re suspension and deposition of radionuclides under various conditions

8/6/2019 Re Suspension and Deposition of Radionuclides Under Various Conditions

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BY0000204

Resuspension and Deposition of RadionuclidesUnder Various ConditionsE. Ga rne r ' , S . Gorde ev 2 , W. Hol land er3 , V. Kash parov 4 ,V. Kashpur1 ,J. Martinez-

Serrano 5 , V . M i r onov s , J . Pe res 7 , J. Tschiersch s, I. Vintersved 9, J. W atterson10Abstract: The resuspension of Cs-137 and Pu-239+240 has been assessed at siteswithin and outside the 30 km exclusion zone around Chernobyl. M easurementswere made during periods of wind-derived resuspension and during simulated andreal agricultural activity. From these data, resuspension rates (fraction of depositremoved in unit time) or emission rates (fraction of deposit removed in unit time orunit area) have been calculated. Resuspension rates of Cs-137 have declined by atleast an order of magnitude 7 years after the accident and were found to be of theorder of 1O"10 s'1. During agricultural activity, the resuspension rate may exceedbackground levels by four orders of magnitude.

IntroductionDuring experimental investigations o f " Experimental Collaboration Project 1(ECP 1 )" in1992-1994 detailed information about the airborne concentration and size distribution of theradioactive atmospheric aerosol was obtained for the resuspension by wind and duringanthropogenic activity. Measurements were made at sites with different surface and soilcharacteristics within and outside the 30 km exclusion zone around Chernobyl. Thesemeasuremen ts have allowed a number of important parameters to b e calculated w hich will helpto predict the spread of contamination and the inhalation dose.Resuspension as a source of Cs-137 in the atmosphereThe data presented in Figs, la and lb demon strate how the atmospheric concentrations of Cs-137 and Ce-144 have changed with time since the accident and h ow resuspension of depositedmaterial has affected the airborne concentrations of these two radionuclides. Themeasurements we re made at tw o sites, Chernobyl, and Pripyat [1], The two straight lines showthe expected atmospheric concentrations of Cs-137 and Ce-144, allowing for radioactivedecay, standardised to the activity concentration in June 1 989. After 1989, the majority of thedecontamination w ork in the 30 km zone w as co mplete, and resuspension by mechanicalactivity would have become less important. The figures show the monthly mean atmosphericconcentrations of Cs-137 and Ce-144 have declined by one or tw o orders during the four yearmeasurement period. During 1987 and 1988, there was a large effort made to decontaminatethe area surrounding the Chernobyl NPP , and there is some evidence that this work enhancedthe atmospheric concentrations of Cs-137 and Ce-144. The decrease in atmosphericconcentration of Ce-144 and Cs-137 is higher than would b e expected from radioactive decayalone and this is particularly well illustrated in Figurela. This feature demonstrates that thereare processes which are responsible for the reduction of the activity concentration in the air,1 UAAS .Ukraine; 2 RSPEAC, Russia ; 3 FHTA, Germany ; 4 UIAR , Ukraine ; 5 CIEMAT, Spain ; & IRBAS , Belarus ; ? CEA,

France; G SF,Ge rman y; FOA, Sweden; AEA,Tecnoiogy,UK

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for example, vertical migration in soil, and run-off of radionuclides with rain or melting waterfrom snow cover.The atmospheric concentrations are apparently influenced by the levels ofanthropogenic activity with atmospheric concentrations attributable to: wind-drivenresuspension, ~ 10 to 10 0u.B qirT ; light agricultural activity, - 2 0 0 to 4 00 uJBq m"3; and aforest fire 17 km from the measurement site, ~ 1000 to 2000 |aBq m"\Studies in Sweden have illustrated the potential for long range transport of Cs-137from the heavily contaminated region around Chernobyl. There are events in Sweden where Csconcentrations increased simultaneously over a large region, and on these occasions, theweather situation in northern Europe has been dominated by an anticyclone over Russia. Thereceptor orientated trajectories calculated for these periods indicate that the Chernobyl area isa possible source for the extra atmospheric activity.

