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1. Introduction
A near-surface disposal facility, which will be the first low-level radioactive waste disposal facility in Korea, is being built in the low and intermediate-level radioactive waste (LILW) disposal center in Gyeongju with a disposal capacity of 32,876 m2 corresponding to waste volume [1]. The near-surface disposal facility is currently under construction, acquiring a construction and operation license from Nuclear Safety and Security Commission in 2022 after applying for a construction license in 2015 and is aimed for completion in 2024 [2]. The near-surface disposal facility as a vault disposal system comprises 20 vaults which are concrete structures for the final disposal of radioactive waste. Once the disposal of radioactive waste is completed, the facility will be closed by constructing a disposal cover on the top of the vaults.
The disposal cover, one of the types of engineered barriers, represents an important component of the disposal facility safety from the operational phase, through the period of the post-closure, and ultimately to the possible free release of the site [3]. The disposal cover is composed of multi-layer to act as a hydraulic barrier to facilitate water run-off and limit infiltration of water due to precipitation. Additionally, it serves as a barrier against intrusion by plants, animal and humans and erosion caused during the post-closure period (300 years). To perform these functions, the performance objective for water infiltration given to the disposal cover is 32 mm per year, and the performance objective for erosion protection is within 2 Ton/Acre/year (soil loss by erosion) [1].
To acquire a realistic forecast for the post-closure period of the near surface disposal facility, it is essential to conduct long-term experimental research under conditions similar to the actual disposal environment. With this purpose in consideration, a performance test facility of the disposal cover was constructed in 2022 at the site of the LILW disposal center as part of an R&D project [4]. Various instruments were installed in the performance test facility to measure the variables related to the safety functions of the disposal cover and to observe how the disposal cover changes over time.
The design of the disposal cover for the performance test facility mirrors that of the near-surface disposal facility, with identical composition and thickness. However, challenges arose during the construction of the performance test facility due to difficulties in sourcing soil materials for the disposal cover. As a result, the performance test facility was built with soil materials that was slightly different from the specifications presented in the design.
In this study, the effect of differences from the specifications presented in the design of the performance facility on the performance objective of disposal cover related to water infiltration from rainfall was evaluated.
2. Performance Test Facility for Disposal Cover
The design of the disposal cover has not undergone verification through actual operation, leading to uncertainties regarding its feasibility, constructability and ability to meet the performance objectives. Consequently, the performance test facility for the disposal cover was constructed in the LILW disposal center in Gyeongju as part of an R&D project. It serves several purposes: verifying the feasibility of research and development technologies, measuring the safety-related variables for the disposal covers, assessing long-term changes in the disposal cover over the time, and providing support for the construction and operation of the near-surface disposal facility through the data acquired from the performance test facility.
The performance test facility, constructed to confirm the long-term performance of the disposal cover post-closure of the near-surface disposal facility, consists of two parts: One is a disposal cover with multi-layer and the other is a monitoring room.
The disposal cover for the performances test facility replicates the same layer composition and thickness as the disposal cover to be installed when the near-surface disposal facility will be closed in the future. Consequently, the performance test facility serves as a downscaled model where the disposal cover maintains consistent height while only the width varies. Furthermore, various measuring instruments are installed in disposal cover to observe how the disposal cover changes over time.
In the monitoring room, data obtained from measuring instruments embedded in the disposal cover can be checked and analyzed. The performance test facility has dimensions of 5 m in width, 26 m in length and 7 m in height. Fig. 1 provides the entire view of the performance test facility.
2.1 Disposal Cover of the Performance Test Facility
The disposal cover is designed with a multi-layer structure, including surface layer, protection layer, drainage layer and barrier layer, mirroring the composition of the actual disposal cover. Each layer serves a distinct function, contributing to the overall effectiveness of the disposal cover. The specific functions of each layer are detailed below:
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• Surface layer
The surface layer is located at the top of the multi-layer and tasked with managing runoff, minimizing erosion and maximizing evapotranspiration. It consists of silty sand.
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• Protection layer
A protection layer, positioned directly beneath the surface layer, works in tandem with the upper layers to shield underlying components from potential degradation. This degradation may result from repeated freeze/thaw cycles, excessive wetting/drying and intrusion by plants, animals, or humans. The protection layer consists of gravelly sand and pea gravel.
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• Drainage layer
The primary function of this layer is to collect water that infiltrates through the surface and protective layers, redirecting it laterally. Drainage layer manages water flow within the disposal cover and consists of sand.
