Detailed chemical analysis of aerosols carrying radioactivity and collected with mica track microfilters

J. Aerosol Sci., Vol. 18. No. 6, pp. 931-934, 1987.

0021-8502/87 $3.00+0.00 Pergamon Journals Ltd.

Printed in Great Britain

DETAILED CHEMICAL ANALYSIS OF AEROSOLSCARRYINGRADIOACTIVITY AND COLLECTED WITH MICA TRACKMICROFILTERS P. Vater, R. Dersch, R. Brandt Kernchemie, FB 14, Philipps-Universit~t, Marburg G. Luthardt, W. Rudolph NUKEMGmbH, Hanau

Introduction Measurements of aerosols carrying radioactivity within an industrial plant producing nuclear reactor fuel elements have to be done to ensure the workers' safety and to make certain that the exposure does not exceed permissible limits. The environmental air near such a working place contains conventional dust particles and a few radioactive particles which consist mostly of uranium compounds having an isotopic composition like the uranium actually used in the fabrication process for nuclear reactor fuel elements. The incorporation of these particles above the permissible limit could endanger the health and has therefore to be carefully controlled. The deposition of aerosols in the lung depends mainly on their particle size, the retention therein on their chemical and physical properties. These qualities must be known for preventive measures to ensure the workers' radiation protection. Knowing these qualities one should be able to infer from the measured activity of aerosol particles in the environmental air the value for the incorporated activity. To study some global properties of aerosols there exist several established methods, but there is composition of individual

a lack of

information concerning the

chemical

aerosol particles and of the size distribution of particles

having a certain chemical composition. We apply a Mica Microfilter Cascade Fractionator (MMCF) to collect aerosol particles from the environmental air near a working place at an industrial plant producing nuclear reactor fuel elements. By using Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Analysis (EDXA) the individual

particles

can be investigated.

Experiment Figure l shows a photograph of the Mica Microfilter Cascade Fractionator (MMCF), and a sketch of the air inlet section. The MMCF contains three or four mica f i l t e r s having different pore sizes which decrease from about lO ~m at the air inlet side down to about l ~m for the last f i l t e r . At the end, a glass fiber f i l t e r acts as an

931

932

P. VATER et aL

absolute f i l t e r .

The outer diameter of the

mm, the diameter of the f i l t e r i n g porosity is

filters

is 50

area 40 mm. The f i l t e r

about 5 %. The use of supporting

grids

and

relatively low pressure differences (up to about 0.25 bar) guarantee for

the mechanical

stability

of

the

filters

during the experiment. By using a bypass the air throughput is controlled to be about lO0 I/h, the total throughput is a few m3. After the aerosol sampling the mass load of the f i l t e r s is determined by weighing and the collected alpha-activity is measured with

a methane flow counter.

After

scanning electron

microscope (SEM) with

attached

dispersive

X-ray

investigate the ~

analysis

system (EDXA) is

that,

a

energy

used

particles collected at the f i l t e r s .

to

Thus,

Roof

the

Cone

chemical composition

(assuming a spherical determined.

In

this

and

the

geometrical

diameter

shape) of individual particles can be way particle

size

distributions

of

uranium and conventional dust particles can be obtained. The Mi¢o Filter Supporting Grid

- I b m --dldB

50 mm

median particle size is characterised by the 50 %-value of the frequency distribution of the particle sizes. As an example, figure 2 shows SEM photographs of(right) an

Figure l:

uranium and ( l e f t )

The MMCFand i t s

a steel-particle together with the

respective X-ray spectra.

air i n l e t section. 1 03 3

I04 i

1

3

Figure 2:

S

2

9

II

13

2

4

6

8

I0

12

14

IG

K.U

SEM photographs of (right) an uranium and ( l e f t ) a steel particle together with the respective X-ray spectra.

Results and Discussion During the last years different experiments at different times have been carried out in the same factory I-3.

The results

given in this

paper are,

to some extent,

preliminary and averaged. The specific mass load and specific a c t i v i t y of air were found to be in the order of (0. I-0.4) mg/m3 and (0.15-0.3) Bq/m3, respectively. Both values are

in

good agreement with

results

of

independent measurements using

different

techniques. As a typical example of measured mass load and alpha-activity of individual f i l t e r s , the results of the last experiment are quoted in the following table.

Chemical analysis of aerosols carrying radioactivity

filter

pore sizes

mass l o a d

(~m)

(mg)

933

alpha-activity (Bq)

l

9.2

0.2

(0.63 + O.Ol)

2

4.4

O.l

(O.ll + O.Ol)

3

1.3

0.3

(0.033+ 0.006)

m

4

glass fiber

not measurable

not measurable

More than three fourth of the activity was found on the f i r s t f i l t e r

which is loaded

only by one third of the total mass load. This indicates that most of the collected mass consists of conventional

dust particles

and the mean size of uranium particles

is

expected to be larger than that of conventional dust particles. This can be seen in figure

3 where the

particle

size distributions

of

uranium and conventional dust

particles collected at the same f i l t e r (in a previous experiment) are plotted. The geometrical particle sizes are characterized by the "median" C, where uranium (C=2.0 Nm) exhibits a significant larger diameter than conventional dust (C=0.35 Nm) particles.

/

Jl:

Figure 3: Frequency distributions

of

geometrical particle sizes of

.

(left)

uranium and

(right)

conventional dust particles.

°°f

U- Particles

4' U - Porticies

C , 2.0 wn

o:s ,:o ;

~

~:lrt~cle S~ZS[iJm)

;o

C - 0.3S pm

~'o.s Z,

~

~ r t l c ( e size (pro)

;o

In the last experiment we found a comparable result of C=l.9 Nm for

uranium

particles. In this experiment we have investigated all particles ~O.2Nm in a certain well defined f i l t e r area. From the number of uranium particles found on this area (29), the median value of the geometrical size (C=l.9 Nm), the isotopic composition of uranium actually used in the fabrication process during the experiment, the total

filter

area

and the assumption that the uranium particles consist of UO2 and are of spherical shape, the total alpha activity of the loaded f i l t e r can be calculated to be (0.52+0.31) Bq. This result is in rather good agreement with the actual measured value of (0.63+0.01) Bq (see Table) indicating that the examined fraction is indeed representativ of the total f i l t e r area. Conclusion The MMCFtogether with the evaluation using SEM/EDXA is proved to be a useful tool to determine the size distribution

of aerosol particles

having a certain

chemical

composition, e.g. of uranium particles. Investigations of uranium particle sizes collected from the environmental air

at an

industrial plant show a rather large value for the median geometrical particle size. Taking into account the large specific gravity of UO2, the AMAD-value (activity median

934

P. VATER et al.

aerodynamic diameter) should be correspondingly large. Respective calculations are in progress to convert geometrical sizes into aerodynamical ones. Acknowledgements We are indebded to Drs. R. Spohr and J. Vetter for kindly arranging the mica irradiations

at

the

UNILAC (GSI, Darmstadt). This

work was

Bundesminister fur Umwelt, Naturschutz und Reaktorsicherheit

supported

by

the

(Bonn).

References Dersch, R., et al.

(1985) "Health Protection

Investigations at a Working Place Using a

Mica Microfilter Cascade Fractionator (MMCF)", GSI 85-I (Scientific Report 1984) 264 Dersch, R., et al. (1986) "Chemical Analysis and Size Distribution of Aerosols Collected with Mica Track Microfilters", GSl 86-I (Scientific Report 1985) 266 Vater, P., et al. Tracks 12, 981

(1986) "Industrial Applications of Mica Track Microfilters", Nuclear