assessed.[9] This will also help to detect any unmonitored releases of radioactivity into the environment[10] and to ensure that the releases from the nuclear facilities are within the limits.[11] Regulatory framework mandates the discharge of radioactive effluents from nuclear facilities to be within permissible limits so that the annual doses to the public in the vicinity of facilities are well within allowable limits.[12] ICRP 103 recommends an annual dose limit of 1 mSv/year above the natural background dose to the public due to operation of nuclear facilities. Using the preoperational data collected during the period 1981–1983, the annual doses[13] only due to natural background radiation have been estimated as 1140 ± 35 μGy/year at Kalpakkam site. Although the annual dose values due to operational releases are usually calculated using theoretical models[14],[15] which are generally conservative, the most realistic calculations of the doses can be made using measured values of activity concentrations in the environmental media and external dose rates.[16] Hence, it is necessary to continuously and independently monitor the area around a nuclear facility, to ensure that the radiation doses are within the prescribed limits. Measurement of annual doses at the site boundary using different systems such as Thermo luminescent Dosimeters[12],[17] are found in the literature.

The Environmental Assessment Division at the Indira Gandhi Center for Atomic Research (IGCAR) carries out the radiation surveillance[18] around the site as one of the essential safety activities since 1997 using a variety of gamma monitors such as GammaTRACERS (GT), Autonomous Gamma Dose Loggers (AGDL)[19] and Local Area Network (LAN) based Environmental Gamma Dose Loggers (EGDL), etc. During the period 1997–2006, the monitoring was performed using off-line detectors such as GT, Area Gamma Monitoring stations. Since 2005 the monitoring has been strengthened with the establishment of LAN/Ethernet based Environmental Gamma Dose Loggers(EGDL)[20] for both radiation surveillance and emergency response applications. During emergencies, a rapid assessment of radiation field can be achieved using data from the environmental radiation monitors.[21] Hence, a Decision Support System (DSS) known as Online Nuclear Emergency Response System (ONERS)[22] has been implemented in 2004 at Kalpakkam site for guidance and decision support in nuclear or radiological emergencies. To provide radiation field inputs to the DSS, a system of AGDLs which can operate from remote locations with solar power is established in different wind sectors. The real-time monitoring data[23] from the AGDLs are used by the DSS for event detection and source term estimation using inverse calculation model[24] for the purpose of assessing the radiation impact in case of any accident situation.

In this article, the data from the GTs (collected continuously from 2013 to 2018) and AGDLs (collected during 2016–2018) were analyzed to assess the natural background radiation levels at various locations within the site and the dose exposure mainly from 41Ar and trace quantities of fission product noble gases (FPNGs) due to the routine continuous releases from operational nuclear facilities (Madras Atomic Power Station [MAPS]-1 and 2). The dose variations at various locations within the site with respect to seasons/wind sectors were studied, and long-term trends were obtained. Further, the cumulative annual doses (effective dose) were calculated for years, 2016–2018 at the site boundary (at distances ranging from 1 km to 2.5 km) using the data of the AGDLs in the outer ring. Detailed description of the spatial distribution of the detectors is given in next section.


  Materials and Methods Top


Brief description of the detectors and their distribution

The present study uses the data collected by various monitors such as GT and AGDLs for analyzing the environmental radiation at Kalpakkam site over a long observation period (2013–2018). All the monitors (GT and AGDL) use the Geiger-Muller-based detectors and measure the ambient equivalent dose rate. The GTs (Berlin Instruments Instruments GmbH, Germany) shown in [Figure 1]a were the first to be employed at Kalpakkam site along with High Pressure Ionization Chambers during 1997 which are portable monitors deployed at specific locations for environmental radiation monitoring. The data recorded by the GTs are stored in an in-built chip and retrieved offline in laboratory using software. In the early monitoring period (1997–2013), the GTs were used as mobile surveying instruments. Based on the prevailing wind direction and requirements, they were moved from place to place. Background dose rates, preoperational monitoring data, and other intermediate releases from the nearby facilities were collected using the GTs. Currently, the GTs are installed in nine permanent locations named as GT1 to GT9 [Figure 2]. As mentioned in previous section, the EGDLs and AGDLs are LAN/Wireless-based gamma monitoring systems indigenously developed[18] at IGCAR [shown in [Figure 1]b]. The EGDL were later decommissioned after the development of enhanced wireless-based AGDLs. The specifications of GTs and AGDLs are given in [Table 1].
Figure 1: (a) GammaTRACERS and (b) autonomous gamma dose loggers installed at EAD

