Concept of Operations (ConOps) — ISO/IEC/IEEE 15289 — Description | IEEE 29148 §6.1
Generated 2026-03-27 — UHT Journal / universalhex.org
Demonstrate that fusion energy can generate net electricity at grid scale, proving the engineering and economic viability of the spherical tokamak as a path to commercial fusion power. STEP bridges the gap between experimental plasma physics devices (JET, ITER) and commercial power stations by integrating plasma confinement, tritium breeding, heat extraction, and power conversion into a single operational plant delivering ~100 MW net electrical output. Without this system, the transition from fusion science to fusion energy remains unproven, and investment in commercial fusion fleet deployment lacks an engineering basis.
| Stakeholder | Relationship | Hex Code |
|---|---|---|
| Control Room Operator | licensed operators managing pulse operations, plasma monitoring, manual intervention during off-normal events. 4 per shift, 24/7. Derived from S-001, S-002, S-004. | — |
| Nuclear Safety Regulator (ONR) | licenses facility, approves safety case, sets dose limits, inspects. First fusion nuclear site licence. Derived from all scenarios. | — |
| Remote Handling Engineer | operates robotic systems for in-vessel maintenance of activated components. Critical path for plant availability. Derived from S-005. | 008532F9 |
| Tritium Plant Operator | manages tritium fuel cycle — separation, storage, fuelling, detritiation, waste. Specific tritium handling certification. Derived from S-001, S-003. | 010D3AF9 |
| Grid Transmission Operator (National Grid ESO) | receives pulsed ~100 MW output, Grid Code compliance, scheduling. Derived from S-001. | — |
| Radiation Protection Adviser | radiological protection ALARP, area classification, dosimetry, environmental discharge monitoring. Derived from S-005, S-004. | 00857AF9 |
| Environment Agency | regulates radioactive waste disposal, atmospheric/liquid discharges, environmental impact. | — |
| Local Community and Public | affected by construction, noise, transport, emergency planning zone, perceived risk. | — |
| Fusion Research Community (UKAEA, EUROfusion) | provides physics basis, validates design assumptions, receives experimental data. | — |
| Decommissioning Authority | end-of-life dismantling, waste categorisation, site remediation. Design for decommissioning. | — |
| Mode | Description |
|---|---|
| Plasma Startup | all pre-pulse interlocks satisfied, cryogenics at 4K, vacuum <1e-6 Pa → gas puff, ECR breakdown, current ramp to ~10 MA via central solenoid, auxiliary heating engaged → plasma reaches Q>1 burn conditions |
| Steady-State Burn | plasma at burn conditions Q>=5 → sustained D-T fusion at ~150 MK, tritium breeding TBR>=1.1, ~100 MW net electrical to grid, real-time plasma control → operator-commanded shutdown or end-of-pulse |
| Planned Shutdown | operator command or schedule → heating ramp-down, current ramp-down over 10-30s, fuel injection ceased, magnets de-energised, tritium purge → vessel in safe standby |
| Emergency Shutdown | disruption detection or manual emergency stop → fast plasma termination, fast magnet discharge, tritium containment isolation, runaway electron mitigation → structural inspection required |
| Remote Maintenance | plasma terminated, vessel cooled, tritium removed → robotic replacement of divertor/blanket/diagnostics, hot cell operations, 2-6 month campaigns → leak test, re-commissioning |
| Commissioning | construction complete, licence granted → subsystem tests, hydrogen/deuterium plasmas, progressive power ramp-up, safety validation, first D-T → full-power D-T operation authorised |
Shift supervisor and 4 operators manage a 6-hour burn pulse at Q>=5. Plasma control handles ELMs and NTMs via ECCD. Pellet injector maintains fuel. Tritium plant processes exhaust. Net 100 MW to grid. Orderly ramp-down at end of pulse.
Locked mode triggers disruption. Mitigation fires shattered pellet injection within 10 ms. 400 MJ thermal quench within design limits. Runaway electrons avoided. 4-hour turnaround to next pulse.
