Introduction
Literature Review
Decommissioning of Nuclear Facilities
Comparative Analysis of the Demolition Process of Nuclear Facilities and Asbestos Structures
Procedure of Asbestos Demolition
Disposal of Asbestos Dismantling Waste
Procedure of Nuclear Facilities Decommissioning
Radioactive Waste Management
Guideline for Safe Decommissioning based on the Comparative Analysis with the Asbestos Dismantlement Process
Conclusions
Introduction
The number of nuclear facilities that are planned to be shut down, as they reach the end of their design lifetime, or earlier for safety or political and social reasons, has been increasing worldwide. This has led to increase the awareness of the importance of developing safety standards and requirements for decommissioning [1]. As a result, industries related to decommissioning nuclear facilities are expanding, and research in various fields is underway to establish guidelines for safe decommissioning and secure the safety of dismantling them. Thus, for countries that have no experience in decommissioning nuclear power plants, it is necessary to establish a strategy to conduct safe decommissioning work through the analysis of existing cases.
However, most of the guidelines for the decommissioning of nuclear facilities focus on radiological risks and are relatively negligent in handling non-radioactive risk factors. In order to protect workers from nuclear decommissioning and prevent accidents, an appropriate decommissioning plan should be formulated, which takes into account both radioactive and non-radiation risks. In this regard, the U.S. Department of Energy (DOE) [2] noted that nuclear decommissioning has similar characteristics to demolishing concrete structures and, likewise, the management of structural hazards is important. Therefore, the purpose of this study is to develop procedures for the safe decommissioning of concrete structures in nuclear facilities by considering radiological risks and structural hazards [3]. Unlike the general dismantling of the construction industry, however, the decommissioning of nuclear power plants cannot be carried out unconditionally. This is because the concrete for radioactive rays shielding has been radiated, so the order and method of decommissioning are applied differently depending on the area.
The process of asbestos dismantling, which is managed as a Group 1 carcinogen, is similar to nuclear decommissioning. As the asbestos dismantling process has many applications, it has to be managed systematically. In this study, we proposed the development of safe decommissioning process of nuclear facilities in comparative analysis of commonalities with asbestos dismantling process and consideration of the specificity of nuclear decommissioning process and verified its adequacy.
Literature Review
Decommissioning of Nuclear Facilities
According to the International Atomic Energy Agency [4], decommissioning is defined as administrative and technical actions involving decontamination, dismantling and removal of radioactive materials, waste, components and structures taken to allow removal of some or all of the regulatory controls from nuclear facilities. Decommissioning is taken on the basis of prior planning and assessment to ensure safety during operations and conducted to achieve a gradual and systematic reduction in radiological risks.
The United States, the largest holder of nuclear facilities, has a lot of decommissioning experience, and the dismantling is still underway for continued nuclear power generation in line with the government's policy of reducing risks and ensuring the protection of the environment and public and workers' health in connection with the dismantling. The U.S. fully regulates all decommissioning projects by the Nuclear Regulatory Commission in the event of dismantling commercial reactors operated by private operators. In addition, in accordance with 10CFR50.82(a)(4)(i) of Code of Federal Regulations [5], the power reactor licensee is required to submit a post-shutdown decommissioning activities report (PSDAR) to the NRC within two years following permanent cessation of operations.
There are three strategies for decommissioning: immediate dismantling, deferred dismantling, and entombment [6]. Immediate dismantling is a form of decontamination and dismantling of equipment, structures, and radioactive wastes immediately after the approval is processed after the plant shuts down. It includes the prompt removal and process of all radioactive materials from the facilities for long-term storage or disposal. Major nuclear power plant-operating countries such as Germany and France prefer immediate dismantling methods, which are characterized by reusing sites, enhancing economic feasibility, and utilizing experienced personnel. Deferred dismantling is a strategy for long-term storage in which final dismantling of a facility is delayed and maintained in a safe condition. In this method, dismantling proceeds when the level of radioactive materials has reached a particular standard for a certain period through safety management and operation to reduce the systematic dose by the radioactive half-life effect following the time delay after obtaining the approval for the dismantling process. The deferred dismantling method is often used at multi-purpose sites if some facilities are shut down and others continue to operate. In addition, some countries may choose it due to the radiation graphite problem of graphite moderated furnace, the cost of dismantling, and the lack of radioactive waste disposal sites. Entombment is a strategy for locking radioactive contaminants in structurally long-lasting substances until radiation decreases from regulatory control to levels that allow the release of facilities. The presence of radioactive materials at the site means that the facility will eventually be designated as a surface waste site, and that it needs to meet the criteria of the facility. Table 1 shows the decommissioning strategies appropriate for each country's situation.
