I. Introduction

In Korea, two types of spent nuclear fuels (SNFs) are generated by two types of nuclear power plants: pressurized water reactors (PWRs) and pressurized heavy water reactor type (PHWR; CANDU). These SNFs are now stored in nuclear power plant sites. The SNFs inevitably generated by utilizing nuclear energy, which is considered radioactive waste, should be managed as high-level radioactive waste. Therefore, it should be safely isolated from the general public and natural environment for a long period to prevent present or future harm from high heat and radiation. Currently, the most widely accepted management method for the long-term isolation of SNFs is disposal in a deep geological repository designed and constructed with multiple barriers composed of engineered and natural barriers so that the waste can be completely isolated in a stable deep geological environment [1].

A long-term R&D program for safe management of SNFs as high-level radioactive waste was initiated in Korea in 1997. Active research has been conducted on the development of the Korean HLW disposal system, and several disposal concepts for SNFs in deep geological formations have been developed through this R&D program according to the nuclear policy environment.

In the early stages of the R&D program, a general disposal concept with data on SNFs from foreign countries such as the USA or Finland was developed because of a lack of data on SNFs and geology in Korea [2]. With the accumulation of SNF and geological data in Korea, the Korean Reference Disposal System for Spent Nuclear Fuels (KRS) was developed [3]. This disposal concept refers to the KBS-3 type concept developed by the SKB (Swedish Nuclear Fuel and Waste Management Co), which is an implementation organization in Sweden, and is currently considered a safe method worldwide [4]. The KRS showed that SNFs from nuclear power plants in Korea can be disposed of safely on the Korean peninsula.

Since then, as nuclear fuels have improved with the development of nuclear technology, the characteristics of SNF discharged from domestic nuclear reactors have improved. In other words, as the uranium enrichment for efficient power generation increased, the burn-up credit of spent nuclear fuel also increased. With these high-burnup data from improved SNFs, a deep geological repository system called KRS-HB (KRS for high-burnup SNF), which reflects the characteristics of high-burnup SNF generated in domestic nuclear power plants, was developed [5]. And next, considering the dimensions of SNFs and the cooling time at disposal time points, an improved disposal concept (KRS⁺) was developed in view of the disposal footprint area [6,7].

Research and development related to the direct disposal of SNFs has mainly focused on PWR SNFs because PWR SNFs have much higher decay heat and radioactivity than CANDU SNFs. However, because the characteristics of CANDU SNFs are significantly different from those of PWR SNFs in terms of the dimensions, number of SNF bundles, and handling systems in nuclear power plant sites, they should be considered differently from PWR SNFs in deep-geological-disposal systems.

In this paper, after reviewing the status of the CANDU SNF disposal concept in Canada and Korea, concepts related to the direct geological disposal of CANDU SNFs are described, and two concepts are proposed based on the results of the development. Analyses comparing these two concepts were conducted in terms of disposal safety, disposal efficiency, and technical maturity. Based on the results of the comparative analyses, a reference concept for the deep geological disposal of CANDU SNFs was determined.

2. Status on Development of Disposal Concepts for CANDU SNFs

2.1 Status in Canada

In Canada, disposal concepts have been developed and improved through case studies. During the 1st to 5th case studies, disposal containers were designed mainly using a Swedish method based on a copper layer with a thickness of 25–50 mm, as shown in Table 1 [8].

Table 1

1st to 5th case studies conducted in Canada

	1st & 2nd Case study	3rd Case study	4th & 5th Case study
Disposal concept

However, from the 6th case study, the Nuclear Waste Management Organization (NWMO) developed a disposal container with 48 bundles of CANDU SNFs instead of the large disposal containers (including a MARK I disposal container with the same specifications as the Swedish and Finnish disposal containers) designed during the 1st to 5th case studies. In the 6th case study, a disposal container named NWMO MARK II was proposed (Table 2). The copper layer thickness of the newly proposed disposal container was 3 mm, determined using electrodeposition technology, and this thickness was estimated to withstand a corrosion depth of 1.21 mm for more than 1 million years in a deep underground environment in Canada [9]. Table 2 presents a schematic diagram of the NWMO disposal container. The mass of the disposal container containing SNFs was approximately less than 3.0 tons, which was much lighter than the that of disposal containers in Sweden and Finland [10].

