AP1000 Verification of Water Sources for Long-Term Recirculation Cooling Following a LOCA

The AP1000 Nuclear Power Plant uses natural recirculation of coolant to cool the core following a postulated Loss Of Coolant Accident (LOCA). Recirculation screens are provided in strategic areas of the plant to remove debris that might migrate with the water in containment and adversely affect core cooling. An evaluation of the potential for debris to plug the AP1000 recirculation screens was performed consistent with the guidance identified in Regulatory Guide 1.82 Revision 3, the Pressurized Water Reactor (PWR) Industry Guidance of NEI 04-07, and the Nuclear Regulatory Commission’s Safety Evaluation on NEI 04-07. The evaluation considered various contributors to screen plugging including debris that could be produced by a LOCA, resident containment debris and post-accident chemical products that might be generated in the coolant pool that forms on the containment floor post-accident. The evaluation accounted for the AP1000 containment design, equipment locations, recirculation screen redesign and containment cleanliness program. A unique aspect of this evaluation is the use of available industry data and its application to the AP1000 design to support the evaluation. Specifically, available industry data from existing plant walkdowns was used to both characterize the fibrous and particulate debris that could comprise resident debris in containment prior to the LOCA, and estimate the amount of that debris. The evaluation also combined generic PWR Industry containment sump chemical effects information with AP1000 specific post-accident coolant chemistry and sump transient conditions to evaluate potential chemical effects. The collection of resident containment debris on the recirculation screens in the post-LOCA recirculation mode may result in a head loss across containment recirculation screens that could restrict the recirculation flow required to cool the core effectively. Using specific design features of the AP1000, the debris loading evaluated from latent containment debris and post-accident chemical effects and conservatively applying an available screen head loss correlation, the head loss across the AP1000 containment recirculation screens due to the collection of latent debris on the screen under natural circulation conditions was calculated. Calculations were also made for pressure drop across the AP1000 In-Containment Refueling Water Storage Tank (IRWST) screens resulting from latent containment debris accumulation on those screens under accident conditions. Finally, calculations were made for pressure drops at the AP1000 core resulting from debris accumulation on the bottom of the fuel. This paper will present the methods used to apply the available PWR Industry data to the AP1000 design, along with the results of the calculations performed which demonstrate the acceptability of the design of the AP1000 recirculation screens.