Application of a cold spray technique to the fabrication of a copper canister for the geological disposal of CANDU spent fuels

Nuclear Engineering and Design 240 (2010) 2714–2720

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Application of a cold spray technique to the fabrication of a copper canister for the geological disposal of CANDU spent fuels Heui-Joo Choi ∗ , Minsoo Lee, Jong Youl Lee Korea Atomic Energy Research Institute, Radioactive Waste Management Technology Development, 150 Dukjin-dong, Yuseong, Daejon, 305-353, Republic of Korea

a r t i c l e

i n f o

Article history: Received 16 June 2009 Received in revised form 4 June 2010 Accepted 30 June 2010

a b s t r a c t A new method was proposed for the manufacture of a copper-cast iron canister for the spent fuel disposal based on the cold spray coating technique. The thickness of a copper shell could be fabricated to be as thin as 10 mm with the new method. Around 6 tons of copper could be saved with a 10 mm thick canister compared with a 50 mm thick canister. The electrochemical properties of the cold sprayed copper layer and forged copper were measured through a polarization test. The two copper layers showed very similar electrochemical properties. The lifetime of a 10 mm copper canister was estimated with a mathematical model based on the mass transport of sulfide ions through the buffer. The results showed that the canister lifetime was more than 140,000 years under the Korean granite groundwater condition. The thermal analysis with a current pre-conceptual design of a CANDU spent fuel canister showed that the maximum temperature between the canister and the saturated buffer was below the thermal criteria, 100 ◦ C. Finally, the mechanical stability of the copper canister was confirmed with a computer program, ABAQUS, under the rock movement scenario. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Four CANDU reactors have been in operation since 1983 in Korea. Even though they show a high utilization efficiency, CANDU reactors generate even larger amount of spent fuels than PWR. Since the CANDU spent fuels include small amount of fissile nuclides, it is thought that they will be directly disposed of without any consideration of reusing them. The specific heat of a CANDU spent fuel is relatively low due to a comparatively lower burnup, which leads the small disposal area. The authors made a conceptual design of a geological disposal system for CANDU spent fuels based on the Swedish copper-nodular cast iron canister (Lee et al., 2007). A canister was designed to hold 297 CANDU spent fuel bundles with a height of 483 cm and a diameter of 102 cm. The CANDU spent fuel canister consists of an inner container of cast iron and outer shell of copper for corrosion resistance. The major function of a canister is to contain the radionuclides during the lifespan. The lifespan of the canister is 100,000 years in the case of Sweden (Andersson, 2002) or Finland (Raiko, 2005). According to SKB’s conclusion (Andersson, 2002) a 30 mm thickness of the copper shell could meet the corrosion resistance over 100,000 years. However, the thickness of the reference copper shell was set at 50 mm for testing of fabrication methods and optimization of

∗ Corresponding author. Tel.: +82 42 868 2274; fax: +82 42 868 8198. E-mail address: [email protected] (H.-J. Choi). 0029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2010.06.038

canister design. It means that a 10 mm thickness of outer shell is sufficient for corrosion resistance if it is possible to fabricate it considering the extremely slow corrosion rate of less than 1 ␮m/year obtained from long-term corrosion experiments (Kim et al., 2006; Taniguchi and Kawasaki, 2008). Generally, a disposal canister is huge to contain a large amount of spent fuels, which is larger than 1 m in diameter and 4 m in height. To fabricate such a big seamless copper canister, several methods such as an extrusion, pierce-draw processing, and forging methods have been suggested in Sweden and Finland (Andersson, 2002; Raiko, 2008). It is not easy to manufacture a thin-walled copper shell of 10 mm thickness in such a large diameter with the suggested methods due to the native flexibility of copper. Thus, the authors tried to search for an effective method to manufacture a thin huge copper shell (Kim et al., 2009). Finally, our decision was to adopt a cold spray coating (CSC) which is a kind of bulk deposition on the inner cast iron container. The copper layer could be formed to drive fine copper powders onto a target plate with a high speed at an elevated temperature but not above its melting point. The copper coating fabricated by cold spray technique is rigid, easy to control the thickness, and free from an oxidation of its layer generally. Another spray coating method called a high velocity oxy-fuel thermal spray (HVOF) was also considered for coating a corrosion-resistant film (DOE/RW-0576, 2005). But this method was considered to be inadequate for creating a thicker copper layer. The spray forming technique had been considered once in Sweden study group (Andersson, 1998), but they discarded it because the

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Fig. 1. Annual discharge burnup distribution of CANDU spent fuels from Wolsong site.

