Sintering densification of CaO-UO2-Gd2O3 nuclear fuel pellets

Article abstract of DOI:10.1016/j.jallcom.2015.05.278

CaO-doped UO₂-10 wt% Gd₂O₃ burnable poison fuel was prepared by co-precipitation reaction method. It was found that 0.3 wt% CaO-doping significantly improved the sintered density, grain sizes and crushing strength of UO₂-Gd₂O₃ fuel pellets at the sintering temperature of 1650 C in the sintering atmosphere of hydrogen for 3.5 h. In addition, homogeneous solid solution without precipitation of free phases of CaO and Gd₂O₃ was successfully achieved. CaO doping in UO₂-Gd₂O₃ fuel pellet system accelerated the thermally activated material transport, so the onset temperature of densification as well as the temperature of the maximum densification rate shifted to a lower temperature region.

Full text of DOI:10.1016/j.jallcom.2015.05.278

Journal of Alloys and Compounds 646 (2015) 626e631
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Sintering densification of CaOeUO eGd O nuclear fuel pellets
2
2 3
a
b
c
,
*
c
c
c
Yun Wang , Huidong Sun , Hui Wang , Xiaoqiang Pan , Tongye Li , Jinhong Liu ,
c
c
Yong Zhang , Xinjie Wang
Fundamental Science on Radioactive Geology and Exploration Technology Laboratory, East China Institute of Technology, Nanchang, 330013, Jiangxi, China
China Nucle Power Engineering Co., Ltd, China
a
b
c
National Key Laboratory for Nuclear Fuel and Materials, Nuclear Power Institute of China, Chengdu, 610041, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 April 2015
Received in revised form
CaO-doped UO -10 wt% Gd O burnable poison fuel was prepared by co-precipitation reaction method. It
2
2 3
was found that 0.3 wt% CaO-doping significantly improved the sintered density, grain sizes and crushing
ꢀ
strength of UO
2
2
eGd O
3
fuel pellets at the sintering temperature of 1650 C in the sintering atmosphere
20 May 2015
of hydrogen for 3.5 h. In addition, homogeneous solid solution without precipitation of free phases of
CaO and Gd was successfully achieved. CaO doping in UO eGd fuel pellet system accelerated the
Accepted 24 May 2015
Available online 22 June 2015
2
O
3
2
2 3
O
thermally activated material transport, so the onset temperature of densification as well as the tem-
perature of the maximum densification rate shifted to a lower temperature region.
Keywords:
Advanced nuclear fuel
Burnable poison
©
2015 Elsevier B.V. All rights reserved.
Sintering densification
Co-precipitation reaction method
High burn-up
1. Introduction
same condition, the additive gadolinium oxide reduces not only the
grain sizes and the density of UO pellets, but also the thermal
2
Developing nuclear power is an inevitable stage to solve the
contradiction between supply and demand of energy, maintain
sustainable economic development and protect ecological envi-
ronment. In order to improve the efficiency of nuclear power,
alleviate the pressure of the treatment and disposal of spent nu-
clear fuel, nuclear fuel elements with high burn-up and long life are
the long-term goals for nuclear power plant.
conductivity [1,7e10], resulting in more fission gas release, irradi-
ation swelling and other issues during nuclear reactor operation,
which reduce nuclear reactor safety and economy.
In order to reduce fission gas release, one often attempted to
produce large grain-sized fuel, which could be obtained by suitable
doping. The results of research on CaO-doped UO
doping significantly improved the sintered density, grain sizes of
UO and the crushing strength of UO , which enhanced the security
performance of the reactor fuel elements [11,12].
The concentration of gadolinium oxide in UO
2
showed that CaO
The goal for a new generation and the future of nuclear power is
in pursuit of inherent safety performance as well as greater eco-
2
2
nomic benefits. UO
2
eGd
2
O
3
burnable poison fuel is widely used in
2 2 3
eGd O fuel pel-
nuclear power reactors in the world, especially reactor cores of
pressurized water reactor (PWR), to control the initial reactivity of
reactors, spread the distribution of core power, increase fuel
burnup and prolong refueling cycle, which thus contribute to the
reduction of nuclear power operation cost and the improvement of
operation safety and reliability [1e6]. Currently the preparation
lets is in the range of 4e8 wt%. However, with the increase of the
fuel cycle length, gadolinium oxide with higher concentration is
needed for UO
oxide concentration would further reduce the sintered density,
grain sizes and thermal conductivity of UO eGd fuel pellets. For
the purpose of increasing fuel burn-up and prolonging fuel cycle
length, it is of great significance to research and develop UO -10 wt
% Gd fuel pellet with high concentration of gadolinium oxide,
high density, large grain sizes and good microstructure.