Table 2

Recent case study (after 6th) conducted in Canada

	6th Case study
Disposal container & buffer box concept
	Disposal container (MARK II) concept	Buffer box concept
Disposal concept
	Disposal tunnel & buffer box emplacement concept

The NWMO buffer material used was highly compacted bentonite. As shown in Table 2, the bentonite buffer box was manufactured to accommodate a MARK II disposal container. When placing a bentonite buffer box with a disposal container in a disposal tunnel, a spacer block was manufactured and filled between the buffer boxes to maintain the distance between them [11]. This NWMO disposal concept is now at the demonstration stage in CANADA [12].

2.2 Status of CANDU SNF Disposal Concept Development in Korea

The development of a deep-geological-disposal concept for CANDU SNFs, which are heavy-water-reactor SNFs generated along with PWR SNFs, was carried out through the development of the KRS [13]. As shown in Table 3, the disposal container of CANDU SNFs has 33 holes, each containing 9 bundles of CANDU SNFs, with a total of 297 bundles. At this time, the disposal container is set to have the same exterior dimensions as the PWR SNF disposal container to ensure easy handling of the disposal container. However, it is not efficient for handling many CANDU SNF bundles.

Table 3

Early version of disposal container concepts for CANDU SNFs in Korea

Disposal container in KRS		Improved disposal container
Concept	Descriptions	Concept	Description
	- Dimension (mm): 1,020 × 4,830 - Capacity: 297 bundles (33 bun./layer × 9 layers) - Copper thick.: 50 mm		- Dimension (mm): 1,240 × 4,170 - Capacity: 420 bundles (60 bun./basket × 7 baskets) - Copper thick.: 50 mm

The CANDU SNF bundle is small, and the number of bundles generated is large. Considering these characteristics and the situation in which CANDU SNFs are loaded into 60-bundle baskets and stored while dry at nuclear power plant sites, various methods have been proposed to facilitate the handling of CANDU SNFs and improve disposal efficiency [14].

First, a disposal container was designed to load seven baskets; thus, the total capacity of the disposal container was 420 bundles (60 bundles × seven baskets; Table 3). The thermal stability and structural integrity of the new disposal container were analyzed for this new disposal concept. The results of these analyses showed that the efficiency was improved by more than 30% in terms of the disposal area, compared with the initial disposal container of KRS [15].

Extensive efforts have been made to enhance disposal efficiency based on the Reference Disposal Concept of SNFs in Korea. In one of these efforts, alternative concepts for CANDU disposal systems were developed by modifying disposal containers. The modified canister accommodates baskets of 60 bundles of CANDU SNFs, as illustrated in Fig. 5. Based on the container capacity for baskets, four types of modified disposal canisters (CANDU-1B, 2B, 4B, and 8B) were designed, and four disposal concepts (DC- 1CAN, 2CAN, 4CAN, and 8CAN) were developed, as shown in Fig. 1. They all must satisfy the thermal requirements of the disposal system. Preliminary thermal analyses were performed to design the appropriate disposal concepts for each kind of disposal container, and the efficiency of each disposal concept was reviewed in terms of the disposal area, uranium density, excavation volume, and volume of copper and cast iron, among other factors [16].

Fig. 1

Various vertical-emplacement-type disposal concepts for CANDU SNFs.

According to the analysis results, the concept of loading four baskets in a disposal container and two disposal containers in a deposition hole (corresponding to DC-2CAN in Fig. 8, which shows a vertical emplacement in a deposition hole) showed the best disposal efficiency [17]. This concept was proposed as a vertical emplacement in a deposition hole for the deep geological disposal of CANDU SNFs.

In addition, with the disposal container concept of accommodating one 60-bundle basket used at a nuclear power plant site for temporary dry storage (CANDU-1B), the concept of the horizontal emplacement of a buffer box integrated with disposal containers in a disposal tunnel (see Fig. 2), similar to the Canada’s NWMO concept, was developed. As shown in Fig. 2, the concept of horizontal emplacement in a disposal tunnel was derived, and the feasibility of its application in Korea was examined. This concept was also proposed for horizontal emplacement in a disposal tunnel for the deep geological disposal of CANDU SNFs [18].

Fig. 2

Horizontal-emplacement-type disposal concepts for CANDU SNFs.

In this paper, two deep geological disposal concepts for CANDU SNFs are proposed and described in detail in the next section.

3. Proposal of Two Disposal Concepts for CANDU SNFs

As mentioned in the previous section, two disposal concepts for CANDU SNFs using a storage basket of 60 bundles were considered as improved concepts for efficiency in terms of the disposal area and handling of SNF bundles. These improved disposal concepts for CANDU SNFs apply the concept of a traditional KBS-3 vertical type and of a Canadian NWMO horizontal type to dispose of containers.

