Gautsch, O.

Mustacchi, C.

Schurenkamper, A.

Wahl, H.

Archive of European Integration

,kv¡m 13. EUR 3267.e KSK EUROPEAN ATOMIC ENERGY COMMUNITY - EURATOM ! M 1 | THE THERMAL RELEASE OF KRYPTON l | i e FROM THIN LAYERS OF UO 9R EB siífeFíiiiieiiliiiil by m:ì m O. GAUTSCH, C. MUSTACCHI, A. SCHÜRENKÄMPER and Η. WAHL Engineering Department - Metallurgy and Ceramics - Physical Chemistry ¡SPS Ρκκΐ ÍIRlSfír^ iiifll«itl Ι>!!,'ί·ϊ*Η 'Jij·' ÍJI:I2¡ Τ»* ì ι!Η3ϋιΐίΙ·"Ί4>>Ι,Ί··. !!MÌ( w NOTICE ft^Jiïatói'Ï avie» This document was prepared under the sponsorship of the Commission of the European Atomic Energy Community my Ji;|T; W Neither the EURATOM Commission, its contractors nor any person acting on their behalf : Make any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this document, or that the use of any information, apparatus, method, or process disclosed in this Ss» document may not infringe privately owned rights ; or li 1M|! Assume any liability with respect to the use of, or for damages resulting from the use of any information, apparatus, method mmBmimì process disclosed in this document. SWAT« m This report is on sale at the addresses listed on cover page 4 FB 40.- DM 3.20 Lit. 500 Fl. 3, When ordering, please quote the EUR number and the title, which are indicated on the cover of each report. affii iislii«*' e i l ^  *v—"e JNEÊ l ' I n i R ™ 0 D 0 ] L . Tonnai·« 1 Qß7 'ί.ί· Brussels, January 1967 feri" lMfW|ïivi'4*.· EUR 3267.e THE THERMAL RELEASE OF KRYPTON FROM THIN LAYERS OF U02 by O. GAUTSCH, C. MUSTACCHI, Α. SCHÜRENKÄMPER and H. WAHL European Atomic Energy Community — EURATOM Joint Nuclear Research Center — Ispra Establishment (Italy) Engineering Department — Metallurgy and Ceramics — Physical Chemistry Brussels, January 1967 — 20 Pages — 7 Figures — FB 40 The release of Kr from thin U0 2 layers was investigated. One type of experiments consisted in annealing sintered U0 2 pellets which were previously bombarded with Kr-ions of a few KeV. In a second type of experiments layers of recondensed U0 2 , containing between 0.04 and 0.1 Kr-atoms per atom of U, were annealed. EUR 3267.e THE THERMAL RELEASE OF KRYPTON FROM THIN LAYERS OF U0 2 by O. GAUTSCH, C. MUSTACCHI, Α. SCHÜRENKÄMPER and H. WAHL European Atomic Energy Community — EURATOM Joint Nuclear Research Center — Ispra Establishment (Italy) Engineering Department — Metallurgy and Ceramics — Physical Chemistry Brussels, January 1967 — 20 Pages — 7 Figures — FB 40 The release of Kr from thin UO„ layers was investigated. One type of experiments consisted in annealing sintered U0 2 pellets which were previously bombarded with Kr-ions of a few KeV. In a second type of experimsnts layers of recondensed UO„, containing between 0.04 and 0.1 Kr-atoms per atom of U, were annealed. In both cases large fractions of gas were released below 900°C within 2 hours. Between 300°C — 900°C gas emission from bombarded pellets was found to be mainly controlled by activation energies in the range of 50 — 70 Kcal/mole. For U 0 2 films, condensed from the vapour phase no gas release could be observed up to 300°C. Between 300°C — 500°C the emission of Kr occurred with activation energies ranging from 40 — 90 Kcal/mole roughly. The dominating values of activation ener-gies could not be established. In both cases large fractions of gas were released below 900°C within 2 hours. Between 300°C — 900°C gas emission from bombarded pellets was found to be mainly controlled by activation energies in the range of 50 — 70 Kcal/mole. For UO, films, condensed from the vapour phase no gas release could be observed up to 300°C. Between 300°C — 500°C the emission of Kr occurred with activation energies ranging from 40 — 90 Kcal/mole roughly. The dominating values of activation ener-gies could not be established. EUR 3267.e