INIS-mf—14775 6Q96£9

INTERNATIONAL ATOMIC ENERGY AGENCY

RCM "The Radiological Impact of Hot Beta-Particles from theChernobyl Fallout: Risk Assessment"

September 1993, Bulgaria

HIGHLY-DISPERSED HOT PARTICLES IN NPP KOZLODUY

V.Angelov, P.Andreev, Ts.Andreev*, Ts.Bonchev, K.Bourin,R.Christov, V.Mavrodiev, T.Semova, N.Todorov*, G.Tonin, ,

Ts.Tzacheva, E.Vapirev, Y.Yanev

Faculty of Physics, Sofia University, 1126 Sofia, Bulgaria*NPP "Kozloduy"

At the beginning of May 1986 scientists from the Sofia Universitydiscovered hot particles (HP) originating from the Chernobyl accidentwhich had been transferred to the territory of Bulgaria. Since the verybeginning the HP were identified and considered as a new radiationagent and the first results were published in tl>2\ The investigationscontinued during the next years, but only a fraction of theinvestigations had been published in international journals [3>91. Later thesame team of investigators started systematic studies of the radioactivecontaminations in NPP Kozloduy and naturally a considerable part of theefforts was allocated on the HP problem. In 1991 the first autoradiographsof highly dispersed HP were obtained and in the. spring of 1992 severalparticles with high activity and considerable dimensions were found onthe NPP site.

It seems naturally for us that the radioactive contaminations in a NPPshould be predominantly in the form of HPs, except for the solublenuclides. Up to now we could not find publications on the problem (exceptfor the already cited references), although the problem with radioactiveaerosols in NPPs has been extensively studied (e .J 1 0b

At the present TCM the results of the investigations of I.Mandjoukov andB.Mandjoukova are presented on highly radioactive HPs which are eithermicroscopic fragments from spent nuclear fuel or activated metal. Our aimis to discuss the problem of highly dispersed HPs.

I. EXPERIMENTAL

I.I. SAMPLING AND IDENTIFICATION

Our experience in the investigation of work areas and facilities withrestricted access in the Nuclear Power Plant at Kozloduy, show thatdetailed information for the surface contamination with alpha-emitterscan be obtained by sampling with plastic adhesive tape. The ready for usestickers (samplers) are pieces of adhesive tape approximately 2.5 x 1.5 cm. Apiece of paper 0.5 cm long is stuck to one of the sampler ends. Thenumbered samplers are stuck to a plastic holder or aluminium from whichthey can be easily taken. The sampling is performed by sticking thesamplers to the investigated surface, after that the sample is returnedback to the plastic holder.

In the laboratory they are taken back from the holder and are covered bythin polymer folio, 0.7 mg/cm2 thick, in order to prevent from fallingapart, except for 1-2 uncovered samples for alpha-spectroscopy. .

1.1.1. Nuclidfi identification

The nuclide identification and activity measurements is performed with aGe detectors with careful activity calibration for the solid angle.

In some cases planar HPGe detector for low energy gamma- and X-rays wasused (4-100 keV). With the planar detector the "exotic" Fe-55 wasdiscovered, which has a TL/2 = 2.7 y and decay into Mn-55 via electroncapture. The only detectable radiation is the 6 keV X-ray from Mn-55 with,yield = 24%. The Fe-55 does not emit anything else except for amonoenergetic neutrino and therefore its presence can be deduced only bythe Mn X-ray. The problem is complicated due to the presence of Mn-54,which emits the same line with yield 22%.

To the search for pure beta-emitters (Sr-89, Sr + Y -90) the methoddescribed in I11] was used (stilbene scintillation detector, energyresolution 10%). Up to now we could not detect even traces of strontiumcontamination.

I.1.2.TMpha-spectroscopy of the samples

The thickness of the alpha-samples proved to be thin enough for alpha-spectroscopy - t12!

The alpha-spectra are obtained with silicon planar detector. The area of thedetector is 50 mm and the energy resolution for alpha-particles is 18-20keV.

239pu

+240pu +Z38pu

Fig.l. Alpha-spectrum of sample from a telemanipulator. Peak relativeintensities: 239Pu + 240Pu . 1 5 % ; 2U A m + 238Pu . 6 0 % ; 244Cm . 15%and 242Cm -10%. Evaluation for the layer thickness of thea-emitter on the plastic adhesive tape - 60 mg/cm2. Evaluation forthe total specific oc-activity -7 x LO3 Bq/g.