3.1 Characteristics of CANDU SNFs and Storage Basket

The reference CANDU SNF for deep geological disposal is the CANDU type-6 fuel bundle of the Wolseong nuclear power plant, which consists of 37 fuel rods of Zircaloyirradiated material and contains 30 pellets per fuel rod [19]. Although there is a slight difference depending on the utilization rate, more than 5,000 bundles of fuel are discharged from the CANDU reactor annually and transferred to wet storage tanks. Fuels that have been sufficiently cooled for more than six years are packed in a basket with a 60-bundle capacity and temporarily stored in a dry storage facility on site. The baskets for the 60 bundles of CANDU SNFs are made of stainless steel (340 L). The outer diameter is approximately 107 cm, the height is 56 cm, and the side and bottom thicknesses are 0.95 cm and 1.9 cm, respectively [20]. Fig. 3 shows the CANDU SNF bundles and the basket to be stored and loaded into a disposal container.

Fig. 3

CANDU SNF and basket.

3.2 Disposal Concept of Vertical Emplacement Type

Disposal containers of this vertical emplacement type of disposal concept are selected using the same material as for the PWR spent fuel disposal containers [21]. The materials used are cast iron inner vessels for structural support at the disposal depth and a copper outer shell for corrosion resistance. In addition, the thickness of the outer copper shell for corrosion resistance are set to 10 mm using cold-spray coating technology or 3-D printing technology [22,23]. Four baskets are loaded into each disposal container. Each disposal container contains 240 CANDU SNF bundles. Fig. 4 shows the CANDU-SNF disposal containers. The disposal containers will be transferred vertically without tilting to ensure the integrity of the spent nuclear fuel in the basket. A deposition hole is required to emplace two disposal containers vertically, and the space between the wall of deposition hole and the disposal containers is filled with compacted bentonite blocks; Fig. 4 shows the deposition hole with two CANDU SNF disposal containers. The disposable tunnel spacing is 40 m, and the deposition hole spacing was determined to be 5 m through a thermal analysis [7], as shown in Fig. 5. As shown in Fig. 5, the structural stability was confirmed by structural analysis of this disposal container at the depth of the disposal environment [24].

Fig. 4

Vertical-emplacement-type disposal concept.

Fig. 5

Thermal and structural stability of V-type disposal concept.

3.3 Disposal Concept of Horizontal Emplacement Type

Another disposal concept for CANDU-SNFs is horizontal- emplacement-type disposal. In this horizontal-type disposal concept, one basket of 60 bundles is loaded into a disposal container (Fig. 6). The container material is the same as the vertical-type concept (containing an inner cast iron vessel and outer copper shell) for the same purposes. The disposal container is surrounded by a compacted bentonite box, similar to a supercontainer [25]. This compacted bentonite buffer box assembly is emplaced horizontally in a disposal tunnel of around 250–300 m length with about 400 buffer box (24,000 SNF bundles) in stable rock at a depth of 500 m. A dense backfill block is installed to maintain the integrity of the disposal system, while maintaining a constant distance between the buffer boxes. The results of thermal and structural analyses confirmed the thermal stability of the horizontal disposal system and the structural integrity of the disposal container, as shown in Fig. 7. These performance analyses determined that the spacing between disposal tunnels for this horizontal disposal system of CANDU SNFs is 30 m and the spacing between the disposal container modules is 2.56 m [7].

Fig. 6

Horizontal-emplacement-type disposal concept.

Fig. 7

Thermal and structural stability of horizontal-type disposal concept.

4. Comparative Analysis of Two Disposal Concepts

As described previously, two types of disposal concepts for CANDU SNFs have been proposed. Assuming that a disposal repository is built in a bedrock with the same site conditions, the difference between the two disposal concepts can be an engineered barrier system (EBS), as shown in Table 4. Therefore, in this paper, the differences in the EBS of these two disposal concepts were compared and a reference disposal concept was selected based on the result of this comparative analyses.

The main components of the EBS in a deep geological disposal system are the disposal container and the buffer material. The disposal containers and buffer materials for the two disposal concepts of CANDU SNFs were compared quantitatively and qualitatively in terms of safety, disposal efficiency (economic feasibility, environmental friendliness), and technical maturity. The results of this comparison are summarized in this paper.

