New Models for UO2 Fuel Structure Evolution under Irradiation in Fast Reactors

TL;DR

This paper introduces new models for the evolution of UO2 fuel structure under fast reactor irradiation, explaining grain elongation, columnar grain formation, and central void growth based on experimental analysis.

Contribution

It proposes novel models for fuel restructuring and coring, improving understanding of microstructural changes during irradiation in fast reactors.

Findings

The restructuring model explains grain elongation and columnar grain formation.

The coring model describes central void growth due to mass transport.

Models predict rapid formation of extended columnar zones during initial irradiation.

Abstract

On the base of analysis of experimental observations and critical assessment of existing models for oxide fuel structure evolution under operation conditions of fast reactors, new models for fuel restructuring and coring are proposed. The restructuring model describes coherent motion in the temperature gradient of various voids (gas bubbles, sintering pores and large lenticular pores) and grain boundaries, to which the voids are attached. As a result, the model explains elongation of thermally growing equiaxed grains and formation of columnar grains, and predicts a rapid formation of extended columnar grain zone during a relatively short initial period of fast reactor irradiation. The coring model describes formation and growth of the central void in the fuel pellet, activated by mass transport from the inner to the outer zone of the pellet under stresses induced by inhomogeneous fuel densification in the initial period of irradiation.