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The system UO2-Eu2O3 at high temperatures
JOURNAL
OF NUCLEAR
THE
SYSTEM
L. N. General
21 (1967) 302-309. 0
MATERIALS
GROSSMAN,
J. E.
Nuclear
LEWIS
Technology
Pleasanton, Received
Phase equilibria determined sphere
in the system
for high
by
thermal
ceramograpby.
analysis,
Extensive
in the fluorite
phase;
fluorite mole
X-ray
solubility Urania
% Eu01.5;
50” C. Lattice
the eutectic
temperature
been
observed
invariant
; a binary
temperature
the
compound
are reported
and phase.
region
are
and
W
has
and
binary
for the Eu303-W
system. Le diagremme
d’bquilibre
des phases a BtB determine
U03-Eu303
et en atmosphitre
neutre
diffraction
des
existe
une
solubiliti!
phase
rayons
fluorine
alors
et
importante qu’il
n’y
Die durch
Eu303 cubique
keit
entre les phases fluorine
pour
80 mole
eutectique
y.
est
de
von
Eu3O3
Uranoxid
sont sign&%
Phase
besteht
UO3 stabilise
Un eutectique
se
et I%303 monoclinique
et
pour
iiber
eine
LB temperature
variante berichtet.
Introduction
Europium oxide is being considered for use in fast reactors to control the increase in reactivity caused by water flooding in a steamcooled reactor that has high neutron leakage Ia). The effectiveness of europium as a thermal or epithermal poison to control routine flooding reactivities in mixed U-Pu oxide fuel has been demonstrated in a subcritical assembly lb). It is anticipated that, for power reactor use, the europium will be in solution in the oxide fuel which will be fabricated by conventional
in
und
Fiir
die
unterhalb
Lijslichkeit der
mit
Solidus-
der
zwischen
Das Soliduslinie postuliert.
Eu303
monoklinen
ca.
80 Mel liegt werden
und
y0 bei die
Liquidus-
Gleichgewichtswird
fiir
Es wurde
die eine
Eu303 und W beobachtet
Verbindung im
im
von UO3
das kubische und
Fluoritphase die
Seite
Temperatur
Grosse L&lich-
Fluoritphase;
Temperatur
angegeben.
binllre
der
keine
Eutektikum
europiumoxydreiche
Les
param&res
ein
Eu203-U02
Riintgenfeinstruktur-
Fluorit
eutektische
Wechselwirkung
System
stabilisiert
1885” C. Zwischen
diagramm
2125 + 50” C.
a 6th
binaire
cornpost
bestimmt.
besteht
featgestellt.
de
ELI 0,5 environ.
la rhgion
und inerte AtmosphLre
Analyse,
Eu3O3 wurde
temperaturen
dans
im
monoklinen
la
& 1885” C environ.
forme
thermische
de Eu303
thermiquo,
solubilit&
un
binaire
analyse und Keramographie
Gitterparameter,
L’oxyde
interaction
Eu303 et W:
Phasengleichgewichte
11
a aucune
Une
wurden fiir hohe Temperaturen
la
la
pour
d’europium. invariante
Des Cquilibres
sugg&&
Eu303-W.
c&amographie.
par l’analyse X
entre
Die EuO1,5. 2125 f 50” C.
aux hautes tempkratures
UO3 dans En303 monoclinique.
1.
observee
bis
pour lo systbme
en oxyde
fluorine.
sont
80
is 2125 &
temperatures
Eu303
riche
du solidus et du liquidus
la phase
du solidus
le systbme
are given for t’ho fluorite between
en dessous
pour
between
for the europia-rich
interaction
les temp&atures
donnkes
une tempkrature
at approximately
solidus
r@ticulaires, sont
Laboratory,
1966
cubic Eu303
Subsolidus postulated.
exists
of UO3 in mono-
liquidus temperatures equilibria
and
of Eu303
is formed
phases
parameters,
have been
Nucleon&s
USA
19 September
diffraction,
stabilizes
1885’ C. An eutectic and monoclinic
CO., AMSTERDAM
and D. M. ROONEY Department,
California,
and neutra,l atmo-
no solubility
clinic Eu303 was found. to about
Eu303-U03
temperature
PUBLISHINQ
UOZ-EuzOs AT HIGH TEMPERATURES
Company,
Electric
NORTH-HOLLAND
und
System
eine
binlire
Eu303-W
; in-
wird
hydrogen sintering. The existence of a large solubility for Euz03 in UOZ and the feasibility of homogenization by hydrogen sintering have been demonstrated 2). The fluorite structure of UOz tolerates large deviations from stoichiometric composition on both the anion- and cation-deficient sides. Hyperstoichiometry is readily produced at temperatures above about 300” C in mildly oxidizing environments. As the temperature is increased, the UOz+, phase boundary broadens by the dissolution of interstitial oxygen ; a 302
THE SYSTEM U02-Euz03 maximum
O-to-U ratio of about 2.26 at about
lo-l.5 atm oxygen and 1450” C has been reported 3). Hypostoichiometry is more difficult it requires temperatures above to produce, about 1600” C and low oxygen pressure. An O-to-U ratio of 1.88 has been reported 4) for Urania equilibrated
at 2400” C in hydrogen
(dew
303
AT HIGH TEMPERATURES after experimentally
establishing
the appropri-
ate calibration curve. It was inferred from weight loss data that no significant compositional changes occurred during sintering. Hot-pressing of blended powders was accomplished Hoyt 8).
in The
apparatus were
described performed
at The
the UOa-s phase reaches a limit of about UO1.65
specimens were held at 5000 psi for lo-20
min
at about 2500” C, ref. 5). Oxides of composition MO2 and of fluorite
at temperature. The hot-pressed pellets were generally crack-free and between 94 and 97 y0
type can dissolve large amounts of sesquioxides when the cations are of similar size. A composition limit of Mol.75 has been observed for these fluorite solutions with anion vacancies 6). At large vacancy concentrations, the vacancies in the fluorite structure can become ordered and form the closely related Type C rare earth (thallium oxide type) structure which is exhibited by most of the rare earth sesquioxides
dense. Previous experience with UOZ pressing has shown that the initially hyperstoichiometric powder is reduced to stoichiometric composition during this procedure. Carbon pickup was confined to the outer pellet surfaces which were cleaned by mild abrasion. The mixed oxide pellets were encapsulated in tungsten by inert arc welding in an argonfilled glove box. Each pellet was cored ultrasonically before welding to accommodate a tungsten tube down the center. A cross section of a specimen capsule after testing is shown in fig. 1.
