가압경수로형 원자로 해체 후 발생된 중준위폐기물의 부피감소를 위한 전해제염 공정 개발

학위논문 (석사)-- 서울대학교 대학원 : 공과대학 에너지시스템공학부, 2019. 2. 심형진.현재, 454기의 원자력 발전소가 전세계적으로 운영 중에 있으며 전체 발전량의 약 11%를 차지하고 있다. 이 중 약 66%의 원자력 발전소는 30년 이상 가동되었으며, 이와 같은 노후원전의 비율이 앞으로 지속적으로 증가할 것으로 예상된다. 국내의 경우, 이미 고리 1호기의 영구정지가 시행되었으며 2030년 전에 7기의 가압경수로형 원자로의 설계수명이 만료가 될 예정이다. 이에 따라 원자력 발전소 해체 준비 필요성이 증가하고 있으며 해체에 따라 발생할 중저준위 방사성 폐기물 관리도 중요해지고 있다. 이 중, 중준위 금속 폐기물이 발생하는 양은 약 66톤으로 주로 방사화 노내 구조물로 이루어져 있다. 노내 구조물은 스테인리스강으로 이루어져 있으며 노심 주변부로써 높은 중성자속에 장기간 노출되어 있다. 스테인리스강이 방사화됨으로써 C14, Nb94, Ni59, Ni63 과 같은 장수명 방사성 핵종 및 Co60과 같은 단수명 방사성 핵종이 발생하게 된다. 하지만 현행 규정 상 방사화 노내 구조물과 같이 장수명 방사성 핵종이 다량 포함된 중준위 방사성 폐기물은 경주 중저준위 방사성 폐기물 처분장의 방사능 총량 규제 기준 한계에 의해 처분될 수 없다. 따라서, 우리나라뿐만 아니라 대다수의 국가들이 관련시설의 임시저장소에 보관 중이며 고준위 방사성 폐기물 혹은 사용후핵연료 처분장의 건설을 기다리고 있는 상황이다. 이를 해결하기 위한 장수명 방사성 핵종을 포함한 중준위 금속 폐기물들의 제염기술 개발이 필요하다. ORIGEN-2를 활용하여 가압경수로 노내 구조물이 32년 유효전출력기간을 가정하여 모델링한 결과, 냉각기간이 15년 지난 뒤 위 핵종들의 방사능 농도는 C14(1.83E+06Bq/g), Nb94(2.88E+04Bq/g), Ni59(5.40E+06), Ni63(1.05E+07Bq/g), Co60(1.35E+09Bq/g)으로 중준위 방사성 폐기물로 평가된다. 국내 가압경수로형 원자로 해체 후 나올 중준위 노내 구조물들이 모두 경주 중저준위 방사성 폐기물 처분장에 처분되기 위해 요구되는 제염계수는 각각 8.2, 259.1, 73.0, 94.7, 36.6이다. 본 논문에서 제시하는 제염공정은 방사화 핵종을 체적제염시킬 수 있으며, 이론적으로 2차폐기물 발생량이 0인 용융염 기반 전해정련이다. 용융염은 불화물(LiF-KF)보다 운전온도가 낮고 부식문제가 적은 염화물(LiCl-KCl)을 선택하였고 핵종들 사이 환원전위차를 이용하여 장수명 핵종들을 남기고 산화경향성이 큰 Fe과 Cr을 회수하는 실험을 수행하였다. 전해정련 실험 전 주요 원소들(Fe, Co, Ni, Cr)의 LiCl-KCl 용융염 내 산화화원 거동을 알아보기 위해 각 원소들의 염화물을 녹인 후 500℃에서 순환전압전류법을 수행하였다. Cr을 제외한 모든 핵종들은 한 개씩의 산환환원피크 쌍을 보였으며, 단순한 거동을 보였다. Cr은 두 쌍의 산화환원피크를 보였으나, Fe의 산화피크가 -0.2~-0.1V[vs. 1 wt. % Ag/AgCl] 에서 발생하는 것으로 보았을 때, Cr2+와 Cr3+ 사이의 산화거동은 무시할 수 있을 것으로 판단했다. 순환전압전류법을 통해 나온 결과를 토대로 각 원소별 Apparent reduction potential과 용융염 내 확산 계수를 획득하였다. 또한, Linear Polarization Method를 통해 각 핵종들의 교환 전류 밀도 및 전하전달계수를 획득함으로써 관련 연구를 위한 데이터베이스를 구축하였다. 구축된 데이터베이스와 1-D 시간 종속 전기화학 모델링코드 REFIN을 활용하여 전해정련 모델링을 수행함으로써 전해정련을 통한 제염계수 확보가능성을 평가하였다. 전해정련을 통해 Nb, Co에 대한 제염계수는 확보가능하다는 것을 보였으며, Ni의 경우 2번의 반복과정을 거치면 제염계수를 달성할 수 있다. 모델링 결과의 검증을 위해 전해정련 실험을 수행하였다. 실험 조건은 LiCl-KCl-3 wt. % FeCl2 용융염을 사용하였고, 양극엔 스테인리스강 막대를 사용하였다. 실험 중 양극, 음극에서 떨어져 나올 수 있는 금속들을 회수 혹은 금속 물질에 의한 오염 방지를 위해 양 극 주변에 용기를 설치하였다. 양극에 걸리는 전위가 -0.2V[vs. 1 wt. % Ag/AgCl], -0.2V[vs. 1 wt. % Ag/AgCl], -0.1V[vs. 1 wt. % Ag/AgCl], 0V[vs. 1 wt. % Ag/AgCl]로 다르게 주어 3번 수행하였다. 전해 정련 셀 내 음극표면, 음극 용기 내부, 양극 용기 내부, 용융염들의 ICP-MS 분석을 수행하였으며, 음극표면의 전착물을 확인하기 위해 XRD 분석을 수행하였다. 분석 결과 음극표면에 Fe가 금속상태로 전착되는 것이 확인되었으며, 양극 인가 전위가 증가하면서 Co, Ni의 제염계수가 감소하는 것을 확인되었다. 이는 양극 인가 전위의 증가로 Fe뿐만이 아닌 Co, Ni의 산화 피크가 일정 부분 포함되면서 양극에서 녹아나와 전착이 된 것으로 예상된다. 하지만 3가지 실험 모두 전해정련 공정이 2번 반복 수행되었을 때, 요구되는 제염계수를 만족시킬 수 있는 것으로 확인되었다. 실제 공정 내에서 한계전류와 확산층 제어를 위해 회전전극을 사용한다는 점을 고려하여 셀 내 용기를 설치한 것이었다. 하지만 전해정련 실험 중 한계전류의 형성이라는 문제점을 확인할 수 있었고 이를 보안하기 위해 CFX코드를 활용하여 새로운 셀 디자인을 설계하였다. 셀 내 추가적인 용기의 디자인을 미세 다공성 구조로 모델링하여 유체 내 입자 거동 해석과 전압 강하에 대한 시뮬레이션을 수행하였다. 미세 다공성 용기를 통해 전압 강하는 2배 이상의 효율을 보였으며, 회전 전극에 의한 유체 거동에 따른 입자의 음극 용기 내 이탈이 효율적으로 제어될 수 있음을 확인하였다. 본 논문의 연구결과를 통해 파일럿 규모의 전해정련로를 제작한다면 경제적으로 1320 억원 이상의 효과를 나타낼 것으로 예상된다.Currently, 454 nuclear power plants are operating globally and account for about 11% of the total electric power generation. About 66% of these nuclear power plants have been in operation for more than 30 years, and the proportion of such old nuclear power plants is expected to continuously increase. In Korea, the Kori Unit #1 reactor has been decided to permanent shutdown, and 7 PWR reactors will reached the design life time before 2030. As a result, the need to prepare for the decommissioning of nuclear power plants is increasing, and the management of intermediate and low-level radioactive waste from decommissioning is also becoming important. Among them, the amount of the intermediate level metal waste from reactor internals periphery to reactor core is about 66 tons. The reactor