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Analysis of fission product ingots formed in uranium-plutonium oxide irradiated in EBR-II
JOURNAL
OF NUCLEAR
MATERIALS
35 (1970)257-266.0 NORTH-HOLLAND
ANALYSIS FORMED
OF FISSION PRODUCT
IN URANIUM-PLUTONIUM D. R.
O’BOYLE,
F. L.
BROWN
Argonne National Laboratory, Received
A
metallic
ingot
an irradiated examined
removed
by means element
in EBR-II
to
analyzed ruthenium,
had
wt y0
12.9
; no
The
is to
the
oxide by
in in
fuel
and
migration
the temperature analyses
the
toward
irradiation.
of
inclusions
columnar
of
Pd
to
have
Ru,
The
results
of the micro-
metallic
ingot
and
crystal
region
structure
to
be
MO, Tc,
A et c0=4,355
inclusions
element 20%
combustible
en poids
la microsonde
par
combustible
avait
EBR-II
having
hexagonal
Gew.-y.
von
6th irradie
ergab
fission/cm3. L’anelyse suivante: lS,S%
48,6%
de technetium,
12,9%
palladium
(pourcentages
detectable
d’umnium,
d’azote
ou
du point *
Work
d’oxygene
de
vue
de
performed
d’un
:
de UO2-PuO2 au moyen
A
en
poids);
n’a
Bte trouvee.
sa composition
beobachtet
le
the auspices
dem
von
de
Molybdgns
Stellen
trace
wandert.
im
Ihre
deuten
und
Probe
Eine Legierung
Plukonn-
iihnelte
in
Oxid-Brenn-
Entstehung
diirfte
Einschliisse
auf
entgegen-
zuriickzufuhren und der Einschhisse
darauf
hin, dass ein Teil zu
wiihrend
der
der gleichen Probe,
im den
die allgemein
Palladiums
Brennstoff
Uran,
bestrahltem
der Probe
zung wie die metallische
lingot,
Die
Temperaturgradienten
mit der Mikrosonde des
12.9 Gew.-y.
Zusammensetzung
metallischer
sein. Untersuchungen
Analyse Gew.-y.
und Sauerstoff
werden.
chemiseher
werden.
Die 20.0
Palladium.
Stickstoff
Stengelkiirnern
gesetzt
Neutronen-
worden.
Spaltprodukt-Einschliissen,
Wanderung
carbone,
Le
und
die
dans
chimique,
gefunden
stoff
et 2,0% de
Kohlenstoff,
y et
L’element
auoune
plutonium,
ao=2,735
Brennelement
Technetium,
2.0 Gew.-y.
den
2O,Oo/ode molybdene,
de
under
weissen
Das
Ruthenium,
16.6 Gew.-y. und
nioht
und
untersucht.
Gew.-y.
de
de 5,6 x 1020
de rhodium
du une
bei 560 W/cm bis zu einem Abbrand
48.6
Aussehen
du lingot donnait la composition
de ruthenium,
avait
am Zentrslkanal
Gammaspektroskopie
Molybdlin,
with
central
& 560 W/cm
que le
PuOZ, wurde mittels
5.6 x 1020 f/cm3 bestrahlt
Rhodium
B un taux de combustion
purete,
du
cristal-
& partir
comprtcte de parametre
entnommen
Ru, MO, Tc, Rh, and
en neutrons.
de haute
Probe,
ingot but which
de la spectroscopic
activation
froide
A.
war im EBR-II
8, base
prepare
et
molyb-
La structure
etait
et Pd
of an alloy
a 6th examine
Blectronique,
de I’analyse reacteur
irradie
de Pu0~
du
vers la region
UOt-20
Mikrosonde,
and palladium
du vide
Les
la m&me composition
qui
on
metallique
partie
Eine metallische
A.
extrait
temperature.
qu’une
von bestrahltem
ten metallique
et
d’inclusions
up
tonium, Un lingot
Rh
de
migre
et
d’oxyde
du lingot
l’irrediation.
ayant
aktivierungs-Analyse
close-packed
A and c0=4.355
alliage
metallique
of the fuel during
as the metallic high purity
durant
qui
been
gradient.
the
gradient
suggerent
structure hexagonale
ir-
inclusions
from
w&s found
ao=2.735
gmins
le
& base
par la migration
et du palladium
line d’un
of metallic
the same composition was prepared
d&e
combustible
chemical
par
par la fission,
dans les grains besalti-
irradie
a Bte forme
des inclusions,
lingot
produites
de microanalyses
wt ‘$&
found.
1970
observQs
de combustible
wt y.
fission-product
13 January
m&talliques
*
60439, USA
aux inclusions
result&s
uranium,
were and
is believed
the cooler
The
2.0
form
pense qu’il
wt y0
16.6
and
suggest that some of the molybdenum migrate
48.6
or oxygen
the
the
of
is
appearance
white
observed
radiated formed
ingot
amounts
nitrogen,
similar
ques
analysis;
at 560 W/cm
molybdenum,
detectable
carbon,
composition
the
Illinois
similaire
IN EBR-II
E. DWIGHT
sont communement
5.6 x 102O fiss/cm3. The
wt Oh rhodium,
plutonium,
commonly
of
of was
microanalysis,
been irradiated of
void
element
IRRADIATED
Argonne,
CO., AMSTERDAM
INGOTS
and A.
1968; in revised
central
fuel
and neutron activation
a burnup
20.0
technetium,
ingot
the
of electron-probe
composition
palladium
from
UOs/20 wt o/o PuOa
gamma spectroscopy, the fuel
16 December
OXIDE
PUBLISHING
den
k<eren
Bestrehhmg Zusammenset-
aber aus hochreinem
Ru, MO, Tc, Rh und Pd hatte die hexagonal dichteste Peckung mit ao=2.735 A und c0=4.355 A.
est
of the United 257
States
Atomic
Energy
Commission.
258
1.
D.
R.
O’BOYLE
Introduction fissioning
AL.
chemical
The fission-product during
ET
of
composition
of a large metallic
elements that are formed
(e
uranium
void of a mixed-oxide
atoms in a fast-neutron
and
plutonium
flux vary widely in half-
life and in chemical properties.
