Nuclear facility decontamination technology

Author: Translator: Zuo Min / et al
Publisher:
Publishing Date: 1997-09-01
Features: Preface by the Translator The development of China's nuclear industry has a history of over 40 years. Since the 1990s, the commissioning of Qinshan Nuclear Power Plant (300MW) and Daya Bay Nuclear Power Plant (2×900MW) marked a new era of development for China's nuclear industry. As the operation time of nuclear power plants progresses, radioactive nuclides inevitably occur and accumulate in the reactor core and are transported to relevant systems through the primary cooling system, causing radioactive contamination of systems and equipment. On the other hand, to ensure the long-term, safe, and stable operation of nuclear power plants, regular inspections and maintenance, or even the replacement of certain components, must be carried out. To reduce the radiation dose received by operators, inspectors, and maintenance personnel, it is necessary to decontaminate nuclear power plants, particularly the primary cooling system. The decontamination technology used should not damage the design function of the facilities being decontaminated, should not accelerate secondary contamination, and should be simple, practical, and economical. Many of China's early nuclear facilities have entered the decommissioning stage, and appropriate decontamination technology is needed to ensure the safe and economical implementation of decommissioning. Therefore, decontamination technology has gained increasing attention in China, and many organizations have been conducting research in this field. In this context, there is a growing demand for a comprehensive book that introduces decontamination technology for nuclear facilities. This book primarily focuses on nuclear power plants and introduces the mechanisms of pollution occurrence and accumulation in various nuclear facilities, as well as ways to reduce pollution sources. It explains the principles, development processes, practical applications, and prospects of various decontamination methods for different purposes, such as decontamination during operation and decommissioning. It also provides insights into preventing secondary contamination after decontamination for operational facilities. The book is primarily based on literature published before 1984 and has been organized and compiled. It contains a wealth of experimental and measured data. Due to its early publication, developments after 1985 are not included. However, considering that the development of decontamination technology can be roughly divided around 1980, with research and development of pollution mechanisms and decontamination technology itself being the focus before this date and application development afterward, this book, written in 1984, provides detailed discussions on the basic aspects of various decontamination methods. Given that decontamination technology is an interdisciplinary field with strong theoretical and practical foundations and is still in its development phase, the content of this book has not become outdated and remains highly valuable for readers in China. This book can serve as a reference for researchers, designers, and operators working on nuclear power plants, reactor engineering, post-processing, waste treatment, and decommissioning of various nuclear facilities. It is also suitable for university faculty and students in related fields. If this book achieves these purposes, the translators will feel immensely gratified. During the translation process, we received assistance and guidance from Professor Shi Zengkuiji, the original editor of the book, as well as from ()テノ·フシト, the publishing company, and Senior Engineer (Professor-level) Ke Youzhi, National Design Master, and Reviewer Guang from South Korea. We extend our sincere gratitude to all of them.
Book Description This book introduces the mechanisms of nuclear pollution and decontamination in light-water reactor nuclear power plants, with coverage extending to other types of nuclear power plants and facilities. It covers the mechanisms of pollution occurrence and decontamination, as well as the approaches and experiences for reducing and preventing pollution. The book also discusses the research, development, and practical effects of various decontamination technologies and equipment, along with their development trends. Additionally, it presents evaluation methods for decontamination technology from technical and economic perspectives and explains the different requirements for selecting decontamination technology for operational and decommissioned facilities. This book is suitable for scientists and engineers engaged in the design, operation, and management of nuclear power plants or other nuclear facilities, as well as for technicians involved in decommissioning and faculty and students at relevant universities.
