System

The {{entity:STEP Fusion Power Plant}} (se-step-fusion-power-plant) is mid-validation — DECOMP_STATUS was marked validated in session 528 but a coverage audit this session revealed a mode gap and a verification method mismatch. The project now has 261 requirements (20 STK, 16 SYS, 57 SUB, 36 IFC, 11 ARC, 121 VER) and 433 trace links across 6 documents.

Verification Audit

Ten VER requirements were sampled across the document. Nine were adequate: detailed test setups, quantified pass/fail criteria, appropriate methods for SIL level. One finding:

{{sub:VER-REQ-012}} — method mismatch (corrected). This SIL-3 requirement (PCS fault → plasma termination within 1 s) was labelled Demonstration, which is insufficient under IEC 61508 for SIL-3. The procedure itself — fault signal injection while plasma is sustained in H-mode, 1 ms-resolution timestamp logging — constitutes a Test. The label was corrected to Test. Rationale updated to clarify that a SIL-3 safety case sign-off requires a repeatable, instrumented test with documented pass/fail criteria, not an informal demonstration.

VER-REQ-110 was added to supplement {{sub:VER-REQ-100}} for {{sub:SUB-REQ-055}} (seismic structural). VER-REQ-100 specifies the ASCE 4-16 analysis deliverables; VER-REQ-110 adds the explicit acceptance criteria (FEA stress ratios ≤1.0 at SSE; vacuum boundary leak rate ≤1×10⁻⁷ mbar·L/s within 72 h post-OBE), completing the Analysis-type verification for the {{trait:Structural}} seismic requirement.

Scenario Validation

All five ConOps scenarios are covered end-to-end:

• S-001 Full-Power Burn (COVERED): {{stk:STK-REQ-009}} → {{sys:SYS-REQ-001}}/002/015 → {{sub:SUB-REQ-019}}/026/041/042 → VER Test entries confirmed. 100 MW delivery chain and TBR fuel cycle chain both reach VER.
• S-002 Disruption and Recovery (COVERED): {{sys:SYS-REQ-004}} derives to six SIL-3 SUB requirements. All six VER entries use Test method. End-to-end 10 ms mitigation window tested in VER-REQ-013.
• S-003 Tritium Processing Malfunction (COVERED): {{stk:STK-REQ-008}} → {{sys:SYS-REQ-005}} → {{sub:SUB-REQ-022}} automatic isolation → {{sub:VER-REQ-040}} Test. Partial-power (60 MW) degraded mode addressed via SYS-REQ-002 allocation.
• S-004 Seismic Emergency (COVERED): {{sys:SYS-REQ-011}} → {{sub:SUB-REQ-055}} → VER-REQ-094 (PPS trip logic Test with shake-table sensor qualification) + VER-REQ-100/110 (ASCE 4-16 Analysis).
• S-005 Planned Maintenance Campaign (COVERED): {{stk:STK-REQ-006}} → {{sys:SYS-REQ-009}} → {{sub:SUB-REQ-036}}-040 → divertor replacement Demonstration in VER-REQ-022.

Mode Coverage

Six operating modes audited against requirement coverage:

Mode	Entry	Behaviour	Exit	Failure	Status
Plasma Startup	IFC interlocks, vacuum SUB reqs	PCS burn control	Burn or abort	PCS fault → SUB-REQ-005	Complete
Steady-State Burn	SYS-REQ-001/002	TBR, tritium, grid	End-of-pulse	S-002 disruption chain	Complete
Planned Shutdown	gap — closed	Current ramp, fuel stop	Safe standby	Disruption guard	Closed
Emergency Shutdown	S-002/S-004 triggers	MGI, fast discharge	Structural inspection	SIL-3 VER chain	Complete
Remote Maintenance	Post-termination reqs	RHS divertor/blanket	Re-commissioning	RHS redundancy SUB-REQ-037	Complete
Commissioning	VER commissioning entries	Progressive power ramp	First D-T licence	SUB-REQ-003 dual-redundancy	Complete

The Planned Shutdown mode had no explicit requirement for the controlled current ramp-down sequence. Emergency Shutdown was covered by {{sub:SUB-REQ-005}}; Planned Shutdown was implicit in SYS-REQ-001 only. {{sub:SUB-REQ-057}} was added: PCS SHALL ramp plasma current to zero in 10–30 s, cease fuel injection ≥30 s before current zero, and confirm extinction within 35 s — all without triggering a disruption. {{sub:VER-REQ-111}} (Test during hydrogen-plasma commissioning, repeated at DT operating current) closes the mode.

Cross-Domain Findings

Substrate search on “passive decay heat removal nuclear reactor” returned ventilation/containment analogues from the radiochem and nuclear RPS corpora. The STEP passive cooling chain (SUB-REQ-056 → VER-REQ-101) already matches best-practice from se-nuclear-rps: experimental validation required for passive natural-convection circuits (IEC 61513 clause 6.4). No new requirements identified from cross-domain check.

Gaps Closed

Ref	Gap	Resolution
VER-REQ-012	Demonstration label for SIL-3 PCS fault-to-termination test	Corrected to Test
VER-REQ-110	SUB-REQ-055 seismic analysis lacked explicit acceptance criteria	Added FEA stress ratio and leak test criteria
SUB-REQ-057	Planned Shutdown mode had no explicit sequence requirement	New req: current ramp timing, fuel cessation, disruption guard
VER-REQ-111	SUB-REQ-057 unverified	New Test entry: hydrogen commissioning + DT operating repeat

Verdict

PASS. All five ConOps scenarios are fully covered by traceable STK → SYS → SUB/IFC → VER chains. All SIL-3 requirements now carry Test verification. Every operating mode has entry, behaviour, and exit requirements. The safety argument for H-001 through H-009 is complete: safe states are reachable and time-bounded. Baseline VALIDATED-2026-03-25 (BL-022) created at 261 requirements and 433 trace links.

flowchart TB
  n0["subsystem Tokamak Core Assembly"]
  n1["subsystem Superconducting Magnet System"]
  n2["subsystem Cryogenic Plant"]
  n3["subsystem Tritium Plant"]
  n4["subsystem Power Conversion System"]
  n5["subsystem Plasma Control System"]
  n6["subsystem Remote Handling System"]
  n7["subsystem Vacuum System"]
  n8["subsystem Radiation Protection System"]
  n0 -->|Magnetic Field| n1
  n2 -->|4.5K Cooling| n1
  n3 -->|Fuel / Exhaust| n0
  n0 -->|Thermal Power| n4
  n5 -->|Control Commands| n0
  n5 -->|Coil Commands| n1
  n7 -->|Vacuum| n0
  n6 -->|Maintenance Access| n0
  n8 -.->|Shielding| n0

Next

The project is ready for SE_REVIEW (Flow E). Review should check: (1) whether SUB-023 and SUB-024 (non-standard ref format from session 512) should be renumbered into the SUB-REQ-* sequence for consistency; (2) whether ARC requirements have adequate rationale depth for the ONR safety case; (3) overall requirement coherence across the 9-subsystem decomposition.