What happened in the reactor fuels of Fukushima Daiichi nuclear power plants. Nuclear fuel behavior when core is exposed

At Fukushima Daiichi Nuclear Power Plant (NPP) accidents, reactor cores of units 1, 2 and 3 under operation were not covered by water due to loss of cooling with emergency cooling system and stoppage of water injection but were exposed, which might lead to a core melt within several hours after core was exposed. Large part of the melted core flowed down to the bottom of the RPV and possibly corroded the reactor vessel (RPV). Strenuous efforts to establish long-term cooling of melt cores were underway toward the end of accidents. Since cooling of melt cores was greatly different from that of intact core, countermeasures of melt core cooling should be appropriate for the state of fuel damage and melt cores. This article presented nuclear fuel behavior from damage to melt when core was exposed based on the knowledge of core melt accident obtained from Three Mile Island (TMI)-2 accident and related research. Unit 1 core was exposed within several hours after reactor shutdown, and the fuel melt was progressed more severe than TMI-2 accident as water was not injected for a long time. A whole core was melt and flowed down to the bottom of the RPV and possibly corroded the RPV. Unit 2 and 3 cores were exposed and melted to the level of TMI-2 accident as water injection was stopped more than three hours. (T. Tanaka)