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Neutron damage and fission product gas release in the initial burst region of uranium monocarbides
Journal of Nuclear Materials 56 (1975) 161-168 0 Nob-polled Publishing Company
NEUTRON DAMAGE AND FISSION PRODUCT GAS RELEASE IN THE INITIAL BURST RBGION OF URANIUM MONOCARBIDES
iiisayuki MATSUI Department of Nuclear Engineering, Faculty of Engineering, Nagoya University,Chikusa-ku,Nagoya, Japan Received 5 June 1974 Revised manuscript received 12 November 1974
Thermal neutron damage and fission product gas ( ‘33Xe) release in a burst region of uranium monocarbides were studied, After neutron irradiation, the electrical resistivity was measured from room temperature to 800°C. Three recovery stages were revealed in the resistivity of UC irradiated to 4.0 X 10r6 nvt. The lattice parameter of UC with the same irradiation also showed three stages of recovery up to 1050°C. The initial burst of Xe from UC was studied in a dose range between 1.6 X 10” and 2.9 X lOi nvt. The burst occurred in three steps for lightly hradiated specimens, while there were two steps of the burst in heavily irradiated specimens. The activation energies for each burst step were calculated. From the results obtained here, we concluded that the burst was correlated with the recovery of damage in the neutronirradiated UC. Les dommages dus aux neutrons the~iqu~ et le degagement de 13aXe, produit de fission darts une r&ion de d&agement brutal de monocarbures d’U ont eti &&i&s. Ap& irradiation neutrordque, la r&stiYit~ electrique est mesu&e depuis la temp&ature ambiante jusqu’a 800°C. Trois stades de restauration apparaissent dam la r&istivite de UC irradie ;i 4 X 1Or6 nvt. Le param&re du tiseau de UC pr&entait pour la m&me irradiation trois stades de restauration jusqu’ & 1050°C. Le dega@ment brutal initial de Xe ii partir de UC a Bte &did pour des doses comprises entre 1,6 X 10’s et 2,9 X 10” nvt. Ce dhgagement se produisait en trois stades pour les ~c~tiRons peu inadi& et en deux stades pour ceux fortement irra dies. Les energies d’activation pour chaque stade ont 6th caJcul&s. Des r&ultats obtenus, on conclut que le digagement brutal de gaz etait en ralation avec la restauration des dommages provoqds dans UC irradi4 par les neutrons. Die Str~en~h~den durch therm&he Neutronen und die Sp~~fre~e~u~ (Xe-133) wurdtm in einem Ausbrmhsbe reich des Uranmonocarbids untersucht. Nach der Neutronenbestrahhmg wurde der elektxische Widerstand zvhhen humtemperatur und 800°C gemessen. Fur den Widerstand des mit 4,0 X 1O’e n/cm’ bestrahlten UC ergeben sich drei Erholungsstufen. Die Gitterkonstante des UC hat bis 1050°C ebenfalls drei Erholungsstufen bei derselben Bestrahlung. Die anf#ngliche Xe-Freisetzung aus dem UC wurde bei einer Dosis im Bereich zwischen 1,6 X 10” und 2,9 X lo’* n/cm2 untersucht. Die Freisetzung tritt bei gering best&Ben Proben in drei Stufen auf, w&rend die Freisetzung in zwei Stufen bei starlr bestrahlten Proben beobachtet wird. Die AktiYierungsenergie fiir jeden Freisetzungsschritt wird berechnet. Aus den gewonnenen Ergebnissen wird geschlossen, dass die Freisetzung im UC mit der Erholung der Strahlenschaden durch Neutronen im Zusammenhang steht.
1. rntroduction Much work has been performed on fission product gas release from neutron-irradiated uranium monocarbide (UC) [l-6]. The studies were concentrated on diffusion of Xe gas and were carried out above 1000°C. Below that temperature, the so-called ‘initial burst’ was observed where the gas was released by a complicated mechanism. Low-temperature releases of ion-bombarded rare
gases from various materials [7- 171, including UC [6,18,19], have been investigated during the past decade. However, experimental studies of the burst release after reactor irradiation [4,20-221 have been limited. The mechanisms for the initial burst of the fission product gas from nuclear fuels have not been clearly understood [23-251, though a lot of proposed models were presented, based on the ion-bombardment studies [26]. In this paper, we studied the behavior of the burst
H. Matsui/Initial burst region of uranium monocarbides
162
and the recovery of neutron damage in UC. It was concluded that the burst of fission product gas in UC occurred during the course of recovery in which extra defects, induced by the Irradiation, were annihilated.
2. Experimental 2.1. Specimen preparation and irradiations Uranium monocarbides were prepared by arcmelting high-purity uranium metal and reactor-grade graphite in an argon atmosphere. In order to eliminate strains caused by quenching during arc-casting, heat treatments were carried out on the specimens for more than 3 h, at 1200-15OO”C, under a high vacuum of less than 10-S torr. The carbon content was analysed using the normal procedure and was found to be 4.6-4.7 wt %. Oxygen impurity was found to be 100-200 ppm by a microbromination method that has been developed in our laboratory [27]. The electrical resistivity and the lattice parameter of UC in this study were 30-40 fl . cm and 4.954-4.956 A respectively, Irradiated specimens (0.1-0.5 g) were sealed in quartz capsules under high vacuum. Irradiations were performed with several reactors, as indicated in table 1 Temperatures during irradiation were estimated to be below IOO’C, except at the highest dose (JMTR).