EUROPEAN ATOMIC ENERGY COMMUNITY - EURATOM THE THERMAL RELEASE OF KRYPTON FROM THIN LAYERS OF UO2 by O. GAUTSCH, C. MUSTACCHI, Α. SCHÜRENKÄMPER and H. WAHL 1967 Joint Nuclear Research Center Ispra Establishment - Italy Engineering Department - Metallurgy and Ceramics - Physical Chemistry SUMMARY The release of Kr from thin U0 2 layers was investigated. One type of experiments consisted in annealing sintered U0„ pellets which were previously bombarded with Kr-ions of a few KeV. In a second type of experiments layers of recondensed UO,, containing between 0.04 and 0.1 Kr-atoms per atom of U, were annealed. In both cases large fractions of gas were released below 900°C within 2 hours. Between 300°C — 900°C gas emission from bombarded pellets was found to be mainly controlled by activation energies in the range of 50 — 70 Kcal/mole. For U 0 2 films, condensed from the vapour phase no gas release could be observed up to 300nC. Between 300°C — 500°C the emission of Kr occurred with activation energies ranging from 40 — 90 Kcal/mole roughly. The dominating values of activation ener-gies could not be established. C o n t e n t s 1,) In t ro duc t i on 2.) Release of Kr trapped by ion bombardment in sintered UOg 3·) Release of Kr trapped in recondensed UOp 4·) Conclusions - k -1. Introduction /,\ 1.1 The scope of the present experiments was to obtain release data for Kr which was embedded in thin layers of UOp. Such data would show whether inert gases which are "attached" (1) (2) (3) to the surface, or buried Just below the surface, are released from polycrystaline UOp with an essentially single activation energy or over a wide range of activation energies. A single activation energy was observed for the release of Kr from UOp mono-crystals (4)· A range was found for the release of fission Xe buried just below the surface of UC (3)· Two types of samples were used: a) sintered UOp wafers, bombarded to saturation with Kr-ions of a few KeV(5), b) thin layers of recondensed UOp, produced by sputtering a UOp wafer with Kr-ions (5). The release data were analyzed by a method describing annealing processes distributed in activation energies. (·) Manuscript received on November 21, 1966 - 5 -2. Release of Kr trapped in sintered UO^ 2.1 UOp wafers were bombarded with known amounts of Oc (Kr + Kr 5)-ions of 1, 51 and 8 KeV. After bombardment the 85 initial beta activity of trapped Kr was measured in a -5 vacuum of about 10 mm Hg, the target being mounted in the release assembly (Pig. 2.1.1). Each target was then annealed in successive temperature steps. The vacuum during heating was better than 10 mm Hg. The preset temperatures were attained within a few minutes. 85 The Kr activity in the target was recorded as a function of time and temperature. The fraction of the gas which remained in the sample was plotted vs time. When the slopes of the release curves became very small, the following annealing step was started at a higher temperature. Some typical curves are shown in Pig. 2.1.2. A significant release was observed already at about 335°C. 2.2 To obtain the activation energy E from the kinetic curves, the relation of Overhauser (6) was used, which is applicable for 2 successive stages of step-annealing, if the processes occur with a single activation energy. If the processes are distributed in activation energy, the above relation will give approximate values for E when the temperature steps are small. - 6 -If T.. and Tp are the temperatures of 2 successive annealing steps ( Κ ) , then 2.3 R T«T9 log V (1) E = , ψ cal/mole. 1 2 R is 2 cal/mole degree and V = (dP/dt) ./(dF/dt)^ The values for dP/dt (P = fraction of gas left in the sample, t = annealing time) were obtained from the slopes of the Ρ vs t curves at the beginning of the successive stage 2, where the value