Although it is more economical to prepare CaO-doped UO
0 wt% Gd fuel pellets by traditional powder metallurgy
method, UO eGd fuel pellets prepared in this way reveal
shortcomings such as poor uniformity of gadolinium oxide,
2 2 3
eGd O fuel pellets. Therefore, rising gadolinium
2
2 3
O
method of UO
similar to that of commercially used UO
difficult to prepare UO eGd fuel pellets than UO
2
2
eGd
2
O
3
fuel pellets in nuclear power plants is
pellets, but it is far more
pellets. In the
2
2
2 3
O
2
2
O
3
2
-
1
2 3
O
2
2 3
O
*
Corresponding author.
E-mail address: yinchanggeng5525@163.com (H. Wang).
http://dx.doi.org/10.1016/j.jallcom.2015.05.278
925-8388/© 2015 Elsevier B.V. All rights reserved.
0

Y. Wang et al. / Journal of Alloys and Compounds 646 (2015) 626e631
627
difficulty in sintering densification, more internal pores and flaws
and a lot of dust, which bring about environmental pollution and
damage to people's physical health.
In view of a lot of advantages by the co-precipitation method,
such as the uniform material chemical composition, good purity
and good crystal structures and low sintering temperatures, it be-
comes the research and development focus of the preparation of
ceramic nuclear fuel and material in the world [11]. As an advanced
technology to produce ceramic materials, however, there are rare
stirred and then heated to the reaction temperature in the pre-
cipitation tank. Then saturated (NH CO solution was added into
4
)
2
3
the tank at a certain flow rate (5 ml/min). When the feeding
finished, a certain volume (60e80 ml) of anhydrous ethanol was
added. After a period of time (0.5e1 h) the precipitation reaction of
the mixed solution finished and the solution lost liquidity. After
sediment filtering, the filter cake was washed three times with
ꢀ
anhydrous ethanol and dried at 65 C for 15 h to get precursor. The
ꢀ
precursor was kept at 600 C for 2.5 h, cooled and ground to pro-
ꢂ2
research reports on the preparation of CaO-doped UO
Gd fuel pellets by co-precipitation reaction method.
(NO , Gd(NO and Ca(OH) were adopted as the initial
experimental materials in this paper. UO -10 wt% Gd fuel with
different contents of CaO (the mass fraction of 0, 0.3%, 0.5% and
.6%, respectively) were prepared by co-precipitation reaction
2
-10 wt%
2 2 3
duce UO eGd O eCaO powder, which was pressed by 4 ton cm
2
O
3
pressure into cylindrical powder bulks with the diameter of about
8 mm and the height of about 10 mm. The pressed powder bulks
UO
2
3
)
2
3
)
3
2
2
2
O
3
were sintered in the reducing atmosphere into CaO-doped UO
10 wt% Gd ceramic fuel pellets. The sintering atmosphere was
, the sintering temperature was 1650 C, and the hold time was
3.5 h.
In order to study the effect of CaO doping on UO
respectively, UO -1 wt% CaO and Gd -1 wt% CaO samples were
2
-
2 3
O
ꢀ
0
H
2
method and sintered in different sintering process conditions. The
paper studied the effects of sintering temperatures and CaO con-
tents on the sintered density, densification dynamics, grain sizes
and crushing strength. In addition, its microstructure and phase
structure were analyzed. The research results provided techno-
2
2 3
and Gd O ,
2
2 3
O
prepared by the above-mentioned method, which were sintered at
ꢀ
1100, 1200, 1300, 1400, 1500 and 1600 C in the sintering atmo-
logical base for the preparation and application of CaO-doped UO
0 wt% Gd
safety for advanced nuclear power use.
2
-
sphere of H
2
with the hold time of 3 h.
