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
a Department of Mining Engineering,Indian School of Mines,Dhanbad e826004,Jharkhand,India
b Environmental Asssment Division,Bhabha Atomi
c Rearch Centre,Trombay,Mumbai e400085,India
a r t i c l e i n f o
Article history:
Received3April2013 Received in revid form
14June2013
Accepted23July2013 Available online23August2013
Keywords:
Uranium mine
222Rn
Radon emanation
Ore grade
Porosity
Emanation 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 inhalation exposure due to short-lived radon progeny.This paper describes the radon emanation studies conducted in the laboratory as well as inside an operating underground uranium mine at Jaduguda,India.Some of the important parameters,such as grade/226Ra activity,moisture content,bulk density,porosity and emanation 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 the laboratory under airtight condition in glass jar.The in situ radon emanation rate inside the mine was measured from drill holes made in the ore body.The in sit
积极分子考试
u222Rn emanation rate from the mine walls varied in the range of0.22e51.84Â10À3Bq mÀ2sÀ1with the geometric mean of8.68Â10À3Bq mÀ2sÀ1.
A significant positive linear correlation(r¼0.99,p<0.001)between in situ222Rn emanation rate and the ore grade was obrved.The emanation fraction of the ore samples,which varied in the range of0.004 e0.089with mean value of0.025Æ0.02,showed poor correlation with ore grade and porosity.Empirical relationships between radon emanation rate and the ore grade/226Ra were also established for quick prediction of radon emanation rate from the ore body.
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1.Introduction
Uranium ore contains natural uranium comprising of99.275%of 238U,0.715%of235U and0.005%of234U.From radiation protection point of view,238U and its decay products are of major concern for uranium mining industry due to the large abundance of238U in natural uranium.The entire spectrum of decay products of238U can be found in the ore depending on the age of the deposit,which has important bearing on cular equilibrium status of the ore (Levinson et al.,1984).External gamma level and inhalation expo-sure due to radon(222Rn),its short-lived progen
测试报告y and long-lived alpha activity associated with ore dust constitute the major source of radiological hazard in uranium mines.However,in low-grade uranium mines(<0.1%U3O8),the hazards due to external exposure and long-lived activity are insignificant.Thus large frac-tion of occupational exposure is attributed to the potential alpha activity/energy of short-lived radon progeny(218Po,214Bi,214Pb and 214Po).Although concentration and activity of214Bi and214Pb are often ud for asssment of the PAEC(Potential Alpha Energy Concentration),the PAEC is usually attributed to alpha emitters such as214Po and218Po.The isotopes220Rn and219Rn having half-lives of54.5s and3.92s respectively can be eliminated from the monitoring system by introducingfilters or other delay techniques (Thompkins,1982).222Rn(t1/2¼3.82days)is found relatively in high concentration in mine atmosphere and can move a substantial distance from its point of origin(Nazaroff and Nero,1988;Mudd, 2008).The incread risk of lung cancer due to the exposure of short-lived decay products of222Rn has been reported elwhere (Field et al.,2000;Gulson et al.,2005;Al-Zoughool and Krewski, 2009).Monitoring of radon concentration inside uranium mines and in the environment has been a matter of concern since last veral decades to minimize the extent of inhalation exposure of occupational workers and the public(IAEA,1992;ICRP,1993,2010).
The radon emanation and concentration profile in mine air due to exhalation of the gas from ore body,underground water coming out through cracks andfissures,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 afterwards on the microstructure of the material.In addition,it depends on the parameters affecting physical process such as diffusion,advection, absorption and adsorption.The amount of radon produced from the grains thatfinally enters into the pore space by recoil effect and diffusion process in the porous system of the material is defined as
*Corresponding author.Tel.:þ919430191673;fax:þ913262296628/2296563.
E-mail address:devi_(D.P.
