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National Conference on GEO-ENVIRONMENTAL ISSUES AND SUSTAINABLE URBAN DEVELOPMENT (GEN-2014) October 11-12, 2014 MNNIT Allahabad, India.

APPRAISAL OF SAFE PLACEMENT DISTANCE BETWEEN CANISTERS IN A TYPICAL DEEP GEOLOGICAL REPOSITORY P.N. Mishra

V.K. Gadi

Graduate Student Department of Civil Engineering NIT Rourkela, Rourkela Email:[email protected]

Graduate Student Department of Civil Engineering NIT Rourkela, Rourkela Email:[email protected]

S.S. Surya

D.N. Arnepalli

Research Scholar Department of Civil Engineering IIT Madras, Chennai Email:[email protected]

Assistant professor Department of Civil Engineering IIT Madras, Chennai Email:[email protected]

Abstract : Energy crisis is one of the most enduring problems in the present day. To address this issue, nuclear power serves as a sustainable as well as an environment friendly alternative. However, the byproducts and the wastes that find their way out of the nuclear power generation are perilous in nature and commonly disposed into deep geological repositories (DGR). A DGR is a multi barrier engineered waste conatianment facility, essentially comprising of waste containing canisters, encapsulating buffer material and the surrounding geologiocal host rock. In the design of a DGR, a safe or critical distance has to be maintained between the canisters so that heat migration occurs from the encapsulated waste to the encompassing geoenvironment effectively. With this in view, modelling of heat migration in a typical DGR is carried out using a commercially available finite element based numerical tool. The present investigation is devoted towards assessing the safe placement distance between multiple canisters, which will assure that both are independent of thermal influence of each other. Keywords:Nuclear waste disposal,DGR,Thermal modelling,Numerical modelling

1 INTRODUCTION Nuclear energy is one of the environmentally benign ways of sustainable energy generation and serves as a white knight in regards to the energy crunch that the present world is undergoing through. However, notable approaches of nuclear energy generation, which include nuclear fusion and fission reactions, generate wastes that are hazardous for human and surrounding biosphere. IAEA (International Atomic Energy Agency) has categorised these wastes as Low Level Waste (LLW), Intermediate Level Waste (ILW) and High Level Waste (HLW), based on when the radioactivity dies down and the exposure risk to geoenvironment, human and animal life [6]. This method of classification is devised considering the time duration until when the waste maintain radioactivity and the amount of menacing impact that it can have on the environment. It has been estimated that, HLW amounts for about 95% of the radioactive wastes generated. Dilution and confinement is one of the widely practised methods for disposal of HLW. The waste is disposed in a multi-layer engineered barrier system, which essentially consists of a host rock, thermal canister and buffer materials. Canisters are steel containers in which the calcinated vitrified wastes are kept and buried deep into the geological formations whereas buffer materials are usually mixture of sand and bentonite put in place so as to isolate the waste from the surrounding geoenvironment. Before placing the canisters in a deep geological repository (DGR), a safe horizontal placement distance between them has to be maintained to make sure that effective heat migration to the surrounding geoenvironment is taking place. 2 OBJECTIVE The objective of the present investigation is to model and analyse transient thermal migration inside a typical deep geological repository (DGR) and to estimate the critical distance at which adjacent canisters in a multiple canister system are independent of thermal influence of each other. This critical distance, as we name it, is the safe horizontal placement distance in a multiple canister system. Sand and bentonite mixture in the dry ratio of 70:30 (SB30) is widely employed as a buffer material in design of deep geological repositories and for the current problem, thermal properties of the same mixture is considered. The modeling is done for optimum moisture content and wet of optimum of compaction curve for the selected mixture.


