ES-0.2 — Natural Circulation Cooldown¶
Entered from ES-0.1, ECA-2.1, or ECA-0.0 (after AC restoration)
when reactor coolant pumps are unavailable but cooldown to cold-shutdown
conditions is required. Natural circulation in a PWR is single-phase
flow driven by the hot-leg-to-cold-leg density gradient through the SGs;
flow rate is ~3-5% of forced-circulation full flow. Cooldown is slower
than forced-flow cooldown (per Vogtle UFSAR §15.2.3 natural-circulation
analysis) but achievable with care.
CSF: core-cooling
CSF: heat-sink
CSF: rcs-integrity
Step 1 verify-natural-circ¶
Check: natural-circulation flow established — hot-leg temperatures «TE-411-HOT» / «TE-421-HOT» / «TE-431-HOT» / «TE-441-HOT» stable; cold-leg temperatures «TE-411-COLD» / «TE-421-COLD» / «TE-431-COLD» / «TE-441-COLD» tracking lower than hot-leg with bounded ΔT (10-30 °F typical); subcooling margin «SUB-MARGIN» ≥ 30 °F
Caution: natural circulation requires steady-state symmetry; unbalanced flow (one loop hot, others normal) suggests a flow-path blockage or asymmetric SG cooling — investigate before proceeding
Note: natural circulation can stall on small perturbations (sudden temperature change, void in upper plenum); maintain steady conditions before manipulating any variable
- Natural circulation established AND symmetric across all loops → #begin-cooldown
Because: natural-circ confirmed; safe to begin cooldown maneuver
- Not established or asymmetric → FR-C.2
Because: degraded-core-cooling response — natural circulation failure approaches ICC territory
Step 2 begin-cooldown¶
Within: cooldown rate must be deliberate — natural-circulation cooldowns typically take 24-48 hours
Action: begin cooldown via SG steam release using ARVs «ARV-A» / «ARV-B» / «ARV-C» / «ARV-D» (condenser dump may not be available depending on entry conditions); release rate sets the primary cooldown rate
Caution: maintain RCS cooldown rate ≤ 50 °F/hr during natural-circulation cooldown — conservative target half of the Tech Spec limit, because temperature distribution lag is longer with natural circulation
Caution: monitor for flow stagnation if loop ΔT widens excessively (>50 °F) OR if hot-leg temperature stops responding to steam release — both indicate impaired natural circulation
Note: per Vogtle UFSAR §15.2.3, natural-circulation analysis assumes the upper-head is solid water (not voided); if upper-head void develops, escalate to FR-I.3
- Cooldown progressing within rate target → #depressurize-rcs
Because: ready to begin coordinated depressurization
- Cooldown stalls (loop ΔT widens, hot-leg-T not responding) → FR-H.1
Because: heat-sink path failure during natural-circulation; bleed-and-feed response engages
Step 3 depressurize-rcs¶
Action: depressurize RCS using pressurizer auxiliary spray (RCP-tripped configuration) OR PORV «PORV-456A» / «PORV-456B» (last resort) per Operations cooldown procedures
Caution: PORV depressurization is rapid and can drive upper-head voiding; coordinate with the natural-circulation flow regime — too-fast depressurization stalls flow
Caution: monitor pressurizer level «PZR-LVL» during depressurization — keep level on-scale by charging «CHG-FLOW» adjustment; off-scale low pressurizer is precursor to forced inventory loss
- Depressurization progressing toward cold-shutdown band (Tcold ≤ 350 °F, P_RCS ≤ 390 psig) → #monitor-cooldown
Because: cooldown trajectory on track
- Loss of pressurizer pressure control (PORV stuck open, normal spray failed) → ECA-3.3
