The first commercial nuclear power plant, Kori Unit 1, was in operation since 1978 and nuclear power has been an important power source for domestic industrial development. Currently, twenty pressurized water reactor type nuclear power plants and three pressurized heavy water reactor type Canada deuterium uranium (CANDU) nuclear power plants are in operation. In addition, according to the 8th power supply and demand plan based on the current energy conversion policy, total of thirty nuclear power plants are planned to operate until the 2080’s according to their lifespan. As long as there is no alternative from innovative alternative energy, the power supply from nuclear power currently in operation and planned is expected to be continued. Therefore, in order to supply electricity stably, the safe and long-term management of spent nuclear fuel, which is inevitably discharged after generating electricity at a nuclear power plant, as well as the safe operation of a nuclear power plant, is essential. These spent nuclear fuels are classified as high-level wastes, and they are disposed of directly or they are considered with a strategy to dispose of radioactive waste generated from the recycling process of spent nuclear fuel for useful substances. Whether considering direct disposal of spent nuclear fuel or disposal of high-level waste generated from the recycling process, it is essential to keep the public safe and to maintain isolation from the human environment for a long time of tens of thousands of years.
The spent nuclear fuels accumulated in Korea are currently being managed in wet or dry temporary storage at each nuclear power plant site. The government established a basic plan for high-level radioactive waste management  in 2016 to prepare a final management plan for spent nuclear fuel, but it is in the re-review phase now.
In contrast to the advanced overseas countries in the radioactive wastes disposal field that have been conducting disposal research since the 1970’s, research and development on high-level radioactive waste in Korea was launched in 1997. In 2007, a deep geological disposal system for spent nuclear fuel, called KRS (KAERI Reference disposal System for spent nuclear fuel), based on the concept of the Swedish KBS-3 type was developed [2, 3]. The KBS-3 concept was a deep geological disposal method for disposing of spent nuclear fuel directly into stable rock with a depth of 500 m, and is currently considered the safest method with current technology . The burn-up of spent nuclear fuel since then has been increasing as the characteristics of spent nuclear fuel released from domestic nuclear reactors have advanced due to the progress of nuclear fuel improvement projects. Therefore, a deep geological disposal system, called KRS-HB (KRS for High Burnup spent nuclear fuel), that reflects the characteristics of high burnup spent nuclear fuel generated by domestic nuclear power plants has been derived .
It is expected that the amount of spent nuclear fuel will continue to increase as it is currently temporarily stored and accumulated at the nuclear power plant sites. Moreover, the operation of the nuclear power plant is required continuously in accordance with the established basic power supply plan. The disposal site area for their safe management is thus also expected to increase significantly. Therefore, research on improving disposal efficiency by reducing the area of the disposal site is essential for efficient use of the national land and public acceptability [6-8].
In this study, based on the existing geological disposal concept (KRS), the concepts of an improved geological disposal concept, which was called KRS+ (the Improved KAERI Reference disposal System for spent nuclear fuel), were presented. They were developed in consideration of the characteristics of pressurized light water reactor (PWR) type and pressurized heavy water reactor type (PHWR, CANDU) spent nuclear fuel in Korea such as the dimension and the time of discharge. In addition, the disposal scenario in accordance with the current high-level radioactive waste basic plan was considered. To this end, the general design requirements and the performance targets for the multi-barrier of the disposal system were derived. Based on this, the characteristics of spent nuclear fuel discharged from domestic nuclear power plants were analyzed to develop the concepts of disposal containers and disposal systems suitable for the characteristics of each type of reactor and specifications. The disposal efficiency of the KRS+ compared with the existing disposal concept was then assessed.
This study was conducted to secure flexibility and adaptability according to the progress of the environment and technology due to the characteristics of the spent nuclear fuel for safe management, which takes a long time from the preparation stage, such as the establishment of policies for managing spent nuclear fuel, to the operation and closure of the final disposal site.
2. Existing Disposal Concepts of spent nuclear fuel
2.1 Reference spent nuclear fuel for existing concept
The reference spent nuclear fuel for the existing disposal concepts was established as follows.