Mishra).Contents lists available at ScienceDirect
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dx.doi/10.1016/j.jenvrad.2013.07.014
Journal of Environmental Radioactivity126(2013)104e114
‘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 radon emanation is proportional to the rate at which radon is produced within the host material,which is a function of the ore grade(ura-nium content of the ore),the radon concentration gradient in the host pores,barometric pressure and diffusion properties such as porosity and emanation fraction of the material.It has been obrved that change in barometric pressure affects the radon gas concentration 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 mine environment,the radon laden airfilling the pores moves out into the mine opening carrying the accumulated radon along with it.Be-sides,radon emanation depends on the bulk properties of rock,such as distribution of the mineral grains,size and specific surface area of the
grains,degree of fracturing andfissuring and prence of water in the cracks(Bochiolo et al.,2012).Radon emanation,which is the fraction of radon-222atoms relead in the connected pore space of a porous material,increas with the water content due to low recoil range 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 prence of discontinuities (fractures)in the rock mass provides potential pathways for radon migration and favours air and water circulation resulting in higher radon exhalation.It has also been reported that the radon emana-tion rate in porous rock is less affected by226Ra content variations than 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 rock surfaces in mine openings unless the ore grades are high.
Numerous studies pertaining to radon emanation have been carried out in uranium mining and ore processing facilities in USA, Australia,Canada,China,India and Japan(Barretto,1973;Rakotoson et 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;Tan et al.,2012).Several rearchers have described different methods for estimation of the radon emanation rate in mines(Khan and Raghavayya,1973;Archibald and Nantel,1979;Nantel and Archibald,1981;Panigrahi et al.,2005;Bochiolo et al.,2012).A technique b
y determining the increa of radon concentration in air between two points in ventilation passage in Japane and Cana-dian uranium mines has previously been studied(Fusamura and Misawa,1963;Thompkins and Rajhans,1967;Keshvani,1970). Thompkins and Cheng(1969)have described a method in which a steel chamber provided with veral valves was cemented on the walls of mine and radon samples were drawn from the chamber at intervals of veral hours up to50h for computation of the emanation rate.A similar technique was ud by Archibald and Nantel(1984)for the radon emanation measurements in Cana-dian uranium mines.However,the aforementioned techniques have drawbacks.The former may give high uncertainties in the results due to various mining operation conditions,contamination of intake air and air leakage,whereas,the later technique is very complex,expensive and time consuming for making the arrange-ment of experimental tup.Dwaikat et al.(2010)investigated the specific radon exhalation from the mine rocks bad on the radon measurements by means of CR-39detectors,in which the uncer-tainty of the measurements depends on veral factors,such as exposure period,etching process and calibration.Keeping the in view,comparatively a simple,quick and less expensive technique giving low uncertainties in the results was ud in the prent study to overcome the aforementioned problems.
In the prent study,we investigate the radon emanation rate from uranium ore samples in the laboratory and from in situ measurements in a uranium mine to obtain relationships between radon emanation and physical properties of the ore body that may be generalized to other low-grade uranium mines.Bad onfield and laboratory data,this study also aims at developing empirical relationships for quick prediction of the radon emanation rate from uranium ore body of similar nature in any low-grade uranium mine.
2.Materials and methods
2.1.Ore samples
We investigated the radon emanation from laboratory and in situ measurements on the ore body of Jaduguda uranium mine located in the Singhbhum shear zone in the eastern part of India. The location of Singhbhum shear zone in India and the transver ction showing the ore body and different lithological units around Jaduguda are shown in Fig.1.Jaduguda mine has two par-allel mineable lodes starting from the surface and lying up to a depth of about905m.The lodes dipping towards north with an average inclination of about40 are parated from each other by a distance of about80m.The footwall and hangwall rocks of both the lodes are quite competent from geotechnical
point of view.Ura-nium-bearing minerals in Jaduguda mine occur in the Precambrian meta-dimentary rocks,which are highly folded and sheared.The principal lithological rock units are autoclastic conglomerate (brecciated quartzite),quartz e chlorite e biotite e magnetite schist, biotite e chlorite schist and epidiorite,of whichfirst two rock units host the mineralisation.The primary uranium minerals of Jaduguda ore are uraninite and pitchblende and most common condary mineral is autunite.The uranium minerals are associated with a wide variety of sulphides of copper,nickel,cobalt,molybdenum, arnic and bismuth.Some prominent ore minerals are magnetite, ilmenite,uraninite,rutile,chalcopyrite,pyrhotite,marcasite, mackinawite,violarite,tellurobismuthite,tetradymite,cubanite and molybdenite(Sarangi and Singh,2006).
