There are serious vibration and noise in the pipes of cooling circle in Reactor Cavity and Spent Fuel Pit Cooling and Treatment System (PTR) system during the commissioning test of a Nuclear Power Plant Unit 1, after the improvement of the cooling ability for PTR was actualized. It has been confirmed that the orifice cavitations is the cause of vibration and noise. Engineering modification plans have been proposed: add the series number of throttle orifices and decrease pressure drop of each throttle orifices. Proper engineering redundancy shall be considered according to the test phenomenon and data to keep the orifices from the zones of cavitations. The field investigations after the modification shows that the vibration and noise have been controlled in the acceptable limit while both system function and structure integrity meet the requirement of design code. Thus it can be said that the system modification is successful.

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