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In-pile tritium release behavior from lithium aluminate and lithium orthosilicate of the VOM-23 experiment
IN-PILE TRITKJM RELEASE BEHAVIOR FROM LITHIUM ALU~INA~ AND L~IU~ U~~~SIL,I~A~ OF THE VUM-23 EXPEDIENT T. KURASAWA’,
H. WATANABE
‘, E. ROTH ’ and D. VOLLATH
3
’ Department of Fuels and Materrals Research, Sopan Atomic Enerm Reseurch Institute. Tokrri-mure, Ibnraki-ken 31%/I, Jrrpun .’ Gommissariat a I’Energie Atomique, DISICP-CEN/Saclay, 91191 Gif-sur- Yvette, Frunce .’ Kerforschungszentrum Karlsruhe GmbH, Postfirch 3640, 7500 Ka&ruhe, Fed, Rep. Germuqt Lithium aluminate from CENjSacIay and lithium orthosificate from KfK/Karlsruhe were irradiated in separate capsules in &heVOM-23 experiment as a part of the IEA breeder exchange matrix (BEATRIX). The behavior of tritium release from lithium orthosilicate was observed to be similar to that of lithium aluminate in spite of differences in grain size. density and specific surface area. The tritium release kinetics was strongly influenced by the content of hydrogen isotopes in the purge gas. Tritiun release was enhanced in purge gases with higher hydrogen contents which is indicative of a significant contribution from the surface adso~t~on-d~so~tion process. The tritium reteast behavior in both materials appeared to be governed by both diffusion and surface adso~~ion-d~o~tion mechanisms. A modef including both mechanisms was developed to analyze the data from this experiment.
1. Introduction The major role of lithium ceramic materials in the blanket regions of fusion reactors is the supply of tritium for the plasma fuel and beat for the power generator systems. In-situ tritium recovery experiments in fission reactors provide a method of evaluating the feasibility of utilizing these breeding materials in a fusion power plant. A high level of tritium recovery in the in-situ tests is one of the most important characteristics in the selection of a tritium breeding material. In-situ tritium recovery experiments were conducted in Japan [1,3,6,11,12], the USA [2,10], Europe [4,5,7-91 and Canada [18]. As a part of the IEA breeder exchange matrix (BEATRIX-I), both lithium aluminate (LiAlO?) and tithium orthosilicate (Li4Si04) were received from CEN/Saclay and KfKjKarlsruhe and irradiated in separate capsules in the VOM experiment. A direct comparison of the in-situ tritium recovery performance of these LiAlO, and Li, SiO, materials is provided and reviewed. 2. MaterEalsand experimentalproeedure Samples of LiAlO, were pressed and sintered into rods which were 4 mm in diameter and 40 mm long, while Li,SiQ samples were sintered and ground into spheres of 3.85 mm diameter. The fine grain size of the LiAlO, (0.49 pm) was equivalent to that of materials examined in the TRIO f2], LILA f4,7] and FUBR-1 [13] experiments. The grain size of Li4Si04 was much larger (14 pm). Densities of the LiAlO, and Li,SiOa samples are 77 and 88% T.D., respectively. Both specimens were encapsuled in a platinum Iined cylinder made of Hasteiloy X-R (19 mm inner diameter x 80 mm length) similar to previous sweep gas type capsules [1,3,6]. Temperatures were measured by the K-type thermocouple located in the center of the solid breeder container. The indicated temperatures in this 0022-3115/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
experiment were not average values but maximum values. The radial temperature gradient cannot be estimated because of the lacking effective conductivity of spheres or rods. This experiment was conducted in the VT-10 of JRR-2 with the same tritium gas analysis system which was previously described [6]. 3. Results
The transient tritium release behavior after an incremental change of temperature was measured in the manner similar to the previous experiment [6]. In fig. I, transient tritium release curves of LiAlO, are compared after changing to seven consecutive temperatures of 800, 750, 700. 650, 600, 550 and 500” C in a 0.1% D, environment. The shape of each curve is representative of its time constant; short time constants at 800 o C are in contrast to long time constants at 500” C. Below 600°C, the tritium release rate did not return to the previous steady state even after several days. The tritium release behavior for Li,SiO, in 0.1% D, purge gas was observed to be similar to that of LiAIOz as shown in fig. 2. From figs. 1 and 2, the behavior of tritium release from lithium aluminate seems to be quantitatively similar to that of lithium orthosilicate in spite of the different sample characteristics. The dashed lines in figs. 1 and 2 represent a curve fitting the theoreticai expression which is described later. 3.2. T~~~s~e~t tritium recotjey purge gas composition
~haui~r after a change
of
An apparent proportion~it~ between the steady state tritium release rate and the hydrogen isotope (D,) content of the purge gas was observed in the previous VOM-22H experiment of Li,O and LiAlO, [ll]. In
545
T. Kurasawa et al. / Tritium release behavior
VOM-23 LiAl02 800°C
VOM-23 LiAl02
0.I % 02 I
I
/3r
b,”
x 0.0BXD2,L=70,D=3E-7
f
. 006 0 004 l a02
I
/
2
4
I
I
1
I
1
1
1
6
8
10
12
14
16
18
TIME
20
(Hr)
x 900°C,L=70,D=3E-7 o 700°C,L=25,D=fE-7
. 750°C,L=35,D=25E-7
q 600~,L=14,D=3%-6 A 5OOC,L= 7,D=6E-9
l
A 650*C,L=20,D=
7E-6
550y,L=fO,D=
IE-6
Fig. 1. Comparison of the transient tritium recovery of LiAlO, after successive incremental temperature changes from 850 to 500°C in the purge gas with 0.1% Dr. The dashed lines represent curve fittings based on eq. (1).
