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1. Introduction
All low- and intermediate-level radioactive waste (LILW) generated from nuclear facilities such as nuclear power plants, research institutes, hospitals, etc. are disposed of at disposal facilities to ensure that there is no impact on human health and the nature environment [1]. Typically, a disposal facility is formed of three representative barriers: waste package, repository, and disposal site. Each barrier plays a role in preventing, delaying, or restricting the release of radionuclides from the disposal facility to the nature environment, and is given proper performance requirements through the disposal facility safety assessment. The safety of a disposal facility is ultimately achieved by the organic and complementary integration of the above multiple barriers [2].
Currently, Korea has been operating a disposal facility in Gyeong-Ju since 2015 to safely dispose of LILW. The disposal facility can dispose of 100,000 drums of LILW and ensures safety by imposing proper performance requirements on multiple barriers such as waste package, disposal container, Silo, and disposal site. Among the barriers at the Gyeong-Ju disposal facility, the performances related to the waste package are described in the waste acceptance criteria (WAC) for the disposal facility. The generator must treat the waste to meet the applicable acceptance criteria and then deliver it to the disposal facility [3].
The representative performance requirement imposed on waste package is the solidification (immobilization) requirement. Legal regulations for solidification are described in Article 3, Sub-paragraph 7 and Article 11 of Nuclear Safety and Security Commission (NSSC) Notice No. 2021-26 [4]. Article 3, Sub-paragraph 7 describes the definitions of solidification (immobilization) and stipulates immobilization as a concept included in solidification. Article 11 stipulates three conditions for solidified waste. Since solid materials such as spent cartridge filters that are subject to immobilization are not fluid, the conditions applicable to immobilization are Sub-paragraph 2 restriction on the release of nuclides and Sub-paragraph 3 structural stability.
The solidification requirements in WAC for the disposal facility distinguish between solidification and immobilization. Solidification is applied to all fluid homogeneous wastes such as spent resin and concentrates waste, regardless of radioactivity, and performance requirements such as compressive strength and leaching rate are given to the solidified waste. On the other hand, immobilization is applied to solid heterogeneous waste with a total radioactivity concentration of 74,000 Bq·g−1 or more of nuclides with a half-life of more than 20 years, and no performance requirements are imposed on the immobilized waste.
Here, in the case of solidification, performance such as compressive strength and leaching rate is imposed on all homogeneous waste regardless of radioactivity, whereas in the case of immobilization, performance is not granted even though it targets only heterogeneous waste with high radioactivity. It is judged that there are contradictions in terms of safety. Since immobilization is a concept included in solidification in the law, it is judged that it makes sense that immobilization should be given similar performance to solidification, such as restriction on the release of nuclides and structural stability. Separately, the level or scale of performance required for immobilization may be different from solidification.
Accordingly, this study aims to investigate the status of immobilization performance application for heterogeneous waste in various foreign countries with extensive experience in operating disposal facilities and radioactive waste management, and to propose immobilization performance requirements for heterogeneous radioactive waste that can be applied in Korea.
2. Overseas Status
The status of immobilization performance for heterogeneous waste applied in 4 leading countries with extensive experience in disposal facility operation and radioactive waste management was investigated. The four overseas countries are the United States, France, Spain, and Japan. Prior to investigating overseas cases, IAEA publications related to immobilization were first reviewed.
2.1 IAEA
The IAEA uses immobilization as a term for the highest- level concept, as shown in Table 1 [5]. Immobilization is described as follows: “Immobilization is the conversion of waste into a waste form by solidification, embedding or encapsulation. The aim of it is to reduce the potential for migration or dispersion of radionuclides during handling, transport, storage and/or disposal”. Comparing IAEA and Korea, solidification is the same, and Korean solidification can be seen as a concept that includes IAEA’s embedding and encapsulation.
Table 1
Classification and definition of immobilization in IAEA
| Term | Immobilization | ||
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| Solidification | Embedding | Encapsulation | |
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| Definition | Immobilization of gaseous, liquid or liquid-like materials by conversion into a solid waste form | Immobilization of solid waste by surrounding it with a matrix material in order to produce a waste form (e.g., metallic materials) | Immobilization of dispersed solids by mixing them with a matrix material in order to produce a waste form (e.g., ash or powder) |
IAEA SSR-5 recommends that engineered barriers, including waste form, should be designed to provide containment of radionuclides in waste and waste form have to ensure the fulfilment of the safety functions with regard to safety in the long term [6].
2.2 United States
In the United States, 10CFR61.55 requires that if waste is classified as Class B or Class C depending on radioactivity concentration regardless of whether homogeneous or heterogeneous, the waste form must meet minimum and stability requirements. The above requirements for waste form are described in 10CFR61.56. The minimum requirements are prohibited items, packaging conditions, etc., and the stability requirements are structural stability, free liquid, and void space restrictions [7].
