Fabrication of UO2-BeO composite pellets with superior thermal conductivity based on multi-parameter theoretical analyses

doi:10.1016/j.jnucmat.2020.152533

Journal of Nuclear Materials

Volume 542, 15 December 2020, 152533

https://doi.org/10.1016/j.jnucmat.2020.152533 Get rights and content

Highlights

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Combing theoretical analyses and experimental investigations, the fabrication process of UO₂-BeO was optimized.
•
Through this method, BeO density was effectively improved and UO₂/BeO interfacial thermal resistance was decreased.
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The thermal conductivity of UO₂-BeO fabricated by this optimized method was improved by 89%.

Abstract

In this paper, the fabrication process of UO₂-BeO composite pellets was optimized for improving the thermal conductivity based on multi-parameter theoretical analyses and experimental investigations. It was found that the density of BeO and UO₂/BeO interfacial thermal resistance (ITR) are crucial parameters that affect the thermal conductivity of UO₂-BeO. To effectively increase BeO density and decrease UO₂/BeO ITR, the fabrication method of pressureless sintering with a spheroidizing process was proposed. Through this method, UO₂-BeO composite with high thermal conductivity was obtained. 89.2% and 71.4% enhancements of the thermal conductivity over UO₂ were achieved at room temperature and 673 K, respectively. This enhancement is higher than all the reported results in the previous literatures that fabricated UO₂-BeO using normal sintering temperatures (<2023 K). The results of finite element modelling showed that the centerline temperatures of our fabricated pellets in the reactor decreased remarkably compared with UO₂ fuel, which would significantly improve the safety of the reactor.

Introduction

UO₂ has been commonly applied in current light water reactors (LWRs) since 1960s [1], [2] due to its high melting point, excellent irradiation stability and good compatibility with the coolant and cladding. Nevertheless, the rather low thermal conductivity of UO₂ enhances the risk of the melting of the reactor core under serious accident conditions [3]. A lot of work has been carried out to increase the thermal conductivity of UO₂ pellets for improving the safety of reactors [4], [5], [6], [7], especially after the Fukushima nuclear accident [8], [9], [10].

Introducing an additive with high thermal conductivity as second phase to UO₂ is a facile method which can be directly applied in current LWRs. Therefore, various materials with high thermal conductivity, such as diamond [11], [12], graphene [13], Mo [14], [15], [16], [17], SiC [18], [19], [20], [21], MAX phase [22] and BeO [23], [24] have been inserted in UO₂ to fabricate composite pellets with improved thermal conductivity. Among these additives, BeO was considered to be a promising candidate because of its high thermal conductivity, good neutron moderation, high resistance to water steam and excellent chemical compatibility with UO₂ [25], [26], [27], [28], [29].

The study on UO₂-BeO composite has been conducted as early as 1960s [30], [31]. In 1996, Ishimoto et al. reported a UO₂-BeO composite pellet with continuous BeO dispersed in UO₂ matrix [32]. An excellent enhancement of thermal conductivity has been achieved by UO₂-BeO pellets fabricated at 2473 K sintering temperature which is higher than the melting point of BeO. The thermal conductivity of UO₂-1.2 wt.%BeO (4.2 vol%) is 89% and 43% higher than that of UO₂ at room temperature (298 K) and 1073 K, respectively. However, such fabrication method is difficult to be applied in the pellets production for the nuclear plant because the sintering temperature is too high. After this work, many efforts have been made for investigating the optimized fabrication method of UO₂-BeO composite pellet with relatively lower sintering temperatures (≤2023 K) [25], [33], [34], [35], [36]. The results showed that a continuous dispersion of BeO in UO₂ matrix is the best structure for enhancing the thermal conductivity. However, based on such structure, different enhancements of UO₂ thermal conductivity were achieved [37], [38], [39]. The best reported performance was that the thermal conductivity of UO₂ was improved by 75% at room temperature after adding 10 vol% BeO [39]. However, this enhancement of thermal conductivity is still far lower than the results reported by Ishimoto et al. or theoretical predictions. This indicates that besides the distribution and volume fraction of BeO, there exist other important parameters that affect the thermal conductivity of UO₂-BeO composite pellets.

