Action class: isolate a faulted or ruptured SG¶
Steam Generator Tube Rupture (SGTR) and steam-line break upstream of an SG each require isolating the affected SG from the secondary inventory chain to prevent containment-bypass release or further inventory loss. The action class covers four coordinated steps:
- Identify the ruptured/faulted SG (rising level + radiation rise for SGTR; falling level + falling pressure for steam-line break).
- Close the affected SG's MSIV.
- Close the affected SG's main feedwater control valve and AFW control valve.
- Verify isolation by trending SG-side parameters and primary-to- secondary leak indicators.
Typical execution time¶
- Cognitive identification: 30–60 s once the diagnostic symptoms are visible. SGTR signature is distinct (level + rad); steam-line break is distinct (level + pressure).
- Motor: ~30 s for the valve closures (4–6 components in rapid sequence). MSIV closure is typically the longest.
Error modes¶
- Wrong-SG isolation — the operator isolates the wrong SG because the SG indicators on the main control board are visually similar across the four loops. Catastrophic if the only intact SG is isolated. Mitigated by peer-check + the procedure's redundant verification step.
- Incomplete isolation — MSIV closes but the SG blowdown valve or atmospheric relief valve is left in service. Inventory continues to leak via the open path. Caught by the verify step.
- Premature isolation — diagnosis is wrong; the operator isolates an intact SG based on instrument anomaly rather than real fault. Heavy cost in heat-sink capacity.
Procedures that invoke¶
Performance-shaping factors¶
| factor | weight | rationale |
|---|---|---|
| diagnostic complexity | high | distinguishing SGTR from steam-line break under noisy indications |
| time pressure | medium | minutes-to-isolate before significant release |
| HSI | high | 4-loop layout invites wrong-loop error |
| peer-check use | high | the procedure explicitly demands it |
| training quality | high | exact-event simulator rehearsal |