Table 1.
There are two ways to decommission nuclear power plants: hot to cold and cold to hot. The first method is usually implemented in countries with adequate experience in decommissioning nuclear power plants, such as the United States and Germany. In this method, high radiation dose areas are dismantled first followed by low radiation dose areas. This method has the advantage of reducing the decommissioning period, cost, and exposure to radiation. However, this requires experience and thorough preparations. The second method is usually implemented in countries with little or no experience in decommissioning nuclear power plants. In this method, dismantling proceeds from low radiation dose areas to high radiation dose areas. From this method, it is possible to gain experience in decommissioning nuclear power plants. However, in this method, there is a possibility of re-contamination in low-contamination areas while dismantling high-contamination areas.
Nuclear power plants generally operate for 30 to 60 years, although they vary depending on the situation in the country or power plants. In Korea, 24 nuclear power plants are in operation and 12 design lifetime is expected to be completed by 2029 [7]. In the case of Gori-1 reactor, immediate dismantling and gradual dismantling are underway. Thus, this study is based on the process of decommissioning concrete structures that make up the reactor in the immediate dismantling method.
Comparative Analysis of the Demolition Process of Nuclear Facilities and Asbestos Structures
Procedure of Asbestos Demolition
Asbestos, which is used in insulation, is a fibrous inorganic material that exists in nature with a very stable chemical structure. However, it is currently prohibited because it is a Category 1A carcinogen [8]. Asbestos is a soft non-flammable material available in the form of fine fiber strands, as shown in Figure 1. When asbestos enters the lungs through the respiratory tract, asbestos fibers cannot be released from the lungs and thereby damages the cell membrane of the lungs causing pneumoconiosis, asbestosis, and fibrosis of the lungs, which eventually results in cancer [9]. The characteristics of asbestos are defined in the Material Safety Data Sheet.
If the sum of the area of the wall, flooring, ceiling, and roof materials containing more than 1% asbestos is more than 50 m2, the Asbestos Safety Management Act stipulates the management guidelines. To dismantle asbestos, preliminary investigation for analyzing asbestos content and pre-training of workers should be conducted. The education of workers handling asbestos includes the effects of asbestos on the human body, the form of disease occurring, measures to prevent disease outbreaks, and safe working methods.
The main contents of the asbestos dismantling process stipulated in the Asbestos Safety Management Act is listed below.
First, as a preliminary investigation, the history of materials used in the building or the analysis of components are investigated for the presence of asbestos in the building. This is not provided in the relevant laws, but prior verification is required in accordance with the procedures prescribed by the Asbestos Disorder Prevention Rules. In the preliminary investigation, samples are collected and analyzed to examine the type of finishing materials used in each room and the shape of asbestos-containing substances. Methods for measuring asbestos concentration include the Bulk Sample analysis method, the X-ray diffraction method, the Air Sample analysis method, and the polarized microscope method. If asbestos content is confirmed as a result of the investigation, the type, location, scope of them are recorded and preserved until the end of the dismantling.
Second, if asbestos content is found to be above the regulations, a work plan for asbestos dismantling and removal can be established, and the dismantling work can be carried out after obtaining approval for submission from the head of a local labor office. The document of work plan includes procedures and methods for dismantling and removing asbestos, removal facilities and disposal methods to prevent scattering of asbestos, and plans for measures to protect workers, and education should be provided to inform workers of the work plan.
Third, a warning board must be installed before dismantling. The warning sign of "Authorized Personnel Only" should be manufactured and installed at least 70cm in width, 50cm in length with a letter size of 8cm in width and 10cm in length in accordance with the regulations.
Fourth, the employer must provide workers with special filter masks or air supplied respirators, and personal protective eyewear, impervious protective clothing, protective gloves, protective boots, etc., and the worker must wear protective gear. When asbestos dismantling activities begins, all but decommissioning workers will be banned from entering the workplaces and smoking or eating food will be prohibited in the workplace.
