PURPOSE: To keep the dry state of sand and contrive the proper maintenance of a spring effect by setting a member for preventing the falling of liquid in a gap provided between a reactor container and a biological shield wall provided along the periphery thereof.

CONSTITUTION: A sand cushion part 3 is filled with sand in the bottom of a gap 4 between a reactor container 1 and a biological shield wall 2. A member 7 is set between the container 1 and the wall 2 at the proper position upward of the sand. The member 7 is structured into bellows so as not to restrain the deformation and the like of the container 1, for example, composed of an intermediate member 7c locked so that the end parts thereof are superposed by a bolt 7d among an attaching member 7a supported by a bolt and the like on the container 1 and another attaching member 7b supported on the wall 2. The wall side of the member 7 is low and inclined and a hole is drill in a part of the member 7b at the side of the wall 2 to connect the tube of drain 16 thereto. Thereby liquid such as water is prevented by the member 7 even if it enters the gap 4 and does not fall in the sand cushion part 3 to hold the environment thereof dry and the spring effect of the sand is not reduced.