A feasibility study of using CeO 2 as a surrogate material during the investigation of UO 2 thermal conductivity enhancement

Abstract

A possible substitution of UO₂ for research purposes is the cerium dioxide (CeO₂) owing to its chemical and physical properties. Neutronic properties are different and fission is absent in the case of CeO₂; however, similarities were studied recently to have a possibility to compare the neutronic influence of secondary additives into the matrix. This paper deals with increasing the thermal conductivity of UO₂ nuclear fuel on surrogate material (CeO₂); the main focus of the research is given on the sintering behaviour of CeO₂. The incorporation of highly thermally conductive material (SiC) is the investigated concept of thermal conductivity enhancement. Conventional sintering and spark plasma sintering (SPS) were applied to compare the behaviour of CeO₂ and UO₂ reported in the literature. High temperature thermal conductivity measurements did not confirm the positive influence of SiC additive inside the CeO₂ matrix mainly due to grain boundary disruptions. Similar behaviour was also previously reported for UO₂ pellets with SiC.

Keywords

SiC nuclear fuel surrogate material thermal conductivity enhancement spark plasma sintering

Introduction

Uranium dioxide (UO₂) is the most common fuel material used in commercial as well as research nuclear reactors. This type of fuel is used for several reasons, such as high melting point, oxidation resistance, reasonable high radiation resistance, limited influence on uranium fission cross-section, limited radiation and chemical creep as well as compression, chemical compatibility with cladding, reasonably low manufacture costs, and unprecedented operational experience. The behaviour of UO₂-based ceramic nuclear fuels during the nuclear fission has been studied most extensively [1 -3].

The main disadvantage of the uranium dioxide as a nuclear fuel is its very low thermal conductivity. Enhancing the thermal conductivity of the UO₂ fuel would allow increasing the power density generated in the fuel and it will improve the inherent safety features of nuclear fuel itself. High thermal conductivity also helps to effective heat removal from fuel pellet to coolant in standard or abnormal operating conditions. Smaller temperature gradient will lead to decrease in thermal tension and possible higher durability of fuel pellets in a reactor core. Such investigations are of extremely high interest after the Fukushima accident, where core melt-down occurred and a significant portion of nuclear fuel was damaged. Higher thermal conductivity of a nuclear fuel is also beneficial during residual heat cooling loss and during abnormal or accidental transients in reactor core. Investigation of so-called accident tolerant fuel is performed world-wide to mitigate fuel damage during Fukushima-like accidents [4 -6]. One of the very challenging types of such fuel is SiC [7] (called SiC fuel or SiC–SiC fuel if it contains SiC also in the cladding). Researchers’ effort is to design and test ceramic nuclear fuel with significantly enhanced thermal conductivity. Some studies propose to change grain type, structure, and scale [8]. It can be done either varying sintering conditions of adding some trace quantities [9] of oxides, such as Cr₂O₃, TiO₂, Nb₂O₅, Y₂O₃ and ZrO₂. The other concept of incorporating higher amount of high thermal conductivity phase into the UO₂ pellet has been investigated in several studies [1,10,11 14]. Some researchers [11,13,14] use beryllium oxide as a high conductivity phase, but the main disadvantage of the BeO is its high toxicity [15], and it is avoided in industrial utilisation and complicates research possibilities. Very challenging materials are carbon nanotubes, nanowires, or nanofibers [16], as well as other complex carbon molecules like graphene. However, chemical compatibility is still problematic owing to the reduction of uranium dioxide to uranium carbide at high temperature [14]. Even better results are still possible by adding microdiamond crystals or polycrystal diamonds [17]. They have a good chemical stability and great thermal conductivity, but intergrain connections and grain boundaries between diamonds and uranium dioxide should be further investigated to get applicable results. A very promising additive with high thermal conductivity is silicon carbide, either of simple powder or of whisker type. The influence of the SiC incorporation into the UO₂ on the thermal conductivity of the nuclear fuel was investigated in numerous studies [1,10,13,14]. The β-SiC has also been chosen by the authors as the high thermal conductivity additive due to its non-toxicity, low neutron absorption, chemical stability, and high melting temperature.

