Patent Number: 
Section: claims

1. A method for preventing growth of unstable density waves in a nuclear power reactor comprising:installing a screen that is free to move between a top plate and a bottom plate in a fuel assembly lower tie plate box;forming and aligning flow holes, during manufacturing, in the top plate and in the bottom plate creating a path for coolant flow;forming disks at a bottom of the screen;aligning the disks with the flow holes in the bottom plate;shaping and forming, during manufacturing, the disks to cover and prevent flow when the disks cover the flow holes in the bottom plate; andthe screen dropping when coolant flow stagnates or reverses direction,causing the disks to rest on the bottom plate blocking coolant flow in the down direction. 2. The method of claim 1 further comprising:forming upstanding tabs at a top surface of the screen;lifting the screen by the hydraulic forces of a normal coolant flow in the upward direction acting on the disks; andstopping the lifting of the screen by the tabs contacting against the top plate so that no flow holes are blocked in the top plate allowing flow in the upward direction. 3. A device to prevent or substantially minimize reverse coolant flow in a nuclear reactor core comprising:a fuel assembly lower tie plate box, having a top plate and a bottom plate, is equipped with a screen that is free to move between the top plate and the bottom plate;flow holes in the top plate and the bottom plate are aligned to form a path for coolant flow;disks are attached to the screen at a screen bottomwith the disks aligned with the flow holes in the bottom plate;the screen is lifted by an upward flow hydraulic force against the disks; andthe screen dropping when coolant flow stagnates or reverses directioncausing the disks to rest on flow holes in the bottom plate blocking coolant flow in the down direction. 4. The apparatus of claim 3 further comprising:the screen has upstanding tabs extending upwardly from the screen at a screen top;lifting of the screen is stopped by the upstanding tabs contacting against the top plate such thatno flow holes are blocked in the top plate andallowing flow in the upward direction; andthe disks are sized to cover the flow holes in the bottom plate; andwhenever the flow stagnates or the flow direction reversesthe screen drops andthe disks cover the flow holes in the bottom plate and restrict downward coolant flow.