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Auxiliary Feedwater System (AFW)

The AFW system delivers feedwater to the steam generators after a reactor trip when main feedwater is unavailable or insufficient. AFW is the secondary-side equivalent of ECCS — it preserves the heat sink during emergencies. The system is the only feedwater source under station blackout (ECA-0.0) because the turbine-driven pump runs on SG steam and DC controls.

Function

Maintains SG inventory above the low-low setpoint following main feedwater isolation, full-load rejection, or station blackout. Provides the heat-removal path that lets RCS decay heat reach the atmosphere via SG → MSL → condenser-or-ARV. AFW is also exercised during normal plant startup and shutdown when feedwater pumps are not yet/no longer sufficient.

Per Vogtle UFSAR §10.4.9 design-basis, AFW capacity is sized to remove decay heat with a single pump operating (single-failure assumption).

Components

flowchart LR
    CST["Condensate Storage Tank<br/>«CST-LVL»"]
    SW["Service Water<br/>(backup suction)"]
    MD_A(("«AFW-PUMP-A»<br/>motor-driven<br/>~700 gpm"))
    MD_B(("«AFW-PUMP-B»<br/>motor-driven<br/>~700 gpm"))
    TD(("«AFW-PUMP-T»<br/>turbine-driven<br/>~700 gpm<br/>steam-supplied"))
    CV_A["«AFW-A-CV»"]
    CV_B["«AFW-B-CV»"]
    CV_C["«AFW-C-CV»"]
    CV_D["«AFW-D-CV»"]
    SG_A["SG-A"]
    SG_B["SG-B"]
    SG_C["SG-C"]
    SG_D["SG-D"]
    MS["Main Steam Header<br/>(TDAFW steam supply)"]

    CST --> MD_A & MD_B & TD
    SW -. "backup" .-> MD_A & MD_B
    MS -. "drives turbine" .-> TD
    MD_A --> CV_A & CV_B
    MD_B --> CV_C & CV_D
    TD --> CV_A & CV_B & CV_C & CV_D
    CV_A --> SG_A
    CV_B --> SG_B
    CV_C --> SG_C
    CV_D --> SG_D

    classDef pump stroke:#1f618d,stroke-width:2px
    classDef tank stroke:#856404,stroke-width:2px
    class MD_A,MD_B,TD pump
    class CST,SW tank
  • Two motor-driven pumps — «AFW-PUMP-A», «AFW-PUMP-B»; AC-powered from emergency 4kV buses; each can feed any two SGs at ~700 gpm.
  • One turbine-driven pump (TDAFW) — «AFW-PUMP-T»; steam-driven from the main steam header (any unisolated SG); can feed all four SGs at ~700 gpm. Survives station blackout because DC controls only.
  • Four AFW control valves — «AFW-A-CV»..«AFW-D-CV»; one per SG; air-operated, fail-open on loss of air.
  • Condensate storage tank (CST) — «CST-LVL»; primary AFW suction source, ~300,000 gal usable.
  • Service-water cross-tie — backup suction for the motor-driven pumps if CST exhausted.

Instrumentation

  • Pump statuses: «AFW-PUMP-A», «AFW-PUMP-B», «AFW-PUMP-T»
  • TDAFW turbine speed: «TDAFW-SPEED»
  • Aggregate AFW flow: «AFW-FLOW»
  • Per-SG control-valve position: «AFW-A-CV», «AFW-B-CV», «AFW-C-CV», «AFW-D-CV»
  • CST level: «CST-LVL»
  • SG narrow-range levels (target indicator): «SG-A-LVL-NR» / «SG-B-LVL-NR» / «SG-C-LVL-NR» / «SG-D-LVL-NR»

Setpoints

Parameter Value Source
AFW auto-start (Low SG level on 2/4 SGs) Low-low SG level «SG-A-LVL-NR» < ~17% NR Vogtle Tech Spec 3.3.2
AFW auto-start (other actuations) SI signal, AMSAC ATWS mitigation, blackout Vogtle UFSAR §10.4.9
TDAFW minimum SG steam supply ~150 psig (turbine min driving pressure) Vogtle UFSAR §10.4.9
CST minimum (Tech Spec LCO) ~70% indicated (~200,000 gal usable) Vogtle Tech Spec 3.7.6
SG no-load level setpoint ~50% NR Vogtle UFSAR §10.3

Normal alignment

  • All three pumps standby; control valves throttled to balance AFW flow with main-feedwater control during low-power operations
  • TDAFW steam-supply valves OPEN (turbine ready to receive steam on demand)
  • CST level >70% (LCO compliance)
  • AFW automatically actuates on SI, low-low SG level, or AMSAC ATWS signal

Failure modes

  • Total loss of feedwater — all AFW pumps unavailable AND main FW isolated. RED-path heat-sink response is FR-H.1; bleed-and-feed via PORVs and HHSI becomes the cooling method.
  • TDAFW failure during station blackout — only available pump under SBO is the turbine-driven one; if that fails, ECA-0.0 cannot maintain the heat sink and the response degrades into FR-H.1 / FR-C.1 territory.
  • SG overfeed by AFW — AFW control valve stuck open or AFW pump speed governor failed. YELLOW-path response is FR-H.3.
  • SG underfeed (low SG level despite AFW running) — control valve stuck closed, pump trip during demand, CST suction loss. YELLOW-path response is FR-H.5; escalates to FR-H.1 if total feedwater is lost.
  • CST depletion in extended event — service-water cross-tie or alternate suction must be aligned per the plant's loss-of-feedwater procedures.

References

  • Vogtle UFSAR §10.4.9 (Auxiliary Feedwater System)
  • Vogtle UFSAR §10.4.7 (Main feedwater — context for AFW interface)
  • Vogtle UFSAR §15.2.7 (Loss of normal feedwater analysis)
  • Vogtle Tech Spec 3.3.2 (AFW actuation logic)
  • Vogtle Tech Spec 3.7.6 (CST level LCO)
  • 10 CFR 50.62 (AMSAC ATWS mitigation requirements, drives AFW auto-actuation logic)