- 40” C); the phase boundary
for
at low temperatures. Three oxides of europium have been reported 7) : Eu203 (monoclinic and body-centered cubic), EuO (face-centered cubic), and Eu304 (orthorhombic); the sesquioxide is the most stable. A fourth reported oxide, designated “Ortho I” has been identified 7) as the orthosilicate EuzSi04. The sesquioxide crystallizes in the Type C rare earth structure below about 1100” C and in the Type B (monoclinic) structure above 1100” C. The suboxides are difficult to form because of their relative instability and of the high volatility 7) of EuO. No analytically significant deviations in composition from the stoichiometric sesquioxide have been reported for monoclinic Euz03. 2. 2.1.
Experimental
procedure
SPECIMEN PREPARATION
Blended UOZ and Eu203 powders were consolidated by two techniques : hydrogen sintering of specimens containing less than 20 mole percent Eu01.5, and vacuum hot-pressing of the other specimens. Powder analyses and sintering details have been reported 2). The molar ratios were confirmed by X-ray fluorescent analysis
2.2.
1400” C in graphite
by
dies.
point about
1300” C and
the
pressings
THERMAL ANALYSIS
The encapsulated mixed oxide specimens were inductively heated in vacuum to obtain heating and cooling curves. The experimental equipment and procedure for thermal analysis are similar to those reported by Rupert 9). Vacuum is maintained at less than 5 x 10-S Torr during heating. The specimen temperature is measured independently of the temperature recording system by optical pyrometry. A manual brightness pyrometer is sighted on the blackbody hole. The pyrometer is frequently calibrated against an NBS secondary standard bulb between 900 and 2300” C. Temperature measurement above 2300” C is made by extrapolation of the correction curve for the pyrometer. Temperatures below 2300” C are accurate to well within & 1 %; the accuracy of temperatures reported between 2300 and 2600” C is believed to be within f lg y. ; between 2600 and 3000’ C, &- 2 o/o is estimated to be the
304
L. N. GROSSMAN.
J. E. LEWIS AND D. M. ROONEY can readily detect temperature than lo C at all temperatures. of the detector
changes of less The linear range
is shifted by the use of neutral
density filters. The response time of the detector is 0.1 set at the middle of the linear range and less than 0.01 set at the high intensity
end of
the linear range ; thermal analysis data were always taken between these limits. Each specimen was heated to above the liquidus several times. Thermal analysis data were obtained on heating and cooling above approximately 2000’ C ; a few samples were
Fig. 1. Cross so&on of UO&% mole o/0 Eu01.5 in tungsten capsule after thermal analysis.
maximum error. The brightness pyrometer readings are corrected for the fused silica window and prism in the light path. For very high temperatures, tungsten evaporation and deposition on the window can affect temperature measurement accuracy. A magnetically operated shutter minimizes this problem. An image of the blackbody tube is projected by a large objective lens onto a cadmium sulfide photoconductor detector. The blackbody image is enlarged to about 40 x so that it completely covers the 3” dia. detector. The detector resistance is monitored with a shielded drop across the dc circuit. The potential photoconductor is monitored, and its first and second time derivatives may be observed. The potential drop across the detector is approximately linear with temperature over about 400” C. Over the linear range, the sensitivity is about 0.15’ C/mV; thus, a millivolt recorder
examined at lower temperatures. Thermal analysis of the specimens resulted in two general types of observed heat effects: thermal arrests, and abrupt changes in slope of the cooling curve. The first type is observed at temperatures corresponding to invariant points (e.g., melting point and eutectic reaction); the second type is characteristic of simple phase boundaries where no reaction occurs. No special care was taken to equilibrate the thermal analysis specimens at any temperature before cooling ; the final cooling was accomplished by shutting off the induction heater while the specimen was between about 2000 and 2500” C. Two specimens (27.8 and 46.2 mole o/o EuOI.5) were equilibrated at 1600” C after melting in the thermal analysis apparatus; shutting off the power cooled these to less than 900” C in about 1 min.
2.3.
X-RAY ANALYSIS
After thermal analysis, the capsules were sectioned longitudinally as shown in fig. 1. One-half the specimen was removed from the tungsten container and crushed to not less than 200 mesh powders. The powders were analyzed by X-ray diffraction with the Debye-Scherrer and diffractometer techniques, employing the reNelson-Riley and sin20 extrapolations, The wavelengths used were spectively. CuK,, (1.54051 A) and CuK,, (1.54433 A). A weighted average CuK, (1.54178 A) was used for the powder camera method. The accuracy of lattice parameter measurement by the
THE
SYSTEM
UOs-EusOs
AT
method was zt 0.005 A; powder camera diffractometer measurements were accurate in most cases to f was
used
specimens
to
0.0003 A. X-ray confirm
containing
the
fluorescence
compositions
of
up to 66 mole o/o EuOi.5
HIGH
305
TEMPERATURES
one between
2650 and 2695’ C, and another
between 2730 and 2780” C. All specimens showed traces tungsten were
too
in the small
oxide to
phases.
measure
interaction
by the Debye-Scherrer
will be discussed separately.
2.4.
CERAMO~RAPHICANALYSIS
The longitudinally sectioned specimens were prepared for ceramography by grinding through 600 grit silicon carbide grinding paper and rough polishing on nylon polishing cloth charged with 6 and 3 pm diamond paste. Final polishing was accomplished with a thick slurry of Hz02 and Linde “B” (0.05 ,um AlaOs) on Buehler Texmet polishing cloth. Medium pressure was used to avoid excess relief between phases. A 1 to 2 min swab-etch with a 9 : 1 Hz02 :HzSOa solution was sufficient to etch grain boundaries and develop substructure. Dark field illumination was used extensively in determining the pattern and location of metallic tungsten inclusions within the structures of the oxide specimens is). The advantage of dark field over bright field is the ability to see below the polished surface of the oxide matrix and therefore see more clearly the distribution of the metallic inclusions. 3.
