radon emanation from low-grade uranium ore

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Radon emanation from low-grade uranium ore

Patitapaban Sahu a, Devi Prasad Mishra a,*, Durga Charan Panigrahi a, Vivekanand Jha b,R. Lokeswara Patnaik b

aDepartment of Mining Engineering, Indian School of Mines, Dhanbad e 826 004, Jharkhand, Indiab Environmental Assessment Division, Bhabha Atomic Research Centre, Trombay, Mumbai e 400 085, India

a r t i c l e i n f o

Article history:Received 3 April 2013Received in revised form14 June 2013Accepted 23 July 2013Available online 23 August 2013

Keywords:Uranium mine222RnRadon emanationOre gradePorosityEmanation fraction

a b s t r a c t

Estimation of radon emanation in uranium mines is given top priority to minimize the risk of inhalationexposure due to short-lived radon progeny. This paper describes the radon emanation studies conductedin the laboratory as well as inside an operating underground uranium mine at Jaduguda, India. Some ofthe important parameters, such as grade/226Ra activity, moisture content, bulk density, porosity andemanation fraction of ore, governing the migration of radon through the ore were determined.Emanation from the ore samples in terms of emanation rate and emanation fraction was measured in thelaboratory under airtight condition in glass jar. The in situ radon emanation rate inside the mine wasmeasured from drill holes made in the ore body. The in situ 222Rn emanation rate from the mine wallsvaried in the range of 0.22e51.84 � 10�3 Bq m�2 s�1 with the geometric mean of 8.68 � 10�3 Bq m�2 s�1.A significant positive linear correlation (r ¼ 0.99, p < 0.001) between in situ 222Rn emanation rate and theore grade was observed. The emanation fraction of the ore samples, which varied in the range of 0.004e0.089 with mean value of 0.025 � 0.02, showed poor correlation with ore grade and porosity. Empiricalrelationships between radon emanation rate and the ore grade/226Ra were also established for quickprediction of radon emanation rate from the ore body.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Uranium ore contains natural uranium comprising of 99.275% of238U, 0.715% of 235U and 0.005% of 234U. From radiation protectionpoint of view, 238U and its decay products are of major concern foruranium mining industry due to the large abundance of 238U innatural uranium. The entire spectrum of decay products of 238U canbe found in the ore depending on the age of the deposit, which hasimportant bearing on secular equilibrium status of the ore(Levinson et al., 1984). External gamma level and inhalation expo-sure due to radon (222Rn), its short-lived progeny and long-livedalpha activity associated with ore dust constitute the majorsource of radiological hazard in uranium mines. However, in low-grade uranium mines (<0.1% U3O8), the hazards due to externalexposure and long-lived activity are insignificant. Thus large frac-tion of occupational exposure is attributed to the potential alphaactivity/energy of short-lived radon progeny (218Po, 214Bi, 214Pb and214Po). Although concentration and activity of 214Bi and 214Pb areoften used for assessment of the PAEC (Potential Alpha EnergyConcentration), the PAEC is usually attributed to alpha emitters

such as 214Po and 218Po. The isotopes 220Rn and 219Rn having half-lives of 54.5 s and 3.92 s respectively can be eliminated from themonitoring system by introducing filters or other delay techniques(Thompkins, 1982). 222Rn (t1/2 ¼ 3.82 days) is found relatively inhigh concentration in mine atmosphere and canmove a substantialdistance from its point of origin (Nazaroff and Nero, 1988; Mudd,2008). The increased risk of lung cancer due to the exposure ofshort-lived decay products of 222Rn has been reported elsewhere(Field et al., 2000; Gulson et al., 2005; Al-Zoughool and Krewski,2009). Monitoring of radon concentration inside uranium minesand in the environment has been a matter of concern since lastseveral decades to minimize the extent of inhalation exposure ofoccupational workers and the public (IAEA, 1992; ICRP, 1993, 2010).

The radonemanation and concentrationprofile inmine airdue toexhalation of the gas fromore body, undergroundwater coming outthrough cracks and fissures, backfill material and broken ore pile(Raghavayya, 1968; Raghavayya and Khan, 1973; Panigrahi et al.,2005; Gherghel and De Souza, 2008; El-Fawal, 2011) depends pri-marily on the radon emanation rate from the grains and afterwardson themicrostructure of the material. In addition, it depends on theparameters affecting physical processes such asdiffusion, advection,absorption and adsorption. The amount of radonproduced from thegrains that finally enters into the pore space by recoil effect anddiffusion process in the porous system of the material is defined as

* Corresponding author. Tel.: þ91 9430191673; fax: þ91 326 2296628/2296563.E-mail address: [email protected] (D.P. Mishra).

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‘effective radium’, and the radon escape to production ratio is called‘radon emanation factor’ (Tanner, 1980; Rogers and Nielson, 1991;Stoulos et al., 2004; Girault and Perrier, 2012). The rate of radonemanation is proportional to the rate at which radon is producedwithin the host material, which is a function of the ore grade (ura-nium content of the ore), the radon concentration gradient in thehost pores, barometric pressure and diffusion properties such asporosity and emanation fraction of the material. It has beenobserved that change in barometric pressure affects the radon gasconcentration in pores of the materials (Schroeder, 1966; Pohl-Rueling and Pohl, 1969; Clements and Wilkening, 1974; IAEA,1981; Zhu and Zhang, 1984). When there is a pressure drop in mineenvironment, the radon laden airfilling the poresmoves out into themine opening carrying the accumulated radon along with it. Be-sides, radon emanation depends on the bulk properties of rock, suchas distribution of themineral grains, size and specific surface area ofthe grains, degree of fracturing and fissuring and presence of waterin the cracks (Bochiolo et al., 2012). Radon emanation, which is thefraction of radon-222 atoms released in the connected pore space ofa porousmaterial, increaseswith thewater content due to lowrecoilrange of radon atom in water compared with air (Semkow, 1991;Ferry et al., 2001; Barillon et al., 2005; Adler and Perrier, 2009).Choubey et al. (1999) reported that the presence of discontinuities(fractures) in the rock mass provides potential pathways for radonmigration and favours air and water circulation resulting in higherradon exhalation. It has also been reported that the radon emana-tion rate in porous rock is less affected by 226Ra content variationsthan the non-porous rock (Thompkins, 1982; Righi and Bruzzi,2006). Therefore, high porosity and micro-fracture are the domi-nant factors that affect the rate of radon gas emanation from rocksurfaces in mine openings unless the ore grades are high.