internals are made of stainless steel and are exposed to high neutron flux for a long period. As the stainless steel is activated, long living activation products such as C14, Nb94, Ni59 and Ni63 and short living activation products such as Co60 are generated. However, according to the current regulations, intermediate level radioactive wastes containing a large amount of long living radionuclides can not be disposed of due to the total activity limits of Gyeongju repository. Therefore, most of the countries as well as Korea kept them in the interim storage on site and waiting for the construction of high-level radioactive waste or spent nuclear fuel repository. In order to solve this problem, it is necessary to develop decontamination technology for long living intermediate level wastes. Based on ORIGEN-2 modeling with some assumptions, pressurized water reactor internals were modeled. The radioactivity of C14, Nb94, Ni59, Ni63 and Co60 were 1.83E+06Bq/g, 2.88E+04Bq/g, 5.40E+06Bq/g, 1.05E+07Bq/g, 1.35E+09Bq/g, respectively. The decontamination factors required for disposal of all reactor internals from 20 units are 8.2, 259.1, 73.0, 94.7, and 36.6, respectively. In this paper, electrorefining process that takes good advantage of theoretically very low secondary waste generation was suggested for decontamination process. LiCl-KCl eutectic salt, which has lower operating temperature than fluoride salt (LiF-KF) and has less corrosion problem, was used as electrolyte. Approach for electrorefining is recovering Fe and Cr with high tendency to oxidation by leaving long-living nuclides using standard potential difference between them. In order to investigate the behavior of the major elements (Fe, Co, Ni, Cr) in the LiCl-KCl, cyclic voltammetry at 500°C was performed. All nuclides except for Cr showed a single pair of redox peak. Cr showed two pairs of redox peaks, but the oxidation peak of Fe was -0.2 ~ -0.1V [vs. 1 wt. % Ag/AgCl]. The oxidation behavior between Cr2+ and Cr3+ was negligible in that region. Apparent reduction potentials and diffusion coefficients in molten salt were obtained based on the results of cyclic voltammetry. In addition, a database for related studies was constructed by acquiring exchange current density and charge transfer coefficient of each nuclide through Linear Polarization Method. The achievability of the decontamination factors through electrorefining was evaluated by conducting REFIN modeling, 1-D time dependent simulation code. It showed that decontamination factors for Nb and Co can be achieved through electrorefining. In case of Ni, it is possible to achieve decontamination factor by 2 successive electrorefining. Electrorefining experiments were performed to verify the modeling results. LiCl-KCl-3 wt. % FeCl2 was used as electrolyte, and a type 304 stainless steel rod was used as anode. During the experiment, baskets were installed around the anode to recover the metals that could come off from the anode and the cathode. The applied potential on anode were -0.2V[vs. 1 wt. % Ag/AgCl], -0.1V[vs. 1 wt. % Ag / AgCl], 0V[vs. 1 wt. % Ag/AgCl]. After electrorefining, the cathode surface, inside the baskets and bulk salts were analyzed by ICP-MS and XRD analysis was performed to confirm the deposition on cathode. As a result, it was confirmed that Fe was electrodeposited on the surface of the cathode and the decontamination factors of Co and Ni was decreased as the applied potential on anode was increased. However, in all three experiments, it was confirmed that the required decontamination factor can be satisfied when electrorefining process is repeated twice. However, in the lab-scale electrorefining experiments, the problem of the formation of the limiting current was confirmed and the new cell design was