Results
A summary of the experimental data obtained by thermal analysis and X-ray diffraction is given in table 1. The two types of thermal effects observed have been abbreviated as follows: A change in slope in the cooling curve is noted by “S”, a thermal arrest is noted by “A”. As a result of gradients in the specimen capsules, a change in slope or an arrest is usually spread over a temperature range of 20 to 50’ C. For example, specimen 10 showed two changes in slope upon heating or cooling,
metallic
The
amounts
accurately
by
ceramographic or X-ray techniques ; X-ray fluorescent analysis of specimens 15 and 17 showed less than 5 o/o total tungsten in the oxide phases. Specimen 18 showed extensive
after thermal analysis. In the pure EuzOs, analysis showed prowhere ceramographic nounced phase segregation (due to reaction with the tungsten capsule), selected areas were scraped with a diamond stylus and analyzed technique.
of
between
pure EusOs and tungsten Ceramography
of
specimens containing up to 57.2 mole y. EuOi.5 showed single phase fluorite which appeared identical to pure UOs. Small metallic tungsten inclusions, similar to those seen lo) in pure UO2, were observed ; a possible trend towards more tungsten with greater EuzOs content was observed. Lattice parameters were obtained from powder films by a Nelson-Riley extrapolation of all diffraction lines with 28 falling above 100” C. For diffractometer scans, a sins0 extrapolation was used. The fluorite lattice parameter data in table 1 and some earlier data 2) are given in fig. 2. Monoclinic europia was observed as a major phase in specimens 15, 17, and 18; line-for-line comparison with the diffraction pattern published by Rau 11) showed near-perfect agreement within the experimental accuracy of the two studies. Body-centered cubic EuzOs was observed in specimens 15 and 17. Identification of the body-centered cubic phase required careful examination to confirm that it was not a second face-centered cubic phase; diffraction lines forbidden in face-centered cubic materials were present. The lattice parameters of the body-centered cubic phases were computed and found to differ significantly from those of pure EuzOs for which ii) ao= 10.869 + 0.003 A. Specimen 15 contained more of the bodycentered cubic phase than specimen 17. Specimen 18, pure Eu203 in tungsten, showed a very nonuniform phase distribution. Obvious reaction had occurred between the tungsten capsule and the oxide, and the reaction had not gone to completion. A mixed oxide structure
306
L.
N.
GROSSMAN,
5.4800
J.
E.
LEWIS
I
I
AND
D.
M.
I
I
ROONEY
I
I .~.
5.4700 !
5.46po
\ ~-
5.4500
-
5.4400
--
"'\,
L
\
;
I
F 2 5.4300 : 'Z -. 5.4200 --
\ 5.4100 -~
I
.... X. x. I... k.
5.4000
~~~
53900 0
I IO
I 20
I 30
I 40
E"O1.5 (Mob
Fig.
2.
Lattice
parameter
versus
composition
Specimen
wt% EuzO3
mole o/0
1
0
0
Not, analyzed
0
0
S, 2755-2787”
ao=5.4707
i
ao =5.4708
& 0.0002
5
7.4
S, about
Not
analyzed
7.4
S, 2760-2850”
C
Not
analyzed
5
15
21.2
S, 2720-2760”
C
Not
analyzed
s,
c
Not
analyzed
equilibrated 10 11
20 35.8
27.8 46.2
S, 2650-2695”
C C
Not
analyzed
46.5
57.2
13
56.6
66.7
15
70
78.2
80
18
100
86.0 100
Not
; at
Fluorite,
ao=5.4100 ao=5.403
S, 2460-2515”
C
Fluorite,
C
Weak
S, 2230-2270”
C
Fluorite,
s,
2350-2390”
C
Weak
trace
S, 2150-2200”
C
Major
monoclinic
2120-2135”
C
A 1870-1895” S,’ 2176-2225” A, 2104-2135” A, 2060-2092”
c C C C
24 S=change
trace
Strong
4
S, 2760-2810’
C
Not
6
8.8
S, 2741-2818”
C
Fluorite,
*
A trace
monoclinic
Euz03
Eu203
bee, ao=10.907
unidentified
Fluorite,
arrest.
EuzO3
5 0.001
Monoclinic Eu203 New fee phase ao=5.395
C
; A =thermel
k 0.001
f
Major monoclinic Euz03 Strong minor bee, ao=10.900
S, 2812-2884”
in slope
& 0.0009
monoclinic
ao=5.400
minor
3.0 6.0
2
0.0009
1600” C
Trace 19 22
f
analyzed
S, 2400-2430”
A, 17
ao=5.4363
0.0002
1600” C
S, 2730-2780” equilibrated 12
Fluorite,
; at
(A)
Fluorit,e,
5
27.8
2800” C
;
present*
Fluorite,
3
20
solutions.
X-ray Phases
C
2770-2795”
solid
da.ta
4
9
j-
fluorite
analysis?
2
1 70
1
of experimental
Thermal
EuO~.~
I 60
50
'I.1
for UOs-Eu01.5
TABLE Summary
I ...... x...................... r-
of W
phase.
ao=5.4660
f
0.005
+ 0.005
& 0.005 See text.
0.0003
analyzed
was found
ao=5.4599 in every
& 0.0003
specimen
examined.
THE
of
apparent
peritectic
UOs-EusOa
SYSTEM
origin
could
be
seen
AT
HIGH
307
TEMPERATURES
respectively.
Solidus and liquidus temperatures
along the tungsten capsule walls and in contact with the tungsten capillary; X-ray diffraction identified the two phases as monoclinic EuaOs
could not be resolved by thermal analysis for compositions rich in UOa. The position of the eutectic is not exactly known, but it must lie
and a new face-centered cubic phase which could not be identified. The new phase has a
between
lattice constant as = 5.395 f similar in structure
0.005 A and is very
of fig. 3 have been inferred
Away from the tungsten in specimen 18 had not
walls, the europia reacted with the
tungsten. A major phase of monoclinic EuzOs and a trace of an unidentified compound were the only phases detected. The diffraction pattern of the unidentified phase is shown in table 2. Ceramography showed this unidentified phase as small nonmetallic islands within the monoclinic EuaOs grains. This phase is dissolved by the HzOa-HaSO4 etchant. TABLE Diffraction
The
specimens
from
side
analysis
of
15 and 17, and from the known 14)
polymorphism
in EuzOs. The invariant temper-
ature observed at about 1885” C in specimen 15 has been interpreted as a peritectoid. (No thermal analysis of specimen 17 was performed below 2000” C.) The peritectoid composition has been estimated from ceramography as has the subsolidus fluorite phase boundary shown in fig. 3. The oxygen pressure dependence of the fluorite phase composition is indicated by
2
pattern of unindentified europium detected in specimen 18
d (8)
y0 Eu01.5.
Subsolidus equilibria on the europia-rich
and size to the compound
2CeO.2..YaOs.
readily
78.2 and 86.0 mole
equilibrated specimens 9 and 11 are represented by two crosses in the fluorite field of fig. 3.