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Figure 2: (a) Location map of environmental radiation monitors at Department of Atomic Energy Complex, Kalpakkam, together with wind sectors (A-P). (b) Location of detectors used in the current study with distances from Madras Atomic Power Station

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Table 1: Detector specifications of autonomous gamma dose loggers s and GammaTRACERS

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Furthermore, the GTs and AGDLs were compared with each other in earlier studies[18] at IGCAR and it is found that the trends of the average dose rates recorded by these detectors match well. The AGDL has shown very good correlation with energy influence monitor[25] tuned to identify 41Ar.

The AGDL detectors are currently installed in 27 locations, initially at 13 places during 2015 and later at 14 more places during 2018. The AGDLs are distributed in different wind direction sectors mainly on the land side in a two-ring fashion with the inner ring approximately at 0.75 km radius and outer ring at 1.5 km radius from the stack of the PHWR also called the MAPS to capture the radiation field from atmospheric releases. Out of the 27 AGDLs, 25 are located on the land side and two detectors on the sea side over the jetties of MAPS and the Prototype Fast Breeder Reactor of Bharativya Navikiya Vidyut Nigam Limited. The data from the AGDLs are transmitted in real time using the wireless communication to the central receiving station, and the data are stored in a server for future analysis purposes. [Figure 2]a shows the location map of the GTs and AGDLs at DAE site, Kalpakkam along with wind sectors labelled A to P, each covering an angle of 22.5° and latest Sentinel-2 satellite imagery of the site downloaded from USGS Earth Explorer website (https://earthexplorer.usgs.gov/).

The DAE site, Kalpakkam is also equipped with a 50-m instrumented meteorological tower[26] for continuous observation of meteorological parameters such as wind speed, wind direction, temperature, humidity, and rainfall required in atmospheric dispersion modeling and other environmental studies. [Figure 2]b gives the locations of the detectors used in the current study as well as their respective distances from MAPS. The releases from other facilities discussed in previous section are discrete and are for very short period of time and some facilities release radioactive effluents only during specific operations. Hence, clear trends in dose variations could not be seen with respect to those facilities and only releases from major nuclear facilities, MAPS (1 and 2) are considered in this work.

Analysis of wind parameters recorded by meteorological tower, Kalpakkam

The radiological status of a nuclear complex depends on both natural and man-made components. The low levels of certain routine radionuclide released from major facilities would enhance the radiation levels above the background in the environment. This man-made component typically depends on the quantity of release from a given facility and the prevailing weather condition such as wind speed, atmospheric stability, wind flow sector, etc., for its dispersion in the ambient atmosphere. Hence, the radiation monitoring data collected by GTs (2013–2018) and AGDLs (2016–2018) are analyzed along with meteorological information to characterize the dose levels and their time of occurrence.

The meteorological data recorded at Edaiyur meteorological tower, Kalpakkam [Figure 2] during 2013–2018 are analyzed for the joint frequency distribution of wind speed and wind direction. The annual wind rose of Kalpakkam site [Figure 3] shows that the wind blows mainly in the North-east, South-west (SW), Southern, and South-eastern sectors during major part of the year. The wind directions are mainly from North-east during December to February (winter), from South during March to May (summer) and from SW during June to September (SW monsoon). As Kalpakkam is situated in the coast of Bay of Bengal, sea breeze[27] develops during summer and SW monsoon seasons due to strong land-sea temperature contrast and consequent local pressure gradient. Thus next to the large scale flows in different seasons, the sea breeze from the direction range 90°–165° (ENE, E, ESE, SE, SSE sectors) is also a major wind flow at Kalpakkam Coastal site.
Figure 3: Time-wind direction rose plot for Kalpakkam site from 2013 to 2018

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Analysis of GammaTRACERS and Autonomous Gamma Dose Loggers data

The GTs are distributed along the DAE site along N-SSW direction and the AGDLs (in outer ring) are distributed mainly on all the land sectors around the site. Hence, for dose rate analysis based on the predominant wind directions prevalent at the site, both GT (GT1-GT9) and AGDLs (AGDL1-AGDL13) are clubbed into four major groups (Group 1–4) as given in [Table 2]. The spatial distribution of the group of detectors is shown in [Figure 4].
Figure 4: Grouping of environmental radiation monitors

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Table 2: Grouping of environmental radiation detectors

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The time series plots of GT and AGDL data acquired for every 10 min are plotted month-wise and are analyzed here in this study. The wind data of sensors located at 50 m and 32 m in meteorological tower, Edaiyur are also analyzed month-wise for the same years along with wind-time rose (wind-direction vs. time) plots.