Experimental work with uranium dioxide is complicated in common laboratory environment owing to its radioactivity, radio-toxicity, and special nuclear material permissions. So, it is convenient to replace UO₂ by another material with similar properties for early stage research. Such an analogue should have similar crystalline structure, density, chemical properties, and low thermal conductivity. Owing to the principal fundamental laws, similar physical and chemical properties indicate tetravalent oxides of lanthanides (Ln), tungsten, and fewer molybdenum dioxides (Table 1) [18]. A tungsten and molybdenum dioxide shows slightly different thermal conductivity behaviour than UO₂, but LnO₂ shows similarities. The only three lanthanides’ tetravalent oxides exist in the nature – TbO₂, PrO₂, and CeO₂; however, TbO₂ cannot be derived in significant amount. Besides these three, NdO₂ is not naturally occurring but can be derived. Unfortunately, it is unstable and decomposes to Nd₂O₃; the same comes for PrO₂ at higher temperature [19]. A typical sintering temperature during UO₂-based ceramic nuclear fuel manufacturing process is around 1750°C, that is why only the cerium dioxide is feasible to be used for proposed investigation. The use of a cerium dioxide (CeO₂) as a UO₂ surrogate material was investigated in several studies, such as [20 -24]. In this study, CeO₂ was used as a surrogate material with a main focus on UO₂ thermal conductivity enhancement. The main reason is its low thermal conductivity, high density and also, with important limitations, similar response to neutron irradiation [25].

Table 1

Possible surrogates for uranium dioxide for thermal conductivity studies [18].

Sample	Density/g cm⁻³	Melting point/°C
UO₂	10.97	2827
CeO₂	7.65	2400
PrO₂	6.80	350*
WO₂	10.80	1500*
MoO₂	6.50	1100*

*Decomposition

The aim of this study is to verify a methodology for the preparation of CeO₂ ceramics analogue to UO₂ fuel suitable for validation of thermal conductivity enhancement by the addition of SiC. Furthermore, experimental results should provide characterisation of microstructures, phase composition, and thermal conductivity of prepared surrogate materials. Two sintering techniques used previously for the enhancement of UO₂ thermal conductivity – conventional sintering and SPS techniques were employed to fabricate CeO₂ and CeO₂ – SiC composite pellets. The goal of the investigation is to test different sintering options, conditions, and atmospheres for various mass concentration of the additive. Then thermal conductivity (diffusivity) and chemical stability is measured and compared to set proper sintering conditions and concentration of the additives.

Experimental materials and methods

Starting powders and milling

Starting powders used in this research were cerium dioxide (99.99% purity; Verochem, China) and silicon carbide (β-SiC GRADE 059; Superior graphite Co., Chicago, IL, U.S.A.). CeO₂ powder was used as pure (0 vol.-%) or doped with 6, 12, and 18 vol.-% of SiC, as an additive with high thermal conductivity.

CeO₂ was tested for sintering behaviour as delivered from producer, as well as after planetary ball milling for 2 h with the addition of isopropanol and ZrO₂ balls was performed to obtain smaller particles of CeO₂ powder and homogenous mixtures of SiC and CeO₂. One milling batch was composed of 60 g of powder (mixture); 200 g of 3 mol-% yttrium-stabilised tetragonal zirconia polycrystals (3Y-TZP) balls and 100 mL of isopropanol.

Green body preparation

For pressureless sintering, pellets were pressed in the form of cylinders, with a diameter of 12 mm and height of 10 mm in hydraulic uniaxial press under a pressure of 50 MPa, to obtain compact green body. The next step was cold isostatic pressing (Autoclave engineering, Inc., USA); applied isostatic pressure was 300 MPa for 1 min. Green body pellets directly used for thermal conductivity measurements were heat-treated at 500°C for 4 h to remove surface water.