Range of measurements madeEuropean and CIS collaborators made measurements of atmospheric concentration anddeposition using a wide variety of equipment. The institutes represented by their initials in thetext and tables are shown in the footnote at the bottom of the first page of this article. Thedevices used to measure atmospheric concentrations included: passively aspirated samplers(fabric 'cone' sampler , UAAS; gauze screen; TRBAS) and actively aspirated size selectivesamplers (impaction surfaces in a wind tunnel, AEAT; Andersen PM10, CIEMAT; 'GRAD',high volume, and 'PK' impactors, UAAS; rotating arm impactor (RAI),GSF and AerodynamicParticle Sizer (APS), G SF).

The teams made measurem ents of the atmospheric concentration of Cs-137 at six sites;three within the 30 km zone and three outside the zone. In some cases, these measurementshave been combined with parameters from a meteorological station (atmospheric stability,wind direction) to allow the calculation of specific resuspension parameters, for example,wind-driven resuspension rates and emission rates from agricultural activity.Intersite variability in the resuspension factor.A simple, easily measured estimate of the level of resuspension is given by the resuspensionfactor [2]:

./? = (Equation 1)where: R = resuspension factor (m 1)

Cam = atmospheric concentration (Bq m"3)oa = soil activity to a specified depth (Bq m"2)

The data in Table 1 summarises the mean resuspension factors for Cs-137 and Pu-239+240 atthe measurement sites. The Cs-137 resuspension factors are mean values calculated from arange of atmospheric samplers. The Pu-239+240 resuspension factors are for particles< 10 u.m in diameter (data from the PM 10 sampler).

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to greater depths. Localised activity at the Beach site, for example people moving around,enhanced the resuspension rate by a factor often .

Table 1: Mean values of the resuspension factor of Cs-137 and Pu-2 39+240 measured between 1992and 1994

Beach- Zapolye Kopachi Novozybkov Mikulichi KovaliPripyat (1993)

Surface charac teristics > 90 % grassla nd grassland bare soil rye field barley fieldsand

Direction [distance (km)] W [4] S [14] S [2] N E[ 15 0] N [45] N [45]from Chernobyl N PPCs-137 contamination 3.3xlO6 5.9xlO 5 2.3xlO 6 l . l x l O 6 4.2xlO 5 5.3xlO 5( B q m ' 2)Mean resuspension factor of .2.2 1 .9 4.4 2.3 2.0 1 . 4 7.7 3.5 3.1 3 .1 6.3 1 7 . 2Cs-137 SD(x lO- 1 0 m - ' )Pu-239+2 40 contamination 5xlO 4 5.7xlO 3 3.9x10"(Bqm"2)Mean resuspension factor of l . lxlO"10 1.8xlO"10 2.4x10""Pu-239+240 (xlO"10 m"1)

3.4xlO 2

l . lx lO"9

Th e resuspens ion fac to rs o f Pu -23 9+2 40 are comparab le to thos e mea sured fo r Cs -137 .The highest mean value was recorded at Koval i , al though i t i s d i ff icul t to ascribe reasons forthis.

/ Resuspension factor in relation to particle sizeMeasurements of the atmospheric concentration of Cs-137 according to particle size weremade by several samplers. Figure. 2 shows the resuspension factor of Cs-137 according tothese two particle size fractions at all the field measurement sites. The figure clearly shows thelarge contribution of particles greater than 15 ujm in diameter to the total resuspension factorat the heavilycultivated sites of No vozy bko v, Mikulichi and Kova li. At Zap olye, duringperiods of wind driven resuspension, there was a good linear correlation between theresuspension factor of Cs-137 and particles below 20 (j.m in diameter. This is shown in Figure3. In general, the resuspension factor increases with particle diameter.

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!

