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Radiation protection aspects of the use of zircon sand
The Science o f the Total Environment, 45 (1985) 135--142
135
Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands
RADIATION PROTECTIONASPECTS OF THE USE OF ZIRCON SAND NATIONAL GROUP FOR STUDYING RADIOLOGICALIMPLICATIONS IN THE USE OF ZIRCON SAND*
ABSTRACT Working processes using zircon sand in a f a c t o r y producing r e f r a c t o r y material were studied from the point of view of r a d i a t i o n protection. Zircon sand contains high concentrations of natural radionuclides and is a t y p i c a l example of an enhanced source. Using various techniques the c h a r a c t e r i s t i c s of the o r i g i n a l material were analysed and environmental r a d i o a c t i v i t y was measured in the d i f f e r e n t s i g n i f i c a n t points of the process. F i n a l l y , a f i r s t assessment was made of e f f e c t i v e dose equivalent values in the areas of highest r i s k . INTRODUCTION The element zirconium i s never found in a pure natural state. I t can be found in oxidised form (ZrO 2 or "baddeleyte") or
as a s i l i c a t e (ZrSiO 4
" z i r c o n " ) and is normally associated with r u t i l e ,
ilmenite and monazite with
or
s i g n i f i c a n t concentrations of natural radionuclides (elements of the secular chains U-235, U-238 and Th-232). Zircon materials used in I t a l y are not extracted in the country i t s e l f but are generally imported from A u s t r a l i a in the form of a sand with a granulometry of 100+200 ~m ( i n 1981 56 000 tons were imported, of which 94.6% came from A u s t r a l i a ) . This granulometry enables i t to be used d i r e c t l y in the production of r e f r a c t o r y material. A f t e r being m i l l e d to 2 ~m i t is used in the production of paints and as a hardening agent and o p a c i f i e r in the ceramics industry. Given the high concentration of natural radionuclides t h i s sand (about which very l i t t l e
has been w r i t t e n ( r e f . I ) ) may represent a health problem in
the various stages of e x t r a c t i o n , p u r i f i c a t i o n , m i l l i n g , storage, transportat i o n and use. I n d i v i d u a l s can be exposed to d i r e c t r a d i a t i o n from the o r i g i n a l
*The group i s formed by: Presidio Multizonale di Prevenzione-USL 2, Piacenza(1); ENEA-PAS-COORBOL, PAS-FIBI-AEROSOL(2), PAS-FIBI-DOSI(3), Bologna; ENEA-PASSCAMB-MISURE, Casaccia (Roma)(4); ENEA-COMB-SAL-FISM, Saluggia(5); ENEA-DISP, Roma; Gruppo Ceramica Regione Emilia Romagna; USL 2, Massa Carrara(6); USL 18, Empoli; Servizio Medicina Preventiva e Ispettorato del Lavoro, Trento; I s t i t u to Superiore di SanitA, Roma(7). The experimental work was done by: M.Bergamini(6), R.Borio(7), G. Campos Venuti(7), T.De Zaiacomo(2), S.Fabbri(1), M.Formignani(2), A.Gazzola(1), R.Giacomelli(5), L.Lembo(3), C.C.Lombardi(2), C.Melandri(2), R.Nanni(3), L . P a l l a v i c i n i ( 1 ) , S.Risica(7), G.Sciocchetti(4),P.Spezzano(5) and G.Tarroni(2).
0048-9697/85/$03.30
© 1 9 8 5 Elsevier Science Publishers B.V.
136
material and the f i n i s h e d product as well as to intake of material with high s p e c i f i c a c t i v i t y . This i s a good example of a case in which the d e f i n i t i o n "enhanced natural r a d i a t i o n " can s t i l l
be useful, even considering the Fro-
blems r e c e n t l y underlined by ICRP ( r e f . 2). A study commission has been formed in I t a l y (created and organised by the administrative regions of Emilia Romagna and Toscana) with the p a r t i c i p a t i o n of both centralised and local I n s t i t u t e s . I t s aim is to study f a c t o r i e s that use or t r e a t zircon sand from the point of view of r a d i a t i o n protection. A sample i n v e s t i g a t i o n has been carried out in a f a c t o r y producing r e f r a c t o r y material. CHARACTERISATION OF THE SAND Specific a c t i v i t y was evaluated by gamma spectrometry on a number of samples from d i f f e r e n t stocks and by alpha spectrometry on one of these samples Gamma spectrometry measurements were made with a GeLi detector (80 cc, e f f i c i e n c y 14%, r e s o l u t i o n 2.2 keV) a f t e r having calibrated the spectrometer, contaminating a sample of the sand (p= 3000 kg m-3) with a multipeak l i q u i d source having an energy range of 88+1836 keV. Mean values with three standard deviations are shown in Table I . TABLE 1 Specific a c t i v i t y in Bq kg -I of sand sample (gamma spectrometry).
