Manufacturing and performance of customized pellets used for buffer and backfill sealing in nuclear waste containment

Document available in extended abstract form only. Bentonite pellets are proposed for use in filling gaps between highly compacted bentonite and the surrounding rock walls. Previous studies typically focused on using commercially available bentonite pellets with good performance results typically being achieved but no comprehensive evaluations were undertaken. This paper summarizes the results of two recent studies completed on gap filling materials and customized pellets that were intended to see to what degree improvement of the pellet materials and placement density could be achieved and what this would mean to system behaviour. Although the joint project covered a wide range of potential materials and sealing applications, in this presentation, the focus is on the use of bentonite filling material in the outer gap between the rock surface and large highly-compacted bentonite buffer blocks used in Posiva's Reference vertical deposition design. The gap between the deposition hole's wall and the buffer is 50 mm, which should be filled with material prior to tunnel backfilling. The required dry density of the outer gap filling is 920 kg/m3, with an average buffer dry density of 1600 kg/m3 at 100% saturation. At these densities, the thermal, hydraulic and mechanical behaviour of the system meet the requirements set for them. In the first part of this study, various types of commercially-available bentonite granular materials were used alone or in combination with finer material. Different placement methods were used to fill vertical gaps of either 25 or 35 mm width in a small-scale experimental mock-up. The sizes of the rectangular gap mock-up elements used in these tests were approximately 1 m in height and 2 m long. The results from the small scale tests suggest that all the filling materials and methods used during the test would achieve as-placed dry density of 800-1200 kg/m3, depending on material and placement method used. The lowest values were noted from free fall pouring of the largest Cebogel QSE pellets and the highest values were obtained from compacted small grain size Minelco bentonite. The highest dry density value observed in these larger-scale tests was slightly lower than in small scale tests because no additional compaction methods (e,g, vibration, dynamic or static compression), were used. With a target as-placed dry density for the pellets of ∼920 kg/m3 it would seem that almost any of the materials and methods examined could be successfully used to fill the buffer-rock gap. The second part of this study was done to further optimize the pellet filling material by customizing the materials and pellet manufacturing. Pellets were made using both and extrusion and roller compaction and methods. The pellets were intended for use in gaps between compacted bentonite and the rock walls in both buffer deposition holes and tunnel backfilling. One of the goals of this project was to evaluate the performance of different types of custom-made pellets with regard to their ease of manufacturing, density, crush strength, abrasion resistance, water holding capacity, free swelling and also their thermal conductivity. These evaluations were done in both Finland (by VTT) and Canada (by AECL). All of the roller-compacted pellets had individual pellet wet densities in the order of 2000 kg/m3. The roller-compacted pellets having the best performance in placement trials were the smallest size (∼9 mm diameter and 6 mm thick). The work done using extrusion pellets has shown that it is possible to manufacture pellets with higher water contents, up to 24% for MX-80. This is higher than was typically possible using roller compaction methods, and allows for closer compatibility with the designed buffer water content