Sintering

Both oxidative pressureless sintering and spark plasma sintering (SPS), also known as plasma-activated sintering or field-assisted sintering technique, processes were employed. For sintering experiments, always two pellets were used. The furnace used in pressureless sintering in air for heating and sintering of green pellets was HT08/17 furnace (Nabertherm, Germany). Pellets were placed on Al₂O₃ plate on the bottom of the furnace chamber and sintered at temperatures from 1300 to 1700°C with step of 100°C and dwell of 60 min. Heating and cooling rates were controlled automatically; heating rates were 300°C h⁻¹ and cooling rates were 600°C h⁻¹.

SPS apparatus (Dr Sinter SPS 2050 fabricated by SPS Syntex, Inc., Japan) was used for sintering of milled CeO₂ and CeO₂ – SiC composites under a mechanical pressure of 50 MPa. First, final sintering temperature was 1500°C, similarly with UO₂ sintering conditions in SPS, when this temperature should be optimal for reaching relative density over 95%. However, chemical reaction occurred at around 1450°C, when high amount of gas was produced. Consequently, final sintering temperature was decreased to 1300°C and heating rates were 6000°C h⁻¹; dwell time at 1300°C was 5 min; mechanical pressure applied on the pellet during sintering was 50 MPa; and vacuum (created by running vacuum pump) was surrounding graphite sintering dies.

Characterisation methods

Densities of samples were measured after sintering by the Archimedes method (EN 623 – 2). Secondary electron images were taken using the FIB/SEM Tescan Lyra3 XMH with digital image capturing system. Images were taken from the surface of fractured samples at low voltage to reveal microstructural characteristics and grain microstructures.

Dilatometric measurements were performed to describe the sintering behaviour of milled and compacted milled CeO₂ powder. The sample for the dilatometric analysis was pressed into the pellet with a diameter of 6 mm and a height of 10 mm. Dilatometric analysis was performed in a L70/1700 dilatometer (Linseis, Germany). The sample was preheated before analysis at 200°C, and exposed to air atmosphere during the thermal dilatometric analysis.

Thermogravimetric analysis (TGA) was performed on CeO₂ containing 6 vol.-% SiC to describe the thermal stability of the silicon carbide in the cerium dioxide matrix. Powder was prepared by milling under previously described conditions. TGA was performed on the 6300 TG-DTA device (Seiko Instruments, Japan) and pure Al₂O₃ crucible was used in air atmosphere. The heating rate was preset on the value of 20°C min⁻¹ and maximum temperature of the analysis was 1300°C.

Phase compositions were investigated using the Rigaku (Cu Kα radiation) X-ray diffractometer (XRD). The XRD was operated at a voltage of 40 kV and a current of 30 mA, and source of X-rays was Cu anode. Results were compared with known data for information about constituents and present phases. All XRD spectra were taken of both the surface of the sintered pellets and in some cases of the crushed powder from sintered pellets.

Thermal diffusivity (D_th) of pellets was measured to determine the thermal conductivity of prepared pellets. The thermal diffusivity was measured using the laser flash method (LFA 1000; Linseis, Germany) at a temperature ranging from 25 to 900°C in vacuum (running pump). Both the front and the back sides of the pellets were coated with the thin layer of carbon before measurements for the better absorption of laser.

The specific heat capacities (c_p), as a function of temperature, were determined from the chemical composition of the pellets using the Kopp–Neumann's law. The heat capacity data of the CeO₂ and SiC were obtained from the HSC chemistry 7 program (Outotec, Finland).

The thermal conductivities (k_th´) of each material (pure CeO₂ and CeO₂ with different content of SiC) were calculated using the equation k_th´ = ρ_·c_p·D_th [26], where ρ is the density of measured samples.