In this paper, combining a multi-parameter theoretical analyses and experimental investigations, two crucial factors that influence UO₂-BeO thermal conductivity were revealed and optimized, which are BeO density in the composite pellet and UO₂/BeO interfacial thermal resistance (ITR). Through the method of pressureless sintering with spheroidizing process, BeO density was improved and UO₂/BeO ITR was reduced. UO₂-BeO composite pellet with superior thermal conductivity has been fabricated. Thermal conductivity of such composite pellet was 89% and 54% higher than that of UO₂ pellet at room temperature and 1073 K, respectively.

Section snippets

Experimental methods

The fabrication method of UO₂-BeO composite pellet via a SPS sintering process has been reported in our previous work [22,37,40]. The fabrication method of UO₂-BeO composite pellet via a pressureless sintering process is as follows. Firstly, pre-sintering process was adopted for the granulation of UO₂. The starting UO₂ powders were 3–5 μm in particle size and more than 99.9% in purity. UO₂ powders were compacted by a pressure of 300 MPa at room temperature. Such samples were ground, sieved to

Theoretical model

A thermal conductivity model established based on equivalent thermal resistance and a 3D grid pattern was used for predicting the thermal conductivity of UO₂ doped with continuous BeO: $\begin{matrix} k_{e f f} = (\frac{1}{l_{m} / l_{p} + l_{p} / l_{m} + 2} + \frac{1}{l_{m} / l_{p} + 1}) \\ k_{p} + \frac{1}{k_{p} / k_{m} + l_{p} / l_{m} + 2 k_{p} R / l_{m}} \frac{1}{1 + l_{p} / l_{m}} k_{p} \\ \frac{l_{m}}{l_{p}} = \frac{{(1 - V_{p})}^{1 / 3}}{1 - {(1 - V_{p})}^{1 / 3}} \end{matrix},$ where k_m and k_p are the thermal conductivities of UO₂ and BeO, respectively, V_p is the volume fraction of BeO including pores, l_m is the granule size of UO₂ surrounded by continuous BeO, and R is the ITR of the interface

Thermal conductivity of UO₂-BeO fabricated by SPS sintering

In the first place, the thermal conductivities of UO₂ containing the same content of BeO with our fabricated samples were predicted using the above model. The measured BeO content was 2.81 wt.% in such composite pellets. Total porosity of UO₂-BeO was set to the value (5%) consistent with the porosity of the sample fabricated by SPS sintering for making comparison. And all the pores were assumed to be distributed in UO₂, the porosity of which was calculated to be 5.49% according to Eqs. (7) and

Discussion

The above results showed that the density of BeO and UO₂/BeO interfacial thermal resistance are both important factors that affect the thermal conductivity of UO₂-BeO composite pellets. To effectively increase BeO density and decrease UO₂/BeO ITR, an optimized fabrication method, pressureless sintering with a spheroidizing process, has been developed. The thermal conductivity of UO₂-BeO fabricated by this method was found to have an 89.2% enhancement over UO₂ at room temperature. There was a

Conclusions

Combining the theoretical predictions and experimental investigations, two key parameters that affect the thermal conductivity of UO₂-BeO composite pellets were revealed, which are BeO density and UO₂/BeO ITR. For improving UO₂-BeO thermal conductivity, the optimized fabrication process that can effectively enhance BeO density and decrease UO₂/BeO ITR was proposed.

First, UO₂-BeO pellet was fabricated by SPS sintering. It was found that the thermal conductivity of this pellet measured by the

Author statement

Rui Gao: Data curation, Writing - original draft. Zhenliang Yang: Investigation, Writing - review & editing. Bingqing Li: Investigation, Writing - review & Editing. Biaojie Yan: Simulation. Liang Cheng: Investigation. Yun Wang: Investigation. Yi Zhong: Investigation. Qiqi Huang: Investigation. Zhiyi Wang: Investigation. Mingfu Chu: Supervision. Bin Bai: Conceptualization, Methodology. Xueyan Zhu: Simulation, Writing - review & editing. Pengcheng Zhang: Conceptualization, Methodology. Rui Li:

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Key Research and Development Program of China [No. 2017YFB0702400], National Natural Science Foundation of China [Nos. 51804285, 21703214, 51604250, 21501156] and CAEP Foundation [No. CX20200018].

