November-December 2017 NPJ

88 www.NuclearPlantJournal.com Product & Service Directory–2018 Upflow Modification By Stewart Morris, Dominion Energy. Stewart Morris Stewart Morris is the Design Engineering Manager at the Dominion Energy North Anna Power Station in Mineral, Virginia. He has over 27 years of experience within the nuclear industry including holding Shift Technical Advisor and Senior Reactor Operator certifications. Stewart holds a B.S. degree in Nuclear Engineering from the University of Virginia. He was the Management Sponsor for the Upflow Modification project. Nuclear Energy Institute’s Top Innovative Practice Process Awards highlight the nuclear industry’s most innovative techniques and ideas. This innovation won the 2017 Westinghouse Vendor Award. The team members who participated included Stewart Morris, Project Sponsor, Dominion Energy; Janelle Madison, Project Manager, Dominion Energy; Chris Allmond, Project Engineer, Dominion Energy; Matthew Paden, Integrated PM, Westinghouse; Nick Rubis, Installation PM, Westinghouse. Summary On September 15, 2014 during core offload for a scheduled Unit 2 refueling outage, the top springs of two fuel rods were found dislodged. Indications of fuel failure had been observed during the previous fuel cycle. Based on the location and characteristics of the fuel failure, it was determined to be caused by “baffle jetting”. Baffle jetting is the process by which water on the outside of the core baffle plate is forced through small open- ings in thebaffle seams and onto the fuel as- semblies. During the previous fuel cycle, baffle jetting caused two rods in the fuel assembly located in core position B11 to begin vibrating. This movement resulted in fuel rod wear and eventual mechanical failure and fuel rod separation. Once sep- arated, a maximumof 15 fuel pellets were released from the two affected rods. To correct future baffle jet impingement, a modification was developed by Westinghouse and NorthAnna personnel. This modification changed the coolant flowpath in thebaffle- barrel region and reduced the differential pressure across the baffle joint which diminished the potential for baffle jetting. Themodificationwas implementedduring the Spring 2016, Unit 2 refueling outage. The project was completed: without any industrial safety events, without any hu- manperformanceclock resets, aheadof the forecasted schedule, on budget, without any quality issues, and with significantly less radiation exposure than planned. An additional benefit of this modification is the reduction of the differential pressure on the baffle plates thus reducing the stress on the baffle bolting. These differential fluid pressures between the core and baffle-barrel region are the largest external forces on the baffle plates, and these pressures differ from the core pressures because of the large inertial and hydraulic resistance associated with former flow holes versus the relatively open flow area of the core. Fluid pressures within the baffle-former-barrel region are influenced by the baffles’ motion; this baffle motion is dependent on the pattern of intact baffle bolts, bolt preload, and whether the baffle-former-barrel region flowis either upflowor downflow. Because of this direct correlation between baffle- former-barrel region flow and intact baffle bolts, the conversion of the downflow design to an upflow design significantly reduces the high differential pressure on the baffle plate; thus minimizing the additional stress on the preloaded baffle bolts; therefore, theupflowconversionmay reduce the potential of baffle bolt failures. Design In addition to supporting the core, a function of the reactor vessel internals assembly is to direct coolant flows within the vessel. While directing the primary flow through the core, the internals assembly also establishes secondary coolant flow paths for cooling the upper regions of the reactor vessel and for cooling the internals structural components. Some of the parameters influencing the mechanical design of the lower internals assembly are the pressure and temperature differentials across its component parts and theflowrate required to remove the heat generated within the structural components due to radiation (e.g., gamma heating). The configuration of the internals provides for flow into the baffle-barrel region to permit adequate cooling capability. The configuration also maintains the thermal gradientswithinand between thevarious structural components (resulting from gamma heating and core coolant temperature changes) within acceptable limits. The fuel assemblies, square in cross-section, are arranged in a circular patternwithin the core barrel.The former and baffle plate system provides the transition from the round barrel to the squared periphery of the core. The former and baffle system is a network of plates that horizontally contain the fuel assemblies at various vertical positions in the core. The baffles bolt to the formers, which have a square inside perimeter but a circular outside perimeter. The outside perimeter of each former plate is bolted to the barrel wall. Westinghouse classifies the reactor internals design by the direction of coolant flow in the region between the core barrel and core baffle plates. This flow is either denoted as upflow or downflow. Prior 88