Results

Conventionally sintered CeO₂ + SiC pellets

Final relative densities of selected sintered pellets are shown in Table 2. Milling significantly promoted CeO₂ densification during pressureless conventional sintering. If it is not stated otherwise, milled powder was used for the preparation of CeO₂ pellets. The microstructure of pure CeO₂ conventionally sintered pellets is depending on powder processing and the pore size of the sintered body (1500°C for 60 min) is significantly different, see Figure 1(a,b).

Figure 1

Microstructure of not milled pure CeO₂ (a) and milled pure CeO₂ (b) sintered at 1500°C/60 min; milled pure CeO₂ (c) and milled CeO₂ + 6 vol.-% SiC (d) pellets sintered in SPS (1300°C/50 MPa/5 min.). Note that white arrow in Fig. 1d indicates SiC.

Table 2

Relative densities of selected sintered samples.

Relative density	Sintering temperature	1300°C	1400°C	1500°C	1600°C
CeO₂ – not milled		72.2%	79.0%	86.1%	91.3%
CeO₂ – milled		81.2%	91.2%	95.3%	95.4%
CeO₂ + 6 vol.-% SiC (air)		75.5%	76.5%	77.4%	78.4%
CeO₂ + 6 vol.-% SiC (SPS – 50 MPa/5 min)		85.4%	…	75.9%	…
CeO₂ – milled (SPS – 50 MPa/5 min)		92.7%	…	…	…

XRD analysis was performed on the CeO₂ + 6 vol.-% SiC pellets to analyse if the SiC remains in the CeO₂ matrix after sintering in oxidative atmosphere at 1600°C for 60 min. The result of XRD analysis of the CeO₂ + 6 vol.-% SiC pellet is shown in Figure 2. The analysis revealed that SiC was oxidised during sintering at high temperatures and the newly formed phase is mainly SiO₂. Thermogravimetric analysis was also performed on the milled CeO₂ + 6 vol.-% SiC powder for better understanding of the silicon carbide thermal stability in air atmosphere inside the CeO₂ matrix. The result of TGA analysis is also shown in Figure 2 and demonstrates thermal stability of SiC inside the CeO₂ matrix up to approximately 1050°C.

Figure 2

XRD analysis of the CeO₂ + 6 vol.-% SiC pellet sintered at 1600°C/1 h; TGA of milled CeO₂ + 6 vol.-% SiC powder.

CeO₂ and CeO₂ + SiC pellets sintered in SPS

Selected samples of CeO₂ and CeO₂ + SiC pellets sintered in SPS were taken and studied in the scanning electron microscope to analyse the influence of SPS on their microstructure.

The comparison of microstructures of the conventionally sintered CeO₂ pellet and the CeO₂ pellet sintered by SPS is shown in Figure 1(b,c). Pellet that was conventionally sintered at 1500°C for 60 min was chosen for the comparison with the pellet sintered in SPS, because their theoretical density are close to require for nuclear fuel pellets (95%). Fine morphology of sintered CeO₂ was caused by lower sintering temperature during SPS.

The microstructure of the CeO₂ + 6 vol.-% SiC pellet sintered in SPS is shown in Figure 1d. Observed SiC particles were evenly distributed inside the CeO₂ matrix; however, the boundary between SiC particles and CeO₂ matrix was not continual and SiC particles were like loosely inserted into the CeO₂ matrix.

The XRD analysis was performed on the CeO₂ + 6 vol.-% SiC pellets sintered in SPS to analyse the presence of SiC after the sintering process to compare it with conventionally sintered pellets. Result of the XRD analysis of the CeO₂ + 6 vol.-% SiC pellet is shown in Figure 3. It was observed that SiC remained in the structure in unchanged state, but CeO₂ was partially reduced to Ce₂O₃.

Figure 3

XRD analysis of the CeO₂ + 6 vol.-% SiC pellet sintered in SPS.

