This section focuses on presenting the results based on the specified steps defined in the workflow. The base case uses the hydrogeological conceptual model derived from the reference case in Taiwan and an example DFN generated according to the site-specific DFN recipe. The sensitivity analysis considers the steady-state groundwater flow and the associated particle traces for the PMs. An additional 48 DFN realizations are employed to conduct the uncertainty analysis. We focus on the Q1 path for all the assessments of the PMs.
4.2. Sensitivity of Hydrogeological Units and EDZ on Flow, Particle Tracking, and PMs
Figure 11 shows the plane and profile views for the steady-state flow and particle traces.
Figure 11a presents the base case results, i.e., the same results as described in
Figure 9a. We use the base case as the basis for the qualitative comparisons between the base case and hydraulic conductivity sensitivity cases.
Figure 11b,c are the hydraulic conductivity sensitivity cases for R0, i.e., the regolith.
Figure 11b shows the hydraulic conductivity decreases to 5.0 × 10
−6 m/s, and
Figure 11c presents the hydraulic conductivity increases to 1.0 × 10
−4 m/s.
Figure 11d,e are the hydraulic conductivity sensitivity cases for R#, i.e., the host rock.
Figure 11d shows the hydraulic conductivity decreases to 1.0 × 10
−12 m/s, and
Figure 11e presents the hydraulic conductivity increases to 1.0 × 10
−9 m/s.
Figure 11f,g are the hydraulic conductivity sensitivity cases for F#, i.e., the fault and fracture zones.
Figure 11f shows the hydraulic conductivity decreases to 1.0 × 10
−8 m/s.
Figure 11g presents the hydraulic conductivity increases to 1.0 × 10
−4 m/s.
Figure 11h,i are the hydraulic conductivity sensitivity cases for EDZ.
Figure 11h shows the case with hydraulic conductivity decreasing to 3.3 × 10
−9 m/s.
Figure 11i results display the hydraulic conductivity increasing to 3.3 × 10
−7 m/s.
Figure 12 shows the CDF of Q
eq and F for cases of hydraulic conductivity sensitivity study.
Figure 12a shows Q
eq of the hydraulic conductivity sensitivity cases. For R0 cases, the decrease of hydraulic conductivity of R0 (i.e., R0_5.0 × 10
−6 m/s) makes all of Q
eq larger than those obtained from the base case by 1.03 times. The minimum Q
eq is 4.69 × 10
−5 m
3/year, while the most significant value is 9.01 × 10
−5 m
3/year. The result indicates that the lower hydraulic conductivity for R0 has no considerable impact on Q
eq. On the other hand, the increased hydraulic conductivity of R0 (i.e., R0_1.0 × 10
−4 m/s) makes all of Q
eq lower than those obtained from the base case from 1.43 to 3.45 times. The minimum Q
eq is 1.32 × 10
−5 m
3/year, while the maximum value is 6.08 × 10
−5 m
3/year. The result indicates that the higher hydraulic conductivity for R0 could lead to a higher flow rate from the top boundary to the repository depth than those of the base case. This behavior makes the Q
eq values higher than those obtained from the base case. In addition, the deviation between the smallest and largest values is 4.61 times greater than that in the base case.
For R# cases, the decrease of hydraulic conductivity of R# (i.e., R#_1.0 × 10−12 m/s) makes all of Qeq smaller than those obtained from the base case. The minimum Qeq is 5.88 × 10−6 m3/year, while the maximum value is 4.12 × 10−5 m3/year. The result indicates that the lower hydraulic conductivity for R# considerably decreases the flow rate in the repository region. On the other hand, the increased hydraulic conductivity of R# (i.e., R#_1.0 × 10−9 m/s) leads all of Qeq to be higher than those obtained from the base case from 2.19 to 3.28 times. The minimum Qeq is 1.49 × 10−4 m3/year, while the maximum value is 1.91 × 10−4 m3/year. The result indicates that the higher hydraulic conductivity for R# could lead to a higher flow rate in the repository region. Specifically, all values are more significant than the safety function indicator for the acceptable hydrogeological condition in the geosphere (i.e., smaller than 1.0 × 10−4 m3/year).
For the F# cases, whether the hydraulic conductivity for F# is lower or higher than the value of the base case (i.e., 5.0 × 10−6 m/s), most Qeq values are lower than those of the base case. When the hydraulic conductivity of F# is lower than the base case value, the minimum Qeq is 4.55 × 10−5 m3/year, while the maximum value is 7.39 × 10−5 m3/year. The result indicates a behavior similar to that in the base case. When the hydraulic conductivity is larger than the recommended value, the lowest Qeq is 3.76 × 10−5 m3/year, while the highest value is 6.07 × 10−5 m3/year. The result indicates that the increased hydraulic conductivity of F# makes all Qeq smaller than those obtained from the base case, varying from 1.21 to 1.44 times. The flow field in the repository region is toward the north, northeast, and northwest, according to the top boundary and terrain. Further, the repository is far from the F#, and the influence might not be significant once the hydraulic conductivity for F# is changed.
