Investigating the Corrosion Behavior of Zircaloy-4 in LiOH under a Thermal Gradient and Two-Phase Flow Regime

Abstract

There are potentially significant cost benefits through plant simplification if a soluble boron-free lithiated primary water chemistry can be demonstrated to be a viable route for small modular reactor operation. However, the corrosion behavior of the clad material under lithiated conditions remains a concern. High levels of lithium (Li) have been demonstrated to be detrimental to the corrosion behavior of zirconium alloys. Under a thermal gradient, as experienced by the clad in pressurized water reactor operation, this becomes more complex; with increasing oxide thickness, the potential for operation under two-phase (nucleate boiling) conditions increases. The significance of this is twofold: first, the concentration of lithium local to the oxide has been shown to increase as boiling occurs within the thick porous oxide, potentially increasing the Li content beyond the threshold for accelerated corrosion; second, the nucleate boiling can result locally in more aggressive (high lithium) chemistry conditions. This paper presents the results from a test program that has investigated the effect of lithium, temperature, thermal gradient, two-phase flow. and stress on the corrosion behavior of Zircaloy-4 and discusses the results in the context of previous work carried out in this area. The data from this work indicate that in nominal lithium concentrations, under certain combinations of a thermal gradient, two-phase flow, and stress, a significant acceleration in corrosion can be observed. Characterization and modeling of the specimens exposed to a thermal gradient, in comparison to isothermally corroded Zircaloy-4 coupons in a range of Li concentrations, has developed the understanding of the processes occurring during this acceleration.

Lithium Stabilization of Amorphous Zro2

“…The build-up of boron within the CRUD (Chalk River Unidentified Deposits) materials on the Axial offset anomaly periphery of the rod that forms in high burn-up regions within the reactor have been found to have a detrimental impact on reactor efficiency (Frattini et al, 2001;Blok et al, 2002). New proposals of small modular reactor (SMR) operation are considering a boron-free coolant design to limit the boron axial offset anomaly (Hulme et al, 2021;Mart et al, 2014;Sanchez and dos Santos, 2021). In the absence of boron, however, the phenomenon known as lithium accelerated corrosion of the zirconium-based alloy cladding has been widely reported (Hulme et al, 2021;Pêcheur et al, 2000Pêcheur et al, , 2009Murgatroyd and Winton, 1967;Ramasubramanian, 1991;Bramwell et al, 1991;Jeong et al, 1999;Motta et al, 2005;Ramasubramanian et al, 2008;Cox et al, 1998;Billot et al, 1993).…”

“…New proposals of small modular reactor (SMR) operation are considering a boron-free coolant design to limit the boron axial offset anomaly (Hulme et al, 2021;Mart et al, 2014;Sanchez and dos Santos, 2021). In the absence of boron, however, the phenomenon known as lithium accelerated corrosion of the zirconium-based alloy cladding has been widely reported (Hulme et al, 2021;Pêcheur et al, 2000Pêcheur et al, , 2009Murgatroyd and Winton, 1967;Ramasubramanian, 1991;Bramwell et al, 1991;Jeong et al, 1999;Motta et al, 2005;Ramasubramanian et al, 2008;Cox et al, 1998;Billot et al, 1993). To understand the mechanism of this accelerated corrosion, all aspects of the oxide that forms on the zirconium alloy require investigation to understand how Li could increase the availability of oxygen to the metal oxide interface.…”