Abstract: A filter medium for strainers used in the emergency sump of a nuclear power plant ECCS. The filter medium is employed with double wall, concentric tube type strainer modules. The filter medium is preferably constructed of a woven or knitted stainless steel wire material and is shaped in a cylinder so that it can be inserted between an external and an internal strainer of a double wall, concentric tube type strainer in such a manner that water that passes through the strainer must pass through and be filtered by the filter medium before the water can be pumped to the reactor core. The filter medium is inserted through the bottom of the strainer module and is held in place by an x-shaped retainer.