Characterization of metallic fuel for minor actinides transmutation in fast reactor

Progress in Nuclear Energy 94 (2017) 194e201

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Progress in Nuclear Energy
journal homepage: www.elsevier.com/locate/pnucene

Characterization of metallic fuel for minor actinides transmutation in
fast reactor
mier a, K. Inagaki b, P. Po
€ml a, D. Papaioannou a, H. Ohta b, T. Ogata b,
L. Capriotti a, *, S. Bre
V.V. Rondinella a
a
b

European Commission, Joint Research Centre, Institute for Transuranium Elements, P.O. Box 2340, 76125 Karlsruhe, Germany
Central Research Institute of Electric Power Industry, 2-11-1 Iwado-kita, Komae, Tokyo 201-8511, Japan

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 7 September 2015
Received in revised form
8 March 2016
Accepted 6 April 2016
Available online 7 May 2016

The METAPHIX programme is a collaboration between the Central Research Institute of Electric Power

Industry (CRIEPI, Japan) and the Joint Research Centre - Institute for Transuranium Elements (JRC-ITU) of
the European Commission dedicated to investigate the safety and effectiveness of a closed nuclear fuel
cycle based on Minor Actinides (MA: Np, Am, Cm) separation from spent fuel, incorporation in metal
alloy fuel and transmutation in fast reactor.
Nine Na-bonded experimental pins of metal alloy fuel were prepared at ITU and irradiated at the
Phenix reactor (CEA, France) achieving 2.5 at.%, 7 at.% and 10 at.% burn-up. Four metal alloy compositions
were irradiated: U-Pu-Zr used as fuel reference, U-Pu-Zr ỵ 5 wt.% MA, U-Pu-Zr ỵ 2 wt.% MA ỵ 2 wt.%
Rare Earths (RE: Nd, Y, Ce, Gd), and ỵ5 wt.% MA ỵ 5 wt.% RE, respectively. RE reproduce the expected
output of a pyrometallurgical reprocessing facility.
Post Irradiation Examination is performed using several techniques, covering properties ranging from
the macroscopic morphology of the fuel matrix to the microanalysis of phases and elemental redistribution/segregation. The irradiated fuel is characterized by many phases occurring along the fuel radius.
The fuel underwent large redistribution of the fuel constituents (U, Pu, Zr) and many secondary phases
are present with a variety of compositions. The distribution of phases in the irradiated fuel containing
minor actinides and rare earths is essentially similar to that observed in the basic ternary alloy fuel.
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND
license ( />
Keywords:
Metallic fuel
Minor actinides
Electron probe micro analysis
Irradiation experiment
Post irradiation examination

1. Introduction
Advanced nuclear reactors and closed nuclear fuel cycles are
important options to achieve sustainable nuclear energy supplies to
satisfy future demands while reducing the long-term radiotoxicity
of high level waste (GIF, 2002; GNEP, 2007; Funasaka and Itho,
2007; Haas et al., 2009; JRC-EASAC, 2014). Spent fuel reprocessing and the subsequent recycling of U and Pu as fuel and transmutation of Minor Actinides (MA) Np, Am, Cm in fast reactors are
necessary steps to achieve this goal (Yokoo et al., 1996; Inoue et al.,

1991; Ohta et al., 2005).
Fast reactors brings different advantages compare to thermal
reactors in term of transmutation of actinides. Hereafter the main
reasons are reported from OECD/NEA, (2012):

* Corresponding author.
E-mail address: (L. Capriotti).

 A favourable neutron balance, which allows to introduce MA of
any type and in significant amounts, without perturbing the
reference performances of the corresponding core without MA.
 A neutron spectrum which allows fissions to dominate captures
for all TRUs. This feature allows limiting with respect to thermal
reactors the build-up of higher mass nuclei, e.g. the build-up of
252 Cf during TRU multi-recycle.
 The flexibility to burn or breed fuel, or to be iso-generator (a
system that has a zero net production of TRU constituents in the
fuel).
 The possibility to benefit from the favourable characteristics
indicated above, whatever the Pu vector, the type of fuel (oxide,
metal, nitride, carbide) and the type of coolant (sodium, heavy
liquid metal, gas).
The METAPHIX programme is a collaboration between the
Central Research Institute of Electric Power Industry (CRIEPI, Japan)
and the Joint Research Centre-Institute for Transuranium Elements

/>0149-1970/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( />

L. Capriotti et al. / Progress in Nuclear Energy 94 (2017) 194e201
Table 1

Average composition (wt.%) of the fuel alloys as fabricated (impurities
content < 0.3 wt.%).
El

