Nuclear fuel undergoes a significant structural restructuration during its lifetime in the nuclear reactor. Especially at the rim of the pellet large UO2 grains disintegrate into a nanosized material. In this paper we focus on the preparation of bulk UO2 with grain sizes below 100 nm to investigate physico-chemical properties of this so-called "High Burn up Structure". 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 UO2 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.
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