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18F presence in neutron-irradiated γ-LiAlO2
Journal of Nuclear Materials 189 (1992) 233-235 North-Holland
Letter to the Editors
“F presence in neutron-irradiated J. Jimenez-Becerril
y-LiAlC&
‘, P. Bosch ‘yband S. Bulbulian a
a lnst~tuto National de ~n~~estigacionesNucleares, A.P. 18-1027, Cal. Escanddn. DelegacGn Miguel Hidalgo, C.P. 11801, Mexico D. F., Mexico ’ Universidad Autbnoma Metropolitana, Iztapalapa, Departamento de Quuimica, Michoaccin Esq. Purisima, Iztapalapa, Mexico D. F., Mexico
Received 21 January 1992; accepted 6 April 1992
1. Introduction
‘H can be generated by neutron irradiation of lithium compounds, but if the compound contains oxygen, then 18F may be simultaneously produced. The recoiling ‘H atoms act on I60 as shown in the following reactions: ‘Li(n, CU)~H,
(I)
‘hO(DH, n)18F.
(2)
Reaction (1) is an exothermic nuclear reaction with a Q-value of 4.78 MeV and a rather large thermal neutron absorption cross section of 940 b. The initial recoil energy of the tritons is 2.74 MeV. At this energy the triton range is approximately 100 p,rn in LizCO,. However, because tritons lose their energy very rapidly by interacting with electrons and nuclei, the maximum practical range to induce ‘sF formation upon bombardment of 160 atoms is only 70 urn. Reaction (2) is also exothermic with a Q-value of 1.26 MeV and a threshold energy of 0.6 MeV. The cross section is a function of the triton energy and decreases very rapidly as the residual energy of the triton decreases. At the initial triton kinetic energy of 2.74 MeV. the reaction cross section is 650 mb;*however, the average cross section over the effective range for ‘*F production is only 87 mb and, as discussed before, the effective triton range over which *sF is produced is approximately 70 pm. Considering this information, it can be found that for every 10” atoms of “H produced in the nuclear reactor as a consequence of reaction (11, at least one atom of “F is also produced as a consequence of reaction (2). Roth and collaborators 11.21have studied the rate of 0022-3115/92/$05.00
tritium extraction from short-term neutron-irradiated y-LiAlO, in laboratory or in-pile experiments. Tritium was recovered from a blanket swept with an inert gas mixed with 0.1% hydrogen. Lithium aluminate was chosen because it satisfied the basic solid breeder blanket performance requirements such as lithium release. It also exhibited thermophysical, chemical and mechanical stabilities at high temperatures. Furthermore, this ceramic was compatible with other blanket and structural materials, and it had the desired irradiation behavior. Roth et al. [2] indicated that “F was not observed in their tritium recovery experiments. They made the point that if “F was produced in the irradiation of y-LiAiO, ceramics it must have reacted with structural members to form a stable fluoride and it remained fixed long enough so that *aF decayed before being observed. Lastly it has to be mentioned that Kwast [3] did not find any “F. In the TRIO experiment care was taken to analyze nuclides in the sweep gas and “F was not among them [4]. Hence, the aims of the present paper are to find out whether “F is formed when neutron-irradiated yLiAIO, is heated and to establish a correlation between the nature of the sweep gas mixture and the ‘sF formed.
2. Experimental 2.1. Neutron
irradiation
of the lithium aluminate
samples
y-LiAlO, prepared as mentioned in a previous work [5] was used. 100 mg samples were sealed in small
0 1992 - Elsevier Science Publishers B.V. All rights reserved
polyethylene capsules and inserted into standard polyethylene irradiation containers for irradiation for 15 min at a flux of 10’h-10’7 n/m’ s in the TRIGA Mark III reactor at Salazar, Mexico. Immediately after being removed from the reactor, the samples wcrc taken to the radiochemistry laboratory where the containers were cut open and the y-LiAlO, analyzed. The neutron-irradiated y-LiAlO, samples were introduced into folded quartz tubes and were heated at various temperatures during IS-150 min. Volatile species were swept with a carrier gas (He, H, or AIwith 0.1% H, added). The outlet gases from the heating quartz tube were collected into a NH,OH trap.
The estimation of sample activities was expressed as the fraction of the total activity of “F. The recovery of fluorine from the solid was determined by measurements in both the NH,OH solution and the solid. “F was measured by integrating the 0.511 MeV positron annihilation peak either with a NaI solid-state detector coupled to a single-channel analyzer or with a Ge/hypcrpure solid-state detector coupIed to a 4096 channet pulse height analyzer. The raw data measurement were corrected for background and for ‘“F decay in the samples. iSF species in the solutions were analyzed by paper chromatography and high voltage paper electrophoresis.