K-40 68~,55
U-238
Th-232
3445.21
763~I0
Alpha spectrometry was done a f t e r d i s s o l v i n g the sample and separating i t radiochemically ( r e f . 3). Thorium and
uranium, electroplated
on small
2 stainless steel discs, were measured by a surface-barrier detector of 450 mm , while Ra-226 was determined using a ZnS(Ag) s c i n t i l l a t o r .
The mean results
of s i x analyses of the same sample are shown with one standard deviation in Table 2. I t can be seen that the chains of natural radionuclides are in e q u i l i b rium. The r e l a t i v e l y small differences between the r e s u l t s of the two tables are caused by the d i f f e r e n t measuring methods used in laboratories that had not been i n t e r c a l i b r a t e d beforehand. In order to see whether the handling of the sand i t s e l f
can create an i n -
halation r i s k i t was also subjected to granulometric measuring. Seiving analysis was carried out, optimising time and mass in order to reduce errors due to
137 TABLE 2 Specific a c t i v i t y in Bq kg -I of sand sample
U-235
U-238 U-238
135~6
(alpha spectrometry).
U-234
Th-232 Th-230
Ra-226
Th-232
Th-228
2 8 0 0 ~ 6 0 2810±100 2800±30 2730±240 678±19 680~18
seive loading,seive time and percentual losses at the various stages,obtaining a mass median diameter of 170~m and a geometric standard deviation of 1.32. Such a small standard deviation encouraged the hypothesis that the sand had a l ready been treated. The large dimensions of the grains lead the authors to bel i e v e that the time spent i n suspension i s extremely short and t h a t , therefore, they do not c o n s t i t u t e a source of r i s k c f i n t e r n a l contamination through inhalation. ANALYSIS OF THE RADIOLOGICAL SITUATION IN THE FACTORY Description of the f a c t o r y The f a c t o r y produces, as
its
main business, electro-fused
refractory
blocks used i n the construction of smelting ovens f o r glass. The zircon sand i s stored in large reinforced-concrete tanks. I t is mixed with alumina and sodium carbonate before being smelted in the oven. The molten material is poured i n t o moulds and, a f t e r the e l i m i n a t i o n of sprues, the f i nished blocks are l e f t to cool f o r f i f t e e n to twenty days in the cooling area. They are then moved to another room where they are sand-blasted, ground, polished with emery, f i n i s h
ground and f i n a l l y
cut. The accurate assembly of a l l
the components of each oven is then checked in the pre-assembly
area. The
sprues and waste products are m i l l e d i n m i l l s outside. Extractor hoods are provided where the mixture is produced and smelted and where sprue is eliminated, as well as in the cooling and grinding areas. The v e n t i l a t i o n makes i t possible to consider environmental conditions regardless of season. Gamma exposure rate Measurements were made inside and outside the f a c t o r y , at points considered representative of the production cycle and the environmental background, res p e c t i v e l y . Four laboratories were involved in measuring with passive dosemet e r s , using four thermoluminescent detectors (LiF: TLD-IO0), exposed f o r three months, at each of the above points. The t o t a l u n c e r t a i n t y of the dosimetric system chosen is in the order of
138
15%, of which about I0% is due to random errors and about 5% to errors of c a l i bration. Exposure rate represents an average value for the conditions of zircon material use, which varies according to the e v o l u t i v e cycle of production. At the same points in which the TL dosemeters were positioned measurements were taken with active dosemeters ( i o n i s a t i o n chamber ( I . C . ) and scintillation
detector (S.D.)). The r e s u l t s are shown in Fig. I . The difference
between the measurements made by active and passive dosemeters at the tops of the sand storage tanks i s due to material management and storage at d i f f e r e n t times.
I) Smelting area 2) Factory e x t e r i o r (environmental background) 3) Cooling area 4) Base of sand storage tanks 5) Top of tanks 6) Pre-assembly area (only LAB4) | 7~ Grinding area (only LAB4)
j ~-| | |
449
29
113
10.3
I08
f
9.0
PASSIVE DOSEMETERS
Fig. 1 -
ACTIV E DOSEMETERS
Mean exposure rate ( i n ~ R h- l ) in representative points.
