• Ⅰ. Introduction
• II. Design premise
• III. Canister Design
• IV. Canister Manufacturing
• V. Corrosion resistance
• VI. Radiation shielding
• VII. Structural stability
• VIII. Summary
• Acknowledgements

Ⅰ. Introduction

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 Currently, a pyroprocessing is being developed by KAERI to cope with the highly accumulated amount of spent nuclear fuel at the temporal storage facilities in Korea, which is a valuable method for reducing the volume of radioactive waste together with the extraction of a reusable energy resource such as uranium or TRU. The pyroprocess produces several kinds of wastes, including metal waste, off-gas waste, and molten-salt waste. Among them, molten-salt waste is solidified by a ceramic binder and is called ceramic waste [1]. Ceramic waste is highly heterogeneous and generates high radioactivity and decay heat, and is thus often classified as high-level radioactive waste (HLW), and should be disposed of at a depth of several hundreds of meters below the surface.

  KAERI has developed a geological disposal system reflecting the geological conditions in Korea. This paper introduces the development of a disposal canister for ceramic waste, concerning its design premises, canister design, manufacturing, corrosion resistance, radiation shielding, and structural stability. Ceramic waste contains a small amount of long-lived radionuclides such as TRUs. Ceramic waste is required to be disposed of at a deep geological repository. Thus, a rigid disposal canister should be supplied for its confinement and integrity for a long disposal period.

  For the canister material, several kinds of metals, such as copper, cast iron, titanium, stainless steel, and nickel alloy, were considered. Copper-cast iron was concluded as the best candidate canister material for a Korean geological environment. The canister structure was designed to resist the hydrostatic pressure of groundwater and the swelling pressure of a saturated compact bentonite. Additionally, other dynamic impacts by fault movement and transportation accidents during the operation were considered in its design.

 After the establishment of the canister design, the canister integrity was confirmed for a long disposal period. The disposal canister should keep its air-tightness to avoid any leakage of the radioactive species. For this purpose, the corrosion resistance of the canister materials under geological repository conditions was empirically evaluated.

II. Design premise

 There have been no regulatory provisions for the disposal of HLW in Korea as of yet. However several disposal premises were suggested in this report to satisfy the purpose of securing HLW at the deep geologic formation.

 The life-time of a canister was set at 1,000 years. The TRU contents were too small to influence the overall activity in ceramic waste. The main radionuclides, such as  ¹³⁷Cs and ⁹⁰Sr, were expected to decay out after 1,000years [2].

 Static loads are originated from the groundwater and surrounding buffer material. The hydrostatic pressure of the groundwater is about 5 MPa at a 500 m depth [2]. The swelling pressure of the saturated buffer is about 7 MPa under experimental conditions at room temperature [2]. The buffer material from Gyeongju in Korea is Cabentonite with a dry density of 1.6 g/cm³ [2]. In practical terms, the swelling pressure of the buffer was supposed to be lower than 7 MPa at a disposal depth because the borehole  emperature was expected to be higher than room temperature due to geological thermal heat and heat dissipation from the disposal canisters. The swelling pressure decreased with an increase of temperature as shown in Figure 1 [3]. In the canister design, a  aximum static load of 12 MPa, the sum of the groundwater pressure and the swelling pressure of compact bentonite, were considered.

Fig. 1. Variation of swelling pressure with temperature from 25 to 70°C for compact Gyeongju bentonite (Dry density 1.6 g/cm³) [3].

 Dynamic loads on a disposal canister can be expected during its transport from an assembly factory to a deposition hole. The transport package for a disposal canister is a Type B container according to the Korean regulations of radioactive material ransportation. The estimated weight of the disposal canister is about 7 tons, which is applicable to a 0.9 m free-drop test in the regulations [4]. For the disposal canister, the free-drop height is set to be 2 m, multiplying the safety factor by 2.

 Fault movement across the borehole after the closure of a disposal system can cause additional dynamic load. The Korean peninsula was not absolutely free from earthquakes. Thus, the occurrence of tectonic movement should be considered in the canister design. A disposal canister should keep its integrity after rock movement of 10 cm crossing a borehole. The degree of impact on the disposal canister is related not only to the tectonic movement but also to the surrounding buffer properties. The validity of the canister design for dynamic loads can be examined using simulated tests.

