ML19011A209
| ML19011A209 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 12/06/2018 |
| From: | Southern California Edison Co |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19011A401 | List: |
| References | |
| Download: ML19011A209 (8) | |
Text
REACTOR
- 4. REACTOR 4.1
SUMMARY
DESCRIPTION 4.1.1 REACTOR SYSTEM The reactor system, listed below, has been removed from service. It no longer has a Design Basis, Licensing Basis, or operational function. Although this system has been removed from service, it may still contain fluids, gases or other hazards such as energized circuits, compressed air, radioactive material, etc. Equipment may not have been physically removed from the plant.
See General Arrangement Drawings, P&IDs, and One Line diagrams for current plant configuration.
STRUCTURES/SYSTEMS/COMPONENTS STATUS Reactor Removed from Service Control Element Drive Mechanism (CEDM) Removed from Service Vibration and Loose Parts Monitoring (VLPM) Removed from Service 4.1.2 FUEL AND ASSOCIATED COMPONENTS The fuel (fuel rods, poison rods, assemblies) and Control Element Assemblies (CEAs), are partially removed from service. SONGS has permanently ceased operation and removed all nuclear fuel and CEAs from both units reactor vessels. The irradiated fuel assemblies and CEAs are being stored in the spent fuel pool (SFP) and in the Independent Spent Fuel Storage Installation (ISFSI) until they are is shipped offsite. As a result, fabrication and in-core operational related information about the nuclear fuel and CEAs has been deleted. A general description of the fuel and CEAs remains in the DSAR.
4.2 FUEL DESIGN 4.
2.1 DESCRIPTION
AND DESIGN This subsection summarizes the mechanical design characteristics of the fuel and associated components. Typical mechanical design parameters are presented in Table 4.2-1 and a typical fuel assembly is presented in Figure 4.2-1.
4.2.1.1 Fuel Assemblies A fuel assembly consists of 236 UO2 fuel, Urania-erbia fuel and poison rods, five control element guide tubes, 11 fuel rod spacer grids, upper and lower end fittings, and a holddown device. The outer guide tubes, spacer grids, and end fittings form the structural frame of the assembly.
November 2018 4-1 Rev 4
REACTOR The fuel spacer grids maintain the fuel rod array by providing positive lateral restraint to the fuel rod but only frictional restraint to axial fuel rod motion. The grids are fabricated from preformed Zircaloy or Inconel strips (the bottom spacer grid material is Inconel) interlocked in an egg crate fashion and welded together. Each cell of the spacer grid contains leaf springs and arches. The leaf springs press the rod against the arches to restrict relative motion between the grids and the fuel rods. The perimeter strips contain features designed to prevent hangup of grids during fuel movement.
The ten Zircaloy-4 spacer grids are fastened to the Zircaloy-4 guide tubes by welding, and each grid is welded to each guide tube at eight locations, four on the upper face of the grid and four on the lower face of the grid, where the spacer strips contact the guide tube surface. The lowest spacer grid (Inconel) is not welded to the guide tubes due to material differences.
Of the ten Zircaloy-4 grids, six are designated the HID-1 or HID-1L design and four are designated the HID-2 or HID-2L design. The higher-strength HID-2 or HID-2L grids are located along the mid-length of the fuel assembly.
The upper end fitting is an assembly consisting of two cast 304 stainless steel plates, five machined posts and five helical Inconel X-750 springs, which attaches to the guide tubes to serve as an alignment and locating device for each fuel assembly and has features to permit lifting of the fuel assembly. The lower cast plate locates the top ends of the guide tubes and is designed to prevent excessive axial motion of the fuel rods.
The springs are of conventional coil design having approximately 14 active coils. The spring material was fabricated in accordance with AMS 5699E.
The upper cast plate of the assembly, called the holddown plate, together with the helical compression springs, comprise the holddown device The lower end fitting is a single piece stainless steel casting consisting of a plate with flow holes and four support legs, which serve as alignment posts.
The four outer guide tubes have a widened region at the upper end, which contains an internal thread. Connection with the upper end fitting is made by passing the male threaded end of the guide posts through holes in the lower cast flow plate and into the guide tubes. When assembled, the flow plate is secured between flanges on the guide tubes and on the guide posts. The connection with the upper end fitting is locked with a mechanical crimp. The center guide tube and each outer guide tube has, at its lower end, a welded Zircaloy-4 fitting. This fitting has a female threaded portion, which accepts a stainless steel bolt, which passes through a hole in the fuel assembly lower end fitting, to secure it. This joint is secured for the outer guide tubes with a stainless steel locking ring tack welded to the lower end fitting in four places. For the center guide tube, the threaded portion and the nut itself are welded.
