FOD in fuel tanks is inevitable. There will always be some in the tanks. But there is a limit to how much the strainers and filters can handle. BTW, the tank access hatches are very small. Typically the tank rats, as we called them, who get inside and work on the tanks are little guys less than 5’6” and 120 lbs.
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Don't they do inspections and final cleaning before buttoning up the tanks during construction? When I was starting up power plants, we always did thorough inspections to make sure no left-over junk (tools, rags, ladders, dead bodies) were left in the systems before commissioning them. Then, upon the first firing up of the boilers, we'd generate high pressure steam and blow out all the lines.