A shutdown cooling (SDC) system was observed to have pressure spikes that lifted relief valves upon alignment with the Reactor Coolant System (RCS) hot leg. Investigations showed that gas could accumulate at several local high points of the system. An evaluation was conducted with the primary objective of determining if the accumulated gas could lift the relief valve and the secondary objective of increased understanding of how the gas affected the system. Modeling was performed to evaluate the effect of various gas pocket sizes on the over pressurization. Pressure data from operation of the LPSI pumps was used to test the model. This investigation determined that pressure magnitudes produced during the alignment of the SDC system would be limited by the acceleration of the fluid mass for small bubbles and by compressibility of the gas pocket for large bubbles. The study results showed that pressure was maximized for small to medium sized gas bubbles at locations outside of containment.

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