Fifth, the interior dismantling space of asbestos shall be equipped with a negative pressure air filtration ventilation system with the high efficiency particulate air filter (HEPA filter) after sealing off openings such as windows and blocking all existing ventilation systems and electrical equipment. At this time, the negative pressure should be measured from start to the end of work using a negative pressure recorder and it should be maintained at –0.508 mmH2O. It is recommended to double cover impermeable polyethylene sheets with a thickness of not less than 0.15 mm on the floor and 0.08 mm on the wall. In addition, decontamination facilities should be installed in the passageways for the entry and exit of equipment and dismantling workers.
Disposal of Asbestos Dismantling Waste
Asbestos waste must be disposed because it contains more than 1% concentration of asbestos by weight. A particular concern is the management of brittle asbestos waste. Moreover, friable materials are more likely to release fiber into the air.
Disposal of waste containing disassembled asbestos is prescribed in the Enforcement Decree of the Occupational Safety and Health Act and special management industrial wastes disposal standards of the Wastes Control Act. The requirements of the person in charge of special management of industrial waste management are stipulated by qualifications and educational background. The container must be wiped or vacuumed with a HEPA vacuum cleaner. Additionally, the material should not be broken into smaller pieces to fill them into containers unless it is performed under negative air conditions with the capability of applying a mist when necessary to reduce fiber levels. Waste materials, including discarded polyethylene sheeting, sealing tape, cleaning materials, protective clothing, vacuum bags, and other contaminated materials, are treated as any other asbestos-containing material for disposal [10]. The regulations stipulate that asbestos waste should be disposed of in containers or packages marked with a label saying “asbestos content.” The final disposal of wastes is managed through reclamation.
The electric machine used for dismantling should be equipped with short circuit breakers to prevent electric shock accidents caused by wet work. After the completion of dismantling work, equipment such as ladders and temporary workbench should be cleaned with a wet mop or vacuum cleaner equipped with a HEPA filter. If a negative pressure-tightening system is installed, it must be operated continuously while cleaning and HEPA filters must be replaced or repaired in an enclosed workplace where negative pressure is maintained.
The transport vehicle for asbestos disposal shall be marked as an industrial waste collection vehicle on the outside of the vehicle body, and measures shall be taken to prevent waste asbestos from scattering or leaking. The container shall be of a structure in which dust cannot be blown and shall be taken to prevent it from falling or moving during transport. The form of the container shall be constructed in a structure suitable for the appearance and shape of the waste so that asbestos dismantling materials are not damaged. The storage container shall be of a structure in which asbestos does not penetrate, and proper marking shall be made so that it can be shown that asbestos waste is included on the outside.
Waste asbestos should be disposed in a landfill pursuant to Article 15 of the Waste Disposal Act. This shall be performed at the final disposal site authorized by the Mayor or Provincial Governor. Additionally, disposal shall not be performed via marine dumping.
Procedure of Nuclear Facilities Decommissioning
It is important to ensure stability against contamination and damage risks to workers and the surrounding environment because the hazard risk caused by the release of radiation is high when the reactor is decommissioned. In Korea, the Nuclear Safety Act has set the standards for decommissioning and provides for management of spent nuclear fuel, interim management, and permanent disposal [11]. However, these regulations only established the general items that should be included in the decommissioning plan and the fact that it should be submitted to the Nuclear Safety and Security Commission before decommissioning the reactor. Moreover, there is no specific licensing procedure for the period between permanent shutdown of the reactor and restoration of the site. The detailed guidelines for preparing the decommissioning plan will be provided in the following sections.
Radioactive Waste Management
As there are several types of radioactive nuclide whose half-life varies from 82 million to 6500 years, proper understanding and precise analysis techniques are required for different types of spent nuclear fuel. Radioactive materials are classified into nuclear fuel materials, spent nuclear fuel, radioisotopes, and nuclear fission products.
The radioactive wastes resulting from nuclear decommissioning should be disposed as radioactive materials or contaminated materials. Depending on the concentration of radioactive materials and the characteristics of dismantling materials, the treatment method of radioactive wastes will also vary. The disposal methods are classified into self-disposal waste and radioactive waste based on the degree of contamination, and radioactive waste is disposed of through solidification and stabilization. Radiation waste is disposed using technologies like heat treatment and liquid waste treatment, and then sealed in a transport container and transported to a storage location. The key technology for disposing radioactive waste, various technologies are applied to separate radioactive materials to reduce volume and reduce risk [12].
