November-December 2017 NPJ

Product & Service Directory–2018 www.NuclearPlantJournal.com 89 to 2016, the North Anna Unit 2 reactor internals were of the downflow design. North Anna Unit 1 was converted to an upflow design in 1996 (reference DCP 95-001). In the downflow design, the coolant exits the reactor vessel inlet nozzle and flows downward in the annulus between the vessel and the core barrel. The vessel surrounds the thermal shield. The downward flow passes over the thermal neutron shield, turns and flows up through the core region. A portion of the downflow stream is diverted to cool the baffle-barrel region components. This diverted flow passes through holes in the core barrel at an elevation between the top two former plates and flows downward through holes in the lower former plates. The top former plate has no holes. At the lower core plate this flow stream combines with the main coolant stream, and flows up through the fuel assemblies in the core region. The downflowdesignresultedinalargepressure differential across the baffle plates at the upper elevation of the core region. Over time as gaps formbetween the boltedbaffle plates, jetting of water can occur through these openings. Jetting of water between the vertical mating surfaces of the baffle plates has caused fuel cladding damage in many Westinghouse designed plants. Baffle jetting is the excessive flow of a fluid jet from the high pressure (baffle- barrel region) to lower pressure (core region) through gaps in baffle plates. Gaps inbaffleplates are influencedby thenormal pressurizationandheat-up that occurs from cold plant to operating conditions. For fuel rod damage to occur as a result of this baffle jetting three elements must occur. (1) Fuel rods susceptible to damage from baffle jetting, (2) Sufficiently large gaps in baffle plates and (3) Relatively large pressure difference across the baffle plates. A number of experimental tests have been performed to study the interaction between baffle joint jetting and the response of fuel rods. Westinghouse has determined that to guard against fuel rod failures from baffle-jetting requires that the cross-flow emanating from baffle joint gaps must be limited to a specific momentumflux. The limitingmomentum fluxdepends upon the specific fuel product and varies along the length of a fuel assembly.Adetailedanalysis is performed to determine the momentum flux present at various gap locations and to compare the momentum flux with the allowable values for a given fuel product. To implement an upflow conversion, coolant flow in the baffle/barrel region is reversed fromdownflow to upflow. This is accomplishedbymachiningholes into the top former plate and plugging the existing holes in the core barrel. The objective of this conversion is to reduce the hydraulic pressure differentials that exist across the baffle joints in the downflow internals configuration. Since the flow direction through the baffle-barrel region of the reactor core is opposite to the direction of the flow through the core, there is an axial pressure gradient across the baffle plates, whichvaries fromamaximumat the topof the core to aminimumat the bottomof the core.Reducing thesepressuredifferentials from the upflow conversion, results in a substantial reduction in the strength of the coolant jetting through the baffle joints. Due to the depthof the refueling cavity at North Anna 2, the Upflow Conversion process required the lower internals to be removed from the reactor vessel and placed onaWestinghouse suppliedLower Internals Storage Structure (LISS). Using the LISS allowed for working on the internals fully submerged, while still having access to the periphery of the core barrel. The Lower Internals Storage Structure (LISS) is a device that allows theLower Internals tobe supportedwithin the reactor vessel in order to gain access for the purpose of performing operations whilekeeping the lower internals shielded underwater due to radiation exposure concerns. Once the plant reached cold shutdown, two special threaded adapters were installed for each LISS instead of RVstudhole plugs at the cardinal location in the vessel flange. The reactor head and upper internals were removed and the entire core was unloaded. The LISS was installed on the reactor vessel flange and the lower internals assembly was raised above the reactor vessel. The LISS was rotated into position and the lower internals were placed on the LISS. This ensured that the top of the lower internals assembly remained underwater and no additionalwater jackets or other shielding was required during upflow conversion North Anna Reactor Internals Configuration – Downflow (Unit 2) & Upflow (Unit 1). (Continued on page 90)