designed using the CFX code to overcome the issues. The design of new baskets in the cell was modeled as a porous structure, and the particle tracking and the IR drop modeling were performed. The IR drop between anode and cathode was more than 2 times greater than that of the non-porous baskets. These findings suggest that if a pilot-scale electrorefiner is manufactured, it would result in reduction of 132.6 billion KRW economically.Contents Chapter 1 Introduction 1 1.1 Background 1 1.2 Problem Statement 4 Chapter 2 Literature Review 10 2.1 Electrochemistry of Decontamination 10 2.2 Electrochemistry Modeling Code 14 Chapter 3 Research Goal and Approach 16 3.1 Research Goal 16 3.2 Research Approach 16 Chapter 4 Decontamination Requirements for Activated Intermediate Level Waste 19 4.1 Inventory Analysis of PWR Internals 19 4.2 Calculation of Decontamination Factor (DF) 23 4.3 Feasibility of Decontamination 24 Chapter 5 Electrorefining Experiments of Stainless Steel 26 5.1 Experimental Setup of Cyclic Voltammetry 26 5.2 Cyclic Voltammetry of Fe, Co, Ni and Cr 29 5.3 Exchange Current Density Determination by Linear Polarization Method 36 5.4 1-D Electrorefining Modeling 42 5.5 Experimental Setup of Electrorefining 45 5.6 Electrorefining Results of Stainless Steel 48 Chapter 6 Pilot-Scale Conceptual Design 56 6.1 Overall Pyrochemical Decontamination Process Flowsheet 56 6.2 Issues for Cell Design 58 6.3 Unit Cell Design 62 6.4 Cost Benefit Analysis 63 Chapter 7 Conclusions and Future Work 65 7.1 Conclusions 65 7.2 Future Work 66 Appendix 68 Bibliography 74 Abstract 76Maste

The effect of blockade of the vasa vasorum upon cholesterol-induced atherosclerosis in rabbits

의과대학/박사[한글] The Effect of blockade of the Vasa Vasorum upon Cholesterol-induced Atherosclerosis in Rabbits by Chung Ho Huh, M.D, Department of Pathology Yonsei University College of Medicine, Seoul, Korea Directed by Professors: Dong Sik Kim, M.D., Kwang sik Min, M.D. Introduction since the vasa vasorum furnish most of the oxygen and nutrients for the arterial wall, many investigators consider them to have an important role in the genesis of degenerative disease of the arteries (Winternitz et al. 1938, Wartman 1938, 1955). Among the degenerative disease of the arteries, the most serious and important one is atherosclerosis. Atherosclerosis has been produced experimentally by the excessive feeding of cholesterol (Duff 1935, Jobling and Meeker 1936, Duff and McMillan 1951). Hypercholesterolemia is regarded as a prime factor in the development of atherosclerosis. However, previous investigations by the authors (Huh and Kim 1965, and chang 1965), as well as other reports in the literature, strongly indicate that many factors other than increased lipids in the blood also play an important role in atherogenesis (Wartman 1955). These factors include local vascular wall changes and such as degeneration of the media, intimal hyperplasia, local anoxia, and local alteration of hemodynamics, etc. Local injuries of the vascular wall have been induced by various methods; injection of bacterial toxins or chemical agents, irradiation, crushing injuries, freezing and by electric cauterization. Any type of local alteration of the vascular wall enhances the development of atheroma formation. However, the mechanism by which local alterations accelerate atheroma formation has not been elaborated. The present investigation was undertaken to investigate the normal distribution of the vasa vasorum in the aorta of rabbit, effect of the blockade of the vasa vasorum on the normal aortic wall, and finally the influence of the blockade of the vasa vasorum upon cholesterol-induced atherosclerosis in rabbits. Materials and Methods Albino rabbits, around 1.8kg of body weight, were divided into four groups