Properties of the high burnup structure in nuclear light water reactor fuel

Abstract

The formation of the high burnup structure (HBS) is possibly the most significant example of the restructuring processes affecting commercial nuclear fuel in-pile. The HBS forms at the relatively cold outer rim of the fuel pellet, where the local burnup is 2–3 times higher than the average pellet burnup, under the combined effects of irradiation and thermo-mechanical conditions determined by the power regime and the fuel rod configuration. The main features of the transformation are the subdivision of the original fuel grains into new sub-micron grains, the relocation of the fission gas into newly formed intergranular pores, and the absence of large concentrations of extended defects in the fuel matrix inside the subdivided grains. The characterization of the newly formed structure and its impact on thermo-physical or mechanical properties is a key requirement to ensure that high burnup fuel operates within the safety margins. This paper presents a synthesis of the main findings from extensive studies performed at JRC-Karlsruhe during the last 25 years to determine properties and behaviour of the HBS. In particular, microstructural features, thermal transport, fission gas behaviour, and thermo-mechanical properties of the HBS will be discussed. The main conclusion of the experimental studies is that the HBS does not compromise the safety of nuclear fuel during normal operations.

Modelling of fine fragmentation and fission gas release of UO2 fuel in accident conditions

“…It has been established that almost all the gas that is depleted from the grains during fuel restructuring is captured in the newly formed pores, and that only a negligible fraction escapes to the rod free volume [86]. This conclusion is based on ceramographic examinations of the HBS, which generally show that the porosity is closed and not connected to the fuel free surface [96,97], and it is also corroborated by SIMS measurements on restructured fuel samples [95,106].…”

“…The restructuring is characterized by grain subdivision (polygonization) into sub-micron grains that are free of extended defects, increase in porosity and depletion of intragranular fission gas [84][85][86]. The high burnup microstructure is often referred to as the "rim zone structure", since it occurs first and foremost at the pellet rim: high local burnup and fission rate in combination with low temperature promotes accumulation of radiation damage in the material at the pellet rim.…”

Scalable Feature Tracking for Finite Element Meshes Demonstrated with a Novel Phase-Field Grain Subdivision Model

“…Under certain temperature and irradiation conditions, the as-fabricated fuel microstructure, which typically has grain sizes on the order of tens of microns, is transformed into a highly porous structure with sub-micron-sized grains and micron-sized pores. 30,31 HBS affects the thermal and mechanical performance of the fuel 31 and can cause a loss of fuel integrity under accident conditions. 32 It is hypothesized that HBS formation is energetically favorable because the reduction in defect energy within the bulk of the material more than compensates for the Fig.…”

“…14,15 As grains recrystallize, they may sweep out fission gases to the grain boundaries, causing the formation of large pores. 14,15,30,31 Here, we propose a novel model of the grain subdivision aspect of HBS formation. As in another phase-field model of HBS (Ref.…”

Improved Protocol for the Quantification of Actinides (Th, U, Np, Pu, Am, and Cm) Using Electron Probe Microanalysis

“…Perhaps the most important nondestructive micro-analytical method that is used to improve the safety and performance of nuclear fuels with the potential of reducing high-level waste is electron probe microanalysis (EPMA) (Walker, 1999; Brémier et al, 2003, 2016; Lamontagne et al, 2007; Walker et al, 2012; Bottomley et al, 2014; Capriotti et al, 2014; Moy et al, 2014; Wiss et al, 2017; Gerczak et al, 2018; Llovet et al, 2020). EPMA is a fast and reliable tool usually applied to earth and material sciences, yet encounters several technical obstacles in nuclear fuel analysis.…”