Thermal conductivity

Green body

Thermal conductivity and thermal diffusivity measurements were implemented on the green body CeO₂ and CeO₂ + SiC pellets prepared by uniaxial and cold isostatic pressing, respectively. No chemical reaction between CeO₂ and SiC is going to happen at room temperature under such conditions. The relative densities of measured materials are shown in Table 2. The thermal diffusivity of green body pellets was measured from laboratory temperature up to 500°C with step of 100°C and three measurements were taken at each temperature. Thermal conductivities of CeO₂ and CeO₂ + SiC pellets were then calculated and their dependence on the temperature is shown in Figure 4. It was observed that the thermal conductivity of green pellets gradually decreases with increasing volume of silicon carbide.

Figure 4

Thermal conductivities of green body CeO₂ and CeO₂ + SiC pellets.

Sintered pellets

Thermal diffusivities of sintered CeO₂ and CeO₂ + SiC pellets were measured from room temperature up to 900°C with step of 100°C, and consequently thermal conductivities were calculated. Changes in thermal conductivities with temperature for conventionally sintered CeO₂ pellets and SPS-sintered CeO₂ and CeO₂ + 6 vol.-% SiC pellets are shown in Figure 5. The thermal conductivity of conventionally sintered CeO₂ pellets was higher than that of CeO₂ sintered in SPS and the lowest thermal conductivity was measured on CeO₂ + 6 vol.-% SiC pellets sintered in SPS. Experimental data demonstrated that the thermal conductivity of CeO₂ is strongly decreasing with a increasing in temperature.

Figure 5

Comparison of thermal conductivities of conventionally sintered CeO₂ pellets and SPS-sintered CeO₂ and CeO₂ + 6 vol. SiC pellets.

Discussion

Green body

Relative densities of green body pellets prepared by uniaxial and cold isostatic pressing are between 61 and 63%. Such high values were achieved thanks to very good compressibility of CeO₂ powder and soft aggregates inside the starting powder, which can be easily milled down. Milling allows changing density and microstructure of the sintered powder as it should be required for surrogate material to copy as many physical parameters as possible. Relatively short milling time (2 h) allows significant change in the morphology of powder and the sintered body. Further prolongation of milling time or its shortening can be used as efficient tool for microstructure tailoring. Therefore, mimicking of UO₂ microstructure and density is possible, and can be used in studies for enhancing the thermal conductivity [27].

Sintering

Pure CeO ₂

An influence of powder milling and sintering temperature on the relative density of sintered CeO₂ pellets was investigated. The relative density measurement results, shown in Table 2, reveal that difference in relative density between pellets prepared from unmilled and milled powder is about 10% at all sintering temperatures, except for 1600°C where the difference is only about 5%. Positive impact of powder milling on relative densities of sintered CeO₂ pellets can be explained by a change in the morphology of powder. However, if temperature reaches 1600°C, the green body morphology does not play so significant role and grain growth is main force of densification.

Target of relative density was 95%, because the density of UO₂ pellets was used as a nuclear fuel. Table 2 shows that this target density was already at a sintering temperature of 1500°C in case that milled powder was used.

Ceo₂ mixture with SiC

Mixture of CeO₂ with SiC was sintered in air conditions to replicate experiments with UO₂ [14]. Relative densities after sintering remain in the range from 75.0 to 78.4% and even a difference of 300°C temperature does not make significant difference. Such densities are low in comparison with relative densities of pure CeO₂ pellets. XRD analysis (Figure 2) disclosed that SiC during conventional sintering in air atmosphere was oxidised into SiO₂, carbon from SiC escaped from the sample in the form of CO₂. This result was confirmed by thermogravimetric analysis, see Figure 2. All the experiments and analysis mentioned above showed that conventional sintering in air atmosphere is not the right procedure to prepare CeO₂ + SiC pellets in which the silicon carbide remains in unchanged state to be able to improve thermal conductivity of pellets. The formation of SiO₂ in CeO₂ + SiC sintered in air atmosphere is similar to the behaviour of UO₂ + SiC reported in the literature, [14] and similar trends for oxidation of SiC in UO₂ and CeO₂ can be expected.

