For the remediation of radionuclides (e.g., ¹³⁷Cs and ⁹⁰Sr) in nuclear wastewater, conventional materials struggle to simultaneously exhibit excellent adsorption performance, high mechanical strength, and good irradiation resistance. Herein, high-strength NaX zeolite microspheres from Guangxi university (GXU-NaXs, compressive strength: 19.21 MPa, Vickers hardness: 216.30) with 75–150 μm particle sizes were prepared for the first time via in situ conversion via geopolymer technology under normal pressure at 105 °C for 12 h. The results revealed that GXU-NaXs achieved removal efficiencies exceeding 90% for Cs⁺ and Sr²⁺ within 20 min at a dosage of 0.8 g·L⁻¹, following pseudo-second-order and Langmuir models with maximum saturated adsorption capacities of 138.30 and 153.60 mg·g⁻¹ at 45 and 30 min, respectively. Moreover, GXU-NaXs maintained > 98% structural stability and adsorption capacity after 500 kGy γ-irradiation, which has the potential for excellent stability in the nuclear environment. GXU-NaXs also exhibited good dynamic adsorption effects at a flow rate of 6 mL·min⁻¹, and the removal efficiency after three cycles remained 97.89% and 56.39% for Cs⁺ and Sr²⁺, respectively, demonstrating that GXU-NaXs have good feasibility for industrial applications. GXU-NaXs showed good removal capacity and selectivity in complex seawater matrices for Cs⁺ and Sr²⁺. Density functional theory (DFT) calculations revealed adsorption energies of −2.43 (Cs⁺) and −3.52 eV (Sr²⁺), whereas scanning electron microscopy (SEM)/energy dispersive spectrometry (EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) methods confirmed that the adsorption mechanism was ion exchange and chemisorption. This study pioneers advanced synthesis technology for nuclear adsorbents, offering a promising direction for radioactive wastewater remediation.