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DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.1 Page 3.1-1 of 3.1-1
3.1 INTRODUCTION
The fuel is slightly enriched uranium dioxide in the form of ceramic pellets contained in a hermetically sealed Zircaloy or M5 rod. The fuel rods are captured in an open Zircaloy and stainless steel cage and are held laterally by spacer grids made of Inconel and/or Zircaloy. Starting with Batch DD reload, the end cap material was changed back to Zircaloy. The fuel rod cladding and instrument tube remain M5. The fuel rod cladding and instrument tube remain M5.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.2 Page 3.2-1 of 3.2-2 3.2 DESIGN BASES 3.2.1 DELETED 3.2.2 DELETED 3.2.3 FUEL DESIGN LIMITS Fuel Bundles The fuel bundles are designed to maintain their structural integrity under steady-state and transient operating conditions, as well as for normal handling, shipping and refueling loads. The design takes into account differential thermal expansion of fuel rods, thermal bowing of fuel rods and guide bars, irradiation effects and wear of all components. Mechanical tolerances and clearances have been established on the basis of the functional requirements of the components. All components including welds are highly resistant to the corrosive action of the reactor environment.
The fuel rod design accounts for internal and external pressure, differential expansion of fuel and clad, neutron fluence-induced growth of materials, fuel swelling, fuel densification, clad creep, fission and other gas release, thermal stress, pressure and temperature cycling and flow-induced vibration. The fuel assembly will meet the following design criteria for the expected conditions and postulated accidents to the design assembly burnup:
- 1. The maximum steady-state cladding and assembly component stresses are within the ASME boiler and Pressure Vessel Code limits.
- 2. The maximum steady-state cladding strain is below the design limit.
- 3. The cladding and assembly component fatigue usage factors are below the design limit.
- 4. Fretting wear of the spacers and fuel rods is precluded.
- 5. Corrosion of the fuel rod and the fuel assembly structural components is below the design limit.
- 6. Fuel rod bowing will be limited so that it has no impact on thermal margins.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.2 Page 3.2-2 of 3.2-2
- 7. Axial growth of the fuel rods and fuel assembly is accommodated within the design clearances.
- 8. The fuel rod internal pressure remains below the criteria limit of reactor system pressure plus 800 psi throughout life (Reference 33, 1).
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-1 of 3.3-9 3.3 REACTOR DESIGN Following the implementation of Permanently Defueled Technical Specifications, the reactor design information herein is considered historical and does not constitute design requirements unless otherwise noted to still be applicable in the defueled condition.
3.3.1 GENERAL
SUMMARY
A general perspective view of the reactor is shown in Figure 3-1.
The fuel is low enriched UO2 (<5 w/o) encapsulated in zircaloy or M5 fuel rods.
In all fuel assemblies, the center fuel rod location is replaced by a captured Zircaloy or M5 instrumentation tube which provides an opening in the fuel lattice for the insertion of incore instrumentation. The cage assembly is made up of Zircaloy guide bars, Inconel and/or Zircaloy spacer grids and stainless steel upper and lower tie plates. This structure axially captures and laterally positions and supports the fuel rods and other assembly components. The outer surface of the guide bars also provides an envelope surrounding the control rod channels in the core.
The 45 control rods are made of rectangular stainless steel tubes containing a silver-indium-cadmium alloy that is hermetically sealed within the tube. The tubes are electron beam-welded into a cruciform structure with stainless steel end fittings. Four of the control blades, known as part length control blades, have neutron absorber (silver-indium-cadmium) only in their lower section.
3.3.2 DELETED 3.3.3 DELETED 3.3.4 MECHANICAL DESIGN The reactor core and internals are shown in perspective in Figure 3-1. A cross section of the reactor core and internals is shown in Figure 3-9. A vertical section of the core and internals is shown in Figure 3-10. Mechanical design features of the reactor internals, the control rod drive mechanisms and the reactor core are described below.
3.3.4.1 Reactor Internals All reactor components were originally considered Class 1 for seismic design.
The structural components satisfied stress values given in Section III of the ASME Boiler and Pressure Vessel Code. As the facility is permanently defueled, these design requirements are no longer applicable.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-2 of 3.3-9 The components of the reactor internals are divided into three major parts consisting of the core support barrel (including the lower core support structure and the core shroud), the upper guide structure (including the control rod shrouds and the incore instrumentation guide tubes) and the flow skirt. These components are shown in Figure 3-10.
Core Support Assembly The major support member of the reactor internals is the core support assembly. This assembled structure consists of the core support barrel, the core support plate and support columns, the core shrouds, the core support barrel to pressure vessel snubbers and the core support barrel to upper guide structure guide pins. The major material for the assembly is Type 304 stainless steel.
The core support assembly is supported at its upper flange from a ledge in the reactor vessel flange. The lower end is restrained in its lateral movement by six core support barrel to pressure vessel snubbers. Within the core support barrel are axial shroud plates and former plates which are attached to the core support barrel wall and the core support plate and form the enclosure periphery of the assembled core. The core support plate is positioned within the barrel at the lower end and is supported both by a ledge in the core support barrel and by 52 columns. Also within the core support barrel just below the nozzles are four guide pins which align and prevent excessive motion of the lower end of the guide structure relative to the core support barrel during operation.
Core Support Barrel The core support barrel carries the entire weight of the core and other internals (about 485,000 pounds). It is a right circular cylinder with a nominal inside diameter of 149-3/4 inches and a minimum wall thickness in the weld prep area of 1 inch. It is suspended by a four-inch-thick flange from a ledge on the pressure vessel. The core support barrel in turn supports the core support plate upon which the fuel bundles rest. Press fitted into the flange of the core support barrel are four alignment keys, three measuring 3.25-inch x 4-inch x 12-inch and one 3.25-inch x 5-inch x 12-inch. The keys are located 90 degrees apart. The reactor vessel, closure head and upper guide structure assembly flanges are slotted in locations corresponding to the alignment key locations to provide proper alignment between these components in the vessel flange region.
At assembly, as the internals are lowered into the vessel, the pressure vessel tongues engage the core support grooves in an axial direction. With this design, the internals may be viewed as a beam with supports at the farthest extremities. The pressure vessel tongues have bolted, lock-welded Inconel shims, and the core support barrel grooves are hard faced with stellite to minimize wear.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-3 of 3.3-9 Core Support Plate and Support Columns The core support plate, 1-1/2 inches thick, is a perforated member with flow distribution and pin locating holes for each fuel bundle. The plate is supported by a ledge and by columns. The ledge on the CSB supports the periphery of the plate, and the plate is pinned, bolted and lock welded to the ledge for maintaining accurate location of the plate. A series of columns are placed between the plate and the beams across the bottom of the core support barrel. The columns provide stiffness and transmit the core load to the bottom of the core support barrel.
Core Shroud Plates and Centering Plates The core shroud follows the perimeter of the core and limits the amounts of coolant bypass flow. The shroud consists of rectangular plates 5/8 inch thick, 145 inches long and of varying widths. The bottom edges of these plates are fastened to the core support plate by use of anchor blocks.
The critical gap between the outside of the peripheral fuel bundles and the shroud plates is maintained by seven tiers of centering plates attached to the shroud plates and centered during initial assembly by adjusting bushings located in the core support barrel. The overall core shroud assembly, including the rectangular plates, the centering plates, and the anchor blocks, is a bolted and lock-welded assembly. In locations where mechanical connections are used, bolts and pins are designed with respect to shear, binding and bearing stresses. All bolts and pins are lock welded.
Flow Skirt The Inconel flow skirt is a perforated (2-1/2 inch diameter holes) right circular cylinder, reinforced at the top and bottom with stiffening rings. The skirt is hung by welded attachments from the core stop lugs near the bottom of the pressure vessel and is not attached to the core support barrel.
Upper Guide Structure Assembly This assembly (Figure 3-11) consists of a flanged grid structure, 45 control rod shrouds, a fuel bundle alignment plate and a ring shim.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-4 of 3.3-9 The upper end of the assembly is a flanged grid structure consisting of a grid array of 18-inch-deep long beams in one direction with 9-inch-deep short beams at 90 degrees to the deeper beams. The grid is encircled by an 18-inch-deep cylinder with a 3-inch-deep flange welded to the cylinder. The periphery of the flange contains four accurately machined and located alignment keyways, equally spaced at 90-degree intervals which engage the core barrel alignment keys. The reactor vessel closure head flange is slotted to engage the upper ends of the alignment keys in the core barrel.
The control rod shrouds are of cruciform configuration and extend from about 1 inch above the fuel bundles to about 2 inches above the top of the pressure vessel flange. The shrouds consist of 4 formed plates, 0.187 inch thick by approximately 138 inches long, which are welded to 4 end bars to form a cruciform-shaped structure. The shrouds are fitted with support pads at the upper end machined for a bolted and lock-welded attachment to the flanged grid structure. The lower ends of the shrouds are also fitted with support pads machined for a bolted and lock-welded attachment to the fuel bundle alignment plate. The cruciform design provides a stiff section, resulting in low stresses and deflections. In the area of maximum cross flow, the shroud is supported between the flanged grid structure and the fuel bundle alignment plate as a beam with fixed ends.
Precision machined and located pins attached to the fuel bundle alignment plate align the fuel bundles. The fuel bundle alignment plate also has four equally spaced slots on its outer edge which engage with stellite hard-faced pins protruding out from the core support barrel to prevent lateral motion of the upper guide structure assembly during operation.
