INSTYTUT FIZYKI JĄDROWEJ INSTITUTE OF NUCLEAR PHYSICS l/lHCTl/mdT FUXEPHOl/1 <=P\A3[AKIA

RAPORT No 1348/B

GROUND DEPOSITION OF LONG-LIVED GAMMA EMITTERS IN POLAND FROM THE CHERNOBYL ACCIDENT

K. KROLAS, M. KUBALA, T. SCIĘŹOR

KRAKOW 1986 GROUND CEPCSITIC;; OF LG!:G-L1VED GAT.: A EMTTERS it; POLA:;D FRO;-1. THE CHERtiOBYL ACCIDENT.

K.KnJlas.I'.Kubala ;.nd T.

JegelIonian University Institute of Physics

Decaiiber 1985

% •'•> WYDANO NAKŁADEM INSTYTUTU FIZYKI JĄDROWEJ W KRAKOWIE UL RADZIKOWSKIEGO 152 NA PRAWACH RĘKOPISU

' Kopią kserograficzną, druk i oprawę wykonano w IFJ Kraków

Wydanie I Zam. 17/87 Nakład 105 egz. Activity composition was measured for the soil contaminated with the fallout from the Chernobyl accident. Soil samples were collected at various areas of Poland. A map showing the 137Cs deposit distribution was drawn for the most contaminated southern part of Poland.

Przedstawione zostały porr.i ary aktywności gleby skażonej pyłem radioaktywnym po wypadku elektrowni jądrowej w Czernobylu. Skład aktywności dfugożyciowych izotopów emitujących promieniowanie gamma ?ostaf wyznaczony dla próbek ziemi zbieranych w różnych rejonach Polski. Wyniki ilustruje mapa skażenia obszaru Polski południowej przez izotop 137Cs.

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The radioactive emission from in.-1 Chernobyl accident was spread over large areas locotao up to thousands of kilometers fro,-,i the place cf the event. Soon aftar the accident it appeared thd the fiiiout deposit wcs ,'clher nonjiifo'wi and strongly de.je^de^ ^n ntieorologicai conditions at the period of the rad i onuc i i de release. First maps summarizing ground-level radiation measurements ,vsre completed and published in some countries in ;.ay,1»oó [i>2]. Long-term exposure and transfer of radioactive substances in the ,'iuian oocy is determined by the anount of lon'j-livea isotopes deposi-'d on the qround. The information on t.vc level 21x1 distribution of 137Cs deposit is particularly ; .-.portart s;ice 137Cs •ios a long nalf-ti.Tie (T1 /2=3C.2y) and it enters vc-jetajles :na other plants via the root-uptake. In the present work tho -leasurenants of VVJ ^ro'i.id-lsvol deposits were performed for savorai isotopes. Sa.:..;l:-s ci' soil ..'ere

collected in ^iany locations in PolanJ and wa are ajie to co'.flet.o a ,nop of gr'ound contamination, /-t tne ti..v; of ..leasjr:-.ent, a f.?,. monms after the Chernobyl accident, the bulk of sr!^rt-iivod isotopes was oeyond tne detection level but tiie activity of the long-lived ones remained significant. Fro;:, snono about 20 isotopes observed soon after the accident [I], we couic ^t nor ,a;.ion on the jround-level distribution of 7 lone-lived "isoto, -;s Decaying with gaiama ray emission.

2.iiEASUR£MEf

Samples of soil were collected in about 15C locations in Poland oy a few students during tlieir holidays in Jjly ana August. As a rule, the soil was taken from open and flat areas preferably not coverad with vegetation. A "c;.*> tnick upp^r layer of scil was cut from a surface of aoout 50cm , corresponding to aoout i5Cg of the soil. From svary location 1 to 5 samples .yere taken and at SO.VK3 places the soil was additionally taken from various depths. For the 137Cs activity determination, a 3"x3" riaO(Tl) scintillator with \\% energy resolution was applied. Soil samples were placed in a standard container and were measured for at least 100 minutes each. The absolute efficiency calibration was performed using the non-radioactive soil sample contaminated with a 137Cs source of known activity. Later, 40 selected samples were examined using a 50ccm Ge(U) detector with the energy resolution 4.5 keV for 662 keV. Each spectrum was recorded for 16 hours. A typical example of the gamma obtained spectrum is displayed in Fig.1. The gamma lines of 4 isotopes: 103Ru, 106Ru, 134Cs and 137Cs are very well seen. Besides, for 7 samples, the gamma rays of 95Zr, 95Hb and 144Ce were also observed. For activity determination the following gamm2 lines were used: 497 keV for 103Ru, 512keV for 106Ru, 662keV for 137Cs, 724keV for 95Zr, 766keV for 95Ub and 796keV for 134Cs. They are indicated in Fig. 1.