The spectrum of
fission products
present in the fuel depends on
the composition
of the fuel (i.e., the relative
uranium and plutonium
concentration
and the
isotopic content of each), the energy spectrum of the neutron flux, the burnup rate, and the time that the fuel has been out of the reactor following irradiation. The most abundant fissionproduct elements include those that form stable oxides (Zr, Nd, Ce, Sr, Ba, La, Pr, Y, Sm, Pm, and Nb), elements the oxides of which are not stable (Tc, Ru, Rh, Pd), elements that are volatile at normal irradiation temperatures (Te, I, Cs), and inert gases (Kr, Xe). The concentration of each of these fission products is greater than one percent of the heavy-metal atoms fissioned, that is, greater than 270 ppm in the fuel examined in this experiment. Studies of the behavior of fission products by means of electron-probe microanalysis have been reported for uranium carbide I), uranium oxide oxide 2, s), and mixed uranium-plutonium fuels 4~5). The results of an investigation of the fission-product distribution in a vibratorily compacted UOs/20 wt% PuOz fuel element irradiated in EBR-II to a burnup of 5.6 x 1020 fiss/cms confirmed 6) that fission-product elements, which form oxides of relatively high thermodynamic stability (Zr, Ba, La, Ce, Pr, and Nd), are found in solution in the mixedoxide matrix ; whereas, fission-product elements the oxides of which are not stable above 1700 “C (MO, Tc, Ru, Rh, and Pd) occur in metallic inclusions within the columnar grains. Davies et a1.7) analyzed ingots formed in oxide and mixed-oxide fuels, which had experienced appreciable center melting during irradiation, and found that the ingots were metallic and contained the fission products ruthenium, molybdenum, rhodium, and palladium ; no uranium or plutonium was detected in any of the ingots. This paper describes an investigation of the
700 ,um diameter) removed
diated
in EBR-II
temperature
density
linear heat
and a maximum
of 570 “C. The U02/20
fuel had an initial effective
fuel pin that was irra-
at a maximum
rating of 560 W/cm
ingot
from the central
cladding
wt y. PuO:!
O/M ratio of 2.00 and an
of 83.3%;
the cladding
was
Type 304 stainless steel with a wall thickness of 0.53 mm. 2.
Experimental technique
A longitudinal cross section through the lower end of the fuel element is shown in fig. 1. The original vibratorily compacted fuel structure is visible in the outermost region of the fuel column. The fuel adjacent to the outer portion was irradiated at a higher temperature and has an equiaxed grain structure. Columnargrain growth, which occurred in the fuel irradiated at the highest temperature, is seen in the fuel adjacent to the central void. The radial and circumferential cracks in the mixedoxide probably occurred while the fuel cooled from reactor temperature.
operating temperatures to room The metallic material at, the
bottom of the central void, shown by gamma to be highly radioactive, autoradiographs consists of individual ingots similar in appearance but larger than the metallic inclusions observed in the columnar- and equiaxed-grain regions of the fuel. Inclusions observed in these regions were generally less than 6 ,um in diameter. Larger inclusions, approximately 60 to 80 pm in diameter, are attached to the wall of the central void and are indicated by arrows in fig. I. A cross section through the ingot analyzed with the electron-probe, presented in fig. 2, shows in greater detail the microstructure of the ingot and the spherical voids, which vary in size up to 30 pm in diameter. The number of voids present is sufficiently large (12 vol%) to affect, the measured density of the ingot; hence, bulkdensity measurements of similar ingots 7, may not indicate the true density of the alloy. The specimen was mechanically removed from the bottom end of the central void and
ANALYSIS
Fig.
1.
irral diated the
Longitudinal to
location
cross
OF
through
section
5.6 x 1020 fiss/cma showing of
,ched to the
fission-product wall
ingots
of the central
FISSION
the
structural Cracks
259
INUOTS
end of a UO420 wt ye PuOs fuel elel ment and that occur in the fuel during irradiation Arrows indicate small fission-product ix1gots
bottom changes
in the central
void.
.PRODTJC!T
void.
were formed irradiation.
during
cooling
follo #wing
of the fuel element
mounted with conductive epoxy resin in a small hole drilled in a brass mount. To lower the gamma activity, the specimen was reduced in thickness
by
grinding
with
600-grit
carbide paper. Hard gamma activity
silicon
measured
30 cm from the polished surface was 1.1 R/h. After the specimen had been mounted and polished in an alpha-gamma hot cell, the gamma activity had been lowered sufficiently to allow the specimen to be transferred to an unshielded alpha glovebox for final cleaning and polishing. The mounted specimen was then handled in the same manner as unirradiated alpha-active
Fig.
2.
product void
Cross
section
ingot removed
in a UO420
through
metallic
formed
during
fission-
of the central
wt o/oPuOa fuel element
to 5.6 x lOao fiss/cms. Round are voids
a
from the bottom
irradiated
black areas in the ingot irradiation
or cooling.
260
D. R. O'BOYLE
01 4.0
I
ET
AL.
I
I
4.5
*
5.0 WAVELENGTH
(8,
(4
WAVELENGTH
(%I
Fig. 3. X-ray spectra emitted by metallic fission products formed during irradiation of a UO2/20 wt o/0 PuOz fuel element in ERR-II to 5.6 x 10” f%s/cm3. The presence of Ru, MO, To, Rh, and Pd are indicated, (a) X-ray spectrum emitted by the metallic ingot. shown in fig. 2. (b) X-ray spectrum emitted by a metallic inclusion in the columnar grain matrix shown in fig. 1. X-ray background in (a) is higher because of the greater y-activity of the specimen.
ANALYSIS
microprobe
OF
FISSION
specimens, except the exposure time
of personnel
was kept
to a minimum.
Pure
elemental standards of molybdenum, technetium, ruthenium, rhodium, and palladium were prepared
in separate mounts for the intensity-
ratio measurements. An unshielded
electron-probe
PRODUCT
decreasing abundance, ruthenium, technetium,
of commercial
design with an X-ray
spectro-
meter take-off
angle of 52.5” was used in this
rhodium,
molybdenum,
and palladium.
Due to the overlap of the RhLpI, RuL& and PdLol lines at 4.378, the PdL& line is difficult to identify
unambiguously.
ladium
was
contributions
microanalyzer
261
INUOTS
confirmed
The presence by
of pal-
calculating
the
of the RhL,$r and RuLba lines
(based on spectral profiles of the pure elements) and then obtaining
by difference
the intensity
due to the PdLol line. Also, the PdLp, line was detected at 4.146A. This line is not apparent in the trace shown in fig. 3,but was identified current of 3.5 ,uA over the wavelength ranges in a wavelength profile recorded at a higher from 1.0 to 10.0 A and from 30 to 93 A. These sensitivity than that used to obtain the spectral wavelength regions include all elements above traces shown in fig. 3. Since the third through atomic number 11 and also boron, carbon, and the sixth most intense palladium L-series lines nitrogen. (PdLyl at 3.7244& PdLpz at 3.9086 A, PdLps at 3. Results 4.0343 A, and PdL,$, at 4.0709 A) lie in the spectral region where the X-ray background The X-ray spectrum emitted by the ingot in due to gamma activity is high, only the PdL/& the wavelength region from 4.0 to 5.5 ,h is line was detected. The L-series spectral lines of shown in fig. 3a. The characteristic X-ray lines molybdenum, technetium, ruthenium, rhodium recorded in this wavelength region included the and palladium used in this analysis are listed in L-series lines of ruthenium, molybdenum, table 1 along with the wavelengths and relative technetium, rhodium, and palladium. Fig. 3b shows the X-ray spectrum (from 4.0to 5.5%I) intensities of the lines. The relative intensities listed for the pure-element standards are those emitted by a metallic inclusion located in the measured on each of the standards and have columnar-grain region of the mixed-oxide fuel been adjusted relative to the Loll line, which was element. A comparison of figs. 3a and 3b shows assigned an intensity of 100. Prior to this that the same fission-product elements are measurement the relative-intensity values for present in both the metallic ingot and the the technetium L-series lines were not available fission-product inclusions. However, the coninvestigation. Measurements were made at an accelerating voltage of 15 kV and a beam
centration
of
molybdenum
and
rhodium
is
slightly lower in the metallic ingot. Over the entire wavelength region, the X-ray background is higher for the ingot than for the metallic inclusion. The background X-ray intensity increases at wavelengths below 4.3d and reaches a maximum at 4.0A of 1100 counts/set for the metallic ingot and 500 counts/set for the metallic inclusion. This increase in background level is due to the higher gamma activity of the metallic ingot (1.1 R/h hard gamma at 30 cm) compared with the gamma activity of the fuel-pin section containing the metallic inclusions (140mR/h hard gamma at 30 cm). The only elements detected in either the metallic ingot or the metallic inclusions are, in order of
in the literature. The technetium-metal standard was prepared by melting the metal powder in an arc furnace normally used for the preparation of plutonium alloys. To determine if dendrites were present in the ingot, which would confirm that the ingot had been molten during irradiation, a linear X-ray scan was recorded across the diameter of the ingot while simultaneously recording the molybdenum and ruthenium X-ray intensities. These data, shown in fig. 4, indicate that there were no periodic variations in molybdenum and ruthenium concentrations to suggest that dendrites were present in the ingot. The locations on the ingot where the molybdenum and ruthenium X-ray intensities decrease sharply