2.2. Procedures A four-probe (dc) potential drop method was employed to measure resistivity in this work. After neutron irradiation an increase in resistivity was measured at room temperature (corrected to 2O’C). A recovery anneal was performed by continuous heating from room temperature to 800°C in a high vacuum and the resistivity was continuously measured at l-2’C intervals during the heating. An activation energy of the recovery of resistivity was calculated using eq. (1) [28] from two recovery temperatures of T, and T2, which were inflection points on each curve of the resistivity versus temperature plot for heating rates of 1 and 2°C per minute, respectively:
(1) where al and a2 are the heating rates and R is the gas constant. In order to determine the lattice parameter of UC before and after irradiation, an X-ray diffractometer was used. Two kinds of specimens, powdered and platelet (prepared from the same arc-melted button), were irradiated for the measurements. Recovery of the lattice expansion in the irradiated UC (powdered) was obtained by pulse heating (isochronal annealing) at intervals of 150°C. The lattice parameter was determined after cooling the specimen to room temperature .
Table 1 Neutron irradiation performance and fluence. Neutron flux (in thermal) is varied at a position in the irradiation facility.
Reactor
Neutron fluence (nvt)
Neutron flux (n/cm2 - set)
Remarks
JRR-1
3.6 X 1014 - 1.6 x 1016
l-5 x 10”
ER a) and LP b,
JRR-2 JRR3
3.3 x 1o16 - 2.1 x 1o19
1012-1013
ER and LP
JMTR
3.1 x 10ZO
1.0 x 1o14
LP
1.6 x 10” - 2.9 x 1o18
2-8 x 1013
FP ‘)
KUR
a) ER: measurement of electrical resistivity. b) LP: measurement of lattice parameter, ‘) FP: fiision-product gas release experiment.
163
H. Matsui/Initialburst region of uranium monocarbides
The experimental procedure to study fission product gas release was that presented in a previous paper [22], with some improvements made in parts of the measurement and calculation procedures. The released 133Xe(y emitter of 8 1 keV) gas was continuously counted by a NaI scintillation detector and the counts were compiled in the memory of a multichannel scaler at one-minute intervals during the heating. In this manner, we could discern the changes in the release rates, which could not have been detected by other procedures. Burst temperatures were determined by continuous heating from room temperature to 14OO”C,with two constant heating rates of 5 and lO”C/min, respectively. The activation energy for each burst was calculated, using eq. (l), in the same manner as the resistivity recovery, because the burst was considered as a first-order reaction. In order to avoid oxidation of the specimens, the He carrier gas was dehydrated by liquid nitrogen and Ti sponge was used as a getter of oxygen in the reaction system. There was no change in appearance of the surface luster of the specimens after the experiments. Oxide layers deposited on the surface of UC were not detected by X-ray analysis before or after the gas release anneals.
3. Results
o-
I
I
l#
0
FIateIet
l
Mered
1P
UC
I
I l#
I
UC
I
IP nvt
Neutron fluence,
Fig. 1. Increase of lattice constant of UC after neutron irradiation.
Recovery of the expanded lattice parameters of powdered UC on subsequent annealing is shown in fig. 2. In a fluence of less than 9.6 X 1016 nvt, three recovery steps were observed, but above that fluence two recovery steps occurred. We believe that a different defect configuration occurs in the range of fluence where the extent of lattice expansion is decreased. Bloch et al. [3 l] also reported the three recovery steps similar to ours with sintered UC, but they used a different range of neutron fluence.
3.1. Effect of radiation and subsequent annealing behavior on the lattice parameter of UC
An increase of the lattice parameter in UC following reactor irradiation was studied in a range of thermal neutron fluence between 3.6 X lOI and 3.7 X 10zo nvt. The results obtained are shown in fig. 1, together with the data on UC,., and UC,, (arc-melt specimens) published by Childs et al. [29]. The platelet specimens indicate a similar trend to the results of Childs et al. except that of the highest dose, which has been irradiated at a different temperature (* 3OO’C)from the others. It should be noted that the lattice parameters of powdered specimens irradiated below 100°C have a tendency to decrease after a saturation value. A similar tendency was observed in UO;! irradiated at 70 and 400°C [30]. Such a phenomenon will be precisely described ln a subsequent paper.
01
0
a
I
I
1
200 400 600 600
1000 lxx)
Annealing tempemtufe
‘C
Fig. 2. Recovery of lattice parameter by isochronal annealing in UC. The recovery was observed in three steps at lower doses, while only in two steps at higher doses.
H. Matsui/Initialburst region of uranium monocarbides
164
3.2. Effect of radiation and subsequent annealing behavior on the electrical resistivity of UC
The electrical resistivity also increases with the neutron fluence as shown in fig. 3 for platelet specimens of UC. The resistivity begins to increase sharply at 1 X 1016 nvt and saturates near 2 X 10” nvt. Our results show a similar trend to that reported by Childs et al. [29] in a range of fluence between 1016 and 101* nvt, although the fractional increase of the resistivity is higher in their study than in this study. The increasing resistivity in UC, following reactor irradiation, resembled that of the lattice expansion (as shown in fig. 1) at least below 10lg nvt. The recovery of the resistivity increase in UC irradiated to 4.0 X 1016 nvt showed three stages upon subsequent annealing from room temperature to 800°C [28]. The behavior resembled the recovery of the lattice parameter of UC, as mentioned in subsect. 3.1. For the three recovery stages in the resistivity, the activation energies obtained were 0.4 (= 200’(Z), 1.O(* 400%) and 1.9 f 0.1 eV (= 650°C). Griffiths [32,33],on the other hand, reported two recovery steps in the resistivity of sintered UC below 1000°C. The increased resistivity in his study, however, was so small (only few /..Al. cm) that he could not detect the second stage that appeared in our study.
s
region of UC
The fractional release of 133Xe from UC irradiated Table 2 Burst temperatures appeared ln S’C/min heat treatment and activation energies of the burst at various neutron doses. Fluence
Burst stage
(nvt)
I
II
50 i
0
Ip
-0
IL?