3 BACKGROUND OF THE STUDY TM

SVHeat , a FEM based numerical modeling tool is used to address the need of the hour. This tool can handle both conductive and convective thermal migration through saturated or unsaturated soil mass. The heat migration modeling can be implemented both in steady as well as in transient state. This software suite incorporates features for assignment of various parameters of the geomaterials pertaining to the model geometry, initial and boundary conditions and provides a means for swift and precise calculation unlike the analytical solutions. 4 MATERIALS AND METHODS As indicated earlier, sand-bentonite mixture, SB30 is considered as the buffer material in the current problem. Thermal properties of the selected buffer were obtained using a thermal probe developed by Joseph (2014), that works on transient heating technique [8]. Thermal properties of the canister and hostrock material are adapated from the literature [7]. 4.1 Material Selection and Characterization In case of a DGR, a buffer material having higher thermal conductivity and lower moisture migration rate is usually provided. This ensures rapid dissipation of heat flux coming out of the waste into the surrounding geoenvironment. Furthermore, higher water retention characteristics of the buffer safeguard the material from being prone to shrinkage and cracking. In addition to the above, thermomechanical behaviour of the buffer material is also not to be overlooked while selecting the suitable buffer material. Researchers endorse sand-bentonite mixture in the ratio of 70:30 by dry weight as a suitable buffer [3,5 & 11]. It has been opined that thermal conductivity increases sluggishly at lower moisture content and shows a drastic increment after a critical moisture content [13]. Thus, the present analysis is carried out for water content on wet side of optimum, as it will reflect higher thermal conductivity values. The geotechnical and physical properties of the selected material is obtaiend and the same is presernted in the following. Figure 1 depicts the particle size distribution characteristic curve for SB30 and Table 1 gives an insight into physical characteristocs of SB30. 100

Percent finer (%)

80

60

40

20

0 -4 10

-3

10

-2

10

-1

0

10

10

1

10

Particle size (mm)

Figure 1: Particle size distribution characteristics of SB30 Table 1: Physical characteristics of SB30 Specific Gravity

Maximum dry unit 3 weight (kN/m )

Optimum Moisture Content (%)

2.67

17.85

13.5

Percent fraction (%) Sand size Silt size Clay size 70.75

1.44

27.81


4.2 Governing Equation for Heat Migration through Soils Out of all the possible modes of heat transfer, for soils, heat migration by conduction predominates. Heat flow due to conduction can be ascribed upon presence of a thermal gradient . For a 2D state of thermal migration, if thermal conductivity is denoted as λ (in W/m.K) and T being the º temperature in C, then the heat flow in x and y directions are given by,

(1) For a transient heat flow problem, the governing differential equation reads, (2) where,

is the volumetric heat capacity of the transfer media.

The equation furnished above can either be solved by an analytical method or a numerical analysis. However, for instances of engineering application where there has to be a tradeoff between accuracy and time lag for computation, numerical analysis easily knocks a victory over the classical analytic solutions. Hence a numerical simulation is attempted to address the current problem. 4.3 Numerical Modeling of Heat Migration in A Typical Deep Geological Repository One of the widely accepted and adopted practices of radioactive waste disposal is diluting and containing the waste. Multi-layer engineered barrier system (EBS) is popular in this regard. This system essentially consists of a host rock, thermal canister, and backfill and buffer materials. A deep geological repository (DGR), which assures a thorough isolation of the hazardous waste from the surrounding atmosphere, can be stated as an example of an EBS. Figure 2 illustrates the layout of a typical DGR.

Figure 2:Layout of a typical DGR (modified from Komine and Ogatta, 2004 [9])

In the current analysis, a DGR is modeled for 200 years for its long term performance assessment. The DGR exists at a depth of 500 m from the ground surface. The prevailing temperature at the ground surface is 30ºC and an increasing thermal gradient of 0.016 ºC/m is imposed along the depth. The model has got three distinct regions i.e. host rock, buffer material and canister. The host rock is assumed to be limestone with a thermal conductivity value of 2.3 W/mK and dry unit weight of 25.5 3 kN/m . The thermal conductivity of the steel canister is taken as 300 W/mK with a dry unit weight of 3 76.52 kN/m . The thermal conductivity value and dry unit weight values are adopted from literature [7]. The buffer material is considered to be sand bentonite mixture with 30 percent bentonite at wet side of optimum. The model incorporates two canister-buffer systems, where diameter of the canister and buffer are 1.25 m and 2.5 m, respectively. A schematic view of the model in presented in Figure 3. Figure 4 enumerates the flowchart of activities followed by the numerical model for the present analysis.