Table 1 Discharge burnup of CANDU spent fuels. Wolsong unit

Mean (MWD/MtU)

Standard deviation (MWD/MtU)

Unit 1 Unit 2 Unit 3 Unit 4

6923 7005 6858 7010

941 1123 1249 1189

Total

6937

1167

lifetime. One of the most important characteristics of spent fuels is burnup. The burnup distribution and discharge burnup of spent fuels for CANDU reactor are presented in Fig. 1 and Table 1. As shown in Table 1, the mean of discharge burnup is 6937 MWD/MtU, and standard deviation is 1167 MWD/MtU. The burnup of a Korean reference CANDU spent fuel is set as 8100 MWD/MtU. The regression equation for the decay heat source from a reference CANDU spent fuel was determined as follows: q(t) = C1 e−A1 t + C2 e−A2 t + C3 e−A3 t + y0

spray forming technique could create pores due to nitrogen gas incorporation in the formed bulk. However, the cold spray technique adopted in this paper could avoid the generation of pores using a supersonic nozzle for copper powder. The main purpose of this study is to propose a new method for manufacturing a disposal canister for CANDU spent fuels with the cold spray technique. For that purpose, a disposal canister for CANDU spent fuels was designed to hold seven baskets, in which 60 CANDU spent fuel bundles were stored. Based on the design, a reduced canister on 1/10 scale was manufactured to demonstrate the possibility of the new technique. In this paper, several fundamental properties were surveyed with the produced samples. The electrochemical properties of the cold sprayed copper were compared with the forged copper through a polarization test. The lifetime of the disposal canister under a Korean reference disposal condition was predicted using a mathematical model based on a mass transport of corrosion agents. Finally, the thermal and mechanical performances of the new canister under limited repository conditions were carried out using FEM computer programs, NISA and ABAQUS. 2. CANDU spent fuels and a disposal canister Since the CANDU spent fuels are to be disposed of directly, it is important to characterize the properties of them. So far around 5000 tU of CANDU spent fuels have been generated from four CANDU reactors and around 16,000 tU equivalent to 842,000 bundles are expected from the four CANDU reactors during their

(1)

where decay heat, q(t) [W/MtU], was calculated using the ORIGENARP program (Gauld et al., 2006) and the coefficients are given in Table 2. Currently, since the wet storage pool is full of CANDU spent fuels at the Wolsong site, CANDU spent fuels are being temporarily stored in a dry storage facility, a concrete silo. To enhance the handling efficiency, all the spent fuels are put in a basket with a capacity of 60 bundles. In consideration of this dry storage condition, a new disposal canister to hold seven baskets shown in Fig. 2 was designed. By changing the configuration as such, the disposal efficiency could be increased remarkably, and the time for handling spent fuel bundles could be shortened. Also, the authors tried to reduce the thickness of a copper outer shell from 50 mm to 10 mm by introducing the cold spray coating technique. The mass of copper in a canister was around 1828 kg for 10 mm thickness, but it was around 8750 kg for 50 mm thickness. In this way we can save around 6 tons of copper for one canister. For the whole CANDU spent fuels in Korea, around 2000 canisters are needed, which means that more than 10,000 tons of copper can be saved. The cold spray coating technique was attempted to fabricate a 10 mm copper shell. Fig. 3 illustrates a schematic of the cold spray coating process. A fine Copper powder with 1–50 ␮m in diameter was used for that purpose. The feed copper powders were preheated up to about 400 ◦ C to make them flexible, then mixed with highly pressurized inert gas, and finally blown out at a supersonic speed of 400–450 m/s onto a metal surface through a narrow nozzle. The carrier gas was also heated to around 600 ◦ C before the

Table 2 The coefficients of regression equation for decay heat from CANDU spent fuels. Cooling time (years)

C1

A1

C2

A2

C3

A3

y0

1 ≤ t ≤ 100 100 ≤ t ≤ 106

251.12 11.50

0.02442 5.96 × 10−5

5431.4 33.255

0.71648 0.00154

0 218.60

0 0.02183

47.23 0.71

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Fig. 2. Schematic of a canister (a) and a CANDU spent fuel basket (b).

coating layer, and the dimension was 0.5 mm in thickness, 2.0 mm in width, and 9.0 mm in length. Three kinds of cold sprayed copper were fabricated with different copper powders, which are Changsung copper powder coating on cast nodular iron plate, Changsung copper powder coating on stainless steel plate, and Tafa copper powder coating on stainless steel plate. In Korea, a pyroprocessing of spent fuels is considered as one of the options for the spent fuel management. A stainless steel container is also expected to Fig. 3. Schematic of a cold spray apparatus.