Numerous studies pertaining to radon emanation have beencarried out in uranium mining and ore processing facilities in USA,Australia, Canada, China, India and Japan (Barretto,1973; Rakotosonet al., 1983; Ferry et al., 2001; Zhuo et al., 2006; Griffiths et al., 2010;Sahoo et al., 2010; Hosoda et al., 2011; Khan and Puranik, 2011; Tanet al., 2012). Several researchers have described different methodsfor estimation of the radon emanation rate in mines (Khan andRaghavayya, 1973; Archibald and Nantel, 1979; Nantel andArchibald, 1981; Panigrahi et al., 2005; Bochiolo et al., 2012). Atechnique by determining the increase of radon concentration in airbetween two points in ventilation passage in Japanese and Cana-dian uranium mines has previously been studied (Fusamura andMisawa, 1963; Thompkins and Rajhans, 1967; Keshvani, 1970).Thompkins and Cheng (1969) have described a method in which asteel chamber provided with several valves was cemented on thewalls of mine and radon samples were drawn from the chamber atintervals of several hours up to 50 h for computation of theemanation rate. A similar technique was used by Archibald andNantel (1984) for the radon emanation measurements in Cana-dian uranium mines. However, the aforementioned techniqueshave drawbacks. The former may give high uncertainties in theresults due to various mining operation conditions, contaminationof intake air and air leakage, whereas, the later technique is verycomplex, expensive and time consuming for making the arrange-ment of experimental setup. Dwaikat et al. (2010) investigated thespecific radon exhalation from the mine rocks based on the radonmeasurements by means of CR-39 detectors, in which the uncer-tainty of the measurements depends on several factors, such asexposure period, etching process and calibration. Keeping these inview, comparatively a simple, quick and less expensive techniquegiving low uncertainties in the results was used in the presentstudy to overcome the aforementioned problems.

In the present study, we investigate the radon emanation ratefrom uranium ore samples in the laboratory and from in situ

measurements in a uranium mine to obtain relationships betweenradon emanation and physical properties of the ore body that maybe generalized to other low-grade uranium mines. Based on fieldand laboratory data, this study also aims at developing empiricalrelationships for quick prediction of the radon emanation rate fromuranium ore body of similar nature in any low-grade uraniummine.

2. Materials and methods

2.1. Ore samples

We investigated the radon emanation from laboratory and insitu measurements on the ore body of Jaduguda uranium minelocated in the Singhbhum shear zone in the eastern part of India.The location of Singhbhum shear zone in India and the transversesection showing the ore body and different lithological unitsaround Jaduguda are shown in Fig. 1. Jaduguda mine has two par-allel mineable lodes starting from the surface and lying up to adepth of about 905 m. The lodes dipping towards north with anaverage inclination of about 40� are separated from each other by adistance of about 80m. The footwall and hangwall rocks of both thelodes are quite competent from geotechnical point of view. Ura-nium-bearing minerals in Jaduguda mine occur in the Precambrianmeta-sedimentary rocks, which are highly folded and sheared. Theprincipal lithological rock units are autoclastic conglomerate(brecciated quartzite), quartzechloriteebiotiteemagnetite schist,biotiteechlorite schist and epidiorite, of which first two rock unitshost the mineralisation. The primary uraniumminerals of Jadugudaore are uraninite and pitchblende and most common secondarymineral is autunite. The uranium minerals are associated with awide variety of sulphides of copper, nickel, cobalt, molybdenum,arsenic and bismuth. Some prominent ore minerals are magnetite,ilmenite, uraninite, rutile, chalcopyrite, pyrhotite, marcasite,mackinawite, violarite, tellurobismuthite, tetradymite, cubaniteand molybdenite (Sarangi and Singh, 2006).

Horizontal cut-and-fill using de-slimed mill tailing as backfill isthe principal stoping method adopted in Jaduguda mine. Twenty-one ore samples were collected from different stopes of the mine.The samples were oven dried to determine their physical propertiesand the activity concentration. The in situ radon emanation ratewas determined from drill holes made within the ore body. Sam-ples of the drill cuttings were collected from these holes to deter-mine the grade of ore in the laboratory.

2.2. Theoretical model for measuring radon emanation rate

The emanation of radon into an enclosed chamber, initially freefrom radon, may be assumed as a steady-state process. The radonconcentration in the chamberwill followanexponential growthuptoa certain build-up period. Thereafter, it reaches a constant value as abalance of the increase due to emanation and decrease due toradioactive decay (Thompkins and Cheng, 1969; Khan andRaghavayya, 1973; Girault and Perrier, 2012). The radon emanationrate can be determined from this build-up pattern. The activity con-centration of radon is estimated by collecting the air sample in scin-tillation cells and using the following equation (Raghavayya, 1981)

C ¼ 6:967� 10�5cEVse�ls

�1� e�lT

� (1)

where c is the total counts during the counting duration “T”, E is theefficiency of the system (%), Vs is the volume of scintillation cell(m3), s is the delay time after end of the sampling (s) and T is thecounting duration (s). Since the volume of scintillation cell is small

P. Sahu et al. / Journal of Environmental Radioactivity 126 (2013) 104e114 105


compared to the volume of accumulation chamber, calculation isneeded to account for the dilution during sampling. The dilutioncorrection factor (df) calculated either by pressure measurementsbefore and after sampling or air volume in the accumulationchamber should be applied to obtain the corrected radon activityconcentration (Cf) (see Appendix A).

It may be mentioned here that the ‘building material commu-nity’ and ‘uranium community’ mostly measure the radonemanation rate with a unit per surface area, while in other domainsthey use preferentially the effective radium concentration withmass-related expression. Effective radium concentration (ECRa) isdefined as the product of the emanation coefficient and radium(CRa) concentration (Stoulos et al., 2004). Once the radon activity

concentration is known, ECRa can be calculated using the followingexpression (Girault and Perrier, 2012):

ECRa ¼ VM

Cf1� e�lt

(2)

where V is the effective volume of chamber (m3), M is the mass ofsample (kg), l is the decay constant of 222Rn (2.097� 10�6 s�1) and tis the build-up time (s).

The radon emanation rate “J” (Bq m�2 s�1) is given by the for-mula (Stoulos et al., 2004)

J ¼ ECRarld (3)

Fig. 1. (a) Map of India showing Singhbhum shear zone and (b) transversion section showing ore body and lithological units of Jaduguda (Sarangi and Singh, 2006).

