water interaction

Fusion Engineering and Design 17 (1991) 193-197 North-Holland

193

Corrosive effects of Pb-17Li/water interaction P. A g o s t i n i a n d G. B e n a m a t i

ENEA, Research Center of Brasimone, Camugnano, Bologna, Italy

The interaction between Pb-17Li and water, as a consequence of a localized tube microcrack, has been studied. Two experiments were performed in which a low quantity of steam was injected into the lithium lead. The artificially machined microcracks, simulating a real microcrack in an AISI 316 heat exchanger tube, had a maximum area of 0.003 mm2. No blockage of the microcrack was observed during the tests. No significant damage was observed on the microcracks geometry, probably because of the short test time. A layer of reaction products having a high melting point was formed around the test section. First analytical results seem to confirm the presence of lithium hydroxide in the reaction products.

1. Introduction

react with water to form hydrogen and lithium hydroxide or oxide following the reactions:

Eutectic lithium lead (Pb-17Li) is one of the most promising breeder materials for a liquid blanket in fusion reactors. Actually two types of cooling systems are foreseen in the existing projects. The first is a self cooled system while the second is a water cooled system. In the water cooled configuration one important aspect is the possible interaction between hot lithium lead and water. From the safety point of view significant work was performed at JRC Ispra simulating a guillotine rupture of a cooling tube in the module [1]. Although this approach is very important for safety studies, usually, this type of damage is considered remote with respect to the production of small leaks associated with microcracks. These defects could be produced during the welding process and their growth could be accelerated by the mechanical cycling of the materials. The presence of these defects is noxious for several reasons. Firstly the chemical interaction between lithium and water produces some reaction products which would decrease the lithium content in the bulk Pb-17Li and could contribute to the formation of plugs. Secondly, the reaction products could have corrosive effects on the materials and could accelerate the self-damage of the microcracks. In order to obtain a good knowledge of the behaviour of a microcrack in lithium lead only the experimental approach is possible. From the thermodynamic point of view, under blanket conditions of Pb-17Li at 1 bar and 593 K and steam at 10 MPa, lithium, from the Pb-17Li, could

Li + H 2 0 ~ LiOH + 0.5H2,

(1)

2Li + H 2 0 ~ Li20 + H 2.

(2)

The final reaction product is dependent on many parameters, but certainly the molar ratio between water and lithium is one of the most important. In fact reaction (1) is more favourable with excess water while reaction (2) is more favourable with excess lithium. Small scale laboratory experiments have confirmed this behaviour with excess lithium [2], but it is very important to verify the situation in more realistic tests. For this reason it was decided to carry out tests consisting of a micro leakage of water into a large quantity of Pb-17Li, about 100 kg, with the total amount of injected water about 300 g. The purpose of the tests was to verify the dimensional evolution of the artificial defect, the nature of corrosion products, and the possible blockage of the microcrack.

2. Experimental tests The experimental rig used for the tests is shown in fig. 1. The test tank was mounted in the floor of an argon atmosphere glove box, so that the experiment and the successive preparation of post-test samples were performed in an oxygen and water free atmo-

0 9 2 0 - 3 7 9 6 / 9 1 / $ 0 3 . 5 0 © 1991 - Elsevier Science Publishers B.V. All rights reserved

194

P. AgostinL G. Benamati / Convsi~e eff[,cts of Pb 17Li / water interaction ARiel

I 2 3 4 5 i

?

-

PRESSURIZED VATER V E S S E L DIFFERENTIAL PRESSURE GAUGE INJECTION DEVICE TEST TANK GLOVE BOX H I O R O G £ H NETER GAUGE O X I G £ U HETER GAUGE

j°

4

vJ

I Fig. I. Experimental rig.

sphere. D u r i n g the tests the oxygen c o n t e n t in the inert a t m o s p h e r e was lower t h a n 8 wppm. T h e d e m o u n t a b l e injection device (DID), m a d e of two parts c l a m p e d together, is shown in fig. 2. T h e D I D was fed by w a t e r coming from the pressurized w a t e r vessel by m e a n s of a c o n n e c t i n g p i p e . T h e p r e s s u r e m e a s u r e m e n t of the feed water was m a d e using a differential pressure gauge c o n n e c t e d to the top a n d the b o t t o m of the pressurized w a t e r vessel. T h e a m o u n t of delivered water was calculated from the difference in the hydrostatic head. Leaks of a b o u t 1 × 10 3 g / s were readily detected; with 1 m m level variation c o r r e s p o n d i n g to 3.084 g. T h e test conditions for the two e x p e r i m e n t s

H20 Feed

Injecfion

Fig. 2. Demountable injection device.

