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Experimental evidence for tritium release-controlling processes in lithium silicates
Journal of Nuclear Materials 179-181 North-Holland
847
(1991) 847-850
Experimental evidence for tritium release-control~ng lithium silicates
processes in
W. Breitung and H. Werle Association KfK-Euratom, Kernforschungszentrum Karlsruhe GmbH, Institut f% Neutronenphysik und Reaktortechnik, Postfach 3640, D-7500 Karixuhe I, Fed. Rep. Germany
Tritium release from lithium ortho- and metasilicate was investigated in in-pile tests and by out-of-pile annealing. Systematic parameter studies and model calculations indicate that for silicates with gram sizes of several ten pm and for blanket relevant temperatures (573-873 K) ttitium release is mainly controlled by first order reactions at the grain surface.
The reaction kinetics above 573 K is sensitive to the surface coverage with oxygen, which depends on the sample pretreatment and the ambient oxygen partial pressure. A strong time dependence in reaction rates was observed, indicating that the release-controlling reactions proceed at changing or energetically heterogeneous surfaces.
1. Introduction Tritium release from lithium orthosilicate and metasilicate is studied in purged in-pile tests and by out-ofpile annealing The main goal of the in-pile tests is the determination of tritium inventories, whereas the annealing experiments try to identify the tritium releasecontrolling processes. Information on release-controlling processes may be obtained by varying parameters which are assumed to influence tritium release. The results of such systematic parameter studies and conclusions drawn on controlling processes are discussed. Processes which are assumed to control tritium release have been modelled and the calculations are compared with experiments.
Generally it is assumed that the tritium inventory of ceramics will depend on gas-solid equilibration (bulk solubility, adsorption on grain surface) and on kinetic processes (diffusion in grain, desorption from grain surface, grain boundary diffusion, transport in porosities and finally desorption from the outer surface of the sample). In the following we summarize experimental observations which seem useful to clarify which of the above mentioned processes might be release-controlling. The inventory due to gas-solid equilibration (solubility, adsorption) is assumed to be proportional either to the tritium partial pressure or to the square root of the tritium partial pressure. Various in-pile tests, at temperatures between 623 and 873 K and at blanketrelevant tritium partial pressures (about 0.1 Pa), demonstrated that the inventory of orthosilicate, metasilicate and aluminate depends, if at all, only weakly on the purge gas flow rate, i.e. the tritium partial pressure [l-4]. If grain boundary diffusion, transport in porosities or desorption from the the outer surface would control ~22-3115/91/$03.50
tritium release, the tritium inventory or the residence time r should depend sensitively on the sample diameter d, according to r - d2 or to T - d. Out-of-pile release from metasiiicate (I 85% theoretical density, d s 1 mm and d = 5 mm) was found to be independent of d, and from orthosilicate (I 95% theoretical density, 50 pm I d I 5 mm) only weakly (appreciably less than linearly) dependent on d. In agreement with these observations, the ratio of in-pile residence times for 1.25 and 0.5 mm diameter orthosilicate spheres (95% theoretical density) was only about 1.5 [2] giving r - do.&. From these results we conclude that even for high-density silicates, the above mentioned processes are not release controlling. Silicates adsorb large amounts of water in a moist atmosphere. Water release investigations indicated that the water is adsorbed at the grain surface and peaks characteristic for desorption of physi- and chemisorbed water could be identified. Information on which of the two processes, bulk diffusion or reactions at the grain surface, control tritium release may be obtained by comparing tritium release from dry and wet samples. For the metasilicate, tritium is released at higher temperatures than water and tritium release is essentially independent of sample moisture. For the orthosilicate, tritium release from dry and wet samples is very different. In constant rate heating experiments, tritium is released from dry orthosilicate at high temperatures in a peak characteristic for che~deso~tion of water, whereas from wet orthosilicate a large fraction of the tritium is released at low temperatures in a peak which is characteristic for physidesorption of water. A reasonable interpretation of these results is that tritium exchanges easily into adsorbed water and that for our samples with grain diameters of several tens of pm, diffusion in orthosilicate is faster and in metasilicate slower than water desorption. The order of the controlling processes is a basic parameter, which is difficult to determine because generally several processes are involved. For all silicates