  Results and Discussion Top


Here, we present the trends in the ambient radiation levels in different groups of detectors followed by annual cumulative doses in different wind sectors during the observation period.

Background radiation analysis

Natural background radiation is present everywhere and the exposure to radiation is an inescapable and continuing feature on earth. The radiation dose to humans due to the exposure to the background radiation varies from place to place depending on contributions from diverse sources[28],[29],[30] present in nature. For any analysis to be carried out with the data from the radiation detectors, the background radiation dose levels at that location need to be identified.

The radiation data recorded by the environmental radiation monitors include contributions of both the releases from nuclear facilities and background radiation. From the analysis, it is found that the doses due to plume exposure are generally short term with sharp rise and fall in the dose rates giving rise to peaks and on the other hand the dose rates recorded only due to the background radiation are long term and are slowly varying without any significant peaks. Dose rate recorded by few detectors, AGDL09, AGDL10, AGDL11, and AGDL12 on a randomly chosen date, February 14, 2018 is shown as an example in [Figure 5]. From [Figure 5], the variations in the dose rates during plume transit can be clearly seen in the case of detectors, AGDL10, AGDL11, and AGDL12. From the joint frequency distribution of Time vs. Wind Direction for the same day, the predominant wind directions are found to be ENE (70%), NE (21%), NNE (8%), and E (1%). Hence, the detectors AGDL11 and AGDL12 located in the downwind direction have recorded lot of peaks. The detectors AGDL09 and AGDL10 have only very small fluctuation in dose rates without any peaks except for a short peak in the morning in AGDL10.
Figure 5: Dose rates recorded by AGDL09 - AGDL11 shown to distinguish the pattern of measured dose rates due to plume transit and natural background

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Hence, for calculating the background dose rates at a detector location, the data collected for every 10 min are first screened month wise for peaks. The dose variation at any detector location is considered only due to the natural background radiation when there are no peaks. Hence, the average value of dose rates when there are no significant peaks and when fluctuations are minimum is taken as the background value at a particular detector location. As an example, the day wise time series of dose rates recorded every 10 min during January 2017 in AGDL04 detector is shown in [Figure 6]. In [Figure 6], there are no peaks and the variation in the dose rate is small, ranging from 180 to 220 nGy/h. Hence, the average value of 200 ± 5 nGy/h is taken as the background dose rate at AGDL04 location. Furthermore, the wind direction information recorded by the local meteorological tower is taken into account to confirm that the wind is not on that sector of detector location as well as on the adjacent sectors. Note that the background dose rate is subjected to change due to factors such as soil disturbances, rainfall, construction activities, or change of top soil.
Figure 6: Time series plot of AGDL04 for January-17

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Similarly, the background values at different detector locations are determined. The overall background dose rate of all the detectors located in DAE site, Kalpakkam is found to be in the range of 150–220 nGy/h with an exception of GT2 having 400 ± 20 nGy/h as background dose rate. Gamma spectrometry observations using Hyper-Pure Germanium Detectors of the soil samples in the GT2 location reveal the presence of thorium and its daughter products contributing to the increase in the background dose rate. Furthermore, it has been extensively cited in the literature that the elevated gamma background radiation in beaches of Kalpakkam is due to the presence of monazite sand.[31],[32],[33] Furthermore, it is observed that some detectors (AGDL05, AGDL11, and AGDL12) initially kept in direct soil which when shifted to concrete platform (due to construction activities), the background dose rate got reduced by ~ 50 nGy/h. The hourly averaged background values of the detectors calculated using above method are summarized in [Table 3] along with standard deviation values.
Table 3: Background dose rates of the environmental radiation monitors

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Trends of gamma dose rates from Group-1