The samples are with low activity and therefore the detector and the sampleare very close one to another, the distance in the vacuum chamber is 1 mm.The close sample-detector geometry is suitable For the application of amethod for estimation of the sample thickness. A rough estimate of thesample thickness for a source without a cover is obtained by the relation :

R_En

(mg/cm2) III

here d is the sample thickness, R is the range of the alpha-particleswith energy EQ, AE is the energy interval from E to the left where thenumber of alpha-particles in the "tail" decrease to 1/2 compared to thenumber of particles in the peak.

A more accurate estimation of the layer thickness is:

121

here (dE/dx) is the averaged specific energy loss in an energy intervalclose to the maximum energy (4.5 - 5.5 MeV).

The specific activity of the contamination in Bq/g can be calculated fromthe sample thickness and the measured total alpha-activity or theactivity of each of the actinides.

The calculated specific alpha-activity of most of the samples is within therange 102-103 Bq/g.

The ratio of activities of the 239Pu/240pu anci 238pu/24iAm p e aks givesinformation on the burn-up and also on the time of the release. 242Cm has ashort lifetime, T1/2 = 163 d, and the relative intensity of the curiumisotopes also gives information about the time of release.

1.2. VISUALIZATION OF HOT PARTICLES

A scanning electron microanalyser is used for the visualization of hotparticles and the identification of the carrier. Two examples for HPsvisualized and identified by this method are shown in Fig.2 and 3.

l o o m , a i " " ' " " ' • • " ' • • • • i • • i • i • • • • i . . 1 1 1 1 1 1 1 1 . . . . i . . . . i . , . 1 1

b.

U t C ( 2 - U | 0 * 1 * LABEL I T ' 100 SCCS.

Fig.2. a - picture of a UO2 hot particle;b - X-ray excitation spectrum of the HP, the lines of uranium are

identified.

l l l . . l l . . , , l , . . , l100000

I I J \ \""t« C C ( ? - U | OAU U K L I t - 1 0 0 SCCS

Fig.3. a - Zr-particles at different magnifications;b - X-ray spectrum of the HP, the Zr lines are clearly observed.

1.3. SELECTIVE AUTORADIOGRAPHY

The radiobiological effect of HPs depend on the activity and the nuclidecontent. For an ensemble of HPs the identification of the different radiationsis not a simple problem. In the present section we present our first resultsfor solving the problem.

The most simple problem is the identification of the HPs containing alpha-emitters since the solid state track detector are not affected by gamma-and beta-radiation.

The solid state track detector which is used is a nitrocellulose detector, typeKodak CN85/B.

For the case when the density of the tracks is approximately 105/cm2 theimage of the contaminated surface can be observed with naked eyev-Fortrack density over 107/cm2 "overexposure" occurs and the image becomesmore transparent^12!. Two examples of such autoradiographs, comparedwith autoradiographs with conventional photo films, are presented in Figs.4 and 5. The autoradiographs are obtained from samples covered withpolymer 0.7 mg/cm2 folio which transmits approximately 60% of the alpha-radiation.

mm,

Atoradiograph on regularblack-and white filmORWO22DIN (DDR), exposure3 days, magnification x3;

Alpha-radiograph, exposure10 days, magnification x3;

Fig.4. Sample from a telemanipulator, the alpha spectrum is shown inFig.l.

Fig.5. The same as in Fig.4 but the sample is from a stand for repair «ofreactor equipment.

The line structure of both autoradiographs of Fig.4. is due to the structureof the sticking tape and the conclusion is that the contaminations arehomogeneous. On both radiographs of Fig.5. grainy structure is observedwhich is due Lo the presence of hot particles.

Further it was attempted to identify the particles which contain Fe-55.That isotope is produced in a (n,y) reaction with thermal neutrons where themacroscopic cross-section is 1.86 x 10'3 cm2/g (iron). It can be calculatedthat the activity of Fe-55 in the metal equipment in and around the coreprevails the activity of the well-known isotope Mn-54 - for a period ofwork 1 y the activity ratio Fe-55/Mn-54 is approx.30, after 6 y the ratio is50, and after 15 y the ratio is 60. One can conclude therefore that in spiteof the relatively low radiobiological value, F-55 could prove to be aserious problem for the nuclear power stations.

The samples with Fe-55 contain also significant activities of Co-60 whoseend-point beta-energy is 318 keV. The problem was to distinguish the 6keV X-ray from Fe-55 and the soft beta-radiation from Co-60. That ispossible because the mass absorption coefficient up of the beta-radiationalmost does not depend on the atomic number while the mass absorptioncoefficient, ux, increases rapidly with the increase of the atomic number.