The hold-down ring (see Figure 3-12) contains 308 holes, all but one of which contain plungers supported by 22 Belleville washers (each) which are contained within a 304 SS frame. The frame, or ring segments, are bolted to the upper guide structure to provide uniform rigidity within the segments. The design loading of the hold-down device will produce a compression resulting in net hold-down force of nominally 700,000 pounds. In addition, a .290-inch shim is located between the upper guide structure and core support barrel flanges to accommodate fuel growth.
The upper guide structure assembly also supports the incore instrument guide tubes. The tubes are conduits which protect the incore instruments and guide them during removal and insertion operations while refueling.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-5 of 3.3-9 3.3.4.2 Control Rod Drive Mechanism There are 45 CRDMs mounted on flanged nozzles on top of the reactor vessel closure head, located directly over the control rods in the reactor core.
Each CRDM is connected to a control rod by a locked coupling. The weight of the CRDMs is carried by the vessel head. In order to provide lateral stability, particularly in resisting horizontal earthquake forces, the CRDMs are supported in the horizontal direction by interconnection.
The CRDM is designed to handle a control rod weighing 215 pounds (dry).
The total stroke of the drive is 132 inches.
The CRDM is of the vertical rack-and-pinion type with the drive shaft running parallel to the rack and driving the pinion gear through a set of bevel gears.
The design of the drive is shown in Figure 3-13. The rack is driven by an electric motor operating through a gear reducer and a magnetic clutch.
Pressure Housing The pressure housing consists of a lower and an upper section joined near the top of the drive by means of a threaded autoclave-type closure. The pressure housing design and fabrication conformed to the requirements of the ASME Pressure Vessel Code,Section III, for Class A vessels (Class 1 vessel for replacement housing). As the facility is permanently defueled, these design requirements are no longer applicable.
The lower housing section has an integral bottom head, which consists of the eccentric reducer and the lower flange which is machined from a single piece bar stock. This flange fits the nozzle flange provided on the reactor vessel closure head and is seal welded to it by an omega-type seal. The upper part of the lower housing is machined to form the closure and is provided with a recessed gasket surface for a spirally wound gasket.
The upper part of the pressure housing has a flange which mates with the lower housing closure, a cavity which contains the drive rotating seal, and a tubular housing extension with a small flange closure which provides access for attaching and detaching the control rod.
The rotating, axially movable member has a carbon-graphite seating surface which in the original design mated to a stationary member made of a carbide alloy. The carbide alloy was replaced with chromium oxide applied directly to the stainless steel body with no bond coat. The carbide alloy was found to present problems because a nickel binder was preferentially leaching out onto the seating surface.
The two parts of the seal are fitted with O-rings to prevent leakage around the seal. The O-rings are static seals.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-6 of 3.3-9 Rack-and-Pinion Assembly The rack-and-pinion assembly is an integrated unit which fits into the lower pressure housing and couples to the motor drive package through the upper pressure housing. This unit carries the bevel gears which transmit torque from the vertical drive shaft to the pinion gear. The vertical drive shaft has splined couplings at both ends and may be lifted out when the upper pressure housing is removed. Ball bearings are provided for supporting the bevel gears and the pinion gear. The rack engages the pinion, and is held in proper engagement with the pinion by the backup rollers which carry the load due to gear tooth reactions. The gear assembly is attached to a stainless steel tube supported by the upper part of the pressure housing. This tube also carries and positions the guide tube which surrounds the rack. The rack is a tube with gear teeth on one side of its outer surface and flats on the opposite side which form a contact surface for guide rollers. Flats are cut on two opposite sides of the rack tube for forming the rack teeth and for a contact surface for the backup rollers.
Motor Drive Package Power to operate the drive is supplied by a synchronous, fractional horse-power, 120-volt, single-phase, 60-hertz motor. The output is coupled to the vertical drive shaft through a magnetic clutch and an antireverse clutch operating in parallel. The motor shaft is fitted with an electrically operated brake which is connected to release the brake when the motor is energized.
The motor, brake, clutches, position indicator and limit switches are all mounted on a common frame for maintaining position and alignment. The frame for the drive package is provided with a flange which is bolted to a flange on the pressure housing for positioning the drive assembly. The electrical connections are located at the top of the drive package and are readily accessible.
Position Readout Equipment Two independent position readout systems are provided for indicating the position of the control rod. One (primary system) is a synchrotransmitter geared to the main drive shaft with readout provided by synchroreceivers connected to the transmitter. The other (secondary system) position indicator consists of a series of accurately located reed switches built into a subassembly which is fastened to the outside of the CRDM along the pressure housing.
