Densification of uranium dioxide fuel pellets prepared by spark plasma sintering (SPS)

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Abstract

An investigation into the influence of processing parameters on densification of UO2 powder during spark plasma sintering (SPS) is presented. A broad range of sintering temperatures, hold time and heating rates have been systematically varied to investigate their influence on the sintered pellet densification process, grain growth, hardness, and Young’s modulus. The results revealed that up to 96% theoretical density (TD) pellets can be obtained at a sintering temperature of 1050 °C for 30 s hold time and a total run time of only 10 min. The resulting UO2 pellets had an average Vickers hardness of 6.4 ± 0.4 GPa and Young’s modulus of 204 ± 18 GPa, which are in excellent agreement with values reported in literature for UO2 processed by other methods.

Introduction

Uranium dioxide (UO2) has been widely used as nuclear fuel in water cooled reactors since 1960s due to its excellent corrosion resistance in water–steam and satisfactory compatibility with the claddings [1]. However, UO2 is a refractory oxide with a melting point in excess of 2800 °C [2], [3] and therefore, requires high sintering temperature around 1700 °C in a hydrogen atmosphere for several hours using conventional sintering methods [4]. Numerous efforts have been made over decades to lower the sintering temperature of UO2. Williams et al. [5] first studied the effect of hyper-stoichiometry and atmosphere on sinterability of UO2 and claimed that high density pellets could be obtained at 1100–1400 °C in neutral atmosphere by sintering the oxide with an oxygen (O) to metal (M) ratio greater than 2.06 and then reducing in a hydrogen atmosphere. Carrea [4] achieved low temperature sintering by increasing the BET specific surface area of UO2 during sintering. A two-step heating cycle was set up to sinter UO2 at 1200 °C for 2 h to achieve 97.6% theoretical density (TD) and the total run time was 8 h. The significance of enhanced uranium diffusion coefficient which accounts for the improved sinterability in hyperstoichiometric UO2+x was discussed by Lay and Carter [6]. Stuart and Adams [7] sintered UO2.20–2.28 in H2–N2–H2O gas mixture at 1300 °C and achieved 96% TD in a single-stage process. However, the total run time was more than 9 h. Chevrel et al. [8] introduced hyperstoichiometric UO2 sintering by mixing U3O8 with UO2.08 to achieve O/U ratio of 2.25 and obtained 96.5% TD under a combined argon and oxygen atmosphere at 1100 °C for 3 h. Kutty et al. [9] studied shrinkage behavior of UO2 under six different atmospheres, from which they concluded that shrinkage may occur at a temperature 300–400 °C lower in an oxidizing atmosphere than in others, but the typical total run time was several hours. Although progress has been achieved in improving sinterability of UO2, sintering by these methods was still a diffusion controlled process, which means long processing time and high temperature are required.

Sintering by non-traditional methods has also been pursued by many researchers. Amato et al. [10] utilized inductive hot pressing while sintering reactor-grade UO2 to achieve more than 95% TD at 1200 °C and 8000 psi (55 MPa). However, the total processing cycle and quality of sintered pellet were not reported and hence, the validation for inductive hot pressing of UO2 remains questionable. In recent years, increasing interest has been shown in sintering UO2 by alternative sintering techniques. Microwave sintering of UO2 has been attempted by Yang et al. [11] who obtained 96.4% TD at 1600 °C under H2 atmosphere for more than 1 h. However rapid sintering has not been achieved due to formation of cracks at high heating rates (20–30 K/min). Later, Yang et al. [12] introduced pressureless induction heating for rapid UO2 sintering within 5 min to produce crack-free 96% TD pellets. However, many critical issues such as crack formation during higher heating rates (442 K/min) and density inhomogeneity continue to be major challenges in the field.

Since 1990s [13], spark plasma sintering (SPS), also known as plasma activated sintering (PAS) [14] or field assisted sintering technique (FAST) [15], has become a popular sintering method for consolidation of powders in various fields [16]. The merits of SPS include rapid processing, homogeneity of final product, low energy consumption, etc. [14]. In SPS, the green body is placed in a graphite die and uniaxial pressure is applied to the powder through graphite punches, as shown schematically in Fig. 1. Low voltage (3–5 V) and high amperage (600–800 A) pulse current are applied to the powder compact via the graphite punches. The current flows through the punches and the die, causing rapid heating. Numerous mechanisms for sintering have been proposed, including resistive heating by dies and punches, direct Joule heating if the powder is electrically conductive, enhanced diffusion and electron migration by the electronic field [17], [18], particle surface cleaning by the plasma [15], surface plasma formation and particle surface heating [19], spark plasma and spark impact pressure [13], [20], [21], [22], power-law creep mechanism by external pressure [18], temperature gradient-driven thermal diffusion [23], [24], etc. There has been an intense debate on whether or not a plasma is created in the SPS process [22]. Regardless, it is now well accepted that pulsed current causes joule heating at the inter-particle contact areas and upon application of pressure, these particles fuse to form the final compact. In the case of dielectric materials, the microscopic level discharge caused by the electrical field is expected to occur and favor densification [25]. Typical sintering times are on the order of 1–10 min at significantly lower processing temperatures than those in conventional sintering method. More details of SPS process are available in reference reports [16], [17], [18], [19], [20], [21], [25], [26].

Although a variety of refractory materials have been successfully processed in SPS [16], to the authors knowledge, literature on processing of nuclear fuels by SPS is limited. In this manuscript, a systematic study of densification during processing of UO2 in SPS is described. The main objectives of the investigation were (i) to successfully sinter UO2 powder into pellets with 96% theoretical density and (ii) to reduce the maximum sintering temperature and sintering duration. Pellets were produced under different heating rates, hold times, and maximum sintering temperatures. The microstructure evolution and densification process under each of these processing conditions are investigated. The density, hardness, and elastic properties of the pellets were determined and compared with those previously published in the literature.

Section snippets

Starting powder

The uranium dioxide powder was supplied by Areva Fuel System, Charlotte, NC. The powder was reported to have a bulk density of 2.3 g/cm3, tap density of 2.65 g/cm3, mean particle diameter of 2.4 μm, and a BET surface area of 3.11 m2/g. The grain size was determined using high resolution SEM to be between 100 and 400 nm, see Fig. 2. The O/U ratio for the starting powder was determined to be 2.11 by measuring the weight change before and after reducing the powder into stoichiometric UO2 using ASTM

Starting powder results

The XRD results in Fig. 5 show that due to the hyper-stoichiometry of the starting UO2.11 powder, small peaks in addition to UO2 peaks are revealed. The reduction process completely reduces the starting powder into the stoichiometric UO2 powder. Also, the spark plasma sintered, polished and then reduced UO2 compact reveals identical peaks to those of reduced UO2.00 powder. This result implied that no residual carbon or formation of carbides (or intermetallic) occurs after the whole process.

Densification

As

Conclusions

The investigation of the influence of processing parameters during spark plasma sintering of UO2 powder revealed that 96.3% theoretical density can be achieved in SPS at a maximum sintering temperature of 1050 °C for only 0.5 min hold time. As long as the sintering conditions are above these value, the variations in parameters such as the maximum sintering temperature (up to 1500 °C), heating rate (200 °C or 100 °C/min) and hold time (0.5–20 min) exhibit little influence on the densification behavior

Acknowledgement

The funding in this research is supported from the DOE Office of Nuclear Energy University Programs.

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