Rel.
oxide
II
intensity
oI
2800
4.225
ww
3.319
MS
2.929
vvw-
2.545
VW
2.373
VW
2.075
M
2600
2400
1.724 Y
Thermal analysis of specimen 18 showed an arrest on heating through the range 2060 to 2090” C which lengthened on each subsequent heating or cooling. Heating above the arrest showed a wide melting region, but no value of the liquidus temperature was obtained. The capsule ruptured after about 15 min above 1800’ C and signs of liquid formation at 2090” C were observed.
r f ;
2200
E 2 2000
1800
1600
4.
Discussion
The data of table 1 have been used to construct the phase diagram shown in fig. 3. The melting points for UOZ and EusOa are those reported by Hausner 12) and Schneider Is),
0
uo2
Fig.
3.
Mol.
r.
UOz-EuOl.5 equilibrium phase high temperatures.
diagram
et
L.
308
oomparison
N.
GROSSMAN,
of this study
with
J.
the work
E.
LEWIS
of
Haug and Weigelis). They studied the UOs.sEusOs system under oxidizing conditions (air
AND
evidence
D.
31.
ROONEY
for only two cation species (Eufs and
sintering at 1100” C followed by an air quench) ;
Uf4) in the fluorite phase. The interaction observed between EusOs and tungsten in specimen 18 bears on the dis-
single
crepancies in reported melting points for EusO3.
phase
fluorite
was
observed
between
about 28 and 64 mole o/o EuOi.5. The fluorite
Wisnyi
phase lattice parameter varied linearly between 5.406 and 5.392 between 38 and 64 mole o/o
for the melting point as measured on a tungsten
EuOi.5 under their oxidizing conditions. The lower lattice parameter observed with increased oxygen content in the fluorite phase is consistent with similar observations on the UOs-UOs system 16) and the UOz-YsOa-UOs system 17). The fluorite phase in the UOs-UaOs-EuzOs system at high temperatures resembles the fluorite phase in the UO2-UsOs-YsOs system 17) in several respects. In both, the fluorite lattice parameter decreases with increasing oxygen content and with increasing lanthanide content. As the oxygen pressure is raised, the region of fluorite phase decreases in both ternaries and gives way to UOs.6 on the Urania-rich side. The linear dependence of lattice parameter with composition shown in fig. 2 is in accord with Vegard’s law as might be expected for random substitution of Eu+s for U+4, and for random anion vacancies. Similar linear behavior 18919) has been observed in the system YsOa-ThOs and LasOs-ThOs where the cations have only one valence and where anion vacancies account for charge neutrality 6). The nonlinear variation of lattice parameter with composition in the UOz-YsOs fluorite solutions has been interpreted by Roberts 6) as evidence of vacancy interaction. Roberts’ conclusion was based on the data of Ferguson and Fogg 20) who equilibrated their specimens with uranium metal at 950 to 1050” C for 2 to 3 days in evacuated vycor tubes. Similar attempts by Aitken 21) to equilibrate high yttria content Urania-yttria fluorite phase with uranium metal did not reach equilibrium after several weeks. Thus it is not clear whether the ordering of vacancies at low temperatures or the excess oxygen caused the nonlinear behavior observed in the YsOs-UOs system. The linear behavior shown in fig. 2 is interpreted as corroborative
and Pijanowski
reported
2050 f
30” C
strip heater 13). Their value is apparently a measure of the invariant temperature observed in specimen 18 for the W-Eu2O3 binary. The value of 2240 f 10” C reported by Schneider is) was obtained using an iridium crucible ; X-ray examination of the residue after melting revealed no new phases. Schneider’s value has been used in the construction of fig. 3, although it may represent an invariant (e.g., monotectic) in the EusOs-Ir system. Foex 22) has reported about 2330” C for the melting point of EusO3 ; his technique involved solar heating of the oxide in air in a crucible of nonmelted EusO3.
5.
Acknowledgments
Grateful acknowledgment is extended to R. M. Harrington, N. R. Mullison, and J. G. Wilson who aided in the fabrication of specimens and in thermal analysis. E. A. Aitken, D. L. Douglass and U. E. Wolff provided stimulating review and discussions. The U.S. Atomic Energy Commission graciously supplied the material from which specimens were fabricated.
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Conf.
Safety,
Fuels
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Report
ANL-‘7120
National
end
Core
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in Thermodynamics of Nuclear Materials (IAEA, Vienna, 1962) E. Aukrust, 1’. Forland and K. Hagemrtrk, ibid. E. A. Aitken, H. C. Brassfield and J. A. McGurty, ANS
Trans.
6, no. 1 (1963)
150
THE
5)
6)
UOs-EuzOs
SYSTEM
AT
HIGH
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(USA)
69 (1965)
1788
L.
Robe&,
E.
J.
pounds,
in Nonstoichiometric
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in Chemistry
Com-
14)
D.C.,
8)
Roy,
9
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A.
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et al., North American Aviation NAA-SR-6765
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DC 59-9-122
‘9
(1959).
Company
Also in ASTM
file of X-ray
diffraction
12)
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J. Nucl.
13)
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15 (1965)
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Company
(1962) and E. (USA)
A.
Aitken,
Report
TM-
1964)
and R.
Mezger,
Z. Phys.
Chem.
201
268 Z. Anorg. Chem.
5
)
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21)
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(1957)
data.
Mat.
(rev.
265 (1951)
(USA) Card
F.
F. Hund and W. Diirrwachter,
9 20
Electric
E.
Electric
E. Hund (1952)
(USA) Report ORNL-
p. 249
J. Nucl. Mat. 9 (1963)
Report
63-7-14
G. 11’. Rupert,
Report
Chem.
355
1962)
U. E. Wolff and D. RI. Rooney, 19”’ Met,allographic Group Meeting (April 20-22, 1965) Oak
R.
10, 1963) p. 12 Phys.
204s
(Germany)
E. W. Hoyt, in Proc. Sec. Intern. Vacuum Congress, Washington, D.C., Oct. 1961, 2 (ed.
TM-1161
J.
1963)
Ridge National Laboratory 11)
R.
15b) H. Haug and F. Weigel,
p. 788 9)
68 (October
and
R. C. Rau, Third rare earth conference (Grand Bahama Island, April, 1963) Gmelin Institute
L. E. Preuss, Pergamon
lo)
Monograph
Warshaw
15a) H. Haug, Doctoral Dissertation (Univ. of Munich,
1963) 7)
I.
(1961)
Series, 39 (ed.
