occupational exposure of phosphate mine workers airborne radioactivity measurements and dose...

7/31/2019 Occupational Exposure of Phosphate Mine Workers Airborne Radioactivity Measurements and Dose Assessment

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Journal of Environmental Radioactivity 75 (2004) 4757

www.elsevier.com/locate/jenvrad

Occupational exposure of phosphate mineworkers: airborne radioactivity measurements

and dose assessment

Ashraf E. Khater

, M.A. Hussein, Mohamed I. HusseinNational Center for Nuclear Safety and Radiation Control, Atomic Energy Authority, P.O. Box 7551,

Nasr City, Cairo 11762, Egypt

Received 1 June 2003; received in revised form 1 September 2003; accepted 1 October 2003

Abstract

Under the Egyptian program for radiation safety and control, airborne radioactivity mea-surements and radiological dose assessment were conducted in some phosphate and uranium

mines. Abu-Tartor mine is one of the biggest underground phosphate mines in Egypt. Air-borne radioactivity, radon (222Rn) and its short-lived decay products (progenies) and thoron(220Rn), were measured in selected locations along the mine. The environmental gamma andworkers dose equivalent rate (mSv/y) were measured inside and outside the mine usingthermo-luminescence dosimeters (TLD). The results were presented and discussed. The cal-culated annual effective dose due to airborne radioactivity is the main source of occupationalexposure and exceeding the maximum recommended level by ICRP-60 inside the mine tun-nels. A number of recommendations are suggested to control the occupational exposures.# 2004 Published by Elsevier Ltd.

Keywords: Occupational exposure; Phosphate; Airborne Radioactivity; Dose calculation

1. Introduction

Among the decay products of uranium, special attention has been directed towards

radon (222Rn), a noble gas, that disseminates into the atmosphere and reaches radio-

active equilibrium with its relatively short-lived daughters in about 2 h. Its high-

energy alpha particles are known to contribute substantially to the induction of lung

neo-plasias and skin cancer (Santo et al., 1995). Phosphate rock is the starting raw

Corresponding author. Tel./fax: +20-2-274-0238.

E-mail address: [email protected] (A.E. Khater).

0265-931X/$ - see front matter # 2004 Published by Elsevier Ltd.doi:10.1016/j.jenvrad.2003.11.001


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material for all phosphate products. The concentration of 238U and its decay pro-ducts tend to be elevated in phosphate deposits of sedimentary origin. A comparisonof the radiological impacts associated with the phosphate industry with those of ura-

nium mining and milling indicates that most impacts are within one order of magni-tude of each other per unit uranium production (Othman et al., 1992). A typicalconcentration of 238U in sedimentary phosphate deposits is 121 mg kg1 (1500 Bq/kg) with a range of 30260 mg kg1 (3723224 Bq/kg) (UNSCEAR, 1993; Altschu-ler, 1980). The uranium contents of some Egyptian phosphate rocks in the Red Seacoast and several Nile valley sites are in the ranges of 19142 mg kg1 (2351761 Bq/kg) and 48185 mg kg1 (5952294 Bq/kg), respectively (Bigu et al., 2000). The aver-age 238U content in Abu-Tartor phosphate rock is about 32.9 mg kg1 (408 Bq/kg)(Khater et al., 2001). The primary potential environmental radiation problem is asso-ciated with phosphate rock mining and processing concerns, mining spoils and pro-

cessing waste products. Occupational exposures mainly occur during mining processand transportation of phosphate rock, as well as during transportation and utiliza-tion of phosphate fertilizers. It has been indicated that 222Rn gas (a decay product of238U226Ra series) and its progeny constitute the largest single contributors to humanradiation exposure from natural and man-made radioactive sources (UNSCEAR,1988, 1977). Inhalation of radon and its short-lived decay products constitutes themost important occupational exposure of workers in mines (Amer et al., 2002).