order to clarify this effect on tritium release, a wide range of deuterium levels from 0.02 to 3% was examined in this experiment (VOH-23) on both the LiAlO, and Li,Si04 samples. After changing the purge gas composition from an initial value of 0.1% D,, a transient tritium release rate along with an asymptotic return to
2
4
6
8 10 12 TIME (Hr 1
” * ‘)
L=45 L=20 L=9
14
16
* Y *
18
i
Fig. 3. Purge gas composition dependence of transient tritium release for LiAlO, at 800 ’ C. The dased lines were obtained by use of the listed variables.
steady state was observed as shown in fig. 3 for LiAlO, at 800°C. A similar behavior was observed at the examined temperatures for both LiAlO, and Li,SiO,. The return to a steady state tritium release rate was faster at higher hydrogen isotope compositions. This behavior cannot be interpreted by the simple diffusion model based on the zero tritium concentration at the surface of irradiated samples but is indicative of the significant contribution of the surface adsorption-desorption process.
3.3. Tritium release behavior at steady state
Li4SiO4 0.1% 02
2
4
6
8
10
TIME x 800%, L=20,D=7E-7 A 6OOt,L=7 ,D=iE-7 0 500'CC,L=i.5,D=3E-6
42
14
16
18
20
(Hr)
0 65O’C, L=i2 ,D=3E-7 .55O*C,L=Z
,D=5E-6
Fig. 2. Comparison of the transient tritium recovery of Li,SiO, after successive incremental temperature changes from 850 to 500°C. The dashed lines represent curve fittings based on eq. (1).
The normalized tritium release rate (R/G) where R is the release rate and G is the generation rate depends on the D, content of the purge gas even under the steady state release as indicated in table 1. Higher steady state tritium release rates were observed in cases of higher hydrogen isotope levels in the purge gas. Hence, these results reflect a singificant contribution of the surface adsorption-desorption processes. In a simple diffusional model which assumed the zero tritium concentration at the surface, the steady state tritium release rate should be independent of the purge gas composition. From table 1, the normalized values of R/G were unity at 800 o C in the 0.1% D, bearing purge gas for both LiAlO, and Li,SiO, samples, hence the release rate is equal to the generation rate under these conditions. In a purge gas environment with greater than 0.1% D,, the normalized values of R/G are greater than unity for LiAlO, at 800 and 7OO”C, and for Li,SiO, at 800 o C. This excess release of tritium may be interpreted as the release of the tritium absorbed in portions of the experimental system. Although the interaction between released tritium and the surface of the system situated in the down stream is quantitatively not
546
Table 1 Normalized
tritium
release rate (R/G)
from LiAl$
and Li,SiO,
at steady state versus temperature
Li&iO
LiAl O2
r
3m Temp. 800°C ( 700°C ' 650°C Cl 2.92% i 2.34 *
1.06 1.06
1
1.05
600°C
1
L
\
292%(
107 1.05 0.58 ) 1.04 010. 0.99 to.03 0.96 ?:0.021093+ 002'085 t 005 0.08' 0.97 0.94 0.82 0.06 ' 096 0.91 090 082 0.04 ( 0.88 091 077 086 002' 0.86 081 074 0.01 ' 073 He i ( 0.29 1 024 ; 0.23 1
116 0.98 097 100
Fourth Cycle, after Fifth cycle, 673’C
3.4. Degradation
073**
; 070** 0.92mt~0.71f0ti
008 * 099 0.06 * 095 004' 0.88 002' 0.87+ 003 001, ~ He * *x
understood, this phenomenon could be interpreted as an isotopic exchange reaction on the surface of the solid breeder and the container materials around the solid breeder [Ill. Hence, these results indicate that an optimized condition of temperature and purge gas is necessary to get the unity of R/G. In the region of R/G < 1, relatively large quantities of tritium must be retained on the surface of the breeder and the experimental system. This point is very important in order to design a ceramic blanket for a fusion reactor. Tritium diffusional inventory requirements would dictate the choice of very small ceramic particles with a high specific surface area which would result in an unduly high tritium inventory due to surface adsorption.