of Ρ is the same. In all annealing experiments only a few with small temperature steps of about 50 C were performed which could be used to calculate E from eq. (1). The values obtained are shown in Table 2.2.1. Table 2.2.1 Activation Energy E [_Kcal / molej Bombardment energy 1 KeV 5 KeV 8 KeV T-step, °C 600-650 31 707-750 46 800-848 83 853-902 74 900-951 60 - 7 -The results obtained do not confirm the assumption that a process with a discrete activation energy controls the gas release. There is some indication for a distribution spectrum of activation energies. A similar behavior was found by A. Auskern (3) for the release of fission Xe from UC powders. 2.3 In order to get more information about the spectrum of activation energies, a method outlined by W. Primak (7) was used. The characteristic activation energy E is defined by (2) EQ = RT In (Bt) where Β is a frequency factor. Β is assumed to be a oonstant in first approximation, since E is not particularly sensitive to changes in Β . If ρ (E ) is the initial activation energy dis­tribution as function of the characteristic activation energy the gas release can be described by the following equation (7) <3) g - - P0 ».) Sr2 - - Ό <V Ψ - 8 -Setting ρ (E ) = Κ eq. (3) can be written: U) K = d (Int) "RT Eq. (2) - (4) require that ρ (E ) be the same for all samples from which the single points of the distribution curve are to be determined. Therefore release curves ob­tained by successive step annealing cannot be analyzed by means of eq. (2) - (4), because after the first annealing step the distribution curve has another shape than before. However, the application of the above equations seems to be justified as an approximation for sufficient large temperature steps and sufficient long annealing times where the influence of the preceeding step is assumed to be small. This means that the slopes dP/dt or dP/d(lnt) have about the same value after a long annealing time, regardless if the sample was brought to a given temperature instantly or if it was already annealed before. Fig. 2.3· 1· shows typical curves, obtained by­re-plotting the data of Fig. 2.1.2. as Ρ vs logt. The slopes of the curves for a given temperature are within experimental errors constant between 1o and 2oo min. Values of Κ were obtained from the slopes of the curves and are shown as a function of temperature in Fig. 2.3· 2. The highest K-value s were found at about 600°C for the 1 KeV-samples, 800°C for 5 KeV, and 85o°C for 8 KeV. - 9 -Por the calculation of the characteristic activation energy E for each temperature by eq. (2), Β must be known. The frequency factor Β may be estimated p using the relation Β = D /l where Β is obtained from diffusion measurements, and the penetration depth 1 from an independent experiment. Por sintered UOp Β = 6.6 · 10~6 cm2/sec (8) and 1 = 2 · 10~8 cm ± 50 $ per KeV (5). Taking log Β = 10, and t = 600-12000 sec, E Q can be calculated. Por the temperatures which correspond to the maxima of K in Pig. 2.3·2. the results are shown in Table 2.3.1. Table 2.3.1. Bombardment energy, KeV 1 5 8 temperature of maximum K,°K 880 1070 1140 Activation energies corresponding to the maximum of the initial distribution, Kcal/mole 52-57 63-69 67-74 The results are in reasonable agreement with those obtained by using the relation (1) outlined by Overhauser, Release of Kr from UOp saturated by ion bombardment is therefore controlled mainly by an activation energy of about 50 - 70 Kcal/mole. - 10 -3· Release of Kr trapped in recondensed UOp 3.1 Thin UOp layers, containing between 0.04 and 0.1 Kr-atoms per atom of U were deposited on Al-collectors in an ion getter assembly (5)· Heat treatment of the collectors was performed as described in Section 2. Typical release curves are shown in Pig. 3.1.1. and 3.1.2. Up to 300°C