ANALYSIS OF NATURAL RADIONUCLIDES IN COAL, SLAG AND ASH IN COAL-FIRED POWER PLANTS IN SERBIA

Transcription

1 J o u r n a l o f J. Mn. Metall. Sect. B-Metall. 47 (2) B (2011) M n n g a n d M e t a l l u r g y ANALYSIS OF NATURAL RADIONUCLIDES IN COAL, SLAG AND ASH IN COAL-FIRED POWER PLANTS IN SERBIA M.M. Jankovć, D.J. Todorovć, J.D. Nkolć # Unversty of Belgrade, Insttute Vnča, Radaton and Envronmental Protecton Department, Belgrade, Serba (Receved 08 February 2011; accepted 25 May 2011) Abstract The radoactvty montorng n the Nkola Tesla, Kolubara, Morava and Kostolac coal-fred power plants was performed by the Radaton and Envronmental Protecton Laboratory, Vnča Insttute of nuclear scences n the perod Montorng ncluded the analyss of sol, water, flyng ash, slag, coal and plants. Ths paper presents the results of the radoactvty analyss of coal, ash and slag samples. Naturally occurrng radonucldes 226 Ra, 232 Th, 40 K, 235 U, 238 U, and 210 Pb as well as the man-made radonuclde 137 Cs were determned by gamma spectrometry usng HPGe detector. The concentratons of pars of radonucldes were statstcally tested to determne the corellaton between them. Based on the obtaned results, health effect due to the actvty of these radonucldes was estmated va radum equvalent (Ra eq ), external hazard ndex (H ex ), external gamma absorbed dose rate ( ) and annual effectve dose. Keywords: Radoactvty; Coal; Ash; Slag. 1. Introducton The concept of technologcally enhanced natural radoactvty (TE NORM) was ntroduced n the md-seventes. It represents the unntentonal exposure to natural sources of radaton whch would not exst wthout the technologcal actvty [1]. Earler studes have shown that the man sources of technologcally enhanced natural radoactvty are coal-fred power plants and artfcal fertlzers appled n agrculture. Coal-fred power plants have been neglected as a radaton source for a long tme. They # Correspondng author: jnkolc@vnca.rs DOI: /JMMB J

2 150 M.M. Jankovć / JMM 47 (2) B (2011) became mportant for nvestgatons as a result of the advancement of the scentfc knowledge of bologcal effects of radaton on humans and after dose lmts reducton n nternatonal recommendatons and standards. Coal combuston n power plants leads to a redstrbuton of natural radonucldes orgnatng from coal, and to ther concentraton n ash and slag. The basc problem of technologcally enhanced natural radoactvty caused by coal-fred power plants s the ncrease of the background gamma radaton level. Therefore, the local populaton s exposed to hgher gamma radaton doses than n absence of coal-fred power plant. In the combuston process, most of the mneral materal n coal s converted nto ash. Coal, lke most materals found n nature, contans trace quanttes of naturally occurrng radonucldes, 238 U, 232 Th and 40 K [2]. Combuston of coal thus enhances natural radaton n the vcnty of the coalfred power plants by release of these radonucldes and ther daughters nto the surroundng ecosystem. Unlke most of the nuclear and hydroelectrc power plants, coalfred power plants n Serba are generally located n areas whch are thckly populated and hence, the envronmental mpact and health mpact experenced by the neghborng populaton s sgnfcant. In the perod as a part of regular control of workng and lvng envronment of coal-fred power plants: Nkola Tesla, Kolubara, Morava and Kostolac, the coal, ash and slag samples taken n the vcnty of the power plants have been analyzed. Based on the part of these analyss, health effect due to the actvty of present radonucldes was estmated va radum equvalent (Ra eq ), external hazard ndedx (H ex ), absorbed dose rate ( ) and annual effectve dose. 2. Expermental The coal, ash and slag samples were collected from varous stes n the vcnty and nsde the power plants. Samples were taken n Nkola Tesla coal-fred power plant durng 2003, 2004, 2007, and 2010, Kolubara durng 2004, 2009 and 2010, Morava durng 2004, 2007, 2008, 2009 and 2010 and Kostolac durng 2005, 2006, 2007, 2008 and The coal and slag samples were provded by the responsble servces n the power plants, whle the ash was sampled n ash ponds n the vcnty of power plants. The samplng stes were chosen n such a manner, that the representatve collecton of samples could be obtaned. Also, samplng stes were denoted by ther GPS coordnates n order to enable us to repeat samplng n the same stes n followng years. Coal, slag and ash samples were dred at C, sfted, weghed and sealed n Marnell beakers. In order to acheve radoactve equlbrum, the sealed samples were left n the laboratory for 30 days pror to the measurement. The samples were counted usng two CANBERRA hgh purty germanum detectors (HPGe) wth relatve effcency of 23 % and 20% and energy resoluton of 1.8 kev at the 1332 kev 60 Co peak. The calbraton was performed usng the sol matrx spked wth 22 Na, 57 Co, 60 Co, 89 Y, 133 Ba, 137 Cs, total actvty of 1.5 kbq kg -1, at