Excerpt: Microscopic corrosion products from structural materials precipitate into the cooling water and are carried to the reactor core, where most of them adhere to the fuel surface and become activated under neutron irradiation. Some of the activated corrosion products detach from the fuel surface and are transported to the core periphery, where they adsorb and accumulate on the surface of pipelines. Although the radioactive accumulation mechanism remains unchanged regardless of the reactor type, the detailed mechanisms of each process differ significantly between pressurized water reactors (PWRs) and boiling water reactors (BWRs) due to differences in water quality and systems. Table 1-2 compares the characteristics of primary cooling water in Japanese PWRs and BWRs. In PWRs, boric acid is added as a chemical compensator, and LiOH is added to control the pH value from the perspective of corrosion inhibition. Since the concentration of boric acid varies with reactor operation, the pH of the reactor water can fluctuate within a certain range, which is a key feature of PWRs. Additionally, H2 is added to inhibit the radiation decomposition of water in the core, another characteristic of PWRs. As a result, almost no O2 exists in the reactor water, and the water is in a reduced state. In contrast, in BWRs, since the core is boiling, it is difficult to control the reactor water environment using additives, so water purification measures are employed. Therefore, the reactor water is controlled by reducing the amount of impurities introduced into the core from the feedwater system. However, an exception is the O2 concentration in the feedwater. recently, it has become clear that O2 exists in the feedwater at a certain concentration, which is essential for inhibiting the corrosion-induced iron precipitation in feedwater system pipelines. It is generally believed that the optimal level of dissolved O2 concentration in the feedwater system is 30–50 ppb. Since H2 is not added in BWRs, radiation decomposition of water occurs in the core, resulting in an O2 level in the reactor water of 200 ppb. Thus, unlike PWRs, the reactor water in BWRs is in an oxidized state. The above discussion details the differences in primary cooling water quality and management between PWRs and BWRs, which are of great significance for considering the behavior of corrosion products and the structure of oxidation films discussed later.
1.2.2 Behavior of Insoluble Corrosion Products (CRUD) The radioactive nuclide accumulation mechanism in the primary cooling system, as briefly described in the previous section, is an extremely complex phenomenon. Although research is actively conducted worldwide, many aspects remain unclear. One of the main reasons for the complexity of this phenomenon is that although the corrosion products entering the primary cooling system are extremely trace amounts, their diverse forms and structures make it difficult to understand the true situation. It is generally believed that corrosion products derived from structural materials consist of water-soluble ions and water-insoluble metal oxides. CRUD is sometimes used to refer to all corrosion products, but the latter, i.e., insoluble corrosion products, are typically referred to as CRUD, with Fe being the main component. In an oxidizing environment like BWR reactor water, iron exists as iron(III) oxide (α-Fe2O3), while in the reduced state of PWR reactor water, it exists as iron(III) oxide (Fe3O4) and nickel ferrite (NiFe2O4) and other metal oxides dispersed in water. On the other hand, although the main component of ionically dissolved corrosion products is also Fe, it is generally considered that the presence of trace amounts of Co ions is more significant. The amount of CRUD currently problematic in actual reactors is 1–10 ppb of iron. What exactly is the significance of 10 ppb? If Japan has a population of 100 million, it would be equivalent to one person having an issue. Thinking in this way may help to understand the scale. As for Co, when the content is as low as 1–10 ppt (10–12), which is three orders of magnitude lower, it becomes problematic. Similarly, as previously mentioned, Co contributes the most to the dose rate on pipeline surfaces, and CRUD plays a crucial role in its accumulation process. It is generally believed that CRUD acts as a medium for the generation and accumulation of nuclides such as 60Co or 58Co. Of course, we know that CRUD is formed due to the corrosion of metal surfaces, but the detailed mechanism of its formation remains unclear. We regard the corrosion surfaces of stainless steel and carbon steel in high-temperature water as having two or more layers, at least with a thin and dense inner layer of the oxide film and a coarse-grained outer layer. The migration of substances through this oxide layer determines the corrosion rate. It is generally assumed that the inner layer is formed by O2 and H2 molecules diffusing to the surface of the parent material and directly reacting with the metal, while the outer layer is formed by Fe ions diffusing through the micropores and grain boundaries outward and precipitating. One mechanism of CRUD formation is the assumption that local impacts caused by flow rate changes detach the coarse-grained outer layer from the corrosion surface and enter the water. However, it is unclear what factors influence this process, how they influence it, and the speed at which it occurs. Additionally, our research group [3] believes that the process of metal ions precipitating in the aqueous phase as tiny colloidal particles is also important. Related to the second process, the following oxidation reaction of Fe2+ ions is significant in an oxidizing environment like BWRs: