Barrier of near-surface LILW disposal facility in Korea :hydraulic model development
Mi Jin Kwon, Ki Jung Kwon and Jin Beak ParkKorea Radioactive Waste Agency (KORAD)

Technology institute, KORAD,
174 Gajeongro, Yuseong-gu, Daejeon 34129, KOREA

Key words; Low-and Intermediate Level Radioactive Waste (LILW) disposal facility, Near-Surface disposal,
Multiple engineered barrier cover system, Hydro-geological modeling

Abstract
The primary objective of the engineered barrier of Low-and Intermediate Level radioactive Waste (LILW)
disposal facility is to limit the amount of water ingress that passes through the engineered barrier and to protect
waste packages. From the mid of 2015, the silo type disposal facility for LILW started for its operation as thefirst stage. As the second stage, the vault type of near-surface disposal facility is being prepared in Korea. The
second stage facility is composed of the multiple engineered barrier cover system so that the effects by rainfall
are reduced and thus leakage of radioactive substances can be minimized. The cover system is composed ofseveral different layers including surface, protective, drainage and barrier layer. In this paper, Hydro-geologicalconcept modeling to predict the rainfall infiltration through the cover system is performed by using Finite
Element subsurface Flow code (FEFLOW).

1. Introduction
Currently, the operation of LILW disposal facility
is ongoing in Gyeongju Wolsong. In December 2014,
the near-surface disposal facility of silo type below
ground level was completed as the first stage, and thelicense for the construction and operation of thenear-surface disposal facility as the second stage isbeing prepared.
For the second stage of near-surface disposal
facility, concrete storages for keeping disposal
containers on the surface will be constructed and thenthe multiple engineered barrier cover system installedon their top so that the influences by rainfall are
reduced, and thus leakage of radionuclide substances
can be minimized [1].
In this paper, rainfall infiltration through themultiple engineered barrier cover system was adaptedand assessed by using the two-dimensional
groundwater flow model with FEFLOW to assess theperformance of the cover system of the second stage
near-surface disposal facility at ground level [2].
2. Design concept of the cover system
The primary objective of cover of LILW disposal
facility is to limit the amount of water that passes
through the cover system and the amount of water
potentially coming into contact with waste. There are
only a few ways in which the cover can limit thepassage of water: by evapotranspiration and diversionof water into runoff or subsurface lateral drainage.
Since precipitation cannot be controlled, seepage canonly be minimized by maximizing the sum ofevapotranspiration, runoff and subsurface lateraldrainage. Other objectives that have a direct impact
on water flow through the cover system are also
important. For instance, it is often necessary for thecover to minimize wind and water erosion and toprevent plant and animal intrusion.
The second stage of the Wolsong LILW disposal
facility was designed as the near-surface disposal.

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

The cover system is composed of multiple layers(surface layer, protective layer, drainage layer andbarrier layer) as engineering barrier elements. The
inclination of each layer should be set to more than
3 % gradient in order to minimize rainfall
infiltration into the disposal vault.
The
permissible infiltration rate is 32 mm/year, and thisrate was calculated taking into consideration thepermeability of the barrier layer. Fig.1 shows theconfiguration of the cover system.

Table 1. Characteristics of cover materials for thesecond stage LILW disposal facility
Layer

Surface layer(silty sand)Protective layer(gravelly sand,pea gravel)
Filter layer(flexiblemembraneliner)
Drainage layer(sand)

Barrier layer(clay)

Figure 1. Configuration of multi-layer cover concept
in the second stage LILW disposal facility
Table. 1 shows the composition and functional
requirement of each layer. The function of each layer
is as follows [3]:-Surface layer
The functions of surface layer are to manage
runoff,minimizing erosion,maximizingevapotranspiration. It consists of silty sand.

-Protective layer
A protective layer is placed directly below thesurface layer. The combined layers function toprotect underlying layers from degradation,
which can occur by repeated freeze/thaw cycles,
repeated excessive wetting/drying, plant, animal,or human intrusion.
The protective layer consists of gravelly sandand pea gravel.

-Drainage layer
The function of the drainage layer is to collectwater infiltrated through the surface andprotective layers and to divert this water laterally.
The drainage layer is composed of sand.

-Barrier layer
The barrier layer is the most critical component,
because this layer plays a role to limit water
infiltration into disposal vault. Functional
requirement of this layer is on the order of a few
cm/year or less: 10-7cm≈3.2cm/year.
The barrier layer is composed of clay.

-Characteristics

Slope : 3 ~ 5 % (humid), 2 %(arid)
Thickness : > 60cm
[60~100 cm(humid), 200 cm(arid)]
Erosion rate : < 2 ton/acre/year
Resistant repeat freeze/thaw cycles

Slope : > 3 %Thickness : > 30 cm
K : > 10-2 cm/s
Hydraulic transmissivity : > 3.0×10-9 cm2/s

Thickness : > 60 cm
Slope : > 3 %K : < 1.0 ×10-7cm/s(=3.2 cm/year)

3. Modeling concept
Two-dimensional analysis was performed on thecross-section as a full scale, which is in the short axis
direction of the second stage near-surface disposal
facility and a multiple engineered barrier disposal
cover installed on top of the disposal storages.
The model was set up by extending 20 m to both theleft and right sides of the model area and 50 m to thebottom where the groundwater was located in order
to exclude external influences in assessing the rainfall
infiltration through the disposal cover as follows Fig.
2.

