Skip to content

Time-pressure profile — SGTR

SGTR is unique among LOCA-family events because the leak path bypasses containment. The crew faces both inventory loss (primary mass migrating to the secondary side) AND a potential atmospheric release pathway (the ruptured SG can vent to atmosphere through ARVs or safety valves). Time pressure is therefore higher than a similarly-sized LOCA inside containment.

Phase 0–5 min — recognition

  • Crew load: high. Trip + SI actuation has occurred; the early symptoms (rising SG level on one loop, falling RCS pressure, air-ejector radiation rise) emerge over seconds.
  • Decisions made: standard E-0 checklist plus the initial diagnostic question — is the ruptured loop identifiable?
  • Failure modes most likely: missing the air-ejector radiation signature (it's a relatively obscure indication on the main control board); misreading SG-level instrumentation under reactor-trip transient.

Phase 5–30 min — isolate + control

  • Crew load: high, focused. Crew enters E-3; identifies the ruptured SG by its distinct level + pressure + radiation signature; isolates it by closing MSIV, MFW valves, AFW valves; begins primary cooldown to minimise the secondary release.
  • Decisions made: identify the ruptured loop with certainty before isolating it; manage cooldown rate to avoid PTS; decide on the depressurisation timing.
  • Failure modes most likely: wrong-loop isolation (4 nearly- identical loops in the control board layout) — see ./hf-action-isolate-faulted-sg.md.
  • HF concerns: high authority-gradient suppression risk. The STA's role is critical here — independent confirmation that the right SG was identified before isolation.

Phase 30–120 min — termination + recovery

  • Crew load: moderate. Primary pressure has been driven below the ruptured SG pressure; the leak has terminated. Crew works through ES-3.1 for backfill and stabilization. EAL classification (typically Alert) is established.
  • Decisions made: when to terminate SI; long-term cooldown planning; coordination with offsite emergency response.
  • Failure modes most likely: SI termination too early (re- opening the leak path); failure to maintain operator awareness of the ruptured-loop status.

Why SGTR is its own profile

The atmospheric-release potential makes SGTR distinct from other LOCAs in two ways:

  1. EAL classification jumps quickly (Alert from RCS leakage, potentially SAE if release isn't isolated). Crew must prioritise emergency response organisation notification as a parallel task to plant control.
  2. Wrong-loop isolation has catastrophic consequences. In other LOCA classes, the wrong-component error is recoverable; isolating the intact SG instead of the ruptured one removes the available heat sink.

Procedures by phase

Phase Active procedure(s)
0–5 min E-0
5–30 min E-3
30–120 min ES-3.1
Beyond recovery + extended cooldown

Cross-reference