Experimental study of 210Po release from 17Li83Pb eutectic

Fusion Engineering and Design 29 (1995) 164-169

ELSEVIER

Fusion Engineedng and Design

Experimental study of 21°po release from 17Li-83Pb eutectic O. Schipakin a, N. Borisov b, S. Churkin b Research and Development Institute of Power Engineering, Moscow, Russia b Karpov's Institute of Physical Chemistry, Moscow, Russia

Abstract

The release of 2~°po from 17Li 83Pb eutectic was experimentally investigated up to 450 °C in flows of noble gases and atmospheric air at different humidity. Aerosol and sorption filters were applied for the quantitative determination of releasing polonium aerosol and gaseous fractions. The experimental results reveal the strong dependence of 2~°Po release rate on temperature and the state of the eutectic surface. In the eutectic temperature range from 350 to 450 °C the polonium release rate in argon is increasing from 10 -11 to 1 0 - 9 C i c m - 2 h -1. In all tests the 21°po activity in the carrier gas was determined in aerosol and gaseous forms. Polonium in the gaseous phase was found to be in the range from 5% to 80% of the overall concentration. The total 2~°po concentration in the carrier gas near the melted eutectic surface at the temperature 450 °C reached 2.7 × 10-'° Ci 1 ~, which exceeds the permissible level by three orders of magnitude. Comparison of experimental data with estimates based on metallic polonium evaporation indicates the formation of chemical compounds of polonium with lead and in some cases with hydroxide.

1. Introduction

The radioactive 2~°po is produced and accumulated in a lithium-lead (17Li-83Pb) eutectic blanket of the fusion reactor by the neutron activation of bismuth impurity and lead. 2~°po has high specific ~ activity, toxicity and volatility and therefore the permitted polonium concentrations in the air and water are very low. The 2t°Po inventory in the ITER eutectic blanket, for example, may achieve several hundred curies after three years of operation and the corresponding specific polonium activity in the eutectic would approach 1 ~tCi cm -3. The polonium hazard is of concern during accidental eutectic spill in working premises or under the depressurization of the cover gas contour. Elsevier Science S.A. SSDI 0920-3796(94)00370-X

To estimate the health risk from polonium it is necessary to obtain experimental data concerning the 21°po release rate from the eutectic as well as polonium concentration levels in the atmosphere under operational conditions of a fusion reactor eutectic blanket. For this purpose, a series of 21°po evaporation tests were conducted in the Aerosol Laboratory of Karpov's Institute of Physical Chemistry together with RDIPE, Moscow, during 1992-1993 [1-3].

2. Test description

The polonium release rate experiments were performed by the stream method in which carrier gas