The mass absorption coefficient can be calculated approximately with theexpression!13!:

^ = 15.5.^-1J3 (cnvVg) /3 /

Emax is in MeV, and for hydrogen |ip is two times greater than for theneighbouring elements!14!. The values for up from /3/ and for ux from t15'are summarized in the Table 1:

TABLE 1

1

4

6

13

22

24

Element/ substance

Hydrogen

Beryllium

Carbon

Aluminium

Titan

Chromium

Air

Water

Polyethylene (CH2)

Up[cm2/g]

0.43

2.65

10.8

111

451

570

22.7

22.8

9.3

[cmZ/g]

154

' 77

77

77

77

77

77

84

84

An example of autoradiographs of a sample with various filters is shownin Fig.6.

The filters with elements with low atomic numbers which can transmit theX-rays and absorb the p-radiation are with low specific densities, andtherefore the linear thickness is considerable and the images are smeared.

The images shown in Fig.6 demonstrate that selective autoradiography ispossible and that will help the identification of HPs with various nuclidecontent.

10

Autoradiograph without radiation selection;

Autoradiograph with polyethylene filter, 28.2 mg/cm2, which transmits 77%of the X-ray radiation and 8.4% of the p-radiation;

£*L

Autoradiograph with titan filter, 8.23 mg/cm2, transmitting 53% of theP-radiation and 2.4% of the X-ray radiation;

Autoradiograph of the sample with nitrocellulose solid state track detector(Kodak) which detects only the alpha-radiation, exposure 24 h.

Fig.6. Autoradiographs from a stand for repair of reactor equipment. Thefirst three autoradiographs are obtained after a 2-hours exposition ona black-white film ORWO22DIN (DDR).

11

II. PRESENCE OF RADIONUCLIDES IN THE AIR AT THE NPP"KOZLODUY1 SITE. INVESTIGATION OF THE AIR FILTERS OF

THE VENTILATION TUBES.

In 1992 the first autoradiographs were obtained of air filters of some ofthe ventilation tubes of units 1 and 2. The air filters were put into operationin 1987. The filter from the auxiliary building of these units, BlCl,demonstrated higher but homogeneous activity. The filter from the mainreactor hall was with lower total activity but the images of HPs with lowactivity were observed, Fig.7.

Fig.7. Autoradiograph of filter B4I3, magnification x 5. Exposure time 36 hon an X-ray film.

The conclusion from these investigations is that the filters clean effectivelythe air from radioactive aerosols. The low activities of both filters is anevidence that the presence of radionuclides in the air in premises withrestricted access is negligible. That observation is confirmed by numerousinvestigations of radionuclides in air at the Kozloduy NPP site (see Table 2).

In the last column of the table the upper limits of the concentrationsaccording to the national regulations (1992) are cited. The actualconcentrations are 104 - 105 times less than the limits.

12

TABLE 2. Aerosol activity of the ground air at the site of NPP "Kozloduy",measurement point SBK-2. The specific activity is in (.iBq/m3 and

• the relative include content in %.

Radio-nuclide

?Be

54Mn

58Co

6°Co

HOm^g

131i

134Cs

137CS

144Ce

uMn60Co

mCs™Cs

Sampling period and sample volume (m3)

11.08.13.08.

2408

710097

8010.1

<6<0.1

600.8

<7<0.1

<5<0.1

100.1

1101.5

-

0.13

0.091

13.08.25.08.

16617

580098

60.1

< 1<0.1

390.7

<10.1

<1<0.1

4.40.1

330.6

50.1

0.15

0.131

25.08.08.09.

18419

400094

240.6

30.1

982.3

110.3

<1.2<0.1

7.20.2

751.8

180.4

0.24

0.10

08.09.10.09.

2551

498095

270.5

-

1102.5

120.2

<5<0.1

110.2

901.7

-

0.24

0.12

10.09.19.09.

12684

469097

130.3

20.1

641.3

100.3

<1<0.1

60.1

340.7

120.3

0.20

0.18

19.09.23.09.

5543

305093

300.9

40.1

702.1

421.3

<2<0.1

110.3

702.1

-

0.43

0.16

24.09.07.10.