Figure 3: Schematics of the model

Figure 4: Algorithm used during numerical modelling

5 RESULTS AND DISCUSSION With the help of thermal probe developed by Joseph (2014), thermal conductivity values for different water contents of the SB30 sample were measured and the same are tabulated in table 2. It can clearly be observed that thermal conductivity value increases significantly with increase in moisture content on the wet side of optimum. Table 2 Thermal condtivity values for SB30 Sample designation SB30

Gravimetric moisture content (%) 18.8 20.7 22

Thermal conductivity (W/m-K) 0.553 1.024 1.831

From the instanses of the water contents considered above, 22% water content for the SB30 sample furnished highest value of thermal conductivity i.e. 1.831 W/m-K. Giving regards to preciseness and consiceness, all the further analysis are carried out for this water content. Figure 5 depicts the variation of temperature with time at the interface of canister and buffer for various placement modes of the canister. A stepped decay in temperature is noticed irrespective of the placement distance. Temperature versus time curve was also plotted at the interface of buffer and the host rock as shown in Figure 6; it also includes the results obtained by Surya et al. (2014) for the single canister model. Surya et al. (2014) demonstrated a maximum rise of temperature upto 95°C at the end of 9.5 years near the buffer host rock interface.The present model also endorses the fact that maximum rise of temperature at the buffer host rock interface occurs after 9.5 years. However, as the distance between canisters go on increasing, the highest temperature observed at the interface reduces. The temperatures observed at the interface when the thermal migration to the hostrock is at its peak are 120°C, 107°C, 102°C and 101°C for placement distances of 2m, 4m, 6m, and 8m, respectively. For all the placement distances considered, an ambient temperature of 60°C is noticed after 150 years. Figure 7 emphasizes on the temperature variation between the canisters when maximum heat migration to the host rock occurs; it can be observed that heat is getting more dispersed as the placement distance increases. The temperature reduced to more than 10°C when the placement distance changed from 2m to 4m. After that for every 2m increase in placement distance temperature is decreasing about 5°C. As the disposal area is restricted placement distance between adjuscent canisters of 4m distance is preferable. Figure 8 exhibits the variation of temperature along distance at the end of analysis period of 200 years. An ambient temperature range of 60°C to 63°C is observed.


140 Placement distance=8m Placement distance=6m Placement distance=4m Placement distance=2m

0

100

80

Placement distance=2m Placement distance=4m Placement distance=6m Placement distance=8m Single canister model (Surya et al.)

120

Temperature ( C)

0

Temperature ( C)

120

100

80

60

40

60 0

20

40

60

80

100

120

Time (years)

140

160

180

20 -20

200

Figure 5: Temperature variation with time at canisterbuffer interface

0

20

40

60

80

100 120 140 160 180 200 220

Time (Years)

Figure 6: Temperature variation with time at bufferhostrock interface

135

64 Placement distance =2m Placement distance =4m Placement distance =6m Placement distance =8m

130 125

Placement distance = 2m Placement distance =4m Placement distance =6m Placement distance =8m

120

63

110

Temperature( C)

105

0

0

Temperature ( C)

115

100 95 90 85 80 75

62

61

70 65 60 40

42

44

46

48

50

52

Radial Coordinates

54

56

58

60

Figure 7: Temperature along the radial co-ordinates when maximum thermal migration to hostrock occurs (9.5 years)

60 50

60

70

80

90

100

110

Radial coordinates

Figure 8: Temperature along the radial co-ordinates at the end of analysis period (200 years)

6 CONCLUSIONS The present study focussed on numerical modelling of heat migration of a multuple canister system. The DGR is ananlysed for a period of 200 years to ascertain the long term performance of the barrier system. It is apparent that maximum thermal migration occurred after a period of 9.5 years and the temperature at the canister-buffer interface at this period reduced as the placement distance between them increased. A more dispersed thermal migration is observed between the canisters with increase in spacing between them. At 8m spacing, the temperature observed at the canister interface is more or less independent of each other. Irrespective of the placement distance, an ambient temperature around 60°C is observed in the host rock. REFERENCES [1]

Ada, M. (2007), Performance assessment of compacted bentonite/ sand mixtures utilized as isolation material in underground waste disposal repositories. Thesis report, The graduate school of natural and applied sciences of Middle East technical university.


[2]

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