.--3, lc?
0’ 1lY
I
1
Id7
Id0
Neutron fluence
ld9
nvt
Fig. 3. Increase of electrical resistivity of UC with neutron dose.
III
1.6 X 1Or5
T a) E b,
200 0.2
450 0.4
925 0.6
8.3 x 10”
T a) E b,
200 0.2
400 0.4
900 0.7
1.6 X 1016
T a) E b,
300 0.3
550 0.7
1000 1.4
5.0 x 1o16
T a) E b,
250 0.2
550 0.7
975 1.5
9.9 x 1o16
T a) E b,
300 0.3
550 0.7
1000 1.7
5.8 X 1017
T a) E b,
325 0.3
800 -
2.9 x lora
T a) E b,
300 0.3
775 1.9
I
a E
“C
3.3. Fission product (133Xe) gas release in the burst
I
100
.!
Annealing temperature
Fig. 4. Xe gas release from UC with a temperature rise of S”C/min. Three burst steps are observed at a low dose (1.6 X 1016 nvt) and two steps at a high dose (5.8 X lOI nvt). Above 12OO”C,a normal release by diffusion is expected.
‘) T = burst temperature CC). b, E = activation energy (ev).
H. Matsui/lnitialburst region of uranium monocarbides
to 1.6 X lOI and 5.8 X lOI nvt, with a heating rate of S*C/min, are presented in fig. 4. Burst temperatures below lOOO”C,obtained from inflection points of the curves, are summarized in table 2. It should be noted that there were three burst temperatures for a dose less than 9.9 X 1016 nvt and that there were two burst temperatures for 5.8 X 1Or7 and 2.9 X 101* nvt [22]. The dependence of the number of bursts on neutron dose corresponds well with the number of recovery steps for the lattice parameter mentioned in subsect. 3.1. The activation energies of each burst step were calculated by eq. (1) and are represented in table 2. In cases where oxide layers were deposited on the surface of the specimen, no gas burst was found below 5OO”C,but above that temperature, an abrupt release of the gas was observed. In addition, the fractional amount of released gas increased about an order of ma~itude, if the oxide layers were present on the surface of UC.
4. Discussion
Many authors [29,31,34,35] have reported on the lattice expansion of UC following reactor irradiation. However, there are some discrepancies in the results after a saturation of the lattice expansion. In our study, a reduction of the lattice expansion of UC was observed at higher doses. Dienst f35f showed a similar reduction at almost the same dose as that employed in the present experiments. He reported, in addition, a second increase and reduction of the lattice parameter of UC at dosages over lo20 nvt. On the other hand, Childs et al. 1291reported that a second increase of the lattice expansion of UC,,, containing U metal, occurred between 4 X 1018 and 2 X 10lg; however, they did not observe any reduction of lattice parameter in UC,, and UC,.,. Bloch et al. [3 11, and Adam and Rogers [34] observed neither the reduction nor the second increase of the lattice parameter in sintered UC below a neutron fluence of 2.2 X 101* nvt. Our experiments indicate that for powdered specimens the reduction of lattice expansion with an increase of dose occurred at a lower dose than for platelet specimens of the
165
same purity. Therefore, even if the purity and stoi~hiomet~ of UC are identical, the behaviors of the defects introduced by fission damage in powder and platelet specimens are different from each other, and the difference depends upon grain size and/or state of grain boundaries in the materials. The subsequent annealing following neutron irradiation was also measured by the above-men~oned authors [29,3 1,34,35], on the lattice parameter of UC. Adam and Rogers [34] showed two recovery stages up to 640°C for sintered UC, in which the expanded lattice parameter of 15% was retained. Bloch et al. 1311also showed three stages in UC irradiated to 1.2 X 101* nvt, but only a single stage for the lower irradiation (9 X 1016 nvt). On the contrary, Childs et al. [29] and Dienst [35] reported two stages of recovery in the lattice parameter for arc-melted specimens of UC. At present, the behavior of the recovery of lattice parameter in UC by subsequent annealing does not agree among the various experiments, probably because of the variations in characteristics of specimens (grain size, stoichiometry and impurities in UC) and in the irradiation conditions. Nevertheless, the temperatures of lattice parameter recovery pub~shed by the above mentioned authors roughly correspond to those of the first and the third stages obtained in our study. The recovery of the resistivity of UC irradiated to less than 9.0 X 1Ol6 nvt also revealed three stages [28] at temperatures corresponding to those of recovery of the lattice parameter. In a previous paper [28 J, we discussed the defects in UC irradiated to 4.0 X 1016 nvt from a consideration of the activation energy of recovery in resistivity. We concluded that the first stage in resistivity recovery might be due to a migration of interstitial atoms, while the second and the third stages were connected to a migration of carbon and uranium vacancies, respectively. Therefore, the defects are probably simple types, such as an interstitial and a vacancy, in UC when the irradiation dose was less than 9.6 X 1016 nvt and movements of these defects contribute to the recovery of the lattice parameter. The two stages of recovery in the lattice parameter were observed in UC irradiated to doses high than 9.6 X 1016 nvt. Eyre and Sole [36] observed, in an electron microscope, clusters of defects in UC after 3.7 X 1017 nvt irradiation and concluded the defects were of an interstitial nature. It is
H. Matsuiflnitial burst region of uranium monocarbides
166
likely, that there are secondary defects induced by fission damage in UC when the neutron dose is higher than 1017 nvt. Simple types of defects are dominant in UC in the low irradiation dose below.9.6 X 1016 nvt. With increasing dose above 9.6 X 1016 nvt, where the saturation of increasing resistivity and lattice parameter occurs in UC, carbon interstitials are combined with each other to form interstitial clusters, as observed by Eyre and Sole [36]. At the same time, carbon vacancies are produced but they can be annihilated by the interstitial clusters, because of their high mobility. Furthermore, a UC di-vacancy [6,37] is also probable. Thus, the concentration of carbon vacancies with high mobility in UC can be reduced, and accordingly the second stage in the recovery of lattice parameter at the higher doses will become obscure. The uranium vacancy, and probably the UC di-vacancy, will remain at higher doses, thus they can move at higher temperature (> 7OO”C),and make the third stage of the recovery. 4.2. Fission product gas burst and the recovery of defects in UC
An integral release of Xe gas from neutronirradiated UC in an isothermal anneal at 800°C was linearly dependent on the annealing time (t), while for an isothermal anneal at 9OO”C,after an 800°C burst, the gas release was dependent upon the square root oft. Thus, the release around 9OO’Cis controlled by a diffusion mechanism in the neutron-irradiated UC. The diffusion-controlled release was also observed even at 800°C when the annealing was prolonged for more than two hours. The burst obtained in this work was the release of fission product gas from the specimen surface in a range of mean free path of the Xe gas in UC. On the other hand, the burst temperatures, obtained here, correspond well to the temperatures of the recovery of lattice parameter. These facts suggest that the burst might be correlated to the recovery of the defects induced by irradiation in UC. As shown in table 3, the burst and the recovery of lattice defects in UC had three steps when irradiated to near 5 X 1016 nvt. Moreover the temperatures at which the gas bursts occur correspond to the recovery of the lattice defects. The activation energies of both phenomena also coincide rather well with each other.