P. Sahu et al. / Journal of Environmental Radioactivity 126 (2013) 104e114106


where r is the bulk density (kg m�3) of sample and d is thethickness of sample which is less or equal to the radon diffusionlength (m).

Alternatively, assuming q be the radon activity in the chamber attime “t”, the rate of change of radon activity is given by

dqdt

¼ JA� lq (4)

where q is the total radon activity in the chamber at any time ‘t’ (Bq)and A is the surface area of the sample (m2).

Integrating Eq. (4) gives the following solution

J ¼ lqA�1� e�lt

� (5)

At the end of pre-determined accumulation period (i.e. a timeperiod at which the radon build-up in the accumulation chamberreaches a measurable activity concentration), the 222Rn sample isdrawn into an evacuated ZnS(Ag) scintillation cell for counting of a-activity to compute the radon emanation rate. The cell beinginitially evacuated, the duration of sample transfer from chamber tothe cell is virtually zero because of the pre-sampling pressuregradient. Therefore, any probable exponential function governingthe variation of 222Rn activity concentration in the chamber and thecell tends to unity (Jha et al., 2001). The 222Rn collected in thechamber is assumed to be uniformly distributed in the entire vol-ume (v þ V). Substituting Cf (Vs þ Ve) ¼ q in Eq. (5) gives

J ¼ lðVs þ VeÞCfA�1� e�lt

� (6)

The radon emanation rate in the laboratory was estimated fromboth the mass-related and surface area-related approaches given inEqs. (3) and (6) respectively, whereas, for in situ measurements onore body, Eq. (6) was only used for determination of radonemanation rate.

2.3. Experimental methods

2.3.1. Radon emanation rate from uranium ore samplesThe radon emanation rate from 21 uranium ore samples was

determined in the laboratory by enclosing the oven dried (at 105 �Cfor 24 h) ore sample in a 1 L capacity jar. The jar was closed with atight fitting lid through which two inlet tubes were inserted intothe jar as shown in Fig. 2. The gap between the lid and jar wassealed with a sealant wax composed of petroleum jelly and purifiedbees wax in ratio of 1:2 by weight during the emanation study. Thegap between the lid and tubes was permanently sealed with aral-dite to prevent leakage of air. The end of the smaller tube waspermanently connected with a filter holder containing filter paper,which prevents entering of the radon progeny into the scintillationcell during sampling. The systemwas flushed thoroughlywith freshair to remove any radon that might be initially present in the jar.The stopcocks in the tubes were closed and the initial time wasrecorded. The air leakage was carefully checked by dipping thewhole system in water. The 222Rn emanated from the ore samplewas allowed to accumulate in the jar. Air samples were drawn fromthe jar into evacuated ZnS(Ag) scintillation cells of 140 ml capacitythrough the smaller inlet tube fitted with filter paper (<0.45 mm) atdifferent time intervals up to five days. Since the scintillation cellwas vacuumed to a pressure of 1.3 Pa before sampling, the pressurein the jar falls on connecting it to the scintillation cell. After eachsampling, the pressure in the jar was allowed to reach normal levelby introducing fresh air before collecting another set of samples.The scintillation cells were connected to the photomultiplier

assembly after a delay period of about 200 min (the instant ofsampling is reckoned as zero time) to ensure equilibrium betweenthe radon and its progeny in the cell. Thereafter, the alpha countswere noted for 10min at 95% confidence level to estimate the radonactivity concentration of each sample (Panigrahi et al., 2005). Theefficiency of the system was 74%, which was calibrated with astandard scintillation cell having activity of 21,500 cpm (counts perminute) and 75% efficiency. Dilution corrections during samplingwere estimated using pressure measurements before and aftersampling and also using the air volume in the jar for obtaining thecorrected radon activity concentration (see Appendix A). In boththe methods, we obtained equal dilution correction factors, whichreflect no air leakage during sampling and from the jar.

The activity concentration of radon in each sample was esti-mated using Eq. (1) and the trend of radon activity concentrationwith build-up time (accumulation curve) is shown in Fig. 3. It wasfound that the radon level in the jar took 5e6 h to reachmeasurableactivity concentration after sealing. Initially the radon build-up inthe chamber increased almost linearly within a period of two days

Swagelok quick connector

Ore sample

Araldite seal

Filter holder

Collection jar

Inlet tube

Filter paper

Stop cock

Scintillation cell

Lid

Flexible tube

Fig. 2. Experimental setup for determination of radon emanation rate from uraniumore sample.

0

500

1000

1500

2000

2500

3000

0 50 100 150

Fig. 3. Variation of 222Rn activity concentration with build-up time in the jar.



and thereafter followed an exponential pattern due to decay factor.This trend reveals no leakage of air in the accumulation jar. Inour study, the measurements of 222Rn activity concentrationswere carried out during linear region of the accumulation curve (i.e.within two days) to determine the 222Rn emanation rate.

As discussed earlier, the estimation of radon emanation raterequires the determination of either the mass or the surface area ofthe ore sample. The mass of the sample was measured with asensitive digital balance having an accuracy of �5 � 10�7 g. Thesurface area of irregularly shaped ore samples is determined usingthe periphery tracing and coating techniques (Raghavayya,1976). Inthis study, the periphery tracing method was used in which theoutlines of the different faces of the ore samples were drawn onlinear graph paper and the surface area was estimated as the in-tegral of all the outlined shapes. Radon emanation rate from the oresample was calculated using Eqs. (3) and (6) (see Appendix B).

