--- type: procedure procedure-md: 0.7 procedure-id: FR-S.2 title: Response to Loss of Core Shutdown profile: nuclear-erg applies-to: Westinghouse-style 4-loop PWR reference-plant: vogtle category: function-restoration csfs-monitored: [subcriticality] entry-triggers: [csf-orange-path] --- # FR-S.2 — Response to Loss of Core Shutdown **CSF subcriticality — ORANGE path.** Entered when shutdown margin is degrading despite a successful reactor trip — boron dilution from unborated water ingress, decay-heat-driven xenon decay, or inadequate initial boron concentration. Detected by rising source-range count rate or boron-concentration samples trending toward criticality. Distinct from FR-S.1 (RED path, active power generation) — here the reactor IS shut down but the margin to recriticality is shrinking. Most common public dilution accident is inadvertent addition of unborated water during boron-thermal-regulation system operation, RCS makeup with mis-aligned valves, or RHR injection from an unborated source during mid-loop operations. See Vogtle UFSAR §15.4.6 (boron dilution analysis) for the design-basis cases. CSF: subcriticality ## Step 1 [id: verify-margin-loss] Check: source-range count rate «NIS-SR» trending up (factor-of-2 increase in any 30-minute window is the conservative recriticality indicator); RCS boron concentration «RCS-BORON» sample trending down; rod-position «ROD-POS-AVG» still bottomed Caution: source-range count rate is logarithmic — an apparent "small" rise may be a doubling already; verify against the trending log, not just instantaneous reading Note: during xenon transients (15–30 hours post-trip) source-range count rate naturally rises as Xe-135 decays; the discriminator is whether the rate of rise exceeds the xenon-decay envelope - Margin loss confirmed (count rate trending up faster than xenon-decay envelope OR boron sample trending down) → #stop-dilution Because: ORANGE-path entry — must identify dilution source and reverse before margin reaches recriticality - False alarm (rise within xenon-decay envelope, boron stable, rod position normal) → [[E-0]] Because: no dilution; return to diagnostic flow ## Step 2 [id: stop-dilution] Action: isolate primary water makeup paths — verify «PRIMARY-WATER-VALVE» closed; verify CVCS makeup is in BORATE mode not DILUTE Action: isolate any unborated-water sources that could leak into RCS — demin water tank cross-connects, condensate makeup, RWST makeup during shutdown modes Action: stop RHR pumps if RHR is in service AND there is any chance the RHR suction is on an unborated source (uncommon but possible during mid-loop / drained-loop operations) Within: 5 minutes of dilution-source-identification — every minute of continued dilution narrows the response window Caution: stopping RHR removes decay-heat removal during shutdown; balance is between recriticality risk (short-term) and core uncovery (longer-term) — only stop RHR if there is genuine reason to suspect RHR is the dilution source - Dilution stopped (verified all unborated-water paths isolated) → #boration Because: dilution must be both stopped AND reversed; boration follows isolation - Dilution source cannot be positively identified → #boration Because: even without identification, immediate boration creates positive margin recovery; isolation continues in parallel ## Step 3 [id: boration] Action: align charging pumps «CHG-PUMP-A» / «CHG-PUMP-B» suction to boric acid tank «BAT-LVL»; initiate emergency boration via «BORATE-FLOW» Action: target the higher of: (a) full restoration to last-known-good boron concentration, or (b) cold-shutdown boron concentration per the curve in Vogtle Tech Spec 3.1.1 figure (typically 1800–2200 ppm for cold shutdown depending on cycle burnup) Within: target 30 ppm boron addition within 30 minutes — sets a floor on margin-restoration rate Caution: rapid boration can cool the RCS via cold borated-water injection — monitor «PT-455» and «SUB-MARGIN» during boration to avoid PTS regime; if PTS limit approached, branch to [[FR-P.1]] Note: sampling for boron concentration confirmation has a 20–45 minute analytic turnaround; do not wait for sample to keep borating — let trending data drive the action - Adequate shutdown margin restored (count rate decaying, boron sample confirms target reached) → [[E-0]] Because: subcriticality CSF restored to GREEN; return to diagnostic flow - Boration in progress but margin still degrading (count rate continues to climb) → [[FR-S.1]] Because: shutdown-margin loss has progressed to RED — power generation imminent or in progress ## Tags - id: NIS-SR description: nuclear instrumentation source range count rate sim-path: nis.source_range.count_rate units: cps equipment: nuclear-instrumentation source: Vogtle UFSAR §7.7 - id: ROD-POS-AVG description: average control rod bottom position sim-path: rcs.rod.position.avg units: steps_withdrawn equipment: rod-control-system source: Vogtle UFSAR §7.7 - id: RCS-BORON description: RCS boron concentration (sampled) sim-path: rcs.boron.concentration units: ppm equipment: rcs source: Vogtle UFSAR §9.3.4 - id: PRIMARY-WATER-VALVE description: primary water makeup isolation valve sim-path: cvcs.primary_water.makeup_valve units: enum[OPEN,CLOSED] equipment: charging-system source: Vogtle UFSAR §9.3.4 - id: CHG-PUMP-A description: charging pump A status sim-path: cvcs.charging_pump.a.status units: enum[STOPPED,RUNNING,FAULT] equipment: charging-system source: Vogtle UFSAR §9.3.4 - id: CHG-PUMP-B description: charging pump B status sim-path: cvcs.charging_pump.b.status units: enum[STOPPED,RUNNING,FAULT] equipment: charging-system source: Vogtle UFSAR §9.3.4 - id: BAT-LVL description: boric acid tank level sim-path: cvcs.bat.level units: percent equipment: charging-system source: Vogtle UFSAR §9.3.4 - id: BORATE-FLOW description: emergency boration flow rate sim-path: cvcs.borate.flow units: gpm equipment: charging-system source: Vogtle UFSAR §9.3.4 - id: PT-455 description: pressurizer pressure (wide range) sim-path: rcs.pressurizer.pressure_wr units: psig equipment: pressurizer source: Vogtle UFSAR §5.4 range: [0, 3000] - 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