8oo”c
2 34 ' 115 175t I I 14 117% 112 058 * 1.11 010, 1oo+o.a
1.75+ 1.17'r
of thr purge g:l!,
4
-’ ..Jemp. :omp --
and I>, content
083 084 084 079
~
0.57
degradation +
065 065 064 062
J
* * * *
049
of release
of tritium release
During this experiment, temperatures in both the LiAlO, and the Li,SiO, were varied from 500°C to 850 o C. The tritium release behavior of Li,SiO, during the fourth cycle did not reproduce the observations during the first cycle as is shown in fig. 4. On the other hand, tritium release curves of LiAlO, during the first and third cycle were almost identical. Since much of the time between cycle 1 and cycle 4 included an operation at 800 o C or higher, the degradation of tritium recovery from Li,SiO, sample may have resulted from either phase dissociation of Li,SiO, or grain growth. Preliminary post irradiation examination supported this interpretation. 4. Data analysis and discussion
j-
I
1
I
2
4
6
I
1
1
8 10 12 TIME (Hr)
I
1
/
14
16
18
Fig. 4. Comparison of tritium release curves during the first cycle and the fourth cycle for Li,SiO, at 650 o C.
The complex phenomenon of tritium release from lithium ceramics can be viewed as a multistep process in which the rate determing step may strongly depend upon the sample size, temperature and purge gas composition. Almost all of the previous experiments have failed to completely characterize purge gas effects. Previously, Johnson et al. pointed out that a majority of the in-situ tritium release experiments were operated in the transition regime of diffusion and desorption control [14]. Those experimental results were affected by the two simultaneous resistances to tritium mass transfer which leads to an overall tritium release slower than diffusion or desorption alone. It is important to conduct experiments which clearly distinguish between diffusion and surface desorption processes. Addition of hydrogen isotopes to the purge gas may simplify the release kinetics by isotopic exchange reactions at the surface. The surface condition can be mod-
541
T. Kurasawa et al. / Tritium release behavior
ified to derive the zero tritium concentration by changing the gas composition, thus providing the classical diffusional release kinetics. The kinetics for the in-situ release of tritium can be postulated as a two-step process; first, diffusion to the sample surface and second, surface desorption from the sample surface into the surrounding purge gas. A model which incorporates both surface desorption and bulk diffusion kinetics has been adopted in the analysis of this experiment in order to provide a better understanding of the tritium release behavior. The governing equation of a new model incorporating both desorption and diffusion for a spherical sample is the same as the equation for simple diffusion with the exception of the boundary condition at the surface. The new boundary condition is described by the ratio of diffusive flux to the surface and the flux leaving from the surface. The solution for this diffusional problem was obtained by rearrangement of a similar heat conduction problem under the condition of constant heat generation and radiation heat transfer at the surface of a sphere as shown by Carslow and Jaeger [15]. From the above solution of the heat conduction model, the normalized tritium release (R/G) after an incremental change of temperature and purge gas composition was obtained as follows; R,G=l-2ah
1
-f n=l
CA;
+
5. Conclusions
exp( -criDt),
ah (ah
-
1)
(1) where h = H/D,
H is called the “surface”
which is referred to the coefficient of surface mass transfer, D is the coefficient of tritium diffusion, a is the sample size and UOL,, is the n th root of aa, cot(acY,,) = 1 - ah. By examining this equation the dimensionless quantity, L = ah, seems to be strongly dependent upon the relative contributions of surface desorption and diffusion. Using eq. (1) D and n are constants. A sample radius of 2 mm for the effective diffusion path and the highest diffusion coefficient obtained [16] were assumed for the purpose of curve fitting. The parameter L was varied until optimum curve fitting was obtained. Curve fitting was good in fig. 1 for LiAlO, but not for Li,SiO, in fig. 2. From both figures, low values of L were obtained at low temperatures of 550 and 500 o C which reflects a major role of surface processes in the low temperature regime. The L values were plotted against the reciprocal temperature in the fig. 5. An activation energy of approximated 49 and 60 kJ/mol suggests a surface adsorption-desorption mechanism. Surface desorption is assumed to be the dominant process for L values lower than 10, while diffusion controls the release for L values higher than 10. Since L indicates the relative contributions from bulk diffusion and surface desorption processes, it is expected that such parameter will define the rate controlling step.