Horizontal cut-and-fill using de-slimed mill tailing as backfill is the 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 properties and the activity concentration.The in situ radon emanation rate was determined from drill holes made within the ore body.Sam-ples of the drill cuttings were collected from the 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 enclod chamber,initially free from radon,may be assumed as a steady-state process.The radon concentration in the chamber will follow an exponential growth up to a certain build-up period.Thereafter,it reaches a constant value as a balance of the increa due to emanation and decrea due to radioactive decay(Thompkins and Cheng,1969;Khan and Raghavayya,1973;Girault and Perrier,2012).The radon emanation rate 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) C¼
6:967Â10À5c
EV s e l s
À
1Àe l
Á(1)
where c is the total counts during the counting duration“T”,E is the efficiency of the system(%),V s is the volume of scintillation cell (m3),s is the delay time after end of the sampling(s)and T is the counti
ng duration(s).Since the volume of scintillation cell is small
P.Sahu et al./Journal of Environmental Radioactivity126(2013)104e114105
compared to the volume of accumulation chamber,calculation is needed to account for the dilution during sampling.The dilution correction factor (d f )calculated either by pressure measurements before and after sampling or air volume in the accumulation chamber should be applied to obtain the corrected radon activity concentration (C f )(e Appendix A ).
It may be mentioned here that the ‘building material commu-nity ’and ‘uranium community ’mostly measure the radon emanation rate with a unit per surface area,while in other domains they u preferentially the effective radium concentration with mass-related expression.Effective radium concentration (EC Ra )is de fined as the product of the emanation coef ficient and radium (C Ra )concentration (Stoulos et al.,2004).Once the radon activity
concentration is known,EC Ra can be calculated using the following expression (Girault and Perrier,2012):
EC Ra ¼
V
C f 1Àe l (2)
where V is the effective volume of chamber (m 3),M is the mass of sample (kg),l is the decay constant of 222Rn (2.097Â10À6s À1)and t is the build-up time (s).
The radon emanation rate “J ”(Bq m À2s À1)is given by the for-mula (Stoulos et al.,2004)
J ¼EC Ra rl d
(3)
Fig.1.(a)Map of India showing Singhbhum shear zone and (b)transversion ction showing ore body and lithological units of Jaduguda (Sarangi and Singh,2006).
P.Sahu et al./Journal of Environmental Radioactivity 126(2013)104e 114
106
where r is the bulk density(kg mÀ3)of sample and d is the thickness of sample which is less or equal to the radon diffusion length(m).
Alternatively,assuming q be the radon activity in the chamber at time“t”,the rate of change of radon activity is given by
d q
d t
¼JAÀl q(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
J¼
l q
A
À
1ÀeÀl t
Á(5)
At the end of pre-determined accumulation a time period at which the radon build-up in the accumulation chamber reaches a measurable activity concentration),the222Rn sample is drawn into an evacuated ZnS(Ag)scintillation cell for counting of a-activity to compute the radon emanation rate.
The cell being initially evacuated,the duration of sample transfer from chamber to the cell is virtually zero becau of the pre-sampling pressure gradient.Therefore,any probable exponential function governing the variation of222Rn activity concentration in the chamber and the cell tends to unity(Jha et al.,2001).The222Rn collected in the chamber is assumed to be uniformly distributed in the entire vol-ume(vþV).Substituting C f(V sþV e)¼q in Eq.(5)gives
J¼lðV sþV eÞC f
A
À
1Àe l
Á(6)形容女孩的词语
The radon emanation rate in the laboratory was estimated from both the mass-related and surface area-related approaches given in Eqs.(3)and(6)respectively,whereas,for in situ measurements on ore body,Eq.(6)was only ud for determination of radon emanation rate.