2.3.2. Ore grade and radium activityThe uranium ore grade was determined from radiometric

analysis. For this purpose, the ore pieces were powdered and aknown quantity (2 g) was enclosed in a leak proof plastic vial. Thevial was sealed after collection of samples to allow growth of 222Rnand its short-lived progeny including prominent gamma emitter214Bi. Before placing the sample in the beg counting machine, themachine was run with an empty sample holder. When the 214Bi (t1/2 ¼ 19 min) activity in this sealed source reached secular equilib-riumwith 226Ra (i.e. after 20 days) (Chiozzi et al., 2000), the gammaactivity in the vial was counted for 200 s using NaI(Tl) scintillationcounter. The gross counts obtained under the 1.76 MeV photo peakof 214Bi were then recorded. The experiment was repeated at leastfive times to enhance the reproducibility of the results. The averagecounts of the ore powder sample (co) recorded within the photopeak were compared with the counts of the standard uranium ore(cs) placed inside an identical geometry. 2 g of standard uraniumore sample (0.0627% U3O8) was taken for the calibration purpose inthis study. Average counts of background radiation (cb) were alsorecorded for the same geometry. From the grade of standard ura-nium ore (Gs) and the standard, background radiation and samplecounts; the average ore grade (Go) was estimated using thefollowing equation:

Goð%Þ ¼ Gsð%Þcs � cb

� ðco � cbÞ (7)

For confirmation of secular equilibrium in the ore, ore samplescollected from different stopes were analyzed separately for naturaluranium and 226Ra content by fluorimetry and emanometry tech-nique respectively. Standard uranium ore grade as mentionedearlier was used for calibration of the system. Representativesamples were analyzed fluorimetrically for natural uranium con-tent, inwhich the leached ore solution after removal of interferencewas extracted in Alamine in Benzene solvent (2% alamine in ben-zene). The organic layer was fused in NaCO3:NaF (85:15) fusionmixture and compared with the standard uranium solution (Hueset al., 1977; Singh et al., 2010). The minimum detection limit ofthis technique is 0.1 mg U g�1 ore. The system was calibratedagainst NBS (National Bureau of Standards) standard pure U3O8powder and BAS certified reference material (STSD-1). The 238Uactivity equivalent was estimated by using the conversion (12.23 Bq238U mg�1 of Unatural).

The emanometry technique was used in this study for mea-surement of 226Ra content in the samples (Raghavayya, 1990; Jhaet al., 2010). The ore samples were dried in an oven at 110 �C for8 h for removing moisture. Then the samples were crushed, sievedthrough standard sieve of 200 mesh size and homologized. A

known quantity (2 g) of the powdered samples was subjected torepeat leaching using conc. HNO3 mixed carefully with a smallquantity of H2O2 for removing organic matter present in the sam-ples. The samples were repeatedly leached and the mixture wasfiltered and made up to 100 ml maintaining the resultant acidnormality at 4 N. Thereafter, 50 ml of the aliquot was transferred toa radon bubbler. The radon already present in the solution wasremoved using a vacuum pump. The solution in the bubbler wasallowed to stand for a desired time period (preferably >20 days toensure secular equilibrium between 226Ra and 222Rn) depending onthe expected 226Ra activity of the sample. The freshly build-upradon was transferred to an evacuated scintillation cell. The scin-tillation cell was left for >200 min to ensure secular equilibriumbetween radon and its short-lived progeny and the alpha countsweremeasured thereafter. The background of the cell was normally0.5 cpm and average efficiency was 85%. Based on the alpha countsand sampling parameters activity, 226Ra activity (Bq kg�1) in the oresamples was estimated at 95% confidence interval using Eq. (8)

226Ra ¼ Cl3Ee�ls

�1� e�lq

��1� e�lT

�� Vts � 1000Vsb �m

(8)

where Vts is the total volume of solution prepared from the sample(100ml), Vsb is the volume of the solution loaded in bubbler (50ml),q is the build-up time in bubbler (s) and m is the weight of thepowder sample (g).

The system was calibrated against NBS standard pure 226Ra so-lution of 583 pCi (21.57 Bq) placed in a bubbler with establishedsecular equilibrium between 226Rae222Rn and also against sec-ondary 226Rae222Rn source prepared in the laboratory using car-rier-free 226Ra solution. It may be mentioned here that theminimum detectable 226Ra activity in the solution taken in thebubbler depends on the factors like duration of radon build-up,efficiency and background count rate of the scintillation cell andcounting duration. Allowing the maximum build-up period, aminimum detectable activity of 6.8 mBq can be obtained.

2.3.3. Migration of radon through the oreThe migration of radon within the ore and its continuous

emanation through rock surface exposed to atmosphere dependson the properties of ore such as specific 226Ra content, bulk density,water content, porosity and emanation fraction (Nazaroff, 1992;Przylibski, 2000; Barillon et al., 2005; Righi and Bruzzi, 2006;Adler and Perrier, 2009). In addition, the bulk properties of ore-bearing rock such as type of mineralization, distribution of themineral grains, size and specific surface area of the grains, degree offracturing and fissuring, presence of water in the cracks, tempera-ture and pressure of the pore-filled fluid influence the transport ofradon (Iskandar et al., 2004; Girault and Perrier, 2011; Bochioloet al., 2012). The presence of water content in the order of 2% wt.in pore space of the materials enhances the probability of release ofradon atoms (Strong and Levins, 1982), whereas, when the poresand fractures in the materials are filled with water, the emanationrate is reduced due to dramatic decrease of the effective diffusioncoefficient (Meslin et al., 2010). Clements and Wilkening (1974)found that the radon emanation rate changes from 60 to 80%with a pressure change of 1e2% in the pore space of the material.Washington and Rose (1990) reported that change in soil temper-ature has less effect on radon concentration in dry soils than inmoist soils. This is due to desorption of radon from solids into theinterstitial space. If the interstitial space is filled with air, radon isfreely available to diffuse to the surface. On the other hand, if it isfilled with moisture; radon will remain dissolved in water. Thefraction of radon atoms migrating from mineral grains to the poresof rock is known as emanation fraction (Tanner, 1964), which can be



used to designate the fraction of radonmigrating to the atmospherefrom the rocks under varying physical conditions such as the porefilling liquid, radon activity concentration in pores and micro-fracturing patterns in mine walls (Choubey et al., 1999; Ferryet al., 2001).

The 222Rn activity concentration is determined by the rate atwhich 222Rn is produced by the decay of 226Ra and the emanationcoefficient (f) or the fraction of radon which escapes from thesource matrix. The emanated radon is free to diffuse through theavailable pore space. Based on the diffusion theory, radon emana-tion rate (J) from the host material can be calculated using thefollowing relation (IAEA, 1992):

J ¼ CRarf ðlDÞ1=2 (9)

where CRa is the specific radium content (Bq kg�1), f is the in situradon emanation factor, l is the decay constant of 222Rn(2.09 � 10�6 s�1) and D is the effective diffusion coefficient(m2 s�1).

The effective radon diffusion coefficient of a material is definedas the ratio of the diffusive flux density of radon activity across thepore area to the gradient of radon activity concentration in thepore or interstitial space. It can be expressed as (Rogers andNielson, 1991):

D ¼ D04 exp�� 6m4� 6m144

�� T273

�0:75

(10)

where D0 is the diffusion coefficient of radon in air at ambienttemperature and pressure (1.1 � 10�5 m2 s�1), 4 is the porosity ofmaterial,m is the fraction of pore space filled with water (moisturesaturation) and T is the absolute temperature (K).

