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Systems > ECCS

ECCS

⚠️ DRAFT

ECCS

Function

The ECCS provides borated water to cool the reactor core in the event of a loss-of-coolant accident (LOCA). It injects borated water into the RCS to: (1) terminate any reactivity increase, (2) provide initial core cooling and reflooding, and (3) replenish coolant lost from the primary system. The boron in the injected water combined with control rods ensures shutdown reactivity. (UFSAR 6.1, 6.3)

Safety Injection Actuation Signals

SignalCoincidenceSource
Low Pressurizer Pressure2/3UFSAR 6.3.2
High Containment Pressure (Hi)2/3UFSAR 6.3.2
High Steam Line Differential Pressure2/3 (between any two SGs)UFSAR 6.3.2
High Steam Line Flow1/2 per line in 2/4 lines, AND (Low-Low Tavg 2/4 OR Low Steam Line Pressure 2/4)UFSAR 6.3.2
Manual1/2UFSAR 6.3.2

The SI signal initiates: reactor trip, diesel generator start, safeguards sequence, and Phase A containment isolation. (UFSAR 6.3.2)

Exam — 2018 Q5
Automatic SI actuation setpoints: PZR Pressure < 1765 psig (2/3 coincidence) OR Containment Pressure > 4 psig (2/3 coincidence). At 11:00 with Containment Pressure 4.1 psig, the Hi Containment Pressure SI setpoint is exceeded — this is the earliest SI signal (PZR pressure at 1864 psig has not yet reached the 1765 psig setpoint). Trap: candidates may look only at PZR pressure and miss the Containment Pressure signal being satisfied first.

ECCS Components

Accumulators (Passive — Cold Leg Injection)

ParameterValueSource
Number4 (one per cold leg)UFSAR T6.3-2
TypeStainless steel clad/carbon steelUFSAR T6.3-2
Design Pressure700 psigUFSAR T6.3-2
Design Temperature300°FUFSAR T6.3-2
Normal Operating Pressure650 psigUFSAR T6.3-2
Minimum Operating Pressure595.5 psigUFSAR T6.3-2
Total Volume1350 ft³UFSAR T6.3-2
Minimum Operating Water Volume831.9 ft³UFSAR T6.3-2
N₂ Gas Volume500 ft³UFSAR T6.3-2
Boron Concentration (nominal/min)2000 / 1900 ppmUFSAR T6.3-2
CodeASME III Class CUFSAR T6.3-2

Isolated from RCS by two check valves in series. Inject when RCS pressure falls below accumulator pressure. For a large LOCA, accumulators begin injecting ~10 seconds after the break. (UFSAR 6.3.2.2)

Centrifugal Charging Pumps (High Head — Cold Leg Injection)

ParameterValueSource
Number2UFSAR T6.3-5
TypeHorizontal multi-stage centrifugalUFSAR T6.3-5
Design Pressure2800 psigUFSAR T6.3-5
Design Temperature300°FUFSAR T6.3-5
Design Flow Rate150 gpmUFSAR T6.3-5
Design Head5800 ftUFSAR T6.3-5
Max Flow Rate560 gpmUFSAR T6.3-5
Head at Max Flow1300 ftUFSAR T6.3-5
Shutoff Discharge Pressure2670 psigUFSAR T6.3-5
Motor HP600UFSAR T6.3-5
MaterialStainless steelUFSAR T6.3-5

Safety Injection Pumps (Intermediate Head — Cold Leg Injection)

ParameterValueSource
Number2UFSAR T6.3-5
TypeHorizontal multi-stage centrifugalUFSAR T6.3-5
Design Pressure1700 psigUFSAR T6.3-5
Design Temperature300°FUFSAR T6.3-5
Design Flow Rate425 gpmUFSAR T6.3-5
Design Head2500 ftUFSAR T6.3-5
Max Flow Rate675 gpmUFSAR T6.3-5
Head at Max Flow1500 ftUFSAR T6.3-5
Shutoff Discharge Pressure1520 psigUFSAR T6.3-5
Motor HP400UFSAR T6.3-5
MaterialStainless steelUFSAR T6.3-5

SI pump discharge is limited to <1500 psig to prevent lifting RCS safety valves. (UFSAR 6.3.2)

Exam — 2023 Q16
SI pump shutoff head is 1520 psig. If RCS pressure is above 1520 psig (e.g., 1700 psig) and SI flow is indicated, this confirms a cold leg discharge leak (flow should be zero above shutoff head). Per EOP-LOCA-6 step 2, close 2SJ135 (Cold Leg Discharge Valve) to isolate SI pump cold leg leak. Note: 21/22 SJ49 valves isolate RHR cold legs, not SI cold legs.