p e r f o r m e d were o b t a i n e d from the design conditions of a water cooled blanket [3] and are given in table 1. Before test start-up, the leak rate was m e a s u r e d by a calibration p r o c e d u r e using steam a n d water. Following calibration the D I D was then c o n n e c t e d to the w a t e r injection system. In the start-up p r o c e d u r e the D I D was h e a t e d by a special heater, a n d the w a t e r feed was started w h e n a t e m p e r a t u r e of 423 K in the first test, a n d 663 K in the second test, was reached. W h e n a first exit of s t e a m was observed the D I D was immediately i m m e r s e d in the lithium lead. T h e P b - 1 7 L i initial t e m p e r a t u r e was a b o u t 593 K in the first test and 653 K in the second test, after few seconds a feeding tube coil i m m e r s e d in the lithium lead e q u i l i b r a t e d the t e m p e r a t u r e s . A fast increase of t e m p e r a t u r e was o b s e r v e d in the reaction vessel b e c a u s e of the reaction b e t w e e n lithium and water. In the first test the increase of t e m p e r a t u r e was a b o u t 30 K while in the second test it was a b o u t 16 K. A few m i n u t e s after the i m m e r s i o n a c o n s t a n t rate of the leak was a t t a i n e d a n d no blockage was observed

Table 1 Cooling system blanket parameters Coolant Coolant Coolant Coolant

inlet temperature (K) outlet temperature (K) tube wall th. (mm) pressure (MPa)

538 578 1.25 l0

195

P. Agostini, G. Benamati / Corrosive effects of Pb-17Li / water interaction Table 2 Tests data Parameters

Test I

Test II

Injection area (mm 2) Expected steam flow rate (g/s) Expected water flow rate (g/s) Observed initial flow rate (g/s) Total water delivered (g) Pb17Li total amount (kg) Molar ratio L i / H : O Pbl7Li and water temperature (K) Mean temperature increase (K) Test duration (h)

-- 0.003

= 0.002

--- 0.009

- 0.006

= 0.07

-- 0.05

0.006 300 87 5

0.002 163 102 11

593 a

653

|

O O

0.o

i.s

s'.o

7'.s

l .O fi.s

TLme I s ]

lS.O xlO 3

Fig. 3. Water level vs. time in the first test. 30 3

16 3

a The initial temperature was about 423 K and the value of regime was reached after 1 minute

during the first test. After the injection of ~ 300 g of water the feed water was halted and the D I D was extracted from the lithium lead. The behaviour of the second test was similar to the first one, but only 163 g of water were injected in the lithium lead. In both test after an initial lag of time with a lower flow rate than the expected,an increase was observed. In both tests, a solid layer was present on the lithium lead surface and around the D I D . The most significant test parameters are reported in table 2. A f t e r its extraction the D I D was dismantled and examined using optical and A u g e r electron microscopys and geometrically controlled. Some samples of reaction products were examined using thermal analysis (TA), X-ray diffraction (XRD), and secondary neutrals mass spectroscopy (SNMS). In both tests the leak path was produced by means of a radial groove which was machined in such way that a smaller radial channel was formed between two mated surface (fig. 2).

3. Results The variation of the water level in the pressurized vessel was recorded during the two tests. The water level variation, in the pressurized water vessel, during the first test is shown in fig. 3. This curve can be divided into three parts. The first part shows an initial high value of flow velocity due to hold up formation in the pipe connecting the feeder valve and the DID. The

second part shows the steady flow rate, reached after the first minutes; of about 0.006 g / s . This value was mantained until a sudden increase in the flow rate appeared. A f t e r the injection of ~ 300 g of water the feeder valve was closed. This resulted in a molar ratio between lithium and water of about 5:1. The water level variation in the pressurized water vessel, in the second test is shown in fig. 4. After an initial period of 2 hours at a 0.002 g / s an increase in the flow rate was observed. During this period the flow rate varied regularly between ~ 0.04 g / s to ~ 0.08 g / s . The experiment was stopped after 3 hours corresponding to 163 g of injected water and a lithium water molar ratio of 11 : 1. The order of magnitude of flow rate in the last part of the tests, seems indicate the presence of a bifasic flow with an high fraction of water. After each test to facilitate the D I D extraction, the lithium lead was heated to a temperature of about 723 K.

E E o

O O

12.o l .O

l .O

l .O

2 .o

2 .o

T [s] Fig. 4. Water level vs. time in the second test.

21.o .10 3

196

P. Agostini G. Benamati / Corrosit~e e]fects of Pb-17Li / water interaction

In both tests when the DID was extracted from the reaction vessel it was completely surrounded by a dull grey solid shell. In its inner part this solid was not completely dull but contained some bright areas. Many points with a large amount of lithium were found in this position, spectrophotometric analysis gave a value of lithium weight content up to 23%. T A of samples, taken from the inside of these solid products, indicated the presence of lithium hydroxide, while X R D was inconclusive, the material being amorphous. First SNMS analyses seem to confirm the presence of hydrogen in the reaction products. It is reasonable to think that the solid shell was formed by lithium oxide, lithium hydroxide (in the inner part), lithium and lead. Geometrical analysis of the microcrack from the first test using an optical system having a sensitivity of 1 #m, didn't show any appreciable variation. However microscopic analysis of the seal surface could not exclude a seal yielding, because the signs of plastic deformation don't seem to be present over all the contacting surface. The microcrack in the second test showed, after chemical cleaning the presence of a red-brown internal surface but no changes in the crack dimensions, larger than 1 /xm, were observed. Nevertheless in this location the surface roughness increased to 0.25 /xm from the original 0.1/xm. In this case the microscopic examinations of the plastic deformation of the seal surface exclude the possibility of a seal failure.

