An Integrated Framework for Simulating Radionuclide Decay Transport ofLow-level Radioactive Waste with Tunnel Disposal in Nearshore Environment
Chin-Chang Lu1, Ming-Hsu Li*1, Yu-Ru Chen1
1Graduate Institute of Hydrological and Oceanic Sciences, National Central University#300 Jhongda Rd., Jhongli, Taoyuan City, Taiwan

Key words: Tunnel disposal, Nearshore, Radionuclide decay transport, Low-level waste
AbstractTunnel disposal at nearshore environments was tentatively proposed as one of final disposal sites
for low-level wastes in Taiwan. The disposal facilities was designed with several disposal tunnels andlocated at 50m~200m below ground surface. Safety functions adopted are containment and retardation
provided by multiple engineered barriers and host rock to prevent leaching of radionuclides intobiosphere. An integrated simulation framework was developed with three levels of finite element grids
to simulate radionuclide decay transport with 3-D HYDROGEOCHEM5.6 numerical models. Steadyflow and transient transport were performed in this study. The coarsest-level grids serve covers thefar-field simulation domain. The middle-level grids described layouts of disposal tunnels serve as gridsof near-field facilities. The finest-level grids were developed to investigate radionuclide migrations atselected tunnel sections. Total radioactive activities of source term and a total of 11 radionuclides(Pu-238, Am-241, Tc-99, Co-60, I-129, Cs-137, Ni-59, C-14, Sr-90, Y-90, Ni-63) with decay chains asgiven in the report of Taiwan Power Company were simulated in this study. Flow boundary conditions
at finer-level grids were obtained from flow simulations of coarser-level grids. Release rates ofradionuclides at coarser grids were obtained from simulations of radionuclide decay transports at finergrids. For the first 100 years, release rates of C-14, Tc-99, and I-129 are higher than the others. After
100 years, release rates of Ni-59, C-14, and Tc-99 are higher than the others. Owing to high sorption ofNi-59 onto bentonite barriers, long-lived Ni-59 with high initial concentration produced low release
rates for the first 100 years. Low release rate of I-129 after 100 years was caused by low initialconcentration of I-129. Results demonstrate complicated interactions among initial concentrations,half-life, decay chains, near-field processes, and far-field processes on radionuclide decay transport.

1.

Introduction
The first nuclear power plant in Taiwan has been operated since 1978. To date, we have a total offour nuclear power plants with three in operations accounting for 13.5% of annual energy generationsin 2016 (TPC web site). The highest percentage of nuclear power in annual energy generations was
52.4% in 1985 and has been gradually reduced to below 20% after 2005. More than 60% of today’s
energy is provided by thermal power generation with coal and gas. Although relatively less greenhousegases were emitted by nuclear power generations, environmental concerns and debates on how theproduced radioactive waste can be properly handled remains great channelings to waste management

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japancommunities. As the government has been committed to phasing out nuclear power in Taiwan by 2025,
the decommission project of the first new clear power plane will be deployed in 2019 and completed by
the end of 2043. Two potential sites for final disposals of low-level wastes (LLW) were officiallyannounced in 2012, one in the Daren Township located in southeast Taiwan and another one in the
Wuciou Township located in an offshore island of Kinmen (Ministry of Economic Affairs, 2012).
Although difficulties of having public votes for site approvals have been encountered for years,
scientific and technical studies of feasibilities on building LLW final disposal facilities have never beensuspended. To both the general public and experts, confidences and acceptances of waste final disposal
repositories require demonstrations of long-term safety with comprehensive numerical simulations.
Near-surface or tunnel disposal is frequently adopted for intermediate and low-level wastes inmany countries, such as UA, France, and USA. Tunnel disposal was tentatively designed for the LLW
in Taiwan. In this study, we proposed an integrated simulation framework having three levels of finite
element grids to simulate radionuclide decay transport with 3-D HYDROGEOCHEM5.6, a special
version of 3-D HYDROGEOCHEM5.5 (Yeh et al., 2009) numerical model, for the LLW disposal at theDaren site. Steady flow and transient radionuclide transport of multiple decay chains in far-field andnear-field were performed for 100,000 years. The objective is to investigate complicated interactions
among initial concentrations, half-life, decay chains, near-field processes, and far-field processes onradionuclide decay transport.