It is obvious that extraction of phosphate ore presents potential health hazardsin addition to its chemical toxicity, particularly when the ore requires building sub-terranean facilities, i.e. underground mines, for its extraction. The problem is a

consequence of poor or inadequate air ventilation which has a close relationship to222Rn concentration in the underground mine tunnels (Altschuler, 1980; Bigu et al.,2000). The increased incidence of lung cancer in uranium miners and fluorsparsminers due to radon daughters concentrations in an underground miner has beendocumented (Boothe, 1977). Radiation monitoring of workers engaged in phos-phate mining and processing activities is essential. In spite of the fact that monitor-ing itself does not improve working conditions, but demonstrates if operationalradiation protection measures function as intended, or whether further protectionmeasures should be considered (Othman et al., 1992). A number of studies have

been made to evaluate the occupational exposure in uranium, phosphate, and coalmines (Bigu et al., 2000; Kenawy et al., 1999, Hussein et al., 1997; Amer et al.,2002). Our work aims at the evaluation of the occupational radiation exposure inAbu-Tartor phosphate mine through airborne radioactivity measurements and,assessment of environmental and personal radiation dose rate. Since the mine isstill in the experimental operation stage, the results of this work are preliminary.The present work has been conducted under the national program for radiationsafety and control of the Egyptian Atomic Energy Authority.

2. Experimental work

This study reports the occupational radiation doses received by the workers inAbu-Tartor phosphate mine. Occupational exposures arise from conventional

A.E. Khater et al. / J. Environ. Radioactivity 75 (2004) 475748


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mining activities. Abu-Tartor mine is a close cast underground mine. The currentreserve estimate in the exhaustively investigated area is in the order of a billiontons of phosphate ore (Said, 1990). The planned annual production is 4 million

tons of ore rock and 2.2 million tons of wet rock. The ore rock are crushed, sieved,and transported to beneficiation plant to produce the wet rock. The wet rocks arestocked in large open piles for sale or transport to a phosphate chemical plant(Khater et al., 2001). The chart plan of the mine site and ore processing activitiesare shown in Fig. 1. There are two mechanical ventilation stations, one for the eastside and another one for the west side. Auxiliary air pumps are used for ventilationof the side tunnels during build up of the side tunnels and long wall retreats (orerock cutting). During sampling, the west side ventilation station was not in oper-ation. As a safety procedure, temperature, humidity and air flow rate in the work-ing places inside the mine tunnels are measured and recorded on a routine basis.

Average ranges from 18 to 46 (36.6)v

C temperatures, from 18 to 56 (39.5)%humidity, and 721 (15) m3/s air flow rates were recorded during sampling (per-sonal communication).

Two types of measurements were carried out in this work: airborne radioactivitymeasurements (222Rn, 222Rn daughters, and 220Rn), and area and personal gammadose rate (mSv/y) measurements. Airborne radioactivity measurements were car-ried out in 20 locations along the mine tunnels, Fig. 2. Radon gas concentrationmeasurements were conducted by the scintillation cell method (Lucas method)(Lucas, 1957). The air sample was sucked into an alpha scintillation chamber. Thescintillation chamber, of 160-ml capacity, has inside walls coated with silver acti-vated zinc sulphide, which emit light flashes when struck by alpha particles. Thescintillations emitted are measured by placing the transparent surface of the cham-ber in contact with a photo-cathode detector in a light tight enclosure. Alpha par-ticle count (radioactive gas and decay products) was done using an alpha particle

Fig. 1. Chart plan of the mine site and ore processing.

49A.E. Khater et al. / J. Environ. Radioactivity 75 (2004) 4757


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counter/scalar; model RDR-511, manufactured by EDA Instruments (Toronto,Canada). A sensitivity of 0.190.37 Bq/l (510 pCi/l) is attainable with a shortcounting period. The radon concentration is evaluated by dividing the counts dueto alpha particles over the cell factor, which is determined using standard radongas source. The standard radon gas source is composed mainly of a standard solidradium slat source model Pylon-150 developed by Pylon Electronic Development(Ottawa, Canada), traceable to the National Institute of Standards and Tech-nology (NIST, USA). The estimated cell factor usually ranges from 1.6 to 1.8.

