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Journal Articles

Modeling of an in-situ diffusion experiment in granite at the Grimsel Test Site

Soler, J. M.*; Landa, J.*; Havlova, V.*; Tachi, Yukio; Ebina, Takanori*; Sardini, P.*; Siitari-Kauppi, M.*; Martin, A. J.*

Materials Research Society Symposium Proceedings, Vol.1665, p.85 - 91, 2014/09

Matrix diffusion is a key process for radionuclide retention in crystalline rocks. An in-situ diffusion experiment in granite matrix was performed at the Grimsel Test Site (Switzerland). Several tracers (HTO, Na$$^{+}$$, Cs$$^{+}$$) were circulated through a borehole and the decrease in tracer concentrations was monitored for 2.5 years. Then, the borehole section was overcored and the tracer profiles in the rock were analyzed. Transport distances in the rock were 20 cm for HTO, 10 cm for Na$$^{+}$$ and 1 cm for Cs$$^{+}$$. The dataset was analyzed with diffusion-sorption models by different teams using different codes, with the goal of obtaining effective diffusivities ($$D$$e) and rock capacity factors. There was a rather good agreement between the values from different teams, implied that $$D$$e and capacity factors in the borehole damaged zone are larger than those in the bulk rock. However, HTO seems to display large discrepancies between measured and modeled results, which have to be investigated in more detail.

Journal Articles

K$$_{rm d}$$ setting approach through semi-quantitative estimation procedures and thermodynamic sorption models; A Case study for Horonobe URL conditions

Tachi, Yukio; Ochs, M.*; Suyama, Tadahiro; Trudel, D.*

Materials Research Society Symposium Proceedings, Vol.1665, p.149 - 155, 2014/09

Sorption of radionuclides (K$$_{rm d}$$) in host rocks depends critically on relevant geochemical conditions, need to correspond to the specific performance assessment (PA) conditions. The integrated approaches for site-specific K$$_{rm d}$$ setting have been developed focusing two transferring procedures, semi-quantitative estimation procedures and thermodynamic sorption models. The present paper focused on illustrating the derivation of K$$_{rm d}$$ values and their uncertainties of Cs, Ni, Am and Th for geochemical conditions from the Horonobe URL. These K$$_{rm d}$$-setting results were compared with the measured K$$_{rm d}$$ values, indicating that the magnitude of sorption can be quantitatively evaluated when adequate data and models were available. These comparative discussions between transferring procedures and measured data are effective to enhance the reliability of K$$_{rm d}$$ setting, and the careful selections from transferred results are needed according to the situation in the existing data and process understanding.

Journal Articles

The Effect of alkaline alteration on sorption properties of sedimentary rock

Shimoda, Satoko*; Nakazawa, Toshiyuki*; Kato, Hiroyasu*; Tachi, Yukio; Seida, Yoshimi*

Materials Research Society Symposium Proceedings, Vol.1665, p.179 - 184, 2014/09

The potential effect of alkaline perturbation caused by the cementitious materials must be evaluated in the performance assessment for HLW geological disposal. In this study, sorption of Cs, Ni and Th was investigated using the altered and unaltered samples of sedimentary rock from Horonobe underground research laboratory. The K$$_{rm d}$$ values of Cs, Ni and Th measured by batch method using synthetic groundwater changed as a function of degrees of alteration. The K$$_{rm d}$$ values of Cs increased with increasing degrees of alteration, indicating secondary minerals contributes to the increase in Cs sorption by ion exchange mechanism. On the other hand, the K$$_{rm d}$$ values of Ni and Th decreased with the increase of degrees of alteration. This change may be caused by dissolution of clay minerals controlling Ni and Th sorption by surface complexation. These results imply that the effect of alkaline perturbation on K$$_{rm d}$$ values of rocks depend on the surface property of the altered rock and sorption mechanism.

