Interpretation of thorium bioassay data

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This study is a comparison between bioassay data of thorium.exposed workers from two different facilities. The first of these facilities is a monazite sand ...
Journal of Radioanalydcal and Nuclear Chemiatry, Articles, VoL 177, No. I (1994) 121-~126

INTERPRETATION OF THORIUM BIOASSAY DATA L- i . Q. C, JULIAO: A. M: G. F. AZEREDO, i . S. SANTOS;DiR, MELO; B.M. DANTAS,J. L. LIPSZTEIN

Instituto de Radioprote~do e Dosimetria/CNEN, A ~. :Salvador Allende, s/n*, via 9'Barra da Tijuccr CEP 22780:!60 ~ de Janeiro (Brazil) (Received October 1, 1993)

This study is a comparisonbetweenbioassay data of thorium.exposedworkers from two different facilities. The first of these facilities is a monazite sand extraction plant. Isotopic equilibrium between Zn'rh and ~

was not observed in excreta smnples of these workers. The

second facility is a gas mantle factory. An isotopic equilibrium between ~

and Z~rh was

observed in excreta samples. Whole body counter m e s s ~ e n t s have indicated a very low intake of thoriumthroughinhalation. As die concentrationof thoriumin feces was ~teryhigh we concluded that die main pathway of entrance of the nuclide was ingestion, mainly via contamination through dirty hands. The comparisonbetween the bioassayresults of workersfrom the two facilities shows that the lack of Th isotopicequilibriumobserved in the exeretion from the workersat the monazite sand plant possiblyoccurreddue to an additional Th intake by ingestionof contaminatedfresh food. This is presumablybecause ZnRa is more effw,iently taken up.from the soil by plants, in comparisonto :ruTh or ~

and subsequently,ruth grows in from its immediate ptn'ent, ~Ra.

Thorium is an element of potential interest for the Brazilian nuclear industry because of its possible use as a fissile material in breeder reactors. In Brazil the largest deposits of the nuclide occur on the coast as monazitc sand. The Brazilian monazite sand is made up of four minerals: iknenite (FeTiO3), ruffle (TiO2), zirconite (SiT_.tO4) and monazite, an orthophosphatr of rare earth containing up to 6% ThO 2 and 0.3% of U30 v The Brazilian monazite sand extraction plant processes about 100 ton/month of the sand to obtain a concentrate o f monazite.

The physical methods t of treatment consist of magnetic

separation of ilmenite, electrostatic separation of zirconite and gravimetric concentration of monazite up to 99%. The monazite concentrate is leached to obtain the rare earth chloride and also Th and U precipitate as by-products. The Th precipitate is purified and concentrated to obtain the thorium nitxate, which is used in the production of gas mantles. In Brazil, thorium is of great interest in terms of radiological protection, 2 especially in the monazite sand extraction and purification of monazite, and in gas mantle facilities.

Elsevier Sequoia S. A., Lausanne Akad~miai Kiad@, Budapest

L. M. Q. C. JULI,~O et al.: INTERPRETATION OF THORIUM BIOASSAY

The present study is a comparison between bioassay data of thorium-exposed workers from two facilities: a monazite sand extraction plant and a gas mantle plant. A group of 150 workers were occupationally exposed in the former facility and about 60 in the latter.

Experimental In order to study the Th behavior in the human ,body, the workers--of t h e two different facilities were monitored through in vitro bioassay and in vivo lung measurements.

In vitro bioassay Twenty-four hour urine and feces excretion samples from workers of the facilities described above were collected.

The samples were analyzed through alpha spectrometry after chemical

treatment. Figure 1 presents a schematic representation of the chemical procedure.

This is a

cSc~mplle~n ~ ' ~

u.oo T L

I I

|

Fusion

~o-precipitallon ~-~--~ I

BCI

IF ....

1

Solvent Extracllon

I Jr I Th~ SaluUon ] i Dooo,'tn I

:[ Counting I -t

Data analysis

I

Figure I, Schematic representation of the chemical procedure for analyzing urine and feces excretion.

routine method 3 at the Bioassay Laboratory of IRD, and evaluates simultaneously thorium and uranium isotopes. Thorium 229 was used as an isotopic tracer to determine the chemical recovery. The lower limit of detection is in the range of l x l 0 -3 Bq/g or 1.