Table 3 Comparison of the activation energies and the temperatures of fission product gas burst and recovery in resistivity of UC irradiated to around 5 X 1016 nvt. The same procedures are applied in both experiments except for the heating rate. The rates are 1 and 2’C/min in resistivity recovery and 5 and lO”C/min in fission product burst. Recovery temperatures of lattice parameters are also presented for comparison.
Remarks
Activation energy (* 0.1 eV) and recovery temperature CC) I
II
III
FP gas release 5.0 X 1016 nvt
0.2 250
0.7 550
1.5 975
Recovery of resistivity 4.0 X 1016 nvt
0.4 200
1.0 400
1.9 650
Recovery of lattice expansion 1.1 X 1016 nvt
225
_
-
550
950
As a consequence, the following cooperative phenomena between the recovery of lattice defects and the burst of fission product gas in UC are proposed. While a lattice of UC, which is distorted by the fission fragment damage, is being recovered at a temperature, an interstitial atom and/or a vacancy, which were introduced by the damage, migrate to a lattice position, or in some fractions to sinks (such as a surface, a grain boundary and a cluster). In the meantime, the relaxation at the lattice is continuing on one side, and on the other a strain field around the fission product gas atom becomes strong. As the strain energy increases, the Xe atom will be unstable and will easily move by itself. Thus an atom near the sink will be pushed out from a site in which the Xe atom is extraneously positioned. Thereafter the whole lattice of UC is relaxed again until the next migration of defects takes place. Accordingly it is concluded here that fusion product gas burst occurs during the course of recovery of lattice defects in UC. We conclude, therefore, that the first step of the gas burst in UC was attributed to a release of Xe atoms related to the movement of interstitial atoms, and the second and the third bursts were connected with migrations of vacancies of carbon and uranium respectively. A variation of the activation energy, espacially in
H. Matsuiflnitialburst region of uranium monocarbides
the third step, indicates that there are many different types of Xe atoms sites surrounded by various defect configurations in UC. Anomalous release mechanism in ion-bombarded rare gases from various materials were interpreted by .many workers [7,12,38,42]. Among those, Jech [7] and Matzke [ 121 explained the anomalous release by the recovery and movement of defects on defect clusters with some discrete stages. Below room temperature, a release of Rn (with an activation energy of 0.1-0.5 eV) from single crystal UC, reported by Matzke [18, 191, is not contradictory. The release of an interstitial Rn atom in UC in Mat&e’s study might be caused in the course of recovery of the lattice (or lattice relaxation) by warming the specimen, and the mechanism could be interpreted by the same consideration of the burst proposed here. It may be reasonably considered that the defects recovered after ion bombardment at a low temperature are different from those of fission damage, as in our experiment, even if the values of the activation energy are roughly equal. The activation energies of the ion-bombarded rare gas release below 1000°C were reported [6, 171, but they do not agree with those obtained in the present study. Among the reports on the Xe gas burst from UC following reactor irradiation [4,20-221, Auskern [20] reported the activation energy of 1.I-- 1.5 eV for the burst of Xe in powdered UC and concluded it resulted from a migration of vacancies. We obtained an almost equal value to that in the third step. We have contributed some new insight on the initial burst of fission product gas in neutron irradiated UC. Further work, however, is expected to clarify more precisely the problem between the damage and the burst release in UC after reactor irradiation.
5. Conclusions We studied fission damage and fission product gas (Xe) burst release of UC. The results obtained are as follows: (1) It was confirmed that fission product gas burst of UC occurred in correlation with the recovery of extra lattice defects. The mechanism of the burst was proposed. (2) Three steps were observed in the burst and
167
also in the recovery of the lattice defects when a neutron fluence was below about 1017 nvt. The recovery was due to the movement of rather simple point defects. (3) Exceeding the dose mentioned above, only two steps appeared both in the burst and in the recovery of the defects because of the formation of clusters of defects and UC double vacancies.
Acknowledgements
The author wishes to thank Prof. T. Kirihara, Nagoya University, for many helpful discussions and suggestions during the course of this work. Special thanks are due to Messrs. M. Tamaki and M. Horiki for their experimental assistance. Part of this work was done at JAERI and the remainder at KUR. The author is indebted to Dr. T. Kikuchi, JAERI, and Dr. T. Tamai, Kyoto University, for their valuable discussions in this study.
