KEPCO KPS is the contractor for the full system decontamination (FSD) of Kori Unit 1 and under preparation such as modification, lay out for equipment installation, setting up tie-in/out point for chemical injection and way to pressurize the system, of its successful performance. In this research, KPS introduced how KPS has designed and prepared for the FSD project and how will the chemical decontamination process be implemented. As described in the previous research, chemical decontamination process is planned to be conducted for three cycles and each cycle is consisted of oxidation, reduction, decomposition, and purification. Oxidation and reduction process were conducted at 90°C. Chemical decomposition and purification process were conducted at 40°C due to the damage of IX by the heat. If the decontamination result does not meet the target DF and the dose rate, additional cycle can be conducted. Expected volume of process water for FSD is 200 m3. Three systems have been designated as decontamination targets: reactor coolant system (RCS), residual heat removal system (RHRS), chemical volume control system (CVCS). For the steady flow rate, existed plant equipment such as reactor coolant pump (RCP) will be operated and modifications on some components will be conducted. Due to the limited space for installation, decontamination equipment and other resources are distributed to three different places. KPS designed the layout of equipment installed inside the containment vessel. The layout contains the information of shielding for highly radiated equipment such as IX and filter skid.

The primary heat transport system consists mainly of the in-core fuel channels connected to the steam generators by a system of feeder pipes and headers. The feeders and headers are made of carbon steel. Feeders run vertically upwards from the fuel channels across the face of the reactor and horizontally over the refueling machine to the headers. Structural materials of the primary systems of nuclear power plants (NPPs) are exposed to high temperature and pressure conditions, so that the materials employed in these plants have to take into accounts a useful design life of at least 30 years. The corrosion products, mainly iron oxides, are generated from the carbon steel corrosion which is the main constituent of the feeder pipes and headers of this circuit. Typical film thickness on CANDU-PHWR surface is 75μm or 30mg/cm2. Deposits on PHWR tends to be much thicker than PWR due to use of carbon steel and also for the source of corrosion products available on the carbon steel surface. Degradation of carbon steel for the feeder pipes transferring the primary system coolant by flow-assisted corrosion in high temperature has been reported in CANDU reactors including Point Lapreau, Gentully-2, Darlington and Bruce NPPs. The formation of Fe3O4 film on a carbon steel surface reduces the dissolution rate of steel substantially. The protectiveness of the Fe3O4 film over the carbon steel is affected by the environmental factors and the operational parameters of the feeder pipes, including the velocity, wall shear stress, solution pH, temperature, concentration of dissolved iron, quality of solution, etc. For effective chemical decontamination of these thick oxides containing radionuclides such as Co-60, it is necessary to understand the corrosion behaviors of feeder pipes and the characteristics of oxide formed on it. In this work, we investigated the growth of oxide films that develop on type SA-107 Gr. B carbon steel in high temperature water and steam environment by scanning electron microscopy (SEM) and glow discharge optical emission spectrometry (GD-OES) for the quantification and the solidstate speciation of metal oxide films. This study was especially focused to set the experimental tests conditions how to increase the oxide thickness up to 50 m by changing the oxidation conditions, such as solution chemistry and thermo-hydraulic conditions both temperature and pressure and so on.

It is crucial to be sure about the safety of nuclear facilities for human resources who are in danger of radioactive emission, also diminishing the volume of the wastes that are buried under the ground. Chemical decontamination of nuclear facilities can provide these demands at the same time by dissolving the oxide layer, which radionuclides such as 60Co and 58Co have been penetrated, of parts that are utilized in nuclear plants. Although there are many commercial methods to approaching its aim and they perform a high decontamination factor, they have some issues such as applying organic acids which have the ability to chelate with radionuclides that can be washed by underground water, have large quantities of radioactive waste and damage to the surface by severe intergranular attack. A new method has been introduced by KAERI’s scientist which is named the HyBRID Process, in this process the main solution is the acidic form of Hydrazine. In this process, like other acid-washing processes, there is a chance of corrosion on the metal surface which is not desired. The metal surface is able to be protected during dissolving process by adding some organic and inorganic corrosion inhibitors such as PP2 and PP3. There is a very new research topic about ionic liquids (ILs) as corrosion inhibitors which illustrates a vast potential for this application due to their tunable nature and the variety of options for cationic and anionic parts. The key factors for ILs corrosion inhibitors such as the hardness properties are summarized. In this study, we review to the fundamentals and development of corrosion inhibitors for chemical decontamination and give an prospect with emphasis on the challenges to be overcome.