D. R.
262
O’BOYLE
ET
AL.
TABLE 1 L-series spectral lines used in the analysis of metallic fission products formed during irradiation PnOz
Molybdenum
fuel
element
Loll LB1 LB4 LB3 LB2 LYS
Technetium
Ln L L;: LB4 L83 LP2 LYl
Ruthenium
Rhodium
( )
indicates an overlap indicates spectral
100 50
5.0485
3
5.0131
4
4.9230
2
4.7256
1
flux
5.519
1
5.115
100
2) 100 50
(<
4.887
50
4.774
3
3
4.737
4
(4)
4.630
4
4.436
2
f f 3
5.5033
1
5.2048
1
cc 2) (< 2)
L&l
4.8455
100
LB1 LB4 LB3
4.6204
51
41
4.5228
3
3
4.4865
LB2 LYl
4.3715
5 A
(5;
4.1820
2
2
Ll Lrr
100
5.2167
1
(<
4.9215
1
4.5972
100
(-= 21 100
4.3739
47
4.2886
2
<2
4.2520
5
(2
4.1305
6
3.9434
2
(6) < 2
2)
(50)
Ll Lr1
4.9522
1
N.D.
4.6601
1
N.D.
LO%
4.3676
100
(100)
4.1459
49
4.0709
3
(50) N.D.
4.0343
5
N.D.
3.9086
8
3.7244
9
< 10 N.D.
LB1 LB4 LP3 Lls3 LYl
N.D.
5.4062 5.1768
LX Ln
Lm LBI LB4 LB3 LP2 LYl Palladium
in a fast-neutron
of spectral lines. line not detected.
of a UO-a/20 wt%
ANALYSIS
OF FISSION
PRODUCT
INGOTS
263
mine if any other elements could be detected. No fission-product elements other than those MOLYBDENUM established by electron-probe microanalysis were detected in any of the specimens examined. A possible source of the voids observed in the 2ingot may be the xenon, krypton, snd other volatile fission products (Cs, Te, I, Rb, and Cd) released from small pieces of mixed-oxide fuel ^b 1 that may have been trapped in the ingot. To i determine if any fragments of (U, Pu)Oa were fi present in the void, ur&nium and plutonium t were sought by means of electron-probe micro5 8 analysis and by alpha spectrometry. No uranium }METALLIC INGOT 4 ;: 3 or plutonium was detected in any of 19 voids I I I I I I examined by microanalysis, nor was any E RUTHENIUM I uranium or plutonium detected by means of z G alpha spectrometry of material that had been t evaporated from the walls of the voids using x2 the laser-beam techniques. An X-ray spectral profile of a void that contained some foreign material showed that both copper and zinc were present in about the same proportion as in the I brass mount. Thus we conclude that the black areas in the ingot, shown in fig. 2, are voids that do not contain (U, Pu)Oz ,but some voids have become filled with brass during the in-cell grinding and polishing operation. DISTANCE, microns The intensity ratios of the L-series lines from Fig. 4. Zineax variations in molybdenum end the four most abundant fission-product elements ruthenium concentrations across a metallio ingot are listed in table 2. The intensity ratio is the formed during irradiation of UOe/20 wt y0 PUOZ in ratio of the number of X-ray photons emitted a fast-neutron flux. A simultaneous decrease in the by the specimen under fixed-instrument condiX-ray int%naitiesof MO and Ru indicates the location of voids in the ingot. tions (constant accelerating voltage and beam current) to the number of photons emitted by correspond to locations where the electron beam a pure elemental standard under the same opercrossed spherical voids. No evidence of in- ating conditions. The concentration of each homogeneity or dendritic structure was observed element is also listed in table 2 after correcting either met&~o~~phic~lly or in the sa;mple- the intensity ratios for absorption B) and secondary fluorescence 10) and normalization to current image of the specimen. To establish if other fission products were 100 percent. The concentration of fissionpresent in the metallic ingot below the detecta- product elements in the metallic ingot is bility limit of electron-probe micro&nalysis, 48.6 wt% ruthenium, 20.0 wt% molybdenum, 16.6 wt% technetium, 12.9 wt% rhodium, and several samples were obtained from the ingot by using a laser-beam microssmpling tech- 2.0 wt% palladium. Estimation of the palladium nique 8) to evaporate material from the surface concentration was based on the calculated of the ingot. Neutron activation analysis was intensity of the PdL& line in the RhL@,, RuL@s, then performed on several specimens to deter- PdLN triplet. 3
b7
o<
D.
R.
O’BOYLE
ET
The
AL.
light
elements,
carbon,
nitrogen,
and
oxygen, were sought in the ingot by using a lead stearate
decanoate
analyzing
96.4 A). The recorded no evidence
of carbon,
any of the locations
crystal
(2d =
spectral profiles showed nitrogen,
examined.
clude that the ingot formed
or oxygen
in
Hence, we conat the bottom
of
the central void is metallic and contains the fission-product elements ruthenium, molybdenum, technetium, rhodium, and palladium, but does not contain detectable amounts of uranium, oxygen.
plutonium,
carbon,
nitrogen,
or
To determine the crystal structure and the lattice parameters of the fission-product phase, an alloy of nominal composition 20 wt 9/, molybdenum, 17 wt”h technetium, 48 wt”/,; ruthenium, 13 wt o/o rhodium, and 2 wt% palladium was prepared from high purity metals. The alloy (approximately 2.3 g) was arc melted on a water cooled copper hearth under an inert atmosphere and annealed for 48 h at 1130 “C. Following annealing, powder was prepared by crushing the alloy and Debye-Scherrer patterns were taken with copper radiation. All diffraction lines were indexed and the alloy was found to have a close-packed hexagonal structure, with cc0= 2.735 + 0.001 A, co = 4.355 & 0.001& c/a =1.592 and unit cell volume of 28.21 By. 4.
