A strainer wall structure that removes foreign substances from a fluid suctioned into a pipe and a re-circulation pump that is part of an emergency core cooling system (ECCS). The strainer wall structure has an inlet side and an outlet side through which cooling water is introduced and discharged, respectively, and includes a body having an opening in a direction of the inlet side, closed side surfaces, and an outlet port disposed at one of the closed side surfaces. The strainer includes a punched plate filter screen inserted into the opening. A modular cassette apparatus including grooved first filter plates is inserted into the body, and second filter plates having second grooves is inserted into the first grooves.