A deep geological disposal system (KRS) for disposal of spent nuclear fuel generated from domestic reactors was developed in 2007. At that time, the reference spent nuclear fuel had values of 4.0wt% initial enrichment, 45 GWd/MtU burn-up, and 40 years cooling time for PWR type. The reference spent nuclear fuel for CANDU type had values of 0.711wt% initial enrichment, 7.8 GWd/MtU burn-up, and 30 years cooling time [9,10]. The characteristics of spent nuclear fuel discharged from nuclear reactors have changed since then due to advances in domestic nuclear power generation technology to improve economic efficiency. In particular, high burn-up spent nuclear fuels are expected to reach about 70% of the total generation from 2010. Setting these high burn-up spent nuclear fuel as new reference spent nuclear fuel (Table 1) for the basis of the geological disposal system was proposed [11,12]. With these reference spent nuclear fuel, the concept of the reference disposal system for high burn-up spent nuclear fuel was developed .
2.2 Reference disposal system
The safety of a geological disposal system for spent nuclear fuel relies mainly on the multiple barriers consisting of a disposal container, buffer, backfill and host rock. According to the Nuclear Safety and Security Commission Notice 2017-74, the total annual risk shall not exceed 10-6 and expected exposure dose shall not exceed 10 mSv‧yr-1 for the representative.
To meet the Notice, the most important requirement in the design of a geological disposal system with multi barrier is the temperature requirement of bentonite, a buffer material. Although some studies on bentonite have evaluated the impact on the safety of the disposal system when the temperature of bentonite exceeds 100°C , the current temperature requirement is to ensure that the temperature does not exceed 100°C at the interface between the disposal container and the bentonite block [14,15]. As shown in Figs. 1 and 2, reference disposal concepts for high burnup spent nuclear fuel that satisfied the above design requirements through thermal stability analyses for disposal of PWR and CANDU spent nuclear fuel in Korea were derived. The disposal container was double layered with a cast iron inner vessel and a copper outer shell. The disposal container with spent nuclear fuel would be emplaced in the deposition hole located at the floor of the disposal tunnel excavated in stable rock at a depth of 500 m. The space between the disposal container and the wall of the deposition hole was filled with bentonite blocks (Fig. 1). In addition, as shown in Fig. 2, the layout for the disposal container with 4-PWR spent nuclear fuel assemblies, which was suitable for design requirements based on the results of a thermal stability analysis, had 40 m disposal tunnel spacing and 9 m disposal hole spacing . The layout for the disposal container with 297-CANDU spent nuclear fuel bundles was also determined by thermal analyses. It had 40 m disposal tunnel spacing and 4 m deposition hole spacing .
3. Analyses of the Characteristics of Spent Nuclear Fuel and the Disposal Scenario
The spent nuclear fuel continues to accumulate in temporary storage facilities at the nuclear power plant sites. Considering the amount of spent nuclear fuel generated based on the energy conversion policy, the area required for a deep geological disposal system is expected to be significant. Therefore, various studies are needed to minimize the area required for disposal of spent nuclear fuel in cases where the national land is very limited, such as in Korea. In this paper, as one of these studies, the characteristics of spent nuclear fuel from domestic nuclear power plants, such as the type, specifications and time of discharge were analyzed. In addition, the disposal scenario was established by referring to the existing HLW management basic plan. Based on this, the concept of an improved disposal concept (KRS+) was developed by calculating the acceptable decay heat per disposal container from setting the cooling time of the spent nuclear fuel at the time of disposal. The KRS+ should also meet the Notice described in previous chapter 2.2, and the temperature requirement of bentonite as a buffer material.
3.1 Specifications of spent nuclear fuel
3.1.1 Specifications of CANDU spent nuclear fuel
The spent nuclear fuel discharged from CANDU nuclear power plants have been loaded into a basket of 60 bundles and temporarily stored in concrete silos or dry storage facilities called Maxtor 400 at a power plant site. In Korea, four CANDU-type reactors have been operated at Wolseong site, and Wolseong Unit 1 is currently suspended. Table 2 presents the specifications and weight per spent nuclear fuel bundle, including the diameter and length of spent nuclear fuel discharged from CANDU reactors. In addition, Fig. 3 shows the decay heat per unit weight (1 tU) of CANDU spent nuclear fuel after release from the reactor with time .