O. Schipakinet al./ FusionEngineeringandDesign29 (1995)164-169

165

Table 1 The experimental 2~°po release rates from 17Li-83Pb eutectic Specimen

Carrier gas

NI

Surface state

Ar Humid air

N2

Dry He, Ar Humid He, Ar Air

N3

Ar Ar

N4

Ar Ar

Release rates (Cicm 2h-t) from solid eutectic

Release rates ( C i c m 2h ~) from molten eutectic

20 c'C

150 cC

220 ~'C

350 >C

400 ~'C

Thin deposit Dense film Clean

---~

1.7 X 10 -13 1.2 × 10 ~3

2.4 x 10 12

< 5 x l0 14

(0.9-1.2) × 10 IJ 1.6 × 10 t2 5.0 x 10 ~2

_

1.6 x 10 9

2.1 × 10 - 9

5.9 × 10 9

Thin deposit Dense film Friable film Film removed Friable film Film removed

--

(0.1

2.5 x 10 -1°

4.0 x 10 1o

(0.5-1.4) x 10 -9

--


2.2x10

ii

1.0xl0

(1.7_6.3) x 1 0 - 1 o

--

8.7x10

4.4x10

lo

1.2x10 9

(0.5_1.1) x 1 0 - 8

8 . 0 x 10 11

2 . 3 x 10 io

5.0x10

< 1 x 10 13 (0.1-2.0) x 10 ii 1.8) x 10 i1 13 12

_ _ _

5.6x10

i1

--

450 ~C

io

to

8.7 X 10 -13

2.1 x 10 lo

4.4 x 10 Io

3.2 X 10 - 9

5.2 x 10 9

1.5 x 10 8

5.0 x 10 13

3.2 × 10 -12

6.8 X 10 -I-'

6.6 X 10 II

3.9 × 10 -10

8.7 × l0 10

Speciman characteristics: N I , irradiated eutectic with (2.2 2.9) x 1 0 - 9 C i g ~ of 21°po; N2, model eutectic with (1.0 1.3) x 10 7 Ci g-~ of 2~°po; N3, model eutectic with 2.4 x 10 6 Ci g ~ of 2mPo; N4, model eutectic with 2.7 × 10 5 Ci g 1 of 2t°po.

-7

450

flowed a b o v e

4OO

------+-~

350

°C

-9

-10.

i-~

-z t .

. -

1.3

.

.

.

. .

-

.

__2_

1.4

3- I

1.5

1.6

.2

1.7

IO00/T, K Fig. 1. The 2Wpo release rate vs. temperature in the different gases (21°Po contents in eutectic, (1.0-1.3) x 1 0 - 7 C i g - I ) : curve 1, dry He, thin deposit on eutectic surface; curve 2, h u m i d He, film on surface; 3, air, dense film on surface.

t h e s u r f a c e o f h e a t e d Li P b eutectic T h e p o l o n i u m c o n t e n t in g a s w a s d e t e r m i n e d b y ~ activity m e a s u r e m e n t s o f a e r o s o l a n d s o r p t i o n filters o n t h e o u t l e t o f t h e c a r r i e r g a s line. T h e l a b o r a t o r y e v a p o r a t i o n device c o n s i s t e d o f t h r e e m a i n parts: ( I ) t h e g a s p r e p a r a t o r y s y s t e m , (2) h e a t e d e v a p o r a t i o n c h a m b e r e q u i p p e d w i t h eutectic s p e c i m e n s a n d (3) p o l o n i u m s a m p l i n g z o n e w i t h r e p l a c e a b l e filter c a r t r i d g e . T h e g a s p r e p a r a t o r y s y s t e m p r o v i d e s for n o b l e g a s e s a n d air w i t h different h u m i d i t i e s a n d flow r a t e s b e t w e e n 50 a n d 90 c m 3 s - t . In t h e low p a r t o f e v a p o r a t i o n c h a m b e r is p l a c e d a h o l d e r w i t h 5 c m 3 p o l o n i u m - c o n t a i n i n g eutectic s p e c i m e n . T h e s p e c i m e n w a s h e a t e d b y t h e electric f u r n a c e . T h e s p e c i m e n s o f eutectic i r r a d i a t e d in t h e e x p e r i m e n t a l c h a n n e l o f t h e fission r e a c t o r I V V 2 M were u s e d . A s well as t h e p u r e eutectic, s p e c i m e n s w i t h 21°po l o a d e d , i.e. " m o d e l s p e c i m e n s " , were u s e d . T h e 21°Po specific activity o f i r r a d i a t e d s p e c i m e n s w a s 2.5 x 10 9 Ci g 1 at t h e test m o m e n t a n d t h e activity o f t h e m o d e l s p e c i m e n s w a s in r a n g e 1.0 x 10 - 7 2.7 x 10 5 Ci g - l .

containing

21°po.

O. Schipakin et al./ Fusion Engineering and Design 29 (1995) 164-169

166

8

i

4-

j

i A3

J t

,,~

.L

•

""

1

1

42

I

0

2

./

/

,0

3b0

i00

soo'¢

Fig. 3. Influence of the friable surface film on release of 21°Po from 17Li-83Pb eutectic in Ar (K is the 2Wpo content in the eutectic): curve 1, friable surface film, K = 2.7 x 10 -5 Ci g ~; curve 2, film is removed, K = 2.7 x 10 -5 Ci g ~; curve 3, friable surface film, K = 2 . 4 x 10 6Cig-l; curve 4, film is removed, K = 2 . 4 x 10-6Cig 1.