Figure 2. Concept model of ground water flow
modeling for the second stage LILW disposal facility
4. Input parameters for concept modeling

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

4.1 Mesh generation
Fig. 3 shows that meshes are composed of a totalof 29,194 elements using triangular elements ofvarious sizes ranging from fine to coarse mesh inreflection of the design of the cover system. The
model domain for cover system is subdivided into 9
areas reflecting the design so that characteristics ofeach layer can be applied. Base on the cover design,thesecondstage
disposalfacilityhas
two-dimensional symmetry structure and for thenumerical safety and computational efficiency, only
the half of model area was simulated.

The bottom of domain was set as constant headboundary condition and the other boundary was set as
no flow boundary. Fig. 5 shows the applied boundary
conditions.

Figure 5. Applied boundary conditions for thecalculation
4.3 Hydraulic properties of cover

Figure 3. Mesh structure of numerical simulation
4.2 Boundary conditions
The ground water flow model requires information
about the head or head gradient at the boundaries ofthe model domain. To set the rain conditions, therainfall data for 30 years of weather station locatednear the second stage LILW disposal facility wasapplied.
The recharge amount excluding evapotranspirationand surface runoff from rainfall was assumed topenetrate through the disposal cover, and thus
estimated amount was 469 mm/year, which was
36.5 % of the average annual rainfall as follows Fig.
4. The rainfall reflected recharge rate was set as thetop boundary condition of domain.

In this study, the hydraulic properties of cover
given in the reference were applied. The hydraulicproperties listed in Table. 2 [4].

Table 2. Hydraulic properties considered for coveras a multi-layer
Van GenuchtenParameters

Residual(-)

Saturated(-)α(cm-1)n(-)

Saturated
Hydraulic
Conductivity(cm/s)

Silty
sand

0.1

0.47

0.044

1.523

1.00e-04

Gravelly
sand

0.02

0.32

0.10082.922

1.00e-02

Peagravel

0.03

0.26

4.695

2.572

1.00e+00

Sand

0.045

0.37

0.06832.08

3.00e-02

Clay

0.00010.36

0.00161.203

1.00e-07

Concrete
Vault

0.08

0.40

0.00631.08

1.00e-8

Water ContentLayer

The hydraulic property parameters were used as
input data in numerical simulation. In order to reflect
the fracture characteristics in unsaturated zone of thesecond disposal facility site, discrete fracture networkmodeling was conducted. The hydraulic conductivity
derived from results of the discrete fracture networkmodeling was applied as the hydraulic conductivityof unsaturated zone. Fig. 6 shows the hydraulicconductivities of media.
Figure 4. The average annual rainfall as the boundary
condition

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, Japan(c)Figure 6. Distribution of hydraulic conductivity
5. Results of hydro-geological concept modelingFig. 7 show the change of saturation distribution
over 300 years and it is possible to judge the behavior
of infiltrated water based on simulation results (after0, 100, 200, 300 years).

(d)Figure 7. Saturation distribution after 0 (a), 100 (b),200 (c), 300(d) years, respectively(a)The percolation of infiltrated water proceeded at avery slow rate along with time. Infiltrated watercollected in the sand layer at the lowermost part ofthe drainage layer (gravelly sand, pea gravel, sand)and most of infiltrated water flow through the slope.
Hence, the amount of infiltration into the clay layer
forming the lower part of the cover layer within 300
years was simulated as extremely insignificant.
6. Conclusion
To assess the performance of the multiple
engineered barrier cover system in the second stage
LILW disposal facility in Korea, KORAD developed
the hydro-geological concept model. The hydraulicevaluation of the cover was conducted using
FEFLOW. As hydraulic evaluation result for thecover system, the infiltration rate into the sand layer
forming the lower part of the cover system wassatisfied with the permissible infiltration rate.

(b)

6th East Asia Forum on Radwaste Management Conference
November 27-29, 2017, Osaka, JapanIn order to improve reliability and validity ofsimulation results, various further studies such as
cover properties suitable for KORAD’s design,
estimation of recharge amount and degradation ofconcrete are required.
The actual flow of rainfall passing through thefacility will be three-dimensional. In this study, it has
been limited to two-dimensional approach. Actual
three-dimensional hydraulic studies will be carriedout at a future date.
7. References[1] ONDRAF/NIRAD, Long-term evolution of themulti-layer cover project near surface disposal ofcategory A waste at Dessel, STB-NF-Version 1(2010).
[2] MIKE Powered by DHI, Finite Element
Subsurface Flow & Transport Simulation System(FEFLOW) 7.0, User guide (2016).
[3] Environmental Protection Agency, Technicalguidance document: Final covers on hazardous waste
landfills and surface impoundments,
EPA/530-SW-89-047 (1989).
[4] Nuclear Regulatory Commission, Applicationof an infiltration evaluation methodology to ahypothetical low-level waste disposal facility,
NUREG/CR-6114 PNL-8842 Vol.1 (1993).

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