For EDZ cases, whether the hydraulic conductivity for EDZ is lower or higher than the value of the base case (i.e., 3.3 × 10−8 m/s), all Qeq are similar to those obtained from the base case. When the hydraulic conductivity of EDZ is lower than the base case value, the minimum Qeq is 4.62 × 10−5 m3/year, while the maximum value is 7.67 × 10−5 m3/year. The result indicates a minor influence of the EDZ hydraulic conductivity on the flow. When the hydraulic conductivity is larger than the value of the base case, the minimum Qeq is 4.48 × 10−5 m3/year, while the maximum value is 7.53 × 10−5 m3/year. The result shows that the increase of hydraulic conductivity of EDZ makes all Qeq similar to those obtained from the base case. Note that the discussion only focuses on Qeq for the Q1 path. Once the target switches to another releasing path based on the KBS-3 disposal concept, Qeq results might be different.
Figure 12b shows F for the hydraulic conductivity sensitivity cases. For R0 cases, the decrease of hydraulic conductivity of R0 (i.e., R0_5.0 × 10
−6 m/s) makes most of F slightly smaller than those obtained from the base case up to 1.43 times. However, only the minimum F (2.54 × 10
6 year/m) is slightly larger than the base case at 1.01 times. The maximum value (6.15 × 10
7 year/m) is higher than that obtained from the base case up to 1.29 times. The result indicates that the lower hydraulic conductivity for R0 has less impact on F. On the other hand, the increased hydraulic conductivity of R0 (i.e., R0_1.0 × 10
−4 m/s) makes F larger than those obtained from the base case, and the values vary from 2.30 to 15.92 times as compared to those obtained from the base case. The minimum F is 5.78 × 10
6 year/m, while the maximum value is 7.61 × 10
8 m
3/year. The result indicates that the higher hydraulic conductivity for R0 might lead to a higher flow rate from the top boundary to the repository depth. The behavior reduces the particle transport velocity from the repository to the top of the model domain. In addition, the deviation between the smallest and largest values is 131.60 times, reflecting the high variations between the traveling traces of particles.
For R# cases, the decrease of hydraulic conductivity of R# (i.e., R#_1.0 × 10−12 m/s) makes F larger than those obtained from the base case, and the differences vary from 6.00 to 12.37 times. The minimum F is 1.51 × 107 year/m, while the maximum value is 5.91 × 108 year/m. The result indicates that the lower hydraulic conductivity for R# considerably decreases the flow rate in the entire numerical domain. It causes a lower travel velocity for each particle and makes considerable deviations in the particle traces. The increased hydraulic conductivity of R# (i.e., R#_1.0 × 10−9 m/s) leads to F values smaller than those obtained from the base case, and the differences vary from 2.05 to 3.24 times. The minimum F is 7.77 × 105 year/m, while the most significant value is 2.33 × 107 year/m. The result indicates that the higher hydraulic conductivity for R# makes the flow rate higher than that obtained from the base case. The relatively high flow rate could reduce the transport time for each particle from the repository to the biosphere. Although all the values are higher than the safety function indicator for the acceptable hydrogeological condition in the geosphere (i.e., higher than 1.0 × 104 year/m), several values are smaller than the 1.0 × 106 year/m, which is relatively low compared to the other cases in this study.
For F# cases, whether the hydraulic conductivity for F# is higher or lower than the value of the base case (i.e., 5.0 × 10−6 m/s), most F values are higher than those obtained from the base case. When the hydraulic conductivity of F# is lower than the value of the base case, the minimum F is 1.41 × 106 year/m, while the largest value is 6.17 × 107 year/m. The result indicates that 20% of F values are slightly smaller than those in the base case, while 80% of F values are slightly larger than those in the base case. When the hydraulic conductivity of F# is larger than the value of the base case, the minimum F is 4.29 × 106 year/m, while the maximum value is 6.83 × 107 year/m. The result indicates that the increased hydraulic conductivity of F# makes F slightly larger than those obtained from the base case, and the differences vary from 1.43 to 1.71 times for 80% of F values. The result could be that the flow field in the repository region is toward the north, northeast, and northwest, according to the top boundary and terrain. Besides, the repository is far from the F#. Therefore, the influence might not be significant when the hydraulic conductivity for F# is changed.
For EDZ cases, whether the hydraulic conductivity for EDZ is lower or higher than the value of the base case (i.e., 3.3 × 10−8 m/s), most F values are very similar to those obtained from the base case. When the hydraulic conductivity of EDZ is lower than the value of the base case, the minimum F is 2.24 × 106 year/m, while the most significant value is 5.33 × 107 year/m. The result indicates that 60% of F values between fractions from 25% to 95% are slightly smaller than those of the base case, while 25% of F values between fractions from 0% to 25% are very similar to those of the base case. When the hydraulic conductivity of EDZ is higher than that of the base case, the minimum F is 1.88 × 106 year/m, while the maximum value is 5.29 × 107 year/m. The result shows that the increased hydraulic conductivity of EDZ makes F values slightly smaller than those obtained from the base case, but the maximum value is higher than those obtained from the base case.
This study evaluated the sensitivity induced by the hydraulic conductivity variations for the hydrogeological units (including R0, R#, F#) and EDZ.
Table 4 lists the maximum and minimum values of Q
eq and F for the base case and cases with hydraulic conductivity sensitivity analysis. The results show that the hydraulic conductivity of R0 dominates Q
eq in the repository area. Specifically, Q
eq will not fulfill the safety function indicator for the acceptable hydrogeological condition in the geosphere when the hydraulic conductivity of R# is 1.0 × 10
−9 m/s. Although all F values are higher than the safety function indicator for the acceptable hydrogeological condition in the geosphere (i.e., higher than 1.0 × 10
4 year/m), there are few values smaller than the 1.0 × 10
6 year/m, which is relatively low in this study.