0 1991 - Elsevier Science Publishers B.V. born-HolI~d)
848
W. Breitung, H. Werle / Tritium release in lithium silicates
studied here the release rate during long-time annealing at high and constant temperatures generally does not decrease exponentially, as expected for a first order process, but shows a curvature characteristic for a second order process. On the other hand, in annealing experiments with ortho- and metasilicate samples, which differed in the specific activity by more than two orders of magnitude, the kinetics was observed to be essentially independent of the specific activity, indicating first order processes. In previous [1,2,4] and in the presently performed in-pile test LILA/LISA-3 for ortho- and metasilicate, metazirconate and aluminate the kinetics was observed to be independent of, and the inventories to be about proportional to the tritium production rate. This too indicates that the controlling processes obey first order kinetics. After weighing the conflicting results, we tentatively assume that the controlling processes of ortho- and metasilicate are first order reactions. Addition of 0.2 ~01% Oz to high-purity He is very detrimental to in-pile tritium release of orthosilicate, metasilicate and aluminate [l]. In agreement with this result, annealing experiments showed that changes of the oxygen partial pressure (accomplished by purging high-purity He over hot Ti or by replacing the stainless steel by a copper sample chamber) have an appreciable effect on tritium release kinetics. Numerous experiments for all investigated lithium ceramics [1,3,5-71 demonstrated that tritium release is accelerated with increasing hydrogen content in the purge gas (up to several 5%).The influence of moisture in the purge gas was hitherto not so carefully studied, but both in-pile tests and annealing experiments indicate that tritium release is accelerated with increasing moisture. It is not clear if purge gas chemistry directly affects the reaction constants of the assumed first order processes, or if parallel isotopic exchange reactions take place. The chemical form of the tritium released from the ceramic is difficult to determine because it can be strongly changed by the sample environment. When annealing orthosilicate in the reducing atmosphere of a
0.0
2.5 2
5.0
7.5 Time
10.0 (seconds)
l2.5
l5.0
n.5
200 20.0 *Id
stainless steel capsule, for both He and He + 1 ~01% H,, essentially all detected tritium is gaseous, whereas in the nonreducing atmosphere of a copper capsule the gaseous fraction is less than lo-20% for He and less than 20-40% for He + 1 ~01% H,. To summarize, we currently assume that for our lithium silicate samples (grain size several tens of pm), equilibration processes, grain boundary diffusion, transport in porosities and desorption from the outer surface are not release controlling. For orthosilicate, tritium release is controlled by first order reactions at the grain surface and for metasilicate it is, in addition, influenced by bulk diffusion. In agreement with ref. [8], we assume that the primary form of the released tritium is mainly water. 3. Evaluation of experiments 3.1. General model assumptions Only HTO release is modeled here. It is further assumed that any rate-limiting reactions proceed at an average grain surface and that they are of first order. Bulk diffusion of tritium to the grain surface is explicitly treated, but it was never found to control release from our samples. The present model differs from other models [9-121 mainly with respect to the chemical form of released tritium (no HT contribution) and with respect to the location of the reactions (grain surface, not the solid-gas interface). 3.2. Analysis of ZYI -tests The ZYl-tests used high-density lithium orthosilicate spheres with an average grain diameter of about 20 pm. The samples were predried by heating at 5 K/min to 1073K. The total tritium inventory after neutron irradiation was about 10’ Bq. The tritium release was measured for a 5 K/min heating ramp up to 1096 K. Test ZYl-K7Pl employed 10 SCCM of He with 1 ~01% of
0.0
2.5
b
5.0
7.5
Time
10.0
l2.5
(seconds)
15.0
17.5
20.0
*Id
Fig. 1. Measured and calculated tntium release rate from high-density, 0.5 mm diameter orthosilicate spheres for a 5 K/min heating rate. The purge gas used was (a) He with 1 vol.% H,, and (b) pure He.
W. Breitung, H. Werle / Tritium release in lithium silicates 4 10
3
@Js L+ B zz ‘2
Grain diameter(p)
5-10
50-100
400-500
Fig. 2. Tritium release from high-density orthosilicate samples with different grain diameters for a 5 K/min heating rate. Purge gas was He.