2.3.Experimental methods
恐龙鱼怎么养
2.3.1.Radon emanation rate from uranium ore samples
The radon emanation rate from21uranium ore samples was determined in the laboratory by enclosing the oven dried(at105 C for24h)ore sample in a1L capacity jar.The jar was clod with a tightfitting lid through which two inlet tubes were inrted into the jar as shown in Fig.2.The gap between the lid and jar was aled with a alant wax compod of petroleum jelly and purified bees wax in ratio of1:2by weight during the emanation study.The gap between the lid and tubes was permanently aled with aral-dite to prevent leakage of air.The end of the smaller tube was permanently connected with afilter holder containingfilter paper, which prevents entering of the radon progeny into the scintillation cell during sampling.The system wasflushed thoroughly with fresh air to remove any radon that might be initially prent in the jar. The stopcocks in the tubes were clod and the initial time was recorded.The air leakage was carefully checked by dipping the whole system in water.The222Rn emanated from the ore sample was allowed to accumulate in the jar.Air samples were drawn from the jar into evacuated ZnS(Ag)scintillation cells of140ml capacity through the smaller inlet tubefitted withfilter paper(<0.45m m)at different time intervals up tofive days.Since the scintillation cell was vacuumed to a pressure of1.3Pa before sampling,the pressure in the jar falls on connecting it to the scintillation cell.After each sampling,the pressure in the jar was allowed to reach
normal level by introducing fresh air before collecting another t of samples. The scintillation cells were connected to the photomultiplier asmbly after a delay period of about200min(the instant of sampling is reckoned as zero time)to ensure equilibrium between the radon and its progeny in the cell.Thereafter,the alpha counts were noted for10min at95%confidence level to estimate the radon activity concentration of each sample(Panigrahi et al.,2005).The efficiency of the system was74%,which was calibrated with a standard scintillation cell having activity of21,500cpm(counts per minute)and75%efficiency.Dilution corrections during sampling were estimated using pressure measurements before and after sampling and also using the air volume in the jar for obtaining the corrected radon activity concentration(e Appendix A).In both the methods,we obtained equal dilution correction factors,which reflect 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 concentration with build-up time(accumulation curve)is shown in Fig.3.It was found that the radon level in the jar took5e6h to reach measurable activity concentration after aling.Initially the radon build-up in the chamber incread almost linearly within a period of two days
氧化反应的定义
Swagelok quick connector
Ore sample
奇忧电影Araldite al
Filter holder
Collection jar
Inlet tube
Filter paper
Stop cock
Scintillation cell
Lid
Flexible tube
Fig.2.Experimental tup for determination of radon emanation rate from uranium ore sample.
500
1000
1500
2000
2500
3000
050100150 Fig.3.Variation of222Rn activity concentration with build-up time in the jar.
P.Sahu et al./Journal of Environmental Radioactivity126(2013)104e114107
and thereafter followed an exponential pattern due to decay factor.
This trend reveals no leakage of air in the accumulation jar.In
our study,the measurements of222Rn activity concentrations
were carried out during linear region of the accumulation
within two days)to determine the222Rn emanation rate.
As discusd earlier,the estimation of radon emanation rate
requires the determination of either the mass or the surface area of
the ore sample.The mass of the sample was measured with a
nsitive digital balance having an accuracy ofÆ5Â10À7g.The
surface area of irregularly shaped ore samples is determined using
the periphery tracing and coating techniques(Raghavayya,1976).In
this study,the periphery tracing method was ud in which the
outlines of the different faces of the ore samples were drawn on
linear graph paper and the surface area was estimated as the in-
tegral of all the outlined shapes.Radon emanation rate from the ore
sample was calculated using Eqs.(3)and(6)(e Appendix B). 2.3.2.Ore grade and radium activity
The uranium ore grade was determined from radiometric
analysis.For this purpo,the ore pieces were powdered and a
known quantity(2g)was enclod in a leak proof plastic vial.The
vial was aled after collection of samples to allow growth of222Rn
and its short-lived progeny including prominent gamma emitter 214Bi.Before placing the sample in th
e b e g counting machine,the machine was run with an empty sample holder.When the214Bi(t1/ 2¼19min)activity in this aled source reached cular equilib-rium after20days)(Chiozzi et al.,2000),the gamma
activity in the vial was counted for200s using NaI(Tl)scintillation
counter.The gross counts obtained under the1.76MeV photo peak
of214Bi were then recorded.The experiment was repeated at least five times to enhance the reproducibility of the results.The average counts of the ore powder sample(c o)recorded within the photo peak were compared with the counts of the standard uranium ore (c s)placed inside an identical geometry.2g of standard uranium ore sample(0.0627%U3O8)was taken for the calibration purpo in this study.Average counts of background radiation(c b)were also recorded for the same geometry.From the grade of standard ura-nium ore(G s)and the standard,background radiation and sample counts;the average ore grade(G o)was estimated using the following equation:
G oð%Þ¼G sð%Þ
c sÀc b
Âðc oÀc bÞ(7)
For confirmation of cular equilibrium in the ore,ore samples collected from different stopes were analyzed parately for natural uranium and226Ra content byfluorimetry and emanometry tech-nique respectively.Standard uranium ore grade as mentioned earlier was ud for calibration of the system.Reprentative samples were analyzedfluorimetrically for natural uranium con-tent,in which the leached ore solution after removal of interference was extracted in Alamine in Benzene solvent(2%alamine in ben-zene).The organic layer was fud in NaCO3:NaF(85:15)fusion mixture and compared with the standard uranium solution(Hues et al.,1977;Singh et al.,2010).The minimum detection limit of this technique is0.1mg U gÀ1ore.The system was calibrated against NBS(National Bureau of Standards)standard pure U3O8 powder and BAS certified reference material(STSD-1).The238U activity equivalent was estimated by using the conversion(12.23Bq 238U mgÀ1of U
natural).