A i r contamination characterisation In order to map a i r contamination during
the stages of production in
various points of the f a c t o r y the f o l l o w i n g measurements were made: I ) alpha a c t i v i t y of the p a r t i c u l a t e ; 2) mass and a c t i v i t y granulometric c h a r a c t e r i s a t i o n ; 3) grab sampling and integrated measurements of radon concentration in a i r . Measurements of alpha a c t i v i t y were carried out by drawing a i r f o r t h i r t h y minutes - using pumps of 18 and 35 l i t r e s per minute flow rate - through c e l lulose t r i a c e t a t e f i l t e r s
of 47 mm diameter and pore diameter O.45pm. A f t e r
enough time f o r the short - l i v e d isotopes to decay and f o r the series of radionuclides inside the p a r t i c l e s to a t t a i n e q u i l i b r i u m , the f i l t e r s
were covnt-
ed f o r t o t a l alpha a c t i v i t y with a ZnS(R,g) detector. The r e s u l t s , with standard deviation are shown in Table 3.
one
139
TABLE 3 Total alpha a c t i v i t y and radon a i r concentration in various areas of the f a c t o r y
Total alpha a c t i v i t yZ lO-LBq m- j
Sampling point Oven operation cabin Pouring area Sprue-elimination area Cooling area central corridor Central oven area Top of sand tanks Grinding area Diamond saw area Pre-assembly area Finish grinding area Storage area of unfinished product M i l l i n g operator cabin Mill-loading
To v e r i f y
4.37 ±0.32 9.72 ±0.47 42.15 ±I.27 2.58 ± 0.27 2.92±0.28
Rn air concentration/Bq m-3 Grab Integrated Sampling measurement
II II
6 20 6
9 13
0.68 ±0.29 0.19 ±0.15
16 17 12
-19 6 18
15
17
1.28 ± 0.21 15.89 ±0.79
i f airborne dust was enriched by Po-210 released during proces-
ses at high temperatures, specific a c t i v i t y of the three most active f i l t e r s in the areas of sprue elimination (1), mill-loading (2) and pouring (3) was monitored. The experimental values, shown in Fig. 2, have been interpolated with a function which is the
sum
of two exponentials that represents the decay of Po-210 and of the very long-lived parents (U-238, U-235 and
,~E40
Th-232). A c t i v i t y values (in lO-2Bq m-3) extrapolated at the moment of
~30
sampling in the same three areas
'920
were 42.68, 12.29, 9.47 for Po-210 and 2.41, 5.27, 0,83 for the very
10-
long-lived radionuclides in equilibrium with their daughters. Po-210
00
50
100
150 200 t / days
250
enrichment was confirmed by direct measurements of the same samples using alpha spectrometry (ref. 4).
Fig. 2 - Calculated decay curves and experimental values of total alpha a c t i v i t y in three areas.
Granulometric measurement of the particulate was carried out in the
140
two most a c t i v e
areas - the sprue-elimination (A)
and the
grinding area (B) -
as well as at areas of more diffused contamination - the central oven
area (C)
and the pre-assembly area (D). Concentration of t o t a l dust was measured by aspiration through g l a s s - f i b r e filters
(diameter 47 mm) at a
flow rate of ten l i t r e s per minute. Mass granu-
lometric d i s t r i b u t i o n was determined by sampling the aerosol at 2 l i t r e s per minute using 8-stage mini SIERRA impactors plus a f i n a l f i l t e r .
Distributions
were bimodal, i n d i c a t i n g more than one contaminating source f o r each sampling point. Total dust concentration, mass median aerodynamic diameter (MMAD) and geometric standard d e v i a t i o n (Sg) f o r each of the two modes of diameter d i s t r i butions are shown in Table 4. TABLE 4 Total concentration and parameters of the mass d i s t r i b u t i o n of sols.
I mode Sampling Point
A B C D
Sampling Time/min
210 150 210 150
Total Dust/ mg.m-3
0.84 3.13 0.65 0.99
Total Mass Fraction % 19.3 30.9 24.5 11.3
I I mode
MMAD/ Sg ~m
0.27 0.27 0.28 0.49
sampled aero-
2.01 2.03 1.95 1.92
Total Mass Fraction %
MMAD/ Sg ~m
80.7 69.1 75.5 88.7
10.3 9.0 9.9 6.2
2.51 2.54 2.69 2.88
At points A and B alpha a c t i v i t i e s associated with p a r t i c l e s with a d i a meter greater and lesser than 0.5 ~ m due to Po-210 and to l o n g - l i v e d r a d i o nuclides were also measured. The measurements of f i l t e r s
of the dichotomous
sampling were taken facing track etching detectors f o r appropriate periods of time. Alpha a c t i v i t y ( i n tracks/mm 2 m3 day) was <0.5pm
> O.5pm
Point A
7.02
3.11
Point B
0.46
0.52
In comparison to Table 4 i t can be noticed that alpha a c t i v i t y can not be correlated to the t o t a l dust concentration suspended in the a i r . Nor can the granulometric spectrum in a c t i v i t y be correlated with that in mass. Radon concentration in the a i r was measured using active instrumentatiop s c i n t i l l a t i o n chambers - and passive dosemeters exposed f o r 86 results are shown in the second and t h i r d columns of Table 3.