122

L M. Q. C. JULL~O et al.: INTERPRETATION OF THORIUM BIOASSAY

Results and Discussion

Mo,nazite Sand Extraction Plant Table 1 summarizes the feces bioassay results of 19 workers from the monazite sand extraction plant. It shows the concentration of ZUTh and 22STh in f e c e s . The concentration of 23~I'h was found to vary in t l ~ r a n g e of 4.1x10 "3 to 7.3x10 "2 Bq/g of a s ~ , z2s'I'h ra nge d from 4 . 6 x 1 0 z to 1.6x10"1 Bq/g of ash. For all workers 228Thp32Th ratios are higher than 1. This ratio varies from 1.4 to 17. An inverse statistical correlation was observed between the 22STh/232Th ratio and the z32"l?h concentration. TABLE 1 232"1"h and 22S'l'h concentration in feces from workers of Monazite Sand Extraction Plant

Code

z"q'h teq/g ash)

ruTh (~q/g ash)

Ratio

7.3 x 102

2

1

1.6 X lO t

2

5.1 x 102

1.4 X 10 ~

3.6

3

5.1xlO:~

1.9 X 102

2.7

4

4.8 x 102

i.9 X 10 a

2.6

5

4.6x 102

1.4 x 102

3.3

6

8.7 x 102

2.7 x 10:

3.2

7

1.0 X IO t

4.5 X102

2.2

8

7.5 x I 0 :

2.6 X102

2.8

9

6.3 x 102

6.8 x 10j

9.2

10

8.9 x 10 :t

1.2 X 102

7.5.

11

9.2 x 102

6.4 x 10 a

1.4

12

5.5 x 102

2.7 X102

2.0

13

4.7 x 102

l.$xlO1

3.1

14

7.2 X 10":

4.1 ~ lOs J

17

15

9.7 X! 0 =

2.5X102

3.9

16

,5,7 x 102

4.7 XIOs

12.1

17

7,5 X102

1.9 X102

3.9

18

1,5 x 10I

3,7 x 102

4.0

1.9 X102

7.4

19

s

1.4xlO t 8.18X102

2.48 x

3.52x!0 a

l .S6 x I 0-~

10a

123

L M.: Q, C JULIAO et al.: INTERPRETATION OF THORIUM BIOASSAY

The lack o f Th isotopic equilibrium observed i n the excretion from the workers o f this facility possibly o c c u r s d u e to an additional Th intake by ingestion o f contaminated fresh food. This is presumably because ~ R a is taken up f r o m t h e soil more efficiently than 23~I'h and subsequent/y, 22STh grows in from its immed/ate parem, 22SRa. All urine results are below t h e lower limit o f detection o f the method.S Gas mantle facility Table 2 contains the results o f feces bioassay from ten workers o f a gas mantle facility, who had potential for being occupationally-exposed~anywhere between 2 and 26 years. In this case two different sampling periods were compared. In the f'LrSt sampling period, individuals were working daily, 8 hours per day and 5 days per week. In the second sampling period, samples were collemted after a 30-day vacation. During the working period, the concentration in feces was found to be in a range o f 3.7x10 3 to 8.8x10 4 Bq/g o f ash for 237"I'hand from 2.0x10 z to 7.9x10 4 Bq/g o f ash for 22STh. After 30 days o f vacation the concentration ranged from 1.1x10 3 to 3.6x10 2 Bq/g of ash for 232Th and from 1.6x10 2 to 3.0x10 "z Bq/g o f ash for 22STh, with the exception o f one worker (6.8X10 "2 Bq/g o f ash). The average concentration for z~z'I'h and 2ZVl'h was one order of magnitude higher during the working period??. All urine results are below the lower limit o f detection.

TABLE 2 ~2Th and zmrh concentration in feces during working period (1) and after 30 days o f vacation (2) from workers of a gas mantle facility I

I

Code

i !

Timeat w o r k ~vean)

~ (0 ON/s ash)

(t) Okl/g ash)

!

2~l'h (2)

0~q/g ash)

' I

='Til f2) (eq/g ash)

t

2

9.6 x 10 .2

9.9 x 10 -~

l.;txl0:2

2.5x10 ~

2

3

9.5 • 102

1.0xl04

6.3 x 10 -)

1.8 • 10 -~

3

3

1.1 • 104

1.3 x lif t

1.1 x 10.3

2.3 X10z

4

9 . 4 x 1 if5

2.0xlO 4

9.0XlO~

3.2 x 10~

5

7

8.3Xt0"

4.0 x 10j

2.0x

6

8

3.3•

7

II

I

II.

