Colloid Formation and Migration (CFM) Introduction

AIST National Instutute of Advanced Industrial Science and Technology (Geological Survey of Japan) ANDRA National radioactive waste management agency, France FZk-INE (BMWi Bundesministerium für Wirtschaft und Arbeit), Germany JAEA Japan Atomic Energy Agency, Japan Nagra National Cooperative for the Disposal of Radioactive Waste

Studies of natural colloids in deep groundwaters from widely ranging environments (eg Yucca Mountain, Wellenberg, Schwarzwald, Oklo, Cigar Lake etc) have been ongoing for several decades. In addition, laboratory programmes on colloid generation, stability and classical batch experiments studying radionuclide uptake on colloids (and the possibilities of subsequent release) have been reported and a significant database already exists.

It is generally accepted that five requirements must be fulfilled (the so-called colloid ladder, see below) to prove that colloid-facilitated transport of radionuclides in a potential repository host rock may be of significance to the long-term performance of a waste repository:

colloid ladder

As has been noted in several studies (including the recently completed CRR, Colloid and Radionuclide Retardation, project in the Grimsel Test Site (GTS)), the answer to the first four questions above is probably affirmative However, before any statements can be made about the likelihood of significant colloid-facilitated radionuclide transport in the vicinity of a deep geological waste repository, two points must be addressed:

• first, the last question about the extent of irreversibility of radionuclide-colloid associations must be investigated (it could, for example, vary for different types of colloids)
  
  
  
• second, this, and the other processes defined in the colloid ladder, must be investigated in repository-relevant systems. Although the in situ work carried out to date has been on a longer temporal and spatial scale than is normally possible in laboratory experiments, the groundwater flow velocities are 10⁴ - 10⁸ times greater than would be expected in a suitable repository host rock. This is common in in situ work ongoing around the world today and is simply a question of practicalities (i.e. conducting in situ experiments within reasonable timescales and budgets). Nevertheless, it could be argued that process and mechanism understanding has now progressed to the stage where such unrealistic experiments can provide little more relevant information and that future work must consider significantly longer time scales than has been the case so far and focus on, for example, semi-stagnant groundwater systems, to better match the conditions in and around a waste repository.
  

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