U-Pu-Zr

U-Pu-Zr 2MA-2RE

U-Pu-Zr 5MA

U-Pu-Zr 5MA-5RE

U
Pu
Zr
MA
Np
Am
Cm
RE
Y
Ce
Nd
Gd

71.00
18.93
10.19
0.03
0.03

66.85
19.80
9.46
2.08
1.23
0.67
0.18
1.73
0.12
0.20
1.25
0.16

66.30
19.35
8.97
4.74
2.97
1.45
0.32
e

63.50
19.75
8.19
4.78
3.04
1.52
0.31

3.40
0.31
0.45
2.30
0.32

e

195

(JRC-ITU) of the European Commission with the support of the
Commissariat 
a l'Energie Atomique et aux Energies Alternatives
(CEA, France). It is dedicated to the study of the safety and effectiveness of a closed nuclear fuel cycle based on MA separation and
irradiation in metallic fuel using fast reactor. In this context, three
assemblies containing nine Na-bonded experimental pins of metal
alloy fuel prepared at ITU (Kurata et al., 1999) were loaded in the
nix reactor in 2003 and irradiated at three different burnups,
Phe
2.5 at.% (METAPHIX-1), ~7 at.% (METAPHIX-2) and ~10 at.% (METAPHIX-3).
Extensive metal fuel irradiation tests were conducted in the USA
in the Integral Fast Reactor (IFR) program (Carmack et al., 2009;
Chang, 1989; Till and Chang, 1991) both on U-10 wt.%Zr binary
alloy fuel and on U-Pu-10 wt.%Zr ternary fuel. Of these test pins, the
highest burnup achieved for the U-19 wt.%Pu-10 wt.%Zr fuel
without pin breach was more than 19 at.% (Crawford et al., 2007).
The first irradiation of MA-bearing metal fuel was conducted in
the X501 test assembly in EBR-II up to 7.6 at.% burnup (Meyer et al.,
2009; Kim et al., 2009). Preliminary post irradiation examinations
revealed that the macroscopic behavior in pile of MA-bearing

metallic fuel is similar to the basic alloy metallic fuel.
The main objective of the PIE studies on the irradiated METAPHIX fuel is to study the safety of this concept during irradiation.
The presence, distribution and behavior of the various phases in the
fuel is a key aspect of these investigations. The possible effects
investigated include abnormal behavior of secondary phases (e.g. in
terms of thermal stability, fuel-cladding chemical interaction, etc.).
The present paper describes some of the recent findings in this
campaign of studies. In particular, it focuses onto the distribution of
phases as evidenced by the PIE. This examination is part of a
broader effort aimed at confirming the safety of the fuel during
irradiation and the achievement of effective transmutation rates.

2. Materials and methods

Fig. 1. Schematic view of the 3 different fuel pins irradiated in PHENIX reactor. The top
of the fuel pin is approximately at the middle of the PHENIX core.

Post Irradiation Examination (PIE) is performed at JRC-ITU.
Nondestructive examinations and fission gas analysis showed
that MA-bearing fuel pin behavior during irradiation was in line
with that of the base alloy (Papaioannou et al., 2012; Ohta et al.,
2011, Rondinella et al., 2010). Destructive examinations, including
optical microscopy, scanning electron microscopy (SEM) and electron probe micro analysis (EPMA) are ongoing for METAPHIX-1 and

Fig. 2. Temperature axial distribution in the metallic fuel alloys at the beginning of irradiation and end of irradiation for the 3 different burn-ups and for fuel centre and periphery
(Ohta et al., 2015a,b).


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L. Capriotti et al. / Progress in Nuclear Energy 94 (2017) 194e201

Fig. 6. Optical microscopy image of the periphery region of the sample, precipitates
are visible in dark grey color (pointed by arrows).

Fig. 3. Optical microscopy macrograph showing a cross-section of a METAPHIX-1
specimen of U-19Pu-10Zr-5MA-5RE, with burnup 2.4 at.%.