The United States provides “Technical Position on Waste Form” guide on how to satisfy the structural stability specified in 10CFR61.56(b). This guide quantitatively provides performance requirements such as compressive strength and leaching rate for homogeneously solidified waste, but heterogeneous waste such as cartridge filter waste that is difficult to solidify requires demonstration to ensure stability. It also suggests the use of High Integrity Container (HIC) or encapsulation within a binder as acceptable alternatives for heterogeneous waste [8].
Encapsulation for heterogeneous waste is covered in the “Technical Position on Concentration Averaging and Encapsulation” guide. In this guide, encapsulation is defined as being different from solidification, which creates a physically homogeneous form of waste. The definition and advantages of encapsulation are as follows.
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(Definition) The process of surrounding discrete items of radioactive waste, such as sealed sources or cartridge filters, in a non-radioactive binding matrix, where the activity remains within the dimensions of the original item of waste.
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(Advantages) It can mitigate waste dispersion to the general environment after disposal, provide additional shielding to limit external radiation, and satisfy the stability requirements of 10CFR61.56(b) [9].
What performance was given to the encapsulation that satisfied the stability requirements could be found in the NRC technical evaluation report on the Class B, C cartridge filter encapsulation of Diversified Technologies. The conceptual diagram for encapsulation is shown in Fig. 1. To meet structural stability, the waste was surrounded by a 10 cm thickness concrete lining, and internal voids were evaluated [10]. In addition, the requirements for encapsulation were confirmed in WAC of Energy Solutions’ Clive disposal facility. Liquid cartridge filters and other acceptable discrete items may be encapsulated, and discrete items must be encapsulated with a minimum 1-inch-thick lining on all sides within the packaging container. Fig. 2 shows a conceptual diagram of discrete item encapsulation [11]. Energy Solutions, which also performs treatment for cartridge filters, a representative heterogeneous waste, crushes and encapsulates Class A waste and encapsulates Class B waste in its original form, as shown in Figs. 3 and 4.
2.3 France (L’Aube Facility)
To ensure the safety of a disposal facility, France assigns performance to a waste package in three categories: mechanical aspect, containment aspect, and sustainability aspect. In addition, to meet the above performance, France divides a package into three components: waste matrix, internal cementitious lining-layer, and concrete container, and allocates performance to each component by selectively combining them. Fig. 5 shows a cross-section of an immobilized heterogeneous waste package (water filters) in France [12,13].
First, the mechanical aspect is divided into the mechanical strength of the package and the mechanical performance requirements of each component. Basically, the mechanical strength given to the package was derived by considering the maximum load applied on the package during the lifetime of the disposal facility. French fundamental safety rules (RFS III.2.e) stipulate the mechanical strength limits of packages as shown in Table 2. To meet the strength limits set out in RFS III.2.e, ANDRA, a French waste management agency, has established various requirements for each component, including compressive strength, tensile strength, crack limit, thickness, porosity, density, thermal cycle, and irradiation. Here, in the case of a homogeneous waste matrix, the compressive strength of the matrix is applied, and in the case of a heterogeneous matrix, the compressive strength of the injection material is applied.
Table 2
Mechanical strength limits
| Contents | Vertical strain rate | Compressive strength |
|---|---|---|
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| Value | < 3% | > 0.35 MPa |
Second, limits on containment performance for package are also specified in RFS III.2.e. The limits are as shown in Table 3, and the values are expressed as the annual fraction of activity released. Moreover, RFS III.2.e allows satisfying the performance limits by combining the performance of two or more components. ANDRA provides various requirements for each component of a package that can meet containment limits. Concrete containers have requirements such as porosity, thickness, and effective tritiated water diffusion coefficient, and the internal lining layer has requirements such as porosity, minimum value of thickness, and limits on the effective tritiated water diffusion coefficient. In the case of waste matrix, containment performance is assigned only to the homogeneous matrix and not to the heterogeneous matrix. ANDRA presents effective diffusion coefficient limits for each nuclide in a homogeneous matrix. The performance of a homogeneous matrix is determined based on the leaching rate.
Table 3
Containment performance limits
| Contents | Nuclide | Specific activity | Limit |
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| Homogeneous package with containment performance | Each beta-gamma nuclide except tritium | 3.7 < SA < 37 MBq·kg−1 | 1.0×10−1 / yr |
| 37 MBq·kg−1 < SA < 370 MBq·kg−1 | 2.8×10−2 / yr | ||
| SA > 370 MBq·kg−1 | 7.3×10−3 / yr | ||
| Each alpha nuclide | - | 2.0×10−4 / yr | |
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| Heterogeneous package with containment performance | Each beta-gamma nuclide except tritium | - | 7.3×10−3 / yr |
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| All package with containment performance | Tritium | - | 5.0×10−2 / yr |
Third, sustainability aspects are considered to ensure that the package maintains its mechanical strength and containment performance throughout the lifetime of the disposal facility. ANDRA considers the performance of concrete as a critical factor and sets the minimum degradable thickness requirements for concrete containers by evaluating the impact of environmental factors such as hydrolysis and corrosion of steel caused by chloride ions.