Radon (222Rn) progeny concentrations were measured using the Rolle method(Rolle, 1972). Air samples were collected for 5 min at a flow rate about 6 l min1

on high efficiency filter paper (Millipore, diameter 2.5 cm), followed by alphacounting after a delay time of about 5 min. The period of delay was selected tominimize the error resulting from variations in radon daughter ratios. The filterpapers were counted using an EDA (RDA-200 Radon Daughter detector, EDAInstrument Inc.) type counting system by placing the filter paper on a scintillationtray coated with silver-activated zinc sulphide. A Pylon RN-190 radon progenystandard source was used for calibration to determine the counting efficiency cali-bration of the scintillation tray. It houses a dry 226Ra source, which emanatesradon gas into a sealed chamber. Radon decays into its daughters, which deposit

on the inner surface of the chamber and on an enclosed filter paper. The RN-190 isdesigned so that the radon progeny are deposited uniformly over the filter and the

chamber surface with an activity deposition of 73:16 Bq cm2 4%. For 4 min

Fig. 2. Sampling locations for airborne radioactivity measurement in the mine tunnels.



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counting time measurement, a radon progeny of 0.03 WL (working level) can bemeasured with a reproducibility of 15%. The radon progeny concentration,expressed as WL, was calculated using the following equation:

WL R=EvtF 1

where R is the alpha count rate in count min1, Eis the counting efficiency, v is the

volumetric sampling rate in l min1, t is the sampling time in min, and F is a con-version factor, which may be approximated by 212 for sampling periods of 120min. One working level represents any combination of short-lived radon daughters

concentration in 1 l of air that results in the ultimate emission of 1 :3 105 MeV ofalpha energy, taking no account of the radon (Amer et al., 2002).

Thoron (220Rn) progeny concentrations were measured using Rock method(Rock, 1975). Thoron progeny was collected on a high efficiency membrane filter

paper for 5 min at a flow rate of about 6 l min1, followed by alpha counting ofthe filter after a delay of 5 h or more after the end of sampling. The 212Pb (ThB), abeta emitter, is in transient equilibrium with its alpha-emitting daughters, enablingits air concentration at the time of sampling to be calculated readily from the alphacount. A minimum detectable activity of 0.02 Bq/l is obtained using such tech-nique. Thoron was calculated using the following equation:

CThB 0:411Re0:001086T=Evt 2

where CThB is the ThB concentration, R is the count rate at T min from the end of

sampling (count min1), T is the interval from the end of sampling until counting(>300 min) in min, E is the counting efficiency, v is the volumetric sampling rate in

l min1 and t is the sampling time in minutes.Forty locations and 45 workers were chosen to carry out area and personal effec-

tive dose measurements using TLD, respectively. The TLD Dosimeters for areamonitoring of the mine tunnels were hung in the middle of the tunnels. The 45workers were provided with TLD Dosimeters. They wore the dosimeter on the partof the body between their neck and waist that was most likely to be exposed to thegreatest amount of radiation. The dosimeter assemblies consist of two parts, aTLD card and a holder. The TLD card consists of four hot-pressed LiF-100

(LiF:Mg,Ti) TL chips of 3 3 0:38 mm3

encapsulated between two sheets ofTeflon 10 mg/cm2 thick and mounted on an aluminium substrate. The holder ismade of durable, tissue-equivalent, ABS plastic, and is sealed to retain the card ina light and moisture excluding environment. It also protects the card from environ-mental damage and retains the filtration media. A Harshaw 6600 reader was used.

