Abstract: Risk management strategy for uncertain systems is proposed and applied to Fukushima Daiichi decommissioning project. It is an uncertain work on which public have significant concern. There exist several risk sources which importance is different in terms of hazard potential and confinement performance. A priority of each risk source is evaluated. Retrieval of fuel assemblies from a spent fuel pool is highly prioritized. Four principles and three risk metrics are defined in the risk assessment. A success path is developed and possible threats are identified for technical/societal/management aspects. As knowledge is limited on the current situations, the decommissioning is performed with wide-scope and long-term perspective. The risk assessment and management process needs to be updated as the most recent information and knowledge on the systems become available.