In September 2004, the Nuclear Regulatory Commission (NRC) issued Generic Letter GL2004-02 “Potential Impact of Debris Blockage on Emergency Recirculation during Design Basis Accidents at Pressurized-Water Reactors” to address Generic Safety Issue 191 (GSI-191) “Assessment of debris accumulation on PWR sump performance.” [1] GL2004-02 requested pressurized water reactor (PWR) licensees to perform a “downstream effects” evaluation of their emergency core cooling (ECCS) and containment spray systems (CSS). GL2004-02 also gave guidance on what analysis had to be completed in order to resolve GSI-191. These evaluations included a wear and plugging assessment of all ECCS and CSS components, including valves. During preliminary “downstream effects” analysis of a plant, it was determined that the positions of ECCS throttle valves could be such that the flow clearances through the valves would be too small to meet the criteria developed for component plugging or wear assessment. This suggested that a modification to the system needs to be made which allows the throttle valves to be more fully opened. In order to allow the throttle valves to be opened more fully, additional hydraulic resistance (i.e. pressure drop at the design flow rate) was added at another location. Several orifice designs were considered to provide the needed resistance. Since the required additional pressure drop was a substantial fraction of the total pressure drop, special design features of the orifice were necessary to preclude system instabilities due to cavitation, degassing and flow swirl. The purpose of this paper is to present a method for assessing the effectiveness of a multi-stage orifice that can be placed in the system to provide the required resistance, thus permitting the throttle valves to be used more efficiently. The paper presents the design aspects of the multi-stage breakdown orifice, CFD modeling used to select the design, and the system condition testing results.

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