no gas release was observed at all (see also ref.9)· If we assume also for these experiments that the gas release is proportional to the annealing of structural defects, an attempt can be made to analyze the release curves by the method of Overhauser (6). The results obtained for experiments with small temperature steps are given in Table 3·1·1 - * Table 3-1.1» Activation Energy E in Kcal/mole T-step, °C E 312-352 62 340-362 87 381-341 42 395-450 46 400-451 56 445-490 89 - 11 -The activation energies for the release seem to be comparable to the values obtained for the release of Kr injected into sintered UO2· 3.2 The logarithmic plot of the release curves shows that after about 40 minutes the slopes of some release curves change abruptly. This behavior would indicate changes of the structure of UOp to which gas release may be related. In fact, electron diffraction micrographs showed that the deposited UOp was initially amorphous. Only during heating by an electron beam rings characteristic for the crystalline structure of UOp appeared. This is shown in Pig. 3.2.1. The values given in Table 3· 1 · 1· can there-fore be considered to be indicative. A more detailed analysis (e.g. by Primak's method) would not be justi-fied. 4. Conclusions 4.1 Large fractions of surface trapped Kr are released from UOp at temperatures below 900°C within 2 hours. Similar release behavior was found by A. Auskern (3) for the release of fission Xe from UC powders. - 12 -4.2 An attempt was made to analyze the obtained results by the methods of Overhauser and Primak. Both methods gave evidence of an activation energy dis-tribution with a maximum at 50 - 70 Kcal/mole in case of Kr-bombarded UOp wafers. 4·3 The release of Kr from recondensed UOp gave some indication of structural changes during annealing at temperatures between 300°C and 500°C. Indicative values for the activation energies involved are 40 - 90 Kcal/mole. The dominating values of the activation energies could not be determined. 4.4 The results lead to the conclusion that the kinetic of gas release at low temperature is different from the kinetic of bulk diffusion. Apparent bursts observed in diffusion measurements may be of the same kind as the release observed in the described experiments - 13 -R e f e r e n c e s 1.) W.B. Lewis et al., AECL-2o19» 1965 2.) R. Kelly, Can. J. Chem. , ¿9_ (1961), p. 2411 3·) A. Auskera, J. Amer. Cer« Soc, àJ7_ (1964), No. 8 4.) R. Kelly et al, J. Nucl. Mat., Y[ (1965), No. 2, p.179 5.) 0. Gautsch, C. Mustacchi, H. Wahl, EUR 2515.e, 1965 6.) A.W. Overhauser, Phys. Rev., <X) (1953), Ρ· 393 7.) W. Primak, Phys.Rev., _100 (1955), p-1677 8.) J. Belle, Uranium Bioxide, USAEC, 1961, p. 512 9.) Β. Bavies et al., AERE-M 969, 1963 io COU n far O - 220V-Supply --ß - scínHLLaíor turgot furnace. ihermo couple Release assembly H'S ¿ 1.1 4 5 6 7 8 9 101 4 5 6 7 8 9 10' 4 5 6 7 8 9 10· O.B α 7 Ob Οό Of ο3 0.2 αϊ ^ 4 ■ς & Ι i Ί5 -S: 4 5 6 7 8 9 tø 4 5 6 7 8 §400 4 5 6 7 8 9 4000 . Teilung \ . . „ „ Einheit \ _-L °9 a r - Division ƒ 1 - 1 0 0° Unité ƒ » m m Τ& 2.3.1 Ed. Aerni-Leuch, Bern. Nr 534 ^ ■ Α η } 1 - ^ « ? ' ! «M» ?,jp. 2. 3. 2 Ed. Aernl-Leuch, Bern Nr. 527 FABRICATION SUISSE ω ■£*■ Ui 0> ~J OD <0 -^ Ol Ol ν ) O! O ^ CO C ol 8 CL = 3 3 3 OJ Α . ο ι σ> - J CO i o Λ? ω *». u i σ ι - J oo t o /¡¡tø Tig. 3.1. 2 L'"'""^.1" > i Co k) ■■ ■■ à. ...JÉ ¡fcf. ' .»*^ SSW! ΙΠΠΕ KiXíHnni n i f l Κ ,·Γ.(Κ ' Lt,;N*!S to­rn (■«•ij m ¡S ■.ïïwmï m KUiU m m iUitøH ÎâfiÊ'ê wÊÊÊÈÈÊÊÊmm ■M mi m. N « m m NOTICE TO THE READER Ail Euratom reports are announced, as and when they are issued, in the monthly periodical EURATOM INFORMATION, edited by the Centre for Information and Documentation (CID). 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The Thermal Release of Krypton from Thin Layers of UO2. EUR 3267.

