ML16258A100

NRR E-mail Capture - (External_Sender) South Texas Project Generic Safety Issue 191 Resolution RAI APLA-3-2 Clarification for September 14, 2016 Public Meeting

ML16258A100
Person / Time
Site:	South Texas, Perkins
Issue date:	09/12/2016
From:	Richards D South Texas
To:	John Klos, Lisa Regner Plant Licensing Branch IV
References
MF2400, MF2401
Download: ML16258A100 (7)

Text

1 NRR-PMDAPEm Resource From:

Richards, Drew <amrichards@STPEGS.COM>

Sent:

Monday, September 12, 2016 6:12 PM To:

Klos, John Cc:

Regner, Lisa

Subject:

[External_Sender] Fw: SNPB-3-2 SG - DRAFT Attachments:

SNPB-3-02 followup - SG.docx

John, See attached. This also needs to go to Josh Kaizer for review.

Thanks, drew

?

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NRR_PMDA Email Number:

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[External_Sender] Fw: SNPB-3-2 SG - DRAFT Sent Date:

9/12/2016 6:12:04 PM Received Date:

9/12/2016 6:12:09 PM From:

Richards, Drew Created By:

amrichards@STPEGS.COM Recipients:

"Regner, Lisa" <Lisa.Regner@nrc.gov>

Tracking Status: None "Klos, John" <John.Klos@nrc.gov>

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STPEGS.COM Files Size Date & Time MESSAGE 103 9/12/2016 6:12:09 PM SNPB-3-02 followup - SG.docx 150053 Options Priority:

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Follow-up SNPB-3-2 Accident Scenario Progression Initial RAI:

Provide a description of the accident progression of the accident scenarios being simulated using the LTCC EM. This description should start at the initiation of the break, define each phase, and the important phenomena occurring in that phase in the various locations of the RCS (e.g., core, reactor vessel, steam generators - both primary and secondary side, loops, pressurizer, pumps, containment)

Follow-up question:

In their response, STPNOC provided a detailed write-up of the accident scenario, along with a number of plots describing key phenomena. However, the following items need to be addressed in the explanation of the accident progression scenario:

The heat stored in the steam generators needs to be appropriately treated to ensure correct flow, as the steam generators become the dominant flow path following full core blockage. Are the levels of auxiliary feedwater used in this analysis consistent with plant procedures following a LOCA? Has all of the secondary metal mass been accounted for the in the simulation? Have the correct material properties been used (e.g., heat capacity of the steam generator tubes)?

STP Response.

The approach used in modeling the steam generators (SGs) in the LTCC EM is adequate to the scope of the simulation proposed. The model of the SGs include sufficient details of the primary and secondary fluid regions to simulate the behavior of the reactor system during the LTCC, including the effects of the SG secondary side on the primary coolant flow reaching the top of the core during the Post-Core Blockage LTCC phase.

Geometry.

The detail included in the RELAP5-3D LTCC EM is adequate to conduct the simulation of the proposed accident scenarios (16, 6, and 2 HLB). All the regions of the SG are modeled in the EM including the primary side (U-tubes and metal walls) and secondary side (downcomer, boiler, steam separator, upper dome, MFW, and AFW injection).

Description of the SG internal volumes (primary and secondary side) dimensions, assumptions, and nodalization technique is described in the steady-state calculation document RC09989 [1]. In the same document adequate comparison with plant operating conditions is presented to verify the adequateness of the model.

Plant LOCA Operations The SGs secondary side boundary conditions used in the LTCC EM are consistent with the plant procedures following a LOCA scenario. In particular:

MFW flow is isolated based on the primary low-pressure signal The MSIV closure is controlled by the reactor trip signal

The AFW injection start is controlled by the reactor trip signals Set points and delays are accounted and aligned with the plant characteristics.

The AFW thermodynamic conditions are consistent with the plant characteristics The AFW flow rate is controlled in order to reach and maintain the liquid level in the SGs secondary side above the U-tubes bundle.

