ML14135A382

South Texas Project, Units 2, Request for Additional Information Reactor Vessel Radiation Surveillance Program, TAC MF0699

ML14135A382
Person / Time
Site:	South Texas
Issue date:	04/30/2014
From:	Powell G T South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NOC-AE-14003131, TAC MF0699
Download: ML14135A382 (7)

Text

Nuclear Operating CompanySouth Texas Project Electric Generatin$ Station PO BY 28,9 Wadsworth Texas 77483 __VV_April 30, 2014NOC-AE-1400313110 CFR 50 Appendix HSTI: 33867989File: G25U. S. Nuclear Regulatory CommissionAttention: Document Control DeskWashington, DC 20555-0001South Texas ProjectUnits 2Docket Nos. STN 50-499Request for Additional InformationReactor Vessel Radiation Surveillance Proaram. TAC MF0699Reactor Vessel Radiation Surveillance Proara TAC MF0699

References:

1. Letter from Marco Ruvalcaba, STPNOC, to NRC Document Control Desk,"Reactor Vessel Radiation Surveillance Program -STP Unit 2," dated February11, 2013. (NOC-AE-13002957) (ML130530263)2. E-mail from B. Singhal, NRC to L. Sterling, STP Nuclear Operating Company,"Request for Additional Information -Reactor Vessel Radiation SurveillanceProgram," dated April 8, 2014. (ML14099A011)By Reference 1, STP Nuclear Operating Company (STPNOC) submitted a Reactor VesselRadiation Surveillance Program letter for South Texas Project (STP) Unit 2. This letter providedsurveillance results of Capsule W, which was removed from the STP Unit 2 reactor duringrefueling outage 2RE1 5 (October 28 -November 22, 2011). By e-mail dated April 8, 2014,Reference 2, the NRC staff has requested additional information regarding Reference 1.STPNOC's response to the requests for additional information is provided in Enclosure 1 to thisletter.There are no commitments in this letter.Should you have any questions regarding this letter, please contact Rafael Gonzales, STPLicensing point-of-contact, at (361) 972-4779 or me at 361-972-7566.I declare under penalty of perjury that the foregoing is true and correct.Executed on ',4'/09' ,o/VDateG.T. PowellSite Vice PresidentRJG

Enclosure:

