Accommodation of Tin in Tetragonal ZrO2

TL;DR

This study uses density functional theory to explore how tin atoms behave in the tetragonal ZrO2 layer on zirconium alloys, revealing defect stability and effects of charge and alloying elements on corrosion resistance.

Contribution

It provides new insights into tin defect behavior and charge interactions in ZrO2, informing the development of corrosion-resistant nuclear fuel cladding alloys.

Findings

$Sn_{Zr}^{\times}$ is the dominant defect across most oxygen partial pressures.

Charge compensation by oxygen vacancies occurs at very low partial pressures.

Adding positive charge increases the stability range of tin defects.

Abstract

Atomic scale computer simulations using density functional theory were used to investigate the behaviour of tin in the tetragonal phase oxide layer on Zr-based alloys. The $Sn_{Zr}^{\times}$ site defect was shown to be dominant across most oxygen partial pressures, with $Sn_{Zr}^{"}$ charge compensated by fully charged oxygen vacancies occurring at partial pressures below $10^{-31}$ atm. Insertion of additional positive charge into the system was shown to significantly increase the critical partial pressure at which $Sn_{Zr}^{"}$ is stable. Recently developed low-Sn nuclear fuel cladding alloys have demonstrated an improved corrosion resistance and a delayed transition compared to Sn-containing alloys, such as Zircaloy-4. The interaction between the positive charge and the tin defect is discussed in the context of alloying additions, such as niobium and their influence on corrosion of cladding alloys.