The emanometry technique was ud in this study for mea-surement of226Ra content in the samples(Raghavayya,1990;Jha et al.,2010).The ore samples were dried in an oven at110 C for 8h for removing moisture.Then the samples were crushed,sieved through standard sieve of200mesh siz
e and homologized.A known quantity(2g)of the powdered samples was subjected to repeat leaching using conc.HNO3mixed carefully with a small quantity of H2O2for removing organic matter prent in the sam-ples.The samples were repeatedly leached and the mixture was filtered and made up to100ml maintaining the resultant acid normality at4N.Thereafter,50ml of the aliquot was transferred to a radon bubbler.The radon already prent in the solution was removed using a vacuum pump.The solution in the bubbler was allowed to stand for a desired time period(preferably>20days to ensure cular equilibrium between226Ra and222Rn)depending on the expected226Ra activity of the sample.The freshly build-up radon was transferred to an evacuated scintillation cell.The scin-tillation cell was left for>200min to ensure cular equilibrium between radon and its short-lived progeny and the alpha counts were measured thereafter.The background of the cell was normally 0.5cpm and average efficiency was85%.Bad on the alpha counts and sampling parameters activity,226Ra activity(Bq kgÀ1)in the ore samples was estimated at95%confidence interval using Eq.(8) 226Ra¼C
l
3Ee l s
À
found
1Àe lq
ÁÀ
1Àe l
ÁÂV ts
Â1000
sb
Âm(8)
where V ts is the total volume of solution prepared from the sample (100ml),V sb 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 the powder sample(g).
The system was calibrated against NBS standard pure226Ra so-lution of583pCi(21.57Bq)placed in a bubbler with established cular equilibrium between226Ra e222Rn and also against c-ondary2
26Ra e222Rn source prepared in the laboratory using car-rier-free226Ra solution.It may be mentioned here that the minimum detectable226Ra activity in the solution taken in the bubbler depends on the factors like duration of radon build-up, efficiency and background count rate of the scintillation cell and counting duration.Allowing the maximum build-up period,a minimum detectable activity of6.8mBq can be obtained.
2.3.3.Migration of radon through the ore
The migration of radon within the ore and its continuous emanation through rock surface expod to atmosphere depends on the properties of ore such as specific226Ra 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 the mineral grains,size and specific surface area of the grains,degree of fracturing andfissuring,prence of water in the cracks,tempera-ture and pressure of the pore-filledfluid influence the transport of radon(Iskandar et al.,2004;Girault and Perrier,2011;Bochiolo et al.,2012).The prence of water content in the order of2%wt. in pore space of the materials enhances the probability of relea of radon atoms(Strong and Levins,1982),whereas,when the pores and fractures in the materials arefilled with water,the emanation rate is reduced due to dramati
c decrea of the effective diffusion coefficient(Meslin et al.,2010).Clements and Wilkening(1974) found that the radon emanation rate changes from60to80% with a pressure change of1e2%in the pore space of the material. Washington and Ro(1990)reported that change in soil temper-ature has less effect on radon concentration in dry soils than in moist soils.This is due to desorption of radon from solids into the interstitial space.If the interstitial space isfilled with air,radon is freely available to diffu to the surface.On the other hand,if it is filled with moisture;radon will remain dissolved in water.The fraction of radon atoms migrating from mineral grains to the pores of rock is known as emanation fraction(Tanner,1964),which can be
垂美四边形P.Sahu et al./Journal of Environmental Radioactivity126(2013)104e114 108