3. Results and discussion

3.1. Airborne radioactivity measurements

The results of airborne radioactivity measurements in Abu-Tartor phosphatemine are shown in Table 1. The average standard deviation (range) values of222Rn (Bq/m3), 222Rn daughters (WL, mSv/y) and 220Rn (Bq/m3) in Abu-Tartor,



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Red Sea phosphate mines, El-Missikat uranium mine, and Erediya uranium mine

are given in Table 2. The 222Rn concentrations in Bq/m3 were measured in five loca-

tions in the side tunnels. All measurements are exceeding the limits for occupational

exposure to radon (1000 Bq/m3

) (IAEA, 1996). The mean222

Rn concentration standard error (SE) is 4187 685 Bq=m3 with a range of 18015535 Bq/m3. Themean 222Rn concentration in other Egyptian phosphate mines is 5772 Bq/m3 where

some of these mines depend on the natural ventilation as in the west side of Abu-

Tartor mine during our field measurements (Bigu et al., 2000). The mean conce

ntration SE (range) of 222Rn decay products in working level unit and itseffective annual dose rate in mSv/y are 0:22 0:05 (0.010.67) and 26:90 5:67(0.6980.99), respectively. The annual effective dose rate due to 222Rn decay pro-

ducts is exceeding the recommended limit, 20 mSv/y (ICRP-60), especially in the

side tunnels locations by a factor of up to 4-fold because of the inadequate andbad ventilation (Bigu et al., 2000; ICRP, 1991). The relationship between the

concentration of 222Rn decay products in units of working level and distance

Table 1Activity concentrations of radon gas (222Rn) and thoron gas (220Rn) in Bq/m3, and radon daughter pro-ducts (222Rn daughters) estimated in level unit (WL) and annual effective dose (mSv/y) in Abu-Tartorphosphate mine

Serial no. Samplecode

Distance(m)a

Distance(m)b

222Rn (Bq/m3) 222Rn daughters 220Rn (Bq/m3)

WL mSv/yc

1 10 451 93 4903:0 242 0.25 30.46 3.402 11 385 66 5153:0 248 0.67 80.99 16.37 3 12 193 40 0.52 63.00 9.054 13 0 0.14 16.34 5.17d

5 14 250 0.10 11.65 6 15 500 0.08 10.10 7 16 800 0.07 7.86

8 17 1000 0.06 7.20

9 18 1200 0.07 7.93 10 19 1400 0.06 7.28 11 20 1600 0.04 4.63

12 6 2148 0.17 20.99 1.9813 3 2532 0.03 3.12 1.0014 7 2632 190 0.23 27.80 5.74d

15 2 2672 51 1801.3 147 0.10 11.61 1.66d

1 1 2723 0.01 0.69 17 5 2865 436 3543:0 208 0.42 50.22 5.15d

18 4 2905 463 0.45 53.52

19 9 3206 776 0.43 51.53 13.1520 8 3232 790 5535.0 258 0.60 71.56 8.81d

Minimum and maximum values are italicized.a Distance from mechanical ventilation station.b Distance from the side tunnel entrance.c C:F: 62:5 lSv h1=WL.d Natural ventilation.



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from the main mechanical ventilation station in the main tunnel, RG3, (sample

codes: 1320) is shown in Fig. 3. This relationship shows clearly the effect of

ventilation efficiency on the concentration of 222Rn daughters and subsequently

Fig. 3. The concentration of 222Rn daughters (WL) in Abu-Tartor phosphate mine tunnels and the dis-tance from the main ventilation station (A).

Table 2The average standard deviation (range) values of 222Rn (Bq/m3), 222Rn daughters (mSv/y and WL)and 220Rn (Bq/m3) in Abu-Tartor phosphate mine and other Egyptian phosphate and uranium mine

Abu-Tartorphosphate mine

Red Seaphosphate mines

El Missikaturanium mine

Erediyauranium mine

222Rn (Bq/m3) 4187 685 (18015535) (131112448)

222Rn daughtersmSv/y 26:90 5:70

(0.6981.99) WL 0:22 0:05

(0.010.67) (1.405.60) (2.266.22)

220

Rn (Bq/m

3

) 6:50 1:48 (1.0016.40)



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on the other airborne radioactivity and worker radiation exposure. This relationdoes not fit to the other locations especially in the side tunnels because of thecomplex layout of the mine and the different ventilation mechanisms. The mean220

