C-14 in Geological Repositories: Source Terms and Transport Models

Abstract Spent fuel management is one of the key issues for the sustainable development of nuclear energy. The deep geological disposal of radioactive waste is a vital aspect of spent fuel management, aiming to isolate radioactive sources from humans and circulating groundwater as much as possible. Buffer materials, as a crucial part of the engineering barrier system, play an important role in preventing the infiltration of groundwater and the outward migration of radioactive nuclides. However, due to a variety of unforeseen circumstances, there remains a risk of leakage in geological repositories, potentially leading to the radioactive sources penetrating the engineering barrier system and causing environmental damage. Carbon-14 (C-14) is one of the significant radioactive elements in China’s geological repositories. In the event of repository damage, C-14 could enter the biosphere and infiltrate the human body through the food chain. It could pose internal radiation hazards to human tissues and organs, which becomes a major source of collective radiation dose for humans. In recent years, there has been significant progress in the study of buffer materials for high-level radioactive waste (HLW) geological repositories in China, including the development of GMZ01 type bentonite as a buffer material. Extensive research has been conducted on the basic physicochemical properties and nuclide adsorption characteristics of these materials. However, studies specifically related to C-14 in these repositories are less common. The methods for assessing the inventory of C-14 in the repositories need to be refined. Although some computational models for simulating the migration process of C-14 from the repositories to the environment have been proposed, there is a lack of comprehensive summaries. Additionally, the overall research framework for the migration process of C-14 in buffer materials is not clear. Therefore, the study of the C-14 source term in geological repositories and its migration and diffusion behavior in bentonite, such as GMZ01 type, is a crucial part of the radiation safety assessment in geological repositories in China. This paper focuses on C-14, which is a typical radionuclide in radioactive source terms in geological repositories, investigates the research progress on the C-14 source term in geological repositories both domestically and internationally, organizes the methods for assessing the inventory of C-14 in repositories and compiles computational models for the migration and diffusion behavior of C-14 in key buffer materials. This paper aims to provide theoretical support for the analysis of the C-14 source term in high-level radioactive waste geological repositories and the study of C-14 migration processes in buffer materials, which could provide an important research basis for the radiation safety evaluation of high-level waste geological repositories in China.