\

10-I~

/

~ -10

6 time, h

4

8

10

Fig. 2. Decreasing 2mpo release rate from molten 17Li 83Pb eutectic in dry Ar vs. time of experiment at 21°Po contents in

eutectic of (1.0 1.3) × l0 7Cig i. The sampling cartridge was equipped with two aerosol filters ( A F A - R M P type), two sorption filters ( A F A S - P type) and a terminal filter. At the end of experiment, the filter cartridge was disassembled and the a activity of each filter was counted. Evaporation tests were conducted at temperatures of 20, 150, 220, 350, 400 and 450 °C. Helium, nitrogen, argon and air at different humidities were used as carrier gases. During the experiment, visual inspection of the surface state of the eutectic was performed.

3. Test results The main results of the experimental study of the 2t°po release rate from 17Li-83Pb eutectic are shown in Table 1 and Figs. 1 5. All the experiments confirm that the 21°Po release rate increases with temperature of the eutectic especially when it is in the molten state.

The measured 21°po release rates for model eutectic specimens were in the same range as those measured for the irradiated specimens (in terms of the same polonium activity in the specimens).

3. I. The influence of the surface film The observed scatter of the results can be explained by the formation of the surface film and the difference in its structure (Fig. 1). The oxide film on the eutectic specimen surface had a different density and colour depending on the composition of the carrier gas: from a thin grey deposit and thicker dense film to friable dark-grey or y e l l o w orange sheet. These observations were confirmed by Jeppson and Muhlestein [4] who determined the appearance and chemcial composition of the oxide films associated with lithium-lead alloys. Our tests found rather quick oxide film formation on the molten eutectic surface and strong influence of this film on the polonium release. The dense film on the surface of the specimen in dry argon decreases the 21°Po release rate by a factor of 5 - 1 0 during 2 8 h (Fig. 2). The formation of a loose layer on the eutectic surface in humid gases (Ar and air) was observed in the next series of evaporation tests. The mechanical removal of the loose layer from the surface of molten eutectic

O. Schipakin et al. / Fusion Engineer&g and Design 29 (1995) 164 169 450

400

350

°C

-4

560

167

450 400 350

250 '~C

-5 -

-6 -to ~

-11 %

-9

•

[~'~ n

4

~J -13

-12

"1.~

~

...4.

-14

1.4

15

£6 1000/T, K

Fig. 4. The 2"~Po concentration (curies per litre) in the gas above the eutectic surface vs. temperature (2t°Po contents in the eutectic are (1.0 1.3) × 10 vCi g ~): curve 1, clean eutectic surface (eperimental); curve 2, dense film on the eutectic surface (experimental): curve A, evaporation of metallic 2~°Po from eutectic (calculated); curve B, evaporation of PbPo from eutectic (calculated); curve C, permissible 2~°Po contamination in the working permises air.

reduced the 2~°Po release rate by a factor of 50 (Table 1 and Fig. 3). The mobilization of 2~°Po release from the porous layer on the eutectic surface may be explained by the increasing free evaporation surface.

3.2. The concentration of polonium in gas The temperature dependence of the total 2~°Po concentration in the carrier gas is shown in Fig. 4. The concentration (in units of curies per litre) corresponds to the partial pressure of 21°po in evaporation chamber for specimens with a polonium content of 10 -7 Ci g t. The total 2~°Po concentration in gas strongly depends on the surface state of the eutectic specimen. The evaporation tests show that polonium release leads to the formation of relatively high 21°po concentrations in gas near the eutectic surface up to 2.7 × 10 ~°Ci 1-1. The estimated concentrations of the metallic polonium and lead polonide in gas as a function of temperature are shown by dashed lines in Fig. 4. The experimental values fall between the calculations for