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NEUTRON DAMAGE AND FISSION PRODUCT GAS RELEASE IN THE INITIAL BURST RBGION OF URANIUM MONOCARBIDES
iiisayuki MATSUI Department of Nuclear Engineering, Faculty of Engineering, Nagoya University,Chikusa-ku,Nagoya, Japan Received 5 June 1974 Revised manuscript received 12 November 1974
Thermal neutron damage and fission product gas ( ‘33Xe) release in a burst region of uranium monocarbides were studied, After neutron irradiation, the electrical resistivity was measured from room temperature to 800°C. Three recovery stages were revealed in the resistivity of UC irradiated to 4.0 X 10r6 nvt. The lattice parameter of UC with the same irradiation also showed three stages of recovery up to 1050°C. The initial burst of Xe from UC was studied in a dose range between 1.6 X 10” and 2.9 X lOi nvt. The burst occurred in three steps for lightly hradiated specimens, while there were two steps of the burst in heavily irradiated specimens. The activation energies for each burst step were calculated. From the results obtained here, we concluded that the burst was correlated with the recovery of damage in the neutronirradiated UC. Les dommages dus aux neutrons the~iqu~ et le degagement de 13aXe, produit de fission darts une r&ion de d&agement brutal de monocarbures d’U ont eti &&i&s. Ap& irradiation neutrordque, la r&stiYit~ electrique est mesu&e depuis la temp&ature ambiante jusqu’a 800°C. Trois stades de restauration apparaissent dam la r&istivite de UC irradie ;i 4 X 1Or6 nvt. Le param&re du tiseau de UC pr&entait pour la m&me irradiation trois stades de restauration jusqu’ & 1050°C. Le dega@ment brutal initial de Xe ii partir de UC a Bte &did pour des doses comprises entre 1,6 X 10’s et 2,9 X 10” nvt. Ce dhgagement se produisait en trois stades pour les ~c~tiRons peu inadi& et en deux stades pour ceux fortement irra dies. Les energies d’activation pour chaque stade ont 6th caJcul&s. Des r&ultats obtenus, on conclut que le digagement brutal de gaz etait en ralation avec la restauration des dommages provoqds dans UC irradi4 par les neutrons. Die Str~en~h~den durch therm&he Neutronen und die Sp~~fre~e~u~ (Xe-133) wurdtm in einem Ausbrmhsbe reich des Uranmonocarbids untersucht. Nach der Neutronenbestrahhmg wurde der elektxische Widerstand zvhhen humtemperatur und 800°C gemessen. Fur den Widerstand des mit 4,0 X 1O’e n/cm’ bestrahlten UC ergeben sich drei Erholungsstufen. Die Gitterkonstante des UC hat bis 1050°C ebenfalls drei Erholungsstufen bei derselben Bestrahlung. Die anf#ngliche Xe-Freisetzung aus dem UC wurde bei einer Dosis im Bereich zwischen 1,6 X 10” und 2,9 X lo’* n/cm2 untersucht. Die Freisetzung tritt bei gering best&Ben Proben in drei Stufen auf, w&rend die Freisetzung in zwei Stufen bei starlr bestrahlten Proben beobachtet wird. Die AktiYierungsenergie fiir jeden Freisetzungsschritt wird berechnet. Aus den gewonnenen Ergebnissen wird geschlossen, dass die Freisetzung im UC mit der Erholung der Strahlenschaden durch Neutronen im Zusammenhang steht.
1. rntroduction Much work has been performed on fission product gas release from neutron-irradiated uranium monocarbide (UC) [l-6]. The studies were concentrated on diffusion of Xe gas and were carried out above 1000°C. Below that temperature, the so-called ‘initial burst’ was observed where the gas was released by a complicated mechanism. Low-temperature releases of ion-bombarded rare
gases from various materials [7- 171, including UC [6,18,19], have been investigated during the past decade. However, experimental studies of the burst release after reactor irradiation [4,20-221 have been limited. The mechanisms for the initial burst of the fission product gas from nuclear fuels have not been clearly understood [23-251, though a lot of proposed models were presented, based on the ion-bombardment studies [26]. In this paper, we studied the behavior of the burst
H. Matsui/Initial burst region of uranium monocarbides
162
and the recovery of neutron damage in UC. It was concluded that the burst of fission product gas in UC occurred during the course of recovery in which extra defects, induced by the Irradiation, were annihilated.
2. Experimental 2.1. Specimen preparation and irradiations Uranium monocarbides were prepared by arcmelting high-purity uranium metal and reactor-grade graphite in an argon atmosphere. In order to eliminate strains caused by quenching during arc-casting, heat treatments were carried out on the specimens for more than 3 h, at 1200-15OO”C, under a high vacuum of less than 10-S torr. The carbon content was analysed using the normal procedure and was found to be 4.6-4.7 wt %. Oxygen impurity was found to be 100-200 ppm by a microbromination method that has been developed in our laboratory [27]. The electrical resistivity and the lattice parameter of UC in this study were 30-40 fl . cm and 4.954-4.956 A respectively, Irradiated specimens (0.1-0.5 g) were sealed in quartz capsules under high vacuum. Irradiations were performed with several reactors, as indicated in table 1 Temperatures during irradiation were estimated to be below IOO’C, except at the highest dose (JMTR).