Sintering in SPS conditions is the environment without SiC degradation; moreover, successful densification of UO₂ with SiC additives by SPS was reported in the literature, when both high density and enhanced thermal conductivity were achieved [28]. Therefore, surrogate material was also sintered in SPS conditions. The relative densities of the CeO₂ and CeO₂ + SiC pellets sintered in SPS were between 85.4 and 92.7% (calculated for CeO₂ and SiC mixture, see Table 2). Such densities look as if they are below the target density; however, due to the simultaneous change of phase composition, its real theoretical value can be different. The main reason for relatively low density was low sintering temperature (1300°C), caused by decomposition of CeO₂. The XRD analysis and microstructure analysis of the CeO₂ + 6 vol.-% SiC pellet (Figure 3) reveal that silicon carbide is generally stable during sintering in SPS, but small part of SiC reacted with CeO₂ and formed silicate (Ce₂Si₂O₇). The XRD analysis also revealed that CeO₂ was reduced to Ce₂O₃ during the sintering in SPS. CeO₂ is reduced in SPS at a relatively low temperature. The UO₂ does not show the tendency to be reduced in SPS conditions [10]. CeO₂ has stronger tendency to be reduced than UO₂; moreover, decomposition of CeO₂ starts at lower temperature. The formation of silicate is also one of the differences in the behaviour of CeO₂ and UO₂ during SPS.

Thermal conductivity

Thermal conductivity was focused in the study of using CeO₂ such as surrogate material of UO₂. First, thermal conductivities of CeO₂ and CeO₂ + SiC green body pellets were measured (Figure 4). The motivation for this measurement was to see the thermal conductivity of starting mixture before any chemical reaction occurred. These measurements should demonstrate a clear trend of SiC addition without the influence of sintering and phase changes during the heat treatment. Thermal conductivities of green body pellets were measured up to 500°C because at temperatures above 500°C sintering of green pellets starts playing an important role, and it would affect measured thermal conductivities. Thermal diffusivity measurements revealed decrease in thermal conductivity with increasing volume of SiC in the pellet. This is exactly an opposite result than expected and changes in the green density cannot explain the trend, see Table 3. This discrepancy between the expectations and measured results can be explained by the fact that with increasing volume of SiC, number of interphase boundaries between CeO₂ and SiC is also increasing and these interphase boundaries act as phonon scattering centres. This enhanced number of phonon scattering centres results in a decrease in thermal conductivity [26,29,30].

Table 3

Calculated and relative densities of selected green body samples.

Sample	Density/g cm⁻³	Relative density/%
CeO₂ + 0 vol.-% SiC	4.55	63.0
CeO₂ + 6 vol.-% SiC	4.28	61.4
CeO₂ + 12 vol.-% SiC	4.14	61.5
CeO₂ + 18 vol.-% SiC	4.02	61.9

Thermal conductivities of sintered CeO₂ and CeO₂ + SiC pellets were measured up to 900°C. Thermal conductivities of pure CeO₂ pellet sintered in air atmosphere, CeO₂ pellet sintered in SPS and CeO₂ + 6 vol.-% SiC pellet sintered in SPS are shown in Figure 5. The difference between thermal conductivity of the CeO₂ pellet sintered in air atmosphere and the CeO₂ pellet sintered in SPS could be found in three differences. First, some microcracks are in the microstructure of CeO₂ pellet sintered in SPS, see Figure 6. Second, the microstructures of pellets are different, and grain size of the CeO₂ pellet sintered in SPS is smaller than that of CeO₂ pellet sintered in air atmosphere. Third, relative densities of pellets are different also due to the presence of Ce₂O₃. The relative density of CeO₂ pellet sintered in air is 95.4% and the relative density of CeO₂ pellet sintered in SPS is 85.4%. Outcome of all these differences is an increase of boundaries in the CeO₂ pellet sintered in SPS, whether it is boundary between grains or boundary between solid material and pores, see the white arrow in Figure 1d. These boundaries act as a phonon scattering centres and therefore thermal conductivity of the CeO₂ pellet sintered in SPS is lower than that of the CeO₂ pellet sintered in air.