2.4 Spain (El Cabril Facility)
Unlike France, Spain places packages into concrete containers and disposes of them in concrete vaults. In addition, Spain classifies waste into level 1 and level 2 according to the specific activity limit of each nuclide as shown in Table 4 and imposes different performance requirements for each level as shown in Table 5. In terms of general waste classification, level 1 and level 2 can be roughly classified as low-level waste and intermediate-level waste, respectively [14,15].
Table 4
Specific activity limit
| Contents | Target nuclide | Specific activity limit |
|---|---|---|
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| Level 1 | Total alpha | 1.85×102 Bq·g–1 |
| Beta-gamma emitters with half-life > 5 y (tritium excepted) by isotope | 1.85×104 Bq·g–1 | |
| Total Beta-gamma activity for isotopes with half-life > 5 y | 7.40×104 Bq·g–1 | |
| Tritium | 7.40×103 Bq·g–1 | |
| Level 2 | Total alpha | 3.70×103 Bq·g–1 |
| 60Co, 90Sr, 137Cs | 3.70×105 Bq·g–1 | |
Table 5
Performance requirements
| Contents | Solidified homogeneous waste (Resins, sludge, evaporator concentrates) | Blocked waste (Cartridge filters, dried sludge, ashes) | ||
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| Level 1 | ||||
| Level 2 | ||||
3. Proposal
Table 6 summarizes the immobilization performance requirements for heterogeneous waste in Korea and the three countries investigated. Similarly to the United States, Korea immobilizes only waste that exceeds a certain specific activity, but France and Spain immobilize all waste. In addition, with regard to performance requirements of immobilization, Korea only has safety of the immobilization material, while other countries have various requirements such as compressive strength, thickness, and tritium diffusion coefficient. On the other hand, due to its heterogeneous characteristics, all countries do not assign performance to the waste matrix itself, but to the internal lining and/or concrete container.
Table 6
Summary of the immobilization performance requirements
| Contents | Korea | United State | France | Spain |
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| Criteria | Heterogeneous wastes exceeding a certain specific activity shall be immobilized. | All class B or C waste must meet the minimum and stability requirements in 10CFR61.55. | All waste is immobilized. Performance requirements depends on specific activity. | All waste is immobilized. Performance requirements depends on specific activity. |
| Method | Immobilization and/or HIC | Encapsulation and/or HIC | Internal lining and/or concrete container | Internal lining and disposal container |
| Performance requirements | Immobilization | Encapsulation | Internal lining | Internal lining |
Based on the overseas application cases, immobilization performance requirements for heterogeneous waste applicable to Korea are proposed. According to the NSSC notice, since immobilization is a part of solidification, it must have performance such as restriction on the release of nuclides and structural stability. To correspond to these provisions, this study proposes a method applicable to Korea in which the immobilization performance for heterogeneous waste is achieved through internal lining as shown in Fig. 6. The performance requirements of this internal lining include compressive strength, thickness, and tritiated water diffusion coefficient. If the performance is achieved with HIC (or a concrete container with containment function), it can be immobilized without an internal lining. And it can be immobilized using a combination of internal lining and HIC (or a concrete container with containment function). Quantitative values for the above requirements of each components need to be derived through quantitative assessment based on the characteristics of domestic heterogeneous waste and disposal facilities. For reference, the immobilization method proposed in this study can be distinguished from the immobilization method without internal lining for heterogeneous waste containing dispersible particulates, as shown in Fig. 7, according to Korea’s WAC. However, if the specific activity of this heterogeneous waste exceeds 74,000 Bg·g−1, the immobilization method proposed in this study should be applied.
4. Conclusions
This study reviewed domestic laws, WAC, and IAEA technical publications, and investigated cases of immobilization application to heterogeneous waste in three foreign countries such as the United States, France, and Spain, and proposed immobilization performance requirements for heterogeneous waste applicable to Korea through comparison with their cases. Currently, in Korea, there is no clear physical form for immobilization, and the only performance requirement is the safety of the immobilization material, it is necessary to specify the immobilization. On the other hand, in overseas cases, the immobilization was applied in a physical form with an internal lining, and immobilization performance requirements were imposed on the internal lining such as compressive strength, thickness, and tritiated water diffusion coefficient. The methods of immobilization by combining internal lining and HIC (or a concrete container with containment function) were also used. But they did not impose performance on the waste matrix due to its heterogeneous nature. Based on the above overseas cases, physical form for immobilization with internal lining, and performance requirements such as compressive strength, thickness, diffusion limit were proposed as a method applicable to Korea.
This study will improve the understanding of stakeholders such as waste generators and repository operators by specifying the immobilization type and performance requirements for domestic heterogeneous waste. Furthermore, it is expected to contribute to improving the safety of disposal facilities by providing performance to immobilized heterogeneous waste packages.
Further research should be conducted in the future to derive quantitative values for the immobilization performance requirements proposed in this study through realistic and quantitative assessments using domestic heterogeneous waste and disposal facility characteristics.