2.

Simulation Setup
The near-field multiple engineering barriers are designed to contain the release of radioactive

waste after the closure of disposal facilities. In this study, we aim to simulate the migration ofradioactive waste in nearby geosphere after failing of near-field engineering barriers. Several SKB
reports for the SR-PSU site (SKB 2014a, 2014b) in Sweden were studied to inspire our design ofsimulation framework. The 3-D HYDROFEOCHE5.6 finite element numerical model was selected forhaving capabilities of resolving coupled flow, thermal, radioactive transport with decay chains. Our
simulation flow chart was shown in Fig. 1. Far-field flow simulation was performed first to provide
boundary conditions of near-field facilities. Subsequently, flow simulation of near-field facilities
generates boundary conditions for simulations of near-field tunnels. For the transport part, we simulate
radioactive decay transport of near-field tunnels to provide radionuclide release rates for radioactive
transport simulations in near-field facilities. Finally, radioactive transport simulations in far-field aresupported with radionuclide release rates simulated in near-field facilities. In this study, total
radioactive activities of source term and a total of 11 radionuclides (Pu-238, Am-241, Tc-99, Co-60,I-129, Cs-137, Ni-59, C-14, Sr-90, Y-90, Ni-63) with decay chains are considered as shown in Table 1(TPC, 2016).
The elevations of far-field simulation domain are from 475m above sea level at the west to -170 m
below sea level at the east. The vertical depth of simulation domain is about 400 m and the top surface
covers a region of 8,600 m times 3,800 m. The far-field domain, the coarsest-level grids, is discretized

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japanwith 5,970 nodes and 4,749 elements as shown in Fig. 2 (left). The near-field facilities designed withfive 828 m long, three 778 m long, and seven 352 m long disposal tunnels is discretized with 6,120
nodes and 4,720 elements in Fig. 2 (right). The near-field tunnel domain considers a single disposal
tunnel section having eight kinds of engineering barriers and is discretized by a total of 873 nodes and1,556 elements as shown in Fig. 3. Hydrogeological and transport parameters were modified from theTPC assessment report (TPC, 2016). Tables 2, 3 and 4 are parameters used for the far-field, near-field
facilities, and near-field tunnel, respectively.

Figure 1 Simulation flow chart

Figure 2 Elements and scales (plan-view) used in far-field (left) and near-field facilities (right)

Figure 3 Elements and scales (side-view) used in near-field tunnels

Table 1 Source term concentrations
Radionuclide

242241

238

137

Concentration
(TBq)

3x104

4x103

1x102

1x106

Cm

Pu

Pu

Cs

129

99

Tc

90

Sr

63

60

Co59

Ni

145x101

2x103

2x106

2x106

5x106

1x105

5x104

I

Ni

C6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan

Table 2 Parameters for far-field grids
Flow parameters

Transport parameters

layer1layer2layer3Kx=Ky

3*10-2(m/day)

3*10-3(m/day)

3*10-5(m/day)Kz

8.9*10-2 (m/day)

8.9*10-3 (m/day)

3*10-5 (m/day)

Porosity

0.08

Longitudinal dispersivity

800(m)

Lateral dispersivity

200(m)

Diffusion coefficient10-4(m2/day)Table 3 Parameters for near-field facilities grids
Flow parameters

RockTransport parameters

Disposal tunnel

Kx=Ky=Kz-5-5

8.64*10 (m/day)

5.16*10 (m/day)

Porosity

0.250.28

Longitudinal dispersivity8(m)Lateral dispersivity
Diffusion coefficient2(m)2

4.89*10-5(m2/day)

1.73*10 (m /day)-4

Table 4 Parameters for near-field tunnel grids
Porosity(-)

Kx=Ky=Kz(m/day)

Rock

0.25

8.64*10

Quadric Lining

0.25

8.64*10Base layer

0.2

8.64*10

Buffer material

0.4

8.64*10

Backfill material

0.3

1.73*10

Disposal

0.2

8.64*10

Cement mortar

0.3

8.64*10

Radioactive waste

0.35

1.73*10Material

3.

Diffusioncoefficient2(m /day)-5

Longitudinaldispersivity(m)

Lateraldispersivity(m)82-4

1.73*10-5-4

1.73*10-6-8

8.64*10-8-5

1.73*10-5-6

8.64*10-6-8

8.64*10-5-5

3.46*10-4-5

6.05*10

Results and Discussions
Simulations of steady flow with steady transport or transient transport for up to 100,000 years

were conducted this study. Figure 4(left) depicts flow field and total head distribution in far-field with aconstant infiltration rate of 0.0003 m/day and a fixed sea level on the right. We do see some upwardvelocities due to terrain cavities. It is noted the regional flow velocities are small and such upward flow
does not represent the occurrence of surface spring. Figure 4(right) gives flow field and total head
distribution simulated for near-field facilities. Figure 5 shows cross-sectional view of flow field andtotal head distribution simulated with near-field tunnel grids. Overall the flow filed is eastward to theocean. Owing to low hydraulic conductivities of engineering barriers, the flow velocities inside

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japandisposal tunnel are about 0.01% of those outside the tunnel. Figure 6 is the distribution of radionuclide
concentrations simulated with steady transport. It is noted regions right above the disposal tunnel haverelative high concentration compared to upward side of the tunnel due to dispersion.

Figure 4 Flow field and total head distributions in far-filed (left) and near-filed facilities (right)

Figure 5 Flow field and total head distribution of near-field tunnel

Figure 6 Relative concentration distributions of radionuclide with steady transport simulation
Source term concentrations given in Table 1 were used as initial concentrations for transient
simulations of radionuclide decay transport with near-field tunnel grids, which will provide release
rates of radionuclides out of tunnels to be simulated at the near-field facilities grids to further support
simulations at far-filed grids. Figure 9 shows radionuclide concentrations in 100,000 years at 200
meters away from disposal facilities. For the first 100 years, release rates of C-14, Tc-99, and I-129 are
higher than the others. After 100 years, release rates of Ni-59, C-14, and Tc-99 are higher than theothers.

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

Figure 9 Radionuclide concentrations in far-field at 200 meters away from disposal facilities
4.

Conclusions
For safety concerns of waste disposal, it is important to quantify the spatial and temporal

distributions of released radionuclides. In this study a 3D numerical model was used to simulate theradionuclide migrations of low-level waste disposal site at nearshore environment tentatively selectedby the Ministry of Economic Affairs in Taiwan. Based on current setting, flow velocities are 0.008 4.8x10-16 (m/day) in far-field, 6.7x10-10 - 7.5 x 10-13 (m/day) in near-field facilities, and 1.7x10-8 to
7.5x10-13 (m/day) on near-field tunnels observed from steady flow simulations. For the first 100 years,release rates of C-14, Tc-99, and I-129 are higher than the others. After 100 years, release rates ofNi-59, C-14, and Tc-99 are higher than the others. Owing to high sorption of Ni-59 onto bentonitebarriers, long-lived Ni-59 with high initial concentration has a low release rate for the first 100 years. A
low release rate of I-129 after 100 years is due to of having low initial concentration. Results
demonstrate complicated interactions among initial concentration, half-life, decay chains, near-fieldprocesses, and far-field processes on radionuclide decay transport.

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.

SKB, 2014a, Engineered Barrier Process Report for the Safety Assessment SR-PSU, TR-14-04.

3.

SKB, 2014b, Geosphere Process Report for the Safety Assessment SR-PSU, TR-14-05.

4.

Taiwan Power Company website, http://www.taipower.com.tw/

5.

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

6.

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, Thermal Transport,and HYDROGEOCHEMical 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