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• Barrier layer
The barrier layer stands out as the most critical component, tasked with restricting water infiltration into the disposal vault. This layer should be at least 0.6 m of compacted, low-permeability soil with an in-place saturated hydraulic conductivity of 10−9 m·sec−1 or less [5]. The barrier layer is composed of clay.
The detailed composition of the multi-layer soil material was designed to meet the performance requirements of the near-surface disposal facility, which is the same as the disposal cover of the 2nd near-surface disposal facility. Based on the performance requirements of the disposal cover, the soil material requirements for the multi-layer are listed in the Table 1 [6].
Table 1
Measuring instruments of the performance test facility
| Measuring item | Measuring instrument (Manufacturer/Product) | Range | Resolution |
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| Soil temperature | Thermal Line sensor (Soam Consultant Co., Ltd /TLS 3D) | −55~125℃ | 0.0625℃ |
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| Relative humidity | Frequency domain reflectometer (Hunan Rika Electronic Tech Co., Ltd/RK520-02) | 0–100%（m3/m3） | 0.10% |
| Electrical conductivity | 0–20 mS·cm−1 | 0.01 ms·cm−1 | |
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| Slope | Inclinometer (BEWIS Sensing Technology LLC/BWH527) | ±30° | 0.0007° |
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| Displacement rate of layer | Acoustic Emission sensor (Soam Consultant Co., Ltd /AEMO system) | 80 dB (Dynamix range) | - |
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| Amount of water infiltration | Flowmeter (Davis Instruments Corp./Rain collector Ⅱ) | 0–9,999 mm [day] | 0.2 mm |
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| Rainfall rate | Meteorological instrument (Davis Instruments Corp./Vantage KW) | 0–9,999 mm [day] | 0.2 mm |
| Outside temperature | −40~65℃ | 0.1℃ | |
| Outside humidity | 1–100% RH | 1% RH | |
| Wind direction | 0–360° | 1° | |
| Wind speed | 1–67 m·s−1 | 0.1 m·s−1 | |
| Solar radiation | 0–1,800 W·m−2 | 1 W·m−2 | |
To fix the shape of multi-layer and maintain their integrity, a concrete retaining wall with a thickness of 1 m was erected around the outside of the disposal cover. A buffer zone was installed next to the concrete retaining wall to prevent rainfall infiltration along the wall. Fig. 2 shows the configuration of the disposal cover in the performance facility. The concrete retaining wall and the buffer zone are included only this facility and will not be included in the construction of the 2nd near-surface disposal facility.
Additionally, various measuring instruments have been installed within the multi-layer of the disposal cover to observe the changing aspects of the disposal cover over time. Measurement items were selected as soil temperature, soil humidity, soil electrical conductivity (EC), slope, displacement rate of layer, weather and the amount of water infiltration. The specifications of installed measuring instruments are listed in Table 2 [4, 7]. The measuring instruments installed in the performance test facility include 402 temperature sensors, 16 FDR sensors, 3 inclinometers, 1 AE sensor, 4 flowmeters, and 1 meteorological instrument. The installation locations of these instruments are shown in Fig. 3. The flowmeters which directly measure the amount of water flowing into the disposal cover was installed under the multi-layer and are located inside the monitoring room.
Table 2
Design requirements for multi-layer soil materials
| Soil material | U.S standard Sieve size (mm) | Percent by weight passing (%) | Remarks |
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| Silty sand | Maximum particle size < 19 mm | ||
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| Gravelly sand | 19.05 | 100 | |
| 9.53 | 75–100 | ||
| 4.76 | 55–100 | ||
| 2.00 | 35–95 | ||
| 0.85 | 20–80 | ||
| 0.43 | 5–10 | ||
| 0.15 | 0–2 | ||
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| Pea gravel | 9.53 | 100 | |
| 4.76 | 0–5 | ||
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| Sand | 9.53 | 100 | |
| 4.76 | 95–100 | ||
| 2.36 | 80–95 | ||
| 1.18 | 50–85 | ||
| 0.60 | 5–60 | ||
| 0.30 | 5–30 | ||
| 0.15 | 0–10 | ||
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| Clay | 4.75 | 90 | |
| 0.075 | 50 | ||
2.2 Monitoring Room
The monitoring room is equipped with a real-time monitoring system. The real-time monitoring system consists of software and hardware that can implement the collection, processing, analysis and visualization of measurement data. Fig. 4 provides an overview of the real-time monitoring system. The SDL (Soam Date Logger) interface is a storage and transmission device that allows a large number of measuring instruments to be connected to a computer, enabling efficient control of various measuring instruments. The real-time monitoring system can store the data of measuring instruments embedded in the disposal cover, check it in real-time and visualize it in 3 dimensions. In addition, the real-time monitoring system predicts the safety of the performance test facility by using measurement data and artificial intelligence techniques.
