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

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Abstract

This study examines the influence of Silicon Carbide (SiC) particle addition on thermal conductivity of UO2–SiC composite pellets. UO2 powder and β-SiC particles of different sizes and of different volume fractions were mechanically mixed and sintered at 1350–1450 °C for 5 min by Spark Plasma Sintering (SPS). The particle size (0.6–55 μm diameter) and volume fraction (5–20%) of SiC were systematically varied to investigate their influence on the resulting UO2–SiC composite pellet microstructure and the thermal properties. It was found that SiC particle size less than 16.9 μm with larger volume fraction is more effective for improving the thermal conductivity of the fuel pellets. Scanning Electron Microscopy examination revealed micro-cracking and interfacial debonding in the composites containing larger size SiC particles (16.9 and 55 μm) which resulted in reduced thermal conductivity. For the UO2–SiC composite pellets containing 1 μm diameter SiC particles, the thermal conductivity increased almost linearly with volume fraction of particles. However, the addition of a larger volume fraction of SiC reduces the amount of heavy metal in the composite pellet and therefore a higher U-235 enrichment is necessary to compensate for the heavy metal loss. The experimental thermal conductivity values of the UO2–SiC composite pellets are in good agreement with the theoretical values based on the available model in the literature.

Introduction

The primary ceramic fuel used in nuclear reactors, uranium dioxide (UO2), has a low thermal conductivity (7.7 W/m K at 300 K [1]) which results in a decrease in the energy output due to low heat transfer efficiency from fuel to coolant. The introduction of high thermal conductivity fuel pellets enables a nuclear reactor to produce more thermal energy while maintaining plant safety due to lower pellet centerline temperature and thermal gradient, resulting in a lower level of fission gas release and thermal cracking.

In recent years, Spark Plasma Sintering (SPS) has evolved as a promising sintering technique for rapid fabrication of UO2 pellets of required shape and size [2]. In our previous research [3] it has been shown that enhanced thermal conductivity of UO2–10 vol%SiC composite fuel pellets can be fabricated by the Spark Plasma Sintering (SPS) technique. In that study, SPS provided higher density composites, better interfacial contact, and reduced chemical reaction between UO2 and SiC particles, compared to conventional sintering. SPS pellets also revealed a thermal conductivity increase of up to 62.1% at 900 °C compared to the literature value of UO2 [1]. Because of its unique and superior properties such as high thermal conductivity, low neutron cross section, high melting point, and great chemical stability, Silicon Carbide (SiC) was chosen as the secondary phase in the UO2 matrix to form heat conducting paths in the ceramic composites.

The influence of particle size and volume fraction of second phase particles on the effective thermal conductivity of a composite and its association with the interfacial thermal resistance have been well documented in literature. Hanada et al. [4] determined that diamond particle size and volume fraction had a significant influence on the effective thermal conductivity of copper–diamond composites. They found that the composites containing larger diamond particles up to 7.7 μm diameter and smaller volume fraction near 1% show higher thermal conductivity. Bai et al. [5] fabricated MoSi2–SiC composites containing 10, 20, and 30 vol% of 0.5 μm and 100 nm SiC particles and found that the composite showed lower thermal conductivity with decreasing SiC particle size and increasing volume fraction due to the interfacial thermal resistance. Chu et al. [6] sintered Cu–Carbon Nanotube (CNT) composites and found that these composites had much lower thermal conductivity than calculated thermal conductivity value based on the rule of mixture which ignored the interfacial resistance. In general, it is found that ceramic composites containing larger particles (low surface to volume ratio) reduce the interfacial thermal resistance, and hence, potentially increase the effective thermal conductivity of ceramic composites.

In the current study, a series of UO2–SiC composite fuel pellets with different sizes and volume fractions of SiC particles was fabricated using the SPS technique. The thermal conductivities of these composites were measured and compared to the values determined from theoretical formulations available in the literature. During the fabrication process, the sintering parameters such as hold time, up/down ramp rate, and pressure were kept constant so as to investigate only the effects of SiC particle size and volume fraction on the resulting thermal conductivity of the composite pellets.

Section snippets

Fabrication of UO2–SiC composite pellets

UO2–SiC pellet fabrication procedure using SPS was described in detail in our recent publication [3]. Therefore, only a brief description is provided here. The uranium dioxide (UO2.11) powder was obtained from AREVA NP, Richland, WA and the SiC powder was obtained from Superior Graphite, Inc., Chicago, IL. The SiC powder was reported to have a high purity (>98%), a surface area from gas absorption method [7] of 1.46 m2/g, a particle density range of 3.1–3.2 g/cm3, and particle mean diameters of

Size effect of SiC particles on UO2–5 vol% SiC composite properties

The micro-morphologies and thermal properties of UO2–5 vol%SiC composite fuel pellets containing SiC particles with five different sizes (see Table 2) were examined. Fig. 1 shows the microstructures of these composites where the SiC particles appear black and the brighter area indicates the UO2 matrix. The SiC particles appear to be homogeneously dispersed in the UO2 matrix in all the composites. However, as shown in Fig. 1(e), in the composite containing 55 μm SiC particles, distinct radial

Conclusion

The microstructure and thermal properties such as thermal diffusivity, specific heat capacity, and thermal conductivity of UO2–SiC composite fuel containing various SiC particle sizes and volume fractions were investigated. The SPS technique was utilized to fabricated high density composite pellets in a relatively short time of 5 min at 1350–1450 °C. While the composite pellets containing 0.6, 1.0, 9.0 μm diameter SiC particles showed higher thermal conductivity, those pellets containing 16.9 and

Acknowledgements

This research is being performed using funding received from the DOE Office of Nuclear Energy’s Nuclear Energy University Programs. Support from AREVA is gratefully acknowledged.

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