The central guide tube inserts into a socket in the upper end fitting and is thus retained laterally by the relatively small clearance at this location. The upper end fitting socket is created by the November 2018 4-2 Rev 4
REACTOR center guide tube post which is threaded into the lower cast flow plate and tack welded in two places. There is no positive axial connection between the central guide tube and the upper end fitting.
The fuel assembly design enables reconstitution, i.e., removal and replacement of fuel and poison rods, of an irradiated fuel assembly. The fuel and poison rod lower end caps are conical shaped to ensure proper insertion within the fuel assembly grid cage structure; the upper end caps are designed to enable grappling of the fuel and poison rod for purposes of removal and handling. Threaded joints which mechanically attach the upper end fitting to the control element guide tubes will be properly torqued and locked during service, but may be removed to provide access to the fuel and poison rods.
Markings provided on the fuel assembly upper end fitting enable verification of fuel enrichment and orientation of the fuel assembly. Identical markings are provided on the lower end fitting to ensure preservation of fuel assembly identity in the event of upper end fitting removal.
Additional markings are provided on each fuel rod during manufacturing to distinguish between fuel enrichments and burnable poison rods, if present.
Starting with Cycle 16 Lead Test Assemblies (LTA) from AREVA and Westinghouse were used in the SONGS 2 and 3 reactor cores. These LTA had the same dimensions as the resident fuel assemblies including grid locations. The LTA have similar characteristics important for handling and storage as the resident fuel. The AREVA fuel cladding and guide tubes were made of AREVA proprietary material, M5, instead of the ZIRLOTM material of the resident fuel.
4.2.1.2 Fuel Rods The fuel rods consist of slightly-enriched UO2 or (U, Er) O2 cylindrical ceramic pellets, a round wire Type 302 stainless steel compression spring, and may contain alumina spacer discs (removed for Batch P fuel), all encapsulated within a ZIRLOTM or Zircaloy-4 tube seal welded with Zircaloy-4 end caps. The fuel rods are internally pressurized with helium during assembly.
During the manufacturing process, each fuel rod is marked in order to facilitate a means of maintaining a record of pellet enrichment, pellet lot and fuel stack weight.
The fuel cladding is cold worked and stress relief annealed ZIRLOTM or Zircaloy-4 tubing 0.025-inch thick The UO2 and (U, Er) O2 pellets are dished at both ends. The density of the UO2 in the pellets is about 10.47 g/cm3, which corresponds to 95.5% of the 10.96 g/cm3 theoretical density (TD) of UO2. The density of the (U, Er) O2 in the pellets is about 10.44 g/cm3, which corresponds to 95.8% of the 10.90 g/cm3 theoretical density of (U, Er) O2. However, because the pellet dishes and chamfers constitute about 1.5% of the volume of the pellet stack, the average density of the pellet stack is reduced to about 10.315 g/cm3 and 10.289 g/cm3 for the UO2 and (U, Er) O2, respectively. This number is referred to as the "stack density."
November 2018 4-3 Rev 4
REACTOR The compression spring located at the top of the fuel pellet column maintains the column in its proper position during handling and shipping.
4.2.1.3 Burnable Poison Rods A small number of fixed burnable neutron absorber (poison) rods may remain in selected fuel assemblies. They replace fuel rods at selected locations. The poison rods are mechanically similar to fuel rods, but contain a column of burnable poison pellets instead of fuel pellets. The poison material is alumina with uniformly-dispersed boron carbide particles. The balance of the column consists of alumina pellets, with the total column length the same as the column length in fuel rods. The burnable poison rod plenum spring is designed to produce a smaller preload on the pellet column than that in a fuel rod because of the lighter material in the poison pellets.
Each burnable poison rod assembly includes a serial number and visual identification mark. The serial number is used to record fabrication information for each component in the rod assembly.
The identification mark is unique to poison rods and provides a visual check on the pellet boron content during fuel bundle fabrication.
