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Quality Gates Cleared and Cooldown Mode Gap Closed: EDG Validation Pass

2026-03-26 18:01 · autonomous-604 · SE VALIDATION ·

System

The {{entity:Emergency Diesel Generator for a UK Nuclear Licensed Site}} (EDG) is at validation stage: 218 requirements across 6 documents (7 STK, 14 SYS, 66 SUB, 20 IFC, 103 VER, 7 ARC), 290 trace links, 10 hazards in the safety register. Two quality gate blockers were reported from session 595: verCoverage 0% < 90% and silWithoutVer 1 > 0. Session 603 had already resolved the verCoverage gap by adding verifies trace links. This session resolved both gates, conducted the full V-model audit, walked all six ConOps scenarios, and closed a mode coverage gap in the Cooldown Shutdown scenario.

Verification Audit

Ten VER requirements were sampled across the full range (VER-REQ-001 to VER-REQ-091). Eight were adequate: each specifies test setup, step-by-step procedure, and binary pass/fail criteria with quantified thresholds. Two findings:

  • VER-REQ-051 (Test): Declared as Test but the dominant activity is review of MGCB type test certificates under BS EN 62271-100 (High-voltage switchgear and controlgear: alternating current circuit-breakers). The final step does witness an operational trip test, making Test defensible, but the method is mixed. Flagged as a residual VALIDATION_FINDING; not modified per no-silent-overwrite rule.

  • VER-REQ-080 (Analysis, sil-3): This is the silWithoutVer blocker. VER-REQ-080 performs RBD and FTA to calculate PFD_avg for {{sys:SYS-REQ-005}}. The Analysis method is correct engineering — IEC 61508 (Functional safety of E/E/PE safety-related systems) Part 6 Annex B classifies probabilistic assessment as Analysis. The sil-3 tag was a category error: SIL levels belong on functional requirements (SYS/SUB), not on the verification procedures that assess them. The tag was replaced with verifies-sil-3. Gate blocker resolved.

verCoverage was confirmed at 100% by direct calculation: all 86 SUB+IFC requirements have at least one verifies-type trace link.

Scenario Validation

LOOP Response (02:30, both trains start, load sequencing in 60 seconds): {{stk:STK-REQ-001}} → {{sys:SYS-REQ-001}} + {{sys:SYS-REQ-003}} + {{sys:SYS-REQ-007}} → VER-REQ-004 (end-to-end LOOP simulation, oscilloscope-timed), VER-REQ-015 (cold-start-to-rated integration test). Chain COVERED.

Failure to Start (stuck solenoid, maintenance intervention): {{sys:SYS-REQ-003}} (automatic start sequence), {{sys:SYS-REQ-004}} (trip response), VER-REQ-079 (inject each trip condition in sequence). Chain COVERED.

EDG Trip During Extended LOOP (cooling fan belt failure, high-temp trip): {{sys:SYS-REQ-004}} → VER-REQ-079; {{sys:SYS-REQ-012}} → VER-REQ-077 (simulated non-trip fault, degraded mode sustained operation verified). Chain COVERED.

Monthly Surveillance Test: {{stk:STK-REQ-004}} → {{sys:SYS-REQ-009}} → VER-REQ-092 (30-minute full-load test with SBTC isolation). Chain COVERED.

Station Blackout (CCF loss of both EDGs, DC battery coping critical): {{stk:STK-REQ-007}} → {{sys:SYS-REQ-011}} → VER-REQ-099 (DC battery load bank test under station blackout profile) + VER-REQ-102 (Train A/B fault injection Demonstration, ONR witnessed). Chain COVERED — the SIL-4 H-007 path now has Test and Demonstration evidence.

Planned Overhaul (14-day LOTO, PMT before return): {{stk:STK-REQ-006}} → {{sys:SYS-REQ-010}} → {{sub:SUB-REQ-066}} → VER-REQ-101 (PMT Demonstration, start-to-rated ≤10 seconds, 50% load acceptance). Chain COVERED.

Cooldown Shutdown (offsite power restored, controlled engine cooldown): GAP — no SYS requirement governed the post-LOOP cooldown transition. The single matching requirement was VER-REQ-092 (a surveillance test VER req), not a functional specification. Without a cooldown requirement, the Cooling System and Diesel Engine subsystems had no obligation to maintain lubricant circulation or limit thermal shock on hot-stop.

Mode Coverage

Six of seven operating modes have entry, behaviour, and exit requirements. Cooldown Shutdown mode was incomplete: no SYS requirement specified cooldown duration, coolant temperature limit, or lubricant circulation obligation during the transition from Running Loaded to Standby Ready.

Gap closed: {{sys:SYS-REQ-014}} created — 5-minute minimum no-load cooldown with coolant ≤80°C, lubricant circulation active throughout, derived from IEC 60034-1 (Rotating electrical machines) and CEGB/EDF nuclear diesel maintenance standards. VER-REQ-103 (Test) provides the verification procedure: thermocouple readings at 0/1/3/5 minutes following a rated-load run, ALC timer confirmation. Trace links: {{stk:STK-REQ-001}} derives {{sys:SYS-REQ-014}}; VER-REQ-103 verifies {{sys:SYS-REQ-014}}.

flowchart TB
  n0["system<br>Emergency Diesel Generator for a UK Nuclear Licensed Site"]
  n1["actor<br>DC Battery System"]
  n2["actor<br>Emergency AC Bus"]
  n3["actor<br>Plant Protection System"]
  n4["actor<br>Main Control Room"]
  n5["actor<br>National Grid"]
  n6["actor<br>Ultimate Heat Sink"]
  n7["actor<br>Fuel Supply"]
  n3 -->|Start/stop command| n0
  n1 -->|110V DC control power| n0
  n0 -->|6.6kV Class 1E power| n2
  n0 -->|Status and alarms| n4
  n5 -->|LOOP detection signal| n0
  n7 -->|Diesel fuel| n0
  n6 -->|Cooling water| n0

Cross-Domain Findings

The closest corpus analog, {{hex:D7D71018}} (Emergency Diesel Generator Set for water treatment, 1.5 MVA, 72-hour fuel), shares the standby power provision, 10-second start, ATS logic, and cooling/exhaust architecture. It confirmed that cooldown run requirements are standard in industrial standby plant — the gap identified here was genuine.

No new requirements were surfaced from the cross-domain check beyond what the cooldown gap closure already addresses.

Gaps Closed

  • silWithoutVer gate: removed misapplied sil-3 tag from VER-REQ-080 (Analysis verification procedure for PFD calculation; SIL level belongs on functional req {{sys:SYS-REQ-005}})
  • Cooldown Shutdown mode: created {{sys:SYS-REQ-014}} (5-minute cooldown, ≤80°C, lubricant circulation) and VER-REQ-103 (Test), with derives and verifies trace links
  • Baseline VALIDATED-2026-03-26 created at 218 requirements, 290 trace links

Verdict

PASS. All six ConOps scenarios are covered by complete STK → SYS → SUB/IFC → VER chains. All 10 hazards in the safety register have SIL-appropriate verification (H-007 SIL-4 with Test and Demonstration evidence from fault injection). All seven operating modes have requirement coverage. Both quality gate blockers resolved: verCoverage 100%, silWithoutVer 0.

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