and treated as follows. Group Ⅰ consisted of 5 normal rabbits which were subjected to the demonstration of the aortic vasa vasorum using the lead acetate and potassium dichromate impregnation method of williams (1948) to study the normal distribution of the vasa vasorum. Group Ⅱ of 14 rabbits were subjected to blockade of the aortic vasa vasorum to investigate the effect of the blockade of vasa vasorum on the normal aorta. Group Ⅲ of 5 rabbits were fed excessive amounts of cholesterol to induce atherosclerosis. Group Ⅳ of 20 rabbits were fed an excessive amount of cholesterol after blockade of the vasa vasorum to study the effect of the blockade of vasa vasorum upon cholesterol-induced atherosclerosis. The blockade of vasa vasrum was achieved by stripping off the adventitia of the abdominal aorta between the renal arteries and the iliac bifurcation. Then the stripped segment of the aorta was wrapped with polyethylene tubing to prevent the formation of new vasa vasorum from the surrounding tissue. The operation was performed under ether anesthesia with aseptic precautions, and 0.25Gm. of penstreptomycin was given for 3 days to prevent postoperative infection. All animals were fed with a basic diet of a bean-curd residue, 200 Gms. per day per animal. Cholesterol, mixed with a small amount of been-curd residue, was given in a fasting state (1.5 Gm. per animal per day). Serum total cholesterol determination was made once a month. Animals in group Ⅱ and Ⅳ were killed at set intervals to make serial studies on the effect of blockade of the vasa vasorum after the operation. The longest observation period lasted 80 days. At the necropsy, a section from the heart, kidneys, adrenals, liver, and thyroid was taken, and sections were taken from the ascending, arch, thoracic, abdominal and operative site from the aorta. All sections were embedded in paraffin after fixation with 4% neutral formalin. Microsections were cut at 5-6 μ. thickness. Hematoxylin-esoin, Verhoeff Van Gieson, and colloidal-iron stains were applied to all aortic sections and hematoxylin-eosin stain alone to the sections from other organs. Results and Summary All animals fed with cholesterol (group Ⅲ and Ⅳ) showed marked and rapid elevation of serum cholesterol concentration during the first month after the beginning of the experiment. However, thereafter, continuous feeding of cholesterol raised the serum cholesterol level more slowly and to a lesser degree than during the first month. There was no statistical difference between the serum cholesterol levels in group Ⅲ and Ⅳ. The elevation of serum cholesterol in both groups was mainly due to the increase in the ester fraction. The distribution of the vasa vasorum in the aorta of nomal rabbits was confined to the outer adventitia with short ramifications into the inner adventitia. No case showed penetration of the vasa vasorum into the media. No direct opening from the intima was noted. Blockade of the vasa vasorum caused a complete ischemic necrosis of the entire thickness of the aortic segment and an acute inflammatory reaction followed by calcification. Finally the aortic wall was reestablisbed by a newly formed hyperplastic fibromuscular intima which later resulted in the narrowing of the lumen. After 80 days rabbits fed cholesterol alone showed a relatively mild degree of atheroma formation at the ascending and arch portions of the aorta. The thoracic and abdominal portions of the aorta showed a minimal amount of atheroma formation around the ostia of the arterial branchings from the aorta. However, at all parts of the aorta as well as in the coronary arteries, atheroma formation in the animals fed with cholesterol after the blockade of the vasa vasorum was greater than that of cholesterol fed animals which did not