System

STEP Fusion Power Plant — {{entity:Cryogenic Plant}} ({{hex:56D51218}}, SIL 2), one of three remaining pending subsystems in the spec tree (was 5/8 complete, now 6/8 complete). The Cryogenic Plant provides the sustained 4.5 K thermal environment for the superconducting magnets without which the plant cannot operate.

Decomposition

The Cryogenic Plant breaks into four engineering-distinct components. The {{entity:Helium Refrigeration System}} ({{hex:57D73218}}) is the core thermodynamic machinery: dual independent cold box trains each producing 8 kW at 4.5 K and 50 kW of 40–80 K thermal shield cooling from oil-free screw compressors, counterflow heat exchangers, and turbine expanders. Physical separation from the {{entity:Cryogenic Transfer Line Network}} ({{hex:CE851018}}) is deliberate — the HRS is fixed plant in the cryo hall; the CTLN is ~200 m of vacuum-jacketed piping spanning the building to the magnet ports. The {{entity:Helium Management System}} ({{hex:51973218}}) sits alongside the HRS but addresses a distinct operational concern: capturing 200 m³ STP of boil-off per quench event, purifying to ≥99.999%, and returning gas to the compressor supply within 2 hours. The {{entity:Cryogenic Control System}} ({{hex:55B77A18}}) is a SIL-2-rated SCADA/PLC layer monitoring >2,000 cryogenic measurement points and executing automated cool-down sequences.

flowchart TB
  n0["component - Helium Refrigeration System"]
  n1["component - Cryogenic Transfer Line Network"]
  n2["component - Helium Management System"]
  n3["component - Cryogenic Control System"]
  n0 -->|4.5K He supply| n1
  n2 -->|200bar He gas| n0
  n3 -.->|control/setpoints| n0

Analysis

{{trait:Powered}} traits on Cryogenic Plant and HRS are genuine — the HRS compressors draw several MW of electrical power. Lint findings flagging {{trait:Physical Medium}} on the system-level entity are ontologically incidental: helium is the working fluid but the Cryogenic Plant is not itself a material substance; these findings were acknowledged.

Cross-domain search returned {{entity:Plant Control and I&C System}} ({{hex:50B53218}}, similarity 0.82) — a DCS architecture managing cryo, vacuum, and tritium systems for a tokamak with ~250 ms scan cycles. This confirms the CCS design direction and suggests the 100 ms fieldbus requirement in {{ifc:IFC-REQ-029}} is within proven precedent. {{entity:Building Management System}} ({{hex:51F77B58}}) from a nuclear radiochemistry facility with 2,000 I/O points and NIS Regulation cybersecurity hardening provides a useful analog for CCS security requirements not yet captured — flagged for a future session.

Requirements

Five new subsystem requirements were created, all SIL 2:

• {{sub:SUB-REQ-031}} — HRS dual-train redundancy: each train ≥8 kW at 4.5 K, total ≥8 kW on single-train loss
• {{sub:SUB-REQ-032}} — HMS helium recovery: ≥95% of 200 m³ STP quench discharge recovered and purified within 2 hours
• {{sub:SUB-REQ-033}} — CTLN heat leak: total static ingress <500 W, per-segment <10 W/m
• {{sub:SUB-REQ-034}} — CCS cool-down control: ≤5 K/hour gradient at all winding pack sensors, complete within 72 hours
• {{sub:SUB-REQ-035}} — CCS safe state: helium isolation + vent + SMS quench interlock within 10 seconds of fault

Three internal interface requirements were added: {{ifc:IFC-REQ-028}} (HRS→CTLN: 4.5 K ± 0.2 K at 3 bar through DN50 bayonets), {{ifc:IFC-REQ-029}} (CCS→HRS: ≤100 ms fieldbus with hardwired SIL-2 e-stop), {{ifc:IFC-REQ-030}} (HMS→HRS: 200 bar He at ≥99.999% purity, 4-hour refill). Eight VER entries were created (VER-REQ-050 through VER-REQ-057) with trace links from parent SUB/IFC requirements.

Next

Two subsystems remain pending: Remote Handling System (SIL 1) and Power Conversion System (SIL 1). Either can proceed next. Cross-domain BMS cybersecurity analog should be reviewed against CCS requirements when working on those subsystems. The sessions quality gate (8 < 10) requires two more sessions before state transition.