2.3 Specifications of Soil Materials Used in Actual Construction
During construction, Challenges arose in procuring soil materials, particularly pea gravel and clay, that met the specified physical properties for design requirements. Consequently, pea gravel was substituted with a material closely resembling the desired specifications. Also a mixture of bentonite and sand, possessing properties comparable to those of clay, was utilized as a substitute for clay.
Prior to construction, tests of soil materials used in the performance test facility were conducted on items related to design requirements. Table 3 Shows the test results of each soil material.
Table 3
Test results of multi-layer soil materials
| Soil material | Suitable or unsuitable for meeting the design requirements | Test results |
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| Silty sand | Suitable | |
| Gravelly sand | Suitable | |
| Pea gravel | Unsuitable | |
| Sand | Suitable | |
| Clay | Unsuitable |
Following the tests, it was found that silty sand, gravelly sand and sand were suitable for the design requirements. However, Pea gravel with a smaller particle size than the design requirements was used and the hydraulic conductivity of clay exceeded the proposed design requirements.
Given that there are significant differences in the hydraulic conductivities of clay, crucial components of the barrier layers responsible for preventing rainfall infiltration in the disposal cover, it is necessary to evaluate this difference impact on water infiltrating into the performance facility.
3. Numerical Simulation of Disposal Cover in Performance Test Facility
This study aims to evaluate the effect of differences from the specifications presented in the design of the performance facility on the performance objective of disposal cover related to water infiltration from rainfall. Therefore, rainfall infiltration evaluation was performed for two cases: One case was configured to meet the design requirements (Designed case) and the other was configured with actual properties used in the construction of the performance test facility (Actual Case).
To fulfill this evaluation for each case, a two-dimensional numerical model of the performance test facility was constructed and the behavior and amount of water flowing into the performance test facility was analyzed. The numerical simulation utilized FEFLOW (Finite Element subsurface FLOW) employing the finite element method to compute fluid flow in saturated and unsaturated porous media.
3.1 Governing Equations
The governing equation for fluid, solved in FEFLOW to simulate flow in various porous media and unsaturated zone, is Richard’s equation [8]. The present finite-element model is based on Richard’s equation written in the following form which has to be solved either for ψ or s.
In equation 1, s(Ψ) represents saturation, ψ is pressure head, t is time, So is specific storage due to fluid and medium compressibility, ε is porosity, Kr (Ψ) is relative hydraulic conductivity Κ is tensor of hydraulic conductivity for the saturated medium, χ is buoyancy coefficient including fluid density effects, e is gravitational unit vector, Q is specific mass supply.
Moreover, constitutive relationships are needed to describe the interconnection between saturation and pressure head or the correlation between pressure and saturation, and to detail how relative hydraulic conductivity varies with either saturation or pressure head. The following empirical relationships are used in FEFLOW. Equation 2 represents the van Genuchten-Mualem parametric model, where Se, Sr and Ss correspond to the effective saturation, the residual saturation and the maximum saturation, respectively.
In equation 3, corresponding to the Brooks-Corey parametric model, kr denotes relative conductivity and α, n and m are the coefficients of the van Genuchten law [9].
3.2 Conceptual Model of Numerical Simulation
To numerical simulation for two case, a domain for the numerical analysis model was established and the conceptual model is depicted in Fig. 5. As shown in the Fig. 5, the multi-layer has a horizontal and symmetrical shape, the behavior of water infiltrating into the performance test facility was assumed to be vertical and symmetrical. Therefore, a two-dimensional conceptual model was set up for a full-scale central cross section of the performance test facility with a symmetrical structure. This domain includes multi-layer, backfill and an unsaturated zone at the base. Since this simulation focus on water infiltration behavior along the vertical direction, a concrete retaining wall installed to maintain structural integrity was excluded from the numerical simulation. The multi-layer was composed of silty sand, gravelly sand, pea gravel, clay and sand. The filling material in backfill, located in between the multi-layer and the unsaturated zone, is silty sand. The total area of model spans 213.06 m2, with a width and height of 27 m and 10.14 m, respectively.
3.3 Boundary Conditions and Input Parameters
To set the rain conditions, the rainfall data for 30 years of weather stations located near the LILW disposal center was applied. The recharge amount excluding evapotranspiration and surface runoff from rainfall was assumed to penetrate through the disposal cover. The estimated amount was 469 mm·year−1, which was accounted for 36.5% of the average annual precipitation (1,287 mm·year−1). The top boundary condition of the domain was defined by the rainfall reflected recharge rate. On the other hand, the bottom of the domain was specified as a constant head boundary condition assuming that the groundwater table exist 70 m below the surface, while the remaining boundaries were designated as no-flow conditions. Fig. 6 illustrates the boundary conditions applied to numerical simulation. The data used to for the boundary conditions setting were based on the values derived through investigation and analysis in the 2nd Near- Surface Disposal Facility safety analysis report.