conductance
Tritium release kinetics on fine grained LiAlO, and coarse grained Li,SiO, materials were similar between 500 and 800 o C in a 0.1% D, purge gas. A combined diffusional and surface adsorption-desorption model was used to fit in-situ tritium recovery data of LiAlO, and Li,SiO,. Surface desorption appeared to have a significant contribution at low temperatures while diffusion appeared to dominate at higher temperatures. The authors are indebted to the entire staff of Fuels Properties Laboratory and Research Reactor Utilization Division of JAERI for assistance in this experiment. Contributions by Drs. B. Rasneur/CEN-Saclay and H. Elbel/KfK on fabrication of the examined samples as part of the IEA breeder exchange matrix (BEATRIX) were appreciated. Also the authors would like to thank to Dr. G.W. Hollenberg of Battelle Pacific Northwest Laboratories for his useful discussion and assistance in preparation of the manuscript and Dr. M. Seki of JAERI for his help concerning the transformation of the heat conduction equation. The authors also wish to express their thanks to Drs. T. Kondo, K. Shiba and K. Iwamoto for their continuous encouragement and support in this work.
8
9
10 l/T
II
12
13
14
References
( i(j4 K-’ )
Fig. 5. Calculated surface parameter (L) from curve based on eq. (I) versus reciprocal temperature.
[II T. Kurasawa, H. Takeshita, H. Watanabe, H. Yoshida, fittings
Naruse, T. Miyauchi, K. Mimura Mater. 122 & 123 (1984) 902.
Y. and H. Umei, J. Nucl.
[2] R.G. Clemmer, P.A. Finn, B. Misra. M.C. Billone, A.K. Fischer, SW. Tam, C.E. Johnson and A.E. Scandora, J. Nucl. Mater. 133& 134 (1985) 171. [3] T. Kurasawa, H. Yoshida, H. Takeshita. H. Watanabe, T. Miyauchi, T. Mauri, H. Umei and Y. Naruse. J. Nucl. Mater. 133 & 134 (1985) 196. [4] E. Roth, J.J. Abassin, F. Botter. M. Briec, P. Chenebault. M. Masson, B. Rasneur and N. Roux. J. Nucl. Mater. 133 & 134 (1985) 238. [S] H. Kwast, R. Conrad and J.D. Elen, J. Nucl. Mater. 133 & 134 (1985) 246. [6] T. Kurasawa, H. Watanabe, G.W. Hollenberg, Y. Ishii, A. Nishimura, H. Yoshida, Y. Naruse, M. Aizawa, H. Ohno and S. Konishi, J. Nucl. Mater. 141-143 (1986) 265. [7] M. Briec. F. Botter, J.J. Abassin, R. Benoit, P. Chenebault, M. Masson, B. Rasneur, P. Sciers, H. Werle and E. Roth, J. Nucl. Mater. 141-143 (1985) 321. [8] H. Kwast, R. Conrad, P. Kennedy, A. Flipot and J.D. Elen, J. Nucl. Mater. 141-143 (1985) 300. [9] H. Werle, J.J. Abassin. M. Briec, R.G. Clemmer, H. Elbel, HE. Hafner. M. Masson, P. Sciers and H. Wedemeyer, J. Nucl. Mater. 141-143 (1985) 321. [lo] R.G. Clemmer, H. Werle and M. Briec, in: Proc. Int.
[II]
[12]
[13]
[14] [15] 1161
j17J [f8]
Symp. on Fusion Reactor Blanket and Fuel C‘ycle I‘echnology, Oct. 27-29. 1986, Tokai. Japan, p. 51. T. Kurasawa, G.W. Hollenberg and H. Watanabe, in: Proc. Int. Symp. Fusion Reactor Blanket & Fuel Cycle Technology, Oct. 27-29. 1986, Tokai, Japan, p. 43. H. Watanabe, T. Kurasawa. E. Roth and D. Vollath. in: Proc. Int. Symp. Fusion Reactor Blanket and Fuel Cycle Technolgy. Oct. 27-29. 1986, Tokai. Japan, p. 33. G.W. Hoilenberg, R.C. Knight. P.J. Densley. L.A. Pember, C.E. Johnson. R.B. Poeppel and L. Yang. J. Nucl. Mater. 141-143 (1986) 271. C.E. Johnson. private communication, from ANL-CMTI8847. p. 24. H.S. Carslow and J.C. Jaeger, Conduction of Heat in Solids. Second Ed. (Clarendon, Oxford, 1959) p. 246. D. Brtinning, D. Guggi and H.R. Ihle, in: Proc. 12th Symp. Fusion Technology, Jiilich, FRG Sept. 13- 17. 1982. p. 543. M. DaBe Donne, Fusion Technol. 9 (1986) 503. R.A. Verrell, J.M. Miller, C.E. Johnson and R.B. Poeppel, to be published in Advances in Ceramics: Proc. Am. Ceramic Sac. Annual Meeting, Pittsbur~, April 26-29, 1987.