Discussion
and conclusions
The metallic elements identified in the fission-product ingot are those elements which have a fairly high fission yield and which form oxides that are not thermodynamically stable at the irradiation temperature of the ingot. The free energies of formation of the oxides of ruthenium, rhodium, and palladium are positive above 2200 “C ; whereas the free energies of formation of the oxides of technetium and molybdenum are slightly negative at this temperature 11). Thus the thermodynamic data suggest that molybdenum, technetium, ruthenium, rhodium, and palladium will be reduced to metals, in preference to the fission product,s that form more stable oxides. The composition of the metallic ingot is
ANALYSIS
OF
FISSION
similar to the composition of the fission-product inclusions that form in the columnar-grain region of irradiated (U, Pu)Oz. The composition of the metallic inclusions in the columnar grain region varied as a function of radial position 6). On moving up the temperature gradient from the interface between the columnar- and the equiaxed-grains to the edge of the central void, ruthenium increased from .40 to 53 wt y0 (48.6 wt.%), ‘molybdenum decreased from 30 to 15 wW3 (20 w%), and ~~hnetium increased from 10 to 18 wt% (16.6 wt%). The numbers in parentheses are the measured weight percentages of the three fission-product elements in the metallic ingot. As summarized in table 2 the chemical composition of the metallic ingot lies within the composition range of the metallic inclusions in the columnar-grain region and is close to the measured composition of the inclusions near the central void. Neither ~anium nor plutonium was detected in either the metallic inclusions or in the fission-product ingot, which agrees with prediotions based on the free energies of formation of UOz, PuOz, and the fission-product oxides. The fission yield of the elements detected in the ingot, expressed as atoms of fission product per 100 uranium and plutonium atoms fissioned, are molybdenum 23.1; ruthenium, 16.9; teohnetium, 6.1; palladium, 4.8; and rhodium, 4.2. Since the relative amounts of molybdenum and pa~~urn detected in the metallic ingot and in the metallic inclusions are less than those expected from the fission-product yield (the yield data indicate a higher concentration of molybdenum than ruthenium and a higher oon~entration of palladium than rhodium), and since neither molybdenum nor palladimn was detected in the (U, Pu)Os columnar-grain matrix (detectability limit M 0.07 wt%), we conclude that some molybdenum and palladium migrates to a cooler region of the fuel element. Recent observations by Stalica 12)of a complete cross section of a (U, Pu)Oz fuel element irradiated in EBR-II has confirmed that molybdenum is concentrated in the cooler equiaxed region of the fuel-element cross section.
PRODUCT
INGOTS
265
The metallographic and chemical evidence suggest that the metallic inclusions in the columnar-grain (U, Pu)Oz matrix (approximately 6 ,um in diameter), the white phase observed along the walls of the central void (60 to 80 pm in diameter), and the metallic ingots found in the bottom of the central void (varying in size up to 700 pm in diameter) all have a similar structure and chemical oomposition. We believe the inclusions in the oolumnargrain matrix, which are formed from fissionproduct elements whose oxides are not thermodynamically stable, migrate up the temperature gradient, preferentially along columnar-grain boundaries to form the larger ingots observed along the wall of the central void. Various mechanisms for the migration of inclusions in a temperature gradient have been discussed by Shewmon 13914). Finally, the ingots which have formed on the walls of the central void, collect in the bottom of the void to form larger ingots, as shown in fig. 1. Recent work by Bramman et al.5) on the melting point of an alloy of molybdenum, ruthenium, technetium, and rhodium of approximately the same composition as the metallic ingot studied in this experiment has determined the melting point to be between 1800 and 1900 “C. These data suggest that the metallic inclusions in the columnar grains near the oentral void (calculated temperature w 2600 “C) and the metallic ingot in the bottom of the central void are molten during irradiation. However, no evidence of dendrites in the ingots or in the inclusions was observed either metallographically or by means of electron-probe microanalyses of the molybdenum and ruthenium distribution. Thus, the physical state of the ingot and the inclusions during irradiation is uncertain and must await a direct determination of the melting point of the ingot. The unit cell structure of the ~ssion-produotalloy prepared from highly pure elements is close-packed hexagonal in agreement with the structure of the “meohanioally extracted” and “chemically extracted” metallio inclusions studied by Bramman et al.5). Bramman found
266 that
D.
both
contained
types
of
fission-product
approximately
40 wt%
whereas the fission-product-ingot examined
by X-ray
ment contained
diffraction
R.
O’BOYLE
inclusions
for his patience and skill in preparing the ingot for microprobe
and the alloy
performing
in this experi-
20 wt o/o molybdenum.
by Bramman
AL.
molybdenum
Listed in table 2 are the unit cell parameters reported
ET
examination,
to J. E. Sanecki for
the microprobe
analysis in a oompe-
tent manner, and to M. D. Adams for laser beam microsampling neutron
the ingot
activation
and performing
the
analysis.
et al.5) and measured in
this study. The unit cell size of the alloy with the higher molybdenum
content
(40 wt %) is
larger than the unit cell size of the alloy with 20 wt”h molybdenum. However, the c/a ratios for the two alloys on which precise lattice parameter data are available are similar, 1.592 (this work) and 1.61 (Bramman et a1.5)). This suggests that the fission-product phase is a solid solution the composition of which varies over a wide range. Further alloy studies are being carried out to experimentally establish the composition range of stability of the hexagonal fission-product phase and to measure its melting temperature. The measured average atomic volume of the fission-product atoms in the hexagonal phase (14.10 As) is close to the average atomic volume of a solid solution (14.17 Aij calculated from the atomic volume of each element. Hence, from the average composition of the metallic inclusions (from microprobe measurements) the contribution of these fission products to fuel swelling can be calculated.
References 1) D.
Quataert and H. W. Schleicher, J. Nucl. Mat.
19 (1966) 2)
B.
T.
221
Bradbury,
J. T.
and D. M. Poole, 3)
B. M. Jeffery,
4)
D. R. O’Boyle, Trans.
5)
J. I.
Am.
6)
8)
J.
Mat.
N.
10 (1967) Sharpe,
462 D.
Thorn and
F. L. Brown and J. E. Sanecki,
Mat.
29 (1969)
27
R. F. Boyle,
B. Weidenbaum
and
M. D. Adams
and S. C. Tong,
J.
Anal.
Chem.
Philibert,
X-ray
(Academic
optics
S. J. B. Reed, Brit. J. Appl. A. Glassner, USAEC National
Laboratory
N. R. Stalica, USAEC National
Laboratory
and
X-ray
Press, New York,
micro-
1963) 379
Phys. 16 (1965) 913
Report ANL-5750,
Argonne
(1965) Report ANL-7427. (1968)
Argonne
102
P. G. Shewmon, Diffusion in solids (McGraw-Hill, New York,
1963) ch. 7
P. C. Shewmon,
Trans.