METAPHIX-2. This paper highlights some recent results concerning
microstructure, morphology and phase distribution for METAPHIX1 and METAPHIX-2.
2.1. Fuel preparation and irradiation experiment characteristics
Table 1 describes the average compositions of four different
metallic alloys ingots prepared using arc melting process (Kurata
et al., 1999): U-19Pu-10Zr used as reference, U-19Pu-10Zr-2MA2RE, U-19Pu-10Zr-5MA-5RE and U-19Pu-10Zr-5MA. RE were added
to reproduce the output of a pyrometallurgical reprocessing facility.
Alloy samples of U-Pu-Zr, U-Pu-Zr-MA, and U-Pu-Zr-MA-RE
were prepared by arc melting in argon atmosphere. The homogenization of the U, U-Pu, or U-Pu-MA alloys was obtained by melting

and mixing with molten Zr. For U-Pu-Zr-MA-RE alloy samples,
metal powders of U-Zr or U-Pu-Zr-MA and RE were blended mechanically before melting (Kurata et al., 1999).
The metallic alloys ingots were cast using yttria molds, which
are compatible with the molten fuel alloys, and cut into rodlets
20e50 mm long.
The metallic fuel present a U-Pu-Zr fuel matrix with homogenous dispersion of RE-MA precipitates (Kurata et al., 1999).
More detailed information and properties on the as fabricated
metallic alloy are reported in Kurata et al., 1999 and Ohta et al.,
2011.
The metallic fuel alloy ingots were loaded into 9 fuel pins in 3
different configurations, as illustrated in Fig. 1, with a total active
fuel length of 485 mm. The cladding utilized was the alloy 15-15Ti

(Seran et al., 1992; Millard et al., 1994; Fissolo et al., 1994). The
fuel pins were bonded with Na to optimize the thermal
conductivity.
The irradiation took place in an irradiation capsule placed in the
core of the PHENIX reactor up to the 3 different burnup levels.

Fig. 4. Optical microscopy images showing the structure along the radius of a METAPHIX-1 specimen of U-19Pu-10Zr-5MA-5RE, burnup 2.4 at.%.

Fig. 5. Central region of the cross-section shown in Fig. 3a) BSE image highlighting the large fission gas bubbles; b) optical microscopy image of precipitates (pointed by arrows) and
gas bubbles.


L. Capriotti et al. / Progress in Nuclear Energy 94 (2017) 194e201

Fig. 7. Optical microscopy images showing the structure along the radius of a METAPHIX-2 specimen of U-19Pu-10Zr-5MA-5RE, with burnup 6.8 at.%.

Fig. 8. Optical microscopy of a METAPHIX-2 specimen showing a macrograph and the radius of U-19Pu-10Zr basic alloy, burnup 6.9 at.%.

Fig. 9. Redistribution of the main fuel components (Pu, Zr, U) in a METAPHIX-2 sample obtained by EPMA (qualitative analysis), (Bremier et al., 2013).

197


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Irradiation conditions were evaluated in order to predict the temperature conditions at the beginning and end of irradiation by
means of the ALFUS code (Ohta et al., 2015a, 2015b). Fig. 2 presents
the axial temperature calculated for the 3 different burnups and for

fuel centre and periphery. According to predictions, at the end of
the irradiation a high temperature phase, g-phase (Ogata, 2012),
should be present in the upper part of METAPHIX-1 fuel pins. This
phase is observed for temperature above 650  C.
2.2. Experimental techniques
Standard metallographic procedure was employed. Fine
grinding of the sample was carried out using diamond paper and
alcohol as a lubricant. Grinding was followed by polishing with a
series of diamond powder suspensions of increasing fineness.
45
40

Zr Conc, Wt.%

35
Outer: U,Pu,Zr fuel
Outer U,Pu,Zr (Zr rich)
Crust
Core Fuel I
Core Fuel II
U-rich Core I
Pu-rich Core II

30
25
20

3. Results and discussion
3.1. Microstructure investigation of U-19Pu-10Zr-5MA-5RE

15
10
5
0
100
90
80

U, wt.%

70
60
50
Outer: U,Pu,Zr fuel

40

Outer: U,Pu,Zr (Zr rich)
Crust

30

Core Fuel I
Core Fuel II
U-rich Core I

20

Pu-rich Core II

10
40

30

Pu, wt.%

Grinding and polishing were performed using a polishing device
specially adapted for operation in a hot cell.
The fuel samples were examined using a Leica Telatom-3 optical
microscope connected to the sample preparation cell by a shielded
tunnel, and the sample is transported using a motorized cart.
The shielded SEM used for analysis of highly radioactive specimens at ITU is a JEOL JSM-6400. The EDX device combined to this
SEM equipment used for the characterization described here was
SAMx Numerix DXD-X10P; the detector was equipped with a lead
collimator.
The background radiation from the different fuel samples was
not negligible; therefore, quantitative evaluation of the results of
EDX measurements is considered as not sufficiently accurate; semiquantitative analysis was achieved.
EPMA was carried out using a state of the art shielded Cameca
SX 100 specially shielded and modified to permit the analysis of
irradiated nuclear fuels (Walker, 1999). The analysis was performed
on a specimen coated with a conducting film of aluminium
approximately 20 nm thick to avoid charging effects. The standards
were also coated at the same time; this removed the necessity for
correction for the film.

