To cool down a reactor and to prevent radioactivity from being diffused in the air when a nuclear power plant is disabled from being controlled by damaging and destroying the nuclear power plant by natural disasters such as earthquakes and tsunami.

A container is a block which has a rectangular shape, is made of iron and has a sealed structure. A valve is attached to the container that is each block in the sealed structure, thereby injecting an aqueous solution into the block as the container. By using such blocks, a levee in a cylindrical shape, a four-sided shape, a rectangular shape or any other shape (in the cylindrical shape hereafter) is constructed around a nuclear power plant. The aqueous solution is injected into the levee constructed by stacking the blocks filled with the aqueous solution to a height equal to or more than the height of the nuclear power plant. Thus, the nuclear power plant is completely submerged, thereby cooling down a reactor and preventing the diffusion of radioactivity.