H, as purge gas, while test ZYI-K6Pl used 10 SCCM of He. Some prominent data points obtained in tests K7Pl and K6Pl are plotted in figs. la and lb, respectively. Five reactions can be distinguished. The first three reactions (labelled HTOl, HT02 and HT03 in the caption) are probably due to desorption of HTO molecules, because very similar peak locations were observed in water release measurements. We assign the fourth reaction (OT/OHl in the legend) to the diffusion of tritium to the grain surface, succeeded by OH/OT recombination to HTO and desorption. The inventory participating in reaction 4 is assumed to be within the grain, because the peak of reaction 4 shifts to lower temperatures with decreasing grain diameter (fig. 2). If the inventory participating in reaction 4 was located at grain surfaces only, no shift should occur in a linear heating experiment. The fifth reaction (OT/OH2 in the legend) could be due to HTO molecules which were generated the same way as those of reaction 4, but which were trapped at the surface. This inventory is assumed to be located at the grain surfaces. For each first order reaction three quantities must be defined: the participating fractional inventory I, the pre-exponential constant k,, and the activation energy AH of the reaction rate constant k = k, exp( - AH/ RT). Values for I, k, and AH were obtained from the measured peak height, the peak temperature and the peak width. The model calculations are compared to measured data in figs. la and lb. The calculations for test K7Pl (fig. la) indicate that the activation energy for the HTO release was about 40, 70 and 80 kJ/mol, respectively. HTO seems to reside in one loosely and two strongly bound states. The corresponding fractional inventories were 22, 9% and 10% of the total inventory. The HTO release is a result of the initial water coverage, which depends on the sample pretreatment. In highly dried samples of orthosilicate (9 h in vacuum at 1173 K), only remnants of reactions 2
849
and 3 are visible (fig. 2). With a hydrogen-containing purge gas, reaction 4 provides the dominant release peak. The peak location is consistent with that of the vacuum-dried samples (fig. 2). In order to fit the measured peak, it is necessary to increase the activation energy slightly during annealing (92 to 97 kJ/mol). About 60% of the total tritium inventory was released via reaction 4. Model modifications to explain the slow release at later times also with reaction 4 were unsuccessful. These attempts included diffusion-controlled release from the average 20 pm grain, surface controlled release from large grains and a second order reaction. Only a fifth reaction (OT/OH2 in the legend), allows the measured late release to be reproduced (fig. la). However, the reaction rate decreased significantly during the course of the experiment. In the calculation the reduction in the reaction rate was ascribed to a AH change only. The apparent activation energy necessary to reproduce the data increased from 75 to 125 M/mole. Fig. lb compares measured and calculated tritium release for a pure He purge gas. The three HTO peaks appeared at practically the same temperature as in test K7Pl. The released inventories were slighty lower (2, 4 and 6% of the total T) and the activation energies were identical. Release from these peaks seems to be insensitive to hydrogen in the purge gas, i.e. to the proton coverage. This is another indication that these reactions represent release of already existing HTO molecules. Reaction 4 involved about the same fractional inventory as in K7P1, namely 55%. Initially the release through reaction 4 shows the same kinetics as in K7P1, however, at later times the reaction velocity decreased significantly. This can be modelled by an activation energy increasing from 92 kJ/mole (as in K7Pl) to around 130 kJ/mole. With He purge gas, the rate of reaction 4 decreases in a similar way as reaction 5 in the test with 1% H, in the purge gas.The apparent inventory of reaction 5 was significantly higher in K6Pl (50% vs 16%) whereas the increase of the activation energy was similar in both tests. It appears that with He purge gas, a much larger fraction of the tritium inventory is first trapped, and later released through reaction 5. 3.3. Interpretation
of ZYI -results
Experiments and calculations dealing with the interaction of H,O molecules with oxide surfaces have shown that water is bonded more strongly to the surface in the presence of co-adsorbed oxygen [13]. Calculations using molecular orbital theory and cluster models of Fe0 found that in the presence of adsorbed oxygen, H,O bonds more strongly to the surface [14]. The very slow reaction 5 may correspond to the release of HTO molecules bonded to surface adsorbed oxygen. With low oxygen partial pressure and small oxygen coverage (test K7Pl) only a small fraction of the tritium inventory is trapped at the surface. With high oxygen partial pres-
850
W. Breitung, H. Weile / Tritium release in lithium silicates
sure (test K6Pl) a large fraction of the tritium is released via reaction 5. Orthosilicate samples dried in vacuum at 900°C for 9 h released almost their total inventory through reaction 4 (fig. 2) because the oxygen coverage was very low. The result of a continuously increasing activation energy can be explained by a heterogeneous surface, which contains a large number of energetically different sites. HTO is released from sites with progressively increasing bonding strength. Both the reaction constant and the activation energy of reaction 5 at long times approach the value observed in-pile (LISA-2). This suggests that the in-pile results may be strongly influenced by adsorbed oxygen, despite the 0.1% Hz in the purge gas. For very low oxygen coverages, reaction 4 seems to represent the rate-controlling process (fig. 2), probably being OH/OT recombination with subsequent desorption from an unoxidized surface. The interpretation of similar tests for metasilicate also required an increase of the activiation energy from initially 70 to 210 kJ/mole at the end of the annealing
WI. 4. Conclusions Both the test series and the calculations indicate that for ortho- and metasilicate, the tritium release is dominated by reactions proceeding at grain surfaces. For the orthosilicate, adsorbed moisture causes low temperature desorption (up to about 573 K) of HTO. The release reactions at higher temperatures (presumably OH/OT and OT/OT recombination) are sensitive to the oxygen surface coverage, which can be influenced by the sample pretreatment and the ambient oxygen partial pressure during the annealing. The strong time dependence of the reaction rates (k4 and k,) suggests
that the reactions proceed heterogeneous surfaces.
at changing
or energetically
References [l] H. Werle, J.J. Abassin, M. Briec, R.G. Clemmer, H. Elbel, H.E. H;ifner, M. Masson, P. Sciers and H. Wedemeyer, J. Nucl. Mater. 141-143 (1986) 321. [2] H. Were, W. Breitung, M. Briec, R.G. Clemmer, H. Elbel, H.E. Hlfner, M. Masson, G. Schumacher and H. Wedemeyer, J. Nucl. Mater. 155-157 (1988) 538. [3] M. Briec, J.J. Abassin, M. Masson, E. Roth, P. Sciers and H. Werle, J. Nucl. Mater. 155-157 (1988) 549. [4] W. Breitung, M. Briec and H. Werle. Fusion Eng. Des. 8 (1989) 323.PI M. Briec, J.J. Abassin, C.E. Johnson, M. Masson, N. Roux and H. Watanabe, in: Fusion Technology 1988, Vol. 1, Eds. A.M. Van Ingen, A. Jijsen and H.T. Klippel (Elsevier, Amsterdam, 1989) p. 1105. WI T. Kurasawa, H. Watanabe, E. Roth and D. Vollath, J. Nucl. Mater. 155-157 (1988) 544. 171 S. Tanaka, A. Kawamoto, M. Yamawaki, T. Terai, Y. Takahashi, H. Kawamura and M. Saito, Fusion Eng. Des. 8 (1988) 155. PI K. Okuno and H. Kudo, Fusion Eng. Des. 8 (1988) 355. [91 J.P. Kopasz and C.E. Johnson, in: Proc. 2nd Specialists’ Workshop on Modelling Tritium Behavior in USA, April 27-28,1989, Ed. C.E. Johnson (Int. Energy Agency) p. 13. WI A.R. Raffray, G. Federici and M. Abdon, ibid. ref. [9], p, 81. 1111 M.C. Billone, ibid. ref. [9], p. 135. WI T. Terai, Y. Takahashi, S. Tanaka and M. Yamawaki, Fusion Eng. Des. 8 (1989) 349. u31 M. Egashira, M. Nakashima, S. Kawasumi and T. Seiyama, J. Phys. Chem. 85 (1981) 4125. [I41 N.C. Debnath and A.B. Anderson, Surf. Sci. 128 (1983) 61. 1151 W. Breitung, J. Lebkucher and H. Werle, ibid. ref. [I], p. 45.
847
(1991) 847-850
Experimental evidence for tritium release-control~ng lithium silicates
processes in
W. Breitung and H. Werle Association KfK-Euratom, Kernforschungszentrum Karlsruhe GmbH, Institut f% Neutronenphysik und Reaktortechnik, Postfach 3640, D-7500 Karixuhe I, Fed. Rep. Germany
Tritium release from lithium ortho- and metasilicate was investigated in in-pile tests and by out-of-pile annealing. Systematic parameter studies and model calculations indicate that for silicates with gram sizes of several ten pm and for blanket relevant temperatures (573-873 K) ttitium release is mainly controlled by first order reactions at the grain surface.
The reaction kinetics above 573 K is sensitive to the surface coverage with oxygen, which depends on the sample pretreatment and the ambient oxygen partial pressure. A strong time dependence in reaction rates was observed, indicating that the release-controlling reactions proceed at changing or energetically heterogeneous surfaces.