The moisture saturation (m) of material can be estimated usingthe following relationship (Rogers and Nielson, 1991):

100m ¼ rMw

rw4(11)

whereMw is the water content of material (%) and rw is the densityof water (kg m�3).

2.3.3.1. Determination of moisture content and bulk density.The moisture content (dry weight basis) of ore samples wasdetermined by heating the sample at 105 � 5 �C for 24 h as per thestandard ASTM D2216. Mass of the oven dried sample was notedand its volume was determined by water displacement method.The bulk density was determined by dividing mass of the ovendried sample by its volume. The moisture content of the Jadugudaore samples varied in the range of 0e5% and the average bulkdensity of the samples was found to be 2770 kg m�3.

2.3.3.2. Determination of porosity. The porosity of the samples wasdetermined as the ratio of volume of water absorbed in the porespace to the total volume of sample. The volume of the ore samples(Vo) was determined by water displacement method. Sample wasoven dried at 105 � 5 �C for 48 h. It was then transferred to vacuumdesiccators and kept there to equilibrate prior to dry weightdetermination. A sensitive digital balance was employed with anaccuracy of �5 � 10�7 g. The dried ore sample was immersed indouble distilled water of near neutral pH for quite significant period(>4 days) to ensure maximum absorption. After removal of excesswater layer, the sample surface was wiped out with a tissue paperand weighed at regular intervals until specimen weight reached afairly constant value. The increase in the weight of the ore samplegives the volume of water absorbed in the pores (Vw) and hence the

total volume of the pore space assuming saturation. The density ofpure water was assumed unity for calculation of the volume ofwater absorption.

2.3.3.3. Determination of emanation fraction. The emanation ofradon (222Rn) into mine air takes place due to the presence anddecay of radium (226Ra) in the stratum of rocks surrounding mineworking. A small part of the radon comes out of the poresdepending on the total magnitude of the internal surface of themineral. Apart from this, the escape is also favored by kinetic en-ergy of the emitted alpha particle from the decaying radon atoms.Due to the difference in activity concentration of radon, most of theatoms will diffuse into the mine atmosphere.

For determination of emanation fraction, a small sample (<5 cmin size) of uranium ore was placed in a jar as described in Section2.3.1 (Archibald and Nantel, 1984). Assuming the activity of parent226Ra (t1/2¼ 1600� 7 y) (Duchemin et al., 1994) constant, the radonactivity q* (Bq) within the ore specimen at any time ‘t’ afterbeginning of the experiment is given by

q* ¼ CRaM�1� e�lt

�(12)

where CRa is the 226Ra content (Bq kg�1) and M is the mass (kg) ofthe sample.

Moreover, a fraction (f) of radon is released into the pores of theore sample and then releases into the jar. From Eq. (5), the radonactivity q (Bq) can be given by

q ¼JA�1� e�lt

�l

(13)

As the size of ore sample is small, the decay of radon within thepores can be neglected. So we can write Eq. (13) as

q ¼ fq* (14)

By combining Eqs. (12)e(14) we obtain

f ¼ JA=CRaMl (15)

Alternatively, from the effective radium concentration (ECRa)expressed in Eq. (2), the emanation fraction can also be determinedusing the following relation

f ¼ ECRa=CRa(16)

Eqs. (15) and (16) were used in this study for determination ofemanation fraction of ore samples (see Appendix B).

2.3.4. In situ measurement of radon emanation rateThe method for measurement of radon emanation rate from

uranium ore body in mine is slightly different from the laboratorymethod. In field studies, the drill hole made in ore body (Fig. 4) actsas emanation chamber into which radon emanates from the wallsof the ore body. The drill cuttings collected from drill hole was usedto determine the ore grade. The drill hole was thoroughly washedwith running water and then dried with a jet of compressed air. Itwas then sealed with a rubber cork inserted with a glass tube fittedwith stopcock. The attached tube reached up to the middle of thehole. The gap between the rubber cork and drill hole was properlysealed with wax to prevent air leakage. The air leakage was alsocarefully checked by spraying soap solution around the sealed drillhole. The radon gas emanated from the rock walls was allowed toaccumulate in the free air space of the drill hole. Air samples weredrawn from the drill hole into evacuated scintillation cells through



the glass tube fitted with filter paper at different time intervals upto six days. The same technique used in the laboratory explainedin Section 2.3.1 was followed to estimate the radon activity con-centration in each sample. The accumulation curve presented inFig. 5 shows a similar trend as that of Fig. 3. Dilution correctionsduring sampling were also estimated to obtain corrected radonactivity concentration using pressure measurements before andafter sampling and also using the air volume in the drill holes. Nouncertainty in dilution correction was found, which confirms noleakage of air during sampling and also from the drill holes. In thiscase, the measurements of 222Rn activity concentrations were car-ried out during the build-up periods varied from 87 to 1445 minlying in the linear region of the accumulation curve to determinethe 222Rn emanation rate (see Appendix B). This experiment wasrepeated with 15 drill holes made within the walls of 11 stopes ofJaduguda uranium mine. The drill holes were of 38 mm diameterand 1.22e3.5 m length.

3. Results and discussions

3.1. Radon emanation rate from laboratory experiments

The statistical parameters of radon emanation rate including theore grade and 226Ra content of 21 uranium ore samples are pre-sented in Table 1. The ore grade and radium content of the oresamples varied in the range of 0.002e0.192% and 259.5e19950.4 Bq kg�1 respectively. The radon emanation rate calculatedusing the surface area and mass of the ore samples varied in theranges of 0.08e12.52 � 10�3 and 0.06e12.34 � 10�3 Bq m�2 s�1

respectively. This shows that the radon emanation rate valuescalculated using both the methods are closely matching andtherefore, it is confirmed that the experiments for radon emanationrate were performed accurately.

