Sensitivity Analysis of Simulating Radionuclide Decay Transport of Low-level
Radioactive Waste in Nearshore Environment
Yu-Ru Chen1, Chin-Chang Lu1, Ming-Hsu Li1
1

Graduate Institute of Hydrological and Oceanic Sciences, National Central University, Taiwan
1#300 Jhongda Rd., Jhongli, Taoyuan City, Taiwan

Key words: Low-level radioactive waste, Sensitivity analysis

Abstract
Tunnel disposal at nearshore environment is one of potential final disposal sites proposed for the
low-level waste in Taiwan. The proposed tunnel disposal facilities were located roughly 800 m awayfrom the shoreline and 50 m below sea levels. Geosphere parameters, sea levels, and infiltration rates
are critical to radionuclide decay transports in far-field environments. A radionuclide decay chain of4N+2, including Pu-238  U-234  Th-230  Ra-226, was simulated by the 3-D
HYDROFEOCHE5.6 numerical model for sensitivity analysis of far-field radionuclide decay transport
for 100,000 years with steady flow and transient transport. Hydraulic conductivities, diffusion
coefficients, and dispersivities were selected for sensitivity analysis of geosphere parameters. With 10times higher of hydraulic conductivities than reference values, the Pu-238 concentrations are higher
and peak concentration appear earlier at shorelines. However, the Ra-226 concentrations simulatedwith 10 times higher/lower hydraulic conductivities are all lower than those with reference values atshorelines. Higher hydraulic conductivities will favor migration of mother radionuclides of Pu-238,U-234, and Th-230 before decaying to Ra-226 at shorelines, while 10 times lower of hydraulicconductivities will hinder migrations of all radionuclides considered in this study. Sensitivities ofdiffusion coefficients on radionuclide decay transport are not significant in this study. Cases with 10times higher/lower dispersivities than reference values deliver higher/lower concentrations for all
radionuclides at shorelines. Considering radionuclide decay transport in association with changes ofgeosphere parameters are much more complicated than single species simulations. Changes of sea
levels and infiltration rates are designed to investigate changes boundary conditions on radionuclide
decay transport through changes of fields. Reliable site investigations are crucial to reliabilities of
far-field radionuclide transport simulations.

1. Introduction
Nuclear power generation is one of Taiwan's major power resources, second only to thermalpower generation. Although relatively less greenhouse gases emitted by nuclear power generations, theresulting radioactive wastes produced great environmental concerns and debates in safety of wastemanagements. Geological waste repositories have been constructed to isolate radioactive wastes fromthe environments in many countries. After regional surveys and preliminary site investigations, the6th East Asia Forum on Radwaste Management ConferenceNovember 27-29, 2017, Osaka, Japan

Ministry of Economic Affairs officially announced two potential sites, One in Daren Township locatedin southeast Taiwan and another in Wuciou Township located in an offshore island of Kinmen, selected
for the final disposal of low-level radioactive waste in Taiwan (2012). Confidences and acceptances ofwaste final disposal in designed repositories require demonstrations of long-term safety with numericalsimulations. The lack of knowledge and other uncertainties in the calculation conditions may create
various uncertainties due to scenario uncertainty, system uncertainty, model uncertainty, parameter
uncertainty, and spatial variations in parameters (SKB, 2014). In this study, we mainly focus ongeosphere parameters of the Daren site by performing sensitivity analysis to investigate influences ofgeosphere parameter uncertainties on radionuclide decay transports in far-field geosphere. Waste
repository of the Daren case was designed with multiple engineering barriers of tunnel disposal instable formations isolated from human inhabitants. Sensitives of geosphere parameters, sea levels, andinfiltration rates on far-field radionuclide decay transport are simulated for 100,000 years by a special
version of a 3-D HYDROGEOCHEM5.5 (Yeh et al., 2009) A radionuclide decay chain of 4N+2 series,including Pu-238 → U-234 → Th-230 → Ra-226, is considered with simulations of steady flow andtransient decay transport. The evaluation of parameter sensitivities is an important part of the safetyassessment to identify individual parameters with a strong influence on radionuclides migrations andsupport the need of detailed site investigations.