3 M.M. Jankovć / JMM 47 (2) B (2011) reference date , ssued by Natonal Offce of Measures, Budapest, MIX-OMH-SZ. The results were then ftted by the exponental curve. All samples were measured for 70000s. The actvty of 226 Ra and 232 Th was determned va ther decay products: 214 B (609, 1120 and 1764 kev), 214 Pb (352 and 295 kev) and 228 Ac (338 and 911 kev), respectvely. 235 U was determned va kev corrected for 226 Ra. 238 U was determned va 234 Th (63 kev) or by 234 Pa (t 1/2 = 1.17 mn, 1000 kev). The actvty of 137 Cs was determned from ts 661 kev energy. The actvtes of 40 K were determned from ts 1460 kev γ-energy. 3. Results and dscusson In the samples of coal, slag and ash, the natural radonucldes ( 226 Ra, 232 Th, 40 K, 235 U, 238 U, 210 Pb) were detected. The mean values and range of obtaned concentratons of naturally occurrng radonucldes n the analyzed samples are presented n Table 1. The combned uncertanty of the results, orgnatng from countng uncertanty, measurng of sample mass and uncertanty arsng from fttng of the effcency calbraton curve, was estmated to range from 10% to 40%. The concentratons of natural radonucldes n coal and slag are lower compared to the concentratons found n ash,.e., maxmum concentratons were obtaned n flyng ash samples. The dfference between concentratons of naturally occurrng radonucldes n ash from passve and actve ash pond are neglgble and do not exceed the statstcal varance. Accordng to UNSCEAR [1], the mean natural radonuclde concentraton expected n coal s 35 Bq kg 1 (range: 17 60) for 226 Ra, 30 Bq kg 1 (range: 11 64) for 232 Th and 400 Bq kg 1 (range: ) for 40 K. As one sees from Table 1, the radonuclde concentratons n coal samples from the Serban power plants are n the range of coal reported n UNSCEAR [1]. The actvty rato 235 U/ 238 U, corresponds to natural uranum n all samples. The range and average of natural radoactvty concentratons n coal, fly ash and slag samples reported n the present study are smlar to those obtaned for the Poland, Inda, Chna and Turkey power plant samples [3-6], except for the slag samples where the actvty concentratons obtaned n Table 1. Range and mean value of measured concentratons of radonucldes n coal, slag and ash samples from nvestgated coal-fred power plants 226 Ra [Bq kg -1 ] 232 Th [Bq kg -1 ] 40 K [Bq kg -1 ] 238 U [Bq kg -1 ] 235 U [Bq kg -1 ] range mean range mean range mean range mean range mean coal Nov slag flyng ash ash actve pond ash passve pond

