System

The {{entity:STEP Fusion Power Plant}} ({{hex:5ED53219}}) is a demonstration fusion power plant using a compact spherical tokamak design to achieve net electricity generation — bridging the gap between experimental devices (JET, ITER) and commercial power stations. The system confines deuterium-tritium plasma at >100 million degrees using superconducting magnets, breeds its own tritium fuel from lithium blankets, and delivers ~100 MW net electrical power to the grid. It operates under a UK nuclear site licence with full tritium inventory management, remote maintenance of activated components, and a defined emergency planning zone.

The mission: prove that fusion energy can generate net electricity at grid scale, providing the engineering basis for commercial fleet deployment. Without this system, the transition from fusion science to fusion energy remains unproven. The UHT trait profile shows {{trait:Synthetic}}, {{trait:Powered}}, {{trait:Structural}}, {{trait:Observable}}, {{trait:Intentionally Designed}}, {{trait:Outputs Effect}}, {{trait:State-Transforming}}, {{trait:System-integrated}}, {{trait:System-Essential}}, {{trait:Rule-governed}}, {{trait:Compositional}}, {{trait:Temporal}}, {{trait:Regulated}}, {{trait:Economically Significant}}, and {{trait:Ethically Significant}} — reflecting a complex, regulated, safety-critical energy production facility.

ConOps

Six operating modes defined: Plasma Startup ({{hex:56F53210}}), Steady-State Burn ({{hex:55F73218}}), Planned Shutdown ({{hex:40B43A10}}), Emergency Shutdown ({{hex:40F53A10}}), Remote Maintenance ({{hex:51853A18}}), and Commissioning ({{hex:50B53A50}}).

S-001 Full-Power Burn: Shift supervisor and four control room operators manage a 6-hour burn pulse at Q>=5. Plasma control handles ELMs via pacing and NTMs via ECCD. Pellet injector maintains fuel mix. Tritium plant processes exhaust gas continuously. Net 100 MW to grid. Mid-pulse NTM detected and stabilised automatically. Orderly ramp-down at pulse end.

S-002 Disruption and Recovery: Locked mode develops from error field. Disruption mitigation fires shattered pellet injection within 10 ms. Thermal quench deposits 400 MJ — within design limits. Runaway electrons avoided. Automated cooldown and structural health check. 4-hour turnaround.

S-003 Tritium Malfunction: Isotope separation column leak detected by room monitors. Automatic isolation, power reduced to 60 MW. Remote repair within shift, <0.1 g tritium released within operational limits.

S-004 Seismic Emergency: Ground acceleration exceeds 0.1g OBE threshold. Fast plasma shutdown within 100 ms. All systems safe-stated via hardwired interlocks. Recovery 2-4 weeks after remote inspection confirms no damage.

S-005 Maintenance Campaign: After 6-month operation, remote handling replaces 8 divertor cassettes (~5 tonnes each) and inspects blanket modules. 4-month campaign including hot cell processing and re-commissioning.

Hazard Register

ID	Description	Severity	Freq	SIL	Safe State
H-001	Plasma disruption — 1 GJ thermal quench, hundreds of MN EM forces	Critical	High	3	Massive gas injection, vessel monitoring
H-002	Tritium release — 1-3 kg inventory through barrier failure	Catastrophic	Rare	3	Building isolation, detritiation
H-003	Magnet quench — 50 GJ energy, rapid helium boil-off	Critical	Low	2	Fast discharge to dump resistors
H-004	Loss of coolant — vessel ingress or decay heat removal loss	Critical	Low	2	Isolation valves, passive cooling
H-005	Loss of vacuum — beryllium-air reaction, toxic aerosol	Critical	Low	2	Vessel isolation, filtered containment
H-006	Runaway electrons — >10 MeV beam, first wall perforation	Critical	Medium	3	Material injection beam dispersal
H-007	Activated dust explosion — Be/W dust exceeds LEL	Critical	Rare	2	Inert gas flood
H-008	Loss of cryogenic cooling — cascading magnet quench	Critical	Low	2	Controlled discharge, evacuation
H-009	Seismic event — simultaneous LOCA and quench	Catastrophic	Rare	3	Seismic trip, passive cooling
H-010	Neutron streaming — dose rates above limits in occupied areas	Major	Medium	1	Radiation interlocks

Cross-domain search surfaced {{entity:Interlock and Emergency Shutdown System}} ({{hex:51B77A59}}) and {{entity:Emergency Shutdown Sequencer}} ({{hex:D7E53A19}}) as relevant analogs for the disruption mitigation architecture.

