ML16321A407

Response to Request for Additional Information Regarding Sensitivity Studies for STPNOC Risk-Informed Pilot GSl-191 Application

ML16321A407
Person / Time
Site:	South Texas
Issue date:	11/09/2016
From:	Connolly J South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
TAC MF2400, TAC MF2401, TAC MF2402, TAC MF2403, TAC MF2404, TAC MF2405, TAC MF2406, TAC MF2407, TAC MF2408, TAC MF2409
Download: ML16321A407 (5)

Text

......

v Nuclear Operating Company South Texas Project E/ectrlc Generating Stat/011 P.Q Box 28!J Wadsworth. Texas 77483 November 9, 2016 NOC-AE-16002916 10 CFR 50.90 U.S. Nuclear Regulatory Commission Attention:. Document Control Desk Washington, DC 20555-0001 South Texas Project Units 1 and 2 Docket Nos. STN 50-498 and STN 50-499 Additional Information Regarding Sensitivity Studies for STPNOC Risk-Informed Pilot GSl-191 Application (TAC NOS. MF2400- MF2409)

Reference:

Letter; J. Connolly to NRC Document Control Desk, "Supplement 3 to Revised STP Pilot Submittal and Requests for Exemptions and License Amendment for a Risk-Informed Approach to Address Generic Safety Issue (GSl)-191 and Response to,.

Generic Letter (GL} 2004-02 (TAC NOS. MF2400 - MF2409)"; October 20, 2016; (NOC-AE-16003401) (ML16302A015).

In the referenced correspondence, STP Nuclear Operating Company (STPNOC) provided a supplement to the STP Risk-Informed GSl-191 application. Following review of the supplement, the NRC staff requested information regarding sensitivity studies on break orientation and reactor core bypass. The requested information is provid~d in the Enclosure to this letter.

There are no commitments in this letter.

If there are any questions or if additional information is needed, please contact Drew Richards at (361) 972-7666 or me at (361) 972-7344.

• I decl9re under penalty of perjury that the foregoing is true and correct.

amr/JWC

Enclosure:

Additional Information Regarding Sensitivity Studies for STPNOC Risk-Informed Pilot GSl-191 Application STI: 34405979

NOC-AE-16002916 Page 2 of 2 cc:

(paper copy) (electronic copy)

Regional Administrator, Region IV Morgan, Lewis & Bockius LLP U.S. Nuclear Regulatory Commission Steve Frantz, Esquire 1600 East Lamar Boulevard Arlington, TX 76011-4511 U.S. Nuclear Regulatory Commission Lisa M. Regner Lisa M. Regner Senior Project Manager U.S. Nuclear Regulatory Commission NRG South Texas LP One White Flint North (08H04) Chris O'Hara 11555 Rockville Pike Jim von Suskil Rockville, MD 20852 Skip Zahn NRC Resident Inspector CPS Energy U. S. Nuclear Regulatory Commission Kevin Pollo P. 0. Box 289, Mail Code: MN116 Cris Eugster Wadsworth, TX 77483 L. D. Blaylock City of Austin Elaina Ball John Wester Texas Dept. of State Health Services Helen Watkins Robert Free

Enclosure NOC-AE-16002916 ENCLOSURE Additional Information Regarding Sensitivity Studies for STPNOC Risk-Informed Pilot GSl-191 Application

Enclosure NOC-AE-16002916 Page 1 of 2 SNPB-3-20 Supplemental Information Follow-up question 1:

The staff understood that STP would provide the conclusions of the break orientation sensitivity study. The staff needs this to complete its SE.

STP Response:

The RELAP5-3D includes a Stratification Entrainment/Pullthrough model to account for changes in void fractions of the mixture flowing through a break from large horizontal pipes or vessels. This model can be enabled at junctions simulating the break on large pipes {such as cold or hot legs) to account for the break orientation {upward, lateral, and downward orientations).

A sensitivity analysis was performed to investigate the effect of the break orientation on the PCT during the post-core blockage LTCC. From the results of the analysis it was concluded that no significant difference in the PCT was observed between the cases analyzed.

Additional considerations on the break size in comparison with the diameter of the hot leg were made to identify the best configuration for each break size analyzed. In particular, the stratification entrainment/pullthrough model was disabled for 16" break scenario, due to the large size of the break, while a downward oriented break was assumed for smaller breaks {6" and 2") to minimize the enthalpy flowing through the break.

Follow-up question 2:

The staff understood that the bypass sensitivity study would assume a true single worst failure. The staff does not believe the single failure of the EGGS train is the worst failure.

Also, the staff believes a top-peaked power shape is necessary and appropriate for this analysis. STP's use of Appendix K decay heat, steam generator tube plugging, and RWST size and temperatures, and sump temperature assumptions appear appropriate.

STP Response:

An additional sensitivity analysis is conducted to investigate the effect of the power shape on the PCT under the condition of core barrel/baffle bypass open during the post-core blockage LTCC. The sensitivity is executed under the same assumptions and conditions applied to the sensitivity B1 described in the answer to SNPB-3-20. The input file is generated from the one used to perform the simulation B1, only changing the core BB bypass Uunction 84501) to be open during the post-core blockage LTCC. As describes in the answer to RAl-SNPB-3-3, this configuration is likely to be the most realistic condition.

The simulation conditions of the reference case {B1) and the sensitivity case {B1 F) are listed below.

Enclosure NOC-AE-16002916 Page 2 of 2 Boundary Conditions and other Assumptions Reference Case (Sensitivity B 1) - Top Skewed, Core BB Blocked The axial power profile is chosen to represent the STP relative axial power distribution at high full power (HFP), beginning of life (BOL) and equilibrium Xenon.

The core BB bypass is assumed to be blocked at the core blockage time and ma inta ined blocked during the post-core blockage LTCC.

Sensitivity Case (B 1F) - Top Skewed, Core BB Bypass Open The axial power profile is the same of the one adopted in the reference case . The core BB bypass is assumed to remain open during the post-core blockage LTCC .

Simulation Results No appreciable differences in PCT during the post-core blockage LTCC are found, as shown in Figure 1. Saturation temperature is also included in the figure.

The PCT of the sensitivity case appears smoother than the one simulated under the refe rence conditions (core bypass blocked) .

- PCT Bl - PCT BlF

• CoreOutSatT Bl .. ..... (oreOUIS.tT BlF - - -

• SunpSw1tcholle1 - - Block.igeTime 1000 900 800

'Li:" 700 LU cc 600

J

~

cc 500 LU a.

~ 400 LU I-300 .. .*

100 1000 1500 1000 1500 lOOO 3500 4500 ssoo 6000 SIMULATI ON TIME [SJ Figure 1. PCT (Reference Case B1 ; Sensitivity Case B1F)