4 152 M.M. Jankovć / JMM 47 (2) B (2011) ths study are slghtly lower than the concentratons n the slag samples reported for the Poland power plant [3]. For all gathered samples n all power plants, basc statstcal analyss was performed. Analyss ncluded calculaton of correlaton factor gven by [7]. xy 1/ n x y rxy...(1) x n 2 x y / 1/ n y The analyss showed an apparent correlaton between the specfc actvtes of detected radonucldes. Strong correlaton s notced between 226 Ra and 238 U (correlaton coeffcent r = 0.86), 226 Ra and 232 Th (correlaton coeffcent r = 0.87), 23 2Th and 40 K (correlaton coeffcent r = 0.87) and 226 Ra and 40 K (correlaton coeffcent r = 0.86), whle a slghtly lower correlaton appears between 238 U and 40 K (correlaton coeffcent r = 0.79). The dstrbuton of 226 Ra, 232 Th and 40 K n envronment s not unform. Unformty wth respect to exposure to radaton has been defned n terms of radum equvalent actvty (Ra eq ) n Bq kg 1 n order to compare the specfc actvty of materals contanng dfferent amounts of 226 Ra, 232 Th and 40 K. It s calculated usng the followng relaton [8, 9]: Ra eq = C Ra C Th C K...(2) where C Ra, C Th and C K are the actvty concentratons of 226 Ra, 232 Th and 40 K n Bqkg 1, respectvely. Whle defnng Ra eq actvty accordng to Eq. (2), t has been assumed that 370 Bq kg 1 of 226 Ra or 259 Bq kg 1 of 232 Th or 4810 Bq kg 1 of 40 K produce the same gamma dose rate. The external gamma absorbed dose rate n the ar at 1m above ground level was calculated from the measured actvtes of 226 Ra, 232 Th and 40 K n ash, assumng that the other radonucldes, such as 137 Cs, 90 Sr and the 235 U seres can be neglected as they contrbute nsgnfcantly to the total dose from envronmental background [10, 11, 12]. The calculatons were performed accordng to the followng equaton 1: D = 0.462C Ra C Th C K..(3) where represents the dose rate n ngy h 1 and C Ra, C Th and C K are the specfc actvtes (Bq kg 1 ) of 226 Ra, 232 Th and 40 K, respectvely. In the above equaton, t s assumed that all decay products of 226 Ra and 232 Th are n radoactve equlbrum wth ther precursors. The external hazard ndex, H ex, s defned as [7]. H ex = C Ra /370 + C Th /259 + C K /4810..(4) where C Ra, C Th and C K are the specfc actvtes (Bq kg 1 ) of 226 Ra, 232 Th and 40 K, respectvely. The value of ths ndex must be less than unty n order to keep the radaton hazard nsgnfcant. The maxmum value of H ex equal to unty corresponds to the upper lmt of radum equvalent actvty (370 Bq kg 1 ). To estmate the annual effectve dose, the followng must be taken nto account: (a) the converson coeffcent from absorbed dose n ar to effectve dose and (b) the ndoor occupancy factor. Usng the dose rate data obtaned from the concentraton values of natural radonucldes n sol, adoptng the converson factor of 0.7 SvGy 1 1 from absorbed dose rate n ar to effectve dose receved by adults and consderng that people, on the average, spend 20% of ther

5 M.M. Jankovć / JMM 47 (2) B (2011) tme outdoors 1, the annual effectve doses are calculated: Annual effectve dose [Sv]= [Gyh -1 ] (5) Obtaned results for Ra eq,h ex,and annual effectve dose are presented n Table 2 n terms of mean values over the nvestgated tme range for every power plant. These ndeces were calculated for ash only (pond ash and flyng ash), snce coal and slag are localzed wthn the power plants, and are not dstrbuted n the envronment. Ra eq ranged Table 2. Range and mean value of calculated radum equvalent (Ra eq ), external hazard ndex (H ex ), external gamma absorbed dose rate () and annual effectve dose n ash samples from nvestgated coal-fred power plants Ra eq [Bq kg 1 ] H ex [Bq kg 1 ] [ngy h 1 ] annual effectve dose [msv] range mean range mean range mean range mean Nkola Tesla A Nkola Tesla B Kostolac Morava Kolubara Fgure 1. Measured concentraton of radonucldes n coal n coal-fred power plant Te A - Nkola Tesla A, Te B - Nkola Tesla B, Ko - Kostolac, Mo - Morava and Kol - Kolubara. Numbers 03, 04, 07, 08, 09 and 10 denote the years 2003, 2004, 2007, 2008, 2009 and 2010.

