ML17037C365

Letter Responding to the August 13, 1971 Letter Regarding the Proposed Fueling of the Reactor Which Niagara Mohawk Hopes to Start on September 19, 1971

ML17037C365
Person / Time
Site:	Nine Mile Point
Issue date:	08/30/1971
From:	Brosnan T Niagara Mohawk Power Corp
To:	Morris P US Atomic Energy Commission (AEC)
References
Download: ML17037C365 (14)

Text

OF OOCUMENT'. DATE RECEIVED

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NIAGARA MOHAWK POWER CORPORATION NIAGARA 'OMAWK 300 ERIE GOUI EVARD WEST SYRACUSE, N.Y. I3ROR eEgj(p)

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/g Dr. Peter A. Morris, Director Division of Reactor Licensing United States Atomic Energy Commissio n

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Dear Dr. Morris:

My letter'f August 20, 1971 provided the answers to three of the four questions in your August 13, 1971 letter regarding the proposed refueling of the Nine Mile Point reactor which we hope to start on September 19, 1971.

We have now been able to assemble the material requested in your fourth question and I am send-ing it to you along with this letter.

Sincerely, T. . Brosnan Vice President and Chief Engineer TJB sn Enc.

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RE: . J. Brosnan August 30, 1971, eg(/(p~'0gfgig response to Dr. P. A. Morris August 13, 1971, letter

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1. guestion: sOcelvsd n/Ln san~~~ I Describe >our evaluation of the consequences of possible loading errors associated with the proposed refueling. Consider errors associated with the multiple enrichments, the gadolinium poison, and the exchange of fuel rods of an assembly, and consider possible errors in, locating and position-ing an assembly in the core in connection with poison curtain position.

Ansne'r:

I. Procedures and Controls Numerous controls discussed below axe employed during fuel design, manufacture,'shipment and installation to assure that loading errors associated with the proposed refueling are highly impxobable. Each such control and/or procedure is backed up or supplemented'y another control and/or procedure.

Pellet Enrichment Controls "Procedures i> -effect 'at 'Genexal 'Electri'c's 1(ilmington Fuel hlanufacturing Facility are designed to maintain control of enrichment at each major step in the manufacturing process. Major line cleanups (disassembly of equipment, wipe down, etc.) are performed on all process equipment when-ever an enrichment change is made, and minor cleanups (vacuum, brush clean, etc.) are performed at least. once every shift on loading stations.

Hach shift, the Quality Control Staff audits the enrichment control procedures. Periodically an overall shop enrichment control procedural audit is conducted'.

Cylinders'of UF6 shipped to Nilmington are analyzed for enrichment and each container is distinctly labeled. Following conversion, U02 powder is placed in cans which are also labeled for enrichment. As the powder is pressed into pellets, sample pellets from each can are checked for enrichment.

Strict controls on enrichment are also maintained on fuel pellets. Each pellet is distinctively marked for that particular enrichment. Pellets of the same enrichments are stored and moved in containers or boats. Each pellet boat has a travel card. Pellet boat cards are color coded with a different color for each enrichment. A large, prominent stamp of the enrichment mark and the specific enrichment in the pellet boat are marked on each boat travel card. The pellet boat card is numbered and the is preceded by a "P" for plain pellets, and "D" for dished pellets. 'umber 1-1

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are stored in a cabinet with one enrichment per cabinet. Prior to rod loading, the authorized control moveman and operator at'he rod loading station check at least one pellet from each tray of pellets for dishing and enrichment. 'nly one enrich-ment is permitted at any one loading station at any given time.

Tubing with the first end plug welded is transferred to the pellet loading station in lots, with a serial number stamped on the bottom end plug. Enrichment, dished pellet and plain pellet identity records are maintained by fuel rod serial number after fuel rods are verified with pellets. Finally, the enrichment of each fuel rod is verified by a procedure which includes 100'o ganIma scanning prior to assembly.

Fuel Rod Placement Controls mechanical design features are employed to assure the proper loading of fuel rods and fuel bundles; . Each fuel rod has two unique characteristic identification features to prevent errors in enrichment location within any fuel bundle:. First, the fuel rods are designed with characteristic mechanical end Sittings, one Sor each of the enrichments and one for each of the gadolinia types. End fittings are designed so that it is not mechanically possible to complete assembly of a fuel bundl~: with high enrichment rods in positions designated by design Sor a lower enrichment. The placement of lower enrichment rods in positions designated Sor higher enrichments is mechanically possible, but would be detected in the inspection process because of a loose Sit between the misplaced rod and the tie plate.

Further, the gadolinium rods'pper end plugs are of a physical config-uration that allows inspection after the bundle is fully assembled.

Secondly, each Suel rod has an identification code number at the lower end plug location; the numbers are recorded as the rods are assembled into the fuel bundle. These data are analyzed by a computer where all rod numbers are accounted for and their location verified, Each fuel bundle is fitted with a handle upon which the project name identification and fuel bundle number are'.inscribed. '*'The handle/tie plate is machined to be assembled in one way only. The handle is further equipped with an orientation lug to aid in positioning in the core.

Gadolinium Controls Fuel rods containing gadolinia is subject to the same controls on enrichment and rod loading as have been previously described. However, all activities related to gadolinia fuel are physically separated from non-gadolinia fuel to prevent cross contamination. Rods, containing gadolinia fuel are identified by pre-assigned fuel rod serial numbers located on the lower end plug of each rod.