CRDM Evaluation The pressure containing members of the CRDM were designed and fabricated in accordance with the ASME Pressure Vessel Code,Section III, Class A (Class 1 for the replacement components). As the facility is permanently defueled, these design requirements are no longer applicable.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-7 of 3.3-9 3.3.4.3 Core Mechanical Design Fuel Bundle Figures 3-14 and 3-15 show a typical fuel bundle which consists of a square (15 by 15) array of 225 positions: 216 fuel rods, 8 Zircaloy-4 guide bars, and 1 Zircaloy-4 or M5 instrument tube. For a gadolinia assembly, typically 3 to 16 of the fuel rods would contain gadolinia mixed with the fuel. Table 3-2 provides further fuel bundle component descriptions.
The guide bars are solid Zircaloy-4 rods with threaded ends. They are located on the perimeter of the fuel bundle and serve three main functions.
First, they serve a structural function. The zircaloy spacer grids are welded to the guide bars at equally spaced intervals and the end fittings are joined to the threaded end of the guide bars with cap screws. Second, they provide a guiding surface for the control rods. The guide bars protrude beyond both the fuel rods and perimeter strip of the spacer grids so that a control rod contacts only the guide bars. Third, they provide guiding surfaces which facilitate refueling and protect fuel rods from damage.
The tie plates and guide bars are connected with Inconel cap screws. The cap screws are torqued during cage assembly. This results in an initial tensile stress that depends upon the initial torque value and coefficient of friction.
The grid spacers (see Figure 3-17) are fabricated in two different designs from Zircaloy-4 strips joined in an "eggcrate" fashion and welded together.
The fuel rods are supported at ten axial locations using arched flow channels.
One demonstration assembly of Batch R design incorporates all-zircaloy HTP spacers in the top and bottom locations. All other Batch R and all S assemblies use inconel high thermal performance spacers in the bottom location and bimetallic spacers in the top location. The remainder of the assemblies in the core (Batch Q and earlier) have bimetallic spacers in the top and bottom locations. Batch T and beyond use zircaloy spacers at all axial locations.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-8 of 3.3-9 Fuel Rod Figures 3-18 and 3-19 show typical fuel rods which consist of a stack of UO2 pellets approximately 132 inches in length with a compression spring at the top end all clad within Zircaloy-4 or M5 tubing and sealed by welding end caps to each end (refer to Table 3-2 for dimensional characteristics). A plenum is provided at the top of the fuel column. The compression spring is located within the plenum to maintain a compact fuel column. For batches N through Q assemblies, the fuel rods have been modified to accommodate debris resistant design features that included a longer solid lower end cap.
Beginning with Batch R assemblies, the active fuel length was increased by 0.8 inch to 132.6 inches and do not use the longer solid lower end cap because of the FUELGUARDTM grid.
Beginning with Batch T, all spacers are the zircaloy HTP design. The theoretical density for Batch T fuel is 95.85%. The theoretical density for Batch Y and beyond is 96%.
For Batch W and beyond, each non-shield assembly incorporates 21 rods which contain a long upper end cap (LUEC). This long upper end cap replaces the top 10.6 inches of active fuel. This reduces the active fuel height to 122.0 inches in these rods. Batch W, AA, and CC shield assemblies incorporate 18 such rods.
Control Rod Design The control rod shown in Figure 3-20 consists of 32 stainless steel clad poison modules and a hanger section. The modules and hanger section are electron beam welded together to form a cruciform blade with a 12.250-inch span in the absorber segment and a total length of 151 inches, including the hanger section.
Each module contains a 131-inch length of absorber material of 80 wt%
silver, 15 wt% indium, 5 wt% cadmium and is clad with 0.020-inch-thick 304 stainless steel. The module cross section is 0.750 inch wide by 0.180 inch thick. End caps are welded to the ends of each module and inspected to ensure integrity.
The hanger section provides a means for handling the blade and for coupling the blade to the CRDM extension shaft. A hanger section is a welded assembly fabricated from a 0.180-inch-thick 304 stainless steel lower section and a 0.312-inch-thick 348 stainless steel upper section.
Four of the 45 control rods contain Ag-In-Cd modules reduced in length to 31 inches.
DSAR CHAPTER 3 - REACTOR Revision 36 SECTION 3.3 Page 3.3-9 of 3.3-9 Source Design Prior to Cycle 12, up to four neutron source assemblies were installed in the reactor to serve as sustainer sources for future start-up service. During the 1993 refueling outage, two of the four sources were transferred to the spent fuel pool. The final two were moved to the spent fuel pool during the 1995 refueling outage. The sustainer source material is antimony-beryllium.
The source pins are stored in the instrument guide tubes of the selected assemblies in the spent fuel pool.
The neutron source rods employ Type 304 stainless steel cladding material with a 0.34-inch OD and a 0.024-inch wall thickness. The sustainer sources contain 72 inches of Sb-Be pellets.