Metal Masses (Passive Structures)

The LTCC EM include heat structures representing the u-tube walls. These heat structures simulate:

The heat transfer mechanisms (convection, conduction) from the primary coolant to the secondary coolant during normal operation (steady-state)

The heat transfer mechanisms (convection, conduction) between the primary and secondary coolants during the accident Masses of the u-tube are modeled in the EM. The material selected for the u-tube represents the STP SGs characteristics. Thermal properties of the material selected (thermal conductivity and heat capacity) are tabulated as function of the temperature in the RELAP5-3D steady-state input file. These thermal properties represent the thermal characteristics of the of the u-tubes walls in the STP plant.

Additional Secondary Side SGs Metal Masses Although the LTCC EM does not model metal masses in the SGs secondary side, a sensitivity study is conducted to demonstrate that the effect of the heat stored in the secondary metal masses on the primary system behavior during the Post-Core Blockage LTCC phase is negligible. The PCT is used as figure of merit in the sensitivity.

Assumptions, boundary conditions, modeling and simulation techniques, and results of the sensitivity are described below.

SG Secondary Side Metal Masses Sensitivity The sensitivity study is conducted to evaluate the effects of the SG secondary metal masses (sensitivity input parameter) on the PCT (figure of merit) during the post-core blockage LTCC phase of a 16 HLB LOCA scenario. The initial conditions for the simulations are the same of the ones produced by the LTCC EM described in the RAI 02. The initial conditions are imposed by using the restart feature of RELAP5-3D.

The sensitivity simulation is started at t=2000 s, which corresponds to the first available restart time from the LTCC EM - Base case immediately before the core blockage time (2099 s). The input file of the sensitivity case is expanded to conservatively include heat structures simulating the total dry mass of the SG. The total dry mass of the SG includes the mass of:

SG pressure shell (including inlet/outlet plena, cylindrical walls, and upper head),

separators and dryers, tube bundle wrapper (shroud) tube support plates and tube plate U-tubes Any other internal structure

The total dry mass of the SG specified in the sensitivity model is equal to 1,045,000 lbm.

This mass is equally split between two passive heat structures (SH-X701 and SH-X702) identified in Figure 1.

Figure 1. Steam Generator Nodalization and Passive Heat Structures (Sensitivity)

These passive heat structures are assumed in contact only with the lower volumes of the secondary side. This maximizes the heat transfer to the secondary fluid during the LTCC phase, when the liquid level is maintained within this region.

SH-X702 is in contact with the hydrodynamic component X76 (simulating the SG downcomer), and isolated on the other side. SH-X701 is in contact with the downcomer (X76) and the boiler (X70) on the left and right sides respectively.

Thermal properties (thermal conductivity and heat capacity) are tabulated as a function of the temperature.

Since the SG U-tubes are simulated with a separate heat structure (see Figure 1, dark profile between X70 and X08), the mass of the U-tubes are accounted twice in the sensitivity model to increase conservatism.

The initial temperatures of the additional heat structures SH-X701 and SH-X702 are conservatively imposed to be equal to the temperature of the secondary side during normal operation.

Simulation Results - PCT The comparison of the PCT of the LTCC EM and the sensitivity case is shown in Figure

2. No major differences are noticeable. The PCT for the two cases is comparable.

Figure 2. PCT during the Post-Core Blockage LTCC Phase The secondary fluid average temperature (average of the temperature of the fluid in the nodes of the boiler (X7001-04) is compared in Figure 3. The comparison shows that the average temperature of the secondary side is higher for the sensitivity case due to the effect on the metal mass simulated.

Figure 3. Secondary Coolant Average Temperature

The total heat transferred between primary and secondary fluids within the first hour after the core blockage time is shown in Table 1.

Case Total Energy Transferred (BTU)

LTCC EM 1.12 E+10 Sensitivity 6.96 E+10 Simulation results confirm that the energy released to the primary coolant flowing through the SGs U-tubes is higher in the sensitivity case due to the presence of the additional metal masses.

Conclusions A sensitivity case is conducted to study the effects of metal structures in the secondary side of the steam generators.

The mass of the metal structures, the initial temperature, and the location of the heat structures are defined to maximize the energy stored in the secondary side at the core blockage time.

The simulation results confirmed that the effects of the secondary metal structures on the on the PCT is negligible.

[1]. RC09989, RELAP5-3D Steady-State Model, Revision 0.