1. STPNOC Response to Requests for Additional InformationjDcs4 NOC-AE-14003131Page 2 of 2cc:(paper copy)(electronic copy)Regional Administrator, Region IVU. S. Nuclear Regulatory Commission1600 East Lamar BoulevardArlington, Texas 76011-4511Balwant K. SingalSenior Project ManagerU.S. Nuclear Regulatory CommissionOne White Flint North (MS 8B1)11555 Rockville PikeRockville, MD 20852Senior Resident InspectorU. S. Nuclear Regulatory CommissionP. O. Box 289, Mail Code: MN1 16Wadsworth, TX 77483C. M. CanadyCity of AustinElectric Utility Department721 Barton Springs RoadAustin, TX 78704Jim CollinsCity of AustinElectric Utility Department721 Barton Springs RoadAustin, TX 78704John W. DailyLicense Renewal Project Manager (Safety)U.S. Nuclear Regulatory CommissionOne White Flint North (MS 011-Fl)Washington, DC 20555-0001Tam TranLicense Renewal Project Manager(Environmental)U. S. Nuclear Regulatory CommissionOne White Flint North (MS 011 F01)Washington, DC 20555-0001A. H. Gutterman, EsquireKathryn M. Sutton, EsquireMorgan, Lewis & Bockius, LLPJohn RaganChris O'HaraJim von SuskilNRG South Texas LPKevin PolioCris EugsterL.D. BlaylockCity Public ServicePeter NemethCrain Caton & James, P.C.C. MeleCity of AustinRobert FreeTexas Department of State Health ServicesRichard A. RatliffAlice RogersTexas Department of State Health ServicesBalwant K. SingalJohn W. DailyTam TranU. S. Nuclear Regulatory Commission NOC-AE-14003131Enclosure 1STPNOC Response to Requests for Additional Information NOC-AE-14003131Page 1 of 4South Texas Project, Unit 2,Request for Additional InformationReactor Vessel Radiation Surveillance ProgramBy letter dated February 11, 2013 (Agencywide Documents Access and Management System(ADAMS) Accession No. ML1 30530263), STP Nuclear Operating Company, submitted WCAP-17636, "Analysis of Capsule W from the South Texas project Nuclear Operating Company Unit2 Reactor Vessel Radiation Surveillance Program," October 2012, documenting the results ofthe examination of Capsule W. The U.S. Nuclear Regulatory Commission (NRC) staff hasreviewed the report and has the following questions:1 .The licensee modeled the reactor and vessel using a three-dimensional representationof the problem geometry, as described in Chapter 6 of WCAP-1 7636-NP (Enclosure toletter dated February 11, 2013). The model used 208 radial, 197 azimuthal, and 190axial intervals. The model is octant-symmetric. The azimuthal nodalization exceedsthe 40 angular intervals suggested in Regulatory Guide (RG) 1.190, "Calculational andDosimetry Methods for Determining Pressure Vessel Neutron Fluence," March 2001(ADAMS Accession No. ML010890301). The recommendations for nodalization in theradial and axial directions are specific to the material and location being modeled. TheRG notes that, "the adequacy of the spatial mesh and angular quadrature, as well asthe convergence criterion, must be demonstrated by tightening the numerics until theresulting changes are negligible."Please provide additional information explaining how the adequacy of the spatial meshwas validated.2. The core neutron source was constructed based on fuel assembly-specific enrichmentand burnup data for each fuel cycle of operation; this is consistent with the guidancecontained in RG 1.190. The fuel assembly-specific neutron sources were derived frompin-wise isotopics represented in Cartesian geometry. The isotopic data wereconverted to cylindrical mesh arrays used in the RAPTOR-M3G transport calculation.Please explain how the uncertainty associated with this conversion was determined andincorporated into the analytic uncertainty analysis, and provide additional supportingdetail (i.e., provide a more detailed description of the H. B. Robinson qualificationeffort).3. Section 6.1 of WCAP-17636-NP includes the statement, "Additionally, the methodsused to develop the calculated pressure vessel fluence are consistent with the NRC-approved methodology described in WCAP-1 4040-A, Revision 4, 'Methodology Used toDevelop Cold Overpressure Mitigating System Setpoints and RCS Heatup andCooldown Limit Curves." Noting this statement, the NRC staff observes that the use ofRAPTOR-M3G is a significant departure from the approved methodology in that thetransport calculations are no longer performed using the suite of Oak Ridge NationalLaboratory discrete ordinates radiation transport codes, typically with a synthesis oflower-dimension calculations to determine the 3-dimensional flux field.Please explain, more specifically, how the calculations are consistent with the WCAP-14040-A, Revision 4, methods. Please also identify any other differences betweenapproved methodology and that described in WCAP-1 7636-NP. NOC-AE-14003131Page 2 of 4NRC RAI Request 1:The licensee modeled the reactor and vessel using a three-dimensional representation ofthe problem geometry, as described in Chapter 6 of WCAP-17636-NP (Enclosure to letterdated February 11, 2013). The model used 208 radial, 197 azimuthal, and 190 axialintervals. The model is octant-symmetric. The azimuthal nodalization exceeds the 40angular intervals suggested in Regulatory Guide (RG) 1.190, "Calculational and DosimetryMethods for Determining Pressure Vessel Neutron Fluence," March 2001 (ADAMSAccession No. ML010890301). The recommendations for nodalization in the radial andaxial directions are specific to the material and location being modeled. The RG notes that,"the adequacy of the spatial mesh and angular quadrature, as well as the convergencecriterion, must be demonstrated by tightening the numerics until the resulting changes arenegligible."Please provide additional information explaining how the adequacy of the spatial mesh wasvalidated.STPNOC RAI Request 1 Response:When constructing the three-dimensional (3-D) RAPTOR-M3G transport model, specialattention has been paid to vary mesh sizes with material and region geometry strictly in linewith the guidance of Regulatory Guide 1.190 (RG 1.190). Specifically, the radial mesh inthe peripheral assemblies is finer than -2 intervals per inch as required by RG 1.190 (meshsize equal to or less than 1 cm per mesh in periphery assembly region). In the excoreregions, again a much finer mesh (5 to 10 intervals per inch in water, and more than 3intervals per inch in stainless steel) is used, which is significantly finer than the -3 intervalsper inch in water and -1.5 intervals per inch in steel as required by RG 1.190. Finer spatialmeshes are also used in regions exhibiting steep gradients, in materials with small meanfree paths, and at material interfaces, to ensure no flux changes in any group will begreater than factor 2 between adjacent intervals. For example, a very fine spatial mesh (inboth r- and 0-directions) is used in modeling the stainless steel baffle plates at theperiphery of the core to adequately describe this rectilinear component in r z geometry(Refer to Figures 6-1 through 6-3 in WCAP-1 7636). Mesh size is selected to ensure thatthe flux in each spatial mesh for all energy groups reaches convergence criterion of 0.001.In Section 1.3.1 of RG 1.190, it is stated "An azimuthal (0) mesh using at least 40 intervalsover an octant in (r, 0) geometry in the horizontal plane should provide an accuraterepresentation of the spatial distribution of the material compositions and source describedin Regulatory Position 1.2." The 3-D RAPTOR-M3G model built for South Texas Unit 2 iscomprised of 197 azimuthal mesh, which exceeds the requirements of RG 1.190. Ingeneral, the finer the mesh, the results are more reliable. The geometrical convergence,i.e., the impact on the discrete ordinate calculation results due to refining the spatial meshsize, should have been achieved in the RAPTOR-M3G model since it used finer meshsizes than those required by RG 1.190.Finally, the comparison of the RAPTOR-M3G results with the measurement data presentedin Appendix A in WCAP-1 7636-NP also shows good agreement, further validating theresults from 3-D RAPTOR-M3G transport calculation are credible. NOC-AE-14003131Page 3 of 4NRC RAI Request 2:The core neutron source was constructed based on fuel assembly-specific enrichment andburnup data for each fuel cycle of operation; this is consistent with the guidance containedin RG 1.190. The fuel assembly-specific neutron sources were derived from pin-wiseisotopics represented in Cartesian geometry. The isotopic data were converted tocylindrical mesh arrays used in the RAPTOR-M3G transport calculation.Please explain how the uncertainty associated with this conversion was determined andincorporated into the analytic uncertainty analysis, and provide additional supporting detail(i.e., provide a more detailed description of the H. B. Robinson qualification effort).STPNOC RAI Request 2 Response:For analyses using the (r, 0, z) coordinate system, the spatial component of the neutronsource is transposed from x, y to r, 0 geometry by overlaying the mesh schematic to beused in the transport calculation on the pin-by-pin array and then computing the appropriaterelative source applicable to each r,0 interval. This is the standard approach that has beenapproved previously by NRC in generating the transport source, whether the multi-channelsynthesis approach is utilized or a 3-D (r, 0, z) transport calculation is utilized. Themethodology employed for this analysis is consistent with WCAP-1 4040-A, Revision 4 (seethe response to Question #3). Per WCAP-14040-A, the uncertainty of the spatialdistribution of the source is 4%. This 4% includes not only the uncertainty attributes fromthe conversion, but also the generation of the pin-by-pin source, itself. The detaileddescription of the H. B. Robinson qualification effort is documented in WCAP-14040-A. NOC-AE-14003131Page 4 of 4NRC RAI Request 3:Section 6.1 of WCAP-1 7636-NP includes the statement, "Additionally, the methods used todevelop the calculated pressure vessel fluence are consistent with the NRC-approvedmethodology described in WCAP-14040-A, Revision 4, 'Methodology Used to DevelopCold Overpressure Mitigating System Setpoints and RCS Heatup and Cooldown LimitCurves." Noting this statement, the NRC staff observes that the use of RAPTOR-M3G is asignificant departure from the approved methodology in that the transport calculations areno longer performed using the suite of Oak Ridge National Laboratory discrete ordinatesradiation transport codes, typically with a synthesis of lower-dimension calculations todetermine the 3-dimensional flux field.Please explain, more specifically, how the calculations are consistent with the WCAP-14040-A, Revision 4, methods. Please also identify any other differences betweenapproved methodology and that described in WCAP-1 7636-NP.STPNOC RAI Request 3 Response:WCAP-14040-A, Revision 4, describes a calculation methodology that includes multi-dimensional discrete ordinates radiation transport calculations. The approved methodologyallows neutron exposure to be derived from either a two-dimensional flux synthesistechnique or a higher-order three-dimensional radiation transport calculation. The use ofthe RAPTOR-M3G code in place of the TORT code does not constitute a departure fromthe approved methodology described in WCAP-14040-A, Revision 4.The RAPTOR-M3G code is a multi-dimensional discrete ordinates radiation transport codethat adheres to the same discrete ordinates methodology as the TORT code. Thesignificant distinction is that RAPTOR-M3G employs a parallel processing technique toobtain its answers. Parallel processing techniques allow large and detailed three-dimensional problems to be solved on a timescale conducive to production engineeringapplications. See the motivating discussion at the beginning of Appendix A of WCAP-16083-NP, Revision 1.Appendix A.1 of WCAP-16083-NP, Revision 1 demonstrates RAPTOR-M3G and TORTproduce answers that are sufficiently similar that they are interchangeable. The remainingsections in Appendix A provide additional comparisons of measurement data tocalculations. RAPTOR-M3G has been designed from its inception as a parallel processingcode, and adheres to the modern best practices of software development. It has beenrigorously tested against the TORT code and benchmarked on an extensive set ofacademic and real-world problems.For these reasons, the use of RAPTOR-M3G is considered to be methodologicallyconsistent with WCAP-14040-A, Revision 4.