days. The
141
The data indicate the presence of
low radon concentrations
and are
confirmed by measurements of potential alpha energy concentration in the same points made by Eberline counters and a Working Level Meter Pylon, which also
recorded very low values ( 0 . 0 4 ÷ 0 . 0 8 ~ J / m 3 ) . Dose evaluation From the experimental data i t can be concluded that environmental levels of radon are comparable with those commonly found in houses b u i l t with low activity
materials. This is also true f o r gamma r a d i a t i o n , apart from the tops of
the tanks, where workers spend only short periods of time. I t can therefore be considered t h a t doses due to external gamma r a d i a t i o n and to radon are n e g l i gSble. Evaluation of the i n h a l a t i o n dose from aerosol was made f o r the two areas of highest a c t i v i t y - sprue-elimination and m i l l i n g . Calculations were based on experimental data and appropriate hypotheses ( r e f . 5) on granulometric d i s t r i bution in the a c t i v i t y of single isotopes (Po-210, U-238, U-235, Th-232 and daughters). In order to calculate the committed dose equivalent to target organs dosimetric and metabolic models recommended by p u b l i c a t i o n 30 of ICRP ( r e f . 6) were adopted, with the f o l l o w i n g assumptions: -
the compounds of Po-210 can be dissociated from inhaled p a r t i c u l a t e s and
therefore be c l a s s i f i e d in W class and absorbed through the walls of the small intestine; - U-238, U-235, Th-232 and t h e i r respective daughters can not be dissociat. ed from the s i l i c a t e matrix. They belong to Y class and can not be absorbed at the level of the g a s t r o - i n t e s t i n a l t r a c t ; - the daughters remain associated to t h e i r grandparents u n t i l they are transferred from the r e s p i r a t o r y system to the body f l u i d s . As soon as they begin to c i r c u l a t e they behave as though they had been injected separately. However daughters produced in the accumulation organs due to the decay of t h e i r parents f o l l o w t h e i r metabolic process ( r e f . 7). The values of committed e f f e c t i v e dose equivalent f o r 8 working hours were 1.81 10 -5 Sv f o r the sprue-elimination area and 2.32 10-5 Sv f o r the m i l l i n g area. CONCLUSIONS The experimental data obtained seem to indicate a low emanation of radon from the sand in both i t s original fcr~ (confirming results in ref. l ) and during i t s transformation in the production process. Po-210 enrichment in atmospheric dust, however, might be connected to the smelting of the material and therefore to the v o l a t i l i s a t i o n temperature of the radionuclide, in a way analogous to that which occurs in coal combustion.
142
The evaluation of e f f e c t i v e dose equivalent t h a t , assuming a working year of 2000 hours, would lead to values of 5 mSv in the most highly contaminated areas, shows the necessity f o r continuing the analysis of the use of t h i s sand in order to define more exactly i t s health effects and to study procedures of optimising r a d i a t i o n protection at some points of the process. Acknowledgments The authors wish to express their gratitude to the other colleagues of the group for the useful discussions they have had: G.Busani, G.Busuoli, P.Comba, A.Cristofolini, P.Gasparini, G.Laffi, E.Ligeri, S.Piermattei, E.Sgrilli, P.Tori.
REFERENCES 1 G.F. Boothe et a l , The Radiological Aspects of Zircon Sand Use. Health Physics, 38 (1980) 393-398. 2 ICRP Publication 29. P r i n c i p l e s f o r Limiting Exposure of the Public to Natural Sources of Radiation. Annals of the ICRP, 14, N.I (1984). 3 P. Spezzano, Determinazione di uranio, t o r i o e radio-226 in sabbie z i r c o n i fere mediante spettrometria a l f a , in press. 4 G. Santori - Laboratorio di Radioecologia delI'ENEA-PAS-SCAMB - private communication. 5 C.C. Lombardi et a l , Caratterizzazione radiologica dei p a r t i c o l a t i aerosospesi in una i n d u s t r i a di lavorazione di sabbie z i r c o n i f e r e , in press. 6 ICRP Publication 30. Limits f o r Intakes of Radionuclides by Workers. Part I , Annals of the ICRP, 2, N.3/4 (1979) Part 2, Annals of the ICRP, 4, N.3/4 (1980) Part 3, Annals of the ICRP, 6, N.2/3 (1981). 7 R.W. Leggett, D.E. Dunning, K.F. Eckerman, Modelling the Behaviour of Chains of Radionuclides inside the Body. Radiation Protection Dosimetry 9 (1984) 77-93.