  A disposal canister dissipates a thermal energy of about 297 W. If the temperature in the buffer rises to above 100°C, the transformation of smectite to illite is a worry because the transformation causes a loss of the swelling capacity [5]. The maximum temperature in the buffer needs to be set to maintain the buffer property. In the design of a disposal system, the maximum temperature was limited to 100°C.

 A disposal canister should keep a safe status for at least 1,000 years regardless of any corrosion behaviors. Therefore, the corrosion depth in an underground environment has to be less than the thickness of the copper shell. The corrosion environment may be an oxidizing condition soon after the closure of the disposal system, and it then becomes an anoxic condition as the trapped oxygen is consumed. Sulfide is considered as the main corrosion agent in an anoxic condition.

 A disposal canister is composed of an inner cast iron container and an outer copper shell. The inner cast iron container is locked by multiple bolts with a lid, and a Viton O-ring is inserted between the inner cast and the lid. The outer copper shell is sealed by cold spray coating or welding. After sealing the canister, its air-tightness should be checked. The permissible helium leak rate is set to be 10^-3 Std.cm³/sec during the inspection.

  The high-energy radiation from a canister can change the borehole environment, producing corrosive agents through radiolysis or heating. Thus, the absorbed dose rate at the canister surface was limited to 1 Gy/hr [6]. The multiplication factor, K_eff , was  estricted to below 0.95 to prevent criticality [6], although there was little probability for the pyroprocessed waste to exceed the criticality.

III. Canister Design

 Ceramic waste is vitrified molten salt from the electrowinning process, which is the final step of the pyroprocess. Molten salt is vitrified into a cylindrical shape using a monazite-based ceramic binder at higher than 1,000°C. The cylindrical block is D 26 cm X H 25 cm in dimension and weighs about 48 kg [2]. The size of the ceramic waste block is determined for effective heat dissipation because high temperature due to decay heat can cause a phase transition of the ceramic binder. The maximum temperature at the center of the block is set to be 200°C [2].

 For the convenience of handling in a maintenance facility, the weight of a storage canister with waste is recommended not to exceed 200 kg. Only 2 waste blocks are placed in a storage canister to make 120 kg of total weight. A commercially available stainless steel 304 L tube (KS250A Sch. 5S) with a 3.4 mm thickness is selected for the cylindrical body. Stainless steel 304 L plates with 5 mm thickness are used for the bottom and lid plates [2]. There is a collar on the lid for lifting. The sealing between the container body and lid is achieved by arc-welding. There is a 10% void space in the storage canister for the attenuation of pressure build-up by decay gases released from the ceramic blocks (Figure 2).

Fig. 2. Semi transparent view of a storage canister containing 2 ceramic waste blocks(right) and its outlook (right).

 One disposal canister accommodates 14 storage canisters by 2 layers (Figure 3). The disposal canister consists of an inner container for the structural strength and radiation shielding, and an outer shell for corrosion resistance. The dimensions of the disposal canister are D 103 cm H X 173 cm, and the total weight is about 7.2 tons. The overall heat power per disposal canister is 594 W [2].

Fig. 3. Cross-sectional top (left) and front (right) views of a disposal canister [7].

 Some technical requirements of "Landfill Disposal for Very Low Level Radioactive Solid Waste" are as follows :

 VLLW should be landfill disposed in the nearby facility. For large amount of VLLW, an on-site facility is recommended. HDPE membrane, GCL, and geotextile can be used as liner system engineering material. The highest water table should be controlled at 2 meters below the liner foundation. Institutional control period is 30 years.

  The inner container is made of cast-nodular iron to resist static loads by the groundwater pressure and swelling pressure of the buffer material. The thicknesses of the cylindrical shell and end plates are set to be 95 mm and 180 mm, respectively. It can resist up to 15 MPa of external pressure according to the Korean Mechanical Standard (KSB 6734, “Shell and head of pressure vessels”). A lid is assembled onto the container body by 6 screws of M27 size which can resist the lifting load. The sealing between the lid and body is achieved using a Viton O-ring (Figure 4).

Fig. 4. Design drawing of the lid part of a disposal canister.

The outer shell is made of copper to resist corrosion loads in the disposal environment. The copper thickness is set to 10 mm assuming that the copper layer would corrode lower than the rate of 1.0 μm/yr [8]. The 10 mm thickness of the copper layer is considered sufficient to keep its airtightness from environmental corrosion for 1,000 years, even if there will be some local fluctuations in the corrosion depth [7]. The copper thickness of the bottom and lid is 30 mm, which is thicker than that of the shell to accommodate good weld-ability and impact resistance during the handling. A screw-type copper lid is attached directly onto a cast iron container, and the body is already covered with a copper shell (Figure 4). The copper lid and copper shell are sealed by cold spray coating or arc-welding. 

