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.

With respect to spent nuclear fuels, disposal containers and bentonite buffer blocks in deep geological disposal systems are the primary engineered barrier elements that are required to isolate radioactive toxicity for a long period of time and delay the leakage of radio nuclides such that they do not affect human and natural environments. Therefore, the thermal stability of the bentonite buffer and structural integrity of the disposal container are essential factors for maintaining the safety of a deep geological disposal system. The most important requirement in the design of such a system involves ensuring that the temperature of the buffer does not exceed 100℃ because of the decay heat emitted from high-level wastes loaded in the disposal container. In addition, the disposal containers should maintain structural integrity under loads, such as hydraulic pressure, at an underground depth of 500 m and swelling pressure of the bentonite buffer. In this study, we analyzed the thermal stability and structural integrity in a deep geological disposal environment of the improved deep geological disposal systems for domestic light-water and heavy-water reactor types of spent nuclear fuels, which were considered to be subject to direct disposal. The results of the thermal stability and structural integrity assessments indicated that the improved disposal systems for each type of spent nuclear fuel satisfied the temperature limit requirement (< 100℃) of the disposal system, and the disposal containers were observed to maintain their integrity with a safety ratio of 2.0 or higher in the environment of deep disposal.

사용후핵연료 심지층처분에 있어서 처분용기의 건전성 확보는 내부에 적재되어 있는 사용후 핵연료로부터 방사성물질이 누출되는 것을 방지하고 격리하여 처분장의 안전성을 보증하기 위한 필수적인 인자이다. 이러한 처분용기는 심지층 처분의 목적인 방사성 독성이 인간 및 자연환경에 영향을 미치지 않도록 장기간 동안 격리하고 누출을 지연시키기 위한 공학적 방벽의 중요한 요소 중의 하나이다. 심지층 처분장 설계시 주요한 요건은 처분시스템의 안전성을 유지를 위하여 처분용기에 적재되어 있는 폐기물로부터 발생된 붕괴열로 인하여 완충재의 온도가 100를 넘지 않도록 하는 것이다. 또한, 처분용기는 지하 심부 500 m 깊이에서의 수압과 완충재의 팽윤압 등 하중에 구조적 건전성을 유지하여야 한다. 본 연구에서는 직접 처분대상으로 고려하고 있는 중수로(CANDU) 사용후핵연료에 대한 처분용기의 개선된 개념을 설정하고, 심지층 처분환경에서의 열적 및 구조적 안정성을 분석하였다. 열적 안정성 해석결과 처분터널 및 처분공 간격이 40 m, 3 m 인 경우 처분 후 37년이 경과한 후에 처분용기 표면온도가 최고 온도에 도달하며, 이때 온도는 88.9 로서 처분장 온도제한 요건(100 )에 만족하였다. 또한, 정상적인 경우와 극한 상황에 따른 하중에 대한 처분용기 구조해석 결과 안전율은 각각 2.9와 1.33 으로 나타나 심지층 처분환경에서 처분용기는 구조적 건정성을 유지하는 것으로 판단되었다.