Table 3 Summary of cold spray coating process parameters used. Main N2 gas pressure (bar) Main gas temperature (◦ C) Powder preheating gas temperature (◦ C) Spray distance (mm) Powder feed rate (kg/h) Gun traveling speed (mm/s)

27 600 400 40 2 20

acceleration. We used a cold spray coating system developed by Taekwang Tech Co. in Korea. A small scale (1/10) copper canister was fabricated with high purity copper powders supplied by Tafa® , whose oxygen content was 0.02% and particle size was distributed in 5–20 ␮m. The applied gas pressure was about 27 bar, and the process temperature was about 600 ◦ C. The processing parameters of the cold spray coating process were listed in Table 3. A 10 mm thick coating layer was fabricated over the inner cylinder of nodular cast iron as shown in Fig. 4. The porosity of the coating layer was 0.3 area% as a result of image analysis, and the density was 8900 kg/m3 . The oxygen content of the copper layer was measured with a LECO TC 136 Nitrogen/Oxygen determinator. The oxygen content of the copper coating layer was 0.019%, and that of original copper powder was 0.02%. As shown in Fig. 5, the copper oxide peaks did not appear in XRD analysis, which indicated that oxidation was not interfered in the cold spray coating process. Mechanical properties of cold sprayed coppers were measured with a tensile test using MTS® . A tensile specimen shown in Fig. 6 was cut horizontally from the

Fig. 4. Fabrication of a small scale (1/10) copper canister by the cold spray coating technique.

Fig. 5. XRD analysis of a Tafa copper coating layer on a nodular cast iron cylinder.

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Fig. 6. Tensile specimen dimension with thickness of 0.5 mm (unit: mm).

be used for the storage of high-level radioactive wastes from the pyroprocessing of spent fuels. Thus, stainless steel was included as coating substrates. As a result of the tensile test shown in Fig. 7, cold sprayed copper was more brittle and stronger than extruded copper. The tensile strength of cold sprayed copper was nearly two times higher than that of general copper, but it had a lower strain to break. The brittleness of the cold sprayed copper is not appropriate property because it needs more carefulness for handling when a disposal canister is made of that. If the brittleness of the copper is severe problem in a geological disposal system, the bottom and lid of the canister could be replaced with forging copper plates. However, the main purpose of this paper is to show the feasibility of manufacturing a disposal canister with the cold spray coating technique. 3. Corrosion properties of copper layer The electrochemical properties of the cold sprayed copper layer were measured through a polarization test at room temperature. The used medium was sea water taken from the Yellow sea (pH 7.13). A reference electrode was Ag/AgCl (saturated KCl), and the cyclic voltage scan was done from −1.5 V to 1.0 V at 10 mV/s. The test results were presented in Fig. 8. The test voltage ascended from −0.5 V to 1.0 V, and then descended to −0.4 V in the test. The Tafa coating layer and the forged copper looked nearly similar in the electrochemical properties. In this study the forged copper was used instead of the copper from the pierce and draw process. The corrosion potentials were around −0.2 V at the ascending step, and the depassivation and reduction potentials were 0.1 V and 0.05 V, respectively, in both copper materials at a descending step. However, the decrease of a current density due to the formation of a

Fig. 8. Polarization curves of copper specimens. Cold sprayed copper layer with Tafa powders and Forged copper.

passive layer was greater in the forged copper than that of the Tafa coating layer, from which it was presumed that the forged copper made a more corrosion-resistant oxide layer than the Tafa coating layer. The lifetime of a copper canister was estimated using a conservative corrosion model (JNCDI, 2000). The corrosion of copper was caused due to sulfide ions in groundwater taken from a deep borehole around the Korea Atomic Energy Research Institute site. The mass transport of sulfide ions through a buffer was modeled by a diffusion equation at a steady state as follows: ∂C 1 ∂ ∂C =D r r ∂r ∂r ∂t

(2)

C(r0 , t) = 0

(3)

