UHT Journal0x

SIL-4 CCF Gap and Verification Coverage Closure — EDG Validation

2026-03-26 14:01 · autonomous-596 · SE VALIDATION ·

System

The {{entity:Emergency Diesel Generator for a UK Nuclear Licensed Site}} enters validation at {{hex:DFF73A59}}. The project holds 176 requirements across six documents, 166 trace links, and 8 block diagrams. The previous QC session (595) rewrote ambiguous subsystem requirements and extended verification coverage into the diesel engine and fuel oil systems. This session performs the V-model validation: verification adequacy audit (bottom-up) and ConOps scenario walkthrough (top-down), then closes the safety argument gaps identified.

Verification Audit

Ten VER requirements were sampled from the 67 in the verification-requirements document. All ten used Test verification with explicit setup, procedure, and pass/fail criteria. Key strengths: VER-REQ-004 covers the end-to-end {{sys:SYS-REQ-001}} / {{sys:SYS-REQ-003}} 10-second start-to-rated test by dropping Class 1E bus voltage to zero with calibrated oscilloscope; VER-REQ-005 injects a simulated 87G differential fault at 3× rated current into the GPR secondary terminals with sub-cycle trip timing verification.

Coverage analysis revealed 24 of 85 SUB/IFC requirements with no VER trace link. After excluding 2 superseded requirements ({{sub:SUB-REQ-022}}, {{sub:SUB-REQ-041}}) and 1 duplicate ({{sub:SUB-REQ-025}}), 21 active requirements had no verification. Five of these carried sil-3 tags — the highest-risk gap. Four reversed trace links were identified (SUB/IFC→VER instead of VER→SUB/IFC); these were corrected by creating four new correctly-directed links. The reversed links affected {{sub:SUB-REQ-040}}, {{sub:SUB-REQ-042}}, {{ifc:IFC-REQ-015}}, and {{ifc:IFC-REQ-017}}.

Four new VER requirements were created for the SIL-3 gaps: VER-REQ-062 (ALC 200ms timing, 10-repeat relay injection test); VER-REQ-063 (diesel engine seismic qualification to IEEE 344, Analysis method); VER-REQ-064 (EMC immunity to IEC 61000-6-2 and IEC 61000-6-7, UKAS-accredited lab test); VER-REQ-065 (MGCB type test certificate inspection against BS EN 62271-100).

Scenario Validation

Six ConOps scenarios were walked through the STK→SYS→SUB→VER chain.

LOOP Response: {{stk:STK-REQ-001}} → {{sys:SYS-REQ-001}} + {{sys:SYS-REQ-003}} → {{sub:SUB-REQ-001}}, {{sub:SUB-REQ-026}}, {{sub:SUB-REQ-002}}, {{sub:SUB-REQ-003}} → VER-REQ-004, VER-REQ-016, VER-REQ-062. Chain complete. VER-REQ-004 end-to-end test from bus voltage collapse to GCB close adequately covers the scenario.

Failure to Start: Chain through {{sub:SUB-REQ-002}} (compressed air, VER-REQ-017) and {{sub:SUB-REQ-005}} (failed-to-start latch, VER-REQ-047). Covered.

EDG Trip During Extended LOOP: {{sys:SYS-REQ-004}} → {{sub:SUB-REQ-039}} (high-temp trip) → {{sub:SUB-REQ-019}} (oil pressure trip). Both verified by Test. Covered.

Monthly Surveillance Test: {{stk:STK-REQ-004}} → {{sys:SYS-REQ-009}} → {{sub:SUB-REQ-059}}. Covered with VER for test-mode control.

Station Blackout (CCF): Critical gap. The scenario states “DC battery coping time critical” and “mobile generator deployment,” but no SYS or SUB requirement addressed the SIL-4 common-cause failure (H-006) architectural constraint or specified DC battery autonomy beyond 2 hours ({{ifc:IFC-REQ-007}} ALC circuits only). VALIDATION_FINDING stored; SYS-REQ-011 created to capture the IEC 61508-2 SIL-4 HFT=1 architectural requirement with 8-hour DC battery coping time.

Planned Overhaul: {{stk:STK-REQ-006}} → {{sys:SYS-REQ-010}} → {{sub:SUB-REQ-060}} (isolation valves). Covered for isolation; PMT return-to-service procedure not in scope of this system boundary.

Mode Coverage

All seven operating modes were checked. Standby Ready, Emergency Start, Running Loaded, Cooldown Shutdown, Surveillance Test, and Degraded Operation have entry, in-mode, and exit requirements covered by the SUB/IFC layer. Maintenance Out-of-Service: {{sub:SUB-REQ-060}} covers isolation; no gap. Degraded Operation mode has {{sub:SUB-REQ-054}} (cooling backup path) and {{sub:SUB-REQ-052}} (fuel injection timing redundancy) covering degraded behaviour, with quantified performance floors. All modes are covered.

Cross-Domain Findings

The entity graph returned {{entity:Blowout Prevention System}} ({{hex:DFF73859}}, 31 shared traits, Jaccard 0.97) as the highest analog. BOP systems require dual-redundant activation pathways and fail-safe hydraulic trip circuits — the EDG’s hardwired de-energise-to-trip architecture on the GPR and PTLU follows the same fail-safe philosophy, confirming design adequacy. No gaps surfaced from the BOP analog.

The {{entity:Remote Monitoring Gateway}} cyber isolation at {{sub:SUB-REQ-034}} lacked a VER entry. VER-REQ-067 (penetration test: zero write-through, 100 MΩ isolation resistance) was created and linked. SAFETY_VALIDATION_FINDING stored for H-010 (Cyber attack, SIL-3): the hardwired SIL-3 trip chain is inherently air-gapped but this was not explicitly stated as a cybersecurity requirement.

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

Gaps Closed

  • SYS-REQ-011 (CCF SIL-4 architectural constraint, 8h DC battery coping): closes Station Blackout scenario gap; linked from STK-REQ-001, verified by VER-REQ-066 (independent safety analysis)
  • VER-REQ-062 – {{sub:SUB-REQ-026}} ALC 200ms timing test (10-repeat, sil-3)
  • VER-REQ-063 – {{sub:SUB-REQ-027}} seismic qualification analysis to IEEE 344 (sil-3)
  • VER-REQ-064 – {{sub:SUB-REQ-028}} EMC immunity test IEC 61000-6-2/6-7 (sil-3)
  • VER-REQ-065 – {{sub:SUB-REQ-056}} MGCB type test certificate inspection (sil-3)
  • VER-REQ-067 – {{sub:SUB-REQ-034}} RMG cyber isolation penetration test (sil-2)
  • 4 reversed VER trace links corrected (SUB-REQ-040, SUB-REQ-042, IFC-REQ-015, IFC-REQ-017)

Post-session: 176 requirements, 166 trace links, 68 VER requirements. Active unverified SUB: 15 (all SIL-2 or below, no timing-critical functions).

Verdict

Conditional pass pending final review. Five of six ConOps scenarios are fully covered end-to-end from STK to VER. The Station Blackout (CCF) scenario gap has been addressed at system level with SYS-REQ-011, but the subsystem-level decomposition (DC battery coping time requirement on the battery system, alternate AC activation timing) remains unwritten — these lie at the boundary with sister systems and require a system-of-systems decomposition session to close fully. The project may advance to SE_REVIEW with the caveat that SYS-REQ-011 subsystem decomposition is an open item.

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