H. WATANABE
‘, E. ROTH ’ and D. VOLLATH
3
’ Department of Fuels and Materrals Research, Sopan Atomic Enerm Reseurch Institute. Tokrri-mure, Ibnraki-ken 31%/I, Jrrpun .’ Gommissariat a I’Energie Atomique, DISICP-CEN/Saclay, 91191 Gif-sur- Yvette, Frunce .’ Kerforschungszentrum Karlsruhe GmbH, Postfirch 3640, 7500 Ka&ruhe, Fed, Rep. Germuqt Lithium aluminate from CENjSacIay and lithium orthosificate from KfK/Karlsruhe were irradiated in separate capsules in &heVOM-23 experiment as a part of the IEA breeder exchange matrix (BEATRIX). The behavior of tritium release from lithium orthosilicate was observed to be similar to that of lithium aluminate in spite of differences in grain size. density and specific surface area. The tritium release kinetics was strongly influenced by the content of hydrogen isotopes in the purge gas. Tritiun release was enhanced in purge gases with higher hydrogen contents which is indicative of a significant contribution from the surface adso~t~on-d~so~tion process. The tritium reteast behavior in both materials appeared to be governed by both diffusion and surface adso~~ion-d~o~tion mechanisms. A modef including both mechanisms was developed to analyze the data from this experiment.
1. Introduction The major role of lithium ceramic materials in the blanket regions of fusion reactors is the supply of tritium for the plasma fuel and beat for the power generator systems. In-situ tritium recovery experiments in fission reactors provide a method of evaluating the feasibility of utilizing these breeding materials in a fusion power plant. A high level of tritium recovery in the in-situ tests is one of the most important characteristics in the selection of a tritium breeding material. In-situ tritium recovery experiments were conducted in Japan [1,3,6,11,12], the USA [2,10], Europe [4,5,7-91 and Canada [18]. As a part of the IEA breeder exchange matrix (BEATRIX-I), both lithium aluminate (LiAlO?) and tithium orthosilicate (Li4Si04) were received from CEN/Saclay and KfKjKarlsruhe and irradiated in separate capsules in the VOM experiment. A direct comparison of the in-situ tritium recovery performance of these LiAlO, and Li, SiO, materials is provided and reviewed. 2. MaterEalsand experimentalproeedure Samples of LiAlO, were pressed and sintered into rods which were 4 mm in diameter and 40 mm long, while Li,SiQ samples were sintered and ground into spheres of 3.85 mm diameter. The fine grain size of the LiAlO, (0.49 pm) was equivalent to that of materials examined in the TRIO f2], LILA f4,7] and FUBR-1 [13] experiments. The grain size of Li4Si04 was much larger (14 pm). Densities of the LiAlO, and Li,SiOa samples are 77 and 88% T.D., respectively. Both specimens were encapsuled in a platinum Iined cylinder made of Hasteiloy X-R (19 mm inner diameter x 80 mm length) similar to previous sweep gas type capsules [1,3,6]. Temperatures were measured by the K-type thermocouple located in the center of the solid breeder container. The indicated temperatures in this 0022-3115/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
experiment were not average values but maximum values. The radial temperature gradient cannot be estimated because of the lacking effective conductivity of spheres or rods. This experiment was conducted in the VT-10 of JRR-2 with the same tritium gas analysis system which was previously described [6]. 3. Results
The transient tritium release behavior after an incremental change of temperature was measured in the manner similar to the previous experiment [6]. In fig. I, transient tritium release curves of LiAlO, are compared after changing to seven consecutive temperatures of 800, 750, 700. 650, 600, 550 and 500” C in a 0.1% D, environment. The shape of each curve is representative of its time constant; short time constants at 800 o C are in contrast to long time constants at 500” C. Below 600°C, the tritium release rate did not return to the previous steady state even after several days. The tritium release behavior for Li,SiO, in 0.1% D, purge gas was observed to be similar to that of LiAIOz as shown in fig. 2. From figs. 1 and 2, the behavior of tritium release from lithium aluminate seems to be quantitatively similar to that of lithium orthosilicate in spite of the different sample characteristics. The dashed lines in figs. 1 and 2 represent a curve fitting the theoreticai expression which is described later. 3.2. T~~~s~e~t tritium recotjey purge gas composition
~haui~r after a change
of
An apparent proportion~it~ between the steady state tritium release rate and the hydrogen isotope (D,) content of the purge gas was observed in the previous VOM-22H experiment of Li,O and LiAlO, [ll]. In
545
T. Kurasawa et al. / Tritium release behavior
VOM-23 LiAl02 800°C
VOM-23 LiAl02
0.I % 02 I
I
/3r
b,”
x 0.0BXD2,L=70,D=3E-7
f
. 006 0 004 l a02
I
/
2
4
I
I
1
I
1
1
1
6
8
10
12
14
16
18
TIME
20
(Hr)
x 900°C,L=70,D=3E-7 o 700°C,L=25,D=fE-7
. 750°C,L=35,D=25E-7
q 600~,L=14,D=3%-6 A 5OOC,L= 7,D=6E-9
l
A 650*C,L=20,D=
7E-6
550y,L=fO,D=
IE-6
Fig. 1. Comparison of the transient tritium recovery of LiAlO, after successive incremental temperature changes from 850 to 500°C in the purge gas with 0.1% Dr. The dashed lines represent curve fittings based on eq. (1).
