General Overview of the Research Project Investigating the Radionuclide Solution Behavior in Mock Mortar Matrix Modeled after Conditions at the Fukushima-Daiichi Nuclear Power Station

Abstract

Decommissioning of the Fukushima Daiichi Nuclear Power Station (F1NPS) in a proper manner requires assessment of the contamination levels and mechanisms for contamination in the concrete structures. Between January 2018 and March 2020, Japan's Ministry of Education Ministry of Education, Culture, Sports, Science and Technology (MEXT) conducted a project called "The Analysis of Radionuclide Contamination Mechanisms of Concrete and the Estimation of Contamination Distribution at the Fukushima Daiichi Nuclear Power Station". In this review, we outline the results of this study. The experimental results from the first project indicate that concrete carbonation, Ca leaching, and drying conditions affected the adsorption behaviors of Cs and Sr and therefore, their penetration depths. Additionally, the studies showed that α-nuclides precipitated on the surface of the samples because concrete causes a high pH. A reaction transport model was developed to assess further the adsorption characteristics of Cs and Sr in carbonated cement paste and on concrete aggregates. The model used real concrete characteristics from the materials used at F1NPS and historical boundary conditions at the site, including radionuclide concentrations and penetration profiles within the turbine pit wall. Capillary water suction resulting from dried concrete was evaluated by considering structural changes in cement hydrates using X-ray CR and 1 H-NMR relaxometry.

Effects of Severe Accident Conditions on Integrity of RPV Pedestal of Fukushima Daiichi Nuclear Power Plant

“…According to Maruyama et al (2021), the concrete strength distribution in the cross section of the RPV pedestal of Hamaoka Unit 1 of Chubu Electric Power Co., Inc., a decommissioned plant, has been obtained, and the compressive strength is about twice as high as 70 MPa in the center, compared to 35 MPa in the surface layer. It has been elucidated that this increase in strength is due to the reaction of silica leached from feldspars with portlandite to form the hydration product of C-S-H. Because ordinary Portland cement is also used in the RPV pedestal of Unit 1 of FD-NPP, and because the aggregate used is igneous rock-based aggre-gate and is thought to contain many feldspars according to Igarashi et al (2021), it is assumed that a similar reaction found in the RPV pedestal of Hamaoka Unit 1 has occurred in the RPV pedestal of Unit 1 of FD-NPP, increasing concrete strength.…”