6 154 M.M. Jankovć / JMM 47 (2) B (2011) Fgure 2. Measured concentraton of radonucldes n slag n coal-fred power plant Te A - Nkola Tesla A, Te B - Nkola Tesla B, Ko - Kostolac, Mo - Morava and Kol - Kolubara. Numbers 03, 04, 07, 08, 09 and 10 denote the years 2003, 2004, 2007, 2008, 2009 and Fgure 3. Measured concentraton of radonucldes n flyng ash n coal-fred power plant Te A - Nkola Tesla A, Te B - Nkola Tesla B, Ko - Kostolac, Mo - Morava and Kol - Kolubara. Numbers 03, 04, 07, 08, 09 and 10 denote the years 2003, 2004, 2007, 2008, 2009 and from Bq kg 1, H ex ranged from Bq kg 1, D from ngy h 1 and the range of annual effectve dose was from msv. As t can bee seen, Ra eq

7 M.M. Jankovć / JMM 47 (2) B (2011) and H ex are below upper lmts (370 Bq kg 1 and 1 respectvely) recommended by [1]. The other two ndeces are smlar to those reported n the lterature for other countres, such as Poland, Inda, Gemany, Turkey and Australa [3-6]. Fgures 1-3 show the results obtaned for the actvty concentratons for coal, slag and ash samples collected n all nvestgated power plants. 4. Conclusons The obtaned concentratons of naturally occurrng radonucldes n all analyzed samples (coal, ash and slag) taken n the vcnty of the power plants n Serba: Nkola Tesla, Kolubara, Morava and Kostolac, are of the same order of magntude compared to the other nvestgated coal-fred power plants n the world. Accordng to the regulatons and recommendatons of IAEA and Serban law, the by-products of coal-fred power plants nvestgated n ths paper can be utlzed as a component for buldng materal, thus provdng practcal use and effectve dsposal of waste. The technology for ths s now n development. Also, t can be concluded that the ash pond does not sgnfcantly endanger human health and envronment, meanng that the producton of energy n Serban coal fred power plants does not contrbute sgnfcantly to TE NORM. Acknowledgements References [1] UNSCEAR, Sources and effects of onzng radaton, Report of the Unted Natons Scentfc Commttee on the Effects of Atomc Radaton to the General Assembly, Unted Natons, New York, USA, (2000). Annex [2] T. Mondal, D. Sengupta, A. Mandal, Current Scence, 91 (2006) [3] H. Bem, P. Weczorkowska, M. Budzanowsk, Journal of Envronmental Radoactvty, 61 (2002) 191. [4] U. C. Mshra, Journal of Envronmental Radoactvty, 72 (2004) 35. [5] L. Xnwe, J. Xaodan, W. Fenglng, Current Scence, 91 (2006) [6] U. Cevk, N. Damla, S. Nezr, Fuel, 86 (2007) [7] B. Ivanovc, Theoretcal Statstcs, (n Serban), Yugoslav Insttute for Economc Research, Belgrade, (1966), 176. [8] J. Beretka, P. J. Mathew, Health Physcs 48 (1985) 87. [9] K.N. Yu, Z. J. Guan, M.J. Stokes, E.C.M. Young, Journal of Envronmental Radoactvty 17 (1992) 31. [10] D. C. Kocher, A. L. Sjoreen, Health Physcs 48 (1985) 193. [11] P. Jacob, H. G. Paretzke, H. Rosenbaum, M. Zankl, Radaton Protecton Dosmetry 14, (1986) 299. [12] K. C. Leung, S. Y. Lau, C. B. Poon, Journal of Envronmental Radoactvty 11, (1990) 279. The nvestgaton was partally supported by the Mnstry of Scence and Technologcal Development of Serba under the Project III43009.