Figure 6

Microstructure of pure CeO₂ pellet sintered in SPS.

Thermal conductivity of the CeO₂ pellet sintered in air reached the value of 14.1 W m⁻¹ K⁻¹ at 25°C. Such value is slightly higher than that reported in the literature. For example, Mogensen [31] stated thermal conductivity of pure CeO₂ reached the value of 12 W m⁻¹ K⁻¹ at room temperature, and Shackelford [32] stated that thermal conductivity of pure CeO₂ is 9.58 W m⁻¹ K⁻¹ at 400 K (126.85°C). At the temperature of 100°C, the thermal conductivity has a value of 10.95 W m⁻¹ K⁻¹ and it is very close to the values reported in the literature. Some differences are expected because of various microstructures of the reported materials.

When comparing the thermal conductivity of CeO₂ pellet sintered in SPS and CeO₂ + 6 vol.-% SiC pellet sintered in SPS, higher thermal conductivity was measured for the CeO₂ pellet. This result was contrary to expectations, and similar trend was observed such as for green body pellets. The reason for such low thermal conductivity of pellet with the addition of high conductive SiC can be found in the microstructure, especially in the grain boundaries. This microstructure is shown in Figure 1d, and there can be seen that the boundary between SiC particles and CeO₂ matrix is not continuously connected. SiC particles are loosely inserted into the CeO₂ matrix, so there is not good contact between SiC and CeO₂. This bad contact between SiC particles and CeO₂ matrix creates once more high number of interphase boundaries and thus high number of phonon scattering centres. This phonon scattering centres greatly reduce the thermal conductivity of CeO₂ + 6 vol.-% SiC pellet. The problem of grain boundary is very similar also for UO₂ + SiC system as reported in the literature [14,33] and similar solutions for both systems can be expected.

Conclusions

Examined sintering behaviour of SiC in CeO₂ matrix has some similarities to the behaviour of SiC in UO₂ matrix observed in the literature. Both mixtures have disruptions on SiC-matrix grain boundary. It was also verified that composite pellets with the addition of SiC cannot be conventionally sintered in air atmosphere without oxidation of SiC to SiO₂. These composites of pellets can be prepared in SPS reductive conditions. The main limitation of CeO₂ is its reduction to Ce₂O₃ during sintering in SPS at relatively low temperatures, which limits the use of CeO₂ as a surrogate material for UO₂.

SiC additive inside the CeO₂ matrix has not positive influence on thermal conductivity at high temperatures. All composite pellets had lower thermal conductivities than pure CeO₂ pellets, which was caused by phonon scattering on imperfect boundaries between CeO₂ matrix and SiC particles. Application of CeO₂ during the investigation of UO₂ thermal conductivity enhancement is feasible, however, within many restrictions given by lower stability of oxygen in CeO₂ than in UO₂.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the authors.

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A feasibility study of using CeO 2 as a surrogate material during the investigation of UO 2 thermal conductivity enhancement

Abstract

Keywords

Introduction

Experimental materials and methods

Starting powders and milling

Green body preparation

Sintering

Characterisation methods

Results

Conventionally sintered CeO2 + SiC pellets

CeO2 and CeO2 + SiC pellets sintered in SPS

Thermal conductivity

Green body

Sintered pellets

Discussion

Green body

Sintering

Pure CeO 2

Ceo2 mixture with SiC

Thermal conductivity

Conclusions

Footnotes

References

Conventionally sintered CeO₂ + SiC pellets

CeO₂ and CeO₂ + SiC pellets sintered in SPS

Pure CeO ₂

Ceo₂ mixture with SiC