In normal nuclear power plants, three fundamental safety functions, i.e., stopping the reactor, cooling the reactor and confining radioactive materials, must be maintained to ensure nuclear safety. Similarly, during the decommissioning of accident-damaged nuclear power plants, they must be maintained. However, in severely accident-damaged nuclear power plants, such as Fukushima Daiichi Units 1 to 3, the stopping function is lost and the confinement function is degraded. In such cases, it can be considered that the cooling function during the decommissioning period before starting fuel debris retrieval becomes more important for nuclear safety than in the other periods. This is because it plays a very important role in maintaining the physical state of the fuel and fuel debris to prevent recriticality and thus suppresses the mobility of radioactive materials. Similarly, the confinement function becomes more important during the fuel debris retrieval. Because of the high radiation level, it is very difficult to access the inside of the primary containment vessel and reactor building to investigate and/or repair equipment or components. Thus, it may be necessary to propose a decontamination and decommissioning project with remaining unknowns and to perform extensive research and development to complete the project. Such a project must be carried out strategically with the efficient and effective use of risk information. In this paper, the results of considering the basic concept and methods of risk management during the decommissioning of accident-damaged nuclear power plants from the viewpoint of safety and economic risks are reported.
International Research Institute for Nuclear Decommissioning is engaged in a METI-funded project with the title ‘Research and Development of Processing and Disposal of Solid Waste’. This R&D project, which aims to build a waste stream for Fukushima Daiichi Nuclear Power Station, is executed by many organizations. The waste stream should be implemented by organizing several steps from generation to storage/disposal, and mutual feedback between each step is important to obtain the desired result. Tools to visualize and share the relations between each R&D theme of this project have been developed to enable the mutual communication of project members who are executing the R&D of different steps.
The cooling rate of molten core materials during solidification significantly affects the segregation of major constituents of fuel debris. To understand the general tendency of the segregation, liquefaction/solidification tests of simulated corium (UO2, ZrO2, FeO, B4C and sim-FP oxides) were performed. The simulated corium was heated up to 2,600℃ in Ar atmosphere and then cooled with two different cooling processes: furnace cooling (average cooling rate of approximately 744℃/min) and slow cooling (cooling from 2,600℃ to 2,300℃ at a rate of 5℃/min and from 2,300℃ to 1,120℃ at approximately 788℃/min). Element analysis detected three oxide phases with different compositions and one metal phase in both solidified samples. The solubility of FeO in these oxide phases was mostly fixed at 12±5 at% in both samples, which is in reasonable accordance with the value estimated from UO2–ZrO2–FeO phase diagrams. However, the significant grain growth of one oxide phase, rich in Zr-oxide, was detected only in the slowly cooled sample. The composition of this particular oxide phase is similar to the initial average composition. The grain growth is considered to be caused by the connection of remaining liquid agglomerates during slow solidification.