Preparation of bulk‐nanostructured UO₂ pellets using high‐pressure spark plasma sintering for LWR fuel safety assessment

Abstract

Nuclear fuel undergoes a significant restructuration during its lifetime in the nuclear reactor. Especially at the rim of the pellet, large UO 2 grains disintegrate into a nanosized material. In this paper, we focus on the preparation of bulk UO 2 with grain sizes below 100 nm to investigate the physico-chemical properties of this so-called "high burn up structure" (HBS). Preparation of bulk nanocrystalline materials is a challenge that can be overcome using the high-pressure spark plasma sintering (HP SPS) technique.In-house developed HP SPS with 500 MPa applied pressure was used for compaction of 11 nm UO 2 powder obtained by oxalate conversion. The procedure yielded dense (>90%) compacts with grain size as low as 34 nm for samples sintered at 800°C. At high burn up, nuclear fuel undergoes a significant restructuration. Especially at the rim of a pellet, large UO 2 grains disintegrate into a cauliflower-like material with a grain size of 100-300 nm and porosity up to 20%.1,2 In the past, properties like morphology, grain size, crystal structure, thermal conductivity, hardness of this so-called "high burn up structure" (HBS) has been studied on real spent fuel 1-6 or yttria-stabilized zirconia surrogates. 7,8 Handling of spent fuel is difficult due to its high radioactivity. Therefore, a surrogate material mimicking the HBS morphology and the same composition would be beneficial for its property studies. Moreover, preparation of a dense bulk nanocrystalline UO 2 with different grain size enables distinguishing effects of grain size vs effects of porosity on the physical behavior of the HBS.The recent development of the spark plasma sintering technique (SPS) allows densification of materials in very short times, at low temperatures, and with limited grain growth.9 Typically, powders are compacted in SPS in graphite dies. Current and pressure are applied through graphite punches. Dense UO 2 pellets can be prepared using classical SPS, 10,11 e.g, under the following conditions:~70 MPa, 1000°C within a 5-min dwell time. 12 The maximum pressure in conventional SPS is limited to~100 MPa by the strength of the graphite dies. Using special SiC or WC inserts, pressures up to 1 GPa are possible. 13,14 This high-pressure variant of SPS (HP SPS) enables production of bulk nanocrystalline materials. CeO 2 compacts with density <99% of theoretical density (TD) and 16 nm grain size were prepared by HP SPS at a pressure of 530 MPa and sintering temperature of 820°C. 13,14 Similarly, dense yttria-stabilized zirconia (~98% of TD) was prepared under a pressure of 800 MPa at 850-900°C with a hold time of 5 minuteThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

A low-temperature synthesis method for AnO2 nanocrystals (An = Th, U, Np, and Pu) and associate solid solutions

“…6 Since the decomposition temperature is usually mild (500-800°C), the final oxide is normally nanosized. 7,8 However, the final powder consists of plate-like agglomerates, as a consequence of the pseudomorphic oxalate-to-oxide conversion. This morphology of the actinide oxalates may be altered by changing the basic precipitation conditions, as recently demonstrated in the thorium case.…”

Preparation of ultrafine fibrous uranium dioxide by electrospinning

“…Uranium oxides attract attention since the dawn of nuclear technology. The current effort is focused on the production of nanosized uranium oxides due to their good properties in lowtemperature sintering [1e4] and spark plasma sintering [5]. Nanocrystalline UO 2 is also studied in connection with the high burnup structures (HBS) in nuclear fuel pellets.…”

Synthesis of nanostructured ThO2 pellets

“…Nanocrystalline ceramic oxides (well below 100 nm grain size) can be obtained with the high‐pressure (HP)‐SPS method, by using special WC or SiC inserts instead of lower strength graphite . In this way, nanocrystalline UO 2 disks (average grain size in the order of 50 nm) were recently achieved …”