PURPOSE: To reduce an installing space by coupling plural slab tank main bodies with a lug keeping a space, connecting them with communicating tubes with each other and filling a neutron absorber among the slab tank main bodies.

CONSTITUTION: The slab tank main body 20 is manufactured, for example, with a thin plate of stainless steel, etc., and three main bodies 20 are arranged in parallel keeping a contact space, and they are coupled with each other by using the lugs 40. The main bodies 20 are connected mutually with the communicating tubes 22. The outside of the main body 20 is enclosed with a frame body 24 having a bottom plane keeping the space, and is fixed at the outside the main body 20 with the lug 40. An overflow tube 26 is provided between the tank 20 and the frame body 24. A shielding material 28 consisting of the neutron absorber is filled between the main bodies 20, and between the main body 20 and the frame body 24. As the shielding material 28, for example, concrete containing boron, etc., can be used.