Indian Journal of Pure & Applied Physics Vol. 48, July 2010, pp. 520-523
Measurement of radon activity, exhalation rate and radiation doses in fly ash samples from NTPC Dadri, India Mamta Gupta1*, A K Mahur2,3, R G Sonkawade4, K D Verma1 & Rajendra Prasad2,3 1
Department of Physics, S V (P G) College, Aligarh 202 001, India
2
Department of Applied Physics, Z H College of Engg & Tech, AMU, Aligarh 202 002, India 3
Vivekananda College of Technology and Management, Aligarh 202 002, India
4
Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India E-mails:
[email protected],
[email protected] Received 22 April 2010; accepted 7 May 2010
Radon activities and radon exhalation rates have been measured in fly ash samples from NTPC (National Thermal Power Corporation), Dadri situated in Uttar Pradesh, using “Can technique”. This technique employs LR-115 type II solid state nuclear track detectors fixed at the top of the “Can” filled with fly ash samples. Radon activity has been found to vary from (222.56 ± 25.8) to (673.68 ± 45.1) Bqm−3 with an average value of (431.71 ± 35.5) Bqm−3. Surface exhalation rate has been found to vary from (80 ± 9) to (243 ± 16) mBqm−2h−1 with an average value (155 ± 13) mBqm−2 h−1, whereas mass exhalation rate has been found to vary from (3.1 ± 0.4) to (9.34 ± 0.6) mBq kg−1h−1 with an average value of (5.98 ± 0.5) mBqkg−1h−1. Indoor inhalation exposure (radon) effective dose has also been estimated which is found to vary from (5.8 ± 0.7) to (17.6 ± 1.2) µSvy−1 with an average value of (11.3 ± 0.9) µSvy−1. Keywords: Radon, Fly ash, LR-115 type II SSNTDs, Radon exhalation rate, NTPC Dadri
1 Introduction Coal contains naturally occurring radio nuclides arising from uranium and thorium series. Burning of coal and the subsequent emission to the environment from the thermal power plants are one of the sources of the technologically enhanced exposure of human beings from the natural radio nuclides. It is well established that Indian coal has high ash contents and an average of ~100 million tones of fly ash is produced per annum in India1-3. Earlier studies have shown that Indian coals contain 1.8-6.0 ppm 238U and 6.0-15.0 ppm of 232Th4. But recent studies have indicated as high as 50 ppm 232Th and 10 ppm 238U in pond ash generated from coal combustion 5. Coal ash is used in a variety of applications. The use of fly ash in the production of concrete bricks and blocks is most widespread. Fly ash based light weight building materials are also being produced. Thus fly ash produced by coal-burning in thermal power stations is important. It may raise the concentration of airborne indoor radioactivity to unacceptable levels, especially in places having low ventilation rates places. A large variation in radon activity is observed in dwellings, as the uranium concentration in natural materials used as building materials vary in a wide range and from place to place.