From Table 1 it is clear that the above three parameters arehighly skewed depicting asymmetrical natures of the distributionand the distribution is probably not normal. For log transformeddata set, the skewness was around 0.11 for ore grade and 226Racontent and �0.8 for radon emanation rate, and kurtosis in all thethree cases were far less than three. The results thus reflect log-normal distribution of the data set. The ore grade and radonemanation rate values are significantly lower as compared to therespective ranges of 0.02e0.24% and 3.7e31.5 � 10�3 Bq m�2 s�1

reported for the same mine by Khan and Raghavayya (1973). Thedifference may be attributed to factors like non-uniformity of dis-tribution of uraninite grains in the ore samples and difference in thesizes (Raghavayya, 1976). Further, with three decade time lag be-tween these two studies, there might have depletion of the oregrade with increased depth of the mine and hence decreasing ac-tivity concentration of 226Ra. Results of the laboratory emanationexperiment plotted in Fig. 6 show a significant positive correlation(r ¼ 0.83) between the grade (G)/226Ra content and the corre-sponding radon emanation rate (J). The error bars in Fig. 6 represent5% positive and negative potential error amounts in the results. Inthis figure, one outlier out of 21 samples reflects greater probabilityof migration of radon. Since the metamorphic rock of Jaduguda isassociated with faults, cracks and fissures, such anomalousemanation rate cannot be ruled out if the samples are collectednear such locations.

3.2. Migration of radon through ore

The mechanism of radon migration through the ore was studiedby determining the parameters such as porosity, grade andemanation fraction of ore. The detailed statistical parameters of oregrade, porosity and emanation fraction are presented in Table 2.The porosity (F) of the ore varied in the range of 0.056e0.79%. Theemanation fraction (f) estimated from the surface-related andmass-related expressions of radon emanation rate varied in theranges of 0.004e0.089 and 0.006e0.079 respectively, which showsthat the values obtained from both the methods agree very well.However, the mass-related expression is comparatively muchsimpler and it would not add possibly large additional un-certainties. The geometric mean values of porosity and emanationfraction were calculated as 0.26% and 0.019 respectively. The datasets of porosity and emanation fraction are highly skewed withvery large value of kurtosis and most probably they are of non-normal distribution. Again their median and geometric meanvalues are very close, so the log-normal assumption is mostappropriate. The results plotted in Fig. 7a show poor correlationbetween the porosity and emanation fraction. Thompkins (1982)also reported that at 10, 20 and 30% porosities, a 5 cm sample

020000400006000080000

100000120000140000160000180000200000

0 50 100 150 200

222 R

n ac

tivity

con

cent

ratio

n(B

q m

- 3)

Buil-up time (Hours)

Fig. 5. Variation of 222Rn activity concentration with build-up time in drill hole.

Table 1Statistical parameters of ore grade, 226Ra content and radon emanation rate inlaboratory studies.

Parameters Oregrade (%)

226Ra content(Bq kg�1)

222Rn emanation rate(�10�3 Bq m�2 s�1)

Surface-related Mass-related

Minimum 0.002 259.5 0.08 0.06Maximum 0.192 19950.4 12.52 12.34Mean 0.032 3334.0 3.65 3.48Median 0.011 1131.4 2.47 2.45Standard deviation 0.042 4418.0 3.68 3.52Kurtosis 10.2 10.2 0.2 0.4Skewness 2.9 2.9 1.08 1.14Geometric mean 0.016 1675.16 1.73 1.62Geometric standard

deviation3.4 3.4 4.6 4.8

Glass tube

Drill hole

Ore body

Ore body

Stopcock

Rubber cork

Wax seal

Filter holder

Filter paper

Scintillation cell

GSwagelok quickconnector

StFlexible tube

Fig. 4. Experimental setup for in situ measurement of radon emanation rate fromuranium ore body.



(hand sized and drill sized) has not significant effect on emanationfraction and hence there is no practical increase in pore radonconcentration in the small ore sample. Similarly, the data sets ofgrade and emanation fraction assume log-normal distribution andthe results plotted in Fig. 7b show poor correlation between the oregrade and emanation fraction. A similar observation was also re-ported by Raghavayya (1976).

The emanation fraction of Elliot Lake ore (0.075% U3O8), Italyore (0.029% U3O8) and Australia ore (0.250% U3O8) were reportedas 0.02, 0.01 and 0.088 respectively (Thompkins, 1982). Funtuaet al. (1997) reported the emanation fraction of rhyolite andgranite uranium ore samples from N. E. Nigeria as 0.22 and 0.31respectively. For Australian uranium rock, Mudd (2008) reportedthe emanation fraction varying in the range of 0.1e0.3. Suchvariations of emanation fraction may be attributed to grain sizedifferences, the spatial distribution of radium, the properties ofporous network, water content and temperature in the materials(Iskandar et al., 2004; Girault and Perrier, 2011, 2012; Sakodaet al., 2011).

3.3. In situ radon emanation rate

In situ radon emanation rate was studied from 15 drill holesmade within the walls of the ore body in 11 stopes of Jadugudauraniummine. Table 3 presents the statistical parameters of the oregrade, 226Ra content, build-up time, 222Rn concentration and in situradon emanation rate which varied in the range of 0.004e0.192%,363.3e19950.4 Bq kg�1, 87e1445min,1658.4e399,200 Bqm�3 and0.22e51.84 � 10�3 Bq m�2 s�1 respectively. The field study resultsalso reflect log-normal distribution of the data set with skewnessesof �0.07, �0.11 and �1.2 respectively for ore grade, 226Ra contentand radon emanation rate and kurtosis in all the three cases werefar less than three. The geometric mean and geometric standarddeviation of radon emanation rate were worked out to be8.68� 10�3 Bqm�2 s�1 and 4.32� 10�3 Bqm�2 s�1 respectively. Anempirical relationship between the ore grade (G) and in situ radon

a)J = 72.416G + 1.3251

r = 0.83

0

2

4

6

8

10

12

14

16

0 0.05 0.1 0.15 0.2 0.25

222 R

n em

anat

ion

rate

(×

10-3

Bq

m-2

s-1)

Ore grade (%)

b) J = 0.0007 226Ra + 1.3288r = 0.83

0

2

4

6

8

10

12

14

0 5000 10000 15000 20000 25000

222 R

n em

anat

ion

rate

(×

10-3

Bq

m-2

s-1)

226Ra content (Bq kg-1)

Fig. 6. Relationship of radon emanation rate with (a) ore grade and (b) 226Ra content inlaboratory studies.

Table 2Statistical parameters of ore grade, porosity and 222Rn emanation fraction.