135
Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands
RADIATION PROTECTIONASPECTS OF THE USE OF ZIRCON SAND NATIONAL GROUP FOR STUDYING RADIOLOGICALIMPLICATIONS IN THE USE OF ZIRCON SAND*
ABSTRACT Working processes using zircon sand in a f a c t o r y producing r e f r a c t o r y material were studied from the point of view of r a d i a t i o n protection. Zircon sand contains high concentrations of natural radionuclides and is a t y p i c a l example of an enhanced source. Using various techniques the c h a r a c t e r i s t i c s of the o r i g i n a l material were analysed and environmental r a d i o a c t i v i t y was measured in the d i f f e r e n t s i g n i f i c a n t points of the process. F i n a l l y , a f i r s t assessment was made of e f f e c t i v e dose equivalent values in the areas of highest r i s k . INTRODUCTION The element zirconium i s never found in a pure natural state. I t can be found in oxidised form (ZrO 2 or "baddeleyte") or
as a s i l i c a t e (ZrSiO 4
" z i r c o n " ) and is normally associated with r u t i l e ,
ilmenite and monazite with
or
s i g n i f i c a n t concentrations of natural radionuclides (elements of the secular chains U-235, U-238 and Th-232). Zircon materials used in I t a l y are not extracted in the country i t s e l f but are generally imported from A u s t r a l i a in the form of a sand with a granulometry of 100+200 ~m ( i n 1981 56 000 tons were imported, of which 94.6% came from A u s t r a l i a ) . This granulometry enables i t to be used d i r e c t l y in the production of r e f r a c t o r y material. A f t e r being m i l l e d to 2 ~m i t is used in the production of paints and as a hardening agent and o p a c i f i e r in the ceramics industry. Given the high concentration of natural radionuclides t h i s sand (about which very l i t t l e
has been w r i t t e n ( r e f . I ) ) may represent a health problem in
the various stages of e x t r a c t i o n , p u r i f i c a t i o n , m i l l i n g , storage, transportat i o n and use. I n d i v i d u a l s can be exposed to d i r e c t r a d i a t i o n from the o r i g i n a l
*The group i s formed by: Presidio Multizonale di Prevenzione-USL 2, Piacenza(1); ENEA-PAS-COORBOL, PAS-FIBI-AEROSOL(2), PAS-FIBI-DOSI(3), Bologna; ENEA-PASSCAMB-MISURE, Casaccia (Roma)(4); ENEA-COMB-SAL-FISM, Saluggia(5); ENEA-DISP, Roma; Gruppo Ceramica Regione Emilia Romagna; USL 2, Massa Carrara(6); USL 18, Empoli; Servizio Medicina Preventiva e Ispettorato del Lavoro, Trento; I s t i t u to Superiore di SanitA, Roma(7). The experimental work was done by: M.Bergamini(6), R.Borio(7), G. Campos Venuti(7), T.De Zaiacomo(2), S.Fabbri(1), M.Formignani(2), A.Gazzola(1), R.Giacomelli(5), L.Lembo(3), C.C.Lombardi(2), C.Melandri(2), R.Nanni(3), L . P a l l a v i c i n i ( 1 ) , S.Risica(7), G.Sciocchetti(4),P.Spezzano(5) and G.Tarroni(2).
0048-9697/85/$03.30
© 1 9 8 5 Elsevier Science Publishers B.V.
136
material and the f i n i s h e d product as well as to intake of material with high s p e c i f i c a c t i v i t y . This i s a good example of a case in which the d e f i n i t i o n "enhanced natural r a d i a t i o n " can s t i l l
be useful, even considering the Fro-
blems r e c e n t l y underlined by ICRP ( r e f . 2). A study commission has been formed in I t a l y (created and organised by the administrative regions of Emilia Romagna and Toscana) with the p a r t i c i p a t i o n of both centralised and local I n s t i t u t e s . I t s aim is to study f a c t o r i e s that use or t r e a t zircon sand from the point of view of r a d i a t i o n protection. A sample i n v e s t i g a t i o n has been carried out in a f a c t o r y producing r e f r a c t o r y material. CHARACTERISATION OF THE SAND Specific a c t i v i t y was evaluated by gamma spectrometry on a number of samples from d i f f e r e n t stocks and by alpha spectrometry on one of these samples Gamma spectrometry measurements were made with a GeLi detector (80 cc, e f f i c i e n c y 14%, r e s o l u t i o n 2.2 keV) a f t e r having calibrated the spectrometer, contaminating a sample of the sand (p= 3000 kg m-3) with a multipeak l i q u i d source having an energy range of 88+1836 keV. Mean values with three standard deviations are shown in Table I . TABLE 1 Specific a c t i v i t y in Bq kg -I of sand sample (gamma spectrometry).
