Severe Accident Issues Raised by the Fukushima Accident and Improvements Suggested

Abstract

The accidents at Fukushima Daiichi nuclear power plants are striking as they not only resulted in simultaneous core damage in multiple units, but also there was a high possibility of failure of the reactor vessels and primary containment vessels in all three reactors. Though the radiological release is estimated to be about 10% of the Chernobyl accident [1,2], the severity of the accident in terms of scale and number of units involved is unprecedented. The accident was classified as International Nuclear Events Scale (INES) level 7 accident [1].The accident progression, including the cause of the accident, the response of the reactor and safety system, recovery actions, and core damage progression leading to a release of radioactive material, were investigated and reported by the Japanese Government [1], TEPCO [2] and international experts [3]. However, the status of the damaged reactor vessel, and damage to the primary containment vessel are still under investigation.Though the occurrence of severe accidents were evidenced in the Three Mile Island (TMI) and Chernobyl accident, the measures for the prevention and mitigation of a severe accident were not strictly regulated. In most countries, severe accident prevention and mitigation measures were recommended only for new builds, as voluntary actions to enhance the safety, while provision of severe accident management guidelines were recommended for operating reactors. It is stated in reference 1 that "While the Japanese National Government recognized that further safety regulations were unnecessary as the safety of nuclear power plant in Japan was fully ensured by the present safety measures, it recommended that electric utilities should perform self-disciplined safety efforts in order to reduce a risk of accident and to further enhance safety." This paper revisits the Fukushima accident to draw lessons in the aspect of nuclear safety considering the fact that the Fukushima accident resulted in core damage for three nuclear power plants simultaneously and that there is a high possibility of a failure of the integrity of reactor vessel and primary containment vessel.A brief review on the accident progression at Fukushima nuclear power plants is discussed to highlight the nature and characteristic of the event. As the severe accident management measures at the Fukushima Daiich nuclear power plants seem to be not fully effective, limitations of current severe accident management strategy are discussed to identify the areas for the potential improvements including core cooling strategy, containment venting, hydrogen control, depressurization of primary system, and proper indication of event progression. The gap between the Fukushima accident event progression and current understanding of severe accident phenomenology including the core damage, reactor vessel failure, containment failure, and hydrogen explosion are discussed.Adequacy of current safety goals are also discussed in view of the socio-economic impact of the Fukushima accident. As a conclusion, it is sugges...

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