TMS-AIME
230
(1964)
1134
Acknowledgments The authors are indebted to B. J. Koprowski
15)
40
1762
10)
14)
227
33
Mat. 25 (1968) 201
11)
13)
22 (1967)
Trans. Am. Nucl. Sot. 9 (1966) 63
analysis
12)
17 (1965)
J. E. Hanson, (1968) 9)
Sot. R.
J. Nucl.
H. Davies,
P. M. Martin
Mat.
F. L. Brown and J. E. Sanecki,
Nucl.
D. R. O’Boyle, J. Nucl.
7)
J. Nucl.
Bramman,
(4. Yates,
Demant,
J. Nucl.
J. I. Bramman,
private communication
(1968)
OF NUCLEAR
MATERIALS
35 (1970)257-266.0 NORTH-HOLLAND
ANALYSIS FORMED
OF FISSION PRODUCT
IN URANIUM-PLUTONIUM D. R.
O’BOYLE,
F. L.
BROWN
Argonne National Laboratory, Received
A
metallic
ingot
an irradiated examined
removed
by means element
in EBR-II
to
analyzed ruthenium,
had
wt y0
12.9
; no
The
is to
the
oxide by
in in
fuel
and
migration
the temperature analyses
the
toward
irradiation.
of
inclusions
columnar
of
Pd
to
have
Ru,
The
results
of the micro-
metallic
ingot
and
crystal
region
structure
to
be
MO, Tc,
A et c0=4,355
inclusions
element 20%
combustible
en poids
la microsonde
par
combustible
avait
EBR-II
having
hexagonal
Gew.-y.
von
6th irradie
ergab
fission/cm3. L’anelyse suivante: lS,S%
48,6%
de technetium,
12,9%
palladium
(pourcentages
detectable
d’umnium,
d’azote
ou
du point *
Work
d’oxygene
de
vue
de
performed
d’un
:
de UO2-PuO2 au moyen
A
en
poids);
n’a
Bte trouvee.
sa composition
beobachtet
le
the auspices
dem
von
de
Molybdgns
Stellen
trace
wandert.
im
Ihre
deuten
und
Probe
Eine Legierung
Plukonn-
iihnelte
in
Oxid-Brenn-
Entstehung
diirfte
Einschliisse
auf
entgegen-
zuriickzufuhren und der Einschhisse
darauf
hin, dass ein Teil zu
wiihrend
der
der gleichen Probe,
im den
die allgemein
Palladiums
Brennstoff
Uran,
bestrahltem
der Probe
zung wie die metallische
lingot,
Die
Temperaturgradienten
mit der Mikrosonde des
12.9 Gew.-y.
Zusammensetzung
metallischer
sein. Untersuchungen
Analyse Gew.-y.
und Sauerstoff
werden.
chemiseher
werden.
Die 20.0
Palladium.
Stickstoff
Stengelkiirnern
gesetzt
Neutronen-
worden.
Spaltprodukt-Einschliissen,
Wanderung
carbone,
Le
und
die
dans
chimique,
gefunden
stoff
et 2,0% de
Kohlenstoff,
y et
L’element
auoune
plutonium,
ao=2,735
Brennelement
Technetium,
2.0 Gew.-y.
den
2O,Oo/ode molybdene,
de
under
weissen
Das
Ruthenium,
16.6 Gew.-y. und
nioht
und
untersucht.
Gew.-y.
de
de 5,6 x 1020
de rhodium
du une
bei 560 W/cm bis zu einem Abbrand
48.6
Aussehen
du lingot donnait la composition
de ruthenium,
avait
am Zentrslkanal
Gammaspektroskopie
Molybdlin,
with
central
& 560 W/cm
que le
PuOZ, wurde mittels
5.6 x 1020 f/cm3 bestrahlt
Rhodium
B un taux de combustion
purete,
du
cristal-
& partir
comprtcte de parametre
entnommen
Ru, MO, Tc, Rh, and
en neutrons.
de haute
Probe,
ingot but which
de la spectroscopic
activation
froide
A.
war im EBR-II
8, base
prepare
et
molyb-
La structure
etait
et Pd
of an alloy
a 6th examine
Blectronique,
de I’analyse reacteur
irradie
de Pu0~
du
vers la region
UOt-20
Mikrosonde,
and palladium
du vide
Les
la m&me composition
qui
on
metallique
partie
Eine metallische
A.
extrait
temperature.
qu’une
von bestrahltem
ten metallique
et
d’inclusions
up
tonium, Un lingot
Rh
de
migre
et
d’oxyde
du lingot
l’irrediation.
ayant
aktivierungs-Analyse
close-packed
A and c0=4.355
alliage
metallique
of the fuel during
as the metallic high purity
durant
qui
been
gradient.
the
gradient
suggerent
structure hexagonale
ir-
inclusions
from
w&s found
ao=2.735
gmins
le
& base
par la migration
et du palladium
line d’un
of metallic
the same composition was prepared
d&e
combustible
chemical
par
par la fission,
dans les grains besalti-
irradie
a Bte forme
des inclusions,
lingot
produites
de microanalyses
wt ‘$&
found.
1970
observQs
de combustible
wt y.
fission-product
13 January
m&talliques
*
60439, USA
aux inclusions
result&s
uranium,
were and
is believed
the cooler
The
2.0
form
pense qu’il
wt y0
16.6
and
suggest that some of the molybdenum migrate
48.6
or oxygen
the
the
of
is
appearance
white
observed
radiated formed
ingot
amounts
nitrogen,
similar
ques
analysis;
at 560 W/cm
molybdenum,
detectable
carbon,
composition
the
Illinois
similaire
IN EBR-II
E. DWIGHT
sont communement
5.6 x 102O fiss/cm3. The
wt Oh rhodium,
plutonium,
commonly
of
of was
microanalysis,
been irradiated of
void
element
IRRADIATED
Argonne,
CO., AMSTERDAM
INGOTS
and A.
1968; in revised
central
fuel
and neutron activation
a burnup
20.0
technetium,
ingot
the
of electron-probe
composition
palladium
from
UOs/20 wt o/o PuOa
gamma spectroscopy, the fuel
16 December
OXIDE
PUBLISHING
den
k<eren
Bestrehhmg Zusammenset-
aber aus hochreinem
Ru, MO, Tc, Rh und Pd hatte die hexagonal dichteste Peckung mit ao=2.735 A und c0=4.355 A.
est
of the United 257
States
Atomic
Energy
Commission.
258
1.
D.
R.
O’BOYLE
Introduction fissioning
AL.
chemical
The fission-product during
ET
of
composition
of a large metallic
elements that are formed
(e
uranium
void of a mixed-oxide
atoms in a fast-neutron
and
plutonium
flux vary widely in half-
life and in chemical properties.