order to clarify this effect on tritium release, a wide range of deuterium levels from 0.02 to 3% was examined in this experiment (VOH-23) on both the LiAlO, and Li,Si04 samples. After changing the purge gas composition from an initial value of 0.1% D,, a transient tritium release rate along with an asymptotic return to
2
4
6
8 10 12 TIME (Hr 1
” * ‘)
L=45 L=20 L=9
14
16
* Y *
18
i
Fig. 3. Purge gas composition dependence of transient tritium release for LiAlO, at 800 ’ C. The dased lines were obtained by use of the listed variables.
steady state was observed as shown in fig. 3 for LiAlO, at 800°C. A similar behavior was observed at the examined temperatures for both LiAlO, and Li,SiO,. The return to a steady state tritium release rate was faster at higher hydrogen isotope compositions. This behavior cannot be interpreted by the simple diffusion model based on the zero tritium concentration at the surface of irradiated samples but is indicative of the significant contribution of the surface adsorption-desorption process.
3.3. Tritium release behavior at steady state
Li4SiO4 0.1% 02
2
4
6
8
10
TIME x 800%, L=20,D=7E-7 A 6OOt,L=7 ,D=iE-7 0 500'CC,L=i.5,D=3E-6
42
14
16
18
20
(Hr)
0 65O’C, L=i2 ,D=3E-7 .55O*C,L=Z
,D=5E-6
Fig. 2. Comparison of the transient tritium recovery of Li,SiO, after successive incremental temperature changes from 850 to 500°C. The dashed lines represent curve fittings based on eq. (1).
The normalized tritium release rate (R/G) where R is the release rate and G is the generation rate depends on the D, content of the purge gas even under the steady state release as indicated in table 1. Higher steady state tritium release rates were observed in cases of higher hydrogen isotope levels in the purge gas. Hence, these results reflect a singificant contribution of the surface adsorption-desorption processes. In a simple diffusional model which assumed the zero tritium concentration at the surface, the steady state tritium release rate should be independent of the purge gas composition. From table 1, the normalized values of R/G were unity at 800 o C in the 0.1% D, bearing purge gas for both LiAlO, and Li,SiO, samples, hence the release rate is equal to the generation rate under these conditions. In a purge gas environment with greater than 0.1% D,, the normalized values of R/G are greater than unity for LiAlO, at 800 and 7OO”C, and for Li,SiO, at 800 o C. This excess release of tritium may be interpreted as the release of the tritium absorbed in portions of the experimental system. Although the interaction between released tritium and the surface of the system situated in the down stream is quantitatively not
546
Table 1 Normalized
tritium
release rate (R/G)
from LiAl$
and Li,SiO,
at steady state versus temperature
Li&iO
LiAl O2
r
3m Temp. 800°C ( 700°C ' 650°C Cl 2.92% i 2.34 *
1.06 1.06
1
1.05
600°C
1
L
\
292%(
107 1.05 0.58 ) 1.04 010. 0.99 to.03 0.96 ?:0.021093+ 002'085 t 005 0.08' 0.97 0.94 0.82 0.06 ' 096 0.91 090 082 0.04 ( 0.88 091 077 086 002' 0.86 081 074 0.01 ' 073 He i ( 0.29 1 024 ; 0.23 1
116 0.98 097 100
Fourth Cycle, after Fifth cycle, 673’C
3.4. Degradation
073**
; 070** 0.92mt~0.71f0ti
008 * 099 0.06 * 095 004' 0.88 002' 0.87+ 003 001, ~ He * *x
understood, this phenomenon could be interpreted as an isotopic exchange reaction on the surface of the solid breeder and the container materials around the solid breeder [Ill. Hence, these results indicate that an optimized condition of temperature and purge gas is necessary to get the unity of R/G. In the region of R/G < 1, relatively large quantities of tritium must be retained on the surface of the breeder and the experimental system. This point is very important in order to design a ceramic blanket for a fusion reactor. Tritium diffusional inventory requirements would dictate the choice of very small ceramic particles with a high specific surface area which would result in an unduly high tritium inventory due to surface adsorption.