3. Results 3.1. Temperature fzow Results for ‘sF recovery from neutron-irradiated y-LiAlO, heated at 500-800°C are given in table 1. “F was found to be mobile at temperatures higher than 500°C. At 600°C the recovered lXF was significant (37.4% after 1.50 min). The release of fluorine in-
(“0
“F removed ( r%)
500 600
0.0 37.4
700 800
72.6 83.1
Temperature
/’
/
/
60 t
/ 1
43c 1
/ ~:_,:
,.,,
;,;I_
,
I /
!
J 0
20
40
60
lx)
80
I40
I60
time (minuE) Fig. 1. Removal of “F at 800°C with Ar plus 0.1 V kt: sweep gas. percentage “F removed versus time.
2.2. “F actil?ty measurements
Table I ‘“F removed through r-LiAlO, ‘. at various temperatures ing 150 min, with Ar plus 0.1% hydrogen sweep gas
1
30
creased rapidly with temperature of “F had been recovered.
until. at 800°C. 83%
3.2. Heating time effect Fig. 1 shows the recovery rate of “F in y-LiAlO, heated at 800°C for 15-150 min. Recovered ‘“F amount is higher for longer heating times. Furthermore the ‘“F recovered is proportional to heating time at this temperature. 3.3. Sweep gas effect Table 2 shows the effect of II1 presence in the sweep gas mixture. If no Hz is included no tluorinc is recovered at 600°C after 150 min. An amount as small as 0.1% of hydrogen in argon gives rise to 38.8% of fluorine. When pure hydrogen is used, the recovered fluorine at 600°C is 100%. Indeed, no “F was found in the solid. 3.4. Analysis
of ,fluorine species
Chromatographic and eIectr(~phoretic analyses ion. During the showed that “F was present at ‘“F
dur-
Table 2 ‘“F removed with different 600°C during 150 min
“F(‘%)
Swept gas He hr+O.l% 11,
sweep gases through
.-.---
-_____H,
0.0 37.4 100.0
y-tiAtOz
-_--~-
:it
J. Jimenez-Becerril et al. / “Fpresence
process the fluorine was, indeed, trapped immediately either in the NH,OH trap or in an iron chip trap.
4. Discussion
and conclusions
lXF release behavior from neutron-irradiated LiAlO, powders has been investigated at various temperatures. It was found that the “F diffuses through the solid and the amount released depended strongly on the composition of the sweep gas, on the temperature, and on the heating time. It was shown that the amount of hydrogen in the sweep gas affects considerably the amount of fluorine swept. The mobilization of “F with the presence of a small amount of H, (0.1%) in argon shows that a chemical reaction between H, and ‘sF very probably produces HF which is easily volatilized. When pure He is utilized, ‘sF_ and 3H+ do not combine to form HF gas although the ratio sH’/ ‘*F- is very large (= 10”). This can be explained considering that the amount of 3H atoms produced in the solid has a very low concentration (one 3H atom per approximately 105-lo6 LiAlO, molecules), and hence the 3H18F formed is insignificant. Tritium release experiments in neutron-irradiated y-LiAlO, using purge gas containing H, are conducted on a regular basis. As yet Roth [1,2] is the only experimentalist to address the issue and he makes a case as to why ‘sF should not have been seen. No one else has reported having observed the presence of lXF in the swept gas, probably because it reacts readily with metal tubing of the tritium recovery equipment.
in neutron-irradiated y-LiAlO,
235
Acknowledgements The authors would like to express appreciation to Dr. E. Roth for having suggested the present work. We would also like to thank the technicians of the chemistry department for their help in the laboratory, the staff of the reactor for the irradiations and V.H. Lara for technical help in the X-ray diffraction measurements.
References
[ll M. Briec, F. Bolter,
J.J. Abassin, R. Benoit, P. Chenebault, M. Masson, B. Rasneur, P. Sciers, H. Werle and E. Roth, J. Nucl. Mater. 141-143 (19860 357. 121E. Roth, F. Botter, M. Briec, M. Rostaing, H. Werle and R.G. Clemmer, J. Nucl. Mater. 141-143 (1986) 275. mentioned by Roth et [31 Dr. Kwast, personal communication al. in ref. [I]. 141 R.G. Clemmer, P.A. Finn, R.F. Malecha, B. Misra, M.C. Billone, D.L. Bowers, A.K. Fischer, L.R. Greenwood, R.F. Mattas, S.W. Tam, R.B. Poeppel, G.T. Reedy, LT. Dudley, F.F. Dyer, E.D. Clemmer, J.S. Watson, P.W. Fisher, J.R. Conlin, R.L. Childs, J.L. Scott, M. Arons and A.E. Scandora, Argonne National Laboratory, Report ANL-8455 (1984). P. Bosch and S. Bulbulian, J. Nucl. [51 J. Jimenez-Becerril, Mater. 185 (1991) 304.