4. Discussion One of the most important questions to be answered from the study of microleaks in Pb-17Li was whether microcracks would block or remain open. The knowledge in the field of steam sodium micro leaks, in fact, couldn't enable a prediction in a lithium lead environment. For this purpose expected values of steam flowrates have been evaluated on the basis of calibration runs and thermodynamic considerations [4,5]. Therefore the first result of the tests is the observation that a complete blockage didn't happen even if an initial flow reduction, consequent to a probable partial obstruction of the microcrack, was observed. Moreover a solid shell was produced around thc DID. For a thermodynamic point of view the formation of lithium hydroxide was not favourable with the lithium water molar ratio adopted in the tests, 5:1 in the first test and 11 : 1 in the second. In fact under these conditions the formation of lithium oxide by reaction (2) would be the most probable [2]. In lithium lead the same results can be obtained if the system enables a contact be-

tween lithium and water by diffusion. Tests performed with a good mixing seem confirm these results [6]. The tests performed at the Brasimone Center show that the presence of a solid layer which formed around the water leak, can modify the former result. The shells presence, in fact, might act as a barrier to diffusion of the reagent and reaction products. Lithium and water would have to diffuse through the solid layer in order to react. In this way it was possible to obtain, in the inner part of the shell, a volume having a high water content resulting in the formation of lithium hydroxide. The presence of solid shell could be explained by various phenomenon. Firstly the water was about 593 K, in the first test, but this temperature would decrease considerably on the expansion through the microcrack. The relatively high lithium lead melting temperature (508 K) could then give a local nucleation of solid material by freezing. However this phenomenon observed in other experiments [1,7], was probably not very important in the these tests because of the low quantity of water injected compared to the large amount of hot lithium lead. The possibility of solid reaction products aggregating in the area surronding the microleak, due to variation of physical and chemical conditions, is suggested by the following considerations. The melting points of lithium hydroxide (approximately 723 K) and lithium oxide (approximately 1073 K) are higher than the maximum observed reaction temperature (approximately 670 K). Besides the low steam flow rate and low hydrogen production rate seem to induce, after a short lag of time, a low turbolence in the bulk Pb-17Li. No data was obtained on the rate of growth of the solid aggregates with time and temperature. The solid aggregates obtained in the second test immersed in hot (723-743 K) lithium lead and stirred for a long time ( ~ 30 minutes) were not completely soluted. Samples of materials taken from the surface of these solid, in contact with Pb-17Li, did not evidence presence of lithium hydroxide. These experimental observations, obtained under water cooled blanket conditions give rise to some questions. The first is, would the behaviour of the reaction bc the samc at higher lithium ]cad temperatures. Some research seems to exclude the accumulation of solid reaction products at temperatures higher than 723 K [7]. The second question is related to the possible damage of the defect after a longer test time. No damage superior to 1 # m was observed using optical microscopy on the artificial microcrack of the first and second test, and a roughness increase was observed in the second microcrack. This fact was probably due to the short test time. In fact 3 h

P. Agostini, G. Benamati / Corrosive effects of Pb-17Li / water interaction is a short time to observe a variation in the geometrical dimension of the leak path caused by corrosion.

5. Conclusions

From these tests some remarkable results are pointed out. Firstly the formation of solid aggregates of reaction products, which seem solid at temperature higher than 723 K. This occurrence is associated with the risk of detachment of solid aggregates from the originating surfaces and consequent lithium lead flow reduction or formation of plugs. Also the presence of lithium hydroxide among the reaction products is an important result. For the behaviour of the microcrack is important to observe that in the performed tests no blockage happened. However this result cannot be considered conclusive because only one certain test is not sufficient to draw a general conclusion. For this reason it is necessary to carry out further tests.

197

References

[1] H. Kottowski, O. Kranert, C. Savatteri, C. Wu, M. Corradini, Studies with respect to the estimation of liquid metal blanket safety, Fusion Engrg. Des. (to appear). [2] P. Hubberstey, T. Sample, M.G. Barker, Determining composition in the Pb-Li system by equilibrium resistance measurement of phase boundaries, 16th SOFT London 1990. (Elsevier, Amsterdam, 1991). [3] G. Casini, The impact of new experimental data on the design of Lil7Pb/Water breeding blanket, 1st ISFNT Tokyo 1988, Furion Engrg. Des. 8 (1988) 139-143. [4] R.H. Perry, D. Green, Chemical Engineers' Handbook (Mc Graw Hill, Singapore, 1984) section 5. [5] A.S. Shapiro, Compressible Fluid Flow, Vol. I (Ronald Press Company, New York, 1953). [6] D.W. Jeppson, L.D. Munlestein, Safety considerations of lithium lead alloy as a fusion reactor breeding material, Fusion Technol. 8 (July 1985) 1385-1390. [7] R. Cuttle, Water-lead lithium reactions, CLM-R278 UKAEA.