The radiation field generated in the primary cooling system of a nuclear power plant tends to increase in intensity as radionuclides bind to the oxide film on the internal surface of the primary system, which is operated at high temperature and pressure, and as the number of years of operation increases. Therefore, decontamination of the primary cooling system to reduce worker exposure and prevent the spread of contamination during maintenance and decommissioning of nuclear power plants uses the principle of simultaneous elution of radionuclides when the corrosion oxide film dissolves. In general, a multi-stage chemical decontamination process is applied, taking into account the spinel structure of the corrosion oxide film formed on the surface of the primary cooling system, i.e. an oxidative decontamination step is applied first, followed by a reductive decontamination step, which is repeated several times to reach the desired decontamination goal. Currently, permanganic acid is commonly used in oxidative decontamination processes to remove Cr from corrosion oxide films. In the reductive decontamination step to remove iron and nickel, organic acids such as oxalic acid are commonly used. However, organic acids are not suitable for the final radioactive waste form. A number of multi-stage chemical decontamination technologies for primary cooling systems have been developed and commercialized, including NP-CITROX, AP/NP-CANDECON, CANDERM, AP/NP-LOMI and HP/CORD-UV. Among these, HP/CORDUV is currently the most actively applied primary cooling system chemical desalination process in the world. In this study, KAERI has developed a new chemical decontamination technology that does not contain organic chemical decontamination agents, with a focus on securing an original technology for reducing the amount of decontamination waste while having equivalent or better decontamination performance than overseas commercial technologies, and compared it with the inorganic chemical agent-based HyBRID (Hydrazine Based Reductive Metal Ion Decontamination) chemical decontamination technology.

As unit 1 of Kori was permanently shut down in June 2017, domestic nuclear industry has entered the path of decommissioning. The most important thing in decommissioning is cost reduction. And volume reduction of radioactive waste is especially important. According to the IAEA report, more than 4,000 tons of metallic waste is generated during the decommissioning of a 1,000 MWe reactor and most of these wastes are LLW or VLLW. To reduce amount of metallic waste dramatically, we should choose efficient decontamination method. In this study, we conducted dry ice and bead blasting decontamination. We prepared Inconel-600 and STS-304 specimen with dimensions of 30 mm × 30 mm × 5 mm. Loose and fixed contamination was applied on the surface of specimen using SIMCON method. Bead and dry-ice blasting was conducted by spraying alumina and dry ice pellet at the same pressure and distance for the same time. The removal of loose contamination was observed using microscope. It was found that contaminants are significantly removed using both dry ice blasting and bead blasting. However, some abrasive material remained on the surface of specimen. The removal of fixed contamination was verified by weight comparison before and after experiment and cobalt concentration comparison before and after experiment using X-ray Fluorescence Spectroscope (XRF). At least 90% of the cobalt was removed, but some abrasive particle was also remained on the surface of specimen. In this study, it is confirmed that the effectiveness of manufacturing a large-scale abrasive decontamination facility, and it is expected that this technology can be used to effectively reduce the amount of metallic waste generated during decommissioning.

Economical radioactive soil treatment technology is essential to safely and efficiently treat of high-concentration radioactive areas and contaminated sites during operation of nuclear power plants at home and abroad. This study is to determine the performance of BERAD (Beautiful Environmental construction’s RAdioactive soil Decontamination system) before applying magnetic nanoparticles and adsorbents developed by the KAERI (Korea Atomic Energy Research Institute) which will be used in the national funded project to a large-capacity radioactive soil decontamination system. BERAD uses Soil Washing Process by US EPA (402-R-007-004 (2007)) and can decontaminate 0.5 tons of radioactive soil per hour through water washing and/or chemical washing with particle size separation. When contaminated soil is input to BERAD, the soil is selected and washed, and after going through a rinse stage and particle size separation stage, it discharges decontaminated soil separated by sludge of less than 0.075 mm. In this experiment, the concentrations of four general isotopes (A, B, C, and D which are important radioisotopes when soil is contaminated by them.) were analyzed by using ICP-MS to compare before and after decontamination by BERAD. Since BERAD is the commercial-scale pilot system that decontaminates relatively large amount of soil, so it is difficult to test using radioactive isotopes. So important general elements such as A, B, C, and D in soil were analyzed. In the study, BERAD decontaminated soil by using water washing. And the particle size of soil was divided into a total of six particle size sections with five sieves: 4 mm, 2 mm, 0.850 mm, 0.212 mm, and 0.075 mm. Concentrations of A, B, C, and D in the soil particles larger than 4 mm are almost the lowest regardless of before and after decontamination by BERAD. For soil particles less than 4 mm, the concentrations of C and D decreased constantly after BERAD decontamination. On the other hand, the decontamination efficiency of A and B decreased as the soil particle became smaller, but the concentrations of A and B increased for the soil particle below 0.075 mm. As a result, decontamination efficiency of one cycle using BERAD for all nuclides in soil particles between 4 mm and 0.075 mm is about 45% to 65 %.