3.1.2 Specifications of PWR spent nuclear fuel
Currently, various types of PWR nuclear fuel are being used in Korea because various PWR type nuclear power plants depending on the supplier country have been operated. The basic specifications of spent nuclear fuel for each type of nuclear power plant scheduled to be operated and constructed in Korea are shown in Table 3. In addition, Fig. 4 shows the decay heat per unit weight (1 tU) of PWR spent nuclear fuel after release from the reactor with time .
Two lengths of spent nuclear fuel, important factor in the design of disposal containers, are applied, 406 cm and 453 cm. The 406 cm-long spent nuclear fuel are discharged from Gori units 1, 2, 3, 4, Yeonggwang units 1, 2 and Uljin units 1 and 2. And, the 453 cm-long spent nuclear fuel are discharged from other nuclear power plants.
In this study, to help understanding and avoid confusions, the 406 cm-long spent nuclear fuel, which is short, was named S-SNF (Short-Spent Nuclear Fuel) and the 453 cm-long spent nuclear fuel was named R-SNF (Regular- Spent Nuclear Fuel).
3.2 Amount of spent nuclear fuel and time of discharge
Since 1979, the nation's first commercial nuclear power plant, Gori 1, generated an assembly of spent nuclear fuel from PWR reactors, and as of the end of the first quarter of 2020, about 19,150 spent nuclear fuel assemblies had been released and stored in the temporary storage pool . For the CANDU spent nuclear fuel, as of the end of the first quarter of 2020, about 465,800 bundles had been released and temporarily stored in on-site storage pool or dry storage facilities (concrete silos and Maxtor-400s) .
The total amount of spent nuclear fuel generated from nuclear power plants could be assessed by predicting the amount of spent nuclear fuel discharged annually from each plant in accordance with the national reactor facility plan. The estimated total amount of spent nuclear fuel generated from the scenarios under the energy conversion policy was as follows.
Based on the 8th power supply and demand plan, 26 PWR reactors and four CANDU reactors will be operated, and the total amount of spent nuclear fuel from the 30 nuclear power plants was estimated. Table 4 shows the estimated total amount of spent nuclear fuel generated. As shown in Table 4, as of 2082, when all 30 nuclear power plants will be shut down, the estimated spent nuclear fuel amount is expected to be a total of 39,500 tons, with about 62,400 assemblies (approximately 26,900 tons) of PWR spent nuclear fuel and about 664,000 bundles (approximately 12,600 tons) of CANDU spent nuclear fuel.
For PWR spent nuclear fuel, about 14,100 assemblies of S-SNF are expected by 2028, and 48,300 assemblies of R-SNF are expected from nuclear power plants operating by 2082 as shown in Table 4. The amount of CANDUtype spent nuclear fuel was 473,600 bundles by 2018 and 664,000 spent nuclear fuel would be expected to be discharged by operating CANDU-type reactors by 2030, and the accumulation of spent nuclear fuel is shown in Table 4.
3.3 Disposal scenario depending on the time of discharge
The Review Committee of the current spent nuclear fuel management policy in accordance with the energy conversion policy was reviewing the high-level radioactive waste management basic plan established in July 2016. Nevertheless, in this study, the disposal scenario was established based on the existing high-level radioactive waste management basic plan. Operation of the disposal repository is set to begin in 2053 based on the schedule outlined in the basic plan. The schedule sets disposal of the CANDU type spent nuclear fuel for the first 10 years, followed by disposal of the PWR type spent nuclear fuel from 2063.
The disposal rate of CANDU spent nuclear fuel is 66,500 bundles per year, which is set to dispose of 1108 baskets of 60 bundle capacity. In addition, the disposal rate of PWR spent nuclear fuel was set to dispose of 250 disposal containers (1,000 assemblies) per year, assuming 250 working days per year. Therefore, out of the total PWR spent nuclear fuel disposal capacity of 62,420 assemblies, 14,133 assemblies of S-SNFs, were set to be first disposed of from 2063 to 2077 after completion of CANDU spent nuclear fuel disposal and 48,287 bundles of R-SNFs were set to be disposed of from 2078 to 2125 after completion of these S-SNF disposal.