C(r1 , t) = C0

(4)

where r0 is the inner radius of buffer [m], r1 is the outer radius of buffer [m], D is the diffusivity of sulfide in buffer [m2 /year], C0 is the concentration of sulfide in groundwater [g/m3 ]. The steady state flux of sulfide at the surface of the copper canister is given as follows:



Jtotal = −De A

∂C  ∂r 

= −2De H r=r0

C0 ln(r1 /r0 )

(5)

where Jtotal is the total flux of sulfide at the surface of a copper canister [g/year], De is the effective diffusivity (pore diffusivity times porosity) in buffer [m2 /year], A is a surface area of a copper canister [m2 ], H is a height of a copper canister [m]. It is assumed conservatively that the number of copper ions is equal to the number of sulfide ions at the surface of the canister to estimate the corrosion rates. Thus, the annual corrosion rate of copper, CR, is calculated using the following equation (6): CR = 2De H

C0 63.5 ln(r1 /r0 ) 32

(6)

where CR is the annual corrosion rate of copper [g/year]. Finally, the corrosion depth of a copper canister could be calculated as follows: CD ∼ =

Fig. 7. Tensile strength of four specimens. Three specimens were taken from the cold sprayed (CS) copper on to stainless steel (STS) and cast iron, and the other is from an extruded copper.

tu × De × C0 63.5  × r0 × ln(r1 /r0 ) 32

(7)

where CD is the corrosion depth [m], tu is the corrosion time [year],  is the density of copper [ton/m3 ]. The corrosion depth of a copper canister for 1000 years was evaluated conservatively to be around 1.40 × 10−5 m with the data

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Table 4 Parameters used for the calculation of life time of a copper canister. Parameter

Value

Outer radius of buffer (m) Inner radius of buffer (m) Pore diffusivity (m2 /year) Porosity Copper density (g/m3 ) Effective Diffusivity of sulfide (m2 /year) Initial concentration of sulfide (g/m3 ) Corrosion time (year)

1.122 0.622 8.67 × 10−3 0.41 8.9 × 106 3.55 × 10−3 6.5 1000

given in Table 4. Even if we consider the pitting corrosion factor of 5, the pitting corrosion depth is around 7.02 × 10−5 m, which means the lifetime of a 10 mm thick copper CANDU canister is more than 140,000 years. The effect of buffer thickness on the copper canister lifetime was calculated under the same condition (Fig. 9). According to the results given in Fig. 9, the minimum buffer thickness to keep the canister lifetime longer than 100,000 years is around 35 cm with the sulfide concentration in Table 4. 4. Thermal analysis According to the concept of the Korean Reference disposal System (Lee et al., 2007), total 842,000 CANDU spent fuel bundles are to be disposed of in a granite repository. It is expected that 420 spent fuel bundles in a proposed canister surrounded by the buffer will be disposed of in a borehole. It is assumed that the gap between the rock and the bentonite buffer is filled immediately due to the swelling of a bentonite buffer for the thermal analysis. The thermal analysis was carried out on the unit cell scale. Once the spent fuels are disposed of in a repository, the heat transfer in crystalline rock and buffer is mainly by conduction, which is somewhat conservative assumption. The schematic view of calculation domain for a quarter of one canister including the deposition hole and the disposal tunnel was given in Fig. 10. The disposal tunnel is located at 500 m level below the surface. The spacing between the disposal tunnels and between the deposition holes is 40 m and 4 m, respectively. For the thermal analysis the bottom boundary was set at the 500 m from the bottom of the deposition hole. Due to the symmetry of the disposal system only a quarter of one deposition hole was modeled for the thermal analysis. That is, the heat transfer through the four lateral sides was neglected (zero heat flux). It was assumed that the temperatures of the ground surface and the bottom of the boundary remained constant, 15 ◦ C and 45 ◦ C, respectively. The geothermal gradient of a site was assumed to be

Fig. 10. Schematic of a geological disposal system for thermal analysis representing a quarter of one deposition borehole.