Parameters Oregrade (%)

Porosity(%)

Emanation fraction

Surface-related Mass-related

Minimum 0.002 0.056 0.004 0.006Maximum 0.192 0.79 0.089 0.079Mean 0.032 0.31 0.025 0.024Median 0.011 0.26 0.02 0.019Standard deviation 0.04 0.19 0.02 0.02Kurtosis 10.24 0.92 3.47 2.61Skewness 2.9 1.15 1.89 1.76Geometric mean 0.016 0.26 0.019 0.019Geometric standard

deviation3.4 1.9 2.2 1.98

a)

00.010.020.030.040.050.060.070.080.090.1

0 0.2 0.4 0.6 0.8 1

Em

anat

ion

frac

tion

Porosity (%)

b)

00.010.020.030.040.050.060.070.080.090.1

0 0.05 0.1 0.15 0.2 0.25

Em

anat

ion

frac

tion

Ore grade (%)

Fig. 7. Relationship of emanation fraction of ore samples with (a) porosity and (b) oregrade.

Table 3Statistical parameters of ore grade, 226Ra content, build-up time, 222Rn concentra-tion and in situ radon emanation rate in field studies.

Parameters Oregrade(%)

226Racontent(Bq kg�1)

Build-uptime(min)

222Rnconcentration(Bq m�3)

222Rnemanation rate(�10�3 Bq m�2 s�1)

Minimum 0.004 363.3 87.0 1658.4 0.22Maximum 0.192 19950.4 1445.0 399200.0 51.84Mean 0.051 5241.9 1215.2 104005.0 16.54Median 0.031 3228.2 1375 60598.3 11.95Standard

deviation0.056 5855.0 456.8 119578.25 15.86

Kurtosis 1.89 1.89 4.29 1.81 0.5Skewness 1.59 1.59 �2.39 1.54 1.16Geometric

mean0.027 2766.46 966.2 46923.2 8.68

Geometricstandarddeviation

3.43 3.52 2.6 4.6 4.32



emanation rate (J) (Fig. 8a) was established and consequently agood fit was also obtained for estimated radon emanation rate (J)and 226Ra content (Fig. 8b). This relationship is based on the factthat the radium content of an uraniferous ore is normally related tothe ore grade when the decay products of uranium series are in astate of secular equilibrium. The error bars in Fig. 8 represent 5%positive and negative potential error amounts in the results.Comparatively, a better correlation between the ore grade andradon emanation rate was observed (Figs. 6 and 8) from the fieldstudies with higher correlation coefficient of 0.99 against 0.83 wasobtained from the laboratory studies. The radon emanation ratefrom the ore sample having ore grade equivalent to mean ore gradeof the samples (0.032%) was obtained as 3.65 � 10�3 Bq m�2 s�1

(Table 1). In contrast, the in situ radon emanation rate corre-sponding to the same ore grade was found to be11.4 � 10�3 Bq m�2 s�1 from Fig. 8a, which is about 3 times morethan the emanation rate of the ore sample. This may be due to themassive size of the ore body and degree of fracturing in the walls ofthe ore body (Bochiolo et al., 2012).

The in situ 222Rn emanation rate from various rock types inunderground uranium mines have been studied by several re-searchers in the past. The 222Rn emanation rate of different porousrock types such as sandstone and shale in American mines werefound to be 18.5 and 5 Bq m�2 s�1 respectively (Thompkins, 1982).Franklin et al. (1982) reported the 222Rn emanation rate of0.185 Bq m�2 s�1 for conglomerate rock in a Canadian uraniummine. Richon et al. (2004) reported the radon flowwithin fracturedgneisses in a tunnel of the French Alps varying in the range of 1.4e9.3 � 10�3 Bq m�2 s�1. Mudd (2008) has reported the 222Rn exha-lation rate in the range of 0.15e1.94 Bq m�2 s�1 from Australianuranium bearing rocks. The radon emanation rate in Jadugudaminewas found to be much lower than the values reported by these

earlier studies. This is mainly due to low ore grade and non-porous(compact and high density) meta-sedimentary characteristic ofJaduguda mine rock.

While comparing the radon emanation rate and 226Ra content ofJaduguda uranium ore with those of the backfill material obtainedby authors, it was found that the radon emanation rate from thebackfill material was maximum up to 167 times higher than that ofthe ore in spite of its low radium content. Similar observations havealso been reported by previous studies (Raghavayya and Khan,1973; Bates and Franklin, 1977). These variations may be attrib-uted to factors like high porosity of tailings/sand used as backfilland large specific surface area of finely divided backfill material.The authors had determined the porosity of the tailings as 33.3%,which is about 107 timesmore than the porosity of the uranium ore(0.31%) found in the present study. Therefore, unless the ore gradeis high, porosity of thematerial is probably the dominant factor thataffects the rate of radon gas emanation from rock surfaces intomine openings.

Further, it may be mentioned here that the 222Rn emanation ratefrom the ore can also be predicted using the theoretical relationshipmentioned in Eq. (9), which includes the parameters such as 226Racontent, bulk density, emanation fraction, decay constant of 222Rnand effective diffusion coefficient. However, this relationship doesnot consider an important mechanism such as adsorption. Undervery dry condition, radon diffusion from the rock decreases due tothe effectof re-adsorptionon the recoiled radonatomson the surfaceof the pores and fractures present in the rock (IAEA, 1981). On theother hand, when the pores and fractures are saturated with water,the recoiling ionescaping into thepores encounters a dense absorberand has a greater probability of remaining in the pores. Thus, esti-mation of radon emanation rate from ore using the theoreticalrelationship based on diffusion theory cannot give accurate resultsbecause of the dramatic variationof the effective diffusion coefficientwhich depends on the water content of the ore (Ferry et al., 2002;Bossew, 2003; Meslin et al., 2010). Therefore, although the resultsobtained from thefieldmeasurements are comparatively lower thanthe theoretically predicted values, they aremore realistic and shouldbe given more priority for radon emanation studies.