Taking into account the disposal rate under the disposal scenario described above and considering that spent nuclear fuel will be disposed of in the order of discharge at the nuclear power plants, it is possible to estimate the cooling time of the spent nuclear fuel at the point of disposal. Based on this, the cooling time at the point of disposal of spent nuclear fuel was calculated as shown in Tables 5 and 6. In addition, when the decay heat was applied accordingly, the decay heat of spent nuclear fuel per disposal container could be determined.
4. Disposal System Design Requirements
The design requirements provides relevant principles for the design of KRS+ and aims to relate requirements to iterative processes of design and performance assessment . The requirements were established to the satisfy the safety principle of the KRS+. The safety principle consists of safety features including isolation, containment, retention, and retardation of radionuclides. The system, together with each system component, should be designed to accomplish and support the safety features in Table 7.
The requirements do not include for the spent nuclear fuel because properties of the fuel could not be designed. Stability of the fuel matrix in an anticipated repository condition, however, should be considered as a safety concept of the KRS+ in the design process.
For the components of the engineered barrier system (EBS), a list of performance targets was derived on the basis of design premises as shown in Table 8 . Likewise, a list of target properties for the host rock was derived and summarized in Table 9. Details of the list and the design premises are provided in reference 21. The design premises of the KRS+, together with the specific design requirements, will be continuously updated and be specified as the siting process in Korea proceeds.
5. Improvement of the Existing Deep Geological Disposal Concepts
The existing PWR and CANDU disposal concepts (KRS) were improved in the direction of improving disposal efficiency as described below, taking into account the characteristics of the spent nuclear fuel discharged from domestic nuclear power plants and the cooling time according to the disposal time of spent nuclear fuel.
5.1 Improvement of the CANDU spent nuclear fuel disposal concept
The CANDU spent nuclear fuel is smaller than the PWR spent nuclear fuel and is currently stored in a basket of 60 bundle at the dry storage facility. The CANDU spent nuclear fuel disposal concepts were thus designed with the concept of loading the basket into the disposal container. As shown in Figs. 5 and 6, the improved disposal concepts for CANDU spent nuclear fuel applied the concept of a traditional KBS-3 vertical type to dispose of containers loaded with these baskets and of a Canadian NWMO type to dispose of them horizontally. Specifications of basket and disposal containers for each type were described in Table 10.
5.1.1 Improved concept of the KBS-3 type vertical disposal for CANDU SNF
Disposal containers were selected with the same material as the PWR spent nuclear fuel disposal containers, and four baskets were loaded in one disposal container [22,23]. One disposal container thus contained 240 CANDU spent nuclear fuel bundles. Figure 5 shows the CANDU spent nuclear fuel disposal containers and deposition holes. The deposition hole was required to contain two disposal containers, and the space between the wall of the deposition hole and the disposal containers was filled with compressed bentonite blocks. The disposal tunnel spacing was 40 m, and the deposition hole spacing was determined to be 5 m through the thermal analysis result (Fig. 6) as described in reference 22, 24.
5.1.2 Improved concept of NWMO-type horizontal disposal for CANDU SNF
The concept of NWMO-type horizontal disposal was applied for the purpose of CANDU spent nuclear fuel disposal. The concept of a disposal container loaded with one basket temporarily stored in the nuclear power plant site was designed, and a buffer box integrated with bentonite and a disposal container was designed so that it could be loaded into a horizontal disposal tunnel (Fig. 7).
A dense backfill block was installed to retain the integrity of the disposal system while maintaining a constant distance between the buffer boxes. Also, in the case of NWMO-type disposal concepts, the highest temperature of the bentonite block as the buffer for the disposal container shall not exceed 100°C. The structural stability of a disposal container should also be maintained in the environment of the disposal depth. The thermal stability of this disposal concept and the structural stability of the disposal containers are described in reference 24. The performance assessment in reference 24 determined that the spacing between disposal tunnels for the NWMO-type horizontal disposal concept of CANDU spent nuclear fuel is 30 m and the spacing between the disposal containers module is 2.56 m through the thermal analysis result (Fig. 8) as described in reference 22, 24.
