System

STEP Fusion Power Plant ({{entity:step fusion power plant}}) — gap-close session against a fully-decomposed 8-subsystem project. All spec tree entries were marked complete entering this session. With 323 requirements and 617 trace links on entry, the work shifted to resolving the 44 lint findings (4 high, 40 medium) identified by airgen lint. No new subsystem decomposition was performed; this session addressed cross-cutting regulatory and safety coverage gaps that prior decomposition sessions had not closed.

Decomposition

The Tritium Plant internal architecture remains as established:

flowchart TB
  n0["component - Plasma Exhaust Processing System"]
  n1["component - Isotope Separation System"]
  n2["component - Tritium Storage and Delivery System"]
  n3["component - Blanket Tritium Extraction System"]
  n4["component - Atmosphere Detritiation System"]

The {{entity:Tritium Plant}} is the anchor for this session’s work: it owns the primary DBA tritium release pathway, the largest occupational radiological hazard, and the main environmental discharge point — making it the correct owner for three of the five new coverage requirements.

Analysis

High-severity findings: All four Physical Object ontological mismatches ({{entity:power conversion system}}, {{entity:cryogenic plant}}, {{entity:vacuum system}}, {{entity:tritium plant}}) were confirmed as already resolved by reclassification in session-552. The lint showed stale hex codes from a session-start cache; acknowledgments stored to suppress recurrence.

Coverage gaps (SYS → SUB): Seven concepts referenced in {{sys:SYS-REQ-016}}, {{sys:SYS-REQ-018}}, {{sys:SYS-REQ-019}}, and {{sys:SYS-REQ-008}} had no subsystem-level decomposition. The most critical gap was the Design Basis Accident pathway: {{sys:SYS-REQ-018}} defines four DBAs (tritium release, in-vessel failure, magnet quench cascade, loss-of-cooling) but none were decomposed to subsystem requirements. The Tritium Plant and Tokamak Core Assembly are the physical systems responsible for bounding the first two scenarios; {{entity:Superconducting Magnet System}} for the third.

System-Essential redundancy: {{trait:System-Essential}} entities {{entity:tritium plant}} and {{entity:superconducting magnet system}} lacked system-level redundancy requirements. {{sub:SUB-REQ-068}} covered Tritium Plant confinement barriers but not process throughput failover. {{sub:SUB-REQ-074}} covered MPSS N+1 converters but not passive quench energy absorption — the critical passive safety function for DBA scenario (c).

Coil set material specification: {{entity:coil set}} carries {{trait:Synthetic}} and {{trait:Physical Medium}} traits, flagging absence of material property requirements. No requirement specified conductor technology, critical current density, or fatigue cycle lifetime — all prerequisites for verifying {{sub:SUB-REQ-025}}.

Requirements

Eight new subsystem requirements created ({{sub:SUB-REQ-078}} through {{sub:SUB-REQ-085}}) with eight corresponding verification entries (VER-082 through VER-089):

• {{sub:SUB-REQ-078}} — Tritium Plant DBA automatic isolation within 30 s, ≤1 g tritium release (derives from {{sys:SYS-REQ-018}})
• {{sub:SUB-REQ-079}} — Tokamak Core Assembly passive decay heat removal for 72 hours post-loss-of-cooling, ≤1 mSv public dose (derives from {{sys:SYS-REQ-018}})
• {{sub:SUB-REQ-080}} — Tritium Plant UK Ionising Radiations Regulations 2017 (SI 2017/1075) compliance, 1 mSv/year dose limit, RPS nominations (derives from {{sys:SYS-REQ-016}})
• {{sub:SUB-REQ-081}} — Vacuum System seal integrity across all operational modes, post-intervention RGA verification ≤1×10⁻⁹ Pa·m³/s per penetration (derives from {{sys:SYS-REQ-008}})
• {{sub:SUB-REQ-082}} — Tritium Plant environmental permitting under EP Regulations 2016 and Nuclear Installations Act 1965, ISO 14001:2015 EMS certification (derives from {{sys:SYS-REQ-019}})
• {{sub:SUB-REQ-083}} — Tritium Plant N+1 process module redundancy, ≥50% throughput (100 Pa·m³/s DT) with any module disabled
• {{sub:SUB-REQ-084}} — SMS passive quench energy absorption for 100% stored magnetic energy (≥10 GJ) without active triggering, single-channel failure tolerant
• {{sub:SUB-REQ-085}} — TF Coil Set Nb3Sn CICC conductor specification: ≥700 A/mm² at 12 T/4.5 K, <5% Ic degradation over 60,000 electromagnetic cycles

All requirements include SIL inheritance: Tritium Plant and Tokamak Core requirements carry sil-3, Vacuum System and SMS carry sil-2. Baseline BL-SESTEPFUSIONPOWERPLANT-028 created at 339 requirements / 631 trace links.

Next

Residual medium-severity findings: regulatory acknowledgment findings for grid code (Digital/Virtual cybersecurity, Institutionally Defined standards references) and national grid (Signalling, Institutionally Defined) remain open. These are interface-level concerns best addressed by adding IFC requirements specifying the ENTSO-E / National Grid ESO grid connection protocol and cybersecurity requirements for the Power Conversion System grid interface. The next session should address these before moving to QC flow.