20

10

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

r/r0
Fig. 10. Concentration and redistribution profiles of the main fuel constituents along
the radius of a METAPHIX-2 sample: (a) Zr, (b) U (c) Pu (Bremier et al., 2013).

Figs. 3 and 4 present optical microscopy macrographs of a pin

cross section and a detailed radius overview of a METAPHIX-1
specimen of U-19Pu-10Zr-5MA-5RE. The sample was cut at an
axial position of 370 mm from the bottom of the fuel stack, corresponding to a burnup of 2.4 at.%.
3 different regions exhibiting various degrees of porosity are
visible. The central part appears to be separated from the rest of the
fuel by a circular crack (or fuel-fuel gap). In this region, large fission
gas bubbles, characteristic of the high temperature phase (g-phase)
that forms above 650  C (Ogata, 2012; Hofman and Walters, 1994),
are present. A back scattered electron (BSE) high magnification
image of the gas bubbles is shown in Fig. 5(a). Second phase precipitates are also visible in this region (Fig. 5(b)) and are inferred to
be agglomerates of MA and fission products (Ohta et al., 2015a). The
intermediate radius region is characterized by a dense phase,
considered to be z-phase (Ogata, 2012), together with relatively
large voids (Fig. 4). In the periphery a highly porous phase and a
second phase are present. This second phase occurs preferentially
around pores and is rich in fission products and rare earths (Ohta
et al., 2015a), as shown in Fig. 6.
Thermochemical and thermal gradients (Hofman and Walters,
1994; Carmack et al., 2009) lead to an increase (compare to nominal value) of the Zr content in the high temperature region. The
SEM/EDX radial scanning confirmed this redistribution of Zr (and of
the others 2 main elements Pu and U) as shown in Ohta et al.,
2015a.
The fuel-cladding gap is already closed at 2.5 at.% burnup and
remains closed at higher burnup. The fuel alloy is in contact with
the cladding owing to fuel swelling and to an increase in the overall
volume of fission gas bubbles (Ohta et al., 2015a).
In the medium burn-up sample (Fig. 7), the high temperature
phase is not present and the central region appears as a dense
phase where no MA precipitates are visible.
The central region appears denser compared to the other regions of the fuel exhibiting both intragranular and intergranular

fine porosity.
In the METAPHIX-2 specimen, the fuel-fuel gap is clearly delineated and thicker compare to the METAPHIX-1 sample.


L. Capriotti et al. / Progress in Nuclear Energy 94 (2017) 194e201

199

Fig. 11. Zr-rich precipitates alloyed with Ru in the fuel central region of a METAPHIX-2 sample.

Fig. 12. Zr-rich quasi-square precipitates at the mid-radius of a METAPHIX-2 sample.

Fig. 13. RE-rich phases in 2 different fuel locations of a METAPHIX-2 sample: a) centre of fuel; b) fuel periphery.

3.2. Redistribution of main elements and secondary phases in U19Pu-10Zr basic alloy
EPMA was performed on a sample of U-19Pu-10Zr basic alloy
from METAPHIX-2, Fig. 8 (Bremier et al., 2013). The sample was cut
480 mm from the bottom of fuel stack; this location corresponds to
the highest burnup, 6.9 at.%.

The calculated temperatures are 600  C in the centre of the fuel
and 500  C at the fuel-cladding interface. The ternary phase diagrams U-Pu-Zr experimentally measured by O'Boyle and Dwight
(1970) and calculated by Kurata (2010) were selected in order to
identify the different phases of the fuel matrix using the quantitative point analysis performed.
For the phase identification of the other intermetallic


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L. Capriotti et al. / Progress in Nuclear Energy 94 (2017) 194e201