I

13

8

,,

9

10

1

,

[

26

u

, 4-$ xlO'*

10 .2

4 . 4 x t 0 "2

2.4•

"~

6.8 x 10:~

3.7x104

1.6x10 2

2.0 x 10.2

2.2 • IO2

8.8x10 4

7.9 xl0 :t

1.3 x 10 -2

1.6 x

!;6Xl04

2,2x10"

2.1 • iO 2

7.3xlff2

7.6 x 10-~

3,6X10-2

3.0 x 10:2

104

1 . 4 8 x 1 0 "l

1 . 0 8 x 102

2.75x 10~

2.63 XI(Yt,

2.3 • I0;'

i .08 x l 0 a

I,.51 X 10"z

1.47 x

] 124

I

~

102

L. M. Q, C. JUI.JAOet aL: INTERPRETATIONOF THORIUMBIOASSAY Tabl~ 3 shows the 2~Th/~2'I'h ratios. They are in a range from 0.9 to 5.7 for the working period and from 0,8 to 28.3 after the vacation period~ The ~Z~'h/mTh ratios for the working period only presented vaiues higher than 1, for the lowest 232Th"activifies,i Le., in the order of 10.2 Bq/g of ash. This low concentration may indicate low occupatiomfl exposure. ~

activities higher

than z~I'h in samples may then be explained by additional T h intake through ingestion of contaminated fresh food, in which the lack of isotopic e q u i t i ~ a m might be expected because of Ra uptake by plants. TABLE 3 ZZSTh/~2Thratio during working period (I) and after 30 days of vacation 9 Codo

from workers of a gas mantle facility

I Tim*at work (yelrO 2 '

'

I

'

1

2.9

[

21

'

3 4

7

t-nttt,,rh

1

3

5

Ratio ('2)

l~tlo (1) ~rhm'rll

39

2 7

s.7

8

1.3

11

4.3

9

0.9

5

1,1 t.2 I

9 84

ts

i

9.5:

i0

26

1

0.83

Equal mTh~2Th ratios in the samples taken during the working period occur in the cases in which ~2Th concentration is in the order of magnitude of 10:~ Bq/g of ash. The same occurs with 22Vl'h. This indicates an occupational exposure which appears to b e independent of the number of working years. After the vacation period, most of the ~ T h / ~ h

arc higher than one. ~ T h concentrations

in these samples a r c low, 1 order o f magnitude lower than i n the working period, (!.lxl(~3 to 3.6x102 Bq]g of ash). This result is expected since no occupational exposure occurred in this period. concentrations in these samples are again higher than zJ~'h concentrations, as a consequence of additional intake by ingestion of fresh food containing Z~Ra and its daughters. 125

L. M. Q. C. JULIAO et al.: INTERPRETATION OF THORIUM BIOASSAY

According to in vivo measurements also performed at the IRD/CNEN Whole Body Counter, 4 inhalation is not the main route o f 2nTh incorporation. As the concentration o f thorium in feces was very high, we concluded that the main pathway o f intake was ingestion, mainly through the contamination o f the hands. Conclusions

In Brazil, Th occupational exposure is relevant. The natural exposure is also important in high background areas. Special care during data analysis should be taken concerning the 22gThintake contribution due to the WaRa decay in plants. Populations exposed to high natural levels provide very important parameters in the study o f the effects o f continuous exposure to low levels o f radiation. Statistical studies are suggested in the region, including food habits, type o f internal consumption, agriculture and local food

production. References

1.

c. COSTA-RIBEIRO, M.A. BARCINSKL N. FIOUEIREDO, E. PENNA FRANCA, N. LOB,~O, Health Phys., 28 (1975) 225o231.

2.

LL. LIPSZTEIN, L,N. BERTELLLC.A.N. OLIVEIRA,A.M.G.F. AZEREDO,D,R. MELO, M.C. LOUKENCO, D. GRYNSPAN, B.M. DANTAS, RadiationProtection Dosimetry, 26, n~

(1989) 57-60.

3.

A.M.G.F.AZEREDO,D.R. MELO, B.M.DANTAS,C.A.N.OLIVEIRA,RadiationProtectionDosimetry,37, n~

4.

(1991) 51-54. B.M. DANTAS, M,C, LOUREN(~O,J.L. LIPSZTEIN, Proc. of 4th Int. Conf. on Low Level Me,as. of Act. and

5.

B. ALTSHUI~ER, B. PASTERNACK,Health Phys., 9 (1963) 293-298.

Long-LivedRadion. in Biol. and Env, Samples, Rio de Janeiro, Brazil, 1992.

126