2. Methodology
The digital elevation model delineated around the Daren waste repository was shown in the top
left of Fig. 1 (left) and discretized with 5970 nodes and 4749 hexahedral elements. The disposal tunnel
having a dimension of 1200 m in length, 800 m in width, and 20 m in height, roughly 800 m away fromthe shoreline and 50 m below current sea levels, was considered as an internal source of releasing
radionuclides. A radionuclide decay chain of 4N+2 series, including Pu-238 → U-234 → Th-230 →Ra-226, was simulated with initial concentrations of unity for Pu-238 and zero for the three daughter
radionuclides. Flow and transport parameters given for the reference case were listed in Table 1. Fourobservation points at different surface locations as show in Fig. 1 (right) were selected to compare therelease of radionuclides in terms of peak concentration values and peak concentration times.
The 3-D HYDROFEOCHE5.5 (Yeh et al., 2009) is a three-dimensional finite element numericalmodel capable to simulate coupled fluid flow, thermal transport, and hydrogeochemical transport
through variable saturated subsurface. A special version of HYDROGEOCHEM5.5, namely
HYDROGEOCHEM5.6, with improved capability of handling decay chains was used in this study for
simulations of steady flow and transient decay transport for 100,000 years. Sensitivities of geosphereparameters were examined with simulation settings as the following,
1.

Changing hydraulic conductivities with values of 10 times (or 0.1 times) to the reference values

2.

Changing diffusion coefficients with values of 10 times (or 0.1 times) to the reference values

3.

Changing longitudinal and lateral dispersivities with values of 10 times (or 0.1 times) to thereference values

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

The second part of sensitivity test involves combinations of changes in infiltration rates, sealevels, and hydraulic conductivities as shown in Table 2. The infiltration rate in the reference case wasset as 3% of annual rainfalls. We increased the infiltration arte to 10% of annual rainfall as the case ofelevated seepages due to changes in precipitation or landuse. Following the studies by the Sinotech
Engineering Consultants, LTD (2013), gradually decreased of sea levels to 66 m below current position
was considered. Therefore, there are 12 simulation cases (i.e., 2 infiltration rates, 2 sea levels, and 3hydraulic conductivities) examined in this study as listed in Table 2.

CDBA
sea level

Figure 1 Elements and scales of far-field simulations in plan view (left) and selected observation points
in vertical view (right)

Table 1 Parameters given in the reference case
Simulation setting
Vertical layer

9Height

25~85(m)Horizontal layer

529

Length & width200(m)*200(m)Total nodes

5970

Total elements

4749

Flow parameters
k=kx

layer1layer2-2-3

3*10 (m/day)yklayer3-5

3*10 (m/day)

3*10 (m/day)-2-3z

8.9*10 (m/day)

8.9*10 (m/day)

3*10 (m/day)-5

Porosity

0.08

Infiltration rate

0.0003(m/day)Transport

Longitudinal dispersivity

800(m)

parameters

Lateral dispersivity

200(m)

Diffusion coefficient10-4(m2/day)Table 2 Lists of combined sensitivity tests with changes of infiltration rates, sea level, and hydraulicconductivities
Case

1234

56789101112Infiltration rates

3%3%3%10%10%10%3%3%3%10%10%10%

Sea levels change╳╳╳╳╳╳○○○○○○HydraulicConductivities(times)