1
2 3
O burnable poison fuel with high burn-up and inherent
2.2. Characterization and measurement
ꢀ
2
. Experimental method
After the cylindrical powder bulks was heated up to 1650 C in
2
, its shrinkage was measured by a lever dilatometer. The densities
H
2
.1. Fuel preparation
of fuel pellets are measured with water immersion method. The
Optical microscope (OM) is used to observe the metallurgical
structures of fuel pellets. Line intercept method is adopted to test
the average grain sizes of pellets. The phase structures of fuel pel-
lets are tested with D/max22400X X-ray diffractometer (XRD).
Electronic universal testing machine was used to measure the
crushing strength of cylinders with height-diameter ratio of 1:1.
The specific measurements were as follows: (1). The pressure sur-
faces sizes of tested samples were measured and the areas were
calculated. The samples for each group are not fewer than 3; (2).
The samples were put in the center of the pressure plate of the
(
1) The preparation of mixed solution of UO
and Ca(NO
2
(NO
3
)
2
, Gd(NO
3
)
3
3 2
)
First, in accordance with the traditional approach to the prep-
aration of nuclear fuel for pressurized water reactors, UO pellets
2
with the ration of O/U ¼ 2.10 are prepared with the raw material of
2
35
ammonium diuranate (ADU,
pellets are broken into UO
U content ¼ 0.2 at%). The UO
2
2
powder with the particle size
ꢀ
ꢁ
1e2
m
m.The UO
2
powder was dried at 350 C for 2.5 h and 99.9%
2
Gd
2
O
3
powder with the particle size ꢁ1
m
m (Ganzhou holmium xin
tester. Load was added at a rate of 2 ꢃ 10 N/s until the sample was
ꢀ
new materials co.,LTD) was dried at 550 C for 2 h. Based on the
destroyed. The maximum load was recorded during that moment;
(3). The test results were put in the following formula to calculate
mass fractions of Gd
amount of dry powder (90 g UO
put into the dissolving tank together with a certain amount
2
O
3
in UO
2
-10 wt% Gd
2
O
3
fuel pellets, a certain
2
powder, 10 g Gd
2
O
3
powder) was
the crushing strength. R
MPa, P is failure load, N, and S is stressed area.
C
¼ P/S, where R
C
is the crushing strength,
(
(
450e500 ml) of deionized water. By rapid stirring, moderate
95e105 ml) concentrated nitric acid was added into the dissolving
3. Results and discussion
tank. After complete dissolution, insolubles were filtered out and
the remaining clear filtrate was mixed solution of UO
Gd(NO
The mass fractions of CaO in doped UO
pellets were 0, 0.3%, 0.5% and 0.6%, respectively calcium hydroxide
was weighed according to the proportion of CaO and stirred with
2
(NO
3
)
2
and
3.1. Phase analysis of CaO-doped UO
2 2 3
eGd O fuel pellets
3 3
) .
2
-10 wt% Gd
2
O
3
fuel
Fig. 1 is XRD diffraction pattern of UO
different CaO contents after sintering at 1600 C in H
time of 3.5 h at the heating rate of 4 K min . There were few dif-
2
eGd
2
O
3
fuel pellets with
with the hold
ꢀ
ꢂ1
2
5
ml water. 1e2 ml concentrated nitric acid was added and dis-
solved to get stable Ca(NO solution. By rapid stirring, Ca(NO
was slowly dropped into the mixed solution of UO
ferences among the XRD diffraction patterns of UO
pellets with different CaO contents as shown in Fig. 1. Almost all
were UO phase diffraction peaks, demonstrating that substitu-
tional solid solution formed in UO -Gd fuel pellets with
different contents of 10 wt% Gd . Fig. 1 shows that when 0.3 wt%
CaO was added into UO eGd fuel pellets, there were not other
2 2 3
-Gd O fuel
3
)
2
3 2
)
and
2
(NO
3
)
2
2
Gd(NO
Gd(NO
3
)
)
3
3
. In the end, uniform mixed solution of UO
and Ca(NO was prepared.
2
(NO
3
)
2
,
2
2 3
O
3
3
)
2
2 3
O
O
2 3
2
(
2) The preparation of saturated (NH
4
)
2
CO
3
solution
peaks in the XRD diffraction pattern, indicating that CaO solved into
the fuel pellets and uniform solid solution formed. And when 0.5%
2
2 g analytically pure solid ammonium carbonate was put into a
CaO was added into UO
showed weak CaO diffraction peaks. When CaO content increased
to 0.6%, the precipitated CaO from UO eGd fuel pellets also
2 2 3
eGd O fuel pellets, the XRD pattern
beaker together with 100 ml deionized water. In the condition of
rapid stirring, saturated (NH
4
)
2
CO
3
solution was prepared.
2
2 3
O
increased. The XRD pattern showed the increase of CaO diffraction
peaks.
(
3) The preparation of fuel pellets
From Fig. 1, the diffraction peaks in XRD diffraction pattern
showed tiny changes with the increase of CaO content. XRD
The mixed solution of UO
2
(NO
3
)
2
, Gd(NO
3
)
3
and Ca(NO
3
)
2
was

628
Y. Wang et al. / Journal of Alloys and Compounds 646 (2015) 626e631
diffusion rate of uranium atoms and the sintered density. According
to the analysis of Fig. 1, however, after the doped CaO content was
more than 0.5 wt%, there was a small amount of CaO free phase
precipitation from fuel pellets sintered at high temperatures. The
precipitated free dispersed particles, perhaps enriching in intra-
crystalline place or at the grain boundary, had a “pinning” effect on
the uranium atoms and inhibited the diffusion rate. Thus, the sin-
tered density rarely changed or even slightly decreased.
2 2 3
Fig. 3 shows the microstructure of UO eGd O fuel pellets with
different doped CaO contents. The sintering temperature was
ꢀ
1
650 C, the sintering atmosphere was H
2
, and the hold time was
3
.5 h. It can be seen from Fig. 3 that the grain size of the undoped
UO eGd pellet was about 4 m. Doped CaO had a distinct effect
on the grain sizes of UO eGd fuel pellets. When the doped CaO
content was more than 0.3 wt%, the grain size of UO eGd pellet
was 10 m. But when the doped CaO contents were 0.5 wt% and
.6 wt%, the grain sizes were similar to those of 0.3 wt% CaO-doped
UO eGd , which also suggested that a small amount of doped
2
2
O
3
m
O
2 3
2
2
2 3
O
m
0
Fig. 1. XRD diffraction pattern for UO
2 2
eGd O
3
pellets with different CaO contents.
2
2 3
O
CaO could significantly increase the grain sizes of fuel pellets.
However, a further increase in the doped amount did not bring
about obvious increase in the grain sizes. Some clusters of small
grains could be seen in Fig. 3(aec). Kim et al. [13] reported that this
cluster was the Gd deficient region. The area fraction of the small-
grained cluster decreased with an increasing CaO content.
Based on the above analysis, a small amount of doped CaO could
significantly improve the densification and increase the grain sizes
of fuel pellets. It is wrong that more doping content will be better.
According to the study in this paper, 0.3 wt% doped CaO played an
obvious role.
2 2 3
diffraction peaks of UO eGd O fuel pellets decreased. According to
Bragg equation and interplanar spacing formula, the formula for
the diffraction direction of tetragonal crystal system is:
2
ꢀ
ꢁ
l
2
2
2
2
sin q ¼
h þ k þ l
(1)
2
a
4
From Formula (1), when the diffraction angle
q
remains the
same, the smaller
the ionic radius of Ca is smaller than that of U , more CaO
content in UO eGd fuel pellets leads to smaller of solid so-
lution and smaller diffraction peak. Fig. 1 provides the direct data.
a is, the smaller the diffraction peak l is. Because
2
þ
4þ
Although small amounts of CaO has a significant effect on the
2
2
O
3
a
sintered density of UO
fluence of CaO in the sintered density of UO
2
eGd
2
O
3
pellets, in order to clarify the in-
and Gd respec-
2
2 3
O
tively, it is required to investigate the sintering temperature of
3
.2. The effect of doped CaO contents on the sintered density and
ꢀ
1100, 1200, 1300, 1400, 1500 and 1600 C, hold time 3 h according
microstructure of UO eGd fuel pellets
2
2 3
O
to our experiment. In this experiment, the sintering temperatures
are low, the hold time is short. Therefore, more amount of CaO
doping is needed in order to obtain a satisfactory effect relationship
2 2 3
Fig. 2 shows the sintered density curve for UO -Gd O fuel
pellets with different doped CaO contents. The sintering tempera-
ture was 1650 C, the sintering atmosphere was H
data. 1wt% CaO doping is chosen for pure UO
shows the effect of different sintering temperatures on the sin-
tered density of UO -1 wt% CaO and Gd -1 wt% CaO. The sin-
tering atmosphere was H , the holding time was 3 h and the
heating rate was 4 K/min. A small amount of doped CaO signifi-
cantly improved the densification and sintered density of UO
2 2 3
or Gd O . Fig. 4
ꢀ
2
, and the hold
time was 3.5 h. It can be seen from Fig. 2 that 0.1 wt% doped CaO
had a great effect on the sintered density of UO eGd fuel pellets,
which significantly improved the sintering. When the doped CaO
content was more than 0.3 wt%, the dopant had little impact on the
sintered density. When the content was over 0.5 wt%, the sintered
density of fuel pellets was slightly lower instead.