,kv¡m 13. EUR 3267.e 
KSK 
EUROPEAN ATOMIC ENERGY COMMUNITY - EURATOM 
! M 1 | 
THE THERMAL RELEASE OF KRYPTON 
l | i e FROM THIN LAYERS OF UO 
9R 
EB siífeFíiiiieiiliiiil by 
m:ì 
m 
O. GAUTSCH, C. MUSTACCHI, A. SCHÜRENKÄMPER and Η. WAHL 
Engineering Department - Metallurgy and Ceramics - Physical Chemistry ¡SPS Ρκκΐ 
ÍIRlSfír^ iiifll«itl Ι>!!,'ί·ϊ*Η 'Jij·' 
ÍJI:I2¡ Τ»* ì 
ι!Η3ϋιΐίΙ·"Ί4>>Ι,Ί··. 
!!MÌ( 
w 
NOTICE 
ft^Jiïatói'Ï avie» 
This document was prepared under the sponsorship of the 
Commission of the European Atomic Energy Community 
my Ji;|T; 
W 
Neither the EURATOM Commission, its contractors nor any 
person acting on their behalf : 
Make any warranty or representation, express or implied, with 
respect to the accuracy, completeness, or usefulness of the 
information contained in this document, or that the use of any 
information, apparatus, method, or process disclosed in this 
Ss» 
document may not infringe privately owned rights ; or li 1M|! 
Assume any liability with respect to the use of, or for damages 
resulting from the use of any information, apparatus, method 
mmBmimì process disclosed in this document. 
SWAT« 
m 
This report is on sale at the addresses listed on cover page 4 
FB 40.- DM 3.20 Lit. 500 Fl. 3, 
When ordering, please quote the EUR number and the title, 
which are indicated on the cover of each report. affii 
iislii«*' e i l ^  *v—"e JNEÊ 
l ' I n i R ™ 0 D 0 ] L . Tonnai·« 1 Qß7 'ί.ί· 
Brussels, January 1967 feri" lMfW|ïivi'4*.· 
EUR 3267.e 
THE THERMAL RELEASE OF KRYPTON FROM THIN LAYERS 
OF U02 by O. GAUTSCH, C. MUSTACCHI, Α. SCHÜRENKÄMPER and H. WAHL 
European Atomic Energy Community — EURATOM 
Joint Nuclear Research Center — Ispra Establishment (Italy) 
Engineering Department — Metallurgy and Ceramics — Physical 
Chemistry 
Brussels, January 1967 — 20 Pages — 7 Figures — FB 40 
The release of Kr from thin U0 2 layers was investigated. One type of 
experiments consisted in annealing sintered U0 2 pellets which were 
previously bombarded with Kr-ions of a few KeV. In a second type of 
experiments layers of recondensed U0 2 , containing between 0.04 and 0.1 
Kr-atoms per atom of U, were annealed. 
EUR 3267.e 
THE THERMAL RELEASE OF KRYPTON FROM THIN LAYERS 
OF U0 2 by O. GAUTSCH, C. MUSTACCHI, Α. SCHÜRENKÄMPER 
and H. WAHL 
European Atomic Energy Community — EURATOM 
Joint Nuclear Research Center — Ispra Establishment (Italy) 
Engineering Department — Metallurgy and Ceramics — Physical 
Chemistry 
Brussels, January 1967 — 20 Pages — 7 Figures — FB 40 
The release of Kr from thin UO„ layers was investigated. One type of 
experiments consisted in annealing sintered U0 2 pellets which were 
previously bombarded with Kr-ions of a few KeV. In a second type of 
experimsnts layers of recondensed UO„, containing between 0.04 and 0.1 
Kr-atoms per atom of U, were annealed. 
