Reactor Coolant System (RCS)¶

The reactor coolant system is the primary pressure boundary and heat- transfer loop between the reactor core and the four steam generators. In normal operation it carries decay heat and fission energy out of the core and into the SGs at high pressure (~2235 psig) and bulk temperature (~547 °F Tavg per Vogtle Tech Spec Mode 3); in emergency operation it is the system that every E-, ES-, ECA-series procedure manages.

Function¶

Transfers reactor decay heat and full-power fission energy from the core to the steam generators while maintaining adequate subcooling margin at the core exit, controlled RCS pressure within the pressure-temperature limit curves, and inventory sufficient to keep the core covered. After a reactor trip the RCS continues to remove decay heat — initially at ~7% of rated thermal power, decaying through ~1% by one hour post-trip per ANS-5.1 decay-heat correlations.

Components¶

Primary-loop topology (one of four identical loops shown explicitly; the other three are abbreviated):

flowchart LR
    RV["Reactor Vessel"]
    PZR["Pressurizer"]
    SG1["SG-A"]
    SG2["SG-B"]
    SG3["SG-C"]
    SG4["SG-D"]
    RCP1(("RCP-1"))
    RCP2(("RCP-2"))
    RCP3(("RCP-3"))
    RCP4(("RCP-4"))

    RV -- "hot leg (T_hot)" --> SG1
    SG1 -- "cold leg (T_cold)" --> RCP1
    RCP1 --> RV
    RV --> SG2
    SG2 --> RCP2 --> RV
    RV --> SG3
    SG3 --> RCP3 --> RV
    RV --> SG4
    SG4 --> RCP4 --> RV

    RV -. "surge line" .-> PZR
    PZR -. "PORVs / safeties" .-> CTMT["Pressurizer Relief Tank → Containment Sump"]

    classDef vessel stroke:#856404,stroke-width:2px
    classDef sg stroke:#1f618d,stroke-width:2px
    class RV,PZR vessel
    class SG1,SG2,SG3,SG4 sg

Reactor vessel — Westinghouse-style 4-loop design, cylindrical carbon-steel shell with stainless-steel cladding on internal surfaces. Vogtle: Combustion-Engineering-fabricated.
Pressurizer — connected to one of the hot legs via a surge line; carries a steam bubble above borated water; controls RCS pressure via heaters, normal/auxiliary spray, and PORVs / safeties.
Four reactor coolant pumps (RCPs) — one per loop, vertical centrifugal, ~88,000 gpm each at rated speed.
Four steam generators (SGs) — vertical U-tube design, ~5,800 tubes each, ~33-ft height; primary side on the tube side, secondary side on the shell side.
Pressurizer relief valves — two power-operated relief valves («PORV-456A», «PORV-456B») with block valves; three spring-loaded safety valves (Vogtle Tech Spec 3.4.10).

Instrumentation¶

Pressurizer pressure: «PT-455» (wide range)
Pressurizer level: «PZR-LVL»
Hot-leg temperatures: «TE-411-HOT» / «TE-421-HOT» / «TE-431-HOT» / «TE-441-HOT»
Cold-leg temperatures: «TE-411-COLD» / «TE-421-COLD» / «TE-431-COLD» / «TE-441-COLD»
RCS Tavg (4-loop average): «TAVG»
Subcooling margin (computed): «SUB-MARGIN»
Core-exit thermocouples: «CET-AVG» (post-TMI NUREG-0737 II.F.2)
Reactor vessel level indication: «RVLS-DYN» (RVLIS dynamic-pressure-compensated channel)

Setpoints¶

Parameter	Value	Source
Normal operating pressure	2235 psig	Vogtle UFSAR §5.1.1
Tavg no-load (Mode 3)	547 °F	Vogtle Tech Spec 1.0
Subcooling margin threshold for RCP trip	30 °F	Vogtle UFSAR §15.6
Cooldown rate limit (≤350 °F)	100 °F/hr	Vogtle Tech Spec 3.4.3
Heatup rate limit	100 °F/hr	Vogtle Tech Spec 3.4.3
Cold shutdown Mode 5	T_cold ≤ 200 °F, P_RCS < 50 psig	Vogtle Tech Spec 1.0

PTS limit curve (P-T limits, heatup and cooldown) is plant- and cycle-specific; for Vogtle see Tech Spec 3.4.3 figures with RTNDT shift for end-of-life vessel embrittlement.

Pressurizer pressure-control feedback (closed-loop in normal operation; PORVs and safeties as overrides):

flowchart LR
    SET["Setpoint<br/>2235 psig"]
    PT["«PT-455»<br/>Pressurizer Pressure"]
    DEC{"Δ = setpoint − actual"}
    HTR["Heaters «PZR-HTR»<br/>(raise pressure)"]
    SPR["Normal Spray<br/>(lower pressure)"]
    PORV["PORVs «PORV-456A»/«456B»<br/>open ≥ 2350 psig"]
    SAF["Safety Valves<br/>lift ≥ 2485 psig"]

    SET --> DEC
    PT --> DEC
    DEC -- "ΔP > 0 (too low)" --> HTR
    DEC -- "ΔP < 0 (too high)" --> SPR
    PT -- "≥ 2350 psig" --> PORV
    PT -- "≥ 2485 psig" --> SAF
    HTR -.-> PT
    SPR -.-> PT

    classDef alarm stroke:#943126,stroke-width:2px
    class PORV,SAF alarm

Normal alignment¶

All four RCPs running (forced convection, ~352,000 gpm total flow)
Pressurizer level controlled at programmed value (~50% at Mode 1, ~25% at Mode 3)
Charging «CHG-FLOW» balanced against letdown «LET-FLOW» (~75 gpm each)
PORVs and block valves: PORVs closed, block valves OPEN (relief path available)
Pressurizer heaters cycling to maintain pressure setpoint

Failure modes¶

Loss-of-coolant accident (LOCA) — break in primary pressure boundary. Diagnosed at E-0 step 4 (LOCA symptoms); managed by E-1. Special cases: ECA-1.1 (recirculation failure), ECA-1.2 (LOCA outside containment).
Steam generator tube rupture (SGTR) — primary-to-secondary leak through SG tube. Diagnosed at E-0 step 16; managed by E-3.
Inadequate core cooling — core exit thermocouples saturated / superheated, vessel level low. Function-restoration response in FR-C.1.
Pressurized thermal shock (PTS) — pressure-temperature trajectory crosses limit curve. Responses FR-P.1 (imminent) and FR-P.2 (anticipated).
RCS overpressure — pressurizer relief valve failures, undetected charging-letdown imbalance, or boron-dilution-driven cold injection with simultaneous heatup. Pressurizer PORVs lift at ~2350 psig, safety valves at ~2485 psig per Vogtle Tech Spec 3.4.10.
Voiding in upper head — natural-circulation flow stagnation, inadequate makeup during depressurization. Response FR-I.3.

References¶

Vogtle UFSAR §5.1 (Reactor Coolant System summary)
Vogtle UFSAR §5.4 (Reactor Coolant System Components — pressurizer, PORVs, RCPs)
Vogtle UFSAR §15.6 (Decrease in Reactor Coolant Inventory)
NRC Westinghouse-style Technology Systems Manual, Chapter 4 (NSSS Overview)
NUREG-0737 II.F.2 (post-TMI instrumentation requirements: CETs + RVLIS)
Todreas & Kazimi, Nuclear Systems Vol. I, Chapter 1 (PWR primary-loop thermal-hydraulics)