2
2 3
O
2
2 3
O
2
2
,
which was in accordance with the data in Fig. 4. Besides, it can be
seen that a small amount of doped CaO significantly improved the
A small amount of CaO could increase the sintered density of
sintered density of Gd
2
O
3
. These samples were almost fully
UO
2
bulks. A tiny amount of Ca element could dissolve in UO
2
and
ꢀ
densified at 1300 C, even at a very low temperature with short
sintering time. It is also seen that the dedensification occurred in
might affect its dislocation structure, thereby enhancing the
ꢀ
both cases at 1600 C.
3.3. The effect of sintering temperatures and CaO contents on the
crushing strength of UO eGd fuel pellets
2
2 3
O
According to Fig. 5, at the sintering temperatures of 1350, 1450,
ꢀ
1
550 and 1650 C, the variation trends of the crushing strength of
UO
the same. The strength of fuel pellets greatly increased when CaO
content in UO eGd fuel pellets increased from 0 to 0.3 wt%.
2 2 3
eGd O fuel pellets with different CaO contents were almost
2
2 3
O
When the CaO content further increased from 0.3 wt% to 0.6 wt%,
the strength of fuel pellets insignificantly changed with a slight
downward trend.
From the analysis of Figs. 1 and 2, when CaO content in
UO
sample significantly increased and the sintering pellet showed a
good microstructure with no free phase. So UO eGd fuel pellets
2 2 3
eGd O fuel pellets was 0.3 wt%, the sintered density of the
Fig. 2. Sintered density of UO
2
eGd
2
O
3
pellets with various CaO contents.
2
2 3
O

Y. Wang et al. / Journal of Alloys and Compounds 646 (2015) 626e631
629
ꢀ
Fig. 3. Microstructures of the undoped and doped UO
2
eGd
2
O
3
pellets sintered at 1650 C for 3.5 h; (a) undoped; (b) 0.3% CaO; (c) 0.5% CaO; (d) 0.6% CaO.
had higher crushing strength. 0.3 wt% CaO-doped UO
pellet sintered at 1650 C showed the highest crushing strength
2
eGd
2
O
3
fuel
ꢀ
(25.8 MPa). The further increasing content of CaO had little effect
on the strength of fuel pellets, which was consistent with the in-
fluence of CaO content on the sintered density.
3.4. The effect of CaO content on the sintering shrinkage rate of
UO eGd fuel pellets
2
2 3
O
2 2 3
Fig. 6 shows the sintering shrinkage curves for UO eGd O fuel
pellets with different CaO contents. Fig. 7 shows their shrinkage
rate curves. It can be seen from Figs. 6 and 7 that the curves of the
sintering shrinkage and the maximum sintering shrinkage rate for
UO
2
eGd
2
O
3
fuel pellets shifted to lower temperatures. The tem-
ꢀ
perature for the maximum shrinkage rate shifted from 1510 C for
ꢀ
Fig. 4. Sintered density of the UO
2
-1 wt% CaO and Gd
2
O
3
-1 wt% CaO pellets after
the undoped UO
doped UO eGd
the temperature for the maximum shrinkage rate increased with
2
eGd
2
O
3
pellet to 1240 C for the 0.6 wt% CaO-
sintering at different temperatures.
2 3
O pellet. The shifts in the shrinkage curves and
2
Fig. 5. The effect of sintering temperatures and CaO content on the crushing strength
of UO eGd fuel pellets.
2 2 3
Fig. 6. Sintering shrinkage curves for UO eGd O fuel pellets with different doped CaO
contents.
