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Main Steam System (MSS)

The main steam system carries steam from the four SG outlets to the main turbine; under accident conditions it carries steam to the condenser dump valves (for cooldown), to atmospheric reliefs (when condenser is unavailable), and to the TDAFW pump turbine. The MSIVs are the boundary between containment-isolated and unisolated states of the secondary side.

Function

In normal operation: delivers ~14 million lb/hr of steam at ~1010 psig (no-load) / 770 psig (full-load) to the main turbine. In emergency operation: provides controlled heat removal from the RCS via SG steam release to condenser, ARVs, or safety valves; carries TDAFW driving steam under station blackout.

Components

flowchart LR
    SG_A["SG-A"]
    SG_B["SG-B"]
    SG_C["SG-C"]
    SG_D["SG-D"]
    MSIV_A["MSIV «MSIV-A»"]
    MSIV_B["MSIV «MSIV-B»"]
    MSIV_C["MSIV «MSIV-C»"]
    MSIV_D["MSIV «MSIV-D»"]
    ARV_A["ARV «ARV-A»"]
    ARV_B["ARV «ARV-B»"]
    ARV_C["ARV «ARV-C»"]
    ARV_D["ARV «ARV-D»"]
    SAF["Safety Valves<br/>(3 per SG)"]
    HEADER["Main Steam Header"]
    TURB["Main Turbine"]
    COND["Condenser<br/>(steam dump)"]
    TDAFW["TDAFW «AFW-PUMP-T»"]

    SG_A --> ARV_A
    SG_A --> MSIV_A
    SG_B --> ARV_B
    SG_B --> MSIV_B
    SG_C --> ARV_C
    SG_C --> MSIV_C
    SG_D --> ARV_D
    SG_D --> MSIV_D
    SG_A & SG_B & SG_C & SG_D -. "if MSIV open" .- HEADER
    HEADER --> TURB
    HEADER -- "steam dump" --> COND
    HEADER -. "steam supply" .-> TDAFW
    SG_A & SG_B & SG_C & SG_D --> SAF

    ARV_A -- "atmospheric" --> ATM["Atmosphere"]
    ARV_B --> ATM
    ARV_C --> ATM
    ARV_D --> ATM
    SAF --> ATM

    classDef sg stroke:#1f618d,stroke-width:2px
    classDef relief stroke:#943126,stroke-width:2px
    class SG_A,SG_B,SG_C,SG_D sg
    class SAF,ARV_A,ARV_B,ARV_C,ARV_D relief
  • Four steam generators (SGs) — see rcs.
  • Four MSIVs — «MSIV-A»..«MSIV-D»; close on Main Steam Line Isolation (MSLI) signal; air-operated, fail-closed.
  • Four atmospheric relief valves (ARVs) — «ARV-A»..«ARV-D»; nitrogen-operated, fail-closed. Modulate for cooldown control when condenser is unavailable.
  • Twelve safety valves — three per SG; spring-loaded, lift at staggered setpoints from ~1085 psig (Vogtle Tech Spec 3.7.1).
  • Main steam header — connects all four SGs upstream of the MSIVs; steam-dump and TDAFW supplies tee off the header.
  • Condenser steam dump — turbine-bypass valves modulate steam to the condenser under load rejection or cooldown.

Instrumentation

  • Per-SG steam pressure: «SG-A-PR», «SG-B-PR», «SG-C-PR», «SG-D-PR»
  • Main steam header pressure: «MS-HEADER-PR»
  • N-16 monitors (SGTR detection): «SG-A-N16», «SG-B-N16», «SG-C-N16», «SG-D-N16»
  • Per-MSIV position: «MSIV-A», «MSIV-B», «MSIV-C», «MSIV-D»
  • Per-ARV position: «ARV-A», «ARV-B», «ARV-C», «ARV-D»
  • ARV nitrogen-supply pressure: «N2-SUPPLY»
  • Steam-dump availability: «STEAM-DUMP»
  • Condenser vacuum: «CONDENSER-VAC»
  • Condenser air-ejector activity: «AEJ-RAD»

Setpoints

Parameter Value Source
Safety valve lift (lowest setpoint) 1085 psig Vogtle Tech Spec 3.7.1
MSLI trip Low steam-line pressure 600 psig Vogtle Tech Spec 3.3.2
ARV control band (no-load) ~1010 psig Vogtle UFSAR §10.3.2
MSL break flow trip High steam flow + low Tavg Vogtle Tech Spec 3.3.2
Tech Spec cooldown rate ≤100 °F/hr below 350 °F Vogtle Tech Spec 3.4.3

Normal alignment

  • All four MSIVs OPEN; steam flowing to turbine through bypass valves during startup, through turbine-stop valves at power
  • ARVs CLOSED; nitrogen supply pressurized
  • Condenser vacuum drawn (~26-28 inHg), steam-dump valves available
  • Safety valves on standby (lift only at setpoint)

Failure modes

  • Faulted SG (steam line break, FW line break inside containment) — rapid SG depressurization on a single SG. Diagnosed at E-0 step 15; managed by E-2.
  • Uncontrolled depressurization of all SGs — main steam header break upstream of MSIVs, common-cause MSIV failure. Response ECA-2.1.
  • SG overpressure (relief failed) — ARV unresponsive AND safety-valve stuck. Response FR-H.2.
  • Loss of condenser dump — condenser vacuum lost, dump valves isolated. ARV becomes the only release path. Response FR-H.4.
  • MSIV will not close on demand — strong indicator of faulted SG (per E-0 step 7 Caution). E-2 isolation path may be impaired.
  • SGTR — N-16 monitor elevated on the ruptured SG; main steam flow carries fission gases. Response E-3.

References

  • Vogtle UFSAR §10.3 (Main Steam Supply System)
  • Vogtle UFSAR §10.3.2 (MSIVs and ARVs)
  • Vogtle UFSAR §10.4.4 (Condenser steam dump)
  • Vogtle Tech Spec 3.3.2 (MSLI logic), 3.7.1 (safety-valve setpoints), 3.4.3 (cooldown rate limit)
  • Vogtle UFSAR §15.1.5 (Steam-system piping failure analysis)