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Chemical and Volume Control System (CVCS)

CVCS performs four functions: (1) RCS volume control via charging / letdown; (2) boron concentration control via boric-acid / primary-water makeup blending; (3) RCP seal injection (continuous flow to RCP labyrinth seals); (4) RCS chemistry control via the volume control tank. Charging pumps are also part of the ECCS — they realign on SI to inject from the RWST.

Function

In normal operation: maintains pressurizer level via charging-letdown balance (~75 gpm each direction); maintains RCS boron concentration via blender (mixes boric acid from BAT and primary water from PWST to setpoint).

In emergency operation: charging pumps realign on SI to take suction from RWST and inject into the cold legs at high pressure. Emergency boration alignment used in FR-S.1 / FR-S.2 aligns charging to BAT-only suction for rapid shutdown-margin restoration.

Components

flowchart LR
    RCS["RCS Cold Leg"]
    LET_ISOL["Letdown Isolation<br/>«LET-ISOL»"]
    LET_ORI["Letdown Orifice"]
    LET_HX["Letdown HX<br/>(CCW-cooled)"]
    VCT["Volume Control Tank"]
    PURIF["Purification<br/>(demineralizers)"]
    PW["Primary Water<br/>«PRIMARY-WATER-VALVE»"]
    BAT[("BAT «BAT-LVL»<br/>(boric acid)")]
    BLEND["Blender"]
    CHG_A(("«CHG-PUMP-A»"))
    CHG_B(("«CHG-PUMP-B»"))
    NORMAL["Normal Charging Line"]
    SEAL["RCP Seal Injection<br/>(continuous)"]
    EMERG["Emergency Boration<br/>«BORATE-FLOW»"]
    RWST_ALT["RWST<br/>(SI realignment)"]

    RCS -- "«LET-FLOW»" --> LET_ISOL --> LET_ORI --> LET_HX --> PURIF --> VCT
    BAT --> BLEND
    PW --> BLEND
    BLEND --> VCT
    BAT -. "emergency boration<br/>direct suction" .-> CHG_A
    BAT -. "emergency boration<br/>direct suction" .-> CHG_B
    VCT --> CHG_A & CHG_B
    RWST_ALT -. "on SI realignment" .-> CHG_A & CHG_B
    CHG_A & CHG_B -- "«CHG-FLOW»" --> NORMAL & SEAL
    CHG_A & CHG_B --> EMERG
    NORMAL --> RCS
    SEAL --> RCS
    EMERG --> RCS

    classDef pump stroke:#1f618d,stroke-width:2px
    classDef tank stroke:#856404,stroke-width:2px
    class CHG_A,CHG_B pump
    class BAT,VCT,RWST_ALT tank
  • Two charging pumps — «CHG-PUMP-A», «CHG-PUMP-B»; positive- displacement; full normal flow on one pump, both required for SI alignment.
  • Boric acid tank (BAT) — «BAT-LVL»; ~7000 gal at 4 wt% boric acid; heat-traced to keep above solubility line (~145 °F minimum).
  • Volume control tank (VCT) — normal charging suction; absorbs RCS inventory excursions; hydrogen-blanketed for radiolytic-oxygen scavenging.
  • Letdown line — three orifices in parallel (low / medium / high flow); letdown HX cooled by CCW; demineralizer beds for chemistry.
  • Primary-water makeup valve — «PRIMARY-WATER-VALVE»; supplies unborated water to the blender for dilution; the FR-S.2 isolation target.

Instrumentation

  • Charging pump statuses: «CHG-PUMP-A», «CHG-PUMP-B»
  • Charging flow: «CHG-FLOW»
  • Letdown flow: «LET-FLOW»
  • Letdown isolation valve: «LET-ISOL»
  • Boric acid tank level: «BAT-LVL»
  • Emergency boration flow: «BORATE-FLOW»
  • Primary water makeup valve: «PRIMARY-WATER-VALVE»
  • RCS boron concentration (sampled): «RCS-BORON»

Setpoints

Parameter Value Source
Normal charging flow ~75 gpm (matched to letdown) Vogtle UFSAR §9.3.4
Charging pump capacity (each) ~150 gpm at SI head Vogtle UFSAR §9.3.4
BAT minimum (cold shutdown reserve) ~70% indicated Vogtle Tech Spec 3.1.2
BAT temperature minimum 145 °F (4 wt% solubility) Vogtle Tech Spec 3.1.2
RCP seal injection flow (per pump) ~8 gpm Vogtle UFSAR §5.4.1
Tech Spec boron for cold shutdown 1800-2200 ppm cycle-dependent Vogtle Tech Spec 3.1.1

Normal alignment

  • One charging pump in service, suction from VCT
  • Letdown OPEN; flow path through orifice → HX → demineralizers → VCT
  • Boric acid blended at setpoint; primary-water valve OPEN at low flow
  • RCP seal injection flow established to all four RCPs

Failure modes

  • Loss of charging — both pumps tripped or VCT depleted. RCP seal cooling lost (5-10 min until seal degradation); pressurizer level drops if letdown still open.
  • Letdown isolation failure (stuck open) — uncontrolled RCS inventory loss to VCT; pressurizer level drops; can challenge FR-I.2.
  • Boron dilution accident — primary-water-valve mis-position during makeup; rising source-range count rate; response FR-S.2.
  • High-concentration boric acid precipitation — BAT temperature drops below solubility line; boric-acid crystallization in charging line; emergency boration capability impaired.
  • VCT overpressure or hydrogen leak — normal charging suction affected; cycle to alternate alignment.

References

  • Vogtle UFSAR §9.3.4 (Chemical and Volume Control System)
  • Vogtle Tech Spec 3.1.1 (RCS boron concentration), 3.1.2 (BAT)
  • Vogtle UFSAR §15.4.6 (Boron dilution analysis)
  • Vogtle UFSAR §5.4.1 (RCP seal injection)