2
2
O
3

630
Y. Wang et al. / Journal of Alloys and Compounds 646 (2015) 626e631
ꢀ
parts of different slopes in the vicinity of 1365 C. The lower slope at
ꢀ
ꢀ
the temperatures between 1240 C and 1365 C was attributed to a
delay of the densification of the undoped UO eGd pellet. A
delay of the densification was associated with the formation of the
U,Gd)O solid solution [15]. CaO-doped UO eGd compacts
2
2 3
O
(
2
2
2 3
O
appeared to have one linear part for each content of CaO and they
had a different initial-stage sintering mechanism from the undoped
UO
compacts seemed to decrease with CaO contents.
Assuming that all the initial-stage sintering mechanisms of CaO-
doped UO eGd pellets were the same, the apparent activation
2 2 3 2 2 3
eGd O compact. The slopes of the CaO-doped UO eGd O
2
2 3
O
energy decreased with increasing CaO contents. For the case of
m ¼ 1 (volume diffusion), the activation energies were estimated to
ꢂ1
3
2,185, 28,640 and 27,475 Cal mol for CaO-doped UO
2
eGd
2
O
3
pellets with CaO contents from 0.3, 0.5 to 0.6 wt%, respectively.
Those values were lower than the activation energy of the undoped
ꢂ1
Fig. 7. Sintering shrinkage rate curves for UO
CaO contents.
2
eGd
2
O
3
fuel pellets with different doped
UO
2
eGd
2
O
3
pellet, 48,000 Cal mol . Further studies are needed to
establish the mechanisms of the enhanced densification in early
stage and the promoted grain growth in CaO-doped UO
pellets.
2 2 3
eGd O
the content of CaO. These results suggested that the addition of CaO
considerably affected the densification in the initial or the inter-
mediate stage.
4. Conclusions
Young [14] developed an equation for an initial-stage sintering
in the condition of a constant heating rate:
(
1) 0e0.3 wt% CaO-doped UO
2
eGd
2
O
3
fuel pellets, prepared by
ꢀ
co-precipitation reaction method at 1650 C in sintering at-
mosphere of hydrogen with the hold time of 3.5 h, had bigger
grain sizes with uniform distribution and good densification.
In addition, homogeneous solid solution formed without
ꢂ
ꢃ
ðDL=L Þ
Q
0
¼
A exp ꢂ
(2)
T
ðm þ 1ÞRT
2 3
precipitation of free phases of CaO and Gd O .
where,
0
DL/L was the relative length change. T was the tempera-
(2) 0.3 wt% CaO-doping significantly improved the sintered
ture. A was a constant depending on the materials parameters and
the sintering mechanisms. The exponent, m, had a value of 0 for
viscous flow, 1 for volume diffusion and 2 for grain boundary
diffusion mechanism.
Eq. (2) was treated with mathematical derivation with natural
logarithms on both sides. The results were as follows:
3
density (Increased from 10.42 to 10.52 g/cm ), grain size
(Increased from 4 to 10
mm) and crushing strength (Increased
from 17.9 to 25.8 MPa) of UO
2
eGd fuel pellets. However,
2 3
O
the further increase of doped CaO content had little effect on
the sintered density, the grain size growth and the
improvement of the crushing strength.
(
2 2 3
3) CaO doping in UO eGd O fuel pellets accelerated the ther-
ðDL=L Þ
Q
1
mally activated material transport, so the onset temperature
of densification as well as the temperature of the maximum
densification rate shifted to the lower temperature region.
The temperature of the maximum shrinkage rate shifts from
0
ln
¼ ln A ꢂ
$
(3)
T
ðm þ 1ÞR T
Based on Eq. (3), lnðDL=L Þ=T versus 1=T at the heating rate of
0
ꢂ1
4
K min was plotted as shown in Fig. 8. If the value of m was
ꢀ
ꢀ
1
510 C for the undoped UO eGd
2
2 3
O pellet to 1240 C for the
known, the apparent activation energy, Q, could be estimated from
the slope of the plots in Fig. 8.
0
.6% CaO-doped UO eGd O pellet.
2
2 3
2 2 3
In the case of the undoped UO eGd O , there were two linear
Acknowledgment
Great thanks for the financial support to our work from the
National Natural Science Foundation of China (Project No.:
21301164) and research fund project in advance from China Na-
tional Nuclear Corp. (Project No.: 416031102).
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