have blockade of the vasa vasorum. Accentuated atheroma formation was noted in the segment of the aorta where the vasa vasorum were blocked. Atheroma at the site of the vasa vasorum blockade showed of proliferative intimal thickening, medial necrosis, medial calcification, and lipid deposition in the thickened intima and degenerated media, simulating the atheroma seen in the human aorta. The enhancement of atheroma formation at the site of vasa vasroum blockade was considered due to the accelerating effect of lipid deposition followed by a fibromuscular proliferation in the intima. Also the blockade of venous vasa vasorum impaired lipid clearance from the aortic wall. The increase of atheroma formation in the remaining areas of the aorta was probably due to increased blood pressure secondary to localized stricture of the aorta at the site at which the vasa vasorum was blocked. [영문] Introduction since the vasa vasorum furnish most of the oxygen and nutrients for the arterial wall, many investigators consider them to have an important role in the genesis of degenerative disease of the arteries (Winternitz et al. 1938, Wartman 1938, 1955). Among the degenerative disease of the arteries, the most serious and important one is atherosclerosis. Atherosclerosis has been produced experimentally by the excessive feeding of cholesterol (Duff 1935, Jobling and Meeker 1936, Duff and McMillan 1951). Hypercholesterolemia is regarded as a prime factor in the development of atherosclerosis. However, previous investigations by the authors (Huh and Kim 1965, and chang 1965), as well as other reports in the literature, strongly indicate that many factors other than increased lipids in the blood also play an important role in atherogenesis (Wartman 1955). These factors include local vascular wall changes and such as degeneration of the media, intimal hyperplasia, local anoxia, and local alteration of hemodynamics, etc. Local injuries of the vascular wall have been induced by various methods; injection of bacterial toxins or chemical agents, irradiation, crushing injuries, freezing and by electric cauterization. Any type of local alteration of the vascular wall enhances the development of atheroma formation. However, the mechanism by which local alterations accelerate atheroma formation has not been elaborated. The present investigation was undertaken to investigate the normal distribution of the vasa vasorum in the aorta of rabbit, effect of the blockade of the vasa vasorum on the normal aortic wall, and finally the influence of the blockade of the vasa vasorum upon cholesterol-induced atherosclerosis in rabbits. Materials and Methods Albino rabbits, around 1.8kg of body weight, were divided into four groups and treated as follows. Group Ⅰ consisted of 5 normal rabbits which were subjected to the demonstration of the aortic vasa vasorum using the lead acetate and potassium dichromate impregnation method of williams (1948) to study the normal distribution of the vasa vasorum. Group Ⅱ of 14 rabbits were subjected to blockade of the aortic vasa vasorum to investigate the effect of the blockade of vasa vasorum on the normal aorta. Group Ⅲ of 5 rabbits were fed excessive amounts of cholesterol to induce atherosclerosis. Group Ⅳ of 20 rabbits were fed an excessive amount of cholesterol after blockade of the vasa vasorum to study the effect of the blockade of vasa vasorum upon cholesterol-induced atherosclerosis. The blockade of vasa vasrum was achieved by stripping off the adventitia of the abdominal aorta between the renal arteries and the iliac bifurcation. Then the stripped segment of the aorta was wrapped with polyethylene tubing to prevent the formation of new vasa vasorum from the surrounding tissue. The operation was performed under ether anesthesia