True cause of Fukushima meltdown accident new analysis about loss of core cooling based on latest data

The purpose of this paper is to clarify the true cause of the Fukushima meltdown accident from the viewpoint of reactor design. There are only two factors. One is that a total loss of battery power was not considered in the basic design, and the other is that the fail-close design was selected in long-term core cooling systems, as the confine function was preferred over the cool function in the three safety functions of stop (nuclear reaction), cool (decay heat) and confine (radioactivity) in the basic criteria. After the loss of all power (battery and station power) in Unit 1, the IC system failed because of the fail-close design. After the loss of all power in Unit 2, the core-cooling function of the RCIC system failed because of the instability of two-phase flow in the turbine steam line, since the core decay heat was confined in the reactor and containment vessel owing to the fail-close design of the vent valves. As loss of station power occurred but battery power was available in Unit 3, operators were able to activate the RCIC system, which was tripped by the trip signal after one day. Then, the HPCI system was started automatically but stopped after about 10 hours because of the loss of battery power. After the loss of core cooling, core meltdown occurred in Units 1 to 3. The lessons learned from this accident are that the core-cooling system should be designed to withstand the loss of all power in its basic design and that the cool function should be preferred over the confine function. (author)