중·저준위 방사성 폐기물 처분을 위한 원전해체 재생시멘트의 고화재로서의 사용성 평가

Abstract

During decommissioning of nuclear power plants, a significant amount of concrete waste is generated. In most cases, all radioactive waste must be stored in a designated disposal facility. For such purpose, radioactive wastes are solidified using cement to prevent leakage of any radioactive elements into the environment. This study aims to assess the viability of recycling irradiated cement paste obtained from concrete in nuclear power plant as a solidifying agent, and thus ultimately reducing the volume of overall concrete waste generated during decommissioning process. To make simulated irradiated cement paste that can be obtained from concrete in decommissioned nuclear power plants, cement paste aged for 2 years was crushed into powder whose size was below 150㎛. To recover its hydraulic reactivity, heat treatment at 500℃, 600℃, and 700℃ for 2 hours were applied. Obtained recycled cement powder was mixed with liquid waste simulants such as 1M solutions of CoCl2, CsCl, and SrCl2, to make wasteform specimens. The 28 day compressive strength, thermal cycling, and leachabilities of Co, Cs, and Sr were measured in accordance with the solidification acceptance criteria set by the Nuclear Regulatory Commission(NRC). Set time and heat of hydration were also measured to understand fundamental characteristics of recycled cement powder. According to the results, specimens made of liquid waste simulants set much faster than specimens made of deionized water. Recycled cement powder, treated at 500℃ showed lower heat of hydration than that treated at higher temperatures. This means 500℃ is not an optimum temperature that can produce necessary reactivity. Recycled cement powders, treated at 600℃ and 700℃, did not show noticeable difference in terms of wasteform performance, meaning that 600℃ is better economical/environmental option for heat treatment. All wasteform specimens met the solidification acceptance criteria, and notably, the leaching test surpassed the standards, showing remarkable stability. This implies the feasibility of utilizing irradiated recycled cement powder as a solidifying agent for immobilization of low and intermediate level radioactive wastes.