2.2. Procedures A four-probe (dc) potential drop method was employed to measure resistivity in this work. After neutron irradiation an increase in resistivity was measured at room temperature (corrected to 2O’C). A recovery anneal was performed by continuous heating from room temperature to 800°C in a high vacuum and the resistivity was continuously measured at l-2’C intervals during the heating. An activation energy of the recovery of resistivity was calculated using eq. (1) [28] from two recovery temperatures of T, and T2, which were inflection points on each curve of the resistivity versus temperature plot for heating rates of 1 and 2°C per minute, respectively:
(1) where al and a2 are the heating rates and R is the gas constant. In order to determine the lattice parameter of UC before and after irradiation, an X-ray diffractometer was used. Two kinds of specimens, powdered and platelet (prepared from the same arc-melted button), were irradiated for the measurements. Recovery of the lattice expansion in the irradiated UC (powdered) was obtained by pulse heating (isochronal annealing) at intervals of 150°C. The lattice parameter was determined after cooling the specimen to room temperature .
Table 1 Neutron irradiation performance and fluence. Neutron flux (in thermal) is varied at a position in the irradiation facility.
Reactor
Neutron fluence (nvt)
Neutron flux (n/cm2 - set)
Remarks
JRR-1
3.6 X 1014 - 1.6 x 1016
l-5 x 10”
ER a) and LP b,
JRR-2 JRR3
3.3 x 1o16 - 2.1 x 1o19
1012-1013
ER and LP
JMTR
3.1 x 10ZO
1.0 x 1o14
LP
1.6 x 10” - 2.9 x 1o18
2-8 x 1013
FP ‘)
KUR
a) ER: measurement of electrical resistivity. b) LP: measurement of lattice parameter, ‘) FP: fiision-product gas release experiment.
163
H. Matsui/Initialburst region of uranium monocarbides
The experimental procedure to study fission product gas release was that presented in a previous paper [22], with some improvements made in parts of the measurement and calculation procedures. The released 133Xe(y emitter of 8 1 keV) gas was continuously counted by a NaI scintillation detector and the counts were compiled in the memory of a multichannel scaler at one-minute intervals during the heating. In this manner, we could discern the changes in the release rates, which could not have been detected by other procedures. Burst temperatures were determined by continuous heating from room temperature to 14OO”C,with two constant heating rates of 5 and lO”C/min, respectively. The activation energy for each burst was calculated, using eq. (l), in the same manner as the resistivity recovery, because the burst was considered as a first-order reaction. In order to avoid oxidation of the specimens, the He carrier gas was dehydrated by liquid nitrogen and Ti sponge was used as a getter of oxygen in the reaction system. There was no change in appearance of the surface luster of the specimens after the experiments. Oxide layers deposited on the surface of UC were not detected by X-ray analysis before or after the gas release anneals.
3. Results
o-
I
I
l#
0
FIateIet
l
Mered
1P
UC
I
I l#
I
UC
I
IP nvt
Neutron fluence,
Fig. 1. Increase of lattice constant of UC after neutron irradiation.
Recovery of the expanded lattice parameters of powdered UC on subsequent annealing is shown in fig. 2. In a fluence of less than 9.6 X 1016 nvt, three recovery steps were observed, but above that fluence two recovery steps occurred. We believe that a different defect configuration occurs in the range of fluence where the extent of lattice expansion is decreased. Bloch et al. [3 l] also reported the three recovery steps similar to ours with sintered UC, but they used a different range of neutron fluence.
3.1. Effect of radiation and subsequent annealing behavior on the lattice parameter of UC
An increase of the lattice parameter in UC following reactor irradiation was studied in a range of thermal neutron fluence between 3.6 X lOI and 3.7 X 10zo nvt. The results obtained are shown in fig. 1, together with the data on UC,., and UC,, (arc-melt specimens) published by Childs et al. [29]. The platelet specimens indicate a similar trend to the results of Childs et al. except that of the highest dose, which has been irradiated at a different temperature (* 3OO’C)from the others. It should be noted that the lattice parameters of powdered specimens irradiated below 100°C have a tendency to decrease after a saturation value. A similar tendency was observed in UO;! irradiated at 70 and 400°C [30]. Such a phenomenon will be precisely described ln a subsequent paper.
01
0
a
I
I
1
200 400 600 600
1000 lxx)
Annealing tempemtufe
‘C
Fig. 2. Recovery of lattice parameter by isochronal annealing in UC. The recovery was observed in three steps at lower doses, while only in two steps at higher doses.
H. Matsui/Initialburst region of uranium monocarbides
164
3.2. Effect of radiation and subsequent annealing behavior on the electrical resistivity of UC
The electrical resistivity also increases with the neutron fluence as shown in fig. 3 for platelet specimens of UC. The resistivity begins to increase sharply at 1 X 1016 nvt and saturates near 2 X 10” nvt. Our results show a similar trend to that reported by Childs et al. [29] in a range of fluence between 1016 and 101* nvt, although the fractional increase of the resistivity is higher in their study than in this study. The increasing resistivity in UC, following reactor irradiation, resembled that of the lattice expansion (as shown in fig. 1) at least below 10lg nvt. The recovery of the resistivity increase in UC irradiated to 4.0 X 1016 nvt showed three stages upon subsequent annealing from room temperature to 800°C [28]. The behavior resembled the recovery of the lattice parameter of UC, as mentioned in subsect. 3.1. For the three recovery stages in the resistivity, the activation energies obtained were 0.4 (= 200’(Z), 1.O(* 400%) and 1.9 f 0.1 eV (= 650°C). Griffiths [32,33],on the other hand, reported two recovery steps in the resistivity of sintered UC below 1000°C. The increased resistivity in his study, however, was so small (only few /..Al. cm) that he could not detect the second stage that appeared in our study.
s
region of UC
The fractional release of 133Xe from UC irradiated Table 2 Burst temperatures appeared ln S’C/min heat treatment and activation energies of the burst at various neutron doses. Fluence
Burst stage
(nvt)
I
II
50 i
0
Ip
-0
IL?
.--3, lc?
0’ 1lY
I
1
Id7
Id0
Neutron fluence
ld9
nvt
Fig. 3. Increase of electrical resistivity of UC with neutron dose.