To reflect the differences from the specifications presented in the design of the performance facility in numerical simulation, the hydraulic conductivity applied to the Actual Case corresponds to the experimental value derived from the soil materials used in the construction of the performance test facility. Other conditions are the same for both cases. The input parameters utilized in the numerical simulation are detailed in Table 4 [10].
Table 4
Hydraulic properties of two cases
| Water content | Van Genuchten parameters | Hydraulic conductivity | ||||
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| Residual [-] | Saturated [-] | α [m−1] | n [-] | Designed Case [m·s−1] | Actual Case [m·s−1] | |
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| Silty sand | 0.1 | 0.47 | 4.4 | 1.523 | 1.00×10−6 | |
| Gravelly sand | 0.2 | 0.32 | 10.08 | 2.922 | 1.00×10−4 | |
| Pea gravel | 0.03 | 0.26 | 469.5 | 2.572 | 1.00×10−2 | |
| Sand | 0.045 | 0.37 | 6.83 | 2.08 | 3.00×10−4 | 3.4×10−3 |
| Clay | 0.0001 | 0.36 | 0.13 | 1.203 | 1.00×10−9 | 1.87×10−8 |
| Backfill | 0.1 | 0.47 | 4.4 | 1.523 | 1.00×10−6 | |
3.4 Mesh of Numerical Model
Meshes were generated using triangular elements of various sizes, ranging from fine to coarse, to accurately represent the performance test facility. The model domain is divided into 10 zones, mirroring the design and allowing for the application of distinct characteristics to each area. As illustrated in Fig. 7, The mesh configuration comprises 10,386 elements and 5,294 nodes. Table 5 provides statistical information regarding the meshes. The mesh applied in the Designed Case and Actual Case and was set identically.
Table 5
Statistical information on the mesh
| Elemental area [m2] | Elemental diameter [m] | |
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| Number of elements | 10,386 | |
| Number of nodes | 5,294 | |
| Minimum | 0.00349386 | 0.0688959 |
| Maximum | 0.121371 | 0.529336 |
| Mean | 0.0205146 | 0.195413 |
| Standard deviation | 0.0171111 | 0.0728401 |
4. Results of Numerical Simulation
The effect of differences between the design specifications (Designed Case) and the actual values (Actual Case) of the performance test facility on water infiltration from rainfall were analyzed using numerical simulation results. This analysis involved examining the distribution of saturation and estimating the flux of water infiltration. The simulation focused on understanding the behavior of water infiltration over the post-closure period of to 300 years. Maintenance and repair activities were accounted for throughout this duration. Consequently, the study assumed that the structural integrity remained intact, considering there would be no degradation of soil materials composing multi-layer in both cases.
4.1 Distribution of Saturation
Fig. 8 illustrates the change of saturation distribution over a 300-year period (at intervals of 0, 75, 125, 225 and 300 years) for both the Designed and Actual cases. Through the distribution of saturation over time the behavior of water infiltration flowing into the performance test facility can be understood. Both cases exhibit nearly identical outcomes.
Over the 300-years period, rainfall gradually infiltrates into the performance test facility, primarily accumulating in the sand layer at the lowermost part of the drainage layer (comprising of gravelly sand, pea gravel and sand). Subsequently, the majority of the water infiltration laterally flows along the slope of the multi-layer.
A significant portion of the water infiltration is directed towards the upper slope terminus of the first clay layer, functioning as a hydraulic barrier.
In the Designed Case, the area where saturation changes under the disposal cover due to lateral drainage along the slope appears broader. Additionally, the saturation of the backfill in the Designed Case surpasses that of the Actual Case. This observation can be attributed to reduced lateral drainage, likely stemming from the fact that the hydraulic conductivity of the sand and clay utilized in the Actual Case is approximately ten times higher than that applied in the Designed Case.
4.2 Flux of Water Infiltration
To ascertain the detailed behavior of water infiltration in both cases, a quantitative analysis of the water infiltration was conducted. The quantitative analysis focused on a specific region surrounding the clay layer, known for its significant influence on the performance objectives of water infiltration. This region was delineated to include three interfaces within the multi-layer system: the initial interface between the first clay layer and the second sand layer, the subsequent interface between the second sand layer and the second clay layer, and the final interface between the second clay layer and the backfill. The flux of water infiltration was calculated by dividing the quantity of water infiltration by the effective area of each interface, with a consistent effective area length of 12.51 meters for all interfaces. Fig. 9 provides a visual representation of these interfaces where the flux of water infiltration was computed.