In both cases large fractions of gas were released below 900°C within 
2 hours. Between 300°C — 900°C gas emission from bombarded pellets 
was found to be mainly controlled by activation energies in the range of 
50 — 70 Kcal/mole. For U 0 2 films, condensed from the vapour phase 
no gas release could be observed up to 300°C. Between 300°C — 500°C 
the emission of Kr occurred with activation energies ranging from 
40 — 90 Kcal/mole roughly. The dominating values of activation ener-
gies could not be established. 
In both cases large fractions of gas were released below 900°C within 
2 hours. Between 300°C — 900°C gas emission from bombarded pellets 
was found to be mainly controlled by activation energies in the range of 
50 — 70 Kcal/mole. For UO, films, condensed from the vapour phase 
no gas release could be observed up to 300°C. Between 300°C — 500°C 
the emission of Kr occurred with activation energies ranging from 
40 — 90 Kcal/mole roughly. The dominating values of activation ener-
gies could not be established. 
EUR 3267.e 
EUROPEAN ATOMIC ENERGY COMMUNITY - EURATOM 
THE THERMAL RELEASE OF KRYPTON 
FROM THIN LAYERS OF UO2 
by 
O. GAUTSCH, C. MUSTACCHI, Α. SCHÜRENKÄMPER and H. WAHL 
1967 
Joint Nuclear Research Center 
Ispra Establishment - Italy 
Engineering Department - Metallurgy and Ceramics - Physical Chemistry 
SUMMARY 
The release of Kr from thin U0 2 layers was investigated. One type of 
experiments consisted in annealing sintered U0„ pellets which were 
previously bombarded with Kr-ions of a few KeV. In a second type of 
experiments layers of recondensed UO,, containing between 0.04 and 0.1 
Kr-atoms per atom of U, were annealed. 
In both cases large fractions of gas were released below 900°C within 
2 hours. Between 300°C — 900°C gas emission from bombarded pellets 
was found to be mainly controlled by activation energies in the range of 
50 — 70 Kcal/mole. For U 0 2 films, condensed from the vapour phase 
no gas release could be observed up to 300nC. Between 300°C — 500°C 
the emission of Kr occurred with activation energies ranging from 
40 — 90 Kcal/mole roughly. The dominating values of activation ener-
gies could not be established. 
C o n t e n t s 
1,) In t ro duc t i on 
2.) Release of Kr trapped by ion bombardment 
in sintered UOg 
3·) Release of Kr trapped in recondensed UOp 
4·) Conclusions 
- k -
1. Introduction /,\ 
1.1 The scope of the present experiments was to obtain 
release data for Kr which was embedded in thin layers 
of UOp. Such data would show whether inert gases which 
are "attached" (1) (2) (3) to the surface, or buried 
Just below the surface, are released from polycrystaline UOp 
with an essentially single activation energy or over a 
wide range of activation energies. A single activation 
energy was observed for the release of Kr from UOp mono-
crystals (4)· A range was found for the release of fission 
Xe buried just below the surface of UC (3)· 
Two types of samples were used: 
a) sintered UOp wafers, bombarded to saturation with Kr-
ions of a few KeV(5), 
b) thin layers of recondensed UOp, produced by sputtering 
a UOp wafer with Kr-ions (5). 
The release data were analyzed by a method describing 
annealing processes distributed in activation energies. 
(·) Manuscript received on November 21, 1966 
- 5 -
2. Release of Kr trapped in sintered UO^ 
2.1 UOp wafers were bombarded with known amounts of 
Oc 
(Kr + Kr 5)-ions of 1, 51 and 8 KeV. After bombardment the 
85 initial beta activity of trapped Kr was measured in a 
-5 vacuum of about 10 mm Hg, the target being mounted in the 
release assembly (Pig. 2.1.1). Each target was then 
annealed in successive temperature steps. 
The vacuum during heating was better than 10 mm Hg. 
The preset temperatures were attained within a few minutes. 
85 The Kr activity in the target was recorded as a function 
of time and temperature. The fraction of the gas which 
remained in the sample was plotted vs time. When the 
slopes of the release curves became very small, the 
following annealing step was started at a higher temperature. 
