Filling the Gap Between Buffer and Rock in the Deposition Hole

The purpose of this study was to investigate different methods and materials which could be used for filling the gap between bentonite buffer and rock in the deposition hole. This work included a short literature survey of the previous gap filling tests which have been done in the recent years. The experimental work done during the study was divided in two phases. In the first phase small scale laboratory experiments were done using several different bentonite materials, material combinations and installation methods for 25 mm and 35 mm gaps. The sizes of these rectangular gap elements used in these tests were approximately 1 m in height and 2 m wide. Based on the first stage results the second sets of experiments were commenced. This time two materials and two methods were used in larger scale with gap widths of 35 mm and 50 mm. The large scale tests were done with cylinder shaped elements which had real size deposition hole radius of 1.75 m but the height was limited to 2 meters. Materials used in these gap filling tests included bentonite pellets and granules and their combination mixtures. Installation methods studied were free fall pouring method, shotcrete spraying method, Proctor hammer compaction method and two vibration methods to verify their functioning with these materials, material types and material forms. The achieved dry density was determined after each filling. The homogeneities of the fillings were also studied with X-ray imaging. The results from the small scale tests suggest that all the filling materials and methods used during the test would have high enough dry density average value needed by current buffer design options. The dry density values ranged from 800-1200 kg/m³ . 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 X-ray images and their analysis showed that Minelco and Ibeco Seal S-FGS had the most homogenised results. The largest differences in homogeneities were determined from Cebogel QSE pellets or from its combination mixture with finer material. Also when these combinations were used with shotcrete spraying installation the separation of the particles could be seen. In large scale experiments the shotcrete spraying tests gave out higher dry density values when compared to free fall pouring. Also there seems to be little or no difference in achieved density values between 35 mm and 50 mm gaps. The increase of the gap size by 15 mm was not a significant factor with the materials and installation methods used in this study. The dry density values ranged from 800-1000 kg/m³. The highest dry density value was lower than in small scale tests because no compaction methods were used. The spraying rates during shotcrete installation method tests varied between 1.6 – 2.0 m³/h depending on the pressure used. Because of the relatively small size of the gap elements used the spraying pressure had to be adjusted lower during installations. In the case of much deeper real size deposition hole the situation may be completely different. If buffer design requires gap filling of certain dry density value range 800 – 1200 kg/m³ to be used it can be achieved in several ways as shown in this study. Pellets could be  used alone and the voids in this type of filling enable artificial wetting design option. Installation method could be the simplest free fall pouring or dry shotcrete spraying. Also there are ways to make more homogeneous filling with fine grain size bentonite materials if the artificial wetting option is not needed.