References (45)

B.G. Childs
The release of stored energy from neutron irradiated uranium oxides
J. Nucl. Mater.
(1962)
L.R. Blake
Irradiation of uranium-metal and uranium-oxide fuel pins to high burn-up at high temperature
J. Nucl. Energy. Parts A/B. React. Sci. Technol.
(1961)
J.K. Fink et al.
Thermophysical properties of uranium dioxide
J. Nucl. Mater.
(1981)
F. Cappia et al.
Postirradiation examinations of low burnup U3Si2 fuel for light water reactor applications
J. Nucl. Mater.
(2019)
F. Cappia et al.
Post-irradiation examinations of UO₂ composites as part of the accident tolerant fuels campaign
J. Nucl. Mater.
(2019)
A.R. Wagner et al.
Fabrication of stoichiometric U3Si2 fuel pellets
MethodsX
(2019)
J. Turner et al.
The use of gadolinium as a burnable poison within U3Si2 fuel pellets
J. Nucl. Mater.
(2018)
W. Li et al.
Innovative accident tolerant fuel concept enabled through direct manufacturing technology
Appl. Energy.
(2020)
I. Greenquist et al.
Grand potential sintering simulations of doped UO₂ accident-tolerant fuel concepts
J. Nucl. Mater.
(2020)
S. He et al.
Thermal hydraulic analysis of the PWR with high uranium density accident tolerant fuels under accident transients with and without reactivity
Nucl. Eng. Des.
(2019)

Z. Chen et al.

Raman spectroscopic investigation of graphitization of diamond during spark plasma sintering of UO₂-diamond composite nuclear fuel

J. Nucl. Mater.

(2016)

P. Medvedev

Effect of diamond additive on the fission gas release in UO₂ fuel irradiated to 7.2 GWd/tHM

Ann. Nucl. Energy

(2020)

S.W. Lee et al.

Performance evaluation of UO₂/graphene composite fuel and SiC cladding during LBLOCA using MARS-KS

Nucl. Eng. Des.

(2013)

H.S. Lee et al.

Numerical and experimental investigation on thermal expansion of UO₂-5 vol% Mo microcell pellet for qualitative comparison to UO₂ pellet

J. Nucl. Mater.

(2019)

H.S. Lee et al.

Numerical characterization of micro-cell UO₂Mo pellet for enhanced thermal performance

J. Nucl. Mater.

(2016)

L. Cheng et al.

Densification behaviour of UO₂/Mo core-shell composite pellets with a reduced coefficient of thermal expansion

Ceram. Int.

(2020)

L. Cheng et al.

Investigation of the residual stress in UO₂-Mo composites via a neutron diffraction method

Ceram. Int.

(2020)

S. Yeo et al.

Enhanced thermal conductivity of uranium dioxide-silicon carbide composite fuel pellets prepared by spark plasma sintering (SPS)

J. Nucl. Mater.

(2013)

A.K. Singh et al.

Processing of uranium oxide and silicon carbide based fuel using polymer infiltration and pyrolysis

J. Nucl. Mater.

(2008)

S. Yeo et al.

The influence of SiC particle size and volume fraction on the thermal conductivity of spark plasma sintered UO₂-SiC composites

J. Nucl. Mater.

(2013)

Z. Chen et al.

Master sintering curves for UO₂ and UO₂-SiC composite processed by spark plasma sintering

J. Nucl. Mater.

(2014)

B. Li et al.

Ti3SiC2/UO₂ composite pellets with superior high-temperature thermal conductivity

Ceram. Int.

(2018)

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^#: R. Gao, Z. Yang, and B. Li contributed equally to this work.

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Fabrication of UO2-BeO composite pellets with superior thermal conductivity based on multi-parameter theoretical analyses

Highlights

Abstract

Introduction

Section snippets

Experimental methods

Theoretical model

Thermal conductivity of UO2-BeO fabricated by SPS sintering

Discussion

Conclusions

Author statement

Declaration of Competing Interest

Acknowledgements

The release of stored energy from neutron irradiated uranium oxides

J. Nucl. Mater.

Irradiation of uranium-metal and uranium-oxide fuel pins to high burn-up at high temperature

J. Nucl. Energy. Parts A/B. React. Sci. Technol.

Thermophysical properties of uranium dioxide

J. Nucl. Mater.

Postirradiation examinations of low burnup U3Si2 fuel for light water reactor applications

J. Nucl. Mater.

Post-irradiation examinations of UO2 composites as part of the accident tolerant fuels campaign

J. Nucl. Mater.

Fabrication of stoichiometric U3Si2 fuel pellets

MethodsX

The use of gadolinium as a burnable poison within U3Si2 fuel pellets

J. Nucl. Mater.

Innovative accident tolerant fuel concept enabled through direct manufacturing technology

Appl. Energy.

Grand potential sintering simulations of doped UO2 accident-tolerant fuel concepts

J. Nucl. Mater.

Thermal hydraulic analysis of the PWR with high uranium density accident tolerant fuels under accident transients with and without reactivity

Nucl. Eng. Des.

Raman spectroscopic investigation of graphitization of diamond during spark plasma sintering of UO2-diamond composite nuclear fuel

J. Nucl. Mater.

Effect of diamond additive on the fission gas release in UO2 fuel irradiated to 7.2 GWd/tHM

Ann. Nucl. Energy

Performance evaluation of UO2/graphene composite fuel and SiC cladding during LBLOCA using MARS-KS

Nucl. Eng. Des.

Numerical and experimental investigation on thermal expansion of UO2-5 vol% Mo microcell pellet for qualitative comparison to UO2 pellet

J. Nucl. Mater.

Numerical characterization of micro-cell UO2Mo pellet for enhanced thermal performance

J. Nucl. Mater.

Densification behaviour of UO2/Mo core-shell composite pellets with a reduced coefficient of thermal expansion

Ceram. Int.

Investigation of the residual stress in UO2-Mo composites via a neutron diffraction method

Ceram. Int.

Enhanced thermal conductivity of uranium dioxide-silicon carbide composite fuel pellets prepared by spark plasma sintering (SPS)

J. Nucl. Mater.

Processing of uranium oxide and silicon carbide based fuel using polymer infiltration and pyrolysis

J. Nucl. Mater.

The influence of SiC particle size and volume fraction on the thermal conductivity of spark plasma sintered UO2-SiC composites

J. Nucl. Mater.

Master sintering curves for UO2 and UO2-SiC composite processed by spark plasma sintering

J. Nucl. Mater.

Ti3SiC2/UO2 composite pellets with superior high-temperature thermal conductivity

Ceram. Int.

Fabrication of UO₂-BeO composite pellets with superior thermal conductivity based on multi-parameter theoretical analyses

Thermal conductivity of UO₂-BeO fabricated by SPS sintering

Post-irradiation examinations of UO₂ composites as part of the accident tolerant fuels campaign

Grand potential sintering simulations of doped UO₂ accident-tolerant fuel concepts

Raman spectroscopic investigation of graphitization of diamond during spark plasma sintering of UO₂-diamond composite nuclear fuel

Effect of diamond additive on the fission gas release in UO₂ fuel irradiated to 7.2 GWd/tHM

Performance evaluation of UO₂/graphene composite fuel and SiC cladding during LBLOCA using MARS-KS

Numerical and experimental investigation on thermal expansion of UO₂-5 vol% Mo microcell pellet for qualitative comparison to UO₂ pellet

Numerical characterization of micro-cell UO₂Mo pellet for enhanced thermal performance

Densification behaviour of UO₂/Mo core-shell composite pellets with a reduced coefficient of thermal expansion

Investigation of the residual stress in UO₂-Mo composites via a neutron diffraction method

The influence of SiC particle size and volume fraction on the thermal conductivity of spark plasma sintered UO₂-SiC composites

Master sintering curves for UO₂ and UO₂-SiC composite processed by spark plasma sintering

Ti3SiC2/UO₂ composite pellets with superior high-temperature thermal conductivity