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Fig.1. Garana spectrum of the soil samples taken at Skawina. The activity values were determined Dasing on intensities of the lines denoted with the gamma ray energy. \ As indicated above tha efficiency calibration takes into account the selfabsorption of 137Cs S62keV garnna rays. The effect for other lines used is estimated to be similar within 3%. The only exception is the 134keV line of H4Ce for which the absorption is rcuch serious. Altnough we clearly observe a small activity of K4Ce tii. .-aningful activity value could not be determinated. • The measured activity was corrected for decay which took place since Hay 1 till the time of measurement. The activity values obtained for 12 samples coining from different regions of Poland are listed in Table.1.

3.RESULTS AND DISCUSSION.

134Cs(T1/2=2.06y) and.137Cs(T1/2=30.2y)

The most complete data were obtained for the 137Cs activity and its distribution over different areas in Poland. The 137Cs conte.it in tne upper layer of soil varied in a broad range from 0.05 Bq/g to 2.55 Bq/g. It appears that the most important factor was a rainfall, which deposited activity on the ground at the end of April. Fig.2. shows the activity of 137Cs in the soil taken from areas with dry and wet fallout depositions, Tre activity of 137Cs is relatively low and it varies from 0.05 Bq/g to O.?1 Bq/g for all the 16 locations at which no rain occured frora April 28 to Kay 2 [4j i.e. in the time when the radioactive cloud was passing aoove Poland [5j. On the contrary, the ground level activity due to the wet deposition, as a rule, is rather high. From among 150' samples, 14 samples were collected at tne places where the stations of the State Hydro-lieteorological Institute registered a rainfall during 24 hours since 7a.m. on April 30 to 7a.m. on May 1 ranging from 0.5 mm to 10 ram. The activity of 137Cs for these locations is drawn in Fig.2 as a function of the rainfall amount. The general tendency is an increase in activity with increasing rainfall. Obviously, the rainfall played a very important part in depositing radioactive isotopes on the ground. All results of the activity measurements may be used for constructing a map of the ground level activity of 137Cs in Poland. 1.6. DRY DEPOSITION WET 1.4 DEPOSITION

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Fig.2. Tfie 137Cs activity in soil samples from different locations in Poland. First part: the locations with dry fallout deposition ordered with respect to decreasing geographical altitucie.*Data from R5f.[cJ. Second part: activity against the amount of the rainfall since 7 a.m. on April 30 to 7 a.m. on May 1 £i]. The average activity value is drawn for each location except Balice (rainfall 7cn) for which 4 points denote the activities of 4 different samples to show the irregularity of activity value.

In view of a rather limited number of the locations examined two additional indications were taken into account. Firstly,a closer inspection of the maps of the rainfall [_4] helped us to determine the borders of the rainfall zones and therefore, we beliava, Uie borders of highly contaminated areas. Secondly, some information was obtained from the first rough measurements of total ground deposition in early Kay. An example of such measurements is presented in Fig.3 which shows the ground level contamination measured on May 2 at the successive railway stations along the railway from Cracow to the east direction. Fig.4 shows a nap of southern Poland only because here tne radioactive deposits were the highest. In the nortnern- and the central part of Poland the 137Cs activity did not exceed 0.22 3q/g .and the average value was equal to 0.12 Oq/g. Thus we found that about 801 of the area of Poland was rather weakly contaminated with 137Cs and that nost of it was deposited in the South. In southern Poland the activity is rather nonuniform, relatively high, and reaches up to 2.55 Bq/g in the Beskidy Mountains. Two characteristic elongated zones of the highest activity extent to the south-west from the points situated 30 km East and 10 km West from Cracow. Isolated areas of the high activity were also found in the Tatra and the Bieszczady Mountains. In view of the limited number of sampling points it is not certain whether all heavily contaminated areas have been recorded and included in the nap.

