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.

Colloid Formation and Migration (CFM) project is being carried out within the Grimsel Test Site (GTS) Phase Ⅵ. Since 2008, the Korea Atomic Energy Research Institute (KAERI) has joined CFM to investigate the behavior of colloid-facilitated radionuclide transport in a generic Underground Research Laboratory (URL). The CFM project includes a long-term in-situ test (LIT) and an in-rock bentonite erosion test (i-BET) to assess the in-situ colloid-facilitated radionuclide transport through the bentonite erosion in the natural flow field. In the LIT experiment, radionuclide-containing compacted bentonite was equipped with a triple-packer system and then positioned at the borehole in the shear zone. It was observed that colloid transport was limited owing to the low swelling pressure and low hydraulic conductivity. Therefore, a postmortem analysis is being conducted to estimate the partial migration and diffusion of radionuclides. The i-BET experiment, that focuses more on bentonite erosion, was newly designed to assess colloid formation in another flow field. The i-BET experiment started with the placement of compacted bentonite rings in the double-packer system, and the hydraulic parameters and bentonite erosion have been monitored since December 2018.

본 연구에서는 국내산 경주벤토나이트를 이용하여 제조한 벤토나이트 콜로이드에 대한 산화환원 반응에 대체적으로 안정한 다가 핵종인 Eu(III)와 Th(IV)의 실험적 수착 연구를 수행하였다. 수착실험에 대한 공시험을 수행하여 반응용기 벽면에 의해, 침전에 의해, 콜로이드 형성에 의해 손실된 핵종들의 양을 평가하였다. 그리고 이러한 손실들을 반영한 Eu(III)와 Th(IV)의 벤토나이트 콜로이드에 대한 수착분배계수 값을 구하고 조사하였다. 세 종류의 손실양을 반영한 벤토나이트 콜로이드의 순수한 수착분배계수 값은 pH 변화에 따라 Eu(III)의 경우 정도의 값을 가지고, Th(IV)의 경우 정도의 값을 가지는 것으로 관측되었다. 특히 Eu(III)의 경우엔 pH 5 이상에서 침전의 영향이 크게 나타났고, Th(IV)의 경우엔 pH 3 이후에 콜로이드 형성과 침전의 영향이 크게 나타났다. 따라서 주어진 농도에서 콜로이드 형성 및 침전 영향이 커지는 pH 이후에는 Eu(III) 및 Th(IV)과 같은 다가 핵종들의 정확한 수착분배계수를 구하기 위해서는 이러한 침전 및 콜로이드 형성과 같은 영향이 반영되어야 할 것이다.