I

Q> 15 micronD < 10 micron

Figure 2: Resuspension factors for Cs-137 associated with airborne particles < 10 /jm a nd> 15 fjm at all the measurement sites

3,5x10'9-3,0x10"9 -2 ,5x10 '9-2,0x1CT9-i , 5 x ic r9 -1,0x10"9 -

5,0x10-10 -0 , 0 -

experimen t 8-11/5/93 experimen t 14-20/5/93

R ( r n > 3. 4 7 x 1 0" " + 5 . 6 8 x 1 0 1 1 . d r=0.9R(m'1)= 4.75 x1 0' 1 0 + 9.41 x1 0" 11 .d r=0.9

10d, particle diameter (

15 2 0

Figure 3: Resuspension factor as a function of particle diameter during wind resuspensionexperiments. Zapolye field site. Atmospheric concentration data from PK impactor.

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Tntersite variability in the resuspension rateThe resuspension rate, potentially, is a more useful measurement of resuspension than theresuspension factor, since it enables downwind inhalation doses and deposition levels to bepredicted. Ho wev er, it is difficult to mea sure in practice. It is defined as [2]:

P* = ^ - (Equation 2)

where: fir = resuspension rate (s'1)ji,,, = resuspension flux (Bq m"2 s"1)s = soil activity to a specified dep th (Bq m'2)

Shortly after the Chernobyl accident in 198 6, it we re made several measurem ents of theresuspension rate within 30 km of the Chernobyl plant in [3]. These measurements were m adeat sites with very different surface characteristics. All the mea surem ents relate to pe riods w henadvection of material from upwind sources was considered to be small, the ground surface dryand with a mo dera te wind (~2 m s"' at 1.0 m ). Table. 2 presen ts thes e data.Table. 2: Resuspension rate according to surface type in 1986Site

ZapolyeForestBeach-Pripyat

Ce-1440.3 + 0.12.1 0.92.2

Resuspension rate SD xCs-137

1.0 + 0.72.1 0.83.7

10"V)Zr-95+Nb-95

0.4 0.23.7 0.92.4

The wo rk suggests that resuspension rates of between 1 and 4x10~9 s"1 were appropriate for afresh deposit of Cs-137 at all the sites, but, the resuspension rates of Ce-144 and Zr-95+Nb-95were approximately three to four times lower at Zapoiie. Table. 3 summarises the resuspensionrates of Cs-137 measured at sites during the field campaigns of the ECP1 programme.The resuspension rates determined are highly variable, both with respect to the

magnitude of estimates at individual sites using the same measurement technique and at thesame site using different measurement techniques. It must be noted that the determination ofthe resuspension rate in this way is prone to high levels of uncertainty. Factors such assystematic measurement differences, including different measurement heights and integrationperiods could account for some of the differences between techniques also. The resuspensionrate would be expected to vary with time, anyway. The technique used by IRBAS provided thehighest estimates of resuspension rate at Novozybkov and Mikulichi .The values reported areof the same order as those recorded shortly after the accident, although the resuspension ratewould have been expected to decline sharply in the first year or two after the accident (see Cs-

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Table. 3: Resuspension rate ofCs-137Site Date Cs-137 mean fax 10"10 SD (s"1)

cone GR AD impactor+PM lO gauze screenZapolycBeach-PripyatZapolyeKopachiNovozybkovMikulichi