Some typical curves are shown in Pig. 2.1.2. A significant 
release was observed already at about 335°C. 
2.2 To obtain the activation energy E from the kinetic 
curves, the relation of Overhauser (6) was used, which is 
applicable for 2 successive stages of step-annealing, if 
the processes occur with a single activation energy. If 
the processes are distributed in activation energy, the 
above relation will give approximate values for E when 
the temperature steps are small. 
- 6 -
If T.. and Tp are the temperatures of 2 successive 
annealing steps ( Κ ) , then 
2.3 R T«T9 log V (1) E = , ψ cal/mole. 
1 2 
R is 2 cal/mole degree and V = (dP/dt) ./(dF/dt)^ The 
values for dP/dt (P = fraction of gas left in the sample, 
t = annealing time) were obtained from the slopes of 
the Ρ vs t curves at the beginning of the successive stage 2, 
where the value of Ρ is the same. In all annealing experiments 
only a few with small temperature steps of about 50 C 
were performed which could be used to calculate E from 
eq. (1). The values obtained are shown in Table 2.2.1. 
Table 2.2.1 
Activation Energy E [_Kcal / molej 
Bombardment energy 1 KeV 5 KeV 8 KeV 
T-step, °C 
600-650 31 
707-750 46 
800-848 83 
853-902 74 
900-951 60 
- 7 -
The results obtained do not confirm the assumption 
that a process with a discrete activation energy 
controls the gas release. There is some indication for 
a distribution spectrum of activation energies. A 
similar behavior was found by A. Auskern (3) for the 
release of fission Xe from UC powders. 
2.3 In order to get more information about the 
spectrum of activation energies, a method outlined 
by W. Primak (7) was used. The characteristic activation 
energy E is defined by 
(2) EQ = RT In (Bt) 
where Β is a frequency factor. Β is assumed to be a 
oonstant in first approximation, since E is not particularly 
sensitive to changes in Β . 
If ρ (E ) is the initial activation energy dis­
tribution as function of the characteristic activation 
energy the gas release can be described by the following 
equation (7) 
<3) g - - P0 ».) Sr2 - - Ό <V Ψ 
- 8 -
Setting ρ (E ) = Κ eq. (3) can be written: 
U) K = d (Int) "RT 
Eq. (2) - (4) require that ρ (E ) be the same for all 
samples from which the single points of the distribution 
curve are to be determined. Therefore release curves ob­
tained by successive step annealing cannot be analyzed by 
means of eq. (2) - (4), because after the first annealing 
step the distribution curve has another shape than before. 
However, the application of the above equations seems to 
be justified as an approximation for sufficient large 
temperature steps and sufficient long annealing times 
where the influence of the preceeding step is assumed 
to be small. This means that the slopes dP/dt or dP/d(lnt) 
have about the same value after a long annealing time, 
regardless if the sample was brought to a given temperature 
instantly or if it was already annealed before. 
Fig. 2.3· 1· shows typical curves, obtained by­
re-plotting the data of Fig. 2.1.2. as Ρ vs logt. The 
slopes of the curves for a given temperature are within 
experimental errors constant between 1o and 2oo min. 
Values of Κ were obtained from the slopes of the curves 
and are shown as a function of temperature in Fig. 2.3· 2. 
The highest K-value s were found at about 600°C for the 
1 KeV-samples, 800°C for 5 KeV, and 85o°C for 8 KeV. 
- 9 -
Por the calculation of the characteristic 
activation energy E for each temperature by eq. (2), 
Β must be known. The frequency factor Β may be estimated 
p 
using the relation Β = D /l where Β is obtained from 
diffusion measurements, and the penetration depth 1 
from an independent experiment. Por sintered UOp Β = 
6.6 · 10~6 cm2/sec (8) and 1 = 2 · 10~8 cm ± 50 $ per 
KeV (5). Taking log Β = 10, and t = 600-12000 sec, E Q can 
be calculated. Por the temperatures which correspond to 
the maxima of K in Pig. 2.3·2. the results are shown in 
Table 2.3.1. 