Despite its advantages such as safety, unnecessary pretreatment, and decontamination of waste with complex geometry, conventional ultrasonic decontamination technology has been only used to remove loose contaminants, oil and grease, not fixed contaminants due to the limitations in increasing the intensity in the high frequency range. Thus, ultrasound has been used as an auxiliary method to accelerate chemical decontamination of radioactive wastes or chemicals were added to the solution to increase the decontamination efficiency. The recently developed high-intensity focused ultrasound (HIFU) decontamination technology overcomes these limitations by combining multiple frequencies of ultrasonic waves in a specific arrangement, making it possible to remove most fixed contaminants, including radioactive micro particles less than 1 micrometer within half an hour. KEPCO NF and EnesG developed mobile HIFU decontamination equipment and successfully demonstrated the decontamination effect on various radionuclides found in nuclear power plants by treating radioactive metal waste to the level below free release criteria. The mobile HIFU decontamination equipment used in the demonstration can be operated anywhere where water is supplied, including controlled area in nuclear power plants, and is expected to be used widely for decontamination and free release of metal radioactive wastes.

Canada’s Pickering Unit 3 was performed a three-stage decontamination from June to August 1989 in preparation for pressure tube replacement. The first step was a reducing CAN-DECON treatment to dissolve the magnetic film inside the reactor, which was applied following partial defueling of the reactor core. The second step was an oxidative dilute alkaline permanganate treatment to remove the chromium-rich oxides of the stainless steel parts. And the final CAN-DECON step was applied continuously after completely removing fuel from the reactor core. In situ pipe gamma-ray spectroscopy techniques were applied to measure radioactivity within feeder piping during various stages of Pickering Unit 3 decontamination. Measurements were performed at a maximum dose rate of 5 mSv/h, and both the detector and the scanned feeder pipe were properly shielded from other neighboring pipes. 60Co was the dominant radionuclide in feeder piping prior to decontamination. And radionuclides 103Ru, 95Zr, 95Nb, 59Fe, 140La and 124Sb were detected. The Co-60 radioactivity was 2.09×105 Bq/cm2 before decontamination and 3.11×103 Bq/cm2 after decontamination in the inlet feeder pipe T18. And in the outlet feeder pipe P21, it is 2.56×104 Bq/cm2 before decontamination and 2.04×103 Bq/cm2 after decontamination.

Radioactive products generated by long-term operation at NPP can become deposited on the surfaces of the system and equipment, leading to radiation exposure for workers during the decommissioning process. Chemical decontamination is one of the methods to reduce radiation exposure of workers, and there are HP CORD UV, CITROX, CAN-DECON. In the chemical decontamination process, organic acids are generally used, and representative organic acids include oxalic acid and citric acid. There are various methods for removing residual organic acid in decontamination liquid waste, such as using an oxidizing agent and an ion exchange methods. However, there is a problem in that oxidizing agent is used excessively or secondary wastes are generated in excess during organic waste treatment. However, when organic acid is decomposed using a UV lamp, the amount of secondary waste is reduced because it tis decomposed into CO2 and H2O. In this study, organic acid decomposition was evaluated as the contact time of the UV lamp. The experimental equipment consists of a UV reactor, a mixing tank, a circulation pump. The experimental conditions involved preparing 60 L of organic liquid waste containing oxalic acid, hydrogen peroxide and iron chloride. Test A was conducted using one UV reactor, and Test B was performed by connecting two UV reactors in series. As a result of the experiment, a decomposition rate of over 95% was shown after one hour for oxalic acid, and it was confirmed that the initial decomposition rate was faster as the contact time increases. Therefore, in order to increase the initial decomposition rate, it is necessary to increase the contact time of the UV lamp by connecting the UV reactors in series.

