ML21067A761

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Pre-Submittal Meeting to Revise Technical Specification 3.7.2, Main Steam Isolation Valves
ML21067A761
Person / Time
Site: Vogtle  Southern Nuclear icon.png
Issue date: 03/05/2021
From:
Southern Nuclear Company
To:
Plant Licensing Branch II
Lamb J
References
Download: ML21067A761 (30)
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Text

Vogtle Pre-Submittal Meeting to Revise Technical Specification 3.7.2, Main Steam Isolation Valves March 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
  • 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
  • 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
  • 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

Current Technical Specification LCO 3.7.2

  • The MSIV system consists of an MSIV and associated bypass valve 9

Current Technical Specification 10

Proposed Design Proposed Design

  • Retain the existing inboard MSIVs - one per steam line
  • Bidirectional gate valve
  • Determine method of eliminating existing outboard MSIV function
  • 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 12

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 Train B 2 inch Function 29.5 inch eliminated To Turbine 26 inch MSIV MSIV Safety related Vent, drain and other lines not shown for clarity Non-safety related Not to scale 13

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

Proposed Design MSIV MSIVs Function Eliminated 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 15

Proposed Technical Specification

  • Changed to remove the requirement for two MSIV systems per steam line
  • One MSIV per steam line is proposed
  • Required Actions and Surveillance Requirements updated accordingly to reflect design change

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 and installation is not changed
  • Two redundant train-oriented steam line isolation signals (SLI-A, SLI-B) are initiated upon receipt of any of the following signals
1. High steam line pressure rate
2. Low steam line pressure
3. Containment high-2 pressure
4. Manual actuation
  • Actuation signals are not changed
  • Actuators are being replaced with new design system media actuators
  • Evaluate any impact of valve closure timing due to change in actuators 18

Proposed Actuator Design

  • System Media Actuated
  • Dual Solenoid
  • Train Separated 19

Single Failure

  • 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

Safety Analysis Chapter 15 and Chapter 6 analyses will be evaluated/investigated to determine impact (if any) 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
  • Impact to reactor response analysis
  • Bounded by current single failure of a train of ESF for boration and stuck highest worth RCCA
  • Impact to radiological analyses resulting from secondary side transients
  • Bounded by current single failure assumption of stuck open ARV
  • SGTR MTO being considered for re-evaluation
  • Impact to environmental qualification of equipment 21

Seismic/ Pipe Rupture

  • Confirm the main steam piping and support system will continue to withstand the dynamic effects of
1. quick valve closure of the MSIVs and turbine stop valves, and
2. the reaction forces of safety, atmospheric, and steam dump valves
  • Determine if the analyses for HELBs outside of containment are affected
  • Determine impacts to pipe stress, pipe support, civil, and seismic calculations
  • Evaluate the need for new supports and structural evaluations for new enclosures and conduits 22

Mechanical Mechanical systems review is needed to determine

  • Impacts to HVAC/heat gain due to addition of electrical components/panels
  • Impacts to Fire Protection/combustibles due to new material/components
  • Impacts to ISI/IST program 23

Containment Isolation Current Licensing Basis

  • 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
  • No changes to the licensing basis for containment isolation are needed 24

Power Supplies

  • Impacts of the change in the actuator will be evaluated
  • Impacts to 125-volt DC battery and battery chargers for providing power to solenoids
  • Impact of valve closure timing due to addition of control circuit surge protection devices 25

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
  • These devices are independent of the electronic flow control logic
  • A reactor trip is one of the actuation signals for the turbine stop valves
  • 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 26

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 27

Milestones Milestones

  • Submit LAR by end of 3Q2021
  • Request NRC approval prior to Vogtle 2R22 (begins 3/5/2022)
  • Implement MSIV change during Vogtle 2R22 and 1R24 (begins 3/12/2023) 29

Questions?

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