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Emergency Core Cooling System (ECCS)

The ECCS is the safety-grade complement of pumps, tanks, valves, and piping that injects borated water into the RCS during accidents to keep the core covered and the reactor subcritical. It is the system that all of E-1 (LOCA), FR-C.x (core cooling), ECA-1.1 (recirculation failure), and ES-1.3/1.4 (recirculation transfer) manage.

Function

Provides emergency makeup and core cooling across the full LOCA spectrum:

  • High-head safety injection (HHSI) for small breaks where RCS pressure remains high
  • Cold-leg accumulator (passive) injection for intermediate breaks
  • Low-head safety injection (LHSI, via RHR) for large breaks at low RCS pressure
  • Long-term recirculation from containment sump after RWST depletion

Per Vogtle UFSAR §6.3 design-basis analysis, ECCS keeps peak clad temperature below the 10 CFR 50.46 limit (2200 °F) across the LOCA spectrum with single-failure assumption.

Components

flowchart LR
    RWST["RWST<br/>«RWST-LVL»<br/>(refueling water<br/>storage tank)"]
    SUMP["Containment<br/>Sump"]
    HHSI_A(("HHSI «SI-PUMP-A»"))
    HHSI_B(("HHSI «SI-PUMP-B»"))
    CHG_A(("Charging «CHG-PUMP-A»"))
    CHG_B(("Charging «CHG-PUMP-B»"))
    RHR_A(("RHR «RHR-PUMP-A»<br/>(LHSI)"))
    RHR_B(("RHR «RHR-PUMP-B»<br/>(LHSI)"))
    ACC1["Accumulator 1<br/>«ACCUM-1»<br/>~600 psig"]
    ACC2["Accumulator 2<br/>«ACCUM-2»"]
    ACC3["Accumulator 3<br/>«ACCUM-3»"]
    ACC4["Accumulator 4<br/>«ACCUM-4»"]
    CL["Cold Legs<br/>(4 loops)"]
    HL["Hot Legs<br/>(hot-leg injection<br/>via «HL-INJECT-A/B»)"]
    RV["Reactor Vessel"]

    RWST --> HHSI_A & HHSI_B & CHG_A & CHG_B
    RWST --> RHR_A & RHR_B
    HHSI_A & HHSI_B --> CL
    CHG_A & CHG_B --> CL
    RHR_A & RHR_B --> CL
    ACC1 & ACC2 & ACC3 & ACC4 --> CL
    CL --> RV
    RV -. "via ECCS hot-leg<br/>recirc path" .- HL
    HL --> RV

    SUMP -. "after ES-1.3<br/>recirc transfer" .-> RHR_A
    SUMP -. "after ES-1.3" .-> RHR_B
    RHR_A -. "piggyback alignment" .-> HHSI_A

    classDef tank stroke:#856404,stroke-width:2px
    classDef passive stroke:#1f618d,stroke-width:2px
    class RWST,SUMP tank
    class ACC1,ACC2,ACC3,ACC4 passive
  • Refueling water storage tank (RWST) — borated-water source for initial injection, ~600,000 gallons (Vogtle Tech Spec 3.5.4).
  • Two high-head SI pumps (HHSI) — «SI-PUMP-A», «SI-PUMP-B»; shutoff head ~1500 psig; high-pressure, low-flow regime.
  • Two charging pumps — «CHG-PUMP-A», «CHG-PUMP-B»; positive- displacement, normally CVCS but realign to SI on accident signal.
  • Four cold-leg accumulators — «ACCUM-1»..«ACCUM-4»; passive, nitrogen-pressurized to ~600 psig, discharge automatically when RCS pressure drops below tank pressure.
  • Two RHR (LHSI) pumps — «RHR-PUMP-A», «RHR-PUMP-B»; shutoff head ~200 psig; high-flow, low-pressure regime; also serves shutdown cooling in Mode 4-5.
  • Containment recirculation sump — long-term suction source after RWST depletion (post-Fukushima sump-strainer integrity reviewed in GSI-191).

Instrumentation

  • RWST level: «RWST-LVL» (low-low triggers automatic recirc-transfer initiation)
  • Containment sump level: «CTMT-SUMP-LVL»
  • Sump-strainer differential pressure: «SUMP-SCREEN-DP»
  • SI signal (latched): «SI-SIG»
  • SI header flow (high-head): «SI-FLOW»
  • Low-head / RHR header flow: «LO-HEAD-FLOW»
  • Accumulator discharge valve positions: «ACCUM-1»..«ACCUM-4»
  • HHSI pump statuses: «SI-PUMP-A», «SI-PUMP-B»

Setpoints

Parameter Value Source
HHSI shutoff head (approximate) 1500 psig Vogtle UFSAR §6.3
Accumulator nitrogen overpressure 600 psig Vogtle UFSAR §6.3.2
LHSI shutoff head (approximate) 200 psig Vogtle UFSAR §6.3
RWST low-low (recirc transfer initiate) 38% indicated Vogtle Tech Spec 3.5.4
RWST normal volume ~600,000 gal Vogtle Tech Spec 3.5.4
10 CFR 50.46 peak clad temperature limit 2200 °F regulatory

Normal alignment

  • HHSI pumps standby; suction aligned to RWST; discharge cold-leg injection valves CLOSED
  • Charging pumps in service (normal CVCS), routinely cycled; SI alignment available on demand via realignment of suction/discharge
  • Accumulator discharge valves OPEN, tank pressure ~600 psig, isolation valves OPEN
  • RHR pumps standby (or in shutdown-cooling alignment in Mode 4-5)
  • RWST level full (>90%)

Failure modes

  • No SI flow despite running pumps — flow-path blockage, sump strainer failure, mis-aligned valve. Inadequate-core-cooling response is FR-C.1; partial-degradation response is FR-C.2.
  • HHSI cannot inject (RCS pressure exceeds shutoff head) — depressurization via PORVs widens injection window; emergency feed-and-bleed if heat sink also lost. See FR-C.1 step 3.
  • Recirculation transfer failure — sump suction unavailable, pump trip during transfer, strainer blockage. Response ECA-1.1.
  • LOCA outside containment (ISLOCA) — ECCS injects into a path that bypasses containment, e.g. RHR V-sequence. Response ECA-1.2.
  • Accumulator failure — nitrogen leak (low pressure → won't discharge at the right pressure) or stuck discharge valve (no injection regardless of RCS pressure). LOCA performance degrades toward FR-C.x territory.

References

  • Vogtle UFSAR §6.3 (Emergency Core Cooling System)
  • Vogtle UFSAR §6.3.2 (RWST, accumulators, recirculation)
  • Vogtle UFSAR §15.6.5 (Loss of Reactor Coolant — Inadequate Core Cooling analysis)
  • 10 CFR 50.46 (Peak clad temperature limit; ECCS performance criteria)
  • NUREG-0737 II.F.2 (RVLIS + CETs — added post-TMI to monitor ECCS performance)
  • NRC GSI-191 (containment sump-strainer integrity; addressed post-Fukushima)