Table 2.3.1. 
Bombardment energy, KeV 1 5 8 
temperature of maximum K,°K 880 1070 1140 
Activation energies 
corresponding to the 
maximum of the initial 
distribution, Kcal/mole 52-57 63-69 67-74 
The results are in reasonable agreement with those 
obtained by using the relation (1) outlined by Overhauser, 
Release of Kr from UOp saturated by ion bombardment is 
therefore controlled mainly by an activation energy 
of about 50 - 70 Kcal/mole. 
- 10 -
3· Release of Kr trapped in recondensed UOp 
3.1 Thin UOp layers, containing between 0.04 and 0.1 
Kr-atoms per atom of U were deposited on Al-collectors 
in an ion getter assembly (5)· Heat treatment of the 
collectors was performed as described in Section 2. 
Typical release curves are shown in Pig. 3.1.1. and 
3.1.2. Up to 300°C no gas release was observed at all 
(see also ref.9)· If we assume also for these experiments 
that the gas release is proportional to the annealing 
of structural defects, an attempt can be made to analyze 
the release curves by the method of Overhauser (6). 
The results obtained for experiments with small 
temperature steps are given in Table 3·1·1 - * 
Table 3-1.1» 
Activation Energy E in Kcal/mole 
T-step, °C E 
312-352 62 
340-362 87 
381-341 42 
395-450 46 
400-451 56 
445-490 89 
- 11 -
The activation energies for the release seem to be 
comparable to the values obtained for the release 
of Kr injected into sintered UO2· 
3.2 The logarithmic plot of the release curves 
shows that after about 40 minutes the slopes of some 
release curves change abruptly. This behavior would 
indicate changes of the structure of UOp to which 
gas release may be related. In fact, electron 
diffraction micrographs showed that the deposited 
UOp was initially amorphous. Only during heating by 
an electron beam rings characteristic for the 
crystalline structure of UOp appeared. This is shown in 
Pig. 3.2.1. The values given in Table 3· 1 · 1· can there-
fore be considered to be indicative. A more detailed 
analysis (e.g. by Primak's method) would not be justi-
fied. 
4. Conclusions 
4.1 Large fractions of surface trapped Kr are 
released from UOp at temperatures below 900°C within 
2 hours. Similar release behavior was found by 
A. Auskern (3) for the release of fission Xe from 
UC powders. 
- 12 -
4.2 An attempt was made to analyze the obtained 
results by the methods of Overhauser and Primak. Both 
methods gave evidence of an activation energy dis-
tribution with a maximum at 50 - 70 Kcal/mole in 
case of Kr-bombarded UOp wafers. 
4·3 The release of Kr from recondensed UOp 
gave some indication of structural changes during 
annealing at temperatures between 300°C and 500°C. 
Indicative values for the activation energies involved 
are 40 - 90 Kcal/mole. The dominating values of the 
activation energies could not be determined. 
4.4 The results lead to the conclusion that the 
kinetic of gas release at low temperature is different 
from the kinetic of bulk diffusion. Apparent bursts 
observed in diffusion measurements may be of the same 
kind as the release observed in the described experiments 
- 13 -
R e f e r e n c e s 
1.) W.B. Lewis et al., AECL-2o19» 1965 
2.) R. Kelly, Can. J. Chem. , ¿9_ (1961), p. 2411 
3·) A. Auskera, J. Amer. Cer« Soc, àJ7_ (1964), No. 8 
4.) R. Kelly et al, J. Nucl. Mat., Y[ (1965), No. 2, p.179 
5.) 0. Gautsch, C. Mustacchi, H. Wahl, EUR 2515.e, 1965 
6.) A.W. Overhauser, Phys. Rev., <X) (1953), Ρ· 393 
7.) W. Primak, Phys.Rev., _100 (1955), p-1677 
8.) J. Belle, Uranium Bioxide, USAEC, 1961, p. 512 
9.) Β. Bavies et al., AERE-M 969, 1963 
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The Thermal Release of Krypton from Thin Layers of UO2. EUR 3267.

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