1. Introduction Tritium release from lithium orthosilicate and metasilicate is studied in purged in-pile tests and by out-ofpile annealing The main goal of the in-pile tests is the determination of tritium inventories, whereas the annealing experiments try to identify the tritium releasecontrolling processes. Information on release-controlling processes may be obtained by varying parameters which are assumed to influence tritium release. The results of such systematic parameter studies and conclusions drawn on controlling processes are discussed. Processes which are assumed to control tritium release have been modelled and the calculations are compared with experiments.
Generally it is assumed that the tritium inventory of ceramics will depend on gas-solid equilibration (bulk solubility, adsorption on grain surface) and on kinetic processes (diffusion in grain, desorption from grain surface, grain boundary diffusion, transport in porosities and finally desorption from the outer surface of the sample). In the following we summarize experimental observations which seem useful to clarify which of the above mentioned processes might be release-controlling. The inventory due to gas-solid equilibration (solubility, adsorption) is assumed to be proportional either to the tritium partial pressure or to the square root of the tritium partial pressure. Various in-pile tests, at temperatures between 623 and 873 K and at blanketrelevant tritium partial pressures (about 0.1 Pa), demonstrated that the inventory of orthosilicate, metasilicate and aluminate depends, if at all, only weakly on the purge gas flow rate, i.e. the tritium partial pressure [l-4]. If grain boundary diffusion, transport in porosities or desorption from the the outer surface would control ~22-3115/91/$03.50
tritium release, the tritium inventory or the residence time r should depend sensitively on the sample diameter d, according to r - d2 or to T - d. Out-of-pile release from metasiiicate (I 85% theoretical density, d s 1 mm and d = 5 mm) was found to be independent of d, and from orthosilicate (I 95% theoretical density, 50 pm I d I 5 mm) only weakly (appreciably less than linearly) dependent on d. In agreement with these observations, the ratio of in-pile residence times for 1.25 and 0.5 mm diameter orthosilicate spheres (95% theoretical density) was only about 1.5 [2] giving r - do.&. From these results we conclude that even for high-density silicates, the above mentioned processes are not release controlling. Silicates adsorb large amounts of water in a moist atmosphere. Water release investigations indicated that the water is adsorbed at the grain surface and peaks characteristic for desorption of physi- and chemisorbed water could be identified. Information on which of the two processes, bulk diffusion or reactions at the grain surface, control tritium release may be obtained by comparing tritium release from dry and wet samples. For the metasilicate, tritium is released at higher temperatures than water and tritium release is essentially independent of sample moisture. For the orthosilicate, tritium release from dry and wet samples is very different. In constant rate heating experiments, tritium is released from dry orthosilicate at high temperatures in a peak characteristic for che~deso~tion of water, whereas from wet orthosilicate a large fraction of the tritium is released at low temperatures in a peak which is characteristic for physidesorption of water. A reasonable interpretation of these results is that tritium exchanges easily into adsorbed water and that for our samples with grain diameters of several tens of pm, diffusion in orthosilicate is faster and in metasilicate slower than water desorption. The order of the controlling processes is a basic parameter, which is difficult to determine because generally several processes are involved. For all silicates
0 1991 - Elsevier Science Publishers B.V. born-HolI~d)
848
W. Breitung, H. Werle / Tritium release in lithium silicates
studied here the release rate during long-time annealing at high and constant temperatures generally does not decrease exponentially, as expected for a first order process, but shows a curvature characteristic for a second order process. On the other hand, in annealing experiments with ortho- and metasilicate samples, which differed in the specific activity by more than two orders of magnitude, the kinetics was observed to be essentially independent of the specific activity, indicating first order processes. In previous [1,2,4] and in the presently performed in-pile test LILA/LISA-3 for ortho- and metasilicate, metazirconate and aluminate the kinetics was observed to be independent of, and the inventories to be about proportional to the tritium production rate. This too indicates that the controlling processes obey first order kinetics. After weighing the conflicting results, we tentatively assume that the controlling processes of ortho- and metasilicate are first order reactions. Addition of 0.2 ~01% Oz to high-purity He is very detrimental to in-pile tritium release of orthosilicate, metasilicate