Date of Award

Summer 2025

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Xinyu Huang

Abstract

Following the Fukushima nuclear accident in 2011, accident tolerant fuel (ATF) became a major research interest to enhance the safety of light water reactors. Because the current fleet of light water reactors and their associated fuel cycle infrastructure are well-established, developing ATF that does not significantly alter the current fuel form or change its supporting infrastructure is ideal. Cr-coated Zircaloy fuel cladding is therefore an excellent short-term solution to enhance accident tolerance. Thin layers of Cr-coating offer notable resistance to corrosion, high-temperature oxidation, physical wear, and hydrogen permeation. Cold spray (CS) and physical vapor deposition (PVD) are two well-established Cr-coating methods for nuclear fuel claddings. However, the quality and microstructure of the Cr coatings deposited with these methods can vary significantly, possibly impacting the in-reactor performance of these coatings.

It is also well known that excessive hydrogen uptake leads to the formation of Zirconium hydrides, which can cause cladding embrittlement. Hydride embrittlement increases risks of cladding fracture and ultimately sets the operational limits of the fuel. Therefore, fully understanding the hydrogen permeability and embrittlement phenomena associated with the new Cr-coated claddings is of great importance. This study specifically aimed to test the hydrogen permeability of Cr-coated fuel claddings produced through CS and PVD coating methods. To better represent in-reactor conditions, some claddings were also oxidized in a pure water autoclave environment before testing. Claddings were subjected to gaseous hydrogen charging and subsequent hydrogen content analyses to quantify the efficacy of Cr coating (and its oxidized form) as a hydrogen permeation barrier. Because fuel claddings experience the greatest stress in the circumferential direction, ring tension testing (RTT) was conducted on the hydrogen-charged claddings to evaluate the effects of coatings and hydrogen uptake on their mechanical strength and ductility. The Cr coatings and oxide layers proved to be highly effective in reducing hydrogen uptake. The Cr- and Zr-oxides had the lowest hydrogen permeability, followed by pulsed PVD and nitrogen-propelled CS Cr coatings, respectively. The studied hydrogen contents (10~190 wppm) had no significant effect on mechanical strength or overall ductility, with variations less than 2% and 10%, respectively. Thicker Cr coatings, specifically cold sprayed coatings, only slightly reduced overall ductility but changed necking behavior.

Rights

© 2025, Daniel Cole Viands

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