13/5/92 to 11/8/9214/7/92 to 11/8/92

06/5/93 to 01/6/9228/7/93 to 03/8/9317/5/94 to 24/5/9413/7/94 to 29/7/94

4 .200 .160.08 0.07

1.60 1 .2 0 0.8 0 .7

0.00110.11

0.017

0.00440.13

0.444. 60.09

137 data in Figure. 1). Although these measurements of resuspension have been made in theabsence of agricultural activity, personnel moving whilst preparing equipment has enhanced theresuspension rate by up to an order of magnitude.Resuspension dur ing agr icul tural act ivi tyAt Zapolie and Kopachi in 1993 approximately half of the experiments were designed tosimulate agricultural activity (harrowing) while the others consisted of vehicles being drivenalong a dirt track. Line sources were prepared (raked bare soil) around the measurement site tocater for different wind directions. Prior to carrying out the experiments, both of these siteswere undisturbed grassland, sparsely vegetated, with generally dry conditions prevailingthroughout. Vehicles used included two different sizes of tractor (pulling a spiked harrow) anda large, six-wheeled army truck. In 1994 the emphasis of the fieldwork was on real agriculturalpractice and measurements were undertaken for a variety of operations at three different sites.At Novozybkov fertilisation, cultivation and planting were carried out on working agriculturalland, i.e. the soil was bare and well mixed. Whole field areas were worked as opposed to asingle strip as at Zapolie. At Mikulichi a rye field was harvested and ploughed. Harvesting (ofbarley) was also carried out at Kovali. To maintain a strict quality control over the results, onlydata from experiments satisfying certain criteria have been interpreted. The basic criteria wereas follows. Experiments were only selected when:(1) The wind direction was consistently blowing across the line (or area) source towards themeasurement site;(2) The wind speed was sufficiently strong to provide a steady, well-mixed plume. Experimentsconducted during variable, thermally generated winds were rejected;(3) The type of vehicle, it 's speed and operation must have been constant throughout theduration of the experiment;(4) The meteorological conditions should be the same for the whole experiment.The Aerodynamic Particle Sizer provided a detailed picture of the dust concentration of thedifferent particles sizes during the experiments ( see Figure 4). T h e d at a ob ta in eddemonst r a t ed tha t the agr i cu l tura l ac t iv i ty increased the a tmospher i c concent ra t ionsby a f ac tor of severa l thousand in compar i son to the background concent ra t ions a tdistan ces of 20 to 30 m from the du st sourc es an d 10 to 100 t ime s at 100 m or m ore ,

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depending on conditions. The increase in particle concentration was not uniform forthe whole particle size range. The increase due to agricultural activity was thehighest in the giant particle fraction. This finding makes the assessment of theactivity connected to the giant particles im portan t.

particle size fraction: : 0.6 - 1 um1 - 3 n m3 - 10 pm10-30 pm

31 1 2 3Date [July/August 1993]

Figure 4: Normalised particle number concentration in four different size ranges as measuredby an Aerodynamic Particle SizerThe measurement of the number concentrations of hot particles showed anincrease of three orders of magnitude, reaching 0.7 to 1.0 hot particles per m J with amaximum activity of 1.5 to 2.0 Bq per particle. The atmospheric concentration ofresuspe nded Cs-137 was found to depen d strongly on soil hum idity r lh . Therelationship is roughly exponential i.e. R=R o exp(^ ) w i th R 03xl0"7m"' and h*60.This mean s that a soil hum idity of 10% redu ces R by a factor 300 as compa red to dr y

soil.1 Em ission rates during agricultural activity

The anal i t ical solut ion of the semi-empir ical di f fusion equat ion for a stat ionaryinf in i t e c rosswind s t r ip source was used for the es t imat ion of the emiss ion r a t esduring agricultural activity [4]. These experiments have been considered as approximatingto a finite dust strip. In reality, the source was neither instantaneous nor stationary as theagricultural equipment moved many times during an experimental period. The emission ratesreached very high values during the work (see Table 4). These rates exceed backgroundresuspension rates by four orders of magnitude for the whole particle size spectrum, and bythree orders of magnitude for the respirable size range.

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Table 4 : Emission rates for various types of agricultural vehicles and agricultural activityderived from PK impactor mea surements and Equation (4.9)

Date (Zapo lye)

12 .05 .93 morn ing13 .05 .93 morn ing13 .05 .93 a f t e rnoon2 2 . 0 5 . 9 3 m o r n i n g22 .05 .93 a f t e rnoon24 .05 .93 a f t e rnoon

Kind of vehicle

M T Z - 8 2T - 1 5 0T - 1 5 0ZBL-131Z I L - 1 3 1Z I L - 1 3 1 a n d Z I L - 1 3 0

Emiss ion ra t e ,10"6 s"1

0 .0271.52.01.01.40.48

Emiss ion ra t e fo rd15 |ampart icles . For an area source the emiss ion rate can be considered equivalent to a resuspensionflux and this enables calculat ion of th e resuspe nsion ra tes presen ted in Tab le 5 . Soi lcon tamina t ion dens i t i es o f 1 .2 , 0 .42 and 0 .56 MBq rtf2 h a v e b e e n u s e d f o r N o v o z y b k o v ,Mikul ichi and Koval i respect ively . Overal l , the mean resuspension rate (for >15 urn part icles)was 1.4 x 10"s s"1 (s.d. = 1.5 x 10 -8 ) . Th i s i s s evera l o rders o f magn i tude g rea te r than thebackground va lues fo r these a reas .

