The radioactive Sr-90, which is formed from beta decay, is well known as one of the most commonly detected nuclides in radioactive waste. In 2015, it was reported that Sr-90 was observed in some soil and metal wastes among the 516 drums of radioactive waste transferred from the decommissioning site of the Korea Research Reactor (in Seoul) to the disposal site (in Gyeongju). Decontamination and sequestration of radionuclides, including Sr, from nuclear waste is important because they are hazardous and harmful to the ecological environment. Immobilization of these nuclides using a zeolite framework is suitable and simple method that has been widely studied. Therefore, it is still necessary to continuously explore the thermal stability of various zeolites and environmental changes around adsorbed cations in zeolite pore for effective immobilization of these radionuclides. In this study, we observed the thermal stability in fully Sr-exchanged natrolite (Sr-NAT), one of small-pore zeolite, from room temperature to 350°C using the in-situ synchrotron X-ray powder diffraction and thermogravimetric (TGA) analysis. In addition, we investigated the structural changes in Sr-NAT during temperature increase by Rietveld analysis. Sr-NAT exhibited apparent zero thermal expansions (ZTE) with the thermal expansion coefficients of -3(1) × 10-6 at the initial stage of increasing the temperature due to dehydration process. In the section from 250°C to 300°C, a phenomenon like negative thermal expansion (NTE) occurs in which the unit cell volume of Sr-NAT decreases despite the increase in temperature. Sr-NAT maintained well its crystallinity up to 350°C, and it became amorphous at 350°C. In this study, we provide a fundamental understanding of the structural changes and thermal stability mechanism of Sr-exchaged zeolite natrolite with increasing temperature.

나트로라이트(Na-제올라이트)의 탈수현상에 따른 변화와 특성을 라만 및 X-선 회절실험을 통하여 고온, 동시적인 고온-고압 하에서 특성을 확인하였다. 고온실험은 최고 온도범위에 따라 ~450℃까지 증온한 XRD 실험에서는 가역적 변화를 관찰하였고, ~630℃까지 증온한 라만분광실험에서는 포스트라트로라이트로 상전이하는 비가역적인 변화를 관찰하였다. 감온과정 약 380℃에서 포스트 나트로라이트 상이 나타나는 온도구간을 본 연구에서 새롭게 확인하였다. 동시적인 고온-고압 하(상온 〈 T 〈 300℃, 0 GPa 〈 P 〈 2.1 GPa)에서 나트로라이트 분광선의 이동은 관찰되지 않았다.