Rn concentration SE was 6:50 1:48 Bq/m3

with a range of 1.0016.40Bq/m3. The inhalation hazard of 220Rn daughters is almost entirely dependenton the air concentration of only one radionuclide, 212Pb. The concentrations of222Rn (Bq/m3), 222Rn daughters (Bq/m3), and 220Rn in units of Bq/m3 insidethe mine tunnels are given in Fig. 4. There is an inverse relationship betweenthe concentration of airborne radioactivity in the mine tunnels and the air venti-lation. In the side tunnels, only auxiliary fans were used for air ventilation,which are not enough to control the airborne radioactivity within the recom-mended limit. For this reason, mechanical ventilation is conventionally used as

Fig. 4. Activity concentration of 222Rn (Bq/m3), 222Rn daughters (WL) and 222Rn (Bq/m3), and the dis-tance (m) from the ventilation station (A).



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an effective way to control airborne radioactivity and other airborne pollutants

in the underground mines (Bigu et al., 2000). The correlations between 222Rnand 222Rn daughters, and 220Rn and 222Rn daughter concentrations are given in

Fig. 5, with correlation coefficient values of 0.87 and 0.86, respectively.

Fig. 5. The correlations between the concentrations of 222Rn (Bq/m3) and 222Rn daughters (WL), and222Rn daughters and 220Rn (Bq/m3) in Abu-Tartor mine tunnels.



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3.2. Dose assessment

The environmental and personal effective dose rate (mSv/y) measured using

TLD are shown in Table 3. Forty locations in three sites in the mine were selectedfor environmental gamma dose rate measurements. The mean environmental effec-tive dose rate was 8.97 mSv/y. Forty-five workers in five sites in different miningactivity areas were selected. The mean value of the workers effective dose rate was11.66 mSv/y. The maximum values were measured for the mine tunnel workers.The average total effective dose rate (due to 222Rn, 222Rn progeny, and 220Rnprogeny) in other Egyptian phosphate mines was 70.2 mSv/y with a range of12.2136.9 mSv/y. The workers response should be considered, which may beaffecting the precision of any survey results in radiation safety and control appli-cation (Hussein, 1998).

3.3. Conclusion and recommendations

The occupational radiation exposure in the underground conventional mines isone of the major aspects in the Egyptian national program of radiation safety andcontrol. This study, in addition to pervious studies in uranium and phosphateunderground mines, implies the urgent need to impose the radiation regulationsand standards through improving the working conditions to reduce the occu-pational radiation exposure to the accepted levels recommended by ICRP-60 andIAEA-Safety series 115. In such working condition, it is a necessity to impose aperiodical radiation-monitoring program in order to continuously define and assesspossible radiological problems and to carry out the proper countermeasures. So,we can summarize our conclusion in the following prevention and remedial-measures and recommendations: efficient ventilation is a must. Job rotation of

Table 3

Environmental and personal effective dose rate (mSv/y) measured in Abu-Tartor phosphate mine tun-nels using TLD

Mean SE SD Minimum Maximum No.a

Worker effective dose rate (mSv/y)

Mine worker 15.55 2.73 12.20 6.78 53.52 20Mine maintenance worker 10.25 0.97 3.64 5.90 18.23 14Ore crushing and transport workers 11.34 1.03 1.78 9.83 13.31 3Beneficiation factory workers 10.95 0.35 0.79 10.09 12.11 5Ore drying and storage workers 10.21 0.15 0.26 9.97 10.49 3

Average 11.66 6.78 53.52

Environmental gamma effective dose rate (mSv/y)Mine 8.51 0.60 3.36 2.19 17.09 31Ore crushing 10.06 0.31 0.70 8.94 10.81 5Beneficiation factory 8.35 0.52 1.08 6.82 9.07 4

Average 8.97 2.19 17.07 SE, Standard error (1 r); SD, standard deviation.a Number of measurements.



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workers is very important to decrease occupational radiation dose. Radiologicalsafety should be considered in conventional mines. Regulations should be issuedand applied by the administration of these sites. Radiological follow up should be

a routine. Medical follow up system should be applied.

Acknowledgements

Authors wish to express their deep gratitude to Abu-Tartor phosphate mine pro-ject authority and Mr. Walid El-Moafy for their assessment and support duringfield measurement activities.

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