8oo”c
2 34 ' 115 175t I I 14 117% 112 058 * 1.11 010, 1oo+o.a
1.75+ 1.17'r
of thr purge g:l!,
4
-’ ..Jemp. :omp --
and I>, content
083 084 084 079
~
0.57
degradation +
065 065 064 062
J
* * * *
049
of release
of tritium release
During this experiment, temperatures in both the LiAlO, and the Li,SiO, were varied from 500°C to 850 o C. The tritium release behavior of Li,SiO, during the fourth cycle did not reproduce the observations during the first cycle as is shown in fig. 4. On the other hand, tritium release curves of LiAlO, during the first and third cycle were almost identical. Since much of the time between cycle 1 and cycle 4 included an operation at 800 o C or higher, the degradation of tritium recovery from Li,SiO, sample may have resulted from either phase dissociation of Li,SiO, or grain growth. Preliminary post irradiation examination supported this interpretation. 4. Data analysis and discussion
j-
I
1
I
2
4
6
I
1
1
8 10 12 TIME (Hr)
I
1
/
14
16
18
Fig. 4. Comparison of tritium release curves during the first cycle and the fourth cycle for Li,SiO, at 650 o C.
The complex phenomenon of tritium release from lithium ceramics can be viewed as a multistep process in which the rate determing step may strongly depend upon the sample size, temperature and purge gas composition. Almost all of the previous experiments have failed to completely characterize purge gas effects. Previously, Johnson et al. pointed out that a majority of the in-situ tritium release experiments were operated in the transition regime of diffusion and desorption control [14]. Those experimental results were affected by the two simultaneous resistances to tritium mass transfer which leads to an overall tritium release slower than diffusion or desorption alone. It is important to conduct experiments which clearly distinguish between diffusion and surface desorption processes. Addition of hydrogen isotopes to the purge gas may simplify the release kinetics by isotopic exchange reactions at the surface. The surface condition can be mod-
541
T. Kurasawa et al. / Tritium release behavior
ified to derive the zero tritium concentration by changing the gas composition, thus providing the classical diffusional release kinetics. The kinetics for the in-situ release of tritium can be postulated as a two-step process; first, diffusion to the sample surface and second, surface desorption from the sample surface into the surrounding purge gas. A model which incorporates both surface desorption and bulk diffusion kinetics has been adopted in the analysis of this experiment in order to provide a better understanding of the tritium release behavior. The governing equation of a new model incorporating both desorption and diffusion for a spherical sample is the same as the equation for simple diffusion with the exception of the boundary condition at the surface. The new boundary condition is described by the ratio of diffusive flux to the surface and the flux leaving from the surface. The solution for this diffusional problem was obtained by rearrangement of a similar heat conduction problem under the condition of constant heat generation and radiation heat transfer at the surface of a sphere as shown by Carslow and Jaeger [15]. From the above solution of the heat conduction model, the normalized tritium release (R/G) after an incremental change of temperature and purge gas composition was obtained as follows; R,G=l-2ah
1
-f n=l
CA;
+
5. Conclusions
exp( -criDt),
ah (ah
-
1)
(1) where h = H/D,
H is called the “surface”
which is referred to the coefficient of surface mass transfer, D is the coefficient of tritium diffusion, a is the sample size and UOL,, is the n th root of aa, cot(acY,,) = 1 - ah. By examining this equation the dimensionless quantity, L = ah, seems to be strongly dependent upon the relative contributions of surface desorption and diffusion. Using eq. (1) D and n are constants. A sample radius of 2 mm for the effective diffusion path and the highest diffusion coefficient obtained [16] were assumed for the purpose of curve fitting. The parameter L was varied until optimum curve fitting was obtained. Curve fitting was good in fig. 1 for LiAlO, but not for Li,SiO, in fig. 2. From both figures, low values of L were obtained at low temperatures of 550 and 500 o C which reflects a major role of surface processes in the low temperature regime. The L values were plotted against the reciprocal temperature in the fig. 5. An activation energy of approximated 49 and 60 kJ/mol suggests a surface adsorption-desorption mechanism. Surface desorption is assumed to be the dominant process for L values lower than 10, while diffusion controls the release for L values higher than 10. Since L indicates the relative contributions from bulk diffusion and surface desorption processes, it is expected that such parameter will define the rate controlling step.
