Emanuele De Bona^1,2

²Olaf Walter, Andreas Scheinost¹, Damien Prieur¹

¹Helmholtz-Zentrum Dresden-Rossendorf; ²European Commission Joint Research Centre Karlsruhe.

Abstract

Spent nuclear fuel (SNF) of light water reactors (LWR) is constituted mainly of unfissioned UO₂. However, the presence of small concentrations of fission products (FP) and minor actinides (MA) strongly affects its chemical and physical properties. Moreover, after reaching a certain burnup level in the reactor, the rim of the fuel pellets undergo a microstructural reorganization. The initial structure formed of grains of 10-15 µm restructures into a pattern of sub-micrometric grains (100-300 nm) surrounding micrometric pores. This region is called high burnup structure (HBS), and due to its position, it would be the first interface of SNF with the external environment in case of failure of the confinement.
When in contact with an oxidizing environment, UO₂ transforms into U₃O₈, resulting in a 36% volume expansion. This could worsen the failure of the confinement due to the stresses induced by the expanding SNF, as well as be detrimental for SNF integrity that would then disperse in the environment more easily. For this reason, in order to establish safe SNF management strategies, deep knowledge of all the aspects of UO₂ oxidation must be acquired, including the role played by grain boundaries and FP on the matrix oxidation.
The aim of the present work is to use Spark Plasma Sintering (SPS) to synthesize Simfuel (UO₂-based material simulating some features of SNF) and to study its oxidation behavior. Small concentrations of FP (Mo,Ru,Ba,Zr,Nd) were included to investigate their effect on the UO₂ matrix oxidation. The use of SPS allows minimizing the coarsening of the microstructure during densification, making it possible to obtain Simfuel of grain size comparable to that of HBS (but without the presence of micrometric pores). In this way, also the effect of the density of grain boundaries on the oxidation of UO₂ could be studied.
The as-produced Simfuel were characterized by XRD, SEM, and HR-XANES. Pure UO₂ samples with the same grain sizes were also produced as benchmark. Repeating the same characterizations after the oxidative treatment will shed some light on the role of FP and grain boundary density on the oxidation of the UO₂ matrix.

Event Timeslots (1)

Tuesday – 14th September 2021
2:20 pm - 2:40 pm
Emanuele De Bona