15149

335096100.3

30.1

621.8

100.3

<1<0.1

70.2

240.7

90.3

0.16

0.29

Upperlimits ofthe con-centra-tion!16)

in air[nBq/m3]

109

6.107

6.107

2.106

6.106

4.106

107

107

106

13

III. SCALING OF RADIOACTIVE CONTAMINATION RELATIVE TO 60Co

The average annual pennissiblc limit concentration of 60Co in air, in Bq/m3,is almost in the middle between the a-emitters and Tritium in reference totheir radiobiological impact. In fact the contamination with 60Co issomething ordinary in NPP. This is the reason that we made an effort of"scaling" the radioactive contaminations in NPP where for one and the samesample we compare the radioactivity and the effect of certain nuclide withthat of 60Co and the obtained ratio we compare with the upper limitconcentration of the radionuclide to the 60Co concentration. The upperlimits of the concentrations are according to the national regulations(1992)t16l In Table 2 the results of one and the same object are averagedconcerning the "relative weight" of each sample measuring its total y-activity in Bq. Data obtained are plotted on nomograms corresponding tothe table contents. In Fig.8 and 9 two examples related to the currentmeasurements in IV and V units of NPP "Kozloduy" are given using "Snismethod.

The nomograms show the problems with the radioactive contaminations. Forunit IV the contaminations with ct-radionucledes and to a certain degreewith 11OmAm1are "ordinary". In the two nomograms the possible limits ofcontaminations with 55Fe are given with bars evaluated by 54Mn.

Of course this method of "scaling" is not especially intended for hot particlesbut as they are the basic contaminators we decided to include it in thispaper.

In the case where a radionuclide from a group happens to be close to the"limit" of 60Co the radionuclide symbol is placed nearly the plotted point.

14

Table of scaling the radioactive contaminationsrelative to 6^

A6 0Coexp

Date: Measured by:

1

2.10- 4

. 0 ,13

0 , 5

1

1,74

2 , 5

5

7,6

10

20

25

50

500

2500

^i, exp

• -

^ lexp

R A D I O N U C L I D E

238p U / 239pU / 240pU / 241A m # 244Cm

90Sx,9:izr

1 0 6 R U / 1 4 4 C e

6 0 C O

131J

HOra^g

B9Srf9iY,93Nb,WCB,WCB

59Fe /65Zn/l25

75S e /90Y /103R U /141C e

54Mn /58Co /

242Cm

5 5 Fe /6 3 Ni , 9 5 Zr /

9 5 Nb, 1 4 0 Ba, 1 4 0 La

7Be, 5 1Cr

T(3H)

OBJECT:

15

'r-3"Pu '

• 2SO

.1

00.5

- I0 05

-2

«°S

- 3 - 2 - 1

p-MSf."Y. "Nb . ntCs.n7Cs

!—. "Fe, "Zn."'Sb,

I I O m A»

0

, ,

r*'Mn, "Co '"Cm.

^ » F g . "Ni»Zf."Nb."0Do,"°lo5

+3

T l ' H I

00.5

- 1

-2

-3

NPP Unit IV,Febr.1933, _ .-u.surfacecontaminations

.5

•3

»2SO

-Ioos

-2

-3

- I

110m,A

- 3 - 2 - 1

Sf

p - "Fe. »2n,"<Sb.

. IO]Ru. u)Ce,

n i l

r- " f e . »Ni.»Zr."Nb."°Bo,w°Lo5

+3

-2

-3

L"'Sb 'Be T l ' H )SlCr

NPP Unit V,Aug.1992.surfacecontaminations

16

IV. CONCLUSION

The presence of highly dispersed HPs was observed in the zone with limitedaccess in Kozloduy NPP especially during repair periods. There can benor doubt that in various extent such particles are present in other NPplants. Therefore during repair and maintenance operations in such areasthe workers wear special suits and respirators. Up to now we could notfind published papers on the problem which most probably has beenextensively studied and published in in-house publications with restrictedcirculation. It seems that the paper of D. Reece (USA) and the presentreport by I.Mandjoukov and B.Mandjoukova raise the question of highlyactive HPs in NPPs for the first time and we think that there should be dueattention to that problem.

The highly dispersed HPs are also important from other point of view,namely: ...*-

the origin of the HPs, or how the alpha emitters have aggregated;

- ' decreasing the spreading of such contaminations especially duringrefuelling and maintenance operations with special systems whichfilter the contaminations;

investigations on the nature and properties of amorphous andpolycrystalline formations with high specific activities;

including the problem of HP contamination in the emergencyplanning.

ACKNOWLEDGEMENT

The results cited in this paper are only part of the data obtained on theproblem of radioactive contaminations in NPP "Kozloduy1'. The studies havebeen performed by the initiative and financial support of the NPPmanagement. Here we would like to express our gratitude to the NPPmanagement and the other experts from NPP with which we arecollaborating and who assist us.