System

Emergency Diesel Generator for a UK Nuclear Licensed Site — final decomposition session. At session start, two subsystems remained pending against a spec tree of seven: {{entity:Fuel Oil System}} (SIL-2) and {{entity:Alternator Subsystem}} (SIL-2). Both have been fully decomposed this session, closing the first-pass decomposition of the entire system at 127 requirements and 94 trace links across 7 diagrams.

Decomposition

The {{entity:Fuel Oil System}} is decomposed into five components: {{entity:Day Tank}} (gravity-feed buffer, ≥8h autonomous reserve), {{entity:Bulk Fuel Storage Tank}} (168h external reserve, bunded and seismically qualified), {{entity:Fuel Transfer Pump Set}} (duty/standby 415V AC pumps), {{entity:Fuel Filtration Assembly}} (duplex 10-micron filter with DP alarm), and {{entity:Fuel Supply Pipework and Valve Assembly}} (fire-isolated, cathodically protected buried sections). The gravity-feed day tank arrangement was chosen to eliminate powered dependency on the fuel supply path during the LOOP event — the design basis failure condition recorded in ARC-REQ-006.

flowchart TB
  n5["external - Bulk Storage Tank"]
  n1["component - Fuel Transfer Pump"]
  n0["component - Day Tank"]
  n2["component - Duplex Fuel Filter"]
  n4["external - Fuel Injection System"]
  n3["component - Fuel Level and Alarm Unit"]
  n5 -->|bulk fuel supply| n1
  n1 -->|diesel fill| n0
  n0 -->|gravity feed 0.3 bar| n2
  n2 -->|filtered fuel 3-6 bar| n4
  n3 -->|level alarm / pump start| n1

The {{entity:Alternator Subsystem}} is decomposed into five components: {{entity:Synchronous Generator Assembly}} (salient-pole, Class H insulation, IP54), {{entity:Automatic Voltage Regulator}} (static AVR, ±0.5% steady-state, PMG-powered), {{entity:Brushless Excitation System}} (PMG → main exciter → rotating diode rectifier), {{entity:Generator Stator Winding and Thermal Protection}} (PT100 RTDs, anti-condensation heaters, 155°C trip), and {{entity:Generator Bearing and Mechanical Support Assembly}} (DE sleeve bearing lubricated from engine lube header, NDE grease-lubricated). Brushless excitation was selected over a brush-type arrangement because slip ring maintenance introduces debris risk in a nuclear building and the PMG-based chain provides voltage build-up independent of terminal voltage — essential for black-start from LOOP.

flowchart TB
  n4["external - Diesel Engine"]
  n0["component - Rotor and Field Winding"]
  n3["component - Brushless Exciter"]
  n2["component - Automatic Voltage Regulator"]
  n1["component - Stator and Armature Winding"]
  n5["external - Generator Protection Relay"]
  n4 -->|shaft torque 1500 RPM| n0
  n0 -->|field rotation| n3
  n3 -->|DC excitation current| n0
  n2 -->|excitation demand signal| n3
  n1 -->|11kV terminal voltage| n2
  n1 -->|11kV 3-phase output| n5

Analysis

Lint at medium severity flagged 59 findings (7 high, 52 medium). The three high-severity findings are ontological mismatches: the {{trait:Physical Object}} trait absent from the UHT classifications of Fuel Oil System, Cooling System, and Automatic Load Controller. Both subsystems were reclassified with context emphasising their physical embodiment (steel tanks, pipework, pump sets). The ALC mismatch is a known prior-session finding. The 52 medium findings are the standard set of implicit interface and safety constraint gap warnings that accumulate across a project at this stage and are appropriate material for the forthcoming QC pass.

Requirements

Six SUB requirements for the Fuel Oil System: day tank 8h capacity ({{sub:SUB-REQ-040}}), bulk tank 168h capacity ({{sub:SUB-REQ-041}}) deriving from {{sys:SYS-REQ-002}}, transfer pump auto-start within 10s ({{sub:SUB-REQ-042}}), 10-micron filtration with DP alarm ({{sub:SUB-REQ-043}}), bulk fuel fire isolation within 10s as the safe state ({{sub:SUB-REQ-044}}, derives from {{sys:SYS-REQ-004}}), and Day Tank thermal management above 5°C for cold-start ({{sub:SUB-REQ-045}}). Three IFC requirements cover the gravity-feed fuel supply pressure interface ({{ifc:IFC-REQ-015}}), fill line flow rate ({{ifc:IFC-REQ-016}}), and fail-safe volt-free level switch contacts to the LAIP ({{ifc:IFC-REQ-017}}). Four VER requirements verified by functional test with the EDG running.

Five SUB requirements for the Alternator Subsystem: AVR voltage regulation ±0.5% steady-state and ±6% transient ({{sub:SUB-REQ-046}}), stator winding 155°C trip ({{sub:SUB-REQ-047}}), bearing 100°C trip with lube oil pressure range ({{sub:SUB-REQ-048}}), excitation build-up to rated voltage within 3s of 95% speed ({{sub:SUB-REQ-049}}), and stator earth fault safe state de-energising excitation within 200ms ({{sub:SUB-REQ-050}}). Three IFC requirements cover the VSMU 4-20mA feedback to AVR ({{ifc:IFC-REQ-018}}), engine-alternator mechanical coupling torsional acceptance criteria ({{ifc:IFC-REQ-019}}), and 3-wire PT100 stator RTD interface to PTLU with fail-to-alarm fault handling ({{ifc:IFC-REQ-020}}).

Next

All seven subsystems are now marked complete in the spec tree. The project is ready for a QC pass (Flow C) to address the 52 medium-severity lint findings, verify trace coverage across the full 127-requirement set, and check for requirements written early in the project (sessions 574-576) that may lack the EARS compliance and rationale depth expected by the current protocol.