R. F. Gould, Am. Chem. Sot., Washington,
309
TEMPERATURES
3679
Technology
Dept.,
communication
179 22
)
M. FBex, C.R.
Pleasanton,
(July
Calif.,
Private
1966)
AC. SC. Paris 260 (1965)
6389
OF NUCLEAR
THE
SYSTEM
L. N. General
21 (1967) 302-309. 0
MATERIALS
GROSSMAN,
J. E.
Nuclear
LEWIS
Technology
Pleasanton, Received
Phase equilibria determined sphere
in the system
for high
by
thermal
ceramograpby.
analysis,
Extensive
in the fluorite
phase;
fluorite mole
X-ray
solubility Urania
% Eu01.5;
50” C. Lattice
the eutectic
temperature
been
observed
invariant
; a binary
temperature
the
compound
are reported
and phase.
region
are
and
W
has
and
binary
for the Eu303-W
system. Le diagremme
d’bquilibre
des phases a BtB determine
U03-Eu303
et en atmosphitre
neutre
diffraction
des
existe
une
solubiliti!
phase
rayons
fluorine
alors
et
importante qu’il
n’y
Die durch
Eu303 cubique
keit
entre les phases fluorine
pour
80 mole
eutectique
y.
est
de
von
Eu3O3
Uranoxid
sont sign&%
Phase
besteht
UO3 stabilise
Un eutectique
se
et I%303 monoclinique
et
pour
iiber
eine
LB temperature
variante berichtet.
Introduction
Europium oxide is being considered for use in fast reactors to control the increase in reactivity caused by water flooding in a steamcooled reactor that has high neutron leakage Ia). The effectiveness of europium as a thermal or epithermal poison to control routine flooding reactivities in mixed U-Pu oxide fuel has been demonstrated in a subcritical assembly lb). It is anticipated that, for power reactor use, the europium will be in solution in the oxide fuel which will be fabricated by conventional
in
und
Fiir
die
unterhalb
Lijslichkeit der
mit
Solidus-
der
zwischen
Das Soliduslinie postuliert.
Eu303
monoklinen
ca.
80 Mel liegt werden
und
y0 bei die
Liquidus-
Gleichgewichtswird
fiir
Es wurde
die eine
Eu303 und W beobachtet
Verbindung im
im
von UO3
das kubische und
Fluoritphase die
Seite
Temperatur
Grosse L&lich-
Fluoritphase;
Temperatur
angegeben.
binllre
der
keine
Eutektikum
europiumoxydreiche
Les
param&res
ein
Eu203-U02
Riintgenfeinstruktur-
Fluorit
eutektische
Wechselwirkung
System
stabilisiert
1885” C. Zwischen
diagramm
2125 + 50” C.
a 6th
binaire
cornpost
bestimmt.
besteht
featgestellt.
de
ELI 0,5 environ.
la rhgion
und inerte AtmosphLre
Analyse,
Eu3O3 wurde
temperaturen
dans
im
monoklinen
la
& 1885” C environ.
forme
thermische
de Eu303
thermiquo,
solubilit&
un
binaire
analyse und Keramographie
Gitterparameter,
L’oxyde
interaction
Eu303 et W:
Phasengleichgewichte
11
a aucune
Une
wurden fiir hohe Temperaturen
la
la
pour
d’europium. invariante
Des Cquilibres
sugg&&
Eu303-W.
c&amographie.
par l’analyse X
entre
Die EuO1,5. 2125 f 50” C.
aux hautes tempkratures
UO3 dans En303 monoclinique.
1.
observee
bis
pour lo systbme
en oxyde
fluorine.
sont
80
is 2125 &
temperatures
Eu303
riche
du solidus et du liquidus
la phase
du solidus
le systbme
are given for t’ho fluorite between
en dessous
pour
between
for the europia-rich
interaction
les temp&atures
donnkes
une tempkrature
at approximately
solidus
r@ticulaires, sont
Laboratory,
1966
cubic Eu303
Subsolidus postulated.
exists
of UO3 in mono-
liquidus temperatures equilibria
and
of Eu303
is formed
phases
parameters,
have been
Nucleon&s
USA
19 September
diffraction,
stabilizes
1885’ C. An eutectic and monoclinic
CO., AMSTERDAM
and D. M. ROONEY Department,
California,
and neutra,l atmo-
no solubility
clinic Eu303 was found. to about
Eu303-U03
temperature
PUBLISHINQ
UOZ-EuzOs AT HIGH TEMPERATURES
Company,
Electric
NORTH-HOLLAND
und
System
eine
binlire
Eu303-W
; in-
wird
hydrogen sintering. The existence of a large solubility for Euz03 in UOZ and the feasibility of homogenization by hydrogen sintering have been demonstrated 2). The fluorite structure of UOz tolerates large deviations from stoichiometric composition on both the anion- and cation-deficient sides. Hyperstoichiometry is readily produced at temperatures above about 300” C in mildly oxidizing environments. As the temperature is increased, the UOz+, phase boundary broadens by the dissolution of interstitial oxygen ; a 302
THE SYSTEM U02-Euz03 maximum
O-to-U ratio of about 2.26 at about
lo-l.5 atm oxygen and 1450” C has been reported 3). Hypostoichiometry is more difficult it requires temperatures above to produce, about 1600” C and low oxygen pressure. An O-to-U ratio of 1.88 has been reported 4) for Urania equilibrated
at 2400” C in hydrogen
(dew
303
AT HIGH TEMPERATURES after experimentally
establishing
the appropri-
ate calibration curve. It was inferred from weight loss data that no significant compositional changes occurred during sintering. Hot-pressing of blended powders was accomplished Hoyt 8).
in The
apparatus were
described performed
at The
the UOa-s phase reaches a limit of about UO1.65
specimens were held at 5000 psi for lo-20
min
at about 2500” C, ref. 5). Oxides of composition MO2 and of fluorite
at temperature. The hot-pressed pellets were generally crack-free and between 94 and 97 y0
type can dissolve large amounts of sesquioxides when the cations are of similar size. A composition limit of Mol.75 has been observed for these fluorite solutions with anion vacancies 6). At large vacancy concentrations, the vacancies in the fluorite structure can become ordered and form the closely related Type C rare earth (thallium oxide type) structure which is exhibited by most of the rare earth sesquioxides
dense. Previous experience with UOZ pressing has shown that the initially hyperstoichiometric powder is reduced to stoichiometric composition during this procedure. Carbon pickup was confined to the outer pellet surfaces which were cleaned by mild abrasion. The mixed oxide pellets were encapsulated in tungsten by inert arc welding in an argonfilled glove box. Each pellet was cored ultrasonically before welding to accommodate a tungsten tube down the center. A cross section of a specimen capsule after testing is shown in fig. 1.
