Description of the Safety Concern
To function properly, the emergency core cooling system (ECCS) pumps need an adequate margin between the available net positive suction head (NPSH) and the required NPSH. Inadequate NPSH margin for the ECCS pumps could result in cavitation and their subsequent failure to deliver the amount of water needed during a design basis accident. The available NPSH is a function of the static head of water above the pump suction, the pressure of the atmosphere above the sump water surface, and the temperature of the pumped water at the suction of the pump.
If a loss-of-coolant accident occurred, piping thermal insulation and other materials in the vicinity of the rupture will be dislodged. A fraction of this dislodged insulation and other materials, such as paint chips and concrete dust, will be transported to the containment floor by the steam/water flows from the rupture and the containment sprays. Some of this debris may be transported to and accumulate on the containment sump screens of the ECCS.
The debris that accumulates on the sump screen forms a bed that acts as a filter. This accumulated debris on the containment sump screen may increase the differential pressure across the sump screen and thus decrease the net positive suction head margin (i.e., head loss) available to ECCS pumps. Excessive head loss across the debris bed may exceed the NPSH margin of the ECCS or containment spray pumps. For sump screens that are only partially submerged by water on the containment floor, excessive head loss across the debris bed may prevent water from entering the sump.
The purpose of the debris screen around containment sumps is to minimize the amount of debris entering the ECCS suction lines. Debris can block restrictions (i.e., openings) or damage components in the systems served by the ECCS pumps. However, excessive head loss, caused by debris accumulation on containment sump screens, can prevent or impede the flow of water into the core or containment (via containment spray).
The NRC is currently implementing its plan to have all pressurized water reactor licensees perform an evaluation of the potential for excessive head loss across the containment sump screen because of the accumulation of debris on the containment sump screen.