During the North-east monsoon and winter seasons, the time variation of dose rates measured by the GTs from GT1 to GT4 and AGDLs from AGDL10 to AGDL12 located in the SW wind sectors indicate increased dose rates above the background level suggesting the influence of plume exposure from operating nuclear facilities such as MAPS (during the normal operating conditions) when the wind blows from NE and NNE directions. The time series of background subtracted hourly dose rates plotted month wise show large number of peaks with dose rates varying from 400 to 900 nGy/h at different locations during the above-mentioned period. As an example, the time series of dose rate plots [Figure 7] for GT1 to GT4 and AGDL10 to AGDL12 and Time-Wind Direction rose plots of February 2017 [Figure 8] are shown below.
Figure 7: Time series of hourly dose rates at GT1-GT4, AGDL10- AGDL12 during February 17

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Figure 8: Time-wind direction plot of February 2017 at 50 m level in Edaiyur Meteorological. Tower

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The black dotted line shown in the plots in [Figure 7] represents the average dose rate at a particular instant of time of all the days for the respective month. From [Figure 7], it can be seen that the overall pattern of the dose rates is similar in AGDL10, AGDL11, AGDL12, and GT4 in terms of time variation (lowest dose rates from 100 to 300 nGy/hr observed during 03:00–10:00 (HH: MM) (Indian Standard Time [IST] and the measured dose rate increasing to 300–600 nGy/h during 10:00–23:00 IST). As the detectors, GT1, GT2, and GT3 are located at farther distances of 1.5–2.5 km from the MAPS, decrease in dose rates is observed (in comparison with the detectors mentioned above) which is due to the diffusion of the plume in the downwind sector. Furthermore, it is observed that the dose rates usually increase usually after 10:00 h and high dose rates prevail till end of the day. On many of the days of this season, the dose rates are around three to four times the natural background radiation. The enhanced dose rates during daytime after 10:00 IST could be due to the formation of Thermal Internal Boundary Layer (TIBL) during the sea breeze hours over Kalpakkam[34] occurs from the NE sector during the North-east monsoon season. Furthermore, it may be noted from wind rose plot [Figure 8] that on most of the days during winter season, the flow before 10:00 IST is from North or North-west, thus the releases due to operation of the reactor are transported and diffused to the sea side and is not affecting the monitors located in the above sectors.

During the remaining period of the year comprising summer (March-May) and SW monsoon (June to September) seasons, these stations have randomly recorded very few peaks with no clear trends. Thus variation in the dose rate is observed to be very minimal during March to September months. From analyzing the wind direction data of these months, it is confirmed that wind blows to the opposite side of the detector location from MAPS, hence only background dose rates have been recorded by these detectors during summer and SW monsoons.

Trends of gamma dose rates from Group-2

The detectors AGDL07, AGDL08, and AGDL09 are located in W and WNW sectors (D and E) near Kunnathur village [Figure 2]. These detectors record the doses due to releases from MAPS during plume transit when the wind blows from E and ESE direction. From the analysis of wind data of 2016–2018, it is observed that wind from E and ESE direction is less prevalent in Kalpakkam coastal site. Wind from these directions is prevailing mostly in Jan-March in the range of 10%–20% distribution and in the remaining months, distribution only within 5% is present. In accordance with the wind data, these detectors have recorded good number of peaks in January to March months and very few peaks in the rest of the year in random fashion and only background dose rates have been recorded during most of the times. The time series of background subtracted hourly dose rates for AGDL07, AGDL08 and AGDL09 are shown for March 2016 in [Figure 9] along with Time-Wind Direction plots in [Figure 10]. From [Figure 9], it can seen that dose rates ranging from 150 to 300 nGy/h above the background have been recorded by the detectors AGDL07-AGDL09 on few days in March 2016 and on the rest of the days, the background subtracted dose rates are close to the zero.
Figure 9: Time series of dose rates at AGDL07, AGDL08 and AGDL09 for March-2016

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Figure 10: Time versus wind direction plot for March-16

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Trends of gamma dose rates of Group 3 detectors

