ML21230A284

SNC Slides - Vogtle MSIV Elimination LAR - Pre-submittal Meeting Part 2 - 08-25-2021

ML21230A284
Person / Time
Site:	Vogtle
Issue date:	08/18/2021
From:	Southern Nuclear Operating Co
To:	NRC/NRR/DORL/LPL2-1
Lamb J, NRR/DORL/LPL2-1, 415-3100
Shared Package
ML21230A287	List: ML21230A284
References
EPID L-2021-LRM-0084
Download: ML21230A284 (39)

Text

Vogtle Pre-Submittal Meeting (Part 2) to Revise Technical Specification 3.7.2, Main Steam Isolation Valves August 2021

Meeting Purpose and Agenda The purpose for this meeting is to discuss proposed amendment request to revise Technical Specification 3.7.2 to remove one of the two MSIV systems from the Limiting Condition for Operation This meeting will cover the following topics:

• Current Design

• Proposed Design

• Proposed Technical Specification

• Technical Topics

• PRA Risk Insights

• Milestones 2

Issue Problem Statement There have been 6 trips at Vogtle since 2012 due to the inadvertent closure of one MSIV in a steam line Proposed Resolution Change the licensing basis and design from 2 MSIV systems per steam line to 1 MSIV system per steam line

• Reduces number of components that could cause inadvertent closure of MSIVs

• Reduces single point vulnerabilities

• Consistent with other PWRs 3

MSIV Safety Function The function of the main steam isolation system is to limit blowdown to 1 steam generator in the event of a steam line break in order to A. Limit the related effect upon the reactor core within specified fuel design limits B. Limit containment pressure to a value less than 90 percent of design pressure The isolation system provides positive shutoff with minimum leakage during postulated line severance conditions either upstream or downstream of the valves Safety function will be preserved 4

Current Design Current Design The main steam line isolation valves, MSIV bypass valves, and piping are designed to prevent uncontrolled blowdown from more than one steam generator

• For main steam line breaks upstream of an isolation valve, uncontrolled blowdown from more than one steam generator is prevented by the isolation valves in the unaffected steam lines and by the isolation valve in the affected line

• For main steam line breaks downstream of an isolation valve, blowdown from more than one steam generator is prevented by the main steam isolation valves on each main steam line

• Each MSIV and MSIV bypass valve in a steam line is actuated from a separate actuation train (SLI-A or SLI-B) 6

Current Design

• Each MSIV is a bidirectional gate valve composed of a valve body which is welded into the system pipeline

• Positive sealing can be maintained in either direction

• The MSIV bypass valves are used when the MSIVs are closed to permit warming of the main steam lines prior to startup

• The bypass valves are air-operated globe valves

• For emergency closure, the valve solenoid, when deenergized, will result in valve closure

• Electrical solenoids are energized from a separate Class IE source 7

Current Design SLI Train A SLI Train B 4 inch 4 inch 4 inch MSIV Bypass MSIV Bypass From Steam Generator LO SLI Train A SLI Train B 2 inch To Turbine 29.5 inch 26 inch MSIV MSIV Vent, drain and other lines not shown for clarity Safety related Not to scale Non-safety related 8

Proposed Design Proposed Design

• Retain the existing inboard MSIVs - one per steam line

• Bidirectional gate valve

• Eliminate existing outboard MSIV function

• Retain outboard valve with capability for manual closure

• Outboard valve will not receive SLI signal

• Retain both existing MSIV bypass valves

• Change actuation of MSIV closure to include both actuation trains on each inboard MSIV

• MSIV bypass actuation does not change, as both valves remain in the steam line 10

Proposed Design SLI Train A SLI Train B 4 inch 4 inch 4 inch MSIV Bypass MSIV Bypass From Steam Generator LC SLI SLI Train A Manual Train B 2 inch Closure Capability 29.5 inch To Turbine 26 inch NO MSIV Safety related Vent, drain and other lines not shown for clarity Non-safety related Not to scale 11

Future Licensing Basis

• SRP 10.3 - Assure that, in the event of a postulated break in a main steam line in a PWR plant, the design will preclude the blowdown of more than one steam generator, assuming a concurrent single active component failure

• In this regard, all main steam shutoff valves downstream of the MSIVs, the turbine stop valves, and the control valves are considered to be functional

• Each MSIV actuator is designed to accomplish its function with a single active component failure 12

Proposed Design Function Eliminated MSIVs Manual Closure Turbine Stop Valves From Steam Generators To Turbine MSIV Bypass Valves To Condenser From Steam Generators To Turbine Turbine Bypass Valves Safety related Vent, drain and other lines not shown for clarity Not to scale Non-safety related 13

Current Technical Specification TS Bases 3.7.2 The MSIV system consists of an MSIV and associated bypass valve 14

Proposed Technical Specification

• LCO changed to replace two MSIV systems per steam line with four MSIVs and associated actuators and bypass valves - reflects one MSIV per steam line (four steam lines total)

• Current Required Actions B and D deleted

• Not needed with only one MSIV per steam line

• Added actuator Conditions in accordance with TSTF-504

• Added bypass valve Condition

• Existing Surveillance Requirement updated to reflect LCO

• Added Surveillance Requirement for actuators

• Changes generally consistent with NUREG-1431 and TSTF-504 15

Proposed Technical Specification 16

Proposed Technical Specification 17

Proposed Technical Specification 18

Proposed Technical Specification 19

Proposed Technical Specification 20

Proposed Technical Specification 21

Proposed Technical Specification 22

Technical Topics Valve/Actuator Design

• Each MSIV is a bidirectional gate valve composed of a valve body which is welded into the system pipeline

• MSIV design function is not changed

• Actuators are being replaced with new design system media actuators

• Two redundant train-oriented steam line isolation signals (SLI-A, SLI-B) are initiated upon receipt of any of the following signals