The spectrum of
fission products
present in the fuel depends on
the composition
of the fuel (i.e., the relative
uranium and plutonium
concentration
and the
isotopic content of each), the energy spectrum of the neutron flux, the burnup rate, and the time that the fuel has been out of the reactor following irradiation. The most abundant fissionproduct elements include those that form stable oxides (Zr, Nd, Ce, Sr, Ba, La, Pr, Y, Sm, Pm, and Nb), elements the oxides of which are not stable (Tc, Ru, Rh, Pd), elements that are volatile at normal irradiation temperatures (Te, I, Cs), and inert gases (Kr, Xe). The concentration of each of these fission products is greater than one percent of the heavy-metal atoms fissioned, that is, greater than 270 ppm in the fuel examined in this experiment. Studies of the behavior of fission products by means of electron-probe microanalysis have been reported for uranium carbide I), uranium oxide oxide 2, s), and mixed uranium-plutonium fuels 4~5). The results of an investigation of the fission-product distribution in a vibratorily compacted UOs/20 wt% PuOz fuel element irradiated in EBR-II to a burnup of 5.6 x 1020 fiss/cms confirmed 6) that fission-product elements, which form oxides of relatively high thermodynamic stability (Zr, Ba, La, Ce, Pr, and Nd), are found in solution in the mixedoxide matrix ; whereas, fission-product elements the oxides of which are not stable above 1700 “C (MO, Tc, Ru, Rh, and Pd) occur in metallic inclusions within the columnar grains. Davies et a1.7) analyzed ingots formed in oxide and mixed-oxide fuels, which had experienced appreciable center melting during irradiation, and found that the ingots were metallic and contained the fission products ruthenium, molybdenum, rhodium, and palladium ; no uranium or plutonium was detected in any of the ingots. This paper describes an investigation of the
700 ,um diameter) removed
diated
in EBR-II
temperature
density
linear heat
and a maximum
of 570 “C. The U02/20
fuel had an initial effective
fuel pin that was irra-
at a maximum
rating of 560 W/cm
ingot
from the central
cladding
wt y. PuO:!
O/M ratio of 2.00 and an
of 83.3%;
the cladding
was
Type 304 stainless steel with a wall thickness of 0.53 mm. 2.
Experimental technique
A longitudinal cross section through the lower end of the fuel element is shown in fig. 1. The original vibratorily compacted fuel structure is visible in the outermost region of the fuel column. The fuel adjacent to the outer portion was irradiated at a higher temperature and has an equiaxed grain structure. Columnargrain growth, which occurred in the fuel irradiated at the highest temperature, is seen in the fuel adjacent to the central void. The radial and circumferential cracks in the mixedoxide probably occurred while the fuel cooled from reactor temperature.
operating temperatures to room The metallic material at, the
bottom of the central void, shown by gamma to be highly radioactive, autoradiographs consists of individual ingots similar in appearance but larger than the metallic inclusions observed in the columnar- and equiaxed-grain regions of the fuel. Inclusions observed in these regions were generally less than 6 ,um in diameter. Larger inclusions, approximately 60 to 80 pm in diameter, are attached to the wall of the central void and are indicated by arrows in fig. I. A cross section through the ingot analyzed with the electron-probe, presented in fig. 2, shows in greater detail the microstructure of the ingot and the spherical voids, which vary in size up to 30 pm in diameter. The number of voids present is sufficiently large (12 vol%) to affect, the measured density of the ingot; hence, bulkdensity measurements of similar ingots 7, may not indicate the true density of the alloy. The specimen was mechanically removed from the bottom end of the central void and
ANALYSIS
Fig.
1.
irral diated the
Longitudinal to
location
cross
OF
through
section
5.6 x 1020 fiss/cma showing of
,ched to the
fission-product wall
ingots
of the central
FISSION
the
structural Cracks
259
INUOTS
end of a UO420 wt ye PuOs fuel elel ment and that occur in the fuel during irradiation Arrows indicate small fission-product ix1gots
bottom changes
in the central
void.
.PRODTJC!T
void.
were formed irradiation.
during
cooling
follo #wing
of the fuel element
mounted with conductive epoxy resin in a small hole drilled in a brass mount. To lower the gamma activity, the specimen was reduced in thickness
by
grinding
with
600-grit
carbide paper. Hard gamma activity
silicon
measured
30 cm from the polished surface was 1.1 R/h. After the specimen had been mounted and polished in an alpha-gamma hot cell, the gamma activity had been lowered sufficiently to allow the specimen to be transferred to an unshielded alpha glovebox for final cleaning and polishing. The mounted specimen was then handled in the same manner as unirradiated alpha-active
Fig.
2.
product void
Cross
section
ingot removed
in a UO420
through
metallic
formed
during
fission-
of the central
wt o/oPuOa fuel element
to 5.6 x lOao fiss/cms. Round are voids
a
from the bottom
irradiated
black areas in the ingot irradiation
or cooling.
260
D. R. O'BOYLE
01 4.0
I
ET
AL.
I
I
4.5
*
5.0 WAVELENGTH
(8,
(4
WAVELENGTH
(%I
Fig. 3. X-ray spectra emitted by metallic fission products formed during irradiation of a UO2/20 wt o/0 PuOz fuel element in ERR-II to 5.6 x 10” f%s/cm3. The presence of Ru, MO, To, Rh, and Pd are indicated, (a) X-ray spectrum emitted by the metallic ingot. shown in fig. 2. (b) X-ray spectrum emitted by a metallic inclusion in the columnar grain matrix shown in fig. 1. X-ray background in (a) is higher because of the greater y-activity of the specimen.
ANALYSIS
microprobe
OF
FISSION
specimens, except the exposure time
of personnel
was kept
to a minimum.
Pure
elemental standards of molybdenum, technetium, ruthenium, rhodium, and palladium were prepared
in separate mounts for the intensity-
ratio measurements. An unshielded
electron-probe
PRODUCT
decreasing abundance, ruthenium, technetium,
of commercial
design with an X-ray
spectro-
meter take-off
angle of 52.5” was used in this
rhodium,
molybdenum,
and palladium.
Due to the overlap of the RhLpI, RuL& and PdLol lines at 4.378, the PdL& line is difficult to identify
unambiguously.