4.1 Disposal Container Lifespan

According to the Notice from Nuclear Safety and Security Commission in Korea, which regulates the performance of engineered barrier system, including disposal containers in high-level radioactive waste disposal facilities, engineered barrier system must prevent radionuclides from leaking, for more than several thousand years [26].

The disposal containers of the two disposal concepts for CANDU SNFs were double-layered with cast iron for the internal vessel and a copper shell for the external vessel. The lifespan of the disposal container is mainly determined by the corrosion capability of the copper shell. The thickness of the external copper shell owing to the corrosion load was considered to be corrosion under aerobic conditions and corrosion under anaerobic conditions. Corrosion under aerobic conditions is caused by the introduction of oxygen into the wick layer due to the construction of the disposal site, which is contained in the buffer even after the disposal site is closed. The thickness of aerobic corrosion due to the introduced oxygen was predicted to be up to 1.15 mm when considering the point corrosion, and it was predicted to corrode up to 2.57 mm when including anaerobic conditions [27]. In addition, in a recent study performed in Canada, the corrosion thickness of approximately 1.3 mm in 1 million years was fairly conservative, and the corrosion thickness was judged to be much thinner [28], while those in Sweden and Finland have evaluated to be 1.3 mm to 2 mm for 1 million years [29].

The disposal containers of the two concepts have the same thickness and use the same method. The external copper containers are constructed by coating the wall of a cast iron container with a thickness of 10 mm by applying 3-D printing technology or cold spray coating technology to the internal cast iron container.

Regarding the corrosion characteristics of disposal containers from a safety viewpoint, the lifespan of the copper container in these two disposal concepts was judged to be of little significance, even though the difference in the bentonite buffer thickness between the two concepts was almost the same and considering the regulation.

4.2 Radionuclide Migration in Bentonite Material

When a disposal container is damaged and loses its function, the main mechanism by which radionuclides leached by groundwater flow out to the bedrock around the disposal site through the buffer material is diffusion. Considering the geometry of the disposal concept, the outflow of radionuclides through the buffer material can be simulated using a two-dimensional diffusion movement that considers radioactive decay. The rate of radionuclide outflow, Ri, defined as the total cumulative flux of radionuclides through the cross-section of the buffer material at the buffer-rock interface, can be expressed as the following Equation 1 [30]:

Here, θ is the porosity of the buffer material, D_p is the pore diffusion coefficient of the nuclide, C_i is the concentration in the solution of nuclide i, and A is the cross-sectional area where nuclide diffusion outflow occurs at the buffer material-rock interface.

Korea Atomic Energy Research Institute (KAERI) evaluated the leakage rate through a buffer of 12 radionuclides, known to be important nuclides for assessing the safety of high-level waste disposal [31]. The total release rate of radionuclides over time after closure of the disposal site was calculated as a function of the radial thickness of the buffer material. The calculated results of the total radionuclide release rate according to the radial thickness of the buffer material 1,000 years after the closure of the high-level waste disposal site are shown in Fig. 8. As shown in Fig. 8, the total release rate of radionuclides decreased rapidly until the radial thickness of the buffer increased to 0.25 m and then decreased gently up to 0.5 m. However, beyond 0.5 m, even if the thickness of the buffer material increased, there was little effect in reducing radionuclide outflow. Regarding radionuclide migration in bentonite materials in the safety evaluation, the delay performance of engineered barrier system for important nuclides after disposal site closure had almost the same delay effect in the two disposal concepts.

Fig. 8

Total release rate of radionuclides as a function of the radial buffer thickness (over 1,000 years after repository closure) [31].

4.3 Disposal Efficiency

To evaluate the disposal efficiencies of the two concepts, the disposal footprint area, economic feasibility, and environmental friendliness were compared and reviewed. For economic feasibility and environmental friendliness, the amounts of material required for one SNF bundle in an engineered barrier system, disposal container, and bentonite buffer for each concept were calculated and compared.

4.3.1 Disposal footprint area

Once SNFs loaded in a disposal container are disposed of in a deep geological disposal repository, the heat transfer from the disposal container to the buffer and crystalline rock occurs mainly by conduction, which is a somewhat conservative assumption. Because the disposal footprint area is related to the decay heat of the SNF and the surface area of the disposal container, as shown in Equation 2 for Fourier’s law of conduction [32], the surface areas of the disposal container per CANDU SNF bundle were compared.

