Abstract: Safe disposal of cesium nuclides in high-level radioactive liquid waste is a critical challenge in the back-end of the nuclear fuel cycle. Pollucite possesses a unique three-dimensional cage framework and excellent chemical durability, making it an ideal mineral matrix for immobilizing radioactive cesium. In this work, high-purity pollucite crystals were successfully synthesized via a low-temperature hydrothermal method. Combined with two characterization techniques, including full-profile Rietveld refinement of X-ray powder diffraction (XRD) and micro-Raman spectroscopy, the phase transformation kinetics and microstructural evolution mechanism of pollucite were systematically investigated. The experimental results reveal that the crystallization of pollucite follows a dissolution-recrystallization mechanism. As a vital structure-directing agent, cesium ion drives the continuous dissolution of sodium-rich metastable phases, which are completely converted into pure-phase pollucite after 60 hours of reaction. Meanwhile, the variation of intrinsic characteristic peaks in the low-frequency region of Raman spectra confirms that cesium ion is successfully immobilized within the cage pores of pollucite. Furthermore, a dual kinetic reservoir model coupled with Python and PHREEQC was established. This model effectively distinguishes the initial rapid leaching behavior of unstable phases on the material surface and accurately determines the long-term intrinsic steady-state leaching rate of the solidified form, which is approximately 3.16×10^-8g·m^-2·d^-1. This study not only verifies the technical feasibility of efficiently immobilizing free cesium by a mild hydrothermal method, but also provides comprehensive spectroscopic experimental evidence and a kinetic prediction model for the long-term geological safety risk assessment of radioactive cesium solidified forms.

Key words: pollucite, hydrothermal synthesis, radioactive cesium, immobilization mechanism