10 CFR 50.71(e) - Maritime Administration - U.S. Department of ...

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Updated Final Safety Analysis Report - (STS-004-002)6 REACTOR CORE6.1 Reactor Core General DescriptionThe reactor core included the fuel elements, the control rods, and the reactor vessel internals. Each ofthese is described in detail below. Table 6-1 and Figure 6-1 describe the reactor vessel fuel elements andcontrol rods as originally designed. During operation, the core was cooled by the primary coolant, whichflowed through the three thermal shields, through the second-pass fuel elements, and finally through thethird-pass fuel elements.6.1.1 Fuel ElementsThe active region of the N.S. SAVANNAH core contained 32 fuel elements. There were 16 fuel elements inthe down flow or second pass and 16 in the up flow or third pass. The reactor was permanently defueledin Fall 1971. The defueling started on August 23, 1971. The last fuel elements were removed September29, 1971.6.1.2 Control RodsThe control rod and follower assembly consisted of three sections: a stainless steel extension, a neutronabsorbingportion, and a Zircaloy-2 follower rod. All three sections were rigidly fastened together in acontinuous assembly. There were 21 cruciform control rods arranged on a square pitch.Following permanent defueling in Fall 1971, all twenty-one control rods are disconnected from theirrespective CRD extension shafts and remain in their fully inserted position between the empty fuellocation channels. The CRD extension shafts are disconnected from the lead screw and are in a fullyinserted position from the reactor head nozzles to the top of the control rods. The CRD lead screws arewithdrawn and pinned in place. One additional control rod (cut into two pieces 6 feet long and onetwelve inch piece) was placed in core. This is the control rod removed during the 1968 fuel shuffle.Steel caps are placed onto the twenty-one reactor head nozzles to completely seal the nozzle from thecontainment vessel atmosphere and mechanically separate the CRD mechanism from the control rod.6.1.3 InternalsThe entire core is mounted in the core support shield, which is supported by a conical support ring weldedto the reactor vessel wall 23.8 inches below the center of the upper flow nozzles. The internals could beremoved either as individual components or as integral assemblies.The fuel container assembly is confined between lower and upper grid plates approximately 90 inchesapart. The fuel elements (removed in 1971) and structural internals were held down against the netupward thrust of the primary coolant flow by the main holddown spring.The main holddown spring transmitted a downward force from the vessel head to the internals tocounteract the upward hydraulic forces during operation. If the ship should capsize, this spring wouldretain the internals. The load is transferred through the upper flow baffle assembly to the core supportring in the vessel.The orifice seal plate is bolted directly to the upper flow baffle assembly and provides a sealing surfacefor the 21 control rod nozzle seals. These seals are maintained by springs in the 21 control rod nozzles.The upper flow baffle assembly, located between the orifice seal plate and the upper grid plate, consists ofa series of 16 flow tubes surrounded by a vertical cylindrical chamber contained between upper and lowercover plates. There are 18 vertical gusset plates, which support the bearing ring around the periphery.Rev. VI 56
Updated Final Safety Analysis Report - (STS-004-002)Control rod guide blocks, which run from the upper cover plate to the bottom cover plate, help positionthe control rods under adverse pitch-and-roll conditions.The flow tubes direct the primary system coolant into the upper plenum of the reactor vessel and preventcross-flow. The flow tubes also prevent lateral forces against the control rod extensions when the rods arewithdrawn. Following defueling in 1971, the fuel element flow nozzles were removed from the fuel andreloaded into fuel channels in the reactor vessel.The upper grid plate is a welded assembly consisting of flow tubes, various vertical baffles, and upper andlower cover plates. The outer region of the grid plate assembly acts as a turnaround manifold for coolantentering the sixteen second-pass fuel elements from the thermal shield area. The flow tubes directed thethird-pass coolant from the fuel elements to the corresponding flow tubes of the upper flow baffleassembly.The first and second coolant passes are separated by the main core seal formed at the perimeter of thelower plate of the grid plate assembly and the top of the inner thermal shield.The fuel container assembly consists of 32 fuel element containers, transition pieces, a base plate, and acylindrical thermal shield. Normally, the fuel container assembly is not removed when the reactor isrefueled.The lower flow baffle is a welded assembly consisting of flow tubes, mounted between upper and lowercover plates and surrounded by a vertical cylindrical shield. Twenty one emergency control rod snubbersare mounted on the bottom of the lower plate.The peripheral flow tubes direct the flow from the second-pass fuel elements into the lower plenumchamber. The center flow tubes conduct the flow from the lower plenum chamber to the third-pass fuelelements.The emergency snubber, a combination of a spring and a hydraulic damper, absorbed the kinetic energyof a control rod if one was dropped during loading of the rods into the reactor.The core support shield, the center one of three thermal shields, had the dual function of protecting thevessel wall from radiation damage and supporting the entire lower flow baffle, the lower plenumchamber, and the fuel container assembly. It is suspended from and bolted to the conical support ring. Aflange at the bottom supports the fuel container assembly and the lower flow baffle assembly. The lowerplenum chamber is bolted to this flange.The lower plenum chamber consists of a circular cylinder closed on the bottom by an elliptical head. Itencloses the region around the lower flow baffle assembly and the emergency control rod snubbers. Thechamber turns the flow from the second pass into the third pass, and it separates the inlet coolant flowfrom the flow that has just left the second pass fuel elements.There are three thermal shields: the inner thermal shield in the fuel element container assembly, the coresupport shield, and the fixed or outer thermal shield permanently attached to the vessel wall. All areconcentric, stainless steel cylinders.Following permanent defueling in Fall 1971, all reactor internal components are in place, i.e., core basket,upper grid plate, upper flow baffle. Thirty-six upper flow transition pieces and one irradiated control rod(cut in three pieces) have been loaded into six fuel element channels in the core basket. Thirty-two upperflow transition pieces were from Core I and 4 were from Core Ia.In 2005, a project was performed to determine the Curie content and radionuclide composition of thereactor pressure vessel, internals and primary (neutron) shield tank by extraction of metal samples atselected locations. Using a precision gun-drilling or boring operation, an access hole was extendedRev. VI 57
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0U.S. Department Savannah Technical
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10 CFR 50.71(e) - Maritime Administration - U.S. Department of ...

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