5.2 Improvement of the PWR spent nuclear fuel disposal concept
5.2.1 Improvement of the disposal container concept
There are two types of spent nuclear fuel in length from domestic PWR nuclear power plants, as mentioned in section 3.2. In the case of 405 cm long spent nuclear fuel, S-SNF, due to the length and the cooling time as shown in Table 6, the disposal container material and the disposal area can be reduced. Therefore, it is possible to improve the disposal efficiency by separating the two types of spent nuclear fuel . The concepts for the improved disposal containers considering the length of spent nuclear fuel are described in Table 11 and Fig. 9.
5.2.2 Improvement of the PWR spent nuclear fuel disposal concept
For the two types of disposal containers classified according to the length of the spent nuclear fuel, the amount of heat that could be accommodated in each disposal container was set according to the disposal scenario. In other words, the cooling time was set based on the time of discharge of spent nuclear fuel from domestic nuclear power plants and the disposal scenario in accordance with the current basic plan. The decay heat capacity acceptable to each type of disposal container was established based on this. With concepts of the disposal containers and the decay heat for each disposal container, analyses of the thermal stability of the disposal system were carried out to check the requirements of the disposal system. Improved disposal concepts for PWR type spent nuclear fuel were designed with the results of the thermal analyses shown in Fig. 10 [23, 24]. With these results, the spacing of the disposal tunnel and the deposition hole of S-SNF were determined to 40 m and 7.0 m for S-SNF and that of R-SNF were determined to 40 m and 7.5 m. Figure 11 shows the disposal concept for S-SNF and R-SNF.
According to the disposal scenario set up in section 3.3 of this paper, for the S-SNF, 97% or more of the spent nuclear fuel’s cooling time at the point of disposal exceeds 50 years, and for R-SNF, 98% or more of the spent nuclear fuel’s cooling time at the point of disposal exceeds 45 years as given in Table 6. Reflecting the results of the thermal performance analyses of the disposal concepts, the cooling period of spent nuclear fuel in the improved disposal concept was set to 50 and 45 years for S-SNF and R-SNF and from the regression equation in Fig. 4, decay heat per disposal container was set to 1,620 W and 1,760 W respectively as shown in Table 12.
6. Efficiency Analyses of the Improved Disposal Concept
6.1 Unit disposal area
In this study, it was assumed that all the spent nuclear fuel generated in Korea will be disposed of in a deep geological repository with a multi-barrier concept consisting of an engineered barrier and a natural barrier. In addition, the layout of the underground disposal area properly set the spacing between the disposal tunnels and the deposition holes so that the maximum temperature of the bentonite block due to the radioactive decay heat emitted from the spent nuclear fuel in the disposal container would meet the temperature requirements for design.
For the efficiency analysis of the improved disposal concept, the definition of unit disposal area was established in consideration of the area between the disposal tun-nel spacing and the deposition hole spacing, as shown in Fig. 12, indicating the area of the disposal area required for the disposal of one disposal container . Thus, the approximate disposal area size of the high-level waste disposal area could be estimated by multiplying the unit disposal area by the total number of disposal containers to be disposed of. From an economic perspective, it is desirable to set the disposal tunnel spacing and deposition hole spacing to minimize the area of underground disposal facilities. Therefore, in this paper, the unit disposal areas of the improved disposal concepts were compared with those of existing disposal concepts for the efficiency analyses.
6.2 Disposal efficiency analysis
After the cooling time at the point of disposal and specifications of spent nuclear fuel generated in Korea were analyzed, two types of disposal concepts for the PWR spent nuclear fuel were established according to the decay heat and the cooling time at the point of disposal. In addition, the disposal efficiency was analyzed by comparing it with that of the existing disposal concept.
The results of the disposal efficiency analysis, such as the disposal area and the amount of uranium disposed of per unit area are shown in Table 13 for the CANDU SNF disposal concept and in Table 14 for the PWR SNF disposal concept.
As shown in Table 13, the improved CANDU SNF disposal concept was assessed to reduce the disposal area by about 20 percent for the KBS-3 type vertical disposal concept and by about 40 percent for the NWMO-type horizontal disposal. In addition, the concept was evaluated to increase the disposal density (U-density) by about 30 percent and 70 percent, respectively.