0.1

1

100.1

1

100.1

1

100.1

1

10

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan3. Results and Discussions
Figure 2 presents comparisons of relative concentrations at surface Point A with different
hydraulic conductivities. Concentrations and peak arrival times of Pu-238, U-234, Th-230 with 10times hydraulic conductivities are higher and faster, respectively, than those at Point A of the referencecase. However, concentrations of Ra-226 with 10 times hydraulic conductivities are smaller than thoseat Pont A of the reference case due to fast migration of mother radionuclides. Differences inconcentrations as changes in hydraulic conductivities observed at Points B, C, and D, are moresignificant than those observed at Point A due to those locations are close to repository tunnel. As well,effects of changes in hydraulic conductivities on peak arrival times at Points B, C, and D are lesssignificant than those observed at Point A. Effects of changes in diffusion coefficients are insignificant
in all cases and observation points as the example depicted for Point A in Fig. 3. Figure 4 presents
Comparisons of relative concentrations at surface Point A with different dispersivities. Concentrations
and peak arrival times of all radionuclides with 10 times dispersivities are higher and faster,
respectively, than those at Point A of the reference case.
The second part of sensitivity simulations involves combinations of changes in infiltration rates,
sea level, and hydraulic conductivities. Figure 5 presents comparisons of relative concentrations ofPu-238 simulated with 12 cases observed at surface Point A. At Point A, changes in hydraulicconductivities have the most significant effects on concentrations and peak arrival time of Pu-238
among both factors of changes in infiltration rates and sea levels in this study. Although changes ininfiltration rates to 10% of annual precipitations may increase seepage rates favoring radionuclide
migrations, the increased infiltrated water, on the other hand, may dilute radionuclide concentrations aswell. Changes in sea levels lower than current positions cause the surface Point A became drier than thereference case, which may hinder radionuclide migration due to less saturated conditions.

Figure 2 Comparisons of relative concentrations at surface Point A with different hydraulicconductivities

Figure 3 Comparisons of relative concentrations at surface Point A with different diffusion coefficients

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Figure 4 Comparisons of relative concentrations at surface Point A with different dispersivities

Figure 5 Comparisons of relative concentrations of Pu-238 simulated with 12 cases observed at surface
Point A

4. Conclusion
This study performs numerical simulations to investigate sensitivities of geosphere parameters,
including hydraulic conductivities, diffusion coefficients, and dispersivities, on far-field radionuclide
decay transports of the potential low-level disposal site in Taiwan. Among three geosphere parameters
examined, hydraulic conductivities are the most important parameters affecting radionuclide decaytransport in views of changes in peak concentrations and peak arrival times. However, increased inhydraulic conductivities may have opposite effects on daughter radionuclide than those on motherradionuclide due to fast migrations of mother radionuclides reaching outside of domain. We further
designed 12 simulation cases with combinations of changes in infiltration rates, sea levels, andhydraulic conductivities. Among three variables examined in the 12 simulation cases, changes inhydraulic conductivities have the most significant effects on radionuclide decay transports than those ofconsidering changes in infiltration rates and sea levels. Increases in infiltration rates may diluteradionuclide concentrations. Decreases in sea levels may cause surface points of observations became
drier than those in the reference sea levels, which may hinder migrations of radionuclides. The
evaluation of uncertainties is an important part of every safety assessment. Uncertainties should be
discussed and examined in depth through sensitivity simulations of selecting calculation cases. This
study only stands as a very preliminary step on sensitivity analysis of radionuclide decay transport inone of potential low-level repository in Taiwan. Further investments including site investigations andmodelling practices are crucial to the confidence and acceptance of our final disposal site.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan5. References
1.

Ministry of Economic Affairs, R.O.C., 2012, Announcement of suggested potential final disposalsites for low-level waste. (http://www.llwfd.org.tw/notice_view.aspx?id=422, in Chinese)2.

Sinotech Engineering Consultants, LTD., 2013, Assessment report of feasible techniques for thefinal disposal of low-level waste (in Chinese)

3.

SKB, Safety analysis for SFR Long-term safety: Main report for the safety assessment SR-PSU,
TR-14-01, 2014.

4.

Taiwan Power Company, 2016, Final Disposal Plan for Low-level Radioactive Waste,
LLWD1-SC-2016-02-V01.

5.

Yeh, G. T., J. T. Sun, P. M. Jardine, W. D. Burger, Y. L. Fang, M. H. Li, and M. D. Siegel, 2009.
HYDROGEOCHEM 5.5: A Three Dimensional Model of Coupled Fluid Flow, ThermalTransport, and HYDROGEOCHEM ical Transport through Variably Saturated Conditions
Version 5.5. Dept. of Civil and Environ. Engineering, University of Central Florida, 4000 Central
Florid Blvd, Orlando, FL 32816.

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan