November-December 2017 NPJ

90 www.NuclearPlantJournal.com Product & Service Directory–2018 operations in the reactor vessel. During the core barrel lift operation, however, the lower internals did break the water level causingradiation levels torisesignificantly (300-700 mRem/hr based on historical data).The same conditionoccurredduring the core barrel re-insertion to the reactor vessel.Verificationof theLISS installation was performed byWestinghouse utilizing cameras tominimize exposure during this high dose evolution. Core Barrel Plug The core barrel flow hole plug is a hydraulically actuated expanding device, whichseals theflowchannel fromthevessel- barreldowncomertothebaffle-barrelregion. The plug utilized for the NorthAnna Unit 2 Nuclear Stationupflowconversion, consists of aplugbodywitha tapered insidediameter, tapered mandrel for expansion, retaining rings, sealing ribs, and indicator pin. The plug is designed and fabricated to accommodate the complete range of expected core barrel hole diameters. The 20corebarrel flowholes areequallyspaced around thecircumferenceof thecorebarrel, at 18-degree intervals starting at 0degrees. Top Former Plate Machining The former plate machining was accomplishedusingtheElectricalDischarge Machining (EDM) process. The size and position of an individual hole relative to the baffle plates and structural bolts is generically applied to all 20 holes. This enables standardization and simplification of the machining equipment since the fixtures will position all of the holes at a common orientation to the baffle plates. Thehydraulic resistanceof thebaffle- barrel region is one of the key hydraulic parametersindeterminingtheflowthrough this region and between adjacent baffle plates for a given fuel design. For the upflowmodification, themajor contributor to this resistance which was changed is the resistance of the top former plate. In downflowplants, this resistance is infinite since there isnoflowthrough thisplate. For converted upflow plants, the number, size andgeometryof theseholesdetermines the hydraulic resistanceof the topformerplate. Implementation Preparation for implementation for this modification began just after completionof the2014 refuelingoutage.A project team consisting of representatives fromeachDominiondepartment involved andWestinghousewas formed. Operating experience from the first implementation of the modification at North Anna Unit 1 in 1996 and similar evolutions at other Westinghouse plants was reviewed and incorporated into the design and implementationplanning. Routineproject meetings were held alternating between Westinghouse locationsandatNorthAnna. Several of the visits to the Westinghouse facilities allowed mockup training to be performed and observed to incorporate changes prior to site implementation. The project finished 65 hours ahead of the baseline schedule. Continuous coverage of personnel in Containment allowed continual motion (i.e. 24/7 coverage) throughout the production portion of the project. Contingency plans prepared prior to the outage were used to reduce delays during execution. Review of the Westinghouse procedures, design change package, and Mechanical Maintenanceproceduresbefore theoutage was instrumental in identifying potential issues before the project started. Final radiation exposure was 2.451 person-REMcompared to 21.727 person- REM for Unit 1 in 1996. This represents a dose expenditure of 11.28%. This reduction in exposure was the result of several key factors to reduce the source term for the workers. First, a shield ring was constructed and installed on top of the core barrel yielding a 40% reduction in dose rates for workers near the Reactor Cavity. Second, the auxiliary work bridge was raised an additional 28 inches with leadshielding installed to reduceexposure. It is estimated that 15-20% reduction was experiencedby this activity.Westinghouse utilized experienced workers who were familiar with the project activities. Approximately85%oftheimplementation team had performed an upflow project in 2015. Finally, reactor cavity water level was maintained at an overfill condition to provide as much shielding as possible. This project had an additional two inches ofwater shielding fromthe1996evolution. All together, theprojectwas completed for 25.1% of the initial 2016 dose projection. Implementation of the Upflow Conversion is complicated by the requirement that activities must be accomplished remotely, underwater, and in the annulus between the core barrel and the thermal shield. Plugging must be accomplished in an area of limited access and all work generated debris must be controlled, and collected for disposal as radwaste. Despite these challenges, all work was completed in a quality manner. Holes machined in the upper former plate were documented via video recordings and successful post modification testing inspections were performed. Holes which had plugs installedwere also documented via video with all locations meeting the acceptance criteria without rework. Contact: Stewart Morris, Dominion Energy, telephone: (540) 894-2835, email: stewart.morris@dom.com . Radiation Shield Ring Installed on Core Barrel Upflow Modification... ( Continued from page 89