and aluminate [l]. In agreement with this result, annealing experiments showed that changes of the oxygen partial pressure (accomplished by purging high-purity He over hot Ti or by replacing the stainless steel by a copper sample chamber) have an appreciable effect on tritium release kinetics. Numerous experiments for all investigated lithium ceramics [1,3,5-71 demonstrated that tritium release is accelerated with increasing hydrogen content in the purge gas (up to several 5%).The influence of moisture in the purge gas was hitherto not so carefully studied, but both in-pile tests and annealing experiments indicate that tritium release is accelerated with increasing moisture. It is not clear if purge gas chemistry directly affects the reaction constants of the assumed first order processes, or if parallel isotopic exchange reactions take place. The chemical form of the tritium released from the ceramic is difficult to determine because it can be strongly changed by the sample environment. When annealing orthosilicate in the reducing atmosphere of a
0.0
2.5 2
5.0
7.5 Time
10.0 (seconds)
l2.5
l5.0
n.5
200 20.0 *Id
stainless steel capsule, for both He and He + 1 ~01% H,, essentially all detected tritium is gaseous, whereas in the nonreducing atmosphere of a copper capsule the gaseous fraction is less than lo-20% for He and less than 20-40% for He + 1 ~01% H,. To summarize, we currently assume that for our lithium silicate samples (grain size several tens of pm), equilibration processes, grain boundary diffusion, transport in porosities and desorption from the outer surface are not release controlling. For orthosilicate, tritium release is controlled by first order reactions at the grain surface and for metasilicate it is, in addition, influenced by bulk diffusion. In agreement with ref. [8], we assume that the primary form of the released tritium is mainly water. 3. Evaluation of experiments 3.1. General model assumptions Only HTO release is modeled here. It is further assumed that any rate-limiting reactions proceed at an average grain surface and that they are of first order. Bulk diffusion of tritium to the grain surface is explicitly treated, but it was never found to control release from our samples. The present model differs from other models [9-121 mainly with respect to the chemical form of released tritium (no HT contribution) and with respect to the location of the reactions (grain surface, not the solid-gas interface). 3.2. Analysis of ZYI -tests The ZYl-tests used high-density lithium orthosilicate spheres with an average grain diameter of about 20 pm. The samples were predried by heating at 5 K/min to 1073K. The total tritium inventory after neutron irradiation was about 10’ Bq. The tritium release was measured for a 5 K/min heating ramp up to 1096 K. Test ZYl-K7Pl employed 10 SCCM of He with 1 ~01% of
0.0
2.5
b
5.0
7.5
Time
10.0
l2.5
(seconds)
15.0
17.5
20.0
*Id
Fig. 1. Measured and calculated tntium release rate from high-density, 0.5 mm diameter orthosilicate spheres for a 5 K/min heating rate. The purge gas used was (a) He with 1 vol.% H,, and (b) pure He.
W. Breitung, H. Werle / Tritium release in lithium silicates 4 10
3
@Js L+ B zz ‘2
Grain diameter(p)
5-10
50-100
400-500
Fig. 2. Tritium release from high-density orthosilicate samples with different grain diameters for a 5 K/min heating rate. Purge gas was He.
H, as purge gas, while test ZYI-K6Pl used 10 SCCM of He. Some prominent data points obtained in tests K7Pl and K6Pl are plotted in figs. la and lb, respectively. Five reactions can be distinguished. The first three reactions (labelled HTOl, HT02 and HT03 in the caption) are probably due to desorption of HTO molecules, because very similar peak locations were observed in water release measurements. We assign the fourth reaction (OT/OHl in the legend) to the diffusion of tritium to the grain surface, succeeded by OH/OT recombination to HTO and desorption. The inventory participating in reaction 4 is assumed to be within the grain, because the peak of reaction 4 shifts to lower temperatures with decreasing grain diameter (fig. 2). If the inventory participating in reaction 4 was located at grain surfaces only, no shift should occur in a linear heating experiment. The fifth reaction (OT/OH2 in the legend) could be due to HTO molecules which were generated the same way as those of reaction 4, but which were trapped at the surface. This inventory is assumed to be located at the grain surfaces. For each first order reaction three quantities must be defined: the participating fractional inventory I, the pre-exponential constant k,, and the activation energy AH of the reaction rate constant k = k, exp( - AH/ RT). Values for I, k, and AH were obtained from the measured peak height, the peak temperature and the peak width. The model calculations are compared to measured data in figs. la and lb. The calculations for test K7Pl (fig. la) indicate that the activation energy for the HTO release was about 40, 70 and 80 kJ/mol, respectively. HTO seems to reside in one loosely and two strongly bound states. The corresponding fractional inventories were 22, 9% and 10% of the total inventory. The HTO release is a result of the initial water coverage, which depends on the sample pretreatment. In highly dried samples of orthosilicate (9 h in vacuum at 1173 K), only remnants of reactions 2