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1000/T, K Fig. 5. The 2~°Po release rate in gas vs. temperature (2H~Po contents in the eutectic are (1.0-1.3) x 10 -7 Ci g - i ) . Calculated: curve A, for Po metal; curve B, for PbPo; curve C, for PoO 2. Experimental: curve 1, in vacuum [6]; curve 2, in argon [6]; band 3, in Ar, He and air (this work); curve 4, in vacuum [5]; curve 5, in He [5],

metallic polonium and lead polonide. The calculation of the 2~°Po concentration C(T) (Ci 1-~) is given by the expression

C(T) = 1.5 × 104Pso/TM Here Pso (mmHg), is the pressure of saturated vapour of Po or PbPo M its molar fraction in the eutectic and T (K) is the temperature. With a 2t°po content in the eutectic of 1 0 - T C l g t the value of M is 95 × 10 11.

3.3. The aerosol and gaseous phases of 21°Po In all the tests, polonium was found in both the aerosol and the gaseous states. The fraction of 2~°Po in gaseous form in the carrier gas is a function of the temperature and gas environment. The gaseous fraction ranged from 5% to 80%. The aerosol fraction increased with eutectic temperature. The simultaneous existence of the 2~°Po in aerosol and gaseous phases is peculiar to polonium. It is possible to assume that the metallic polonium and PbPo after evaporation from the eutectic and cool-

168

O. Schipakin et al. / Fusion Engineering and Design 29 (1995) 164 169

ing in the carrier gas were condensed and formed aerosols while the gaseous phase of polonium appeared as a consequence of the formation of highly volatile polonium compounds, hydride for instance.

4. The comparison of evaporation tests with other experiments and theory 4. I. Results o f comparison Direct experimental Studies of 2~°Po evaporation from eutectic were published earlier by Feuerstein et al. [5] and Tupper et al. [6]. These researcher used the Langmuir method when the polonium evaporates in vacuum or in quasi-stationary gas. The evaporation tests were conducted at temperature ranges of 400850 °C [5] and 235-550 °C [6]. The evaporating polonium was collected on the surfaces in the gas space and on silver foils. The 2~°Po content in eutectic specimens was 6.2× 10-7Cig i [5] and 3.4× 10 7 C i g - I [6]. In the first case lead-lithium was used and in the second case lead-bismuth eutectic. The experimental results are presented in Fig. 5 (curves 1, 2, 4 and 5). Also shown in Fig. 5 are the calculated release rates for evaporation of the metallic polonium (curve A), lead polonide PbPo (curve B) and polonium oxide PoO2 (curve C). All comparable values were obtained for a polonium activity in the eutectic of 0.1 laCi g-1 which corresponds to molar contents of 21°po in eutectic of 1.95 × 10 - ~ . The calculated release rate JP°(T) (Ci cm 2 h-~) for evaporation of 21°Po can be expressed by

JP°(T) = 4.3 × 105P~°(T)(1000/T) °5 Here Pso(T) Po (mmHg) is the saturated vapour pressure of evaporating metal at temperature T(K). The numerical coefficient in this expression are 3.0 x 105 for PbPo and 4.0 z 105 for PoO2. The pressure Pso(T) of saturated vapour for the metallic polonium, lead polonide and polonium oxide were obtained by Abakumov [7]. Brooks [8] and Nesmejanov [9].

4.2. Discussion When comparing our results with those from other reseachers it is necessary to stress that different experimental methods were used, namely evaporation in gas flow (our results) and evaporation in vacuum or in quasi-stationary gas in Refs. [5] and [6]. As shown in Fig. 5 the release rates of 21°po in gas presented here are in region of the evaporation in vacuum obtained by