Residual Heat Removal Pumps (Low Head — Cold Leg Injection)

ParameterValueSource
Number2UFSAR T6.3-5
TypeVertical single-stage centrifugalUFSAR T6.3-5
Design Pressure600 psigUFSAR T6.3-5
Design Temperature400°FUFSAR T6.3-5
Design Flow Rate3000 gpmUFSAR T6.3-5
Design Head350 ftUFSAR T6.3-5
Max Flow Rate4500 gpmUFSAR T6.3-5
Shutoff Discharge Pressure170 psigUFSAR T6.3-5
Motor HP400UFSAR T6.3-5

RHR pumps deliver when RCS depressurizes to ~170 psig. Inject via accumulator discharge lines to cold legs. (UFSAR 6.3.2)

Refueling Water Storage Tank (RWST)

ParameterValueSource
Number1UFSAR T6.3-4
Tank Capacity400000 galUFSAR T6.3-4
Minimum Volume364500 galUFSAR T6.3-4
Operating PressureAtmosphericUFSAR T6.3-4
Operating Temperature40–100°FUFSAR T6.3-4
Design Temperature120°FUFSAR T6.3-4
Outside Diameter~38 ftUFSAR T6.3-4
Height (straight side)48 ftUFSAR T6.3-4
MaterialASTM A-240 Type 304L SSUFSAR T6.3-4
Boron Concentration (nom/min/max)2400 / 2300 / 2500 ppmUFSAR T6.3-4
Seismic ClassificationClass IUFSAR 6.3.2

Provides suction source for all ECCS pumps during injection phase. Minimum volume based on refueling canal fill requirement. (UFSAR 6.3.2)

Exam — 2020 Q42
RWST level reference: 20 ft = 190000 gallons. With 2A Vital Bus lost (losing "A" bus loads), ECCS draw rate = 1100 gpm (both charging pumps) + 650 gpm (1 SI pump) + 2600 gpm (1 CS pump) = 4350 gpm total. After 6 minutes: 190000 - 26100 = 163900 gallons (>16 ft, still above 15.24 ft). If "B" bus loads were lost instead, the higher draw rate would drop RWST to 147700 gallons in 6 minutes — below 15.24 ft. Knowing which bus powers which ECCS/CS pumps is critical for RWST depletion rate calculations.
Exam — 2023 Q29
Automatic swapover of charging pump suction from VCT to RWST requires 2/2 coincidence (LT-112 AND LT-114) at low-low VCT level. If LT-112 fails high, the 2/2 logic is not satisfied and no auto swapover occurs — the VCT drains and the charging pump loses suction. See [[CVCS]] for full VCT level control logic.