849
and 3 are visible (fig. 2). With a hydrogen-containing purge gas, reaction 4 provides the dominant release peak. The peak location is consistent with that of the vacuum-dried samples (fig. 2). In order to fit the measured peak, it is necessary to increase the activation energy slightly during annealing (92 to 97 kJ/mol). About 60% of the total tritium inventory was released via reaction 4. Model modifications to explain the slow release at later times also with reaction 4 were unsuccessful. These attempts included diffusion-controlled release from the average 20 pm grain, surface controlled release from large grains and a second order reaction. Only a fifth reaction (OT/OH2 in the legend), allows the measured late release to be reproduced (fig. la). However, the reaction rate decreased significantly during the course of the experiment. In the calculation the reduction in the reaction rate was ascribed to a AH change only. The apparent activation energy necessary to reproduce the data increased from 75 to 125 M/mole. Fig. lb compares measured and calculated tritium release for a pure He purge gas. The three HTO peaks appeared at practically the same temperature as in test K7Pl. The released inventories were slighty lower (2, 4 and 6% of the total T) and the activation energies were identical. Release from these peaks seems to be insensitive to hydrogen in the purge gas, i.e. to the proton coverage. This is another indication that these reactions represent release of already existing HTO molecules. Reaction 4 involved about the same fractional inventory as in K7P1, namely 55%. Initially the release through reaction 4 shows the same kinetics as in K7P1, however, at later times the reaction velocity decreased significantly. This can be modelled by an activation energy increasing from 92 kJ/mole (as in K7Pl) to around 130 kJ/mole. With He purge gas, the rate of reaction 4 decreases in a similar way as reaction 5 in the test with 1% H, in the purge gas.The apparent inventory of reaction 5 was significantly higher in K6Pl (50% vs 16%) whereas the increase of the activation energy was similar in both tests. It appears that with He purge gas, a much larger fraction of the tritium inventory is first trapped, and later released through reaction 5. 3.3. Interpretation
of ZYI -results
Experiments and calculations dealing with the interaction of H,O molecules with oxide surfaces have shown that water is bonded more strongly to the surface in the presence of co-adsorbed oxygen [13]. Calculations using molecular orbital theory and cluster models of Fe0 found that in the presence of adsorbed oxygen, H,O bonds more strongly to the surface [14]. The very slow reaction 5 may correspond to the release of HTO molecules bonded to surface adsorbed oxygen. With low oxygen partial pressure and small oxygen coverage (test K7Pl) only a small fraction of the tritium inventory is trapped at the surface. With high oxygen partial pres-
850
W. Breitung, H. Weile / Tritium release in lithium silicates
sure (test K6Pl) a large fraction of the tritium is released via reaction 5. Orthosilicate samples dried in vacuum at 900°C for 9 h released almost their total inventory through reaction 4 (fig. 2) because the oxygen coverage was very low. The result of a continuously increasing activation energy can be explained by a heterogeneous surface, which contains a large number of energetically different sites. HTO is released from sites with progressively increasing bonding strength. Both the reaction constant and the activation energy of reaction 5 at long times approach the value observed in-pile (LISA-2). This suggests that the in-pile results may be strongly influenced by adsorbed oxygen, despite the 0.1% Hz in the purge gas. For very low oxygen coverages, reaction 4 seems to represent the rate-controlling process (fig. 2), probably being OH/OT recombination with subsequent desorption from an unoxidized surface. The interpretation of similar tests for metasilicate also required an increase of the activiation energy from initially 70 to 210 kJ/mole at the end of the annealing
WI. 4. Conclusions Both the test series and the calculations indicate that for ortho- and metasilicate, the tritium release is dominated by reactions proceeding at grain surfaces. For the orthosilicate, adsorbed moisture causes low temperature desorption (up to about 573 K) of HTO. The release reactions at higher temperatures (presumably OH/OT and OT/OT recombination) are sensitive to the oxygen surface coverage, which can be influenced by the sample pretreatment and the ambient oxygen partial pressure during the annealing. The strong time dependence of the reaction rates (k4 and k,) suggests
that the reactions proceed heterogeneous surfaces.
at changing
or energetically
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