ladium
was
contributions
microanalyzer
261
INUOTS
confirmed
The presence by
of pal-
calculating
the
of the RhL,$r and RuLba lines
(based on spectral profiles of the pure elements) and then obtaining
by difference
the intensity
due to the PdLol line. Also, the PdLp, line was detected at 4.146A. This line is not apparent in the trace shown in fig. 3,but was identified current of 3.5 ,uA over the wavelength ranges in a wavelength profile recorded at a higher from 1.0 to 10.0 A and from 30 to 93 A. These sensitivity than that used to obtain the spectral wavelength regions include all elements above traces shown in fig. 3. Since the third through atomic number 11 and also boron, carbon, and the sixth most intense palladium L-series lines nitrogen. (PdLyl at 3.7244& PdLpz at 3.9086 A, PdLps at 3. Results 4.0343 A, and PdL,$, at 4.0709 A) lie in the spectral region where the X-ray background The X-ray spectrum emitted by the ingot in due to gamma activity is high, only the PdL/& the wavelength region from 4.0 to 5.5 ,h is line was detected. The L-series spectral lines of shown in fig. 3a. The characteristic X-ray lines molybdenum, technetium, ruthenium, rhodium recorded in this wavelength region included the and palladium used in this analysis are listed in L-series lines of ruthenium, molybdenum, table 1 along with the wavelengths and relative technetium, rhodium, and palladium. Fig. 3b shows the X-ray spectrum (from 4.0to 5.5%I) intensities of the lines. The relative intensities listed for the pure-element standards are those emitted by a metallic inclusion located in the measured on each of the standards and have columnar-grain region of the mixed-oxide fuel been adjusted relative to the Loll line, which was element. A comparison of figs. 3a and 3b shows assigned an intensity of 100. Prior to this that the same fission-product elements are measurement the relative-intensity values for present in both the metallic ingot and the the technetium L-series lines were not available fission-product inclusions. However, the coninvestigation. Measurements were made at an accelerating voltage of 15 kV and a beam
centration
of
molybdenum
and
rhodium
is
slightly lower in the metallic ingot. Over the entire wavelength region, the X-ray background is higher for the ingot than for the metallic inclusion. The background X-ray intensity increases at wavelengths below 4.3d and reaches a maximum at 4.0A of 1100 counts/set for the metallic ingot and 500 counts/set for the metallic inclusion. This increase in background level is due to the higher gamma activity of the metallic ingot (1.1 R/h hard gamma at 30 cm) compared with the gamma activity of the fuel-pin section containing the metallic inclusions (140mR/h hard gamma at 30 cm). The only elements detected in either the metallic ingot or the metallic inclusions are, in order of
in the literature. The technetium-metal standard was prepared by melting the metal powder in an arc furnace normally used for the preparation of plutonium alloys. To determine if dendrites were present in the ingot, which would confirm that the ingot had been molten during irradiation, a linear X-ray scan was recorded across the diameter of the ingot while simultaneously recording the molybdenum and ruthenium X-ray intensities. These data, shown in fig. 4, indicate that there were no periodic variations in molybdenum and ruthenium concentrations to suggest that dendrites were present in the ingot. The locations on the ingot where the molybdenum and ruthenium X-ray intensities decrease sharply
D. R.
262
O’BOYLE
ET
AL.
TABLE 1 L-series spectral lines used in the analysis of metallic fission products formed during irradiation PnOz
Molybdenum
fuel
element
Loll LB1 LB4 LB3 LB2 LYS
Technetium
Ln L L;: LB4 L83 LP2 LYl
Ruthenium
Rhodium
( )
indicates an overlap indicates spectral
100 50
5.0485
3
5.0131
4
4.9230
2
4.7256
1
flux
5.519
1
5.115
100
2) 100 50
(<
4.887
50
4.774
3
3
4.737
4
(4)
4.630
4
4.436
2
f f 3
5.5033
1
5.2048
1
cc 2) (< 2)
L&l
4.8455
100
LB1 LB4 LB3
4.6204
51
41
4.5228
3
3
4.4865
LB2 LYl
4.3715
5 A
(5;
4.1820
2
2
Ll Lrr
100
5.2167
1
(<
4.9215
1
4.5972
100
(-= 21 100
4.3739
47
4.2886
2
<2
4.2520
5
(2
4.1305
6
3.9434
2
(6) < 2
2)
(50)
Ll Lr1
4.9522
1
N.D.
4.6601
1
N.D.
LO%
4.3676
100
(100)
4.1459
49
4.0709
3
(50) N.D.
4.0343
5
N.D.
3.9086
8
3.7244
9
< 10 N.D.
LB1 LB4 LP3 Lls3 LYl
N.D.
5.4062 5.1768
LX Ln
Lm LBI LB4 LB3 LP2 LYl Palladium
in a fast-neutron
of spectral lines. line not detected.
of a UO-a/20 wt%
ANALYSIS
OF FISSION
PRODUCT
INGOTS
263
mine if any other elements could be detected. No fission-product elements other than those MOLYBDENUM established by electron-probe microanalysis were detected in any of the specimens examined. A possible source of the voids observed in the 2ingot may be the xenon, krypton, snd other volatile fission products (Cs, Te, I, Rb, and Cd) released from small pieces of mixed-oxide fuel ^b 1 that may have been trapped in the ingot. To i determine if any fragments of (U, Pu)Oa were fi present in the void, ur&nium and plutonium t were sought by means of electron-probe micro5 8 analysis and by alpha spectrometry. No uranium }METALLIC INGOT 4 ;: 3 or plutonium was detected in any of 19 voids I I I I I I examined by microanalysis, nor was any E RUTHENIUM I uranium or plutonium detected by means of z G alpha spectrometry of material that had been t evaporated from the walls of the voids using x2 the laser-beam techniques. An X-ray spectral profile of a void that contained some foreign material showed that both copper and zinc were present in about the same proportion as in the I brass mount. Thus we conclude that the black areas in the ingot, shown in fig. 2, are voids that do not contain (U, Pu)Oz ,but some voids have become filled with brass during the in-cell grinding and polishing operation. DISTANCE, microns The intensity ratios of the L-series lines from Fig. 4. Zineax variations in molybdenum end the four most abundant fission-product elements ruthenium concentrations across a metallio ingot are listed in table 2. The intensity ratio is the formed during irradiation of UOe/20 wt y0 PUOZ in ratio of the number of X-ray photons emitted a fast-neutron flux. A simultaneous decrease in the by the specimen under fixed-instrument condiX-ray int%naitiesof MO and Ru indicates the location of voids in the ingot. tions (constant accelerating voltage and beam current) to the number of photons emitted by correspond to locations where the electron beam a pure elemental standard under the same opercrossed spherical voids. No evidence of in- ating conditions. The concentration of each homogeneity or dendritic structure was observed element is also listed in table 2 after correcting either met&~o~~phic~lly or in the sa;mple- the intensity ratios for absorption B) and secondary fluorescence 10) and normalization to current image of the specimen. To establish if other fission products were 100 percent. The concentration of fissionpresent in the metallic ingot below the detecta- product elements in the metallic ingot is bility limit of electron-probe micro&nalysis, 48.6 wt% ruthenium, 20.0 wt% molybdenum, 16.6 wt% technetium, 12.9 wt% rhodium, and several samples were obtained from the ingot by using a laser-beam microssmpling tech- 2.0 wt% palladium. Estimation of the palladium nique 8) to evaporate material from the surface concentration was based on the calculated of the ingot. Neutron activation analysis was intensity of the PdL& line in the RhL@,, RuL@s, then performed on several specimens to deter- PdLN triplet. 3
b7
o<
D.
R.
O’BOYLE
ET
The
AL.
light
elements,
carbon,
nitrogen,
and
oxygen, were sought in the ingot by using a lead stearate
decanoate
analyzing
96.4 A). The recorded no evidence
of carbon,
any of the locations
crystal
(2d =
spectral profiles showed nitrogen,
examined.
clude that the ingot formed
or oxygen
in
Hence, we conat the bottom
of
the central void is metallic and contains the fission-product elements ruthenium, molybdenum, technetium, rhodium, and palladium, but does not contain detectable amounts of uranium, oxygen.
plutonium,
carbon,
nitrogen,
or
To determine the crystal structure and the lattice parameters of the fission-product phase, an alloy of nominal composition 20 wt 9/, molybdenum, 17 wt”h technetium, 48 wt”/,; ruthenium, 13 wt o/o rhodium, and 2 wt% palladium was prepared from high purity metals. The alloy (approximately 2.3 g) was arc melted on a water cooled copper hearth under an inert atmosphere and annealed for 48 h at 1130 “C. Following annealing, powder was prepared by crushing the alloy and Debye-Scherrer patterns were taken with copper radiation. All diffraction lines were indexed and the alloy was found to have a close-packed hexagonal structure, with cc0= 2.735 + 0.001 A, co = 4.355 & 0.001& c/a =1.592 and unit cell volume of 28.21 By. 4.