with aseptic precautions, and 0.25Gm. of penstreptomycin was given for 3 days to prevent postoperative infection. All animals were fed with a basic diet of a bean-curd residue, 200 Gms. per day per animal. Cholesterol, mixed with a small amount of been-curd residue, was given in a fasting state (1.5 Gm. per animal per day). Serum total cholesterol determination was made once a month. Animals in group Ⅱ and Ⅳ were killed at set intervals to make serial studies on the effect of blockade of the vasa vasorum after the operation. The longest observation period lasted 80 days. At the necropsy, a section from the heart, kidneys, adrenals, liver, and thyroid was taken, and sections were taken from the ascending, arch, thoracic, abdominal and operative site from the aorta. All sections were embedded in paraffin after fixation with 4% neutral formalin. Microsections were cut at 5-6 μ. thickness. Hematoxylin-esoin, Verhoeff Van Gieson, and colloidal-iron stains were applied to all aortic sections and hematoxylin-eosin stain alone to the sections from other organs. Results and Summary All animals fed with cholesterol (group Ⅲ and Ⅳ) showed marked and rapid elevation of serum cholesterol concentration during the first month after the beginning of the experiment. However, thereafter, continuous feeding of cholesterol raised the serum cholesterol level more slowly and to a lesser degree than during the first month. There was no statistical difference between the serum cholesterol levels in group Ⅲ and Ⅳ. The elevation of serum cholesterol in both groups was mainly due to the increase in the ester fraction. The distribution of the vasa vasorum in the aorta of nomal rabbits was confined to the outer adventitia with short ramifications into the inner adventitia. No case showed penetration of the vasa vasorum into the media. No direct opening from the intima was noted. Blockade of the vasa vasorum caused a complete ischemic necrosis of the entire thickness of the aortic segment and an acute inflammatory reaction followed by calcification. Finally the aortic wall was reestablisbed by a newly formed hyperplastic fibromuscular intima which later resulted in the narrowing of the lumen. After 80 days rabbits fed cholesterol alone showed a relatively mild degree of atheroma formation at the ascending and arch portions of the aorta. The thoracic and abdominal portions of the aorta showed a minimal amount of atheroma formation around the ostia of the arterial branchings from the aorta. However, at all parts of the aorta as well as in the coronary arteries, atheroma formation in the animals fed with cholesterol after the blockade of the vasa vasorum was greater than that of cholesterol fed animals which did not have blockade of the vasa vasorum. Accentuated atheroma formation was noted in the segment of the aorta where the vasa vasorum were blocked. Atheroma at the site of the vasa vasorum blockade showed of proliferative intimal thickening, medial necrosis, medial calcification, and lipid deposition in the thickened intima and degenerated media, simulating the atheroma seen in the human aorta. The enhancement of atheroma formation at the site of vasa vasroum blockade was considered due to the accelerating effect of lipid deposition followed by a fibromuscular proliferation in the intima. Also the blockade of venous vasa vasorum impaired lipid clearance from the aortic wall. The increase of atheroma formation in the remaining areas of the aorta was probably due to increased blood pressure secondary to localized stricture of the aorta at the site at which the vasa vasorum was blocked.restrictio

Analysis of protein sequences in each domain of G-protein-coupled receptors based on pattern driven approaches

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