III
1.6 X 1Or5
T a) E b,
200 0.2
450 0.4
925 0.6
8.3 x 10”
T a) E b,
200 0.2
400 0.4
900 0.7
1.6 X 1016
T a) E b,
300 0.3
550 0.7
1000 1.4
5.0 x 1o16
T a) E b,
250 0.2
550 0.7
975 1.5
9.9 x 1o16
T a) E b,
300 0.3
550 0.7
1000 1.7
5.8 X 1017
T a) E b,
325 0.3
800 -
2.9 x lora
T a) E b,
300 0.3
775 1.9
I
a E
“C
3.3. Fission product (133Xe) gas release in the burst
I
100
.!
Annealing temperature
Fig. 4. Xe gas release from UC with a temperature rise of S”C/min. Three burst steps are observed at a low dose (1.6 X 1016 nvt) and two steps at a high dose (5.8 X lOI nvt). Above 12OO”C,a normal release by diffusion is expected.
‘) T = burst temperature CC). b, E = activation energy (ev).
H. Matsui/lnitialburst region of uranium monocarbides
to 1.6 X lOI and 5.8 X lOI nvt, with a heating rate of S*C/min, are presented in fig. 4. Burst temperatures below lOOO”C,obtained from inflection points of the curves, are summarized in table 2. It should be noted that there were three burst temperatures for a dose less than 9.9 X 1016 nvt and that there were two burst temperatures for 5.8 X 1Or7 and 2.9 X 101* nvt [22]. The dependence of the number of bursts on neutron dose corresponds well with the number of recovery steps for the lattice parameter mentioned in subsect. 3.1. The activation energies of each burst step were calculated by eq. (1) and are represented in table 2. In cases where oxide layers were deposited on the surface of the specimen, no gas burst was found below 5OO”C,but above that temperature, an abrupt release of the gas was observed. In addition, the fractional amount of released gas increased about an order of ma~itude, if the oxide layers were present on the surface of UC.
4. Discussion
Many authors [29,31,34,35] have reported on the lattice expansion of UC following reactor irradiation. However, there are some discrepancies in the results after a saturation of the lattice expansion. In our study, a reduction of the lattice expansion of UC was observed at higher doses. Dienst f35f showed a similar reduction at almost the same dose as that employed in the present experiments. He reported, in addition, a second increase and reduction of the lattice parameter of UC at dosages over lo20 nvt. On the other hand, Childs et al. 1291reported that a second increase of the lattice expansion of UC,,, containing U metal, occurred between 4 X 1018 and 2 X 10lg; however, they did not observe any reduction of lattice parameter in UC,, and UC,.,. Bloch et al. [3 11, and Adam and Rogers [34] observed neither the reduction nor the second increase of the lattice parameter in sintered UC below a neutron fluence of 2.2 X 101* nvt. Our experiments indicate that for powdered specimens the reduction of lattice expansion with an increase of dose occurred at a lower dose than for platelet specimens of the
165
same purity. Therefore, even if the purity and stoi~hiomet~ of UC are identical, the behaviors of the defects introduced by fission damage in powder and platelet specimens are different from each other, and the difference depends upon grain size and/or state of grain boundaries in the materials. The subsequent annealing following neutron irradiation was also measured by the above-men~oned authors [29,3 1,34,35], on the lattice parameter of UC. Adam and Rogers [34] showed two recovery stages up to 640°C for sintered UC, in which the expanded lattice parameter of 15% was retained. Bloch et al. 1311also showed three stages in UC irradiated to 1.2 X 101* nvt, but only a single stage for the lower irradiation (9 X 1016 nvt). On the contrary, Childs et al. [29] and Dienst [35] reported two stages of recovery in the lattice parameter for arc-melted specimens of UC. At present, the behavior of the recovery of lattice parameter in UC by subsequent annealing does not agree among the various experiments, probably because of the variations in characteristics of specimens (grain size, stoichiometry and impurities in UC) and in the irradiation conditions. Nevertheless, the temperatures of lattice parameter recovery pub~shed by the above mentioned authors roughly correspond to those of the first and the third stages obtained in our study. The recovery of the resistivity of UC irradiated to less than 9.0 X 1Ol6 nvt also revealed three stages [28] at temperatures corresponding to those of recovery of the lattice parameter. In a previous paper [28 J, we discussed the defects in UC irradiated to 4.0 X 1016 nvt from a consideration of the activation energy of recovery in resistivity. We concluded that the first stage in resistivity recovery might be due to a migration of interstitial atoms, while the second and the third stages were connected to a migration of carbon and uranium vacancies, respectively. Therefore, the defects are probably simple types, such as an interstitial and a vacancy, in UC when the irradiation dose was less than 9.6 X 1016 nvt and movements of these defects contribute to the recovery of the lattice parameter. The two stages of recovery in the lattice parameter were observed in UC irradiated to doses high than 9.6 X 1016 nvt. Eyre and Sole [36] observed, in an electron microscope, clusters of defects in UC after 3.7 X 1017 nvt irradiation and concluded the defects were of an interstitial nature. It is
H. Matsuiflnitial burst region of uranium monocarbides
166
likely, that there are secondary defects induced by fission damage in UC when the neutron dose is higher than 1017 nvt. Simple types of defects are dominant in UC in the low irradiation dose below.9.6 X 1016 nvt. With increasing dose above 9.6 X 1016 nvt, where the saturation of increasing resistivity and lattice parameter occurs in UC, carbon interstitials are combined with each other to form interstitial clusters, as observed by Eyre and Sole [36]. At the same time, carbon vacancies are produced but they can be annihilated by the interstitial clusters, because of their high mobility. Furthermore, a UC di-vacancy [6,37] is also probable. Thus, the concentration of carbon vacancies with high mobility in UC can be reduced, and accordingly the second stage in the recovery of lattice parameter at the higher doses will become obscure. The uranium vacancy, and probably the UC di-vacancy, will remain at higher doses, thus they can move at higher temperature (> 7OO”C),and make the third stage of the recovery. 4.2. Fission product gas burst and the recovery of defects in UC
An integral release of Xe gas from neutronirradiated UC in an isothermal anneal at 800°C was linearly dependent on the annealing time (t), while for an isothermal anneal at 9OO”C,after an 800°C burst, the gas release was dependent upon the square root oft. Thus, the release around 9OO’Cis controlled by a diffusion mechanism in the neutron-irradiated UC. The diffusion-controlled release was also observed even at 800°C when the annealing was prolonged for more than two hours. The burst obtained in this work was the release of fission product gas from the specimen surface in a range of mean free path of the Xe gas in UC. On the other hand, the burst temperatures, obtained here, correspond well to the temperatures of the recovery of lattice parameter. These facts suggest that the burst might be correlated to the recovery of the defects induced by irradiation in UC. As shown in table 3, the burst and the recovery of lattice defects in UC had three steps when irradiated to near 5 X 1016 nvt. Moreover the temperatures at which the gas bursts occur correspond to the recovery of the lattice defects. The activation energies of both phenomena also coincide rather well with each other.