Because: alternative depressurization control procedure (ECA-3.3 covers SGTR-without-pressurizer-control but the procedure applies)
Step 4 monitor-cooldown¶
Check: cooldown trajectory (T_cold trends, P_RCS trends); ARV release rate; PTS curve margin
Within: re-evaluate every 30 minutes during long natural-circulation cooldown
- Cold-shutdown conditions reached (Mode 5: T_cold ≤ 200 °F, P_RCS < 50 psig per Vogtle Tech Spec) → END
Because: natural-circulation cooldown complete; transfer to Operations shutdown procedures
- Continuing cooldown → #monitor-cooldown
Because: monitoring loop is the operating regime for natural-circ cooldowns
- PTS concern (trajectory approaching curve) → FR-P.2
Because: anticipated-PTS response is required
Tags¶
-
id: TE-411-HOT
description: RCS loop 1 hot-leg temperature
sim-path: rcs.loop1.t_hot
units: degF
equipment: rcs-loop-1
source: Vogtle UFSAR §5.1.1 -
id: TE-421-HOT
description: RCS loop 2 hot-leg temperature
sim-path: rcs.loop2.t_hot
units: degF
equipment: rcs-loop-2
source: Vogtle UFSAR §5.1.1 -
id: TE-431-HOT
description: RCS loop 3 hot-leg temperature
sim-path: rcs.loop3.t_hot
units: degF
equipment: rcs-loop-3
source: Vogtle UFSAR §5.1.1 -
id: TE-441-HOT
description: RCS loop 4 hot-leg temperature
sim-path: rcs.loop4.t_hot
units: degF
equipment: rcs-loop-4
source: Vogtle UFSAR §5.1.1 -
id: TE-411-COLD
description: RCS loop 1 cold-leg temperature
sim-path: rcs.loop1.t_cold
units: degF
equipment: rcs-loop-1
source: Vogtle UFSAR §5.1.1 -
id: TE-421-COLD
description: RCS loop 2 cold-leg temperature
sim-path: rcs.loop2.t_cold
units: degF
equipment: rcs-loop-2
source: Vogtle UFSAR §5.1.1 -
id: TE-431-COLD
description: RCS loop 3 cold-leg temperature
sim-path: rcs.loop3.t_cold
units: degF
equipment: rcs-loop-3
source: Vogtle UFSAR §5.1.1 -
id: TE-441-COLD
description: RCS loop 4 cold-leg temperature
sim-path: rcs.loop4.t_cold
units: degF
equipment: rcs-loop-4
source: Vogtle UFSAR §5.1.1 -
id: SUB-MARGIN
description: RCS subcooling margin (T_sat at PT-455 minus hot-leg temperature)
sim-path: rcs.subcooling_margin
units: degF
equipment: rcs
source: Vogtle UFSAR §15.6 -
id: ARV-A
description: SG-A atmospheric relief valve position
sim-path: secondary.arv.a.position
units: enum[OPEN,CLOSED,INTERMEDIATE]
equipment: sg-a-msl
source: Vogtle UFSAR §10.3.2 -
id: ARV-B
description: SG-B atmospheric relief valve position
sim-path: secondary.arv.b.position
units: enum[OPEN,CLOSED,INTERMEDIATE]
equipment: sg-b-msl
source: Vogtle UFSAR §10.3.2 -
id: ARV-C
description: SG-C atmospheric relief valve position
sim-path: secondary.arv.c.position
units: enum[OPEN,CLOSED,INTERMEDIATE]
equipment: sg-c-msl
source: Vogtle UFSAR §10.3.2 -
id: ARV-D
description: SG-D atmospheric relief valve position
sim-path: secondary.arv.d.position
units: enum[OPEN,CLOSED,INTERMEDIATE]
equipment: sg-d-msl
source: Vogtle UFSAR §10.3.2 -
id: PORV-456A
description: pressurizer PORV 456A position
sim-path: rcs.pressurizer.porv.456a.position
units: enum[OPEN,CLOSED,INTERMEDIATE]
equipment: pressurizer
source: Vogtle UFSAR §5.4 -
id: PORV-456B
description: pressurizer PORV 456B position
sim-path: rcs.pressurizer.porv.456b.position
units: enum[OPEN,CLOSED,INTERMEDIATE]
equipment: pressurizer
source: Vogtle UFSAR §5.4 -
id: PZR-LVL
description: pressurizer level
sim-path: rcs.pressurizer.level
units: percent
equipment: pressurizer
source: Vogtle UFSAR §5.4 -
id: CHG-FLOW
description: charging flow rate
sim-path: cvcs.charging.flow
units: gpm
equipment: charging-system
source: Vogtle UFSAR §9.3.4