Development and performance evaluation of accident-tolerant coated fuel claddings for light water reactors

Doctoral thesis, 2023

The development of accident tolerant fuel has the aim of providing nuclear fuels able to endure severe accident conditions. Research in this field has also sparked a wave of material renewal in the nuclear industry that had been delayed for the last few decades. Climate change is an ever-growing public concern, and policies about greenhouse gas emissions are becoming more stringent both at the national and international level. Nuclear energy produces very low carbon emissions and the successful development of new accident tolerant materials might play a role in making this technology a viable solution to this global issue. Coated zirconium claddings are one of the most promising candidates as a near-term response to the need for accident tolerant materials. These coatings can be produced via a range of different technologies, but two main techniques are being currently employed for the development of such coatings: cold spray deposition and physical vapour deposition (PVD). In cold spray, high pressure gas is fed through a nozzle together with a powder. Powder particles are accelerated up to 1200 m/s and directed to the substrate, in this case cladding tubes made of zirconium alloy. In PVD, the coating material is evaporated from a target and sputtered onto the substrate as atoms or ions. The resulting coated zirconium claddings are the subject of this work.

As-fabricated samples, autoclave tested material, and specimens tested under simulated accident conditions are characterized with atom probe tomography and a range of electron microscopy techniques. The scope of the investigation is to evaluate the performance of the coated claddings under operating conditions and in simulated accident conditions. Metallic Cr has emerged as the best candidate for accident tolerant coatings for pressurised water reactors. In this work, the focus was on Cr-coatings deposited with cold spray, which performed well in both tested environments. A passivating layer of chromia formed during autoclave exposure, and a protective scale of chromia prevented any oxidation of the Zrsubstrate for up to 40 min in 1200  C steam. The nature of the adhesion in cold spray coatings and the effects of this deposition method on the substrate were studied. The formation of a Cr2Zr phase at the Cr/Zr interface was observed both under autoclave and simulated accident conditions. The search for an effective accident tolerant coating for boiling water reactors is more challenging and many coatings have been tested in autoclave, where PVD (Cr,Nb)N coatings showed the best performance under operating conditions. A 200 nm thick passivating oxide film composed of an outer Cr, Nb, Ni phase and an inner layer of oxidised coating was found after exposure.