4.3 Control Element Assembly Description and Design Figures The San Onofre Units 2 and 3 have three different types of CEAs: full-length five-element, full-length four-element, and part-length five-element. Each CEA interfaces with the guide tubes of one fuel assembly, with the exception of the four-element CEA, which straddles two adjacent fuel assemblies. Part-length CEAs are differentiated from full-length CEAs by the following identifying features:
CEA Engraved Identification Grooves on Type Number (on Spider) Control Rod Full-length 1, 2, 3, etc. (1-in. high) None, smooth OD Part-length A, B, C, etc. One per rod (1-1/2-in. high)
The control elements of a full-length CEA consist of an Inconel 625 tube loaded with a stack of cylindrical absorber pellets. The absorber material consists of boron carbide (B4C) pellets, with the exception of the lower portion of all full length CEAs, which contain silver-indium-cadmium (Ag-In-Cd) alloy cylinders.
Each full-length control element is sealed by welds, which join the tube to an Inconel 625 nose cap at the bottom, and an Inconel 625 end fitting at the top. The end fittings, in turn, are threaded and pinned to the spider structure, which provides rigid lateral and axial support for the November 2018 4-4 Rev 4
REACTOR control elements. The spider hub bore is specially machined to provide a point of attachment for the CEA extension shaft.
The control elements of a part-length CEA consist of solid Inconel 625 over the bottom section of their length, an Inconel 625 tube open to the reactor coolant over the next section and a sealed chamber containing B4C pellets in the top section. See Table 4.2-1 for dimensions.
The lower ends of the four outer fuel assembly guide tubes are tapered gradually to form a region of reduced diameter.
November 2018 4-5 Rev 4
REACTOR Table 4.2-1 (Sheet 1 of 2)
MECHANICAL DESIGN PARAMETERS Spacer Grids And Fuel Rods Spacer Grid Type Leaf spring Material Zircaloy-4 10 (6 HID-1 Number per assembly and 4 HID-2) 3.6 HID-2 Weight each, lb 1.8 HID-1 Bottom Spacer Grid Type Leaf spring Material Inconel 625 Number per assembly 1 Weight each, lb 2.6 Weight of fuel assembly, lb 1,500 Outside dimensions Fuel rod to fuel rod, inches 7.972 x 7.972 Fuel Rod Fuel rod material (sintered pellet) UO2 (U,Er)O2 Pellet diameter, inches 0.3255 Pellet length, inches 0.390 Pellet density, g/cm3 10.47 10.44 Pellet theoretical density, g/cm3 10.96 10.90 Pellet density (% theoretical) 95.5 95.8 Stack height density, g/cm3 10.315 10.289 Clad material Zircaloy-4 or ZIRLOTM Clad ID, inches 0.332 Clad OD, (nominal), inches 0.382 Clad thickness, (nominal), inches 0.025 Diametral gap (cold, nominal), inches 0.0065 Active length, inches 150.0 Plenum length, inches 9.138 Fuel rod pitch, inches 0.506 Fuel rod array arrangement 16 x 16 November 2018 4-6 Rev 4
REACTOR Table 4.2-1 (Sheet 2 of 2)
MECHANICAL DESIGN PARAMETERS Control Rods (Control Element Assemblies)
Control Element Assembly (CEA) Full Length Part Length 79/5 element 8/5 element CEAs CEAs 4/4 element Number/Absorber Elements per ass'y CEAs Type Cylindrical Cylindrical rods rods Clad material Inconel 625 Inconel 625 Clad thickness, inches 0.035 0.035 Clad OD, inches 0.816 0.816 Diametral gap, inches 0.009 0.009 Poison Material B4C/Ag In Inconel/H2O/Spacer/B4C Cd/Inconel Length, in.
5 element CEAs 136/12.5/0.6 76.4/55/2/16 4 element CEAs 126.5/12.5/10.13 B4C Pellet Diameter, inches 0.737 0.737 Density, % of theoretical 73 73 density of 2.52 g/cm3, nominal Weight % boron, minimum 75 75 Finger Array Dimensions 5 element CEAs, inches 4.050 x 4.050 4.050 x 4.050 4 element CEAs, inches 4.050 x 4.130 NOTE: This table presents typical nominal values to provide familiarity with the Mechanical Nuclear Fuel Design.
November 2018 4-7 Rev 4
REACTOR Figure 4.2-1 FUEL ASSEMBLY TYPICAL November 2018 4-8 Rev 4