6 .2 Depos i t ion ve loc i ty

The values of deposi t ion veloci ty measured during agricul tural work are sys temat ical ly higher(by nearly a fac to r o f th ree) compa red w i th na tu ra l cond i t ions . Close to the source (x=2 2m)the mean dep os i t ion ve loc i ty i s (0 .1 44 0 .136) m s"1 whi le a t g rea te r d i s t ances (x >130m) , ava lue o f (0 .113+0 .087) m s"1 has been calculated. The large scat ter in values of v ; d o e s n o tpermit detai led discuss ion of the dependence of deposi t ion veloci ty on dis tance. If Stoke'sformula is used to es t imate the mean radius of part icles wi th observed v ; = 0.115 m s' \ then avalue of r 20 um is obtained. Using the equat ion v ; = bu.+vsed [5 ] , whe re b=0 .01 -0 .08 fo rpart icles of d iameter grea ter than 5 u,m depo si t ing to a dry gras s surface and with a soi ldensi ty of 2 .3 g cm" 3 , g ives a va lue for the mean rad ius o f 18 um. Fro m impac to rmeasurements i t was found that part icles of th is order of s ize had the second highest act iv i ty .

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Table. 5: Summ ary of results from area source equation (Novozybkov andBragin 1994)Experiment (reference)

NovozybkoV13.5(Agr ic2/Nov94)14.5 (Agric 3/Nov 94)14.5 (Agric 4/Nov 94)17.5(Agr ic7/Nov94)

Mikulichi30.7 (Agric 1/Mik 94)31.7 (Agric 2/Mik 94)01.8 (Agric 3/Mik 94)02.8 (Agric 4/Mik 94)

Kovali06.8 (Agric 1/Kov 94)

c (mBq m"3)

19.343.539.911.7

25.68.730.7127.0

5.6

Q ( < 1 0 u m )(mBq s"1 m"2)

-

0.20.41.60.9

0.3

Q ( > 1 5 u m )(mBq s"1 m"2)

8.120.416.35.3

5.51.24.720.6

0.8

Resuspensionrate (total) (s"1)

6.8xlO"91.7xlO"81.4xlO"84.4xlO"9

1.4xl0"83.8xlO"91.5xl0"s5.1xl0"8

2.6x10"9

N ot e :a Novozybkov data are for particles >15 |am only

7 . Co ntam inat io n of the a tmosph er i c sur face layer and y ie ld for agr i cu l tura l wo rk

Measurement s of the t emporal change of the mean concent ra t ion of Cs-137 showeda sharp increase in concen t ra t ion a t t he s t ar t of harves t ing . Co ncent ra t ions foragr i cul tura l work were h igher by a f ac tor of t en over background concent ra t ions .The mean background deposi t ion f lux dens i ty was 0 .003 mBq m" 2s ' at Mikul ichiand 0.009 mBq m~ 2s" ' a t Koval i . The ma xim um depo si t ion f lux den s i t i es we re 1.4m B q m ~2s~' and 0.53 mBq m~ 2s~ ' for rye harves t ing and bar l ey harves t ingrespect ive ly . Thus , for an th ropo gen ic ac t iv it ies the deposi t ion f lux dens i ty w as 60-400 t imes h igher than natura l background l evel s .

Sum m ar y :The atmospheric concentrations of Cs-137 have declined by over two orders of magnitudesince the accident. However, long term measurements extending to some years after theaccident show clear evidence that resuspension influences the atmospheric concentrations. Thedata highlight the potential for events such as forest fires to raise the atmosphericconcentrations by up to 100 times.

