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Reactor Protection System (RPS)

The reactor protection system monitors plant parameters and trips the reactor when any monitored condition exceeds its safety setpoint. Failure of RPS to trip on demand is ATWS (Anticipated Transient Without Scram); the response is in FR-S.1. The 10 CFR 50.62 AMSAC backup is independent of RPS and is required as an ATWS mitigation.

Function

Generates a reactor trip signal that opens the reactor trip breakers, de-energizes the rod-control magnetic jacks, and drops all control rods into the core under gravity within ~2.2 seconds. Trip signals come from ~20 different parameter channels organized in 2-of-4 coincidence logic.

Components

flowchart LR
    NIS["Nuclear Instrumentation<br/>«NIS-PR-AVG» / «NIS-IR» / «NIS-SR»"]
    RCS_P["RCS Pressure<br/>«PT-455»"]
    RCS_T["RCS Temperature<br/>«TE-411-HOT»"]
    SG_LVL["SG Levels<br/>«SG-A-LVL-NR» etc."]
    OTHER["Other trips<br/>(turbine, ESF, manual)"]
    BISTABLE["Bistable Logic<br/>(2-of-4 coincidence<br/>per parameter)"]
    PB["Manual Trip<br/>«RT-PB»"]
    BKR_A["Trip Breaker A<br/>«TRIP-BKR-A»"]
    BKR_B["Trip Breaker B<br/>«TRIP-BKR-B»"]
    ROD_PWR["Rod Magnetic Jacks<br/>(motor-generator set)"]
    RODS["Control Rods<br/>«ROD-POS-AVG»"]

    NIS & RCS_P & RCS_T & SG_LVL & OTHER --> BISTABLE
    BISTABLE --> BKR_A & BKR_B
    PB --> BKR_A & BKR_B
    BKR_A & BKR_B -- "OPEN" --> ROD_PWR
    ROD_PWR -- "de-energized" --> RODS
    RODS -- "fall under gravity" --> CORE["Core (subcritical)"]

    classDef logic stroke:#856404,stroke-width:2px
    classDef breaker stroke:#943126,stroke-width:2px
    class BISTABLE logic
    class BKR_A,BKR_B breaker
  • Reactor trip breakers — «TRIP-BKR-A», «TRIP-BKR-B»; two in series in the rod-control power feed. Opening either de-energizes the rod magnetic jacks.
  • Bistable logic cabinets — 4 redundant trains; each generates trip signals from sensor inputs.
  • Manual trip pushbuttons — «RT-PB»; two on the main control board.
  • Rod-control magnetic jacks — energized continuously during operation; de-energizing drops the rods.
  • Source-range / intermediate-range / power-range neutron detectors — «NIS-SR», «NIS-IR», «NIS-PR-AVG».

Instrumentation

  • Trip breaker positions: «TRIP-BKR-A», «TRIP-BKR-B»
  • Manual trip pushbutton state: «RT-PB»
  • Reactor power (4-channel power-range): «NIS-PR-AVG»
  • Intermediate range: «NIS-IR»
  • Source range count rate: «NIS-SR»
  • Average rod bottom position: «ROD-POS-AVG»

Setpoints

Trip parameter Setpoint Source
High power (Power range) 109% rated Vogtle Tech Spec 3.3.1
Overtemperature ΔT (OTΔT) f(T_avg, P_RCS) Vogtle Tech Spec 3.3.1
Overpower ΔT (OPΔT) f(T_avg) Vogtle Tech Spec 3.3.1
Low RCS flow 90% nominal in 2/4 loops Vogtle Tech Spec 3.3.1
Low pressurizer pressure 1865 psig Vogtle Tech Spec 3.3.1
Low-low SG level ~17% NR on 2/4 SGs Vogtle Tech Spec 3.3.1
Manual trip depress both «RT-PB» always available
Rod drop response ≤ 2.2 s to full insertion Vogtle UFSAR §15.4

Normal alignment

  • All bistable cabinets in service; trip-permissive logic at power
  • Both trip breakers CLOSED, rod-control power energized
  • Manual trip pushbuttons enabled (not bypassed)
  • Source range high-flux trip bypassed above P-6 (intermediate-range permissive); intermediate-range trip bypassed above P-10 (power-range permissive)

Failure modes

  • Failure to trip on demand (ATWS) — bistables fail OR breakers fail to open. Response FR-S.1; AMSAC provides backup turbine trip + AFW initiation on independent logic.
  • Spurious trip — single-channel failure exceeding bistable threshold without real plant condition. Two-of-four coincidence reduces but doesn't eliminate spurious trips.
  • Loss of shutdown margin (post-trip) — boron dilution or xenon decay drives reactor toward criticality despite all rods inserted. Response FR-S.2.
  • Inadvertent rod insertion — rod-control system malfunction. Can challenge axial flux distribution; managed by Operations procedures (not an EOP).

References

  • Vogtle UFSAR §7.2 (Reactor Trip System)
  • Vogtle Tech Spec 3.3.1 (RPS Instrumentation)
  • Vogtle UFSAR §15.4.9 (ATWS analysis)
  • 10 CFR 50.62 (AMSAC ATWS mitigation requirements)
  • NRC Westinghouse-style Technology Systems Manual, Chapter 12 (RPS)