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[HCG24-P05] Study to establish methodology for 4D SDM development for Post-Closure safety assessment of the repository
Keywords:Geological disposal, 4D SDM, Geological environment characteristics, Long-term evolution
1. Introduction
In order to isolate radioactive waste for geological disposal from the human environment for a long period of time (tens of thousands of years or more) and to be able to contain it by multi-barrier system consisting of engineered and natural barriers, it is necessary to select a geological environment that avoids significant effects of natural phenomena and that maintains favorable characteristics in terms of geological disposal in a stable manner for a long time. Therefore, it is important to develop the 4D SDMs (site descriptive models considering 3D space and time axis) to take into account temporal and spatial changes in the geological environment characteristics due to long-term topographical changes and climatic and sea-level change, while coordinating with the design and safety assessment of a repository. In this paper, we introduce the current status of our activities in this development.
2. Outline of activities
(1) Development of topographic and geological models
Considering the geological characteristics of the three rock types widely distributed in the deep underground of Japan (plutonic rocks, Neogane sedimentary rocks and Pre-Neogene sedimentary rocks), as summarized in the NUMO Pre-siting SDM-based Safety Case (NUMO, 2021), topographic and geological models were developed for each rock type. The modeling area is approximately 100 km by 50 km, taking into account the range of the regional groundwater flow system and the extent of the land area due to sea-level decline. The long term evolution of topography and geological structure at 20 time sections is modeled for the period from the present to about 1 Ma, assuming the maximum seaward movement and maximum seaward retreat associated with sea level change (Figure 1).
(2) Development of a hydrogeological model
The hydrogeological model was developed by setting hydraulic properties to the elements of geological structure represented in the geological model at each time section.
(3) Groundwater flow and solute transport simulations
3D Groundwater (GW) flow and solute transport simulations were conducted to calculate the spatio-temporal distribution of GW flow and salinity. Specifically, the following studies were carried out using the hydrogeological model (Figure 2).
・Transient 3D GW flow and solute transport simulations considering continuous changes of topography, climate and sea-level change.
・GW flow and solute transport simulations considering both advection/dispersion and matrix diffusion in fractures.
(4) Study for reflection in design and safety assessment
Based on the results of (1) to (3), the following studies were carried out in order to consider how to specifically reflect the results in the design and safety assessment.
・Extraction of potential areas for the installation of underground facilities in a repository, taking into account the distribution of faults and geological formation, travel time of GW and temporal and spatial changes in salinity, etc..
・The solute transport simulations from the extracted potential repository area.
・Organization of the information to contribute to the development of storyboards illustrating the evolution of the geological disposal system condition and how its evolution with time impacts safety functions.
3. Results
The tasks to be carried out in the construction of a 4D SDMs have been subdivided and the work procedures have been summarized based on the result of this study.
・Concept of time section setting taking into account the time evolution of long-term topography and geological structure
・Concept of the topographical and geological modeling
・Concept of hydrogeological modeling and GW flow and solute transport simulations
Reference
NUMO (2021), The NUMO Pre-siting SDM-based Safety Case, NUMO-TR-21-01.
In order to isolate radioactive waste for geological disposal from the human environment for a long period of time (tens of thousands of years or more) and to be able to contain it by multi-barrier system consisting of engineered and natural barriers, it is necessary to select a geological environment that avoids significant effects of natural phenomena and that maintains favorable characteristics in terms of geological disposal in a stable manner for a long time. Therefore, it is important to develop the 4D SDMs (site descriptive models considering 3D space and time axis) to take into account temporal and spatial changes in the geological environment characteristics due to long-term topographical changes and climatic and sea-level change, while coordinating with the design and safety assessment of a repository. In this paper, we introduce the current status of our activities in this development.
2. Outline of activities
(1) Development of topographic and geological models
Considering the geological characteristics of the three rock types widely distributed in the deep underground of Japan (plutonic rocks, Neogane sedimentary rocks and Pre-Neogene sedimentary rocks), as summarized in the NUMO Pre-siting SDM-based Safety Case (NUMO, 2021), topographic and geological models were developed for each rock type. The modeling area is approximately 100 km by 50 km, taking into account the range of the regional groundwater flow system and the extent of the land area due to sea-level decline. The long term evolution of topography and geological structure at 20 time sections is modeled for the period from the present to about 1 Ma, assuming the maximum seaward movement and maximum seaward retreat associated with sea level change (Figure 1).
(2) Development of a hydrogeological model
The hydrogeological model was developed by setting hydraulic properties to the elements of geological structure represented in the geological model at each time section.
(3) Groundwater flow and solute transport simulations
3D Groundwater (GW) flow and solute transport simulations were conducted to calculate the spatio-temporal distribution of GW flow and salinity. Specifically, the following studies were carried out using the hydrogeological model (Figure 2).
・Transient 3D GW flow and solute transport simulations considering continuous changes of topography, climate and sea-level change.
・GW flow and solute transport simulations considering both advection/dispersion and matrix diffusion in fractures.
(4) Study for reflection in design and safety assessment
Based on the results of (1) to (3), the following studies were carried out in order to consider how to specifically reflect the results in the design and safety assessment.
・Extraction of potential areas for the installation of underground facilities in a repository, taking into account the distribution of faults and geological formation, travel time of GW and temporal and spatial changes in salinity, etc..
・The solute transport simulations from the extracted potential repository area.
・Organization of the information to contribute to the development of storyboards illustrating the evolution of the geological disposal system condition and how its evolution with time impacts safety functions.
3. Results
The tasks to be carried out in the construction of a 4D SDMs have been subdivided and the work procedures have been summarized based on the result of this study.
・Concept of time section setting taking into account the time evolution of long-term topography and geological structure
・Concept of the topographical and geological modeling
・Concept of hydrogeological modeling and GW flow and solute transport simulations
Reference
NUMO (2021), The NUMO Pre-siting SDM-based Safety Case, NUMO-TR-21-01.