conductance
Tritium release kinetics on fine grained LiAlO, and coarse grained Li,SiO, materials were similar between 500 and 800 o C in a 0.1% D, purge gas. A combined diffusional and surface adsorption-desorption model was used to fit in-situ tritium recovery data of LiAlO, and Li,SiO,. Surface desorption appeared to have a significant contribution at low temperatures while diffusion appeared to dominate at higher temperatures. The authors are indebted to the entire staff of Fuels Properties Laboratory and Research Reactor Utilization Division of JAERI for assistance in this experiment. Contributions by Drs. B. Rasneur/CEN-Saclay and H. Elbel/KfK on fabrication of the examined samples as part of the IEA breeder exchange matrix (BEATRIX) were appreciated. Also the authors would like to thank to Dr. G.W. Hollenberg of Battelle Pacific Northwest Laboratories for his useful discussion and assistance in preparation of the manuscript and Dr. M. Seki of JAERI for his help concerning the transformation of the heat conduction equation. The authors also wish to express their thanks to Drs. T. Kondo, K. Shiba and K. Iwamoto for their continuous encouragement and support in this work.
8
9
10 l/T
II
12
13
14
References
( i(j4 K-’ )
Fig. 5. Calculated surface parameter (L) from curve based on eq. (I) versus reciprocal temperature.
[II T. Kurasawa, H. Takeshita, H. Watanabe, H. Yoshida, fittings
Naruse, T. Miyauchi, K. Mimura Mater. 122 & 123 (1984) 902.
Y. and H. Umei, J. Nucl.
[2] R.G. Clemmer, P.A. Finn, B. Misra. M.C. Billone, A.K. Fischer, SW. Tam, C.E. Johnson and A.E. Scandora, J. Nucl. Mater. 133& 134 (1985) 171. [3] T. Kurasawa, H. Yoshida, H. Takeshita. H. Watanabe, T. Miyauchi, T. Mauri, H. Umei and Y. Naruse. J. Nucl. Mater. 133 & 134 (1985) 196. [4] E. Roth, J.J. Abassin, F. Botter. M. Briec, P. Chenebault. M. Masson, B. Rasneur and N. Roux. J. Nucl. Mater. 133 & 134 (1985) 238. [S] H. Kwast, R. Conrad and J.D. Elen, J. Nucl. Mater. 133 & 134 (1985) 246. [6] T. Kurasawa, H. Watanabe, G.W. Hollenberg, Y. Ishii, A. Nishimura, H. Yoshida, Y. Naruse, M. Aizawa, H. Ohno and S. Konishi, J. Nucl. Mater. 141-143 (1986) 265. [7] M. Briec. F. Botter, J.J. Abassin, R. Benoit, P. Chenebault, M. Masson, B. Rasneur, P. Sciers, H. Werle and E. Roth, J. Nucl. Mater. 141-143 (1985) 321. [8] H. Kwast, R. Conrad, P. Kennedy, A. Flipot and J.D. Elen, J. Nucl. Mater. 141-143 (1985) 300. [9] H. Werle, J.J. Abassin. M. Briec, R.G. Clemmer, H. Elbel, HE. Hafner. M. Masson, P. Sciers and H. Wedemeyer, J. Nucl. Mater. 141-143 (1985) 321. [lo] R.G. Clemmer, H. Werle and M. Briec, in: Proc. Int.
[II]
[12]
[13]
[14] [15] 1161
j17J [f8]
Symp. on Fusion Reactor Blanket and Fuel C‘ycle I‘echnology, Oct. 27-29. 1986, Tokai. Japan, p. 51. T. Kurasawa, G.W. Hollenberg and H. Watanabe, in: Proc. Int. Symp. Fusion Reactor Blanket & Fuel Cycle Technology, Oct. 27-29. 1986, Tokai, Japan, p. 43. H. Watanabe, T. Kurasawa. E. Roth and D. Vollath. in: Proc. Int. Symp. Fusion Reactor Blanket and Fuel Cycle Technolgy. Oct. 27-29. 1986, Tokai. Japan, p. 33. G.W. Hoilenberg, R.C. Knight. P.J. Densley. L.A. Pember, C.E. Johnson. R.B. Poeppel and L. Yang. J. Nucl. Mater. 141-143 (1986) 271. C.E. Johnson. private communication, from ANL-CMTI8847. p. 24. H.S. Carslow and J.C. Jaeger, Conduction of Heat in Solids. Second Ed. (Clarendon, Oxford, 1959) p. 246. D. Brtinning, D. Guggi and H.R. Ihle, in: Proc. 12th Symp. Fusion Technology, Jiilich, FRG Sept. 13- 17. 1982. p. 543. M. DaBe Donne, Fusion Technol. 9 (1986) 503. R.A. Verrell, J.M. Miller, C.E. Johnson and R.B. Poeppel, to be published in Advances in Ceramics: Proc. Am. Ceramic Sac. Annual Meeting, Pittsbur~, April 26-29, 1987.