Detailed results of the flux and the corresponding amount of water infiltration derived from each interface are presented in Table 6.
Table 6
Calculation results of infiltration water observed for 300 years
| Interface | 1 | 2 | 3 | |||
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| Infiltration water | Amount [m3] | Flux [mm·year−1] | Amount [m3] | Flux [mm·year−1] | Amount [m3] | Flux [mm·year−1] |
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| Designed Case | 28.99 | 7.73 | 25.96 | 6.92 | 22.89 | 6.10 |
| Actual Case | 298.18 | 79.47 | 263.91 | 70.33 | 230.96 | 61.55 |
The flux of water infiltration exhibits a consistent trend in both cases, showing a decrease towards the lower sections of the performance test facility. Over the 300-year period, the estimated flux of water infiltrating into the first sand layer was 7.73 mm·year−1 in the Designed Case and 79.47 mm·year−1 in the Actual Case. Similarly, the flux into the second clay layer was 6.92 mm·year−1 in the Designed Case and 70.33 mm·year−1 in the Actual Case, while the flux into the second sand layer was 6.10 mm·year−1 in the Designed Case and 61.55 mm·year−1 in the Actual Case.
In the Designed Case, these flux values satisfied the performance objectives (32 mm·year−1) for the disposal cover. However, in the Actual Case, the flux values exceed the threshold of 32 mm·year−1. These differences in water infiltration between the two cases can be attributed to variations in the hydraulic conductivity of sand and clay. Specifically, the hydraulic conductivity applied in the Actual Case is approximately ten times higher than that used in the Designed Case. Consequently, the amount of water infiltration in the gravitational direction at each interface is roughly ten times greater in the Actual Case compared to the Designed Case.
5. Conclusion
This study is focused on evaluating the effect of differences from the specifications presented in the design of the performance facility on the performance objective of disposal cover related to water infiltration from rainfall. To conduct this rainfall infiltration evaluation, numerical simulations were executed for two cases: one using the design-specified material properties (Designed Case), and the other reflecting the actual construction materials (Actual Case). Numerical simulations were conducted over a 300-year post-closure period using FEFLOW. The hydraulic conductivity of sand and clay applied to the Actual Case corresponds to the experimental value from the construction materials and all other conditions and input data applied to two cases were the same.
The results over 300 years showed similar water infiltration behavior. In the Actual Case, there was an influx of approximately 10 times more water infiltration into each layer interface compared to the Designed Case. This difference can be attributed to the hydraulic conductivities of sand and clay, pivotal factors influencing the behavior of water infiltration, being approximately ten times greater in the Actual Case. Based on these findings, when the multilayer of disposal cover functions as intended, water mainly flows laterally through the sand layer over the clay layer, reaching the unsaturated zone. Hydraulic conductivity, particularly of the clay layer acting as a barrier, significantly affects water infiltration.
This facility was constructed not only to confirm that it meets the performance objective related to water infiltration but also to verify the feasibility, constructability, stability and so on of the multi-layer for the disposal cover. Therefore, it was built even though the multi-layer soil materials associated with the water infiltration did not meet the design requirements. However, when constructing the disposal cover for the closure of a near-surface disposal facility with radioactive wastes, it becomes imperative to identify soil materials that meet the specific requirements of not solely for layers related to water infiltration, but also for the other layers.
Additionally, Planning the project schedule to address potential material supply issues is crucial, considering the increased quantity of soil materials needed.
In this study, only hydraulic conductivities of sand and clay from experiments were utilized in numerical simulations and the insufficient measurement data from the performance test facility limited its application in numerical simulation. For a future work, it is expected that more realistic and accurate predictions of rainfall infiltration will be possible by applying various experimental values for soil materials. Additionally, it would be better to compare results from numerical simulation using the rainfall measurement data at the performance test facility and the results with measurement data related to rainfall infiltration.
The numerical model developed in this study can analyze the water behavior flowing into the disposal cover using various hydraulic properties from multi-layer. Therefore, in the future, it is expected to be utilized in a variety of ways, such as evaluating various rainfall scenarios and reflecting hydraulic properties from experiments. The construction and operation insights gained from the performance test facility, along with the findings of this study and the measurement data collected during its operation, will be utilized for both the construction and operation of the 2nd near-surface disposal facility.
