Summary of Fukushima nuclear accident and future nuclear safety

One year passed after the Fukushima nuclear accident and based on more correct data obtained at the accident, overview of accident progression and response, and lessons learned from the accident and future nuclear safety were summarized. The earthquake off the Pacific coast in the Tohoku region and tsunami, impacts of earthquake and tsunami on plant, state of accident response after tsunami struck, causes of hydrogen explosions and state of spent fuel pools were outlined. The complete power loss and insufficient provision against severe accident resulted in core cooling systems failure causing the water level in the reactors to drop and exposing the cores, which led to meltdowns of the cores and release of hydrogen due to a reaction between the zirconium and water. The hydrogen contained volatile radioactive materials and leaked into the reactor buildings through containment vessels resulting in hydrogen explosions in the reactor buildings of Units 1, 3 and 4, which contaminated the area around the facility. Based on lessons learned from the analysis of accident causes and issues derived, countermeasures had been proposed such as (1) complete waterproofing against tsunami, (2) prevention of fuel failures at complete power loss and losses of final heat sink with combination of flexible measures, (3) impact mitigation measures after the core damaged, (4) enforcement of infrastructure and emergency support system and (5) other measures to upgrade safety as middle and long term technical issues. Definite applicable examples of safety measures were also presented. (T. Tanaka)