Approximate analytic equation of the surface complexation based on Basic Stern Model was derived and a simple procedure to estimate of the sorption distribution coefficient K_d was presented. The K_d values of Cs, I, and Pu on sandstone were evaluated and compared to the data in the sorption data base compiled by JAEA. The calculated values are almost equal to those given by the geochemical speciation code MICROQL II and acceptable accordance with experimental data were found. Sensitivity analysis and uncertainty analysis were also conducted by using the presented procedure for the case in which the minimum and maximum K_d values differed by a factor of 7 even though the experimental conditions were almost the same. The result revealed that the variation of the logarithm of the intrinsic surface complexation constant within 10% gave rise the possible scatter of several fold in K_d.

  In this study, a realistic radionuclide transport model was developed by assuming the fault zone to consist of distinct sub-regions: the gouge zone, the damage zone, and the fracture zone, and by taking into account the specific characteristics of each region. Numerical simulations were conducted using this model, and the results were employed as training data to construct a predictive model through machine learning of the neural network. This predictive model enables fault impact assessments across the entire repository. Furthermore, focusing on major uncertainty factors related to fault scenarios, specifically fault size, occurrence timing, and location, a total of 1,000 uncertainty analysis cases were performed using various combinations of these factors and the developed predictive model. The results revealed that fault magnitude exerts the greatest influence on the maximum total dose, and that the dominant contributing region to the maximum total dose varies depending on the conditions. These findings suggest that, for an appropriate evaluation of the maximum total dose, it is essential to consider the number of waste packages located within each fault zone region.

  The accident at the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station resulted in the release of radioactive cesium (r-Cs) into the environment, contaminating concrete structures. Surface dose rates were measured on concrete columns at the former Fukushima Prefectural Aquafarming Research Institute, located along the Pacific coast, and were found to have decreased by half between December 2015 and January 2024. Concrete cores were collected, and immersed in pure water and artificial seawater for four months, and the surface radioactivities were evaluated using an imaging plate. There were no changes before and after immersion in pure water and stored in a plastic bag, but a 21% decrease was observed in artificial seawater immersion on average. The r-Cs reached the concrete in March 2011 is believed to have been through an ion exchange between rainwater and concrete and adsorbed onto the concrete. Although it is unlikely that r-Cs would leach from the concrete by exposure to rain with low salt concentrations, the concrete structures under evaluation are located facing the Pacific Ocean and may have been affected by sea salt, leading to leaching.