- 40” C); the phase boundary
for
at low temperatures. Three oxides of europium have been reported 7) : Eu203 (monoclinic and body-centered cubic), EuO (face-centered cubic), and Eu304 (orthorhombic); the sesquioxide is the most stable. A fourth reported oxide, designated “Ortho I” has been identified 7) as the orthosilicate EuzSi04. The sesquioxide crystallizes in the Type C rare earth structure below about 1100” C and in the Type B (monoclinic) structure above 1100” C. The suboxides are difficult to form because of their relative instability and of the high volatility 7) of EuO. No analytically significant deviations in composition from the stoichiometric sesquioxide have been reported for monoclinic Euz03. 2. 2.1.
Experimental
procedure
SPECIMEN PREPARATION
Blended UOZ and Eu203 powders were consolidated by two techniques : hydrogen sintering of specimens containing less than 20 mole percent Eu01.5, and vacuum hot-pressing of the other specimens. Powder analyses and sintering details have been reported 2). The molar ratios were confirmed by X-ray fluorescent analysis
2.2.
1400” C in graphite
by
dies.
point about
1300” C and
the
pressings
THERMAL ANALYSIS
The encapsulated mixed oxide specimens were inductively heated in vacuum to obtain heating and cooling curves. The experimental equipment and procedure for thermal analysis are similar to those reported by Rupert 9). Vacuum is maintained at less than 5 x 10-S Torr during heating. The specimen temperature is measured independently of the temperature recording system by optical pyrometry. A manual brightness pyrometer is sighted on the blackbody hole. The pyrometer is frequently calibrated against an NBS secondary standard bulb between 900 and 2300” C. Temperature measurement above 2300” C is made by extrapolation of the correction curve for the pyrometer. Temperatures below 2300” C are accurate to well within & 1 %; the accuracy of temperatures reported between 2300 and 2600” C is believed to be within f lg y. ; between 2600 and 3000’ C, &- 2 o/o is estimated to be the
304
L. N. GROSSMAN.
J. E. LEWIS AND D. M. ROONEY can readily detect temperature than lo C at all temperatures. of the detector
changes of less The linear range
is shifted by the use of neutral
density filters. The response time of the detector is 0.1 set at the middle of the linear range and less than 0.01 set at the high intensity
end of
the linear range ; thermal analysis data were always taken between these limits. Each specimen was heated to above the liquidus several times. Thermal analysis data were obtained on heating and cooling above approximately 2000’ C ; a few samples were
Fig. 1. Cross so&on of UO&% mole o/0 Eu01.5 in tungsten capsule after thermal analysis.
maximum error. The brightness pyrometer readings are corrected for the fused silica window and prism in the light path. For very high temperatures, tungsten evaporation and deposition on the window can affect temperature measurement accuracy. A magnetically operated shutter minimizes this problem. An image of the blackbody tube is projected by a large objective lens onto a cadmium sulfide photoconductor detector. The blackbody image is enlarged to about 40 x so that it completely covers the 3” dia. detector. The detector resistance is monitored with a shielded drop across the dc circuit. The potential photoconductor is monitored, and its first and second time derivatives may be observed. The potential drop across the detector is approximately linear with temperature over about 400” C. Over the linear range, the sensitivity is about 0.15’ C/mV; thus, a millivolt recorder
examined at lower temperatures. Thermal analysis of the specimens resulted in two general types of observed heat effects: thermal arrests, and abrupt changes in slope of the cooling curve. The first type is observed at temperatures corresponding to invariant points (e.g., melting point and eutectic reaction); the second type is characteristic of simple phase boundaries where no reaction occurs. No special care was taken to equilibrate the thermal analysis specimens at any temperature before cooling ; the final cooling was accomplished by shutting off the induction heater while the specimen was between about 2000 and 2500” C. Two specimens (27.8 and 46.2 mole o/o EuOI.5) were equilibrated at 1600” C after melting in the thermal analysis apparatus; shutting off the power cooled these to less than 900” C in about 1 min.
2.3.
X-RAY ANALYSIS
After thermal analysis, the capsules were sectioned longitudinally as shown in fig. 1. One-half the specimen was removed from the tungsten container and crushed to not less than 200 mesh powders. The powders were analyzed by X-ray diffraction with the Debye-Scherrer and diffractometer techniques, employing the reNelson-Riley and sin20 extrapolations, The wavelengths used were spectively. CuK,, (1.54051 A) and CuK,, (1.54433 A). A weighted average CuK, (1.54178 A) was used for the powder camera method. The accuracy of lattice parameter measurement by the
THE
SYSTEM
UOs-EusOs
AT
method was zt 0.005 A; powder camera diffractometer measurements were accurate in most cases to f was
used
specimens
to
0.0003 A. X-ray confirm
containing
the
fluorescence
compositions
of
up to 66 mole o/o EuOi.5
HIGH
305
TEMPERATURES
one between
2650 and 2695’ C, and another
between 2730 and 2780” C. All specimens showed traces tungsten were
too
in the small
oxide to
phases.
measure
interaction
by the Debye-Scherrer
will be discussed separately.
2.4.
CERAMO~RAPHICANALYSIS
The longitudinally sectioned specimens were prepared for ceramography by grinding through 600 grit silicon carbide grinding paper and rough polishing on nylon polishing cloth charged with 6 and 3 pm diamond paste. Final polishing was accomplished with a thick slurry of Hz02 and Linde “B” (0.05 ,um AlaOs) on Buehler Texmet polishing cloth. Medium pressure was used to avoid excess relief between phases. A 1 to 2 min swab-etch with a 9 : 1 Hz02 :HzSOa solution was sufficient to etch grain boundaries and develop substructure. Dark field illumination was used extensively in determining the pattern and location of metallic tungsten inclusions within the structures of the oxide specimens is). The advantage of dark field over bright field is the ability to see below the polished surface of the oxide matrix and therefore see more clearly the distribution of the metallic inclusions. 3.
