Upscaling of cesium and strontium sorption from crushed to intact rock

Distribution coefficients (Kd) are used in determining retardation factors (R) utilized in the evaluation of radionuclide migration in the geosphere (Hakanen et al., 2012). Distribution coefficients are typically determined in static batch experiments and as the rock is crushed for these experiments they may exaggerate the sorption in the intact rock due to increase in surface area. The aim of this study was to produce conversion factors for distribution coefficients determined with crushed rock to those for intact rock. Sorption experiments with cesium or strontium were performed with both intact and crushed rock. The intact rock samples were about 10−30 mm thick and 52 mm in diameter and the migration of the cesium and strontium into them was enhanced by using an electrical current through the samples. The batch method was used to study the sorption of cesium and strontium on crushed rock. Previous studies have indicated that biotite is the most import mineral on which cesium and strontium are sorbed but it is assumed that muscovite, also found in the rocks at lower depths, play a similar role. The sorption mechanism of cesium and strontium on minerals is ion exchange. In the first phase four typical rock types of Olkiluoto were used in the electromigration experiments of cesium. These were T series mica gneiss (T-MGN), drill core ONK-PP131; P series tonalite-granodiorite-granite gneiss (P-TGG), drill core OL-KR43; T series tonalitegranodiorite-granite gneiss (T-TGG), drill core OL-KR43 and pegmatitic granite (PGR), drill cores ONK-PP131 and ONK-PP249. No determinations of minerals were done to these samples but according to report of Kärki and Paulamäki (2006) the content of biotite varies in these rocks. In this phase typically only two parallel rock samples were studied and the results of the parallel studies varied in a considerable range. In the second phase several parallel experiments were done with same rock type to improve statistics of the results. In addition to cesium these experiments were done also for strontium. The biotite contents of each rock were individually determined. The experiments were done with the rock samples from mica gneiss (MGN), drill cores ONK-PP219 and ONK-PP309 and from tonalite-granodiorite-granite gneiss (TGG), drill core ONK-PP175. These samples originated from the drillholes of the underground rock characterization facility, ONKALO. In addition, a granitic variety of tonalite-granodiorite-granite gneiss drill core sample from deep drillhole OL-KR56 in Olkiluoto was included in the study. The electromigration experiments of strontium were done with rock samples from ONK-PP219 and ONK-PP175. The mineral contents of the rock samples used in this study were studied with several methods and their specific surface areas were determined. Batch sorption experiments were done as a function of grain size and as a function of cesium and strontium concentration. For MGN the distribution coefficient (Kd) of cesium for intact rock was 0.0042±0.014 m3/kg and for crushed rock 0.023±0.013 m3/kg. For TGG, Kd values were 0.0035±0.0042 m3/kg and 0.009±0.015 m3/kg, respectively. The values are for equilibrium cesium concentrations around 0.05 mM. The ratios of crushed rock distribution coefficients to those of intact rock, to be used as upscaling factors, were 5.4±3.5 for MGN and 2.4±1.0 for TGG. For the two studied PGR drill core samples, intact rock distribution values differed largely, being 0.022±0.04 m3/kg for the other and 0.00016±0.00004 m3/kg for the other. Upscaling factors were 50±17 and 1.4±0.2, respectively. For strontium the upscaling factors were more or less identical to those of cesium, being 7.4±5.0 for MGN and 6.7±2.6 for TGG. The strontium distribution coefficients were 0.017±0.0001 m3/kg with crushed rock and 0.0044±0.0033 m3/kg with intact rock for MGN and 0.040±0.001 m3/kg with crushed rock and 0.0073±0.0036 m3/kg with intact rock for TGG. These values are for strontium equilibrium concentration of around 0.3 μM. As a result, the upscaling factors for MGN and TGG and for cesium and strontium did not differ very much from each other and a general conservative upscaling factor of 12 was derived for all radionuclides sorbing on MGN and TGG. This value is a geometric mean of all cesium and strontium values plus two standard deviations of mean thus representing 95.5 % statistical certainty level. For PGR a conservative upscaling factor of 69 is recommended at 68.3% statistical certainty level, but this value contains uncertainty from the great variation of the results with the two parallel PGR drill cores. In addition to electromigration experiments the upscaling was evaluated by determining the cesium distribution coefficients for various grain size fractions of rocks. These experiments suggested lower upscaling factors, at about half of those from electromigration experiments, for MGN and TGG. In addition to sorption experiments, porosities were measured using water saturation method and formation factors using electrical method. The formation factor results were further converted to effective diffusion coefficients. The average porosities for MGN, TGG and PGR samples were 0.23±0.12 %, 0.15±0.08 %, and 0.30±0.15 %, respectively. The average effective diffusion coefficient for MGN samples was (18±13)×10-14 m2/s and for TGG samples (22±29)×10-14 m2/s. These values were found to be typical for rock samples from Olkiluoto.