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Engineered Safety Features (ESF) Actuation

The ESF actuation system is the logic layer that aggregates accident- detection signals (low pressurizer pressure, high containment pressure, manual actuation, etc.) into actuated functions: SI, Phase A containment isolation, Phase B containment isolation, containment spray, MSLI, FWI, AFW initiation, and DG start. ESF outputs are latched signals that hold their outputs until manually reset by the operator.

Function

Generates latched actuation signals on 2-of-4 coincidence logic from sensor inputs. Each ESF signal drives a specific set of components to their safety-protective positions:

  • Safety injection (SI) «SI-SIG»: starts HHSI / charging in SI alignment / accumulator isolation valves OPEN / RHR ready
  • Phase A containment isolation: closes non-essential containment penetrations (ventilation, sample lines, instrument air)
  • Phase B containment isolation «PHASE-B-SIG»: closes all containment penetrations not required for accident mitigation
  • Containment spray: starts spray pumps + sodium hydroxide additive
  • Main Steam Line Isolation (MSLI): closes all MSIVs
  • Feedwater Isolation (FWI): closes main feedwater isolation valves
  • AFW Actuation: starts MD-AFW + TDAFW; opens AFW control valves
  • Emergency Diesel Generator (DG) start: from undervoltage or SI signal

Components

flowchart LR
    SENSORS["Sensor Inputs<br/>«PT-455» / «CTMT-PR» /<br/>«SG-A-LVL-NR» / etc."]
    MANUAL["Manual Initiation<br/>(operator pushbuttons)"]
    BISTABLE["2-of-4 Coincidence<br/>Logic Cabinets<br/>(4 redundant trains)"]
    LATCH["Latched Output<br/>(holds until reset)"]
    SI["SI Actuation «SI-SIG»"]
    PA["Phase A Isolation"]
    PB["Phase B Isolation «PHASE-B-SIG»"]
    SPRAY["Containment Spray"]
    MSLI["MSLI"]
    FWI["FWI"]
    AFW["AFW Actuation"]
    DG["DG Start"]

    SENSORS & MANUAL --> BISTABLE
    BISTABLE --> LATCH
    LATCH --> SI & PA & PB & SPRAY & MSLI & FWI & AFW & DG

    classDef logic stroke:#856404,stroke-width:2px
    classDef out stroke:#1f618d,stroke-width:2px
    class BISTABLE,LATCH logic
    class SI,PA,PB,SPRAY,MSLI,FWI,AFW,DG out
  • Solid-state protection system (SSPS) — Westinghouse-style logic cabinets implementing the coincidence logic. Four redundant trains.
  • Manual ESF actuation pushbuttons — main control board; can initiate each ESF function independently.
  • Master / Slave relays — drive component-level actions per ESF signal.

Setpoints (actuation thresholds)

Signal Setpoint Source
SI on low pressurizer pressure «PT-455» < 1815 psig Vogtle Tech Spec 3.3.2
SI on high containment pressure «CTMT-PR» > 3 psig Vogtle Tech Spec 3.3.2
SI on low steam-line pressure «SG-A-PR» < 600 psig (2/4 lines) Vogtle Tech Spec 3.3.2
Phase A on SI automatic with SI Vogtle UFSAR §6.2
Phase B on high-high containment «CTMT-PR» > 17 psig Vogtle Tech Spec 3.3.2
Containment spray Same as Phase B Vogtle Tech Spec 3.3.2
MSLI on high steam flow + low Tavg high flow on 1/4 line + low Tavg Vogtle Tech Spec 3.3.2
MSLI on low SG pressure < 600 psig (2/4 lines) Vogtle Tech Spec 3.3.2
FWI on SI automatic with SI Vogtle UFSAR §10.4.7
AFW on low-low SG level < 17% NR on 2/4 SGs Vogtle Tech Spec 3.3.2
DG start on undervoltage bus voltage < 75% nominal Vogtle Tech Spec 3.3.5

Normal alignment

  • All bistable logic cabinets in service
  • No ESF signals latched (all actuated outputs reset)
  • Manual ESF pushbuttons available
  • Master/slave relays in standby

Failure modes

  • Single-channel failure — 2-of-4 logic tolerates one bad channel; reduces to 1-of-3 for that signal
  • ESF actuation when not needed (spurious) — common cause: sensor drift; can drive plant into accident response (SI initiation, containment isolation)
  • Failure to actuate on demand — drives EOPs to verify automatic actions at E-0 steps 4 (SI), 6 (Phase A), 7 (MSL isolation), 8 (FWI); manual initiation is the backup at each
  • Reset confusion — ESF outputs latch until manually reset; premature reset before conditions clear can cause re-actuation

References

  • Vogtle UFSAR §7.3 (Engineered Safety Features Actuation System)
  • Vogtle UFSAR §6.2 (Containment isolation logic)
  • Vogtle Tech Spec 3.3.2 (ESF Instrumentation)
  • NRC Westinghouse-style Technology Systems Manual, Chapter 14 (ESF)