Results
A summary of the experimental data obtained by thermal analysis and X-ray diffraction is given in table 1. The two types of thermal effects observed have been abbreviated as follows: A change in slope in the cooling curve is noted by “S”, a thermal arrest is noted by “A”. As a result of gradients in the specimen capsules, a change in slope or an arrest is usually spread over a temperature range of 20 to 50’ C. For example, specimen 10 showed two changes in slope upon heating or cooling,
metallic
The
amounts
accurately
by
ceramographic or X-ray techniques ; X-ray fluorescent analysis of specimens 15 and 17 showed less than 5 o/o total tungsten in the oxide phases. Specimen 18 showed extensive
after thermal analysis. In the pure EuzOs, analysis showed prowhere ceramographic nounced phase segregation (due to reaction with the tungsten capsule), selected areas were scraped with a diamond stylus and analyzed technique.
of
between
pure EusOs and tungsten Ceramography
of
specimens containing up to 57.2 mole y. EuOi.5 showed single phase fluorite which appeared identical to pure UOs. Small metallic tungsten inclusions, similar to those seen lo) in pure UO2, were observed ; a possible trend towards more tungsten with greater EuzOs content was observed. Lattice parameters were obtained from powder films by a Nelson-Riley extrapolation of all diffraction lines with 28 falling above 100” C. For diffractometer scans, a sins0 extrapolation was used. The fluorite lattice parameter data in table 1 and some earlier data 2) are given in fig. 2. Monoclinic europia was observed as a major phase in specimens 15, 17, and 18; line-for-line comparison with the diffraction pattern published by Rau 11) showed near-perfect agreement within the experimental accuracy of the two studies. Body-centered cubic EuzOs was observed in specimens 15 and 17. Identification of the body-centered cubic phase required careful examination to confirm that it was not a second face-centered cubic phase; diffraction lines forbidden in face-centered cubic materials were present. The lattice parameters of the body-centered cubic phases were computed and found to differ significantly from those of pure EuzOs for which ii) ao= 10.869 + 0.003 A. Specimen 15 contained more of the bodycentered cubic phase than specimen 17. Specimen 18, pure Eu203 in tungsten, showed a very nonuniform phase distribution. Obvious reaction had occurred between the tungsten capsule and the oxide, and the reaction had not gone to completion. A mixed oxide structure
306
L.
N.
GROSSMAN,
5.4800
J.
E.
LEWIS
I
I
AND
D.
M.
I
I
ROONEY
I
I .~.
5.4700 !
5.46po
\ ~-
5.4500
-
5.4400
--
"'\,
L
\
;
I
F 2 5.4300 : 'Z -. 5.4200 --
\ 5.4100 -~
I
.... X. x. I... k.
5.4000
~~~
53900 0
I IO
I 20
I 30
I 40
E"O1.5 (Mob
Fig.
2.
Lattice
parameter
versus
composition
Specimen
wt% EuzO3
mole o/0
1
0
0
Not, analyzed
0
0
S, 2755-2787”
ao=5.4707
i
ao =5.4708
& 0.0002
5
7.4
S, about
Not
analyzed
7.4
S, 2760-2850”
C
Not
analyzed
5
15
21.2
S, 2720-2760”
C
Not
analyzed
s,
c
Not
analyzed
equilibrated 10 11
20 35.8
27.8 46.2
S, 2650-2695”
C C
Not
analyzed
46.5
57.2
13
56.6
66.7
15
70
78.2
80
18
100
86.0 100
Not
; at
Fluorite,
ao=5.4100 ao=5.403
S, 2460-2515”
C
Fluorite,
C
Weak
S, 2230-2270”
C
Fluorite,
s,
2350-2390”
C
Weak
trace
S, 2150-2200”
C
Major
monoclinic
2120-2135”
C
A 1870-1895” S,’ 2176-2225” A, 2104-2135” A, 2060-2092”
c C C C
24 S=change
trace
Strong
4
S, 2760-2810’
C
Not
6
8.8
S, 2741-2818”
C
Fluorite,
*
A trace
monoclinic
Euz03
Eu203
bee, ao=10.907
unidentified
Fluorite,
arrest.
EuzO3
5 0.001
Monoclinic Eu203 New fee phase ao=5.395
C
; A =thermel
k 0.001
f
Major monoclinic Euz03 Strong minor bee, ao=10.900
S, 2812-2884”
in slope
& 0.0009
monoclinic
ao=5.400
minor
3.0 6.0
2
0.0009
1600” C
Trace 19 22
f
analyzed
S, 2400-2430”
A, 17
ao=5.4363
0.0002
1600” C
S, 2730-2780” equilibrated 12
Fluorite,
; at
(A)
Fluorit,e,
5
27.8
2800” C
;
present*
Fluorite,
3
20
solutions.
X-ray Phases
C
2770-2795”
solid
da.ta
4
9
j-
fluorite
analysis?
2
1 70
1
of experimental
Thermal
EuO~.~
I 60
50
'I.1
for UOs-Eu01.5
TABLE Summary
I ...... x...................... r-
of W
phase.
ao=5.4660
f
0.005
+ 0.005
& 0.005 See text.
0.0003
analyzed
was found
ao=5.4599 in every
& 0.0003
specimen
examined.
THE
of
apparent
peritectic
UOs-EusOa
SYSTEM
origin
could
be
seen
AT
HIGH
307
TEMPERATURES
respectively.
Solidus and liquidus temperatures
along the tungsten capsule walls and in contact with the tungsten capillary; X-ray diffraction identified the two phases as monoclinic EuaOs
could not be resolved by thermal analysis for compositions rich in UOa. The position of the eutectic is not exactly known, but it must lie
and a new face-centered cubic phase which could not be identified. The new phase has a
between
lattice constant as = 5.395 f similar in structure
0.005 A and is very
of fig. 3 have been inferred
Away from the tungsten in specimen 18 had not
walls, the europia reacted with the
tungsten. A major phase of monoclinic EuzOs and a trace of an unidentified compound were the only phases detected. The diffraction pattern of the unidentified phase is shown in table 2. Ceramography showed this unidentified phase as small nonmetallic islands within the monoclinic EuaOs grains. This phase is dissolved by the HzOa-HaSO4 etchant. TABLE Diffraction
The
specimens
from
side
analysis
of
15 and 17, and from the known 14)
polymorphism
in EuzOs. The invariant temper-
ature observed at about 1885” C in specimen 15 has been interpreted as a peritectoid. (No thermal analysis of specimen 17 was performed below 2000” C.) The peritectoid composition has been estimated from ceramography as has the subsolidus fluorite phase boundary shown in fig. 3. The oxygen pressure dependence of the fluorite phase composition is indicated by
2
pattern of unindentified europium detected in specimen 18
d (8)
y0 Eu01.5.
Subsolidus equilibria on the europia-rich
and size to the compound
2CeO.2..YaOs.
readily
78.2 and 86.0 mole
equilibrated specimens 9 and 11 are represented by two crosses in the fluorite field of fig. 3.
