A method and device is claimed for preventing reverse coolant flow in a BWR Power Reactor. The device comprises a screen that is free to move between a top plate and a bottom plate in a fuel assembly lower tie plate box. Flow holes are formed and aligned in the top plate and in the bottom plate, creating a path for reactor coolant. Disks at a bottom of the screen are aligned with the flow holes in the bottom plate, and are shaped and formed to cover flow holes in the bottom plate. When flow stagnates or reverses, the screen drops causing the disks to rest on the bottom plate blocking downward flow. Upstanding tabs at the top surface of the screen contact the top plate when normal coolant flow is in the upward direction, allowing flow through the flow holes in the top plate.