The Colloid Formation and Migration (CFM) international joint research initiative continues as a part of the GTS’s Radionuclide Retardation Programme, which has been in progress since 1984. This project focuses on examining the formation of colloids from a bentonite-engineered barrier system and exploring how these colloids impact the migration of radionuclides in fractured host rock when subjected to advective flow. Phase 1 of the project was launched in 2004 and concluded in early 2008, focusing on preliminary studies related to in-situ boundary conditions, predicting models, and supplementary lab works. Following that, Phase 2 spanned from 2008 to 2013 and aimed at fortifying the field setup by adding three new monitoring boreholes and suitable instrumentation in both the boreholes and tunnel. This phase also tested the system’s resilience while mapping the flow domain. Phase 3 kicked off in January 2014 and extended until December 2018. During this period, the Long-term In-situ Test (LIT) was introduced in May 2014, featuring a set of compacted bentonite rings laced with radionuclide tracers. These were placed in a borehole to serve as a colloid and radionuclide source. CFM Phase 4 initiative commenced in January 2019, marking the successful deployment of the i-BET (In-situ Bentonite Erosion Test). This project component involves placing approximately 50 kg of compacted bentonite in a natural water-conducting shear zone. Korea Atomic Energy Research Institute (KAERI) joined CFM in 2008 to examine the behavior of colloid generation and migration with radionuclides in the Underground Research Laboratory. The fourth phase of the CFM project was also scheduled to include a post-mortem evaluation of the LIT and additional tracer experiments in the well-mapped MI shear zone. This study aims to provide an interim update on the ongoing i-BET, a key component of Phase 4 of the CFM project. We will also discuss the current status of the post-mortem analysis for the LIT experiment. In addition, we will outline plans for the forthcoming Phase VI of the project. These plans will continue to advance our understanding of radionuclide migration and the influence of bentonite-based disposal systems.

• Sang-Ho Lee(Korea Atomic Energy Research Institute (KAERI)) Corresponding author
• Jin-Seok Kim(Korea Atomic Energy Research Institute (KAERI))
• Seung Yeop Lee(Korea Atomic Energy Research Institute (KAERI))
• Jang-Soon Kwon(Korea Atomic Energy Research Institute (KAERI))