IV. Canister Manufacturing

 A storage canister can be easily manufactured using a currently available technique with commercially available components. The inner container of the disposal canister can be fabricated simply at a casting foundry. An inner container has been successfully made at half scale using a casting method. However, it was difficult to make the 10 mm thick outer container with copper since this was too thin to apply conventional manufacturing methods such as piercing and drawing, extruding, and forging due to the flexibility of pure copper. Thus, a cold spray coating method was studied to make a 10 mm thick copper shell onto the inner cast-iron container directly rather than making a separate container [9].

 Cold spray coating is a relatively new technology. It spurts soft metal powders ranging from 1 to 50 ·Ïm in diameter onto a target surface at supersonic speed with the aid of a carrier gas [10]. A solid state coating is then achieved by the plastic deformation and stacking of metal powders. Cold spray coating has advantages in that the layer formed is clean, dense, and thick compared with the thermal sprayed coating. Cold spray coated copper showed a high modulus and its breaking strength is 2-3 times higher than general copper; however, it was slightly brittle. This coating also had an attractive advantage in that the copper is not oxidized entirely, and maintains its basic purity.

 A small disposal canister was manufactured at a 1/10 scale applying the cold spray coating method. The inner container was fabricated using casting and machining. The thickness of the copper layer formed on the surface using the cold spray coating method was over 10 mm (Figure 5). The bottom part was also formed by cold spray coating.

Fig. 5. Copper-cast iron disposal canister at 1/10 scale fabricated by cold spray coating ( 12 cm x H 43 cm x t 10 mm) [10].

 Another disposal canister was made at the 1/20 scale, in which the bottom and lid parts were from the extruded copper plate. The plate parts were tightly combined by screwing with the inner cast iron container. The cylindrical surface was then coated with copper reaching a 5 mm thickness (Figure 6). In practical application, the lid part should be attached to the body at the last stage unlike in this manufacturing.

Fig. 6. Copper-cast iron disposal canister at 1/20 scale fabricated by cold spray coating ( 5.1 cm x H 18.1 cm x t 5 mm).

 Using a cold spray coating method, several Cu canisters were successfully manufactured in a reduced scale. From this experience, it was believed that the cold spray coating is technically reliable to fabricate the outer copper shell.

 Several welding techniques were considered for sealing a disposal canister. Firstly, laser welding was considered because it was a widespread technique in industrial applications. However, laser welding had a demerit of shallow welding depth. Secondly, electron beam welding was considered as a dominant candidate because it could form a thick welding for a relatively short period of time. However, electron beam welding could cause a local phase transition at the welding joint due to the high heat generation and residual stress after the welding. Finally, friction stir welding had been considered, which had been adopted in Sweden and Finland. This  ethod hasda merit in that the thermal deformation was only limited to the welding surfaces. However, it had demerits in that its application becíà more difficult as the welding depth became thicker, and the welding could be coarse at the beginning and finishing regions at which the friction stir pin came in and out. Currently, cold spray coating is considered more promising for the sealing of a copper canister than thermal welding methods mentioned above. The welding part has the same microstructure as that of a cylindrical body because cold spray coating does not require high temperature, which can cause a phase transition of the copper.

  The practicality of cold spray coating for sealing a canister was investigated together with a helium leak test. A small stainless steel container was mechanically closed with a screw-type copper lid. The cylindrical shell was treated with cold spray coating to form a 5 mm thick copper layer. The helium leak test confirmed that no leakage was detected along the welding line of the sealed canister (Figure 7). The sealed canister was sectioned and the weldline was examined. It was verified that there are no voids or cracks in the canister. Consequently, it was concluded that the sealing was successful.

Fig. 7. Sealing of copper stainless steel container by forming a 5 mm thick copper layer by cold spray coating.

V. Corrosion resistance

 The copper coating was studied to determine if it can resist corrosion for 1,000 years at a deep disposal environment. The disposal canister would be disposed of into a deep geological formation at a depth of 500 m below ground.

  The major ionic components of the groundwater sampled from KURT at a depth of 200 m were given in Table 1. The Eh of the groundwater was about -380 mV, which was less than the corrosion potential of copper, -100 mV. The pH of the groundwater  indicated a weak alkaline of pH 8.1. The temperature was constant at about 15.4°C throughout the whole year.