Journal Articles

Dose estimate in treatment and disposal of contaminated materials due to the accident at the Fukushima Nuclear Power Plant

Takeda, Seiji; Sawaguchi, Takuma; Kimura, Hideo

Materials Research Society Symposium Proceedings, Vol.1665, p.103 - 109, 2014/09

Some kinds of material in the environment due to the accident at the Fukushima Nuclear Plant have been contaminated by radioactive cesium, which are represented by dehydrated sludge, surface soil, disaster wastes generated by the 2011 off the Pacific coast of Tohoku Earthquake and so on. Treatment (transportation, temporary storage or incineration) and disposal of the contaminated materials should be carried out while ensuring the safety of radiation for the workers and the public. In this study, in order to provide the technical information for making the guideline, the dose estimate for scenarios on the treatment and disposal is conducted, based on the method used for driving the clearance levels in Japan. Minimum activity concentration in contaminated material is calculated from the dose results, corresponding to the effective dose criterions indicated by the Nuclear Safety Commission of Japan (June 3, 2011). From the calculation result, it is suggested that it is necessary to forbid reusing the disposal site as construction, resident and agriculture in which the calculated doses for the public are higher than those in the other exposure pathways. Minimum activity concentration of radioactive cesium is derived to be 8.9 Bq/g for exposure pathway in landfill work under the condition of limited reuse of the site. In the case of the activity concentration below 8.9 Bq/g, the calculated dose of the resident due to direct and sky-shine radiation from the temporary storage place is less than 1 mSv/y, irrespective of the distance from the storage place.

Journal Articles

The Behaviour of radiocaesium deposited in an upland reservoir after the Fukushima Nuclear Power Plant accident

Funaki, Hironori; Hagiwara, Hiroki; Tsuruta, Tadahiko

Materials Research Society Symposium Proceedings, Vol.1665, p.165 - 170, 2014/09

Journal Articles

"Relative rates method" for evaluating the effect of potential geological environmental change due to uplift/erosion to radionuclide migration of high-level radioactive waste

Ebashi, Takeshi; Kawamura, Makoto*; Inagaki, Manabu*; Koo, Shigeru*; Shibata, Masahiro; Itazu, Toru; Nakajima, Kunihiko*; Miyahara, Kaname; Apted, M. J.*

Materials Research Society Symposium Proceedings, Vol.1665, p.39 - 45, 2014/07

In Japan, the likelihood of uplift/erosion on repository performance and waste isolation can be typically greatly reduced or excluded by careful siting, however, the inability to completely exclude the uplift/erosion scenario may require an analysis of the consequences of such a scenario. For this purpose, an assessment approach has been developed to more realistically treat the effect of uplift/erosion for a hypothetical repository located in sedimentary host rock. A key advantage to this approach is the extrapolation of the geohistory of modern analogue sites to develop credible initial inputs for future volunteer sites that may be poorly characterized at the initial stages of site investigation. In addition, the approach provides a systematic basis for bounding the range of possible evolution in thermal-hydrological-mechanical-chemical conditions of a repository experiencing different uplift/ erosion rates.

Journal Articles

Diffusion modeling in compacted bentonite based on modified Gouy-Chapman model

Yotsuji, Kenji; Tachi, Yukio; Nishimaki, Yuichiro*

Materials Research Society Symposium Proceedings, Vol.1665, p.123 - 129, 2014/07

The integrated sorption and diffusion (ISD) model has been developed to quantify these processes in compacted bentonite. The ISD model assuming averaged narrow porespace and the electric double layer (EDL) theory could give quantitative agreement with diffusion data of monovalent cations and anions under wide range of conditions. However, the systematic disagreements were observed for multivalent cations, anions and complex species. In this study, the excluded volume effect and the dielectric saturation effect were introduced into the current ISD model and the modified Poisson-Boltzmann equations were numerically solved. The ionic distributions were influenced at the near surface, however, these changes were canceled by averaging for the porespace. As a result, these modified models had little effect on the effective diffusivity. On the other hand, the modified model considering hydrated ions with the effective electric charge could give good reproducibility of diffusion data.

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