ECCS Operation Phases

Injection Phase

  1. SI signal actuates all ECCS pumps and opens injection valves
  2. All pumps take suction from RWST
  3. Accumulators inject passively when RCS pressure < accumulator pressure (~650 psig)
  4. Charging pumps inject at high RCS pressures (highest head)
  5. SI pumps inject when RCS < ~1500 psig
  6. RHR pumps inject when RCS < ~170 psig
  7. Time delay: ~25 seconds for active pumps to deliver flow
Exam — 2020 Q3
SBLOCA with 2A 4KV bus faulted and LOOP: only one SI pump available (the SI pump on the remaining vital bus). With only 1 SI pump running and all RCPs stopped, EOP-LOCA-2 Table C adverse containment subcooling requirement is 141°F for charging pump reduction. At 1035 psig (1049.7 psia), T-Sat = 550.56°F → max CET for stopping one charging pump = 409°F. Trap: with 2 SI pumps running the subcooling requirement would be 135°F, not 141°F. Candidates must determine the number of available SI pumps based on 4KV bus alignment.
Exam — 2023 Q31
During a SBLOCA, a cold leg break produces the highest peak clad temperature. Because ECCS pumps inject into the cold legs, a cold leg break allows injected ECCS water to be lost directly out the break before reaching the core, reducing the effectiveness of core cooling. Hot leg, CRDM housing, and PZR steam space breaks are analyzed scenarios but do not produce the worst PCT.
Exam — 2022 Q33
ECCS flow at 300 psig RCS with 2C 4KV bus de-energized: 22 SI Pump (on 2C bus) unavailable. Remaining ECCS: 21 Charging Pump at runout = 560 gpm + 21 SI Pump at runout = 675 gpm = 1235 gpm total. RHR pumps at shutoff head (>170 psid) — no RHR flow at 300 psig. RHR pumps only deliver flow when RCS pressure < 170 psig during injection. 300 psig is below RHR initiation for shutdown cooling (~340 psig) but above RHR pump shutoff head for LOCA injection.
Exam — 2022 Q14
SEC Mode 2 (Blackout only) — load sequencer starts Charging Pumps ONLY. SI and RHR pumps are NOT started without an SI accident signal. SI pumps powered from 2A and 2C 4KV Vital buses (not 2B). SEC Modes: Mode 1 = Accident only, Mode 2 = Blackout only, Mode 3/4 = Accident plus Blackout.
Exam — 2019 Q12
ECCS pump power supply mapping: 22 SI Pump is powered from 2C 4KV Vital Bus; 22 RHR Pump is powered from 2B 4KV Vital Bus. During a Reactor Trip + SI coincident with LOOP and EDG 2C unavailable: 22 RHR Pump is running (2B bus powered by EDG 2B), but 22 SI Pump is stopped (2C bus de-energized). Pump number does NOT indicate bus assignment — 22 RHR is on B bus, 22 SI is on C bus.
Exam — 2022 Q18
SJ49s (Cold Leg Isolation Valves) are normally open and NOT Phase A valves. To operate: only CMC switch on 2RP4 to VALVE OPERABLE (no Phase A reset needed). Monitor RCS pressure rise to determine if LOCA outside containment is isolated.
Exam — 2020 Q32
Per EOP-FRTS-1 bases: ECCS flow may contribute to RCS cooldown or prevent subsequent reduction in RCS pressure during a PTS condition. FRTS-1 directs SI termination to reduce cold ECCS injection, and RCP restart to mix cold incoming ECCS water with warm reactor coolant, decreasing the likelihood of brittle fracture. SI termination is NOT because a "soak" requires it — some SBLOCA conditions prevent SI termination even in FRTS-1.

Semi-Automatic Switchover (Unit 2)

At 15.2 ft RWST level (2/4 RWST level channels), the following automatic actions occur on Unit 2:

  • 21 & 22 SJ113s (SI to Charging Pump Crossover Valves) OPEN automatically — these valves are “armed” in the normal 100% power ECCS lineup for semi-automatic switchover
  • SJ44s (RHR Pump Sump Suction Valves) do NOT open automatically — operator must manually “arm” them if sump level > 62%, then they will auto-open
  • RH4s (RHR Pump Suction Valves) do NOT close automatically at this level
  • CS36s (RHR Discharge to Containment Spray Header) are manipulated manually at LO-LO RWST level
Exam — 2020 Q41
At RWST level < 15.2 ft on 2/4 channels, Unit 2 semi-automatic switchover opens SJ113 valves (SI to Charging Pump Crossover) automatically because they are "armed" in the normal 100% power ECCS lineup. SJ44s (RHR Pump Sump Suction) require manual arming by the operator first — they are NOT automatically armed at normal power. Unit 1 has an interlock requiring RH4 closed before SJ44 opens.
Exam — 2018 Q4
RWST Level LO alarm triggers the start of cold leg recirculation transfer (IAW EOP-LOCA-1). 21SJ44 and 22SJ44 (Containment Sump Suction Valves) each serve one RHR pump independently. With 21SJ44 closed, ONLY 22 RHR pump can draw suction from the containment sump. Trap: unlike some other ECCS suction arrangements (e.g., SJ1/SJ2 for CCPs where both can take suction from RWST if one fails), the containment sump suction valves are NOT cross-connected — each SJ44 serves only its associated RHR pump.

