<p>Zirconium-based alloys are widely used as primary structural materials in water-cooled nuclear reactors due to their favorable mechanical properties, corrosion resistance, and low neutron absorption cross section. However, hydrogen embrittlement resulting from hydriding remains a critical issue that limits the service life of these components. Understanding the migration behavior of hydrogen (and its isotope deuterium) is therefore essential for predicting material performance and ensuring effective aging management. In this study, the hydrogen diffusion behavior in the pressure tube of a Pressurized Heavy Water Reactor (PHWR), made of Zircaloy-2, and in the clad tube made of Zircaloy-4, is investigated. Hydrogen (H)/ deuterium (D) concentration profiles were measured using the hot vacuum extraction quadrupole mass spectrometer (HVE-QMS) technique, and diffusion coefficients were evaluated over the temperature range of 523–673&#xa0;K. The corresponding activation energies were determined using Arrhenius analysis. The results show that Zircaloy-2 exhibits a higher activation energy and lower diffusion coefficient compared to Zircaloy-4. This behavior may be attributed to the significantly higher Ni content in Zircaloy-2, leading to the formation of Ni-containing second-phase particles that may act as effective hydrogen trapping sites and may retard hydrogen migration. In contrast, Zircaloy-4, with negligible Ni content, shows relatively higher diffusivity. Overall, this study highlights the dominant role of alloying elements, particularly Ni, in governing hydrogen transport behavior in Zr-based alloys and provides important insights for predicting hydrogen-induced degradation in nuclear reactor materials.</p>