K-40 68~,55
U-238
Th-232
3445.21
763~I0
Alpha spectrometry was done a f t e r d i s s o l v i n g the sample and separating i t radiochemically ( r e f . 3). Thorium and
uranium, electroplated
on small
2 stainless steel discs, were measured by a surface-barrier detector of 450 mm , while Ra-226 was determined using a ZnS(Ag) s c i n t i l l a t o r .
The mean results
of s i x analyses of the same sample are shown with one standard deviation in Table 2. I t can be seen that the chains of natural radionuclides are in e q u i l i b rium. The r e l a t i v e l y small differences between the r e s u l t s of the two tables are caused by the d i f f e r e n t measuring methods used in laboratories that had not been i n t e r c a l i b r a t e d beforehand. In order to see whether the handling of the sand i t s e l f
can create an i n -
halation r i s k i t was also subjected to granulometric measuring. Seiving analysis was carried out, optimising time and mass in order to reduce errors due to
137 TABLE 2 Specific a c t i v i t y in Bq kg -I of sand sample
U-235
U-238 U-238
135~6
(alpha spectrometry).
U-234
Th-232 Th-230
Ra-226
Th-232
Th-228
2 8 0 0 ~ 6 0 2810±100 2800±30 2730±240 678±19 680~18
seive loading,seive time and percentual losses at the various stages,obtaining a mass median diameter of 170~m and a geometric standard deviation of 1.32. Such a small standard deviation encouraged the hypothesis that the sand had a l ready been treated. The large dimensions of the grains lead the authors to bel i e v e that the time spent i n suspension i s extremely short and t h a t , therefore, they do not c o n s t i t u t e a source of r i s k c f i n t e r n a l contamination through inhalation. ANALYSIS OF THE RADIOLOGICAL SITUATION IN THE FACTORY Description of the f a c t o r y The f a c t o r y produces, as
its
main business, electro-fused
refractory
blocks used i n the construction of smelting ovens f o r glass. The zircon sand i s stored in large reinforced-concrete tanks. I t is mixed with alumina and sodium carbonate before being smelted in the oven. The molten material is poured i n t o moulds and, a f t e r the e l i m i n a t i o n of sprues, the f i nished blocks are l e f t to cool f o r f i f t e e n to twenty days in the cooling area. They are then moved to another room where they are sand-blasted, ground, polished with emery, f i n i s h
ground and f i n a l l y
cut. The accurate assembly of a l l
the components of each oven is then checked in the pre-assembly
area. The
sprues and waste products are m i l l e d i n m i l l s outside. Extractor hoods are provided where the mixture is produced and smelted and where sprue is eliminated, as well as in the cooling and grinding areas. The v e n t i l a t i o n makes i t possible to consider environmental conditions regardless of season. Gamma exposure rate Measurements were made inside and outside the f a c t o r y , at points considered representative of the production cycle and the environmental background, res p e c t i v e l y . Four laboratories were involved in measuring with passive dosemet e r s , using four thermoluminescent detectors (LiF: TLD-IO0), exposed f o r three months, at each of the above points. The t o t a l u n c e r t a i n t y of the dosimetric system chosen is in the order of
138
15%, of which about I0% is due to random errors and about 5% to errors of c a l i bration. Exposure rate represents an average value for the conditions of zircon material use, which varies according to the e v o l u t i v e cycle of production. At the same points in which the TL dosemeters were positioned measurements were taken with active dosemeters ( i o n i s a t i o n chamber ( I . C . ) and scintillation
detector (S.D.)). The r e s u l t s are shown in Fig. I . The difference
between the measurements made by active and passive dosemeters at the tops of the sand storage tanks i s due to material management and storage at d i f f e r e n t times.
I) Smelting area 2) Factory e x t e r i o r (environmental background) 3) Cooling area 4) Base of sand storage tanks 5) Top of tanks 6) Pre-assembly area (only LAB4) | 7~ Grinding area (only LAB4)
j ~-| | |
449
29
113
10.3
I08
f
9.0
PASSIVE DOSEMETERS
Fig. 1 -
ACTIV E DOSEMETERS
Mean exposure rate ( i n ~ R h- l ) in representative points.