Isotope separation column leak detected. Automatic line isolation. Power reduced to 60 MW. Remote repair by tritium operators. <0.1 g release within limits. Full power resumed within shift.
Ground acceleration exceeds 0.1g OBE. Fast plasma shutdown within 100 ms. Magnets discharged, coolant isolated. Post-event remote inspection, leak test, structural assessment. Recovery 2-4 weeks.
6-month operation complete. Tritium removed, vessel purged. Remote handling replaces 8 divertor cassettes and inspects blanket modules. 4-month campaign. Re-commissioning includes leak test, magnet cool-down, first hydrogen plasma.
| Category | Constraint |
|---|---|
| Physical | tokamak hall temperature 15-35°C, seismic design to SSE 0.15g, biological shield minimum 2m concrete equivalent, building footprint ~200m x 150m, total mass ~40,000 tonnes |
| Vacuum | base pressure <1e-6 Pa in ~1000 m3 vessel, leak rate <1e-9 Pa·m3/s per seal, all in-vessel materials UHV-compatible, bake-out to 200°C |
| Cryogenic | magnets at 4.5K, ~80 kW cooling capacity, liquid helium inventory ~50 tonnes, cool-down time ~2 weeks, warm-up time ~1 week |
| Radiation | 14.1 MeV neutron flux ~1e18 n/m2/s at first wall, activation dose rate >10 Sv/hr on in-vessel components post-operation, worker dose limit 20 mSv/year (ALARP target <1 mSv/year) |
| Regulatory | UK Nuclear Installations Act, ONR nuclear site licence, REPPIR emergency planning, Environmental Permitting Regulations for radioactive discharges, COMAH for tritium inventory |
| Electromagnetic | toroidal field 3-4T at plasma centre, pulsed poloidal fields generating significant eddy currents in vessel structures, EMC shielding required for diagnostics and control systems |
| Operational tempo | target 50% availability, pulse duration 2-8 hours, inter-pulse turnaround 30 min to 4 hours, 6-month operational campaigns between 4-month maintenance periods |
| System | Interface | Hex Code |
|---|---|---|
| National Electrical Grid | 400 kV connection, exports ~100 MW during burn, imports ~50 MW for auxiliaries, Grid Code compliance, pulsed power profile coordination | 50C57A58 |
| Helium Supply Chain | commercial helium procurement for make-up, on-site storage and purification, critical for magnet operations | — |
| Cooling Water System | river or sea water abstraction for condenser cooling, environmental thermal discharge limits, drought contingency planning | 56C51018 |
| Radioactive Waste Disposal | low-level and intermediate-level waste routes, spent component storage, decommissioning waste strategy | — |
| Emergency Services | site emergency plan, off-site emergency planning zone coordination, mutual aid agreements with local fire/ambulance | — |
| IAEA Safeguards | tritium accountancy reporting, inspection access for nuclear materials oversight | — |
flowchart TB n0["system<br>STEP Fusion Power Plant"] n1["system<br>STEP Fusion Power Plant"] n2["actor<br>Control Room Operators"] n3["actor<br>Nuclear Safety Regulator (ONR)"] n4["actor<br>National Grid (400kV)"] n5["actor<br>Cooling Water Supply"] n6["actor<br>Helium Supply Chain"] n7["actor<br>Emergency Services"] n8["actor<br>Radioactive Waste Disposal"] n9["actor<br>IAEA Safeguards"] n1 -->|Plasma status, alarms, commands| n2 n1 -->|Safety case, dose reports, incidents| n3 n1 -->|100 MW export / 50 MW import| n4 n5 -->|Condenser cooling water| n1 n6 -->|Liquid helium supply| n1 n1 -->|Emergency alerts, mutual aid| n7 n1 -->|LLW/ILW packages| n8 n1 -->|Tritium accountancy| n9
STEP Fusion Power Plant — Context