30 ◦ C/km. Thermal properties of materials were given in Table 5. The thermal conductivity of bentonite buffer given in Table 5 was for wet bentonite. The heat transfer is represented with the following 3dimensional heat conduction equation (Sizgek, 2005):



∂ ∂T ∂ k (Cp T ) = ∂t ∂x ∂x





+

∂ ∂T k ∂y ∂y



+

∂ ∂z



k

∂T ∂z



+ q(t)

(8)

where T is the temperature [◦ C], t is the time [s],  is the density [kg/m3 ]; Cp is the specific heat [J/kg ◦ C], k is the thermal conductivity [W/m ◦ C], q(t) is the time-dependent volumetric heat source [W/m3 ]. The maximum temperature between the canister and the buffer strongly depends on the heat generation rates of the spent fuels. The decay heat was calculated using the regression equation (1). In this study, the cooling time for CANDU spent fuels was assumed to be 30 years. The evolution of the temperature distribution around the disposal canister was calculated over a period of 100 years with the heat sources given in Eqs. (8) and (1) using a computer program, NISA (EMRC, 1998), based on the finite element method. The Table 5 Thermal properties of materials in the proposed disposal system.

Fig. 9. Effect of buffer thickness on the canister lifetime.

Material

Properties

Values

Nodular cast insert

Density (kg/m3 ) Thermal conductivity (W/m ◦ C) Specific heat (J/kg ◦ C)

7200 52 504

Copper outer shell

Density (kg/m3 ) Thermal conductivity (W/m ◦ C) Specific heat (J/kg ◦ C)

8900 386 383

Bentonite buffer

Density (kg/m3 ) Thermal conductivity (W/m ◦ C) Specific heat (J/kg ◦ C)

1970 1.0 1380

Backfill

Density (kg/m3 ) Thermal conductivity (W/m ◦ C) Specific heat (J/kg ◦ C)

2270 2.0 1190

Rock

Density (kg/m3 ) Thermal conductivity (W/m ◦ C) Specific heat (J/kg ◦ C)

2650 3.2 815

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Table 6 Mechanical properties for EBS. Material

Density (kg/m3 )

Young’s modulus (MPa)

Poisson’ ratio

Yield stress (MPa)

Nodular cast iron Copper Bentonite

7200 8900 2000

180,000 230,000 150.22

0.29 0.21 0.25

200 245 1.82

mum von-Mises stress was 201 MPs for 15 cm displacement which was lower than the yield stress, 245 MPa. 6. Conclusions

Fig. 11. Evolution of temperature along the centerline between the deposition boreholes.

maximum temperature occurred along the centerline between the deposition holes. Fig. 11 showed the change of temperatures at the points along the centerline with time after disposal. The maximum temperature on the surface of the canister was 84.0 ◦ C 13 years after the disposal. According to Raiko (2008), the peak temperature for the dry buffer might be greater by more than 10 ◦ C than that for the wet buffer. However, it was expected that the peak temperature could be lowered by the thermal design of buffer if necessary. 5. Mechanical analysis The robustness of a proposed copper canister should be confirmed against the external stress. In spite of 10 mm thickness, the proposed copper canister could show a better mechanical stability since the copper shell was attached to the inner cast iron structure whereas there was a gap between the inner structure and the outer shell for a Swedish copper canister. For the numerical simulation, a rock movement scenario (Werme, 1998) was selected and simulated using a computer program, ABAQUS. The strength of the canister was evaluated with mechanical properties given in Table 6 for three cases of rock movement speed with several displacements. The buffer was modeled with a Drucker–Prager model (Drucker and Prager, 1952). The von-Mises stresses were calculated with different shear displacements. As shown in Fig. 12, the maxi-

A new method for the manufacture of a copper-cast iron canister was proposed in order to effectively dispose of CANDU spent fuels, and demonstrated successively by making it at reduced scale. The thickness of a small scale copper shell was fabricated to be as thin as 10 mm with the new method based on the cold spray coating technique with high purity copper powders. XRD analysis showed oxidation was not interfered in the cold spray coating process, and the porosity of the coating layer was only 0.3 area%. It was expected that around 6 tons of copper could be saved with a 10 mm thick canister compared with a 50 mm thick canister. The electrochemical properties of the cold sprayed copper layer and forged copper were measured through a polarization test. The two layers showed very similar electrochemical properties. The lifetime of a copper canister was estimated with a mathematical model based on the mass transport of sulfide ions through a buffer. The lifetime of a 10 mm copper canister was estimated to be more than 140,000 years with the granite groundwater taken from a deep borehole around Korea Atomic Energy Research Institute. The thermal analysis with a current pre-conceptual design of a CANDU spent fuel canister was carried out using a computer program based on a finite element method. The results showed that the peak temperature between the canister and the saturated buffer was around 84 ◦ C below the thermal criteria, 100 ◦ C. The mechanical performance of the copper canister was analyzed using a computer program, ABAQUS. According to the calculation, the copper canister showed a good mechanical stability under the shear displacements. Even though the paper focused on the feasibility of the cold spray coating technique for manufacturing a copper shell, it is believed that many things still remains to be conducted through the further study. Very long-term corrosion test, effects of thermal stress due to differential thermal expansion of bimetal, and welding of the lid should be demonstrated or carried out on a real scale. Acknowledgements This study was performed under the long-term nuclear research and development program sponsored by the Ministry of Education, Science and Technology. The authors would like to express their sincere thanks to Ms. Hyun Ah Kim for her mechanical analysis with ABAQUS. References