Table 3 Comparison of the activation energies and the temperatures of fission product gas burst and recovery in resistivity of UC irradiated to around 5 X 1016 nvt. The same procedures are applied in both experiments except for the heating rate. The rates are 1 and 2’C/min in resistivity recovery and 5 and lO”C/min in fission product burst. Recovery temperatures of lattice parameters are also presented for comparison.
Remarks
Activation energy (* 0.1 eV) and recovery temperature CC) I
II
III
FP gas release 5.0 X 1016 nvt
0.2 250
0.7 550
1.5 975
Recovery of resistivity 4.0 X 1016 nvt
0.4 200
1.0 400
1.9 650
Recovery of lattice expansion 1.1 X 1016 nvt
225
_
-
550
950
As a consequence, the following cooperative phenomena between the recovery of lattice defects and the burst of fission product gas in UC are proposed. While a lattice of UC, which is distorted by the fission fragment damage, is being recovered at a temperature, an interstitial atom and/or a vacancy, which were introduced by the damage, migrate to a lattice position, or in some fractions to sinks (such as a surface, a grain boundary and a cluster). In the meantime, the relaxation at the lattice is continuing on one side, and on the other a strain field around the fission product gas atom becomes strong. As the strain energy increases, the Xe atom will be unstable and will easily move by itself. Thus an atom near the sink will be pushed out from a site in which the Xe atom is extraneously positioned. Thereafter the whole lattice of UC is relaxed again until the next migration of defects takes place. Accordingly it is concluded here that fusion product gas burst occurs during the course of recovery of lattice defects in UC. We conclude, therefore, that the first step of the gas burst in UC was attributed to a release of Xe atoms related to the movement of interstitial atoms, and the second and the third bursts were connected with migrations of vacancies of carbon and uranium respectively. A variation of the activation energy, espacially in
H. Matsuiflnitialburst region of uranium monocarbides
the third step, indicates that there are many different types of Xe atoms sites surrounded by various defect configurations in UC. Anomalous release mechanism in ion-bombarded rare gases from various materials were interpreted by .many workers [7,12,38,42]. Among those, Jech [7] and Matzke [ 121 explained the anomalous release by the recovery and movement of defects on defect clusters with some discrete stages. Below room temperature, a release of Rn (with an activation energy of 0.1-0.5 eV) from single crystal UC, reported by Matzke [18, 191, is not contradictory. The release of an interstitial Rn atom in UC in Mat&e’s study might be caused in the course of recovery of the lattice (or lattice relaxation) by warming the specimen, and the mechanism could be interpreted by the same consideration of the burst proposed here. It may be reasonably considered that the defects recovered after ion bombardment at a low temperature are different from those of fission damage, as in our experiment, even if the values of the activation energy are roughly equal. The activation energies of the ion-bombarded rare gas release below 1000°C were reported [6, 171, but they do not agree with those obtained in the present study. Among the reports on the Xe gas burst from UC following reactor irradiation [4,20-221, Auskern [20] reported the activation energy of 1.I-- 1.5 eV for the burst of Xe in powdered UC and concluded it resulted from a migration of vacancies. We obtained an almost equal value to that in the third step. We have contributed some new insight on the initial burst of fission product gas in neutron irradiated UC. Further work, however, is expected to clarify more precisely the problem between the damage and the burst release in UC after reactor irradiation.
5. Conclusions We studied fission damage and fission product gas (Xe) burst release of UC. The results obtained are as follows: (1) It was confirmed that fission product gas burst of UC occurred in correlation with the recovery of extra lattice defects. The mechanism of the burst was proposed. (2) Three steps were observed in the burst and
167
also in the recovery of the lattice defects when a neutron fluence was below about 1017 nvt. The recovery was due to the movement of rather simple point defects. (3) Exceeding the dose mentioned above, only two steps appeared both in the burst and in the recovery of the defects because of the formation of clusters of defects and UC double vacancies.
Acknowledgements
The author wishes to thank Prof. T. Kirihara, Nagoya University, for many helpful discussions and suggestions during the course of this work. Special thanks are due to Messrs. M. Tamaki and M. Horiki for their experimental assistance. Part of this work was done at JAERI and the remainder at KUR. The author is indebted to Dr. T. Kikuchi, JAERI, and Dr. T. Tamai, Kyoto University, for their valuable discussions in this study.
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H. Matsui/Initial burst region of uranium monocarbides
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