i

Fitj.3. Total ground activity measured on liay 2 at ths successiv? raiLway stations along the railway fran Cracow to east, units) Fig.4. l-ia? of 137Cs activity in the upper layer of soil In Southern! Poland. Activity for the whole remaining ore.i of Poland is lovar thai" 0.2 Uq/g or, in limited areas, it slightly exceeds 0.2 Bq/g Nevertheless, we believe that the map depicts the real situation since the meteorological stations do not report heavy rainfall at noninvestigated places. The average activity of 157Cs in the South of Poland was estimated as equal to 0.4 3q/g. Of course, the activity of 137Cs is always accompanied by the activity of 134Cs. Within limits of statistical error the activity ratio 134Cs/137Cs was the same for each sample. The average value equaled 0.57(2) for activities extrapolated to.May 1. Consequently, the map presented in Fig.4 is also valid for the 134Cs distribution with appropriate changes in the activity scale.

103Ru(T1/2=39.4d) and 106Ru(T1/2=368d)

Activity of ruthenium isotopes was observed fcr all 40 samples investigated with the Ge(Li)-detector. The determined value of isotopic ratio 106Ru/103Ru equaled 0.21(3) on Kay 1. The distribution of the rutheniun isotopes did not differ significantly from that for the cesium isotopes. Tiie initial activity ratio 103Ru/137Cs ranged from 1.6(1) to 2.6(1), out it was similar for neighbouring sampling points. Us have not found a common feature for these locations where the ground deposit was enriched with the ruthenium isotopes with respect 137Cs. Anyway, 103Ru and 106Ru always accompanied volatile isotopes like cesiun and some short-lived isotopes of tellurium and iodine. This suggests that ruthenium, with its high melting point, was released from the reactor in its volatile oxide form.

95Zr(T1/2=64d), 95Kb(T1/2=35d) and 144Ce(T1/2=2G5d)

The gamma.rays emitted in 952r, 95Kb and 144Ce decays were detected in all samples collected in East and North-East Poland but in 2 samples only from southern Poland. The 95Zr activity extrapolated to the "May 1" value ranged fra;i a trace anount to C.7 3q/g. As it war already mentioned tne 1 ••'in¬ activity couid not bti jetor.Tiinoo ću-: ;o strorv. sc-lf; ;,c:"tion of the 134kcV lina in tho samples. The 2t;o is a riucj.it.~-,- ;:rodur/. 3r the 95Zr decay ar>l independent}' of ti'e starting values c<" t';-.r activities the S5!to/-o7.r activity 2:10 ni" Z.c is P''a'.r:ci!ly to reached after a few months of their decays [?]. Indeed, a similar activity ratio was observed in our samples. The initial activities of 95N& and 144Ce, however, could be suggested basing on the constant ratio 95Nb/95Zr and U4Ce/95Zr observed for the hot particles separated from the radioactive fallout following the Chernobyl accident [7J. One can assume that the initial activities of 95Zr, 95Nb and 144Ce were equal with 30% accuracy. It has to he emphasized that the distribution of the isotopes discussed above is completely different from the 137Cs distribution in Poland. In the South, there was practically no activity of 95Zr, 95Nb and 144Ce in contrast to the usually high ground contamination with cesium and ruthenium isotopes. In the samples taken from Iwrth-East of Poland the activity of 95Zr- was relatively high while the activity of 137Cs was low (see Table.1).

Table I. Radioactive composition in the upper layer of soil taken at 12 different locations in July and August. Activity values in (Bq/gl are extrapolated to May t, 1SS6. Statistical error equals about 5%, while the activity of two samples taken at the same location differed up to 50%.

Location 137Cs 134Cs 103Ru 106Ru 95Zr

Puszcza Aug. 0.05 0.03 0.1 0.C3 0.4 Szypliszki 0.08 0.03 0.1 0.05 0.6 Białystok 0.12 0.07 0.1 0.02 0.3 PrzemySl ' 0.15 C.11 0.4 0.08 •Cielce 0.1S 0.10 0.43 0.10 Gliwice 0.23 0.13 0.52 0.08 Kraków 0.45 0.26 0.92 0.18 Mszana D. 0.47 0.26 0.76 0.12 Balice 1.15 0.65 1.9 0.46 0.1 Andrycndw 1.55 0.88 2.0 0.46 Szczyrk U65 0.90 2.5 C.58 p.Jafowiecka 2.55 1.35 *.O 1.0

11 Total fallout deposit

The radioactivity deposited on the ground surface at the and of April has penetrated into deep layers of soil with a rate depending on soil properties. Fig.5 shows the 137Cs activity as a function of depth at which the soil was taken. In November 1986 more than 90% of the total activity stayed in the upper one centimetre thick layer of clay-like soil while only 40% of the total activity remained near the surface in the sand-like soil. Much low penetration was observed for the samples collected in July. It was estimated that, in average, about 70% of the total deposited activity of 137Cs was still in the upper layer of soil collected in July and August to investigate the activity distribution.

