<p>Refractory high-entropy alloys demonstrate exceptional potential for primary loop pipe, which is mainly attributed to their excellent oxidation resistance and thermal stability under high-temperature conditions. However, there are limited studies on their corrosion behavior in reactor environments, and the underlying corrosion resistance mechanisms remain unclear. In this study, the corrosion behavior of equimolar ZrNbTiVHf refractory high-entropy alloy in a pressurized water reactor primary loop water was investigated. After 1500-h corrosion testing, it was found that three distinct oxide layers formed on the surface of the alloy: a corrosion particle layer, a porous deposit layer, and an internal oxidation layer. A growth model for the corrosion particles was developed. The interface effect between oxide layers was revealed by characterization and analysis. The competitive growth between the corrosion particle layer and the porous deposit layer inhibited the inward growth of the corrosion particles. The innermost amorphous oxide layer hinders the propagation of corrosion medium along grain boundaries and other defects by virtue of its disordered structure, which effectively prevents the penetration of corrosion elements. This work elucidates the corrosion resistance mechanism of refractory high-entropy alloys in reactor environments, enhances the understanding of their corrosion behavior, and contributes to the design of new primary loop pipe materials.</p> Graphical abstract <p></p>