Fuel Bundle Location and Orientation Controls Proper orientation and location of fuel bundles:. in the reactor core is readily verified by visual observation and is assured by formal verification procedures during core loading. There are five separate visual indications of proper fuel bundle orientation: 1) the channel 1-2

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fastener assemblies, including the spring and guard used to maintain cloarances between channels, are located at one corner of each fuel bundle adjacent to the center of the control rod; 2) identification bosses on the fuel bundle handles all point toward the adjacent control rod; 3) the channel spacing buttons are adjacent to the control blades;

4) the bundle identification numbers on the fuel bundle handles are all readable fxom the direction of the center of the cell; and 5) there is cell-to-cell symmetry.

During refueling, "all fuel bundle movements are under the direction of the Shift Control Operator. Separate fuel bundle location tag boards are maintained in both the fuel loading area and the control room. Location and movements of each fuel bundle are monitored on these boards by tags assigned to each bundle. Pexsonnel are assigned to both of these locations Sor the purpose of verification and recording of the location and movement of each fuel bundle at all times. Experience at Nine Mile Point has demonstrated that each fuel bundle can be clearly identified so that even should a fuel bundle be misoriented or incorrectly located, it would be readily detected during formal core loading verifications.

Review and Audit General Electric's quality control program has been reviewed by both Niagara Mohawk and its quality assurance agent for this project, the Nuclear Audit and Testing Company. In addition, quality control specialists from Nuclear Audit and Testing frequently visited General Electric's Nilmington facility during fabrication of the fuel to verify compliance with specified quality control procedures.

Upon xeceipt at the site, fuel will again be inspected by Niagara Mohawk and General Electric personnel, at which time proper loading of rods in the bundles can be verified. The loading of bundles into the core will be formally rechecked by pexsonnel other than those actually performing the movement in order to assure their proper location and orientation.

Evaluation of Loadin Errors Despite the multiplicity of design Seatuxes, procedural controls and inspections, all of which tend to preclude loading erxors, analyses have been performed as xequested to determine the consequences of such events at rated power opexation.

Pellet Enrichment Deviations Any large deviation in a pellet enrichment which might cause failuxe of the fuel rod would likely be detected during the gamma scanning process.

However, even if a failure were to occur because of enrichment deviations, such a failure would not be of great consequence since the Station is designed to safely accommodate Sailed fuel rods. Off-gas releases are continually monitored at various points. Any significant increase would be readily detected and evaluated by the Station Staff.

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'adolinium Deviations As a worst case, the efiect of complete omission of gadolinium from a reload fuel bundle located on the periphery of the core has been considered'. Because of the low lineal heat generation, rate at which the gadolinium bearing fuel will be operated, as discussed in our July, 27, 1971, letter, it is assured that the bundle would remain substan-tially below the fuel damage limit even with complete omission of gadolinium.

Fuel Rod Placement Errors II Interchange of fuel rods of a given bundle are restricted to rods of similar enrichments because of the physical characteristics previously described. Three possible rod interchanges have been identified.

Pellets in selected individual fuel rods are dished to provide additional volume to accommodate irradiation swelling of U02 fuel pellets thereby minimizing cladding stress and strain. If a fuel rod containing undished fuel pellets were to be loaded into the most limiting position in a fuel bundle (which should by design contain dished pellets) the fuel rod would not ordinarily be expected to Sail because even without dishing the fuel rod would not reach the clad damage limit of one percent plastic strain.

Further, if such a failure did occur, it would be limited to the individual rod and consequences would be minimal as discussed above for pellet enrichment deviations.

Fuel rod loading errors involving mislocation of gadolinium rods while maintaining a symmetxic array were analyzed. In no case would fuel failure be expected. Specifically, the results show that the maximum lineal heat generation rate remains below 17.5 kw/ft and minimum critical heat flux ratio remains above 1.9.

Physical limitations and .detailed procedures with independent checks assure that the exchange of rods in reconstituted fuel bundles will be limited to rods of the same initial enrichment and specific exposure differences.

Replacement of a defective fuel rod of high exposure with an initial fuel rod of minimum exposure was considered. Results from these analyses indicate that the reconstituted bundle would operate with a maximum lineal heat generation rate < 18.8 kw/ft and a minimum critical heat flux ratio

> 1.77.

Fuel Assembl Placement Errors Procedures and inspections as previously described are designed to prevent errors associated with placement of fuel bundles during the wefueling. The analyses of two errors in placement of reload fuel are described below.

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Analyses have been performed assuming that a reload fuel bundle is incorrectly loaded on the periphery of the core such that the bundle is rotated 180'rom the proper orientation. Results show the bundle would operate without failure of fuel rods. Specifically, the maximum lineal heat generation rate is less than 17.5 kw/ft and the minimum critical heat flux ratio greater than 1.?S.

Analyses have also been made assuming a reload fuel bundle is incorrectly placed in the most limiting central region of the core, and that the temporary control curtains are left in the area adjacent to the bundle.

Results indicate that the limiting fuel rod in this reload bundle would reach a maximum lineal heat generation rate < 19.7 and a minimum critical heat flux ratio > 1.49.

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