Changeover to Recirculation Phase

  1. Containment sump level shows adequate submergence of sump strainer
  2. RWST level low alarm alerts operator
  3. Operator initiates switchover to cold leg recirculation
  4. RHR pumps realigned from RWST to containment sump
  5. SI and charging pumps take suction from RHR pump discharge (piggyback)

Hot Leg Recirculation

Initiated at ~14.0 hours (Unit 1) or ~6.5 hours (Unit 2) after switchover to cold leg recirculation to assure termination of boiling. Minimum one SI pump aligned for hot leg recirculation flow. (UFSAR 6.3.2)

Exam — 2022 Q24
EOP-FRCC-1 step 18 RCP start criteria: if CETs >1200°F and at least one SG NR level >9% (15% adverse containment), start one RCP at a time for each idle loop with SG NR >9%. If no SG meets criteria, reset Phase A, open both CA330s, open both PZR PORVs and Rx Head Vents.

Single Failure Criteria

  • During injection phase: tolerant of any single active failure
  • During recirculation phase: tolerant of one active or one passive failure (but not in addition to an injection phase failure)
  • All active ECCS components located outside containment in Auxiliary Building (UFSAR 6.1.1.4, 6.3.2)

Tech Spec LCOs

  • TS 3/4.5 — ECCS — ECCS subsystem operability
  • TS 3/4.5 — ECCS|TS 3/4.5.1 — Accumulators (Modes 1, 2, 3 with pressurizer pressure ≥1000 psig)
  • TS 3/4.5 — ECCS|TS 3/4.5.2 — ECCS Subsystems (Tavg ≥350°F)
  • TS 3/4.5 — ECCS|TS 3/4.5.3 — ECCS Subsystems (Tavg <350°F)
  • TS 3/4.5 — ECCS|TS 3/4.5.4 — RWST
Exam — 2023 Q65
SI indication on one train failure: "SI & FW ISOL" Red lamp on 2RP4 FLASHES when SSPS trains disagree (one train actuated, one did not). LIT solid means both trains actuated. The "SI RESET" Green bezel light is LIT on the train that FAILED to actuate, and extinguished on the train that DID actuate.
Exam — 2023 Q76
ECCS injection at 1450 psig: both charging pumps (shutoff head 2670 psig) AND SI pumps (shutoff head 1520 psig) are injecting. RHR pumps (shutoff head 170 psig) are NOT injecting at this pressure. Common trap: believing only charging pumps inject at 1450 psig because it is "close" to SI pump shutoff head — 1450 psig is below 1520 psig, so SI pumps deliver flow.
Exam — 2023 Q87
EOP-TRIP-4 CAS directs SI actuation when PZR level cannot be maintained > 4% (or subcooling reaches 0 degrees F). With charging maxed out (2CV55 and 2CV71 fully open) and PZR level still lowering at 10%, the CAS criterion is met — actuate SI and return to EOP-TRIP-1. Trap: TRIP-4 does NOT direct transition to EOP-LOCA-1 — it directs SI actuation and return to TRIP-1.
JPM — 2023 SRO-A1.a
EOP-LOCA-5 Step 16.1: minimum ECCS injection flow from Figure 1 at 100 minutes post-trip is 290 gpm (280-320 gpm acceptable). With one SI pump at 160 gpm and one charging pump at 220 gpm, neither alone meets 290 gpm — NONE can be stopped.
JPM — 2023 Sim-b
EOP-LOCA-4 hot leg recirculation: realign 21 SI pump from cold leg to hot leg injection — STOP pump → CLOSE 21SJ134 (cold leg discharge) → OPEN 21SJ40 (hot leg discharge, key-locked) → START pump.
JPM — 2019 Sim-b
EOP-LOCA-4 hot leg recirc with two alternate paths: (1) 21SJ45 CLOSED for 21 RHR pump maintenance — RHR-to-SI cross-connect unavailable; (2) 22 SI pump tripped during LOCA-3 — must use 21 SI pump via Step 6.1. Starts in EOP-LOCA-1 Step 27: close 21/22 SJ40, remove lockouts, verify Valve Operable backlights. LOCA-4 alignment: CLOSE 22CS36 (RHR supply to CS) → OPEN 22SJ49 (cold leg isolation) → realign 21 SI pump for hot leg injection.