The equipment and tools used for dismantling the facility are disposed in the same manner as the structure on the premise of contamination by exposure.
The permanent storage of radioactive waste is prescribed by the Radioactive Waste Management Act along with the characteristic criteria for waste disposal, radiological characteristics of waste package, and chemical properties of waste. The method of checking the suitability of wastes should include preliminary inspection, acceptance test of disposal facilities, application method of preliminary inspection and acceptance test, physical characteristics of waste packaging, and chemical characteristics of waste packaging, etc. Moreover, the safety assessment should be conducted by stipulating the measures for nonconforming waste packages in case of nonconformity.
Guideline for Safe Decommissioning based on the Comparative Analysis with the Asbestos Dismantlement Process
The similarity between nuclear decommissioning and asbestos dismantling process is that it must comply with the prescribed regulations to ensure safety from the preparation to the implementing phase in the process of handling hazardous wastes. Although the preparation steps and process for decommissioning nuclear power plants have several differences in compliance with larger structures and procedures, the overall procedure is similar to that of asbestos. In this study, the process for maintaining safety was derived by comparing the commonalities with the asbestos dismantling process, whose performance data were accumulated and systematic management was performed for several years, and the specificity of the nuclear decommissioning process alone. The commonalities and differences between asbestos and nuclear facility dismantling processes are summarized in Table 2.
Table 2.
As shown in Table 2, both activities have some common features, i.e., they are both subjected to intensive management through special regulations because they produce carcinogens and are strictly managed from the pre-preparatory stage by special law. Furthermore, maintaining the stability to prevent the respiratory disease caused by scattering is a major requirement. However, this is different in the case of nuclear facilities, which are larger in scale and pose a higher safety risk in the working environment, such as protection of workers against radiation exposure during the dismantling of concrete structures. Additionally, for decommissioning nuclear facilities, the disposal plan should be established, provided that the equipment deployed is contaminated with radiation.
Based on the empirical data from the asbestos dismantling process and the similarities/differences in each phase of the dismantling technology of radioactive structures, a comparative analysis was performed based on the process of deriving the risk factors in each phase of decommissioning the radioactive concrete structure.
A work breakdown structure draft was presented based on a comparative analysis aforementioned using the empirical data from the asbestos dismantling process the decommissioning procedure of radioactive structures and overseas related data [13]. As shown in Table 3, the overall process was subdivided into three levels. The first level, i.e., from the preparation stage to the dismantling and transportation stage, was divided into seven stages, whereas the second level was subdivided based on the required processes of each stage. In level 3, the work was detailed and classified into a progress process.
Table 3.
Additionally, the risk factors were derived for each process. In this study, the risk factors were derived from surveys and in-depth interviews of 12 experts in the relevant field, including research institutes, construction management experts, structural engineering experts, and dismantling-related mechanical experts. Furthermore, we presented procedures and risk avoidance measures for each detailed process based on the decommissioning process of the structure through the in-depth interviews with experts. Finally, after reviewing the procedures presented in ISO criteria [14, 15, 16], the scope of application was confirmed through verification by external members specializing in ISO certification and evaluation of applicable items and procedures. The process of revising the procedures for problems derived from the assessment was repeated.
The work was initially divided into five stages: (1) Preparation Work, (2) Decontamination work, (3) Drilling Work, (4) Cutting Work, (5) Transport work. However, we divided the work into seven stages as above, as we had obtained results from in-depth expert interviews that countries with little or no experience in decommissioning should cut the areas of non-radiation and radiation, respectively, to ensure safety.
In the preliminary investigation and preparation phase, the decommissioning plan should be established by analyzing the characteristics of the reactor structure and separating the radioactive and non-radioactive parts. The process should start by dismantling the non- radioactive area and end by dismantling the radioactive area. The activities in the exterior part of the reactor, which is the non- radioactive area, shall be planned in accordance with the safety standards of general construction work, whereas the activities in the radioactive area should be planned by considering the exposure to radioactive materials. The process of analyzing the exposure dose criteria and exposure volume by location in advance is the key to the safety assessment for decommissioning.
During the dismantling phase, it is important to establish a carry-in plan for equipment, ventilation and lighting facilities, etc. The suitability of filters for ventilation of the dismantling space should be analyzed. The dismantling of structures in non-radioactive areas should be performed using small and medium-sized dismantling equipment among the existing equipment.