It is reported that 48 pressurized heavy water reactors (PHWRs) are in operation, and 10 PHWRs including Wolsong-1 NPP have been permanently shut down in the world. In the case of PHWRs, which have been permanently ceased, they are managed through the delayed decommissioning method, but there are no cases of dismantling. Therefore, technology development is urgent for the effective decommissioning of PHWRs. Unlike PWRs, PHWRs are separated into coolant system and moderator system. Most of pipes and systems of coolant system are mainly composed of carbon steel, expect of the steam generator tubes which are composed of nickel alloy. On the other hand, the moderator system is composed of stainless steel. In the case of stainless steel, the inner layer of the oxide film is composed of chromium oxide, and the outer layer is composed of iron and nickel oxide in enriched. To remove two oxide layers, it is needs to different decontamination method, the coolant system can perform the system decontamination process through a reduction process, but in the case of the moderator system, the oxidation/reduction process is required because it has a material and oxide film similar to PWRs. In this study, this is evaluated the oxide film removal rate according to the type of stainless steel and temperature in order to remove the oxide film deposited in the moderator system. The experiments were carried out at temperatures of 60, 70, 80 and 90°C, with a concentration of 200 ppm of permanganic acid and nitric acid, and 2,000 ppm of oxalic acid, respectively. The results of the oxide film removal rate test for SUS304 showed 29% at 60°C, 38% at 70 and 80°C, and 41% at 90°C. For SUS403, the oxide film removal rate experiment results showed 62% at 60°C, 85% at 70°C, 94% at 80°C, over 99% at 90°C. The results showed that the removal efficiency of the oxide film increased as the temperature increased. Following the results of experimental, the optimum temperature of oxide removal in composed of the stainless steel material is to be 90°C for decontamination of PHWR.

Laser scabbling has the potential to be a valuable technique capable of effectively decontaminating highly radioactive concrete surface at nuclear decommissioning sites. Laser scabbling tool using an optical fiber has a merits of remote operation at a long range, which provides further safety for workers at nuclear decommissioning sites. Furthermore, there is no reaction force and low secondary waste generation, which reduces waste disposal costs. In this study, an integrated decontamination system with laser scabbling tool was employed to test the removal performance of the concrete surface. The integrated decontamination system consisted of a fiber laser, remote controllable mobile cart, and a debris collector device. The mobile cart controlled the translation speed and position of the optical head coupled with 20 m long process fiber. A 5 kW high-powered laser beam emitted from the optical head impacted the concrete block with dimensions of 300 mm × 300 mm × 80 mm to induce explosive spalling on its surface. The concrete debris generated from the spalling process were collected along the flexible tube connected with collector device. We used a three-dimensional scanner device to measure the removed volume and depth profile.

KAERI has been developing a new decontamination process that does not contain any organic chemicals in the decontamination solution and minimizes the use of ion exchange resin in the solution as a purifying step. The process is hydrazine based reductive metal ion decontamination for decommissioning (HyBRID) and consists of N2H4, H2SO4 and Cu+ ions. The primary system of the LWR is composed of materials with high corrosion resistance, such as stainless steel and Inconel, but among the materials, the feeder and header of the primary system of the PHWR are composed of carbon steel (SA106Gr.B) with low corrosion resistance. Therefore, when decontamination of PHWRs, attention should be paid to corrosion of carbon steel. Since Fe3O4, a contaminating oxide film formed on the surface of carbon steel dissolves faster than ferrite or chromite formed on the surface of Inconel or stainless steel, the base material is exposed to the solution and is corroded during decontamination. When a large amount of iron ions is eluted into the decontamination agent due to corrosion of carbon steel, not only the soundness of the base metal is adversely affected, but also the amount of decontamination waste increases. The purpose of this study is to develop inhibitors that can minimize corrosion of carbon steel when decontamination of PHWRs using the HyBRID decontamination process. CG, 570S and PP3 were selected as corrosion inhibitors. In addition, corrosion tests of carbon steel were conducted in the HyBRID solution with corrosion inhibitors. The best corrosion inhibitors and optimal operating conditions were selected, and HyBRID decontamination agents with corrosion inhibitors were much better in corrosion resistance than existing commercial decontamination agents.

Nuclear weapon generates huge amount of radioactive fallout which is extremely dangerous. The fallout gradually falls to the ground and then covers every surface in city and nature. A hydrogel decontamination medium has been developed to clean the surface polluted by the fallout. The hydrogel is soluble in water so the used hydrogel can be simply removed from the surface by washing. However, significant amount of waste water, containing the radioactive fallout, is generated with this process. In this respect, it is necessary to secure alternative technical options for the used hydrogel recovery. In this study, a steam-suction process was suggested for the used hydrogel recovery. Contaminated stainless steel surface, with fixed simulated fallout particles, was prepared for test. The simulated fallout particles were obtained by high-temperature treatment of a mixture of natural soil, used concrete, and Fe2O3. The hydrogel, composed of poly-vinyl alcohol and borax, was spread onto the contaminated stainless steel surface. The hydrogel was soft at first and it gradually becomes rigid with time. The used hydrogel was recovered by suction with a simultaneous steam spraying to soften the rigid gel. As a result, the clean surface of the stainless steel without the simulated fallout particles was obtained, showing the feasibility of this technique for the used hydrogel recovery.