Discussion
and conclusions
The metallic elements identified in the fission-product ingot are those elements which have a fairly high fission yield and which form oxides that are not thermodynamically stable at the irradiation temperature of the ingot. The free energies of formation of the oxides of ruthenium, rhodium, and palladium are positive above 2200 “C ; whereas the free energies of formation of the oxides of technetium and molybdenum are slightly negative at this temperature 11). Thus the thermodynamic data suggest that molybdenum, technetium, ruthenium, rhodium, and palladium will be reduced to metals, in preference to the fission product,s that form more stable oxides. The composition of the metallic ingot is
ANALYSIS
OF
FISSION
similar to the composition of the fission-product inclusions that form in the columnar-grain region of irradiated (U, Pu)Oz. The composition of the metallic inclusions in the columnar grain region varied as a function of radial position 6). On moving up the temperature gradient from the interface between the columnar- and the equiaxed-grains to the edge of the central void, ruthenium increased from .40 to 53 wt y0 (48.6 wt.%), ‘molybdenum decreased from 30 to 15 wW3 (20 w%), and ~~hnetium increased from 10 to 18 wt% (16.6 wt%). The numbers in parentheses are the measured weight percentages of the three fission-product elements in the metallic ingot. As summarized in table 2 the chemical composition of the metallic ingot lies within the composition range of the metallic inclusions in the columnar-grain region and is close to the measured composition of the inclusions near the central void. Neither ~anium nor plutonium was detected in either the metallic inclusions or in the fission-product ingot, which agrees with prediotions based on the free energies of formation of UOz, PuOz, and the fission-product oxides. The fission yield of the elements detected in the ingot, expressed as atoms of fission product per 100 uranium and plutonium atoms fissioned, are molybdenum 23.1; ruthenium, 16.9; teohnetium, 6.1; palladium, 4.8; and rhodium, 4.2. Since the relative amounts of molybdenum and pa~~urn detected in the metallic ingot and in the metallic inclusions are less than those expected from the fission-product yield (the yield data indicate a higher concentration of molybdenum than ruthenium and a higher oon~entration of palladium than rhodium), and since neither molybdenum nor palladimn was detected in the (U, Pu)Os columnar-grain matrix (detectability limit M 0.07 wt%), we conclude that some molybdenum and palladium migrates to a cooler region of the fuel element. Recent observations by Stalica 12)of a complete cross section of a (U, Pu)Oz fuel element irradiated in EBR-II has confirmed that molybdenum is concentrated in the cooler equiaxed region of the fuel-element cross section.
PRODUCT
INGOTS
265
The metallographic and chemical evidence suggest that the metallic inclusions in the columnar-grain (U, Pu)Oz matrix (approximately 6 ,um in diameter), the white phase observed along the walls of the central void (60 to 80 pm in diameter), and the metallic ingots found in the bottom of the central void (varying in size up to 700 pm in diameter) all have a similar structure and chemical oomposition. We believe the inclusions in the oolumnargrain matrix, which are formed from fissionproduct elements whose oxides are not thermodynamically stable, migrate up the temperature gradient, preferentially along columnar-grain boundaries to form the larger ingots observed along the wall of the central void. Various mechanisms for the migration of inclusions in a temperature gradient have been discussed by Shewmon 13914). Finally, the ingots which have formed on the walls of the central void, collect in the bottom of the void to form larger ingots, as shown in fig. 1. Recent work by Bramman et al.5) on the melting point of an alloy of molybdenum, ruthenium, technetium, and rhodium of approximately the same composition as the metallic ingot studied in this experiment has determined the melting point to be between 1800 and 1900 “C. These data suggest that the metallic inclusions in the columnar grains near the oentral void (calculated temperature w 2600 “C) and the metallic ingot in the bottom of the central void are molten during irradiation. However, no evidence of dendrites in the ingots or in the inclusions was observed either metallographically or by means of electron-probe microanalyses of the molybdenum and ruthenium distribution. Thus, the physical state of the ingot and the inclusions during irradiation is uncertain and must await a direct determination of the melting point of the ingot. The unit cell structure of the ~ssion-produotalloy prepared from highly pure elements is close-packed hexagonal in agreement with the structure of the “meohanioally extracted” and “chemically extracted” metallio inclusions studied by Bramman et al.5). Bramman found
266 that
D.
both
contained
types
of
fission-product
approximately
40 wt%
whereas the fission-product-ingot examined
by X-ray
ment contained
diffraction
R.
O’BOYLE
inclusions
for his patience and skill in preparing the ingot for microprobe
and the alloy
performing
in this experi-
20 wt o/o molybdenum.
by Bramman
AL.
molybdenum
Listed in table 2 are the unit cell parameters reported
ET
examination,
to J. E. Sanecki for
the microprobe
analysis in a oompe-
tent manner, and to M. D. Adams for laser beam microsampling neutron
the ingot
activation
and performing
the
analysis.
et al.5) and measured in
this study. The unit cell size of the alloy with the higher molybdenum
content
(40 wt %) is
larger than the unit cell size of the alloy with 20 wt”h molybdenum. However, the c/a ratios for the two alloys on which precise lattice parameter data are available are similar, 1.592 (this work) and 1.61 (Bramman et a1.5)). This suggests that the fission-product phase is a solid solution the composition of which varies over a wide range. Further alloy studies are being carried out to experimentally establish the composition range of stability of the hexagonal fission-product phase and to measure its melting temperature. The measured average atomic volume of the fission-product atoms in the hexagonal phase (14.10 As) is close to the average atomic volume of a solid solution (14.17 Aij calculated from the atomic volume of each element. Hence, from the average composition of the metallic inclusions (from microprobe measurements) the contribution of these fission products to fuel swelling can be calculated.
References 1) D.
Quataert and H. W. Schleicher, J. Nucl. Mat.
19 (1966) 2)
B.
T.
221
Bradbury,
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and D. M. Poole, 3)
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4)
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5)
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6)
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10 (1967) Sharpe,
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F. L. Brown and J. E. Sanecki,
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R. F. Boyle,
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(Academic
optics
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Laboratory
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Report ANL-5750,
Argonne
(1965) Report ANL-7427. (1968)
Argonne
102
P. G. Shewmon, Diffusion in solids (McGraw-Hill, New York,
1963) ch. 7
P. C. Shewmon,
Trans.
TMS-AIME
230
(1964)
1134
Acknowledgments The authors are indebted to B. J. Koprowski
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1762
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(4. Yates,
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J. I. Bramman,
private communication
(1968)