compounds containing RE and noble metals, the binary phase diagrams of Okamoto (1993) were used.
The distribution of the fuel constituents (U, Pu, Zr) and selected
fission products (e.g. Nd, Mo, La, Ru, Xe, Cs) was assessed by X-ray
mapping. Fig. 9 shows the qualitative (re)distribution of U, Pu, Zr in
the irradiated fuel. In Fig. 10(a)e(c) the results of the quantitative
concentration analysis for the three main fuel constituents are
plotted.
In the centre of the fuel an almost complete depletion of Zr is
observed (Fig. 10 (b)); moreover, Pu and U (Fig. 10(a) and (c))
formed two heterogeneously distributed phases (core phase I and
II, respectively) with different U/Pu ratio. The almost complete
depletion of Zr in this region is the main reason why it was assumed
that the temperature at the end of the irradiation was around
600  C; this assumption is not in contradiction with the temperature profile calculated by ALFUS because it is within the 15% uncertainty affecting the irradiation parameters (Ohta et al., 2009).
Taking in consideration this aspect, the ternary phase diagram at
595  C from Kurata (2010) identifies as b-U and z phases the 2
distinct phases, core phase I and II. No molten phases were visible
in the centre of the fuel and according to the U-Pu phase diagram
(Okamoto H., 1993), for this range of compositions, the melting
temperature is significantly higher than 600  C.
In the “dense phase” or “crust” stripe at mid-radius (Fig. 8), the
Zr content is still very low; this region consists of one single phase,
which results in z phase, considering the ternary diagram at 550  C.
From the mid-radius to the periphery of the fuel, the microstructure is very porous. U and Pu exhibit symmetric radial profiles;
the Pu concentration is decreasing close to the periphery (Fig. 10
(c)) and the Zr content in the fuel matrix is close to its nominal,
as-prepared value. Zr-rich phase is present also in the outer radial
region, as shown in Fig. 10 (a) (“outer Zr-rich”).
Taking in consideration the U-Pu-Zr phase diagrams at 550  C

(close to mid-radius) and 500  C (radial periphery of fuel) from
O'Boyle and Dwight (1970), the matrix fuel phases are estimated to
be a mixture of z, d and a-U; the appearance of the fuel in this region is quite heterogeneous.
Zr-rich phases or Zr segregations are found throughout the fuel
radius. Fig. 11 shows Zr-rich precipitates found in the central region
with a concentration of Zr up to 25 wt.% alloyed with noble metals
(Ru, Pd, Rh). Fig. 12 shows at mid-radius precipitates appearing in
quasi-squares shapes with Zr concentration up to 40 wt.% combined with Ru, U, Rh, Mo, Pd, Pu in different quantities. Zr-rich
precipitates are found also in other studies such as in D.D. Keiser,
2012.
RE-rich phases, incorporating also noble metals, are observed at
different radial locations; in Fig. 13 lanthanum precipitate are
shown at 2 different locations, in the centre region (Fig. 13 (a)) and
at the fuel periphery (Fig. 13 (b)). From the ratio between RE and
noble metals these precipitates are inferred to be of two kinds:
RE7(Pd,Rh)3, found throughout the fuel, and RE3(Pd,Rh)2 found only
at the radial periphery. These RE-rich precipitates are well studied
in literature and of concern for the integrity of the cladding (Kim
et al., 2009; Carmack et al., 2009).
4. Conclusion
Destructive post irradiation examinations were performed on
low and medium burn up METAPHIX fuel pins. The morphology,
composition and distribution of fuel matrix and secondary phases
were characterized by optical and scanning electron microscopy
and by electron probe micro-analysis. The irradiated fuel is characterized by many phases occurring along the fuel radius. The
distribution of phases in the irradiated fuel containing minor actinides and rare earths is essentially similar to that observed in the

basic ternary alloy fuel (Ogata, 2012; Hofman and Walters, 1994). In
the sample exhibiting a g-phase zone (i.e. achieving higher irradiation temperature) some large precipitates, estimated to be inclusions of MA and RE, are observed. Second phases are present also
in the lower temperature region at the radial periphery of the fuel.

EPMA analysis was performed on U-Pu-Zr fuel irradiated to 7 at.%
burnup, providing more detailed insight in the complex configuration of irradiated ternary alloy fuel. The redistribution behavior of
the fuel constituents (U, Pu, Zr) is in line with findings reported in
the literature, and many secondary phases are present with a variety of compositions.
PIE performed so far confirms a substantially positive irradiation
behavior of MA-containing alloy fuel, in line with what is known
about U-Pu-Zr irradiation behavior. Ongoing and planned investigation campaigns will extend the range of compositions and
burnup levels analyzed with the aim of obtaining a complete picture to assess the irradiation behavior of ternary alloy fuel containing minor actinides. The outcome of the post-irradiation
examination studies will be integrated with the ongoing experimental and modeling activities carried out in the METAPHIX project to assess the overall safety and performance of this closed fuel
cycle concept.
Acknowledgments
Important contributions to the PIE were provided by R. Nasyrow,
R. Hasnaoui, R. Gretter, G. Paperini, (JRC-ITU); many other colleagues in JRC-ITU and CRIEPI deserve warm acknowledgements for
their support, input and contributions in the various stages of this
project.
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