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ATWS

Reactor at 100% rated thermal power. Loss of main feedwater at t = 0 generates an automatic reactor trip signal at t = 5 s, but the trip breakers fail to open (common-mode mechanical fault). Pressurizer pressure rises rapidly as the moderator-feedback negative reactivity insertion is the only power-reduction mechanism. AFW initiates automatically. Crew recognizes the ATWS at E-0#verify-reactor-trip (trip breakers still closed despite trip signal), transitions to FR-S.1 for emergency boration and manual trip. Manual trip at FR-S.1#manual-trip succeeds at t = 60 s; emergency boration follows per FR-S.1#emergency-boration.

The classifier triggers Alert (SA1) for the ATWS condition itself — power > 50% while trip breakers remain CLOSED. Once manual trip succeeds and breakers open at t = 60 s, the SA1 predicate becomes false, but NEI 99-01's persistence rule applies (once declared, the Alert is held until termination criteria are met by the emergency director).

Initial state

{
  "PT-455": 2235,
  "NIS-PR-AVG": 100,
  "TRIP-BKR-A": "CLOSED",
  "TRIP-BKR-B": "CLOSED",
  "SG-A-LVL-NR": 50,
  "RCS-TEMP-HOT": 612,
  "SUB-MARGIN": 35
}

Injections

[
  { "tag": "PT-455", "value": 2450, "at-time-s": 10 },
  { "tag": "NIS-PR-AVG", "value": 92, "at-time-s": 10 },
  { "tag": "PT-455", "value": 2520, "at-time-s": 30 },
  { "tag": "NIS-PR-AVG", "value": 85, "at-time-s": 30 },
  { "tag": "TRIP-BKR-A", "value": "OPEN", "at-time-s": 60 },
  { "tag": "TRIP-BKR-B", "value": "OPEN", "at-time-s": 60 },
  { "tag": "NIS-PR-AVG", "value": 5, "at-time-s": 90 },
  { "tag": "PT-455", "value": 2200, "at-time-s": 180 }
]

Expected traversal

[
  "E-0#verify-reactor-trip",
  "FR-S.1#verify-power-generation",
  "FR-S.1#manual-trip",
  "FR-S.1#emergency-boration",
  "FR-S.1#verify-shutdown"
]

Expected terminal state

{
  "TRIP-BKR-A": "OPEN",
  "TRIP-BKR-B": "OPEN",
  "NIS-PR-AVG": 5,
  "PT-455": 2200
}

EAL classification

Alert (SA1) at t = 0 — the categorical ATWS predicate fires immediately because NIS-PR-AVG is at 100% while both trip breakers are CLOSED. The condition persists until t = 60 s when manual trip succeeds (breakers open). The classifier returns the highest class reached, which remains Alert.

The classifier does not currently model emergency-director judgment for classification termination. NEI 99-01 Alert persists until termination criteria are formally met; the scenario's terminal state shows breakers open and power decayed, but the formal Alert termination is an out-of-scope operator decision.

References

  • Vogtle UFSAR §15.8 (ATWS analysis)
  • WCAP-11993 (ATWS mitigation in W-style PWRs)
  • 10 CFR 50.62 (ATWS rule)
  • NEI 99-01 SA1 (RPS failure to scram)
  • Westinghouse-style ERG procedures E-0, FR-S.1