Prediction of Potential for the Ingression of Crude Oil Covering the Sea Water Surface Into the Sea Water Cooling System of Nuclear Power Plant

When an incident of crude oil spill from an oil carrier occurs in the sea near the nuclear power plants, the spilled oil can be transported to the intake pit, where the NSCW (nuclear service cooling water) pumps locate, by sea current and wind drift (induced) current. The NSCW pumps take the essential service water from the sea being used as the ultimate heat sink and supply to the component cooling water heat exchangers to remove the decay heat generated from reactor. The NSCW system shall provide sufficient cooling capacity during the reactor normal operation, transients, and loss-of-coolant accidents (LOCAs). In this regard, it is very important to confirm if the cooling function of the NSCW system can be threaten due to spilled oil which may overspread the surface of sea water in the intake area. Thus, in this work, when an incident of crude oil spill occurs in the sea near a nuclear power plant using sea water as the ultimate heat sink, possibility of crude oil ingression into the component cooling water heat exchangers through the NSCW pumps has been evaluated in a conservative manner. To do this, for a flow field surrounding a NSCW pump equipped in an intake pit where a limited volume of sea water is initially contained and its surface is covered with spilled crude oil, a numerical simulation is performed using a CFD (Computational Fluid Dynamics) code. The objective of the present simulation is to find out the critical sea water level at or below which oil or air floating on the surface of sea water begins to be sucked in the pump inlet nozzle. The numerical simulation results show that in general, the oil covering the free surface of sea water in the intake can hardly flow into the pump inlet until the floating oil changes into tars which are heavier than oil as long as the sea water level does not fall below the limit value specified in the Technical Specification of each nuclear plant. It is also shown that the critical sea water level increases as the pump flowrate increases. This is physically plausible considering the effect of gravitational force.