A i r contamination characterisation In order to map a i r contamination during
the stages of production in
various points of the f a c t o r y the f o l l o w i n g measurements were made: I ) alpha a c t i v i t y of the p a r t i c u l a t e ; 2) mass and a c t i v i t y granulometric c h a r a c t e r i s a t i o n ; 3) grab sampling and integrated measurements of radon concentration in a i r . Measurements of alpha a c t i v i t y were carried out by drawing a i r f o r t h i r t h y minutes - using pumps of 18 and 35 l i t r e s per minute flow rate - through c e l lulose t r i a c e t a t e f i l t e r s
of 47 mm diameter and pore diameter O.45pm. A f t e r
enough time f o r the short - l i v e d isotopes to decay and f o r the series of radionuclides inside the p a r t i c l e s to a t t a i n e q u i l i b r i u m , the f i l t e r s
were covnt-
ed f o r t o t a l alpha a c t i v i t y with a ZnS(R,g) detector. The r e s u l t s , with standard deviation are shown in Table 3.
one
139
TABLE 3 Total alpha a c t i v i t y and radon a i r concentration in various areas of the f a c t o r y
Total alpha a c t i v i t yZ lO-LBq m- j
Sampling point Oven operation cabin Pouring area Sprue-elimination area Cooling area central corridor Central oven area Top of sand tanks Grinding area Diamond saw area Pre-assembly area Finish grinding area Storage area of unfinished product M i l l i n g operator cabin Mill-loading
To v e r i f y
4.37 ±0.32 9.72 ±0.47 42.15 ±I.27 2.58 ± 0.27 2.92±0.28
Rn air concentration/Bq m-3 Grab Integrated Sampling measurement
II II
6 20 6
9 13
0.68 ±0.29 0.19 ±0.15
16 17 12
-19 6 18
15
17
1.28 ± 0.21 15.89 ±0.79
i f airborne dust was enriched by Po-210 released during proces-
ses at high temperatures, specific a c t i v i t y of the three most active f i l t e r s in the areas of sprue elimination (1), mill-loading (2) and pouring (3) was monitored. The experimental values, shown in Fig. 2, have been interpolated with a function which is the
sum
of two exponentials that represents the decay of Po-210 and of the very long-lived parents (U-238, U-235 and
,~E40
Th-232). A c t i v i t y values (in lO-2Bq m-3) extrapolated at the moment of
~30
sampling in the same three areas
'920
were 42.68, 12.29, 9.47 for Po-210 and 2.41, 5.27, 0,83 for the very
10-
long-lived radionuclides in equilibrium with their daughters. Po-210
00
50
100
150 200 t / days
250
enrichment was confirmed by direct measurements of the same samples using alpha spectrometry (ref. 4).
Fig. 2 - Calculated decay curves and experimental values of total alpha a c t i v i t y in three areas.
Granulometric measurement of the particulate was carried out in the
140
two most a c t i v e
areas - the sprue-elimination (A)
and the
grinding area (B) -
as well as at areas of more diffused contamination - the central oven
area (C)
and the pre-assembly area (D). Concentration of t o t a l dust was measured by aspiration through g l a s s - f i b r e filters
(diameter 47 mm) at a
flow rate of ten l i t r e s per minute. Mass granu-
lometric d i s t r i b u t i o n was determined by sampling the aerosol at 2 l i t r e s per minute using 8-stage mini SIERRA impactors plus a f i n a l f i l t e r .
Distributions
were bimodal, i n d i c a t i n g more than one contaminating source f o r each sampling point. Total dust concentration, mass median aerodynamic diameter (MMAD) and geometric standard d e v i a t i o n (Sg) f o r each of the two modes of diameter d i s t r i butions are shown in Table 4. TABLE 4 Total concentration and parameters of the mass d i s t r i b u t i o n of sols.
I mode Sampling Point
A B C D
Sampling Time/min
210 150 210 150
Total Dust/ mg.m-3
0.84 3.13 0.65 0.99
Total Mass Fraction % 19.3 30.9 24.5 11.3
I I mode
MMAD/ Sg ~m
0.27 0.27 0.28 0.49
sampled aero-
2.01 2.03 1.95 1.92
Total Mass Fraction %
MMAD/ Sg ~m
80.7 69.1 75.5 88.7
10.3 9.0 9.9 6.2
2.51 2.54 2.69 2.88
At points A and B alpha a c t i v i t i e s associated with p a r t i c l e s with a d i a meter greater and lesser than 0.5 ~ m due to Po-210 and to l o n g - l i v e d r a d i o nuclides were also measured. The measurements of f i l t e r s
of the dichotomous
sampling were taken facing track etching detectors f o r appropriate periods of time. Alpha a c t i v i t y ( i n tracks/mm 2 m3 day) was <0.5pm
> O.5pm
Point A
7.02
3.11
Point B
0.46
0.52
In comparison to Table 4 i t can be noticed that alpha a c t i v i t y can not be correlated to the t o t a l dust concentration suspended in the a i r . Nor can the granulometric spectrum in a c t i v i t y be correlated with that in mass. Radon concentration in the a i r was measured using active instrumentatiop s c i n t i l l a t i o n chambers - and passive dosemeters exposed f o r 86 results are shown in the second and t h i r d columns of Table 3.