Table 5 lists the surface areas of the disposal containers for a bundle of CANDU SNFs for each disposal concept. As shown in the table, the surface area of the horizontal disposal is larger than that of the vertical disposal. However, The proportion of the CANDU SNF disposal area in the entire disposal area, including PWR SNF generated domestically, is extremely low. Therefore, it was judged that the difference in the disposal area would not be significant from the viewpoint of disposal areas with the same disposal volume.

4.3.2 Economic feasibility and environmental friendliness

In view of economic feasibility and environmental friendliness, two disposal concepts were compared and reviewed in terms of engineered barrier system resource requirements.

As an engineered barrier system resource requirement, the amount of copper material required for a bundle of CANDU SNFs in the disposal container and that of bentonite material as a buffer/backfill material required for a bundle were calculated and compared based on disposal concepts. The calculated results are listed in Tables 6 and 7, respectively.

As shown in the tables, owing to the differences in the way in which the unit modules are packaged, the vertical emplacement disposal concept was judged to be more advantageous in terms of the material quantity required to produce an engineered barrier system.

4.4 Technology Maturity

For the direct disposal of SNFs, a vertical disposal concept (KBS-3V type) was proposed in Sweden in the 1980s [33], and this concept was developed in the commercialization stage in Sweden and Finland. In Finland, the construction of a disposal facility was in progress after securing a construction license in 2015, and an operation license was applied for at the end of 2021. In Sweden, where this disposal concept was developed, a construction license was obtained in January 2022 [34]. Canada, which developed the CANDU nuclear reactor, established NWMO, an agency dedicated to SNF management, in 2002. Subsequently, concepts suitable for the direct disposal of CANDU SNFs were reviewed, and a horizontal tunnel emplacement concept (NWMO type) was developed. This concept is currently in full-scale production and the demonstration test stage. The selection process for a disposal site began in 2010; however, currently, two candidate sites have been selected, and the final site has not yet been decided upon [35].

In Korea, research on the direct disposal of SNFs has been conducted since 1997 by KAERI. To date, design and safety evaluation methodology development have been conducted based on the Swedish KBS-3V-type disposal concept and KURT site characteristics. It appears that the vertical disposal concept is slightly more mature than the horizontal disposal concept.

However, although there was much experience in the development of the KBS-3V-type concept, it was judged that there would be no significant difference between the KBS-3-vertical-type disposal concept and the NWMO-horizontal- type disposal concept in terms of technical maturity. Additionally, it was judged that the two concepts would be similar in terms of licensing.

4.5 Selection of Reference Disposal Concept for CANDU SNFs

Based on the criteria selected and analyzed in this study, the reference disposal concept for CANDU SNFs was determined, which may change depending on the disposal site environment. As mentioned previously, considering the current situation in which no project has yet been processed on a real disposal site, a concept was selected by analyzing the characteristics of the engineered barrier system of the disposal concepts. The comparison results of the two disposal concepts are summarized in Table 8. Although the superiority of the two disposal concepts cannot be quantitatively compared, considering the current status of the comparison analysis results, as shown in Table 8, the vertical-emplacement-disposal-type disposal concept was judged to be slightly superior and would be selected as the reference disposal concept.

5. Conclusions

There are two types of SNFs generated from two types of nuclear reactors, PWRs and CANDU, to be disposed of in Korea. CANDU SNFs have much lower decay heat and radioactivity but much larger numbers of bundles than PWR SNFs. Because the characteristics of CANDU SNFs are much different from those of PWR SNFs in the dimension and handling systems at nuclear power plants site, they should be considered in deep-geological-disposal system.

In this paper, CANDU SNFs were considered to be disposed of in the two disposal concepts located at a depth of 500 m. These two disposal concepts were compared and analyzed in terms of disposal safety, disposal efficiency, and technical maturity. Based on the results of these analyses, a reference disposal concept for CANDU-SNFs was determined.

The summary and the conclusions of this paper are as follows,

In this study, considering the current situation whereby no project has yet been processed on a real disposal site, a concept was selected as a reference disposal concept by analyzing the characteristics of the EBS of the disposal concepts. Once the disposal site is determined in the future, comparison of construction costs by site and additional analysis of the impact of adjacent disposal containers are necessary. In addition, considering the various changes that are difficult to predict in the future, it is advisable to conduct additional research on safety issues, including thermal-hydraulic-mechanical behavior, gas generation in the basket, and damage to disposal containers during handling on horizontal disposal and vertical disposal for CANDU spent nuclear fuels as an alternative disposal concept, if necessary.