4. Conclusions

The emanation of 222Rn gas into mine atmosphere is primarilycaused by diffusion through the mineral-bearing host rock andsubsequent exhalation through the mine walls and surface ofbroken ore samples. This article reports the characterization ofradon emanation from a uranium ore deposit occurring in meta-sedimentary rocks at Jaduguda, India with various measurementsfrom ore samples in the laboratory (radon emanation rate, bulkdensity, porosity, ore grade, emanation fraction) and in situ mea-surements (radon emanation rate from the walls inside the mine).The entire set of experiments revealed that the radon emanationrate is largely dependent on the grade/226Ra content of ore. Aremarkable relationship between in situ radon emanation rate andore grade/226Ra concentrationwas obtained, whichmay be used forthe prediction of radon emanation rate from the ore body insideuranium mines. The radon emanation rate from ore samplesdetermined in the laboratory did not reflect the in situ radonemanation rate due to massive size of the ore body and micro-fracturing pattern of the ore walls. The laboratory study alsorevealed poor correlation between the radon emanation fractionwith ore grade and porosity. In small ore samples, since porosityhas not significant effect on the emanation fraction and there is nopractical increase in pore radon concentration, the emanationfraction measured in the laboratory can be approximately treatedas the in situ emanation fraction.

a)

J = 278.52G + 2.4797r = 0.99

0

10

20

30

40

50

60

0.00 0.05 0.10 0.15 0.20 0.25

In si

tu22

2 Rn

em

anat

ion

rate

(×

10-3

Bq

m-2

s-1)

Ore grade (%)

b)J = 0.0027 226Ra + 2.4797

r = 0.99

0

10

20

30

40

50

60

0 5000 10000 15000 20000 25000

In si

tu 22

2 Rn

eman

atio

n ra

te

(×10

-3B

q m

-2s-1

)

226Ra content (Bq kg-1)

Fig. 8. Relationship of in situ radon emanation rate with (a) ore grade and (b) 226Racontent in field studies.



Acknowledgements

The authors wish to express their sincere thanks to Mr. D.Acharya, Chairman andManaging Director, Uranium Corporation ofIndian Ltd for granting necessary permission to carry out theresearchwork in Jaduguda uraniummine. Authors are also thankfulto the Health Physics Unit, BARC, Jaduguda for providing the facil-ities and assistance during the course of investigation.

Appendix A. Determination of dilution correction duringsampling with scintillation cells

The dilution correction estimated during sampling is usuallyapplied to obtain the corrected radon activity concentration in theaccumulation chamber. This correction was calculated from the airvolume in the accumulation chamber and pressure measurementsbefore and after sampling in the following manner (Girault andPerrier, 2012):

Let the volume of free air in the jar be Vj (1 L), the volume of thescintillation cell be Vs (140 ml ¼ 0.140 L), the initial pressure in thejar be Pj, which is equivalent to atmospheric pressure (101.3 kPa)and the initial pressure in the scintillation cell be Ps(10�2 torr¼ 1.33� 10�3 kPa). When the jar and the scintillation cellare connected, the pressures equilibrate to a new pressure (Pn).Assuming isothermal conditions and perfect gases, we can writethe following relation:

Pn�Vj þ Vs

� ¼ PjVj þ PsVs (A1)

Vj

Vs¼ Pn � Ps

Pj � Pn(A2)

The quantity of air extracted from the accumulation jar is pro-portional to (PjVj� PnVj). The fraction of initial air quantity in the jar(f) is given by

f ¼ 1� Pn=Pj(A3)

Assuming Cj be the initial radon concentration in the jar, theextracted radon activity that will reach in the scintillation cell willbe fCjVj. Thus measured radon concentration (Cs) is given by

Cs ¼ fCjVj

Vs(A4)

The dilution correction factor (DF) can be calculated using thefollowing relation

DF ¼ CjCs

¼ Vs

fVj(A5)

From Eqs. (A1) and (A3), Pn and f were calculated as 88.85 kPaand 0.12 respectively. From Eq. (A5), DF was calculated to be 1.14. Bycombining Eqs. (A2), (A3) and (A5), the dilution correction factorwas also be computed as 1.14 from the following equation

DF ¼ PjPn � Ps

(A6)

Since in both the methods, we obtained a same dilution correc-tion factor of 1.14, it is evident that the accumulation experimentswere performed in well manner without any air leakage. Similarly,for in situ measurements, with free air volume of the drill holesvarying in the range of 1.3e3.8 L and the pressure inside these holesat 106.0 kPa, the estimated dilution correction factors varied in therange of 1.04e1.14 depending on the air volumes in the drill holes.

Appendix B. Examples for calculation of radon emanationrate and emanation fraction in laboratory and inside mine

B1. Determination of radon emanation rate and emanation fractionin laboratory

The 226Ra content (CRa), mass (M), bulk density (r), thickness(d), volume (Vo) and surface area (A) of a particular ore samplewere determined as 3217.8 Bq kg�1, 27.5 � 10�2 kg, 2770 kg m�3,0.8 cm, 1 � 10�4 m3 and 125 � 10�4 m2 respectively. This oresample was placed in a jar of 1 L (1 � 10�3 m3) capacity (Vj). Theeffective volume of the jar (Ve) was calculated asVj � Vo ¼ 9 � 10�4 m3. Air sample was drawn from the jar by anevacuated scintillation cell of capacity (Vs) 0.140 � 10�3 m3 after aradon build-up time (t) of 19,800 s. This cell was later connected tothe photomultiplier assembly having efficiency (E) of 74% after adelay period (s) of 74,100 s. The total alpha counts (c) wereobserved to be 112 for counting duration (T) of 600 s at 95% con-fidence level.

From the above parameters, using Eq. (1), the radon activityconcentration (C) was estimated as 699.8 Bq m�3. Applying thedilution correction factor of 1.14 (Appendix A), the corrected radonactivity concentration (Cf) was estimated to be 797.8 Bq m�3. FromEq. (2), the effective radium concentration (ECRa) was calculated tobe 71.9 Bq kg�1. The radon emanation rate (J) of the ore sample wasestimated as 3.4 � 10�3 Bq m�2 s�1 using Eq. (3) or (6). Finally, theemanation fraction (f) of the ore sample was calculated to be 0.022using Eq. (15) or (16).

B2. Determination of in situ radon emanation rate

The radon emanation rate from the drill hole inside the minewas calculated in a similar way as explained for the laboratorystudy. Let us assume the volume (V ¼ Ve) and surface area (A) of aparticular drill hole be 3.4�10�3 m3 and 36� 10�2 m2 respectively.Considering efficiency of the system (E) ¼ 80%, build-up time(T) ¼ 82,200 s, total counts (c) ¼ 4690, counting duration(T) ¼ 600 s, counting delay time (s) ¼ 17,100 s, the radon activityconcentration (C) was estimated to be 24052.7 Bqm�3 using Eq. (1).Applying the dilution correction factor of 1.04, the corrected radonactivity concentration (Cf) in the drill hole was computed to be25014.8 Bq m�3. From Eq. (6), the in situ radon emanation rate (J)was estimated as 3.14 � 10�3 Bq m�2 s�1.

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