open access

https://doi.org/10.14943/doctoral.k14231

https://hdl.handle.net/2115/79784

廃止措置と原子力安全評価のための溶融コリウム・燃料デブリの挙動研究

英語

CFD

2020-09-25

Hokkaido University

145p

One of the most severe accidents for Nuclear Power Plants (NPPs) is called a Loss of Coolant Accident (LOCA). During this kind of accident, the water used to cool down the reactor core is lost. If this coolability is not restored, it often ends with the meltdown of the reactor fuel into what is called, in the Nuclear Energy field, corium. This corium is a mixture of molten steel structure components, and molten fuel debris. Under the effect of its weight, it tends to re-locate to the bottom of the Reactor Pressure Vessel (RPV). If not properly cool when it is there, the corium will melt-through the RPV metallic layer and will spread out of it in the form of a molten material jet. From there, it will impinge and spread on the Primary Containment Vessel (PCV) concrete structures that can be found under the RPV.Thus, the present study focuses on investigating the jet’s formation and impingement phenomena involved in such kind of scenario. The purpose of this study is to provide insightful information that will help to improve the Nuclear Safety Assessment and Decommissioning for this accidental scenario. Firstly, working on the Decommissioning part of the subject, the present study treats the case of Fukushima Daichi Nuclear Power Plant (NPP) and its debris removal project. The project’s purpose was to determine the heat transfer and temperature’s profile of the reactor building during the 2011’s accident, through the use of simulations. This knowledge will allow us to know how deep the radioactive contamination is in the different concrete structures inside the reactor building. The simulation’s results revealed that deep contamination, roughly 25 cm depth, is expected in the concrete structure directly under the RPV. As for the other concrete structures, it was found that only superficial radioactive contamination is expected in those areas. This information will help the debris removal planning and also, the decommissioning of Fukushima Daiichi. Through the above project, it was found that the simulation’s results were highly dependent on the debris field geometry used. That geometry was built according to data provided by Tokyo Electric Company (TEPCO) and generated by accidental simulation software, introducing inaccuracy through the assumption using in this software. From that 3 assessment, it was assumed that a way to improve those simulations will be to deepen the knowledge on debris field formation. Therefore, the present study secondly focuses on creating a formulation which predicts the debris field formation accurately. Through the experimental work realized here, it was possible to build formulation for the maximum spreading ratio 𝝃𝒎𝒂𝒙 and finger number N. Their uses to predict accurately the debris field geometry. In addition, simulations were built to investigate the key parameters for debris field formation simulation. From them, it was found that the surface roughness is a detrimental key parameter for such simulations. Therefore, through these two works, it was possible to find insightful information that both help the Nuclear Safety Assessment and Decommissioning as they provide new and highly valuable information on debris field formation.

2020-09-25

甲第14231号

博士(工学)

(主査) 教授 澤 和弘, 教授 大島 伸行, 特任教授 中島 宏, 准教授 三輪 修一郎

工学院（エネルギー環境システム専攻）

博士論文

Version of Record