Table. 1. Major ionic components of groundwater at KURT.

  The corrosion potential was measured for several kinds of copper samples including cold spray coated copper using polarization tests in a 3 M NaCl solution (Figure 8). The corrosion potential was not affected by the formation method, but the purity of the copper influenced the corrosion potential. High pure copper had the highest corrosion potential as shown in Figure 8. The corrosion currents of the coated coppers were slightly higher than those of other coppers regardless of the purity.

Fig. 8. Polarization curves of several kinds of copper in a 3.0 M NaCl solution [9].

  Several immersion tests were performed for the copper specimens in an active environment such as a concentric HCl solution, humid air, and seawater. The results of the tests indicated that the coating copper was slightly more vulnerable to corrosion than normal coppers such as extruded and forging coppers. However, the suppression of corrosion was detected for the coating coppers as in the other copper specimens according to the results of the humid air test. A rapid increase in weight loss was only limited to several of the early days, presumably due to the formation of a passive oxide film (Figure 9). Therefore, it was conceived that a cold spray coating method could be applicable to the manufacturing of a disposal canister because the corrosion resistance of the coating copper becomes stronger as time goes by.

Fig. 9. Weight loss curves of copper specimens in humid air [9].

  Environmental corrosion tests were performed for various coppers in a simulated repository condition in a laboratory. The test specimens were placed between Gyeongju compacted bentonite blocks in a water- permeable titanium vessel, and then immerged in groundwater collected from KURT. The results of a 2-year test showed that all the coppers exhibited extremely low corrosion rates of less than 1.0 μm/y, and there were no signs of local corrosion on the surfaces (Figure 10).

Fig. 10. Corrosion depths of copper specimens in a simulated environment in a laboratory.

 A long-term corrosion test was performed at KURT, in which groundwater collected from a deep borehole was then transferred to a titanium vessel continuously to simulate a deep geological repository condition. Figure 11 shows the results of a 2-year test. The corrosion rate was less than 0.2 μm/y. The corrosion depths of the coated coppers were not as different as other normal coppers.
Consequently, it was concluded that a 10 mm thick coated copper shell is sufficient as a corrosion barrier for 1,000 years under the disposal environments.

Fig. 11. Corrosion depth of copper specimens for a long-term corrosion test at KURT.

 Using the corrosion rates measured at KURT, the lifetime of a copper canister was estimated. The corrosion depth of most coppers appeared to be less than 0.2 μm for 365 days, and 0.3μm for 700 days as shown in Figure 11. Even if the corrosion rate was conservatively set to 0.2 μm/y, the lifetime of a 10 mm thick copper canister was expected to be much longer than 10,000 years.

 Generally, oxygen is a major corrosion agent for copper corrosion under an oxidizing condition. However chloride and sulfide can be the major corrosion agents under an anoxic condition like a deep geological disposal environment. Since the corrosion potential of chloride for copper is much higher than the underground Eh value, the corrosion is impossible thermodynamically. Also, the concentration of sulfide in deep KURT groundwater is very low. Although the microbial activity or ferrous ions (Fe²⁺) can reduce sulfate into sulfide, the corrosion by sulfide has no significant effect on the overall corrosion rate of copper. Another corrosion agent is the corrosive products from radiolysis. However, radiolysis produces the same amount of reductive agent, too. It is not clear that radiation is harmful for a copper canister in the current status.

VI. Radiation shielding

 A ceramic waste disposal system consists of waste blocks, storage canisters, disposal canisters, and a buffer. The absorbed dose due to gamma rays and neutrons at the boundary between the disposal canister and buffer was calculated. Generally, a HLW disposal canister should be designed to obtain a low surface activity to suppress the generation of radiolysis products, which can act as a corrodant for the canister.

  The total photon and neutron rates for a storage canister that contains 2 ceramic waste blocks were 1.51 × 10¹³ (photons/sec) and 1.079 × 10³ (neutrons/sec), respectively, for the 44 energy groups. The Flux-to-Dose factor of ICRP-74 was used for the dose  onversion. Dose calculations for a disposal canister were performed, the results of which were given in Table 2. The absorbed dose in a buffer calculated at a 1.0 cm bentonite layer was around 0.0115 Gy/hr which was much lower than the maximum permissible value of 1.0 Gy/hr in the disposal canister design of KAERI [6].

Table 2. Absorbed dose calculation results for ceramic waste disposal system.

VII. Structural stability

 A disposal canister needs to be designed and handled so that the postulated operational conditions during encapsulation, storing, transport, and deposition do not cause any damage or change in properties that may affect the disposal canister’s ability to isolate its contents.

  During normal operation, a disposal canister is lifted a few times from the lifting shoulder of the copper lid. The grip is made with a special device that gives a secured grip and a widely distributed surface load. Thus, the contact pressure should not exceed the copper strength. The payload of the gripper is the total weight of the disposal canister and its contents (Table 3).

Table 3. Loads used for the stability analysis during handling.

 For a lifting operation, an additional dynamic factor was added according to the lifting device design verification practice and respective standards. In addition to normal operation, some disturbance effects were also analyzed, such as the operation of an emergency braking system of the lifting device in the disposal canister installation vehicle (Table 4). The computation results with a safety factor of 2 had satisfied the standard criteria (yield strength) of 45 MPa for both normal and deceleration cases. In addition, the results from a free fall situation, which was an extreme case for the handling of a disposal canister, were confirmed to be below the standard criteria value as well [11].

Table. 4. Results of a stability analysis during handling [11].

  The design pressure was assumed to be evenly distributed and acting on all faces of the canister. For an abnormal condition, the bentonite swelling pressure was assumed to also be unevenly distributed in the saturated condition due to a tilted canister in the disposal hole, the variations of bentonite density, heterogeneous rock properties, or a curved disposal hole. The results of the stability analysis were given in Table 5. The computation results with a safety factor of 2 had satisfied the standard criteria of 235 MPa for a normal case. In addition, the results from an extreme case were also confirmed to be below the standard criteria as well [12].

VIII. Summary

 The pyroprocess is currently being studied by KAERI, and will produces a ceramic waste classified as high-level radioactive waste (HLW). Such ceramic waste should be disposed of at a depth of several hundreds of meters below the surface. In this paper, an overview of disposal canister development for the ceramic waste was introduced, mainly regarding its design premises, constitution, manufacturing, corrosion resistance, radiation shielding, and structural stability.

 In the design premises, the required life-time of a canister, the expected maximum static and dynamic loads, the maximum temperature at the canister surface, the permissible leak rate on the canister, the maximum dose rate at the canister surface, and the multiplication factor (K_eff ) were discussed to design a reliable disposal canister.

  The disposal canister dimension was designed to be D 103 × H 173 cm, and the total weight is about 7.2 tons. The heat power per disposal canister was 594 W. The disposal canister had double layer structure. The inner layer was made of cast-nodular iron to resist mechanical loads, and the out layer was made of copper to resist corrosion loads in a disposal environment. The copper  hickness was set to be 10 mm, which was considered to be thick enough to resist the environmental corrosion for 1,000 years.

 To manufacture a double layered disposal canister, a cold spray coating method was adopted. A disposal canister at the 1/10 scale was successfully manufactured by applying a cold spray coating method. And cold spray coating was also considered a promising technique for the sealing of a copper canister, rather than common thermal welding methods, since cold spray coating did not bring about a thermal transition of the metal phase at the weld region.

  The corrosion behavior of the coating copper was studied to know if it can resist environmental corrosion for 1,000 years at a deep geologic condition. As a result of several corrosion tests, it was concluded that the copper formed by cold spray coating is not much different in corrosion behavior compared with other common coppers. A long-term corrosion test was being performed at KURT, simulating a deep geological repository condition. The estimated lifetime of a 10 mm thick copper canister was much longer than 10,000 years at a deep geological condition.

 The absorbed dose due to gamma rays and neutrons at the boundary between the disposal canister and buffer was calculated. The calculated dose rate in a bentonite layer was around 0.0115 Gy/hr, which was much lower than the maximum permissible value of 1.0 Gy/hr.

  The structural safety of a disposal canister was estimated using a computational analysis. For a lifting operation, the analysis results had satisfied the standard criteria (yield strength) of 45 MPa for both normal and deceleration cases with a safety factor of 2. In addition, the abnormal emplacement of a disposal canister, in which the swelling pressure around a disposal canister was assumed to be unevenly distributed, was also studied. The analysis results for an abnormal emplacement had satisfied the standard criteria of 235 MPa.

Acknowledgements

 This study was performed under the long-term nuclear research and development program sponsored by the Ministry of Education, Science and Technology.