Mineral composition characteristics of radiocesium sorbed and transported sediments within the Tomioka river basin in Fukushima Prefecture

The deposited radiocesium in the Fukushima river basin is transported in the river systems by soil particles and redistributed in the downstream areas. Although predicting the behaviors of minerals that adsorb radiocesium and of radiocesium dissolved in river water within the river systems is essential, the dominant mineral species that adsorb radiocesium have not yet been comprehensively identified. We identify herein such mineral species by investigating the ¹³⁷Cs distribution and the mineral species in each size fraction that are found in the bedload sediments from an upstream reservoir to an estuary within the Tomioka river basin located east of Fukushima Prefecture in Japan. In the fine sand sediment, which is the dominant fraction in terms of the ¹³⁷Cs quantity in the river bedload, the ¹³⁷Cs concentrations of the felsic and mafic minerals are comparable to that of micas. The mafic minerals contain 62% of the ¹³⁷Cs in the fine sand fraction in the upstream area, while the felsic minerals contain the highest quantities of ¹³⁷Cs in the downstream area. These results suggest that the quantification of the mineral species and the ¹³⁷Cs concentration of each size fraction are critically important in predicting the behaviors of the minerals and radiocesium within the Fukushima river basin in the future.

Batch sorption experiments of cesium and strontium on crushed rock and biotite for the estimation of distribution coefficients on intact crystalline rock

The distribution coefficient (K_d) of radionuclides on bedrock is one of the key parameters used in the safety analysis of spent nuclear fuel repositories. Typically, distribution coefficients have been determined using crushed rock. However, recent studies have shown that crushing of the rock increases considerably the distribution coefficient compared with the values of intact rock. This study aimed to test if batch sorption experiments using different grain sizes (i.e. mean diameter of grains) can be used to evaluate the K_d of strontium (Sr) and cesium (Cs) on intact crystalline rock, which would decrease the needed experimental time compared with transport experiments. Here we report the results of the batch sorption experiments with crushed rocks and compare the results with those from a recent study performed using electromigration experiments with intact drill core samples (Puukko et al., 2018). The batch sorption experiments were done for rock samples from Olkiluoto, Finland, as a function of grain size and of Cs and Sr concentration. Furthermore, the specific surface areas of the same rock samples with different grain sizes were determined. It was shown that Cs distribution coefficients correlate with specific surface areas of the studied rocks and biotite, the correlation coefficient being 0.95. The Cs distribution coefficient was highest for biotite at about 0.1 m³/kg at 10⁻⁴ M cesium concentration and increased systematically to about 1 m³/kg at 10⁻⁸ M. Distribution coefficients for rocks were up to about two orders of magnitude lower, being lowest with the rock with the lowest biotite content (3.3%). The distribution coefficient of Sr varied from 0.04 m³/kg to 0.007 m³/kg and behaved in a different manner: it remained constant in two out of three studied rocks in the concentration range of 10⁻⁸-10⁻⁴ M and only in the case of one rock a decreasing trend was seen at the higher concentration range. It was also shown that batch sorption experiments overestimate the distribution coefficient in respect to intact rock. The decrease of the distribution coefficient as a function of grain size can be estimated using a power law function. It was also shown that estimation of distribution coefficients of Cs and Sr for intact rock by extrapolation of distribution coefficients determined for different grain sizes is not possible without increasing grain size, but in that case diffusion into the grains would also affect the results. A new method was developed for estimating the fraction of the inner surface area of the total surface area of crushed grains. For the mean grain sizes of 0.25 mm and 0.75 mm the fraction of the inner surface was found to be 35–70% and 60–90%, respectively. The inner specific surface area was highest with biotite at 1.2 m²/g and lowest with the rock with lowest biotite content (3.3%) at 0.07 m²/g. The surface area analysis revealed that crushing creates and/or allows access to additional inner surface area that is not measured in intact rock. Furthermore, it was demonstrated that sorption of Cs on crushed rock was dominated by mica minerals in multiple concentrations while the effect of mica minerals on the K_d of Sr was not as straightforward.

Through diffusion experiments to study the diffusion and sorption of HTO, 36Cl, 133Ba and 134Cs in crystalline rock

The spent nuclear fuel in Finland will be deposited in crystalline granitic rock in Olkiluoto, Finland. As a part of the safety assessment of the repository, series of extensive in-situ sorption and diffusion experiments and supplementary laboratory work has been done in the Olkiluoto site. Through Diffusion Experiment in a laboratory (TDElab) aims to provide applicable data for the ongoing in-situ experiment in Olkiluoto. This laboratory scale experiment resembles the in-situ experiment and aims to gain information on possible effects in values of distribution coefficients, effective diffusion coefficient and porosity that are caused by differences in laboratory and in-situ conditions. The through diffusion and sorption of tracer solution with known activities of HTO, ³⁶Cl, ¹³³Ba and ¹³⁴Cs were studied in a decimeter scale sample of veined gneiss, which is one of the main rock types in Olkiluoto. The measured breakthrough curves were modeled taking into account the porosity of the rock and diffusion and sorption of the radionuclides using Time-Domain Random Walk (TDRW) simulations. The porosities of 0.7-0.8% were determined for the rock and effective diffusion coefficients of (3.5 ± 1.0) × 10^-13 m²/s and (3.0 ± 1.0) × 10^-13 m²/s were determined for HTO and ³⁶Cl, respectively. The porosity and effective diffusion coefficients were found to be in agreement with previous results for veined gneiss. Furthermore, distribution coefficients of (1.0 ± 0.3) × 10^-4 m³/kg and (2.0 ± 0.5) × 10^-3 m³/kg were determined for ¹³³Ba and ¹³⁴Cs, respectively, using information about the effective diffusion coefficient determined for HTO. The distribution coefficients were found to be significantly smaller than the ones determined for crushed rock in previous studies and slightly smaller than the ones from previous in-diffusion experiments.

Imaging connected porosity of crystalline rock by contrast agent-aided X-ray microtomography and scanning electron microscopy

We set out to study connected porosity of crystalline rock using X-ray microtomography and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) with caesium chloride as a contrast agent. Caesium is an important radionuclide regarding the final deposition of nuclear waste and also forms dense phases that can be readily distinguished by X-ray microtomography and SEM-EDS. Six samples from two sites, Olkiluoto (Finland) and Grimsel (Switzerland), where transport properties of crystalline rock are being studied in situ, were investigated using X-ray microtomography and SEM-EDS. The samples were imaged with X-ray microtomography, immersed in a saturated caesium chloride (CsCl) solution for 141, 249 and 365 days and imaged again with X-ray microtomography. CsCl inside the samples was successfully detected with X-ray microtomography and it had completely penetrated all six samples. SEM-EDS elemental mapping was used to study the location of caesium in the samples in detail with quantitative mineral information. Precipitated CsCl was found in the connected pore space in Olkiluoto veined gneiss and in lesser amounts in Grimsel granodiorite. Only a very small amount of precipitated CsCl was observed in the Grimsel granodiorite samples. In Olkiluoto veined gneiss caesium was found in pinitised areas of cordierite grains. In the pinitised areas caesium was found in notable excess compared to chloride, possibly due to the combination of small pore size and negatively charged surfaces. In addition, elevated concentrations of caesium were found in kaolinite and sphalerite phases. The findings concerning the location of CsCl were congruent with X-ray microtomography.