Letter to the Editors
“F presence in neutron-irradiated J. Jimenez-Becerril
y-LiAlC&
‘, P. Bosch ‘yband S. Bulbulian a
a lnst~tuto National de ~n~~estigacionesNucleares, A.P. 18-1027, Cal. Escanddn. DelegacGn Miguel Hidalgo, C.P. 11801, Mexico D. F., Mexico ’ Universidad Autbnoma Metropolitana, Iztapalapa, Departamento de Quuimica, Michoaccin Esq. Purisima, Iztapalapa, Mexico D. F., Mexico
Received 21 January 1992; accepted 6 April 1992
1. Introduction
‘H can be generated by neutron irradiation of lithium compounds, but if the compound contains oxygen, then 18F may be simultaneously produced. The recoiling ‘H atoms act on I60 as shown in the following reactions: ‘Li(n, CU)~H,
(I)
‘hO(DH, n)18F.
(2)
Reaction (1) is an exothermic nuclear reaction with a Q-value of 4.78 MeV and a rather large thermal neutron absorption cross section of 940 b. The initial recoil energy of the tritons is 2.74 MeV. At this energy the triton range is approximately 100 p,rn in LizCO,. However, because tritons lose their energy very rapidly by interacting with electrons and nuclei, the maximum practical range to induce ‘sF formation upon bombardment of 160 atoms is only 70 urn. Reaction (2) is also exothermic with a Q-value of 1.26 MeV and a threshold energy of 0.6 MeV. The cross section is a function of the triton energy and decreases very rapidly as the residual energy of the triton decreases. At the initial triton kinetic energy of 2.74 MeV. the reaction cross section is 650 mb;*however, the average cross section over the effective range for ‘*F production is only 87 mb and, as discussed before, the effective triton range over which *sF is produced is approximately 70 pm. Considering this information, it can be found that for every 10” atoms of “H produced in the nuclear reactor as a consequence of reaction (11, at least one atom of “F is also produced as a consequence of reaction (2). Roth and collaborators 11.21have studied the rate of 0022-3115/92/$05.00
tritium extraction from short-term neutron-irradiated y-LiAlO, in laboratory or in-pile experiments. Tritium was recovered from a blanket swept with an inert gas mixed with 0.1% hydrogen. Lithium aluminate was chosen because it satisfied the basic solid breeder blanket performance requirements such as lithium release. It also exhibited thermophysical, chemical and mechanical stabilities at high temperatures. Furthermore, this ceramic was compatible with other blanket and structural materials, and it had the desired irradiation behavior. Roth et al. [2] indicated that “F was not observed in their tritium recovery experiments. They made the point that if “F was produced in the irradiation of y-LiAiO, ceramics it must have reacted with structural members to form a stable fluoride and it remained fixed long enough so that *aF decayed before being observed. Lastly it has to be mentioned that Kwast [3] did not find any “F. In the TRIO experiment care was taken to analyze nuclides in the sweep gas and “F was not among them [4]. Hence, the aims of the present paper are to find out whether “F is formed when neutron-irradiated yLiAIO, is heated and to establish a correlation between the nature of the sweep gas mixture and the ‘sF formed.
2. Experimental 2.1. Neutron
irradiation
of the lithium aluminate
samples
y-LiAlO, prepared as mentioned in a previous work [5] was used. 100 mg samples were sealed in small
0 1992 - Elsevier Science Publishers B.V. All rights reserved
polyethylene capsules and inserted into standard polyethylene irradiation containers for irradiation for 15 min at a flux of 10’h-10’7 n/m’ s in the TRIGA Mark III reactor at Salazar, Mexico. Immediately after being removed from the reactor, the samples wcrc taken to the radiochemistry laboratory where the containers were cut open and the y-LiAlO, analyzed. The neutron-irradiated y-LiAlO, samples were introduced into folded quartz tubes and were heated at various temperatures during IS-150 min. Volatile species were swept with a carrier gas (He, H, or AIwith 0.1% H, added). The outlet gases from the heating quartz tube were collected into a NH,OH trap.
The estimation of sample activities was expressed as the fraction of the total activity of “F. The recovery of fluorine from the solid was determined by measurements in both the NH,OH solution and the solid. “F was measured by integrating the 0.511 MeV positron annihilation peak either with a NaI solid-state detector coupled to a single-channel analyzer or with a Ge/hypcrpure solid-state detector coupIed to a 4096 channet pulse height analyzer. The raw data measurement were corrected for background and for ‘“F decay in the samples. iSF species in the solutions were analyzed by paper chromatography and high voltage paper electrophoresis.