DEPTH [cm]

Fig.5. Activity dependence on dephs for two types of soil (clay- and sand-like soil) taken in November, 1986 at Skawina near Cracow.

The total deposit of 137Cs in Poland was estimated as follcws: the average activity 0.12 Dq/g assured for CO* cf i!v Po.isr, territory corrssponcs tc the ground surface activity 0.12 Bq/g x 30000 g/r,i x (G.7)"2s5k3q/ni.

12 la c!ie sarae uay, one could ootain the grounc surface activity ec.uai if co 20 k3q/tn for the regaining territory. This gives 2.5 x 1G 3q of 137Cs as the total deposit ail over Poland. It is roughly naif c-f t;:o Lota! deposit of 137Cs in Sweden [8] and about 2 times -nore .: total activity of 137Cs deposited in tiis country .ju- •.,._ the nuclear weapon uL-;ospi;aric test period [ćj. AjTiong 7 identified long-lived gamma emitters, tne activities. 1Ó7Cs and 134Cs reached the level at which their contributions to trie annual radiation doses should be considered. Annjcl extinul dose calculated for the mst contaminated ćrea (100 !<3<";/n cf 137Cs) is equal to 300 r.irad per year as seeing 24 hours exposurr: every day. This value is aooot 2 tiir.es Icfgsr than the overage aiViual dose fron naturai background redistion. however, irulivicudl doses j;-pe;io on ho-.; r.uch tirr.e people have spe.it outdoors a;U .;ili i>.2 sijnificsntly jew. It is uLCh ;.iore difficult to sstinat; t!-.t intdff.d! dose Jue to cesiun isotopes sntsrinc; uod/ •..•it'i focj. .,.- O2l;?ve t;iat tne present determination of t.ic ;uj!i3st activity deposit orea will stinulate neu studies on 137Cs usliavioi..- i.i the fooii-cr.air, and on its abundance in the food produced in contaminated zones. Annual doses frm the content of the other identified isotopes ar? -;jcn snaller. Firstly, their halflives are relatively short and secondly they do not accumulate neither in plants nor in hucian oociy. iiowever, there is an evidence thót the activity of S5Z.- -vti UiCe is often accompanied by alpha a.iittars [sj. Our results indicate that while the fallout of cesiu.", isotopes was mostly deposited in Southern Poland tfi3 nortn-oastern part "r.^y be wee/.ly containinated with sonie alpha emitters.

This v»ork was perfomed in cooperation with the Institute of i.uclear Physics in Cracow. The authors are grateful to R.Sroda.R.il-aiecY-.Suryjtk and W.'..'aiuś for many useful discussions, i.'ann tiianks te ...Jóiefowsiu, T.l.'fodek and S.iiróbel who nelped us to collect the samples.

13 References: 1. T.Bennerstedt, M.Holmberg end L.Lindborg, Chernobyl- fa! lbut, measurements and consequences, SSI-report 85-13, Stockholm,(1906).

2. C.Hohenemser, D.Deicher, A.Ernst, 6.Lindner and E.Recknagel, ENVIRONMENT 20 NO 5 (1986).

3. R.Broda, J.Grebosz and J.Wrzesirfski, Report Mo 1330, Institute of Nuclear Physics, Cracow (1985).

4. Archive of the State Hydro-Meteorological Institute, Warsaw.

5. Report of the Government Council, Warsaw, June 1986.

5. J.Jastrzebski, to be published

7. R.Broda, Raport to 1342, Institute of Nuclear Physics, Cracow (1935), Acta Phys. Pol., to be published. 3. J.O.Snihs, Consequences in Sweden of the Chernobyl accident, SSI-report 86-21, Stockholm, (1536) 9. R.Broda, Z.Szeglowski, K.Zuber, to be'published.