PURPOSE: To transfer reactor core cooling members even when an earthquake breaks out by providing a sloshing liquid level regulating means such as partition plates for dividing, a baffle plate for converting direction, and a trap for generating a liquid dropping time delay, at the upper part inside an upper plenum, so as to regulate the liquid level.

CONSTITUTION: Reactor core cooling member is accumulated in an upper plenum 3, and a free liquid level 1 is formed there. When an earthquake breaks out, and a sloshing is generated in an atomic reactor, the ratio of the span length of spans and the amplitude of the wave is made three times of the case of no partition plate 2a because the liquid level is divided into three by the partition plates 2a, and a nonlinear effect is to be generated at 1/3 amplitude, reducing the liquid level drop remarkably. Then, the inclination of a baffle plate is made 45°, and the scattering liquid drops from the inside of the plenum 3, and together with a flow moving upward along the plenum wall, the liquid level effect is reduced. Furthermore, a time delay is generated when the liquid drops by providing a trap, and thereby, the liquid level drop is reduced. Consequently, the reactor core cooling member can be transferred even when an earthquake breaks out.