In the previous measurements6, fly ash was found to contain enhanced level of uranium as compared to coal. Because of its small size and hence large surface area, it has greater tendency to absorb trace elements that are transferred from coal to waste products during combustion7. Radon exhalation rate is of prime importance for the estimation of radiation risk from fly ash samples, used for building construction materials. Due to low level of radon emanation from building materials, long term measurements are required for which solid state nuclear track detectors can be used effectively and conveniently. In the present study, radon activity and radon exhalation rate have been measured in fly ash samples collected from NTPC (National Thermal Power Corporation) Dadri, situated in state of Uttar Pradesh in India. Indoor inhalation exposure (radon) effective doses have also been estimated from the radon exhalation rate. The aim of study is the possible health risk assessment in the area associated with the utilization of fly ash. 2 Experimental Details 2.1 Radon exhalation rate measurement
Fly ash samples collected from NTPC, Dadri, were dried and sieved through a 100 mesh sieve. For the
GUPTA et al.: RADON ACTIVITY, EXHALATION RATE AND RADIATION DOSES IN FLY ASH
measurement of radon exhalation rate, “Sealed can technique” was used. Equal amount of samples (100 gm) were placed in the cans (diam 7.0 cm and height 7.5 cm) similar to those used in the calibration experiment8. LR-115 type II solid state nuclear track detector (2 cm × 2 cm) was fixed on the top inside the cylinder can. The cans were sealed for 95 days. Sensitive lower surface of the detector is freely exposed to the emergent radon so that it could record the tracks of alpha particles resulting from the decay of radon in the remaining volume of can. Radon and its daughters reach an equilibrium concentration after 4 h and hence the equilibrium activity of emergent radon can be obtained from the geometry of can and time of exposure. After the exposure for 95 days, the detectors were etched in 2.5 N NaOH at 60°C for a period of 90 min in a constant temperature water bath. The resultant alpha-particle tracks were counted using an optical microscope at a magnification of 400X. From the track density; the radon activity was obtained using a calibration factor of 0.056 track cm−2d−1 (Bqm−3)−1 obtained from an earlier calibration experiment8. The surface exhalation rate of radon is obtained from the following expression9: EA =
CVλ 1 AT + {e−λT − 1} λ
… (1)
This formula is also modified to calculate the mass exhalation rate of radon:
EM =
CV λ 1 M T + {e−λT −1} λ
521
… (2)
where, EA is radon surface exhalation rate (Bqm−2h−1); EM is radon mass exhalation rate (Bqkg−1h−1); C is a integrated radon exposure as measured by LR-115 solid state nuclear track detectors (Bqm−3 h); V is the effective volume of can (m3); λ is the decay constant (hr−1); T is the exposure time (hr); A is the area of the can (m2) and M is mass of the sample. 3 Results and Discussion Health risks are especially high in the area downward of the thermal power plant. Being very small in size, fly ash particles may tend to remain airborne for long periods leading to serious health problems as the airborne ash can enter the lungs through inhalation and may stick to lung tissues. Due to higher radon emanating power, the lung tissues may be irradiated with α- particles from radon progeny to a high degree, increasing the possibility of lung cancer. The radio nuclides in the fly ash may migrate from the waste disposal site to the underlying ground water body and may also accumulate in the top soil giving sufficient chances for the radio nuclides to become enriched in the soil. Radon activity and radon exhalation rate are measured in a number of fly ash samples collected from NTPC, Dadri, U.P. and are presented in Table 1. The radon activities were found to vary from
Table 1 — Radon activity, radon exhalation rate and indoor inhalation exposure (radon) effective dose in fly ash samples from NTPC, Dadri India Samples