The detectors GT5, GT6, AGDL3, AGDL4, AGDL5, and AGDL6 of Group 3 are located in the NW and NNW (C and D) sectors. Out of the six detectors, the GTs GT5 and GT6 are located close to the source (MAPS) at distances 0.6 km (GT5) and 1.0 km (GT6), whereas the AGDLs, AGDL3 to AGDL6 are located at the site boundary at distances 2.5–1.75 km from the source (MAPS). From the analysis of dose rates measured by these detectors, it is seen that these detectors have recorded radiation doses above the normal background level mainly during summer (March to May) and SW monsoon (June to September) season with large number of peaks. The reason for higher dose rates at these stations during summer has been verified using the wind data recorded by Meteorological tower [Figure 10] and [Figure 11]. From the meteorological observations of the period, 2016–2018, it is seen that 25%–50% of the wind in summer seasons, 5%–30% of the wind in SW monsoon seasons blows from SSE and SE directions. The major contributor is wind from SSE direction. These winds are recognized as the sea breeze. The SSE wind prevails mostly from 10:00 to 00:00 (HH:MM) IST hours. In summer seasons, it is observed that the sea breeze sets up early due to hot climates[35] and extends beyond 24 h. During SW monsoon seasons, the sea breeze sets up late and does not sets on some of the days due to large-scale flows from the west. In accordance with the wind direction data, these detectors got exposed to 41Ar plume from MAPS by sea breeze circulation and have recorded large number of peaks from 10:00 to 00:00 IST in summer. The occurrences of increased dose rates above the background level are found to be lesser during June-September months (SW season) and are usually seen after 14:00 IST. As an example, time series of background subtracted hourly dose rates of the detectors in Group-3 [Figure 12] and Joint Frequency Distribution plot of Time versus Wind Direction [Figure 11] are shown for May 2018. From the [Figure 12], peaks with dose rates above the background level are seen from 10:00 IST till the end of the day and are sometimes extended to early hours of next day. However, maximum dose rates are seen from 10:00 to 18:00 IST. Furthermore, it is observed that the dose rates corresponding to the peaks are higher in GT5 and GT6 (twice to thrice the background dose rates) than the AGDLs which are located farther away (twice the background dose rates).
Figure 11: Time versus wind direction plot for May 2018

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Figure 12: Time series of dose rates at (i) GT5 (ii) GT6 (iii) AGDL3 (iv) AGDL4 (v) AGDL5 and (vi) AGDL6 in May 2018 respectively

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In the remaining months of the years (October-February), these detectors have recorded dose rates equivalent to background dose rates with one or two random peaks as there are no upwind sources corresponding to this wind sector [Refer [Figure 8] for Time-Wind Direction plot of February 2017].

Analysis of data of Group-4

The detectors GT7, GT8 and GT9, AGDL1, AGDL2 and AGDL13 located in the N and NNE (A and P) sectors form Group 4. Similar to the detectors in Group 3, it is seen that the peaks due to high dose rates at the detectors in Group 4 were recorded during summer and SW monsoon seasons when the wind blows from S and SSW directions indicating the influence of the 41Ar plume from MAPS. From the analysis of wind data from 2016 to 2018, it is observed winds from S and SSW directions are present from 15% to 50% in summer, from 20% to 30% in SW monsoon, <10% in the remaining period of the year. (Joint Frequency Distribution Plot of Time-Wind Direction shown in [Figure 8], [Figure 10] and [Figure 11] can be referenced as examples). [Figure 13] shows the time series of background subtracted hourly dose rates recorded by Group 4 detectors for May-2018. As the detector GT7 is located close to MAPS, higher dose rates are observed compared to other detectors. Generally the southerly winds are prevalent in the morning and hence peaks are observed in the morning hours in [Figure 13]. During North-east monsoon and winter seasons, these detectors have recorded very less variations in hourly dose rates and on most of the days, only background dose rates were recorded.
Figure 13: Time series of background subtracted hourly dose rates recorded by (a) GT7 (b) GT8 (c) GT9 (d) AGDL1 (e) AGDL2 and (f) AGDL13 in May 2018

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Cumulative annual doses at the site boundary

The AGDLs in the outer ring are located at the site boundary at distances ranging from 1 km to 2.5 km. Due to geographical extent of the DAE site, Kalpakkam and closeness of the MAPS reactors to the coast, the AGDLs located in the site boundary in the northern and north-western wind sectors are at distances greater than 1.5 km and those at the western and south-western sectors are at distances lesser than 1.5 km. Detector wise distances from the source (MAPS) is given in [Table 4]. The total doses (effective dose) received by the these detectors in a year may be useful in estimating how much radiation dose is received by any person staying outside a nuclear complex due to normal operational releases. Hence the total annual doses recorded by the detectors, AGDL1 to AGDL13 were computed here in this study for the years 2016–2018. To compute the annual doses, the raw data of each AGDL in nGy/h units was visually screened and filtered for anomalies such as low dose values, erratic values, dose values accompanied by some special characters, values recorded during testing and calibration etc., and were compiled in uniform format. After filtering the anomalies, on an average 70%–90% of the data were utilized for computing the annual doses for the years 2016 and 2017 and for the year 2018, 60%–75% of the data were used for computing the annual dose.
Table 4: Annual Effective Doses at AGDLs in Outer Ring, DAE Complex, Kalpakkam