JPM — 2022 Sim-c
EOP-LOCA-1 Step 14 accumulator isolation: when at least two RCS T-Hots < 405F, remove lockout and close 21-24 SJ54. If 24SJ54 fails to close (alternate path), vent the unisolated accumulator: open 2NT35 (N2 HDR Valve) and 24SJ93 (N2 Supply Valve) to depressurize the accumulator to zero.
Scenario — 2022 #1
LBLOCA with auto SI failure on both trains (CT#1, CT-2): RO must manually actuate SI before transition out of TRIP-1. FSAR analyses assume at least one train of safeguards actuates. Additionally, 22 RHR pump (low-head ECCS) fails to start on SEC signal (CT#2, CT-5) — RO must manually start 22 RHR before transition out of TRIP-1. FSAR LBLOCA analysis requires minimum pumped injection from one each high-head, intermediate-head, and low-head pump. In EOP-LOCA-5, crew reduces ECCS to one train (CT#3): stops all but one charging pump, runs only one SI pump.
Exam — 2020 Q77
ECCS subsystem composition: each of the two independent ECCS subsystems includes one centrifugal charging pump (high-head), one SI pump (intermediate-head), and one RHR pump + HX (low-head). The 23 Charging Pump (positive displacement) is NOT part of an ECCS subsystem — it counts only toward TS 3.1.2.4 (Charging Pumps for reactivity control). Loss of one centrifugal charging pump = one ECCS subsystem inoperable → TS 3.5.2 applies (72 hours to restore, then Hot Shutdown in 12 hours).
JPM — 2020 Sim-d
SI Accumulator makeup using 21 SI Pump (S2.OP-SO.SJ-0002 Section 5.2): prerequisite ALL RCS Cold Leg Temps >312°F. Start 21 SI Pump → open 2SJ53 (discharge test line) → open 2SJ123 (test line to CVCS HUT) → open 21SJ20 (accumulator fill). Close 21SJ20 when level reaches 60%. CAUTION: do NOT cross-tie accumulators while OPERABILITY required. If RCS pressure <2000 psig, must close 21SJ134 (Cold Leg Discharge) first — may require TSAS 3.5.2 entry.
JPM — 2018 Sim-c
EOP-TRIP-6 Step 12 — Isolate SI Accumulators: with RCS pressure <1000 psig, remove lockout from 21-24 SJ54 Accumulator Outlet Valves at 2RP4 panel. Close all four SJ54s. 24SJ54 fails at intermediate position (alternate path) — consult TSC, then vent 24 Accumulator: verify RCS pressure > accumulator N2 pressure, open 2NT35 (N2 HDR VALVE) and 24SJ93 (N2 SUPPLY VALVE), observe accumulator pressure lowering to zero, then close both vent valves.
JPM — 2019 SRO-A1
EOP-LOCA-5 Figure A — Minimum ECCS Flow Versus Time After Trip: at 50 minutes post-trip, minimum flow = 390 gpm. With 21 SI pump at 420 gpm and 22 Charging pump at 360 gpm: stop 22 Charging pump (360 < 390, cannot meet minimum alone), keep 21 SI pump (420 > 390, exceeds minimum alone). Contrast 2023 SRO-A1.a at 100 min (290 gpm minimum) where neither pump could be stopped.
Scenario — 2018 #2
LBLOCA with both trains of auto SI failing. One SI keyswitch also fails to manually actuate — RO uses other train keyswitch to initiate SI on both trains (CT#1: CT-2). After SI, containment pressure >15 psig — Phase B and Spray auto-initiated. Cold leg recirculation transfer (CT#2: CT-36): at RWST lo level alarm (15.2 feet), crew performs EOP-LOCA-3. Time-critical steps: close 2SJ69 within ~3.7 min, stop 22 CS pump within ~5.5 min, complete recirc alignment (Step 15: close 2SJ67/2SJ68, open 21SJ45/22SJ45, close 2SJ30/2SJ1/2SJ2, place 21RH29/22RH29 in manual/closed) within ~11.2 min.

Connections