Table 14 shows the results of a comparative analysis of the disposal efficiency due to a reduction of acceptable decay heat in the disposal container, considering the cooling time (S-SNF: 50 years, R-SNF: 45 years) for each type of PWR spent nuclear fuel. As shown in Table 14, the unit disposal area was 4.61 km2 based on 14,133 assemblies (3,533 containers) of S-SNF and 48,287 assemblies (12,072 containers) of R-SNF. The unit disposal area decreased by roughly 20 percent compared with that (5.62 km2) of the existing disposal concept. It was also confirmed that the disposal density of the improved disposal concept was 5.83 kU‧m-2, which is about 20% higher than that (4.79 kU‧m-2) of the existing disposal concept.
7. Concluding Remarks and Future Plan
Nuclear power plants have stably supplied the energy needed for domestic industries, and it is expected that they will continue to operate in the future. Therefore, the amount of spent nuclear fuel discharged after generating electricity has been steadily increasing and accumulating, but the national management policy for this has not been decided yet. As the disposal area for spent nuclear fuel is expected to increase continuously if they are disposed of directly, various studies are required to improve the disposal efficiency and reduce the disposal area in order to efficiently utilize the national land and enhance the public acceptance. As part of this study, based on the existing spent nuclear fuel disposal concept, improved disposal concepts considering the characteristics of spent nuclear fuel discharged from domestic nuclear power plants were proposed as described below.
• By analyzing the specifications and other characteristics of spent nuclear fuel discharged from domestic nuclear power plants, two types of disposal containers for the KBS-3V concept and NWMO concept for CANDU SNF were developed. Also, the PWR SNF was classified into two types, S-SNF (length of 406 cm) and R-SNF (length of 453 cm) respectively, and the concepts of suitable disposal containers were developed.
• Based on the current high-level waste management basic plan, a disposal scenario was established to determine the cooling time for each type of PWR SNF, as shown in Table 15, depending on the time of discharge from the nuclear power plants and the point of spent nuclear fuel disposal.
• With the derived disposal container concepts and decay heat, the existing disposal concepts were improved and the thermal analysis results were reflected to ensure that the improved disposal concept meets the design requirements of not exceeding 100°C buffer temperature. According to the analysis results, 40 m disposal tunnel spacing and 5 m deposition hole spacing for a KBS-3 vertical type disposal concept of CANDU SNF were set. For the horizontal disposal concept of CANDU SNF, which is a Canadian NWMO concept, 30 m disposal tunnel spacing and 2.56 m disposal containers distance were determined. It was also confirmed that for PWR spent nuclear fuel, the maximum temperature of the buffer material should be maintained at around 95°C when the spacing of the S-SNF spent nuclear fuel disposal container is 7 m and the spacing of the R-SNF spent nuclear fuel disposal container is 7.5 m with 40 m disposal tunnel spacing.
• The improved disposal concept was compared and analyzed with the existing disposal concept, in terms of characteristics such as unit disposal area and disposal density (uranium density per area). For CANDU spent nuclear fuel, the improved vertical disposal concept area of 0.28 km2 was reduced by about 20% compared with the existing disposal concept area of 0.36 km2, and the horizontal disposal area was improved by about 30% with a size of 0.21 km2. In addition, in the case of PWR SNF, the improved disposal concept area was reduced by about 20% to 4.61 km2 compared to the existing disposal concept area of 5.62 km2. And the disposal density improved by about 20% from 4.79 kU‧m-2 of the existing concept to 5.83 kU‧m-2 of the improved concept.
As described above, from the comparative analysis of the disposal efficiency between the existing and the proposed disposal concepts that improved the existing disposal concept considering the characteristics of the spent nuclear fuel, it was confirmed that the disposal area expected to be required by the continued increase and accumulation of spent nuclear fuel could be reduced relatively. Therefore, it was judged that the proposed disposal concept can be applicable as an improved concept for a meaningful disposal system that could efficiently utilize the limited national land.
Also, it is estimated that the present research results will be used not only to establish management policy for spent nuclear fuel but also to design a commercial deep geological disposal system for spent nuclear fuel.