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Hourly averages were then calculated from the cleaned up 10 min data and were compiled year-wise. The hourly average dose values were then analyzed for each detector to arrive at the annual cumulative dose. The background dose values were computed for each detector location [refer section 3.1 and [Table 3]]. For each detector, the records which exceed the sum of background dose and 3 σ of the background dose (bkg + 3 σ) were identified, and the total sum of the identified records was computed for each year after subtracting the background dose values. On analyzing the associated hourly wind direction recorded at 50 m level of local Meteorological tower at Kalpakkam site, 70%–90% of high doses above the background level (used for calculating the cumulative dose) match with the corresponding downwind sectors as well as with nearby sectors. [Table 4] summarizes the annual doses measured at each detector for the years 2016, 2017, and 2018. From [Table 4], it is observed that the annual effective doses vary from a minimum of 40.43 μSv (AGDL07) to maximum of 114.3 μSv (AGDL10) in 2016 and vary from 39.96 μSv (AGDL07) to 189.52 μSv (AGDL12) in 2017 and vary from 11.27 μSv (AGDL07) in 74.64 μSv (AGDL11) in 2018. In all the 3 years (2016–2018), the minimum doses were received by the detectors in Group 2 and the maximum doses were received by the detectors in Group 1. It has to be noted that the detectors in Group 1 (AGDL10-AGDL12) located in SW wind sector are located at distances (1.0 km) which is 500 m short of site exclusion boundary (1.5 km) for dose rate measurements, could be one of the reasons slightly higher doses than the other detectors. Furthermore, it is observed that annual effective doses recorded in all the detectors for the year 2018 are less compared to the doses in previous years as one unit of MAPS was not under operation for the year 2018.


  Conclusions Top


The data from various environmental gamma radiation monitors such as GammaTRACERS (GT1-GT9) and Autonomous Gamma Dose Loggers (AGDL01-AGDL13) installed around the DAE Complex, Kalpakkam in different wind sectors are categorized into four groups, Group1 to Group-4 and are analysed season-wise and sector-wise. It is observed that the detectors generally measure the normal background doses and at certain times higher doses above the background due to exposure to plume from MAPS reactor (mainly 41Ar and traces of FPNGs) under favourable wind conditions.

The background dose rates at different detector locations are computed during the absence of the external plume and only when very small variations are present. The background dose rates are found to vary from 100 - 220 nGy/hr except at a southern location, GT2 where the dose rate is around 400±20 nGy/hr due to the presence of thorium rich soil. It is observed that the detectors in Group-1 (located in SW sector) recorded higher radiation doses above the background during the winter & north-east monsoon seasons and detectors in Group 3 & 4 recorded higher doses above background during summer & south-west monsoon seasons. Large number of peaks were observed in these detectors from 10:00 IST till end of the day during the above seasons with maximum dose rates up to 600-900 nGy/hr. The detectors in Group-2 (Western sector) recorded peaks with higher dose rates above background usually during January-March when the plume from MAPS was transported by easterly winds to their location. All the detectors have mostly recorded background doses or very few peaks during the remaining period of the year. The releases from other facilities at the site are usually for short duration and are taking place in discrete manner. Hence trends in the dose rates could not be correlated and qualitatively analysed.

The cumulative annual doses (effective doses) at the site boundary due to operational releases from the nuclear facilities (mainly MAPS) were calculated from 2016- 2018 using the data of 13 AGDLs, AGDL01 -AGDL13. In this three year period, the annual doses (above the background dose) varies from 11 μSv to 189 μSv which is very less when compared with the annual dose limit (1000 μSv) for the general public. The annual doses are found to be generally lesser in Group-2 detectors and higher in Group-1 detectors. It is noted that the Group-1 detectors are located at relatively shorter distances (~1.0 km) from MAPS stack which may be one of the reasons for slightly higher doses. It is also observed that the annual effective doses computed during the year 2018 are lesser (nearly half) than the years 2016 and 2017 at all the detectors as one reactor unit of MAPS was not under operation during 2018.

Acknowledgments

The authors sincerely thank Dr. A.K. Bhaduri, Director, IGCAR for providing constant support and encouragement for carrying out this work.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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