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Team members of have made extensive measurements of atmospheric concentrationsseveral sites in the CIS and have combined these with meteorological measurements to provideestimates of the resuspension in these areas. This wo rk has included calculating resuspensionfactors and also resuspension rates.Atmospheric concentrations of Cs-137 were assessed in all the field campaigns, butonly a few measurements of Pu were made. Resuspension factors were found to be in therange 2.0 to 8xlO"10 m"1 for 137Cs for the particle size range covering a few microns up to tensof microns in diameter. Resuspension factors were similar for Pu, ~ 10' 9 to 10"10 m'1, but thesemea surem ents only relate to particle sizes less than 10 urn in diameter.Despite the wide range of soil types and environmental conditions at the sites whereresuspension was assessed , there was only a four-fold difference between the lowest andhighest estimates of the mean resuspension factor of Cs-137. The inter-equipment variability in

the resuspension factor was around an order of magnitude, and this needs to taken intoaccount when comparing data from a range of types of equipment. It is not an easy task torelate the magnitude of the resuspension factor to environmental parameters such as soilmoisture content and wind, since the effect of individual parameters cannot be controlled.However, large scale human movement, for example, preparing the field sites, increased theresuspension factor of Cs-137 by up to an order o f magnitude.Resuspension rates of Cs-137 have declined by at least one order of magnitude 7 yearsafter the accident. For example, at a grassland site, the resuspension rate has fallen froml.OxlO"9 s"1 in 1986 to 1.6xlO"10 s"1 in 1993 for the particle size range covering a few micronsup to tens of microns in diameter. The variability in the resuspension rates was much greaterthan in the resuspension factors. Values of the resuspension rate, calculated from the sameinstrument, varied by up to two orders of magnitude at individual sites and a similar variabilitywas observed in the values of the mean resuspension rate calculated from different instruments.The magnitude of the measured resuspension rates was of the same order as thosefound during a review of the literature, taking into account the age of the Chernobyl deposit.From the limited data available, the resuspension rate for Pu appears comparable to that of Cs-137 but these Pu m easurem ents only relate to particle sizes less than 10 urn in diameter.The da ta from field based m easurement cam paigns of agricultural work have been usedin conjunction with models to predict the airborne radioactive contamination in the atmosphereat ground level. The fraction of material removed from the surface in either unit time or perunit area have been calculated. Values varied from 2.7xlO" 8 to 2.0x10"* for all particle sizes andfrom 4.0xl0"9 to 2.0xl0"7 for particles of 12 um in diameter.Harvesting, harrowing, cultivation, planting, etc., are operations which give an increase

in secondary contamination of the underlying ground surface at distances of between 50-200 mfrom the agricultural work being undertaken. On the ground the contamination is increased bya factor of between 60-200 while in the surface layer of the atmosphere levels are increased by10-20 times.The creation of a buffer zone (an untreated agricultural area) of 200 m width between asettlement and nearby farmland would allow a decrease of approximately two orders ofmagnitude in the transfer of secondary radioactive substances into the settlement duringagricultural work on the bordering fields.

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References[1] Garger, E.K., Kashpur, V.A., Gurgula, B.I., Paretzke, KG. and Tschirsch, J. (1994)Statistical characteristics of the activity concentration in the surface layer of the atmosphere inthe 30 km zo ne of Chernobyl. J. Aerosol Set., 25(5), 767-777.[2] Nicholson, K.W. (198 8) A review of particle resuspension. Atmos. Environ., 22(12),2639.[3] Garger, E.K., Zhukov, G .P. and Sedun ov, Y u.S. (1990) Estimating parameters of wind liftof radionuclides in the zone of the Chernobyl Nuclear Po wer P lan t Soviet Meteorology andHydrology, N 1, 5-10 (in Russian).[4] Onikul, R.I. and Kchurshudyan, L.G. (1983) K voprosu o rasprostranenii pyli ot eenasemnych plochshadnych istochnikov . Trud y Glavnoy Geoph ysicheskoy Observatorii, N467, pp. 27-36 (i n Russia).[5]Chamberlain A.C. (1967) Transport of lycopodium spores and other particles to roughsurfaces. Proc. Roy. S oc. Ser. A, Vol. 226, N 1444, pp. 63-70.

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re suspension and deposition of radionuclides under various conditions

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