Fig. 12. von-Mises stresses calculated with ABAQUS program.

Andersson, C.-G., 1998. Test Manufacturing of Copper Canisters with Cast Inserts, SKB TR-98-09, Sweden. Andersson, C.-G., 2002. Development of Fabrication Technology for Copper Canisters with Cast Inserts, SKB TR-02-07, Sweden. DOE/RW-0576, 2005. Science and Technology Program Plan. DOE, USA. Drucker, D.C., Prager, W., 1952. Soil mechanics and plastic analysis for limit design. Q. Appl. Math. 10, 157–165. EMRC, 1998. NISA User’s Manual. Engineering Mechanics Research Corporation. Gauld, I.C., Bowman, S.M., Horwedel, J.E., 2006. ORIGEN-ARP: Automatic Rapid Processing for Spent Fuel Depletion, Decay, and Source Term Analysis, ORNL/TM2005/39. ORNL, USA.

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Japan Nuclear Cycle Development Institute, JNCDI, 2000. H-12: Project to Establish the Scientific and Technical Basis for HLW Disposal in Japan, JNC-TN-1410/2000. JNCDI. Kim, S.-S., Chun, K.-S., Yeon, J.-W., Choi, J.-W., 2006. Corrosion of the candidate materials for the disposal container of a high-level radioactive waste. J. Korea Soc. Waste Manage. 23, 97–101. Kim, S.K., Lee, M.S., Choi, H.J., Choi, J.W., Kwak, T.-W., 2009. Progress of a cost optimization for an HLW repository in Korea. Prog. Nucl. Energy 51, 401–408. Lee, J., Cho, D., Choi, H.-J., Choi, J., 2007. Concept of a Korean Reference Disposal System for spent fuels. J. Nucl. Sci. Technol. 44, 1565–1573. Raiko, H., 2005. Disposal Canister for Spent Nuclear Fuel-Design Report, POSIVA 2005-02, Finland. Raiko, H., 2008. Manufacturing of the Canister Shells: T54&T55, POSIVA Working Report 2008-73, Finland. Sizgek, G.D., 2005. Three-dimensional thermal analysis of in-floor type nuclear waste repository for a ceramic waste form. Nucl. Eng. Des. 235, 101–109. Taniguchi, N., Kawasaki, M., 2008. Influence of sulfide concentration on the corrosion behavior of pure copper in synthetic seawater. J. Nucl. Mater. 379, 154–161. Werme, L., 1998. Design Premises for Canister for Spent Nuclear Fuel, SKB TR-98-08, Sweden.

Heui-Joo Choi was born on January 30, 1961 in Cheongju, Korea. He did his B.S. in nuclear engineering in HanYang University during the period March 1979–February 1983. He did his M.S. in nuclear engineering at KAIST during the period March 1983–February 1985. He did his Ph.D. in nuclear engineering at KAIST during the period March 1991–February 1996. During the period November 1987–October 1988 he worked as an attached staff at Chalk River Nuclear Laboratories in Canada. During that period he had worked on the colloid transport and studied numerical techniques in partial differential equations. During the period March 1985–December 1996 he worked in the field of safety assessment of the radioactive wastes disposal as a researcher and senior researcher of Korea Atomic Energy Research Institute. During the period January 1997–1999 he worked in the field of safety assessment of the radioactive wastes disposal as a senior researcher of Nuclear Environment Technology Institute of Korea Electric Power Corporation. From January 2000to till date he has been working in the field of modeling of radionuclides in the environment and design of the high-level radioactive waste repository as a principal researcher of Korea Atomic Energy Research Institute. His fields of interest include modeling of groundwater movement in both saturated and unsaturated geologic media, mass and heat transfer modeling, characterization of colloids, measurement of radioactivity and development of measurement systems, development of numerical techniques in partial differential equations.