To perform the decontamination work, a scaffold or mobile gondola should be installed for decontamination. As the decontamination equipment and workers are located on the scaffolding, the risk factors for decontamination work includes the risk of exposure due to damage to protective clothing, delay of work, and risk factors, such as falls and stenosis. The decontamination technology is divided into mechanical, physical, chemical, and electrochemical decontamination. However, mechanical decontamination is usually applied to structures. The removal of contaminants by polishing, among various physical methods, consists of devices with abrasives attached to them and devices that can collect the powder of polished contaminants. The abrasive can be distinguished from the automatic operation method and worker's direct operation. The dust collection method is also similar.
During the dismantling phase of the radioactive area, removal of radioactive contaminated parts of the reactor is made up of unmanned or manned and is under construction and demonstration of various equipment for the development and application of dedicated equipment, some of which are practical. The demolition of the radioactive parts of the structure is executed using special equipment, such as wire saw, after decontamination. To ensure stability against scattering dust during cutting, the primary dust should be collected by directly inhaling from the cutting surface. In the secondary collection, it is important to maintain the performance of filters in ventilation facilities for external pollution prevention. Additionally, continuous pollution measurement is required to ensure safety.
After cutting, the volume is reduced through heat treatment and stored in a transport container. Based on the type of radioactive materials and the limit of radioactivity, transport types and containers are classified into types L, IP, B, C, and fissile materials. Type L is classified as a radiation drug, Type IP is a low-level waste, Type A is a new fuel, Type C is air transport, and for fissile material, regulations not in the IAEA are classified as separate regulations in Korea. The safety of transport containers shall be ensured using the main test of transport containers to prevent the risk of movement or leakage of contaminants during long-term storage. Additionally, technologies, such as evaporation concentration, chemical sedimentation, organic ion exchange, reverse osmotic pressure, electrodialysis, ultrafiltration, and microfiltration are applied with liquid waste treatment technology. Generally, melting technology is used for disposing metal waste and to reduce the volume of waste for disposal by melting metal. Finally, the treatment techniques of concrete should be decontaminated on the dismantling structure, and mechanical decontamination, chemical decontamination, volume reduction technology, post-corrosion recycling, and hardening techniques should be applied.
In Figure 2, 6.2.1 correspond to the code number of the detailed process. It displays the preceding work to complete and cooperate with the preceding work and scheming the detailed work in chronological order. Risk factors derived for each process include activities that are distinct from asbestos dismantling, resulting in dangerous factors, such as collapse, fall, eye attack, fire, etc. Thus, the guidelines for risk avoidance and reduction based on the scenarios for decommissioning concrete structures in nuclear power plants based on the derived risk factors and checklists were deduced.
Finally, the process and guidelines were verified through the risk assessment of asbestos dismantling techniques, self-comparative verification that considers the characteristics of nuclear power plant concrete structures, and consultation with experts in the nuclear field and specialist in ISO standard procedure certification assessment.
Conclusions
In this study, the basic guidelines for deriving hazards and avoiding safety risks in the decommissioning process of radioactive concrete structures in nuclear facilities were presented by comparing it with the asbestos dismantling process, which is has similar processes from the pre-stage to the disposal stage due to lack of empirical verification.
As the measure for each phase, the preparation phase derived the risk factors that could occur during decontamination and dismantling based on the scenario and the safety management. Moreover, the decommissioning procedure was derived considering the structural safety of the decontamination and dismantling process. In the cutting work, the need to design an equipment and manpower layout and introduce exposure prevention technologies, such as unmanned dismantling technology, in the radioactive concrete area were discussed. The need for differentiation based on the size and degree of contamination of storage containers after dismantling, volume reduction technology of wastes, and safety assessment through secondary decontamination work were also discussed.
As decontamination and dismantling of radioactive structures are an important process that determines cost and safety, the preparation of decommissioning process is indispensable. Moreover, additional research is consistently required to develop procedures and guidelines for safety assessment in each phase of decommissioning.
In this study, there were limitations in comparative analysis with the asbestos dismantling process due to security issues preventing access to drawings of nuclear facilities. However, we hope that future studies will develop complementary guidelines by demonstration actual decommissioning and segmentation of processes.