F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 F-10 F-11 Average value S.D. Rel. Std. %
Tracks cm−2d−1
Radon activity (Bqm-3)
Exhalation rate EA (mBq m−2h−1)
Exhalation rate EM (mBq kg−1h−1)
Indoor inhalation exposure (radon) effective dose (µSv y−1)
34.19 ± 2.4 36.38 ± 2.5 12.46 ± 1.4 37.73 ± 2.5 17.01 ± 1.7 21.05 ± 1.9 16.67 ± 1.7 24.42 ± 2.0 26.11 ± 2.1 24.08 ± 2.0 15.83 ± 1.6 24.18 ± 2.0 8.32 ± 0.35 34.41 ± 17.5
610.53 ± 42.7 649.62 ± 44.2 222.56 ± 25.8 673.68 ± 45.1 303.76 ± 30.1 375.90 ± 33.4 297.70 ± 30.1 436.10 ± 36.2 466.20 ± 37.3 430.10 ± 36.1 282.70 ± 29.1 431.71 ± 35.5 148.50 ± 6.2 34.40 ± 17.5
220 ±15 234 ± 16 80 ± 9 243 ± 16 109 ± 11 135 ± 12 107 ± 11 157 ± 13 168 ± 13 155 ± 13 102 ± 11 155 ± 13 54 ± 2 34.50 ± 17
8.46 ± 0.6 9.0 ± 0.6 3.1 ± 0.4 9.34 ± 0.6 4.21 ± 0.4 5.21 ± 0.5 4.13 ± 0.4 6.04 ± 0.5 6.46 ± 0.5 5.96 ± 0.5 3.92 ± 0.4 5.98 ± 0.5 2.06 ± 0.08 34.45 ± 16
16.0 ± 1.1 17.0 ± 1.2 5.8 ± 0.7 17.6 ± 1.2 7.9 ± 0.8 9.8 ± 0.9 7.8 ± 0.8 11.4 ± 0.9 12.2 ± 1.0 11.2 ± 0.9 7.4 ± 0.8 11.3 ± 0.9 3.89 ± 0.18 34.42 ±19.6
INDIAN J PURE & APPL PHYS, VOL 48, JULY 2010
522
Table 2 — Average radon exhalation rate in fly ash samples from different thermal power plants in India Name of thermal power plant
Kasimpur, Aligarh (U.P.) Parichha, Jhansi (U.P.) Obra, Mirjapur (U.P.) Kolaghat (W.B.) Durgapur (W.B.) NTPC,Dadri (U.P.), Present study
Average radon exhalation rate (± SD) (mBqm−2h−1) 275.3 ± 45.7 257.6 ± 48.7 239.9 ± 24.8 1310 ± 430 406.2 ± 37.7 155 ± 54
EP WLM . y −1 =
−3
(222.56±25.8) to (673.68 ± 45.1) Bqm with an average value of (431.71 ± 35.5) Bqm−3. Mass exhalation rate vary from (80 ± 9) to (243 ± 16) mBqm−2h−1 with an average value (155 ± 13) mBqm−2h−1 whereas, surface exhalation rate is found to vary from (3.1 ± 0.4) to (9.34 ± 0.6) mBq kg−1h−1 with an average value of (5.98 ± 0.5) mBq kg−1h−1. Radon activity and radon exhalation rate are lower than previous measurement on fly ash samples from different thermal power plants situated in two different states of the country8 as shown in Table 2. Mahur et al.9, observed that in coal and fly ash samples from thermal power plant at Kolaghat (W.B.), radon exhalation rate in fly ash (average value, 1310 mBq m−2h−1) is higher than in coal samples. It is worth mentioning that it is difficult to predict the radon exhalation rate from the concentration of uranium or its decay series products in a sample, since the radon exhalation rate depends also on the texture and grain size composition. In recent years, fly ash has found diversified applications in construction activities. Thus, it is quite important to estimate the radiation risk to the population from the radon exhalation rate. 3.1 Indoor internal exposure due to radon inhalation
The risk of lung cancer from domestic exposure of radon and its daughters can be estimated directly from the indoor inhalation exposure (radon) effective dose. The contribution of indoor radon concentration from fly ash samples can be calculated from the following expression10:
C Rn =
Ex × S V × λv
exhalation area (m2); V is room volume (m3), and λv is air exchange rate (h−1). The maximum radon concentration from the building material was assessed by assuming the room as a cavity with S/V = 2.0 m−1 and air exchange rate of 0.5 h−1. The annual exposure to potential alpha energy Ep (effective dose equivalent) is then related to the average radon concentration CRn by following expression:
…(3)
where, CRn is radon concentration (Bqm−3); Ex is radon exhalation rate (Bq m−2h−1); S is radon
8760 × n × F × CRn 170 × 3700
… (4)
where, CRn is in Bqm−3; n is the fraction of time spent indoor; 8760, the number of hours per year; 170, the number of hours per working month and F, the equilibrium factor for radon, is taken as 0.42 as suggested by UNSCEAR11. Radon progeny equilibrium is the most important quantity, where dose calculations are to be made on the basis of the measurement of radon concentration, it may have value 0
