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Residual Heat Removal System (RHR)

RHR has two distinct operating modes: (1) shutdown cooling in Modes 4-5, removing decay heat from the RCS at low pressure to take the plant to refueling temperature; and (2) low-head safety injection (LHSI) during a LOCA, including the long-term cold-leg and hot-leg recirculation path after RWST depletion.

Function

In shutdown cooling: suction from one hot leg, discharge through RHR heat exchanger (cooled by component cooling water), return to cold legs. Brings the RCS from RHR cut-in conditions (Mode 4, ~350 °F, ~390 psig) down to Mode 5 conditions for refueling.

In LHSI / recirculation: provides low-pressure ECCS flow from the RWST initially, then from the containment sump after ES-1.3 recirculation transfer; in piggyback alignment supplies high-head SI pump suction during long-term recirculation.

Components

flowchart LR
    HOT["Hot Leg<br/>(Loop 1 or 3)"]
    SUCT_VLV["Suction Isolation<br/>(motor-operated)"]
    PUMP_A(("«RHR-PUMP-A»"))
    PUMP_B(("«RHR-PUMP-B»"))
    HX_A["RHR HX A<br/>(CCW-cooled)<br/>«RHR-HX»"]
    HX_B["RHR HX B"]
    CL["Cold Leg<br/>(injection or<br/>shutdown return)"]
    HL_INJ["Hot-Leg Injection<br/>«HL-INJECT-A»"]
    RWST["RWST<br/>«RWST-LVL»"]
    SUMP["Containment Sump"]
    HHSI[("HHSI Pump<br/>(piggyback)")]

    HOT -- "Mode 4-5<br/>shutdown cooling" --> SUCT_VLV
    SUCT_VLV --> PUMP_A & PUMP_B
    PUMP_A --> HX_A
    PUMP_B --> HX_B
    HX_A & HX_B --> CL
    HX_A & HX_B -. "long-term recirc" .-> HL_INJ
    RWST -. "initial LHSI" .-> PUMP_A & PUMP_B
    SUMP -. "after ES-1.3" .-> PUMP_A & PUMP_B
    PUMP_A -. "piggyback" .-> HHSI

    classDef pump stroke:#1f618d,stroke-width:2px
    classDef vessel stroke:#856404,stroke-width:2px
    class PUMP_A,PUMP_B,HHSI pump
    class RWST,SUMP vessel
  • Two RHR pumps — «RHR-PUMP-A», «RHR-PUMP-B»; centrifugal, ~4500 gpm each at low head.
  • Two RHR heat exchangers — shell-and-tube; tube side carries RCS flow, shell side carries component cooling water (CCW).
  • Hot-leg suction isolation valves — motor-operated, interlock with RCS pressure (cannot open above ~400 psig per Tech Spec to prevent overpressurization of the low-pressure RHR piping).
  • Crossties — to high-head SI suction for piggyback recirculation alignment; to containment-spray header for alternate spray suction in some configurations.

Instrumentation

  • RHR pump statuses: «RHR-PUMP-A», «RHR-PUMP-B»
  • Low-head header flow: «LO-HEAD-FLOW»
  • RHR heat-exchanger outlet temperature (representative): «RHR-HX»
  • Suction-source alignment (RWST vs sump): typically two-valve indication

Setpoints

Parameter Value Source
RHR cut-in temperature (Mode 4) ≤ 350 °F Vogtle Tech Spec 1.0
RHR cut-in pressure ≤ 390 psig Vogtle Tech Spec 3.4.6
Hot-leg suction valve interlock Closes if RCS pressure > 400 psig Vogtle UFSAR §5.4.7
LHSI shutoff head (approximate) 200 psig Vogtle UFSAR §6.3
RHR pump NPSH (sump suction) per pump curve + sump strainer DP Vogtle UFSAR §6.3.2

Normal alignment

In Modes 1-3 (at-power and hot standby):

  • Both RHR pumps standby; suction aligned to RWST
  • Hot-leg suction isolation valves CLOSED (interlock at RCS pressure)
  • Cold-leg discharge valves CLOSED; ready for SI on actuation signal

In Modes 4-5 (shutdown cooling):

  • One or both RHR pumps running; suction on hot leg (loop 1 or 3)
  • Discharge to cold legs through RHR HX
  • CCW supplying HX shell side

Failure modes

  • Recirculation transfer failure — pump trip, sump suction blockage, valve failure. Response ECA-1.1.
  • Loss of RHR in Mode 4-5 — pump trip during shutdown cooling drives RCS heatup; alternate decay-heat removal via SG steam dump may be needed.
  • Sump strainer blockage — debris from LOCA insulation in containment; addresses GSI-191; if DP rises during recirc, pump cavitation risk.
  • Hot-leg suction valve fails to close on RCS repressurization — overpressurizes the low-pressure RHR piping; Tech Spec interlock required.

References

  • Vogtle UFSAR §5.4.7 (Residual Heat Removal System)
  • Vogtle UFSAR §6.3.2 (RHR as LHSI; recirculation sequence)
  • Vogtle Tech Spec 3.4.6 (RHR loops — Mode 4)
  • Vogtle Tech Spec 3.5.5 (RHR ECCS — Modes 1-3)
  • NRC GSI-191 (sump strainer integrity)