3. Results 3.1. Temperature fzow Results for ‘sF recovery from neutron-irradiated y-LiAlO, heated at 500-800°C are given in table 1. “F was found to be mobile at temperatures higher than 500°C. At 600°C the recovered lXF was significant (37.4% after 1.50 min). The release of fluorine in-
(“0
“F removed ( r%)
500 600
0.0 37.4
700 800
72.6 83.1
Temperature
/’
/
/
60 t
/ 1
43c 1
/ ~:_,:
,.,,
;,;I_
,
I /
!
J 0
20
40
60
lx)
80
I40
I60
time (minuE) Fig. 1. Removal of “F at 800°C with Ar plus 0.1 V kt: sweep gas. percentage “F removed versus time.
2.2. “F actil?ty measurements
Table I ‘“F removed through r-LiAlO, ‘. at various temperatures ing 150 min, with Ar plus 0.1% hydrogen sweep gas
1
30
creased rapidly with temperature of “F had been recovered.
until. at 800°C. 83%
3.2. Heating time effect Fig. 1 shows the recovery rate of “F in y-LiAlO, heated at 800°C for 15-150 min. Recovered ‘“F amount is higher for longer heating times. Furthermore the ‘“F recovered is proportional to heating time at this temperature. 3.3. Sweep gas effect Table 2 shows the effect of II1 presence in the sweep gas mixture. If no Hz is included no tluorinc is recovered at 600°C after 150 min. An amount as small as 0.1% of hydrogen in argon gives rise to 38.8% of fluorine. When pure hydrogen is used, the recovered fluorine at 600°C is 100%. Indeed, no “F was found in the solid. 3.4. Analysis
of ,fluorine species
Chromatographic and eIectr(~phoretic analyses ion. During the showed that “F was present at ‘“F
dur-
Table 2 ‘“F removed with different 600°C during 150 min
“F(‘%)
Swept gas He hr+O.l% 11,
sweep gases through
.-.---
-_____H,
0.0 37.4 100.0
y-tiAtOz
-_--~-
:it
J. Jimenez-Becerril et al. / “Fpresence
process the fluorine was, indeed, trapped immediately either in the NH,OH trap or in an iron chip trap.
4. Discussion
and conclusions
lXF release behavior from neutron-irradiated LiAlO, powders has been investigated at various temperatures. It was found that the “F diffuses through the solid and the amount released depended strongly on the composition of the sweep gas, on the temperature, and on the heating time. It was shown that the amount of hydrogen in the sweep gas affects considerably the amount of fluorine swept. The mobilization of “F with the presence of a small amount of H, (0.1%) in argon shows that a chemical reaction between H, and ‘sF very probably produces HF which is easily volatilized. When pure He is utilized, ‘sF_ and 3H+ do not combine to form HF gas although the ratio sH’/ ‘*F- is very large (= 10”). This can be explained considering that the amount of 3H atoms produced in the solid has a very low concentration (one 3H atom per approximately 105-lo6 LiAlO, molecules), and hence the 3H18F formed is insignificant. Tritium release experiments in neutron-irradiated y-LiAlO, using purge gas containing H, are conducted on a regular basis. As yet Roth [1,2] is the only experimentalist to address the issue and he makes a case as to why ‘sF should not have been seen. No one else has reported having observed the presence of lXF in the swept gas, probably because it reacts readily with metal tubing of the tritium recovery equipment.
in neutron-irradiated y-LiAlO,
235
Acknowledgements The authors would like to express appreciation to Dr. E. Roth for having suggested the present work. We would also like to thank the technicians of the chemistry department for their help in the laboratory, the staff of the reactor for the irradiations and V.H. Lara for technical help in the X-ray diffraction measurements.
References
[ll M. Briec, F. Bolter,
J.J. Abassin, R. Benoit, P. Chenebault, M. Masson, B. Rasneur, P. Sciers, H. Werle and E. Roth, J. Nucl. Mater. 141-143 (19860 357. 121E. Roth, F. Botter, M. Briec, M. Rostaing, H. Werle and R.G. Clemmer, J. Nucl. Mater. 141-143 (1986) 275. mentioned by Roth et [31 Dr. Kwast, personal communication al. in ref. [I]. 141 R.G. Clemmer, P.A. Finn, R.F. Malecha, B. Misra, M.C. Billone, D.L. Bowers, A.K. Fischer, L.R. Greenwood, R.F. Mattas, S.W. Tam, R.B. Poeppel, G.T. Reedy, LT. Dudley, F.F. Dyer, E.D. Clemmer, J.S. Watson, P.W. Fisher, J.R. Conlin, R.L. Childs, J.L. Scott, M. Arons and A.E. Scandora, Argonne National Laboratory, Report ANL-8455 (1984). P. Bosch and S. Bulbulian, J. Nucl. [51 J. Jimenez-Becerril, Mater. 185 (1991) 304.