Rel.
oxide
II
intensity
oI
2800
4.225
ww
3.319
MS
2.929
vvw-
2.545
VW
2.373
VW
2.075
M
2600
2400
1.724 Y
Thermal analysis of specimen 18 showed an arrest on heating through the range 2060 to 2090” C which lengthened on each subsequent heating or cooling. Heating above the arrest showed a wide melting region, but no value of the liquidus temperature was obtained. The capsule ruptured after about 15 min above 1800’ C and signs of liquid formation at 2090” C were observed.
r f ;
2200
E 2 2000
1800
1600
4.
Discussion
The data of table 1 have been used to construct the phase diagram shown in fig. 3. The melting points for UOZ and EusOa are those reported by Hausner 12) and Schneider Is),
0
uo2
Fig.
3.
Mol.
r.
UOz-EuOl.5 equilibrium phase high temperatures.
diagram
et
L.
308
oomparison
N.
GROSSMAN,
of this study
with
J.
the work
E.
LEWIS
of
Haug and Weigelis). They studied the UOs.sEusOs system under oxidizing conditions (air
AND
evidence
D.
31.
ROONEY
for only two cation species (Eufs and
sintering at 1100” C followed by an air quench) ;
Uf4) in the fluorite phase. The interaction observed between EusOs and tungsten in specimen 18 bears on the dis-
single
crepancies in reported melting points for EusO3.
phase
fluorite
was
observed
between
about 28 and 64 mole o/o EuOi.5. The fluorite
Wisnyi
phase lattice parameter varied linearly between 5.406 and 5.392 between 38 and 64 mole o/o
for the melting point as measured on a tungsten
EuOi.5 under their oxidizing conditions. The lower lattice parameter observed with increased oxygen content in the fluorite phase is consistent with similar observations on the UOs-UOs system 16) and the UOz-YsOa-UOs system 17). The fluorite phase in the UOs-UaOs-EuzOs system at high temperatures resembles the fluorite phase in the UO2-UsOs-YsOs system 17) in several respects. In both, the fluorite lattice parameter decreases with increasing oxygen content and with increasing lanthanide content. As the oxygen pressure is raised, the region of fluorite phase decreases in both ternaries and gives way to UOs.6 on the Urania-rich side. The linear dependence of lattice parameter with composition shown in fig. 2 is in accord with Vegard’s law as might be expected for random substitution of Eu+s for U+4, and for random anion vacancies. Similar linear behavior 18919) has been observed in the system YsOa-ThOs and LasOs-ThOs where the cations have only one valence and where anion vacancies account for charge neutrality 6). The nonlinear variation of lattice parameter with composition in the UOz-YsOs fluorite solutions has been interpreted by Roberts 6) as evidence of vacancy interaction. Roberts’ conclusion was based on the data of Ferguson and Fogg 20) who equilibrated their specimens with uranium metal at 950 to 1050” C for 2 to 3 days in evacuated vycor tubes. Similar attempts by Aitken 21) to equilibrate high yttria content Urania-yttria fluorite phase with uranium metal did not reach equilibrium after several weeks. Thus it is not clear whether the ordering of vacancies at low temperatures or the excess oxygen caused the nonlinear behavior observed in the YsOs-UOs system. The linear behavior shown in fig. 2 is interpreted as corroborative
and Pijanowski
reported
2050 f
30” C
strip heater 13). Their value is apparently a measure of the invariant temperature observed in specimen 18 for the W-Eu2O3 binary. The value of 2240 f 10” C reported by Schneider is) was obtained using an iridium crucible ; X-ray examination of the residue after melting revealed no new phases. Schneider’s value has been used in the construction of fig. 3, although it may represent an invariant (e.g., monotectic) in the EusOs-Ir system. Foex 22) has reported about 2330” C for the melting point of EusO3 ; his technique involved solar heating of the oxide in air in a crucible of nonmelted EusO3.
5.
Acknowledgments
Grateful acknowledgment is extended to R. M. Harrington, N. R. Mullison, and J. G. Wilson who aided in the fabrication of specimens and in thermal analysis. E. A. Aitken, D. L. Douglass and U. E. Wolff provided stimulating review and discussions. The U.S. Atomic Energy Commission graciously supplied the material from which specimens were fabricated.
References J. W. Hallam, R. K. Haling, P. Killian and G. T. Peterson,
Proc.
Conf.
Safety,
Fuels
Design in Large Fast Power Reactors, Ill.,
1965, Argonne
Report
ANL-‘7120
National
end
Core
Argonne,
Laboratory
(USA)
(1965)
E. Kiefhaber and K. Ott, ibid. H. W. Hill, J. E. Lewis and L. N. Grossman. General Electric Company (USA) Report GEAP5008 (Jan. 1966) T. L. Markin, L. E. J. Roberts
and A. Walter,
in Thermodynamics of Nuclear Materials (IAEA, Vienna, 1962) E. Aukrust, 1’. Forland and K. Hagemrtrk, ibid. E. A. Aitken, H. C. Brassfield and J. A. McGurty, ANS
Trans.
6, no. 1 (1963)
150
THE
5)
6)
UOs-EuzOs
SYSTEM
AT
HIGH
of the metal oxides, National Bureau of Standards
A. E. Martin and R. K. Edwards, J. Phys. Chem.
(USA)
69 (1965)
1788
L.
Robe&,
E.
J.
pounds,
in Nonstoichiometric
Advances
in Chemistry
Com-
14)
D.C.,
8)
Roy,
9
ref. no. AED-CONF-63-040-18
Press, New York,
W.
A.
(USA)
65
Rev.
Sci. Instr.
36 (1965)
1629
(1966),
Young
et al., North American Aviation NAA-SR-6765
17) S. Bartram, General
C. Rau,
General
DC 59-9-122
‘9
(1959).
Company
Also in ASTM
file of X-ray
diffraction
12)
H. Hausner,
J. Nucl.
13)
S. J. Schneider, Compilation of the melting points
15 (1965)
Juenke
Company
(1962) and E. (USA)
A.
Aitken,
Report
TM-
1964)
and R.
Mezger,
Z. Phys.
Chem.
201
268 Z. Anorg. Chem.
5
)
I. F. Ferguson and P. G. T. Fogg, J. Chem. Sot.
21)
E. A. Aitken, General Electric Company, Nuclear
(1957)
data.
Mat.
(rev.
265 (1951)
(USA) Card
F.
F. Hund and W. Diirrwachter,
9 20
Electric
E.
Electric
E. Hund (1952)
(USA) Report ORNL-
p. 249
J. Nucl. Mat. 9 (1963)
Report
63-7-14
G. 11’. Rupert,
Report
Chem.
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