Feuerstein et al. [5] and Tupper et al. [6]. At the same time our results are essentially higher than those obtained by these workers for the evaporation of polonium in the noble gases at atmospheric pressure. This fact may be explained by the absence of saturation conditions during the evaporation of polonium in our tests. Under such a case the conditions of evaporation in a gas flow approach the conditions of evaporation in vacuum. This assumption will be checked in our future experiments. The comparison of our experimental 2~°Po release rates with the calculated results (Fig. 5) for the release rate of lead polonide and polonium oxide were good. The calculated release rate of the metallic polonium was 103-104 times higher than our experimental data in the range 350 450 °C. In our opinion polonium can react with lead in the eutectic and partly with oxygen in the surface film. As lead polonide and polonium oxide have lower saturated vapour pressures than pure polonium, the release rates of chemical compounds of polonium are about 10 3_ l 0 4 times smaller than that of pure polonium. Thus, the chemical activity of polonium decreases its evapration rate from eutectic. In estimates of the polonium evaporation rate Feuerstein et al. [5] use the coefficient of chemical activity of 2~°Po in Li Pb eutectic. They obtain this coefficient empirically through comparison of experiment with calculation. 4.3. Estimate o f the consequence o f accidental eutectic

spill The radiation consequences of an accidental eutectic spill from the ITER blanket have been estimated on the basis of the experimental data on 21°po release rate. According to the accident scenario, 25 m 3 of eutectic with a specific 21°Po activity of 1 I~Cicm 3 will spill at 350 °C into the reactor premises. The surface area of the spilled eutectic evaporation is 1000 m 2 and the free volume of the premises is 6 × 103m 3. In a few minutes up to 2 × 10-5 Ci of 21°po will be released in the room atmosphere and the concentration will reach 60 times the permissible value.

5. Summary The release of 21°po from solid and molten 17Li 83Pb eutectic into the flow of noble gases air at normal pressure was investigated at temperatures up to 450 °C. Aerosol and sorption filters were used for determina-

O. Schipakin et al. / Fusion Engineering and Design 29 (1995) 164-169

tion of the phase state of the released polonium. In all tests the activity of 21°Po in the carrier gas was found in gaseous aerosol forms. Our results show that surface film strongly affected the 2J°Po release rate and its concentration in the carrier gas. The comparison of results of the experiments and the evaporation theory show that the 21°Po release rate from the eutectic depends on the formation of polonium chemical compounds having different vapour pressures. Lead polonide probably evaporates instead of metallic polonium at high temperatures (350-450 °C), but polonium hydride vaporizes at lower temperatures (150-220°C). In the considered experiments insufficient attention was paid to the chemical aspects of the polonium evaporation process owing to the difficulty in conducting of chemical analysis. Effects in future experiments to measure the 2~°Po release rate from eutectic will be (1) the continuation of tests to determine the effect of surface film on the 2t°Po release rate from 17Li-83Pb eutectic, (2) the determination of the chemical form of the releasing polonium compounds by thermochromatographical methods, and (3) the study of effect of the carrier gas velocity on polonium release from the eutectic.

169

References [1] I. Petrajonov, N. Borisov an S. Churkin, Research of 2"~Po release in gas from model 17Li 83Pb eutectic and determination of polonium phase state in gas, Rep. N14-d, Karpovs Institute of Physical Chemistry, Moscow, January 1992. [2] N. Borisov, S. Churkin and O. Schipakin, Research of 21°po release in gas from irradiated and model Li-Pb eutectic and study of the polonium deposition into tubes from gas flow, Rep. N215-a, Karpovs Institute of Physical Chemistry, Moscow, December 1992. [3] O. Schipakin, N. Borisov and S. Churkin, Polonium-210 problem for eutectic blanket of a fusion reactor, Rep. N160-ot-4511, Research and Development Institute of Power Engineering, Moscow, March 1994. [4] D.W. Jeppson and L.D. Muhiestein, Safety considerations of lithium lead alloys as a fusion reactor breeding material, Fusion Technol. 8 (July 1985) 1385 1391. [5] H. Feuerstein, J. Oschinski and S. Horn, Behavior of 2"JPo in molten Pb-17Li, J. Nucl. Mater. 191 194 (1992) 288 291. [6] R.B. Tupper et al., Polonium hazards associated with lead bismuth used as a reactor coolant. [7] A.S. Abakumov, Thermal reactions with polonium, Usp. Khim. 51(7) (1982). [8] L. Brooks, J. Am. Chem. Soc. 77 (1955) 3211. [9] A.N. Nesmejanov, Vapour Pressure of the Chemical Elements, Academy of Sciences Press, Moscow, 1963.