Stakeholders

Role	Relationship	Hex	Key Concerns
Control Room Operator	Primary ops, 24/7	{{hex:01AD72F9}}	Plasma monitoring, disruption response
Nuclear Safety Regulator (ONR)	Licensing, oversight	{{hex:00857AFD}}	First fusion nuclear site licence
Remote Handling Engineer	In-vessel maintenance	{{hex:008532F9}}	Plant availability critical path
Tritium Plant Operator	Fuel cycle management	{{hex:010D3AF9}}	Tritium accountancy, ALARP dose
Grid Transmission Operator	Power export/import	{{hex:00A53AF8}}	Pulsed profile, Grid Code compliance
Radiation Protection Adviser	ALARP, dosimetry	{{hex:00857AF9}}	Worker/public dose limits
Environment Agency	Discharge regulation	—	Tritium/activation product releases
Local Community	Emergency planning zone	—	Perceived risk, public acceptance
Fusion Research Community	Physics basis	—	Experimental data, design validation
Decommissioning Authority	End-of-life	—	Design for decommissioning

Operating Environment

Physical: Tokamak hall 15-35°C, seismic design to SSE 0.15g, biological shield >=2m concrete, ~200m x 150m footprint, ~40,000 tonnes total. Vacuum: UHV <1e-6 Pa in ~1000 m3 vessel, leak rate <1e-9 Pa·m3/s. Cryogenic: 4.5K magnets, 80 kW cooling, ~50 tonnes liquid helium. Radiation: 14.1 MeV neutron flux ~1e18 n/m2/s at first wall, >10 Sv/hr contact dose post-operation. Electromagnetic: 3-4T toroidal field, significant eddy currents. Regulatory: Nuclear Installations Act, ONR site licence, REPPIR, COMAH, Environmental Permitting Regulations. Operational tempo: 50% availability target, 2-8 hour pulses, 6-month campaigns.

External Interfaces

External System	Interface	Protocol/Notes
National Grid	400 kV connection	~100 MW export / ~50 MW import, Grid Code
Cooling Water	River/sea abstraction	Thermal discharge limits
Helium Supply	Commercial procurement	Critical for magnet operations
Waste Disposal	LLW/ILW routes	Storage and decommissioning strategy
Emergency Services	Mutual aid	Off-site emergency planning zone
IAEA Safeguards	Tritium accountancy	Inspection access

flowchart TB
  STEP["STEP Fusion Power Plant"]
  CRO["Control Room Operators"]
  ONR["Nuclear Safety Regulator (ONR)"]
  GRID["National Grid (400kV)"]
  CW["Cooling Water Supply"]
  HE["Helium Supply Chain"]
  ES["Emergency Services"]
  RW["Radioactive Waste Disposal"]
  IAEA["IAEA Safeguards"]
  STEP -->|Plasma status, alarms, commands| CRO
  STEP -->|Safety case, dose reports| ONR
  STEP -->|100 MW export / 50 MW import| GRID
  CW -->|Condenser cooling water| STEP
  HE -->|Liquid helium supply| STEP
  STEP -->|Emergency alerts, mutual aid| ES
  STEP -->|LLW/ILW packages| RW
  STEP -->|Tritium accountancy| IAEA

Next

The scaffold session should derive STK requirements from the five ConOps scenarios, focusing first on the safety-critical stakeholder needs arising from H-001 (plasma disruption, SIL 3), H-002 (tritium release, SIL 3), and H-006 (runaway electrons, SIL 3). The plasma control and disruption mitigation subsystem is the highest-risk, most architecturally significant area — it should be the first subsystem decomposed, ahead of the tritium plant and remote handling. The cross-domain analog with {{entity:Emergency Shutdown Sequencer}} systems suggests the disruption mitigation architecture should be explored for proven sequencing patterns from process safety.