days. The
141
The data indicate the presence of
low radon concentrations
and are
confirmed by measurements of potential alpha energy concentration in the same points made by Eberline counters and a Working Level Meter Pylon, which also
recorded very low values ( 0 . 0 4 ÷ 0 . 0 8 ~ J / m 3 ) . Dose evaluation From the experimental data i t can be concluded that environmental levels of radon are comparable with those commonly found in houses b u i l t with low activity
materials. This is also true f o r gamma r a d i a t i o n , apart from the tops of
the tanks, where workers spend only short periods of time. I t can therefore be considered t h a t doses due to external gamma r a d i a t i o n and to radon are n e g l i gSble. Evaluation of the i n h a l a t i o n dose from aerosol was made f o r the two areas of highest a c t i v i t y - sprue-elimination and m i l l i n g . Calculations were based on experimental data and appropriate hypotheses ( r e f . 5) on granulometric d i s t r i bution in the a c t i v i t y of single isotopes (Po-210, U-238, U-235, Th-232 and daughters). In order to calculate the committed dose equivalent to target organs dosimetric and metabolic models recommended by p u b l i c a t i o n 30 of ICRP ( r e f . 6) were adopted, with the f o l l o w i n g assumptions: -
the compounds of Po-210 can be dissociated from inhaled p a r t i c u l a t e s and
therefore be c l a s s i f i e d in W class and absorbed through the walls of the small intestine; - U-238, U-235, Th-232 and t h e i r respective daughters can not be dissociat. ed from the s i l i c a t e matrix. They belong to Y class and can not be absorbed at the level of the g a s t r o - i n t e s t i n a l t r a c t ; - the daughters remain associated to t h e i r grandparents u n t i l they are transferred from the r e s p i r a t o r y system to the body f l u i d s . As soon as they begin to c i r c u l a t e they behave as though they had been injected separately. However daughters produced in the accumulation organs due to the decay of t h e i r parents f o l l o w t h e i r metabolic process ( r e f . 7). The values of committed e f f e c t i v e dose equivalent f o r 8 working hours were 1.81 10 -5 Sv f o r the sprue-elimination area and 2.32 10-5 Sv f o r the m i l l i n g area. CONCLUSIONS The experimental data obtained seem to indicate a low emanation of radon from the sand in both i t s original fcr~ (confirming results in ref. l ) and during i t s transformation in the production process. Po-210 enrichment in atmospheric dust, however, might be connected to the smelting of the material and therefore to the v o l a t i l i s a t i o n temperature of the radionuclide, in a way analogous to that which occurs in coal combustion.
142
The evaluation of e f f e c t i v e dose equivalent t h a t , assuming a working year of 2000 hours, would lead to values of 5 mSv in the most highly contaminated areas, shows the necessity f o r continuing the analysis of the use of t h i s sand in order to define more exactly i t s health effects and to study procedures of optimising r a d i a t i o n protection at some points of the process. Acknowledgments The authors wish to express their gratitude to the other colleagues of the group for the useful discussions they have had: G.Busani, G.Busuoli, P.Comba, A.Cristofolini, P.Gasparini, G.Laffi, E.Ligeri, S.Piermattei, E.Sgrilli, P.Tori.
REFERENCES 1 G.F. Boothe et a l , The Radiological Aspects of Zircon Sand Use. Health Physics, 38 (1980) 393-398. 2 ICRP Publication 29. P r i n c i p l e s f o r Limiting Exposure of the Public to Natural Sources of Radiation. Annals of the ICRP, 14, N.I (1984). 3 P. Spezzano, Determinazione di uranio, t o r i o e radio-226 in sabbie z i r c o n i fere mediante spettrometria a l f a , in press. 4 G. Santori - Laboratorio di Radioecologia delI'ENEA-PAS-SCAMB - private communication. 5 C.C. Lombardi et a l , Caratterizzazione radiologica dei p a r t i c o l a t i aerosospesi in una i n d u s t r i a di lavorazione di sabbie z i r c o n i f e r e , in press. 6 ICRP Publication 30. Limits f o r Intakes of Radionuclides by Workers. Part I , Annals of the ICRP, 2, N.3/4 (1979) Part 2, Annals of the ICRP, 4, N.3/4 (1980) Part 3, Annals of the ICRP, 6, N.2/3 (1981). 7 R.W. Leggett, D.E. Dunning, K.F. Eckerman, Modelling the Behaviour of Chains of Radionuclides inside the Body. Radiation Protection Dosimetry 9 (1984) 77-93.