• High steam line pressure rate

• Low steam line pressure

• Containment high-2 pressure

• Manual actuation

• Actuation signals are not changed 24

New Actuator Design

• System Media Actuated

• Dual Solenoid

• Train Separated 25

Single Failure

• No single active component failure will result in the failure of more than one main steam isolation valve to operate

• MSIV actuator closes valve assuming a single failure

• Dual paths to supply steam for closure

• Train A and B closure signal to actuator

• As with current design, signals are generated from separate instrumentation and powered by separate instrumentation control systems

• New design provides safety function assuming single active failure

• New design credits operation of turbine stop valves, turbine bypass valves to limit blowdown to only one steam generator 26

Safety Analysis Chapter 6 and Chapter 15 analyses have been evaluated/investigated to determine impact of the removal of the closure function of the outboard MSIV, specifically:

• Impact to mass and energy release and associated containment response

• Reverse flow into containment due to single MSIV failure in ruptured loop

• Small increases in peak containment temperature and pressure - FSAR to be updated to reflect updated analysis

• Impact to reactor response analysis

• No impact

• Impact to radiological analyses resulting from secondary side transients

• Bounded by current single failure assumption of stuck open atmospheric relief valve (ARV)

• SGTR Margin To Overfill (MTO) Analysis

• Bounded by current single failure assumption of stuck open ARV

• Impact to environmental qualification of equipment

• Increases in peak containment P/T are being addressed for impacted equipment 27

Seismic/ Pipe Rupture

• Confirm the main steam piping and support system will continue to withstand the dynamic effects of:

• Quick valve closure of the MSIVs and turbine stop valves, and

• Reaction forces of safety, atmospheric, and steam dump valves

• The piping containing the MSIVs qualifies as No Break Zone piping both before and after MSIV actuator replacement

• Pipe stress and pipe supports are within allowable load limits

• Seismic Category I design and evaluations complete for new enclosures and conduits 28

Mechanical Mechanical systems review determined:

• HVAC has margin to accommodate heat gain due to addition of electrical components/panels

• Combustible load of affected fire areas/zones due to new material/components remain with allowable limits

• ISI/IST program to complete 10 CFR 50.55a evaluation for ISI/IST plan changes 29

Containment Isolation Current Licensing Basis

• The containment penetrations associated with the secondary side of the steam generators are not subject to General Design Criterion 57

• The valves associated with these penetrations do not receive a containment isolation signal and are not credited with effecting containment isolation in the safety analyses

• The barriers against fission product release to the environment are the steam generator tubes and the piping associated with the steam generators

• No changes to the licensing basis for containment isolation are needed 30

Power Supplies

• Redundant power supplies and power trains operate the MSIVs and MSIV bypass valves to isolate safety and non-safety related portions of the system

• Impacts of the change in the actuator were evaluated

• Battery chargers and 125-volt DC battery have adequate margin to accommodate the increase in load for providing power to solenoids

• Control circuit surge protection devices add a short time delay prior to valve stroke which is accounted for in the valve closure time curve 31

Secondary System Performance - Turbine Stop Valves

• The flow of the main steam entering the high-pressure turbine is controlled by four stop valves and four governing control valves

• Each 28-in. stop valve is controlled by an electrohydraulic actuator, so that the stop valve is either fully open or fully closed

• The function of the stop valves is to shut off the steam flow to the turbine, when required

• The stop valves are closed within 0.3 s by actuation of the emergency trip system devices which are independent of the electronic flow control logic

• A reactor trip is one of the actuation signals for the turbine stop valves and is generated following a main steam line break

• The turbine stop valves are closed when the Unit is in Mode 2 or below

• The turbine stop valves are designed to close against full steam pressure supplied to the turbine 32

Secondary System Performance - Turbine Bypass Valves

• The system consists of a manifold connected to the main steam lines upstream of the turbine stop valves and of lines from the manifold with regulating valves to each condenser shell

• The system is designed to directly bypass 40 percent of the valve-wide-open main steam flow to the condenser

• The turbine bypass valves fail in the closed position on loss of instrument air or electrical supply to a valve

• The valves are capable of going from the fully open to the fully closed position within 5 sec after de-energization of the solenoid valves over the pressure range of 100 to 1185 psig

• Position switches are provided on the bypass valves, and fully open and fully closed position signals are transmitted to the main control room 33

PRA Risk Insights PRA Risk Insights

• Not a risk-informed LAR, will include PRA insights consistent with SRP 19.2 Appendix D

• Modification will be assessed per model maintenance procedures for consideration of immediate update or scheduled update

• LAR will include a summary of design change risk significance (i.e., delta CDF and delta LERF associated with current MSIV configuration versus future modified MSIV configuration), and RICT impact

• internal events (<1% increase in CDF & LERF)

• internal flooding (<0.1% increase in CDF & LERF)

• fire (<0.1% increase in CDF & LERF)

• seismic (2.2% increase in CDF, 4% increase in LERF) 35

PRA Risk Insights LAR will include information supporting RICT option for one MSIV inoperable in MODE 1 condition (current TS has RICT option for one or more steam line with one MSIV system inoperable in MODE 1) for example:

• PRA success criteria, comparison with Design Basis success criteria

• Sample RICT calculations using conceptual impact model

• Total estimated CDF and LERF values from conceptual impact model

• Review for impact on seismic penalty factor

• Review for impact on analyses of external hazards

• Review for impact on uncertainty analysis 36

Milestones Milestones

• Submit LAR by end of 3Q2021

• NRC approval requested 12 months after submittal

• Implement during Vogtle 1R24 (Spring 2023) and 2R23 (Fall 2023) 38

Questions?