In order to establish disposal plans for sludge, which is one of the untreated waste materials from domestic nuclear power plants, it is necessary to determine the radioactivity concentration of radioactive isotopes. In this study, we aim to evaluate the gross alpha radioactivity of sludge containing radioactive contaminants after pre-treatment, in order to assess the level of sludge waste and obtain analytical data for discussing disposal methods. Samples of sludge generated from nuclear power plants were pre-treated, solutionized, and prepared as analysis samples for evaluating the gross alpha radioactivity.

Typically, the bottom of the effluent treatment facility at a nuclear power plant contains sediment, which is low-contamination waste consisting of sludge, gravel, sand, and other materials from which radioactive contaminants have been removed. Among these sediments, sludge is an irregular solid form consisting of small particles that are coagulated together, with radioactive isotopes containing cobalt attached. Currently, there is a record of disposing of dry active waste from domestic nuclear power plants, and efforts are underway to gather basic data for the disposal of untreated waste such as sludge, spent filter, and spent resin. In particular, the classification and disposal methods of waste will be determined based on the radioactivity concentration. Therefore, plans are being made to determine the radioactivity concentration of radioactive isotopes and establish disposal plans for sludge samples. In this study, pre-treatment and solutionization were carried out for the analysis of radioactive isotopes in sludge sampels from nuclear power plants. The deviation of the gamma radioisotope analysis results was derived to obtain an optimal sample quantity that represents the sludge.

To analyze the activity concentration of radionuclides in radioactive sludge samples generated from low- and intermediate-low-level radioactive waste from domestic nuclear power plant, a pretreatment process that dissolves and homogenizes the sample is essential. However, this pretreatment process requires the use of hydrofluoric acid, which makes analysis difficult and challenges users to handle harmful chemicals. Therefore, we aim to minimize the use of hydrofluoric acid by measuring gamma nuclides in the sludge sample without pretreatment process and compare the differences of measurement results according to the sample matrix with and without pretreatment process. We will collect about 0.1 g of the sludge sample, and dissolve it using an acid treatment process after using microwave decomposition. We will then use gamma spectroscopy to check the concentration of nuclides present in the sludge before and after dissolution and consider the effect of the sample matrix.

Various types of tanks are used in nuclear power plants, and sludge composed of various organic substances and inorganic oxides contaminated with radioactive materials may be present at the bottom of a tank of a radioactive waste treatment device. In addition, glassy and fixative oxide contamination layers are accumulated on the inner wall of the tank depending on the tank material, usage and degree of oxidation. Such contaminated sludge is the main cause of radiation exposure to workers when dismantling nuclear power plant tanks. In addition, the waste filters generated by filtration of contaminated sludge is treated as secondary radioactive waste, and this radioactive waste not only occupies a lot of disposal space, but also the disposal cost is continuously increasing. Therefore, it is necessary to develop a technology that does not generate waste filters as much as possible. To solve this problem, NILEPLANT Co., Ltd. registered a patent named “Filtering apparatus” based on previous research and manufactured a rotary filtration membrane device through detailed design. The rotary filtration membrane device is composed of three or more multiple rotary filtration membranes, and can remove fine particles in wastewater as well as sludge accumulated inside a radioactive contamination tank. In addition, considering the site characteristics of special conditions such as nuclear power plants, it was designed to show excellent performance in removing fine particles while minimizing the area where the device is installed. The rotary filtration membrane device is designed and manufactured as a double cylinder structure that combines a hydro cyclone filter type body and an inner partition wall, and is equipped with a filter cloth-based rotary cylinder filter to process sludge through the filter cloth in addition to inertial. In addition, the patented principle enables self-backwashing without stopping the filtration process, extending the life of the filter and minimizing waste filters. The filtration performance, self-backwashing function, and sludge behavior of the rotary filtration membrane device manufactured based on the detailed design were evaluated through experiments, and improvements to obtain more effective filtration performance were derived. Accordingly, it is expected that the more improved rotary filtration membrane device can be effectively used to remove sludge generated during the dismantling of nuclear power plants in the future.

The mixing powder of vitrification material and metallic oxide sludge was solidified by hot isostatic press method and was tested to check whether the solidified waste disposal acceptance criteria were met or not. From various contaminated tank in nuclear power plants, and other nuclear energy facilities, radioactive sludge based on metallic oxide can be generated. The most of tank consist of stainless steel can be oxidated by the long-term exposure on oxygen and moisture, and then can be made sludge layer based on metallic oxide on the inner wall of contaminated tank. Radioactive sludge waste should be solidified and disposed. Melting and hardening is the most basic method for solidification. The melting points of metallic oxide of stainless steel as Fe3O4, NiO, Cr2O3 are 1597, 1955, 2435, respectively. Those are very high temperature. To melt these metallic oxides, a furnace capable of raising the temperature to a very high temperature is required, which requires a lot of thermal energy, which may lead to an increase in disposal cost. Therefore, it is necessary to lower the melting point and solidify non-melted metallic oxide powder by adding vitrifying material powder as Na2O, SiO2, B2O3. The more vitrification material is added, the easier it is to solidify the sludge based on metallic powder at a low temperature, but there is a problem in that the total waste volume increases due to the addition of vitrification material. In this study, the mixing ratio and temperature conditions that can fix the sludge while adding a minimum amount of vitrification material will be confirmed. Mixing ratio conditions of the vitrification material and sludge powder are 10:90, 15:85, 20:80, 25:75. To fix the metallic oxide sludge by melting only the vitrification material without completely melting the metallic oxide, compression by external pressure is required. Therefore, the HIP (Hot Isostatic Pressing) method was used to solidify the metallic oxide sludge by simultaneously heating and pressurizing it. Because the softening points of most of vitrification based on Na2O, SiO2, B2O3 are ranged from 800 to 1000, temperature conditions are 800, 900, 1000. Since the compressive strength for disposing of the solidified materials was 3.4 MPa, the maximum pressure condition was set to 5000 psi (about 34 MPa), which is 10 times 3.4 MPa. And optimal mixing ratio, temperature, pressure conditions that meet the solidified waste disposal acceptance criteria will be found.