STEP Fusion Power Plant

Rationale: Control Room Operator, S-001 Full-Power Burn: operators must continuously monitor plasma parameters during 6-hour burn pulses to detect instabilities and decide on intervention timing.

Rationale: test update

Rationale: Nuclear Safety Regulator (ONR), all scenarios: ONR must license the first fusion nuclear facility; the safety case must meet established nuclear standards adapted for fusion-specific hazards.

Rationale: Nuclear Safety Regulator (ONR), S-003 Tritium Processing Malfunction: tritium is the principal nuclear material; loss-of-accountability triggers regulatory escalation and potential licence conditions.

Rationale: Remote Handling Engineer, S-005 Planned Maintenance: in-vessel dose rates exceed 10 Sv/hr post-operation, making human entry impossible; all maintenance must be robotic with hot cell support.

Rationale: Remote Handling Engineer, S-005 Planned Maintenance: 4-month maintenance campaign target drives plant availability to 50%; exceeding this timeline directly reduces energy output and economic viability.

Rationale: Tritium Plant Operator, S-001 Full-Power Burn: tritium self-sufficiency is a fundamental mission requirement; external tritium supply is limited and expensive, so breeding and recycling must close the loop.

Rationale: Tritium Plant Operator, S-003 Tritium Processing Malfunction: automatic isolation prevents inventory loss and environmental release; 30s threshold limits release to <0.1 g per scenario analysis.

Rationale: Grid Transmission Operator (National Grid ESO), S-001 Full-Power Burn: the fundamental mission of STEP is to demonstrate net electricity generation; Grid Code compliance is mandatory for grid connection.

Rationale: Grid Transmission Operator (National Grid ESO), S-001/S-004: pulsed power profile requires grid scheduling; unplanned loss of 100 MW generation affects grid stability.

Rationale: Radiation Protection Adviser, S-005/S-004: IRR17 dose limits and ALARP principle are non-negotiable regulatory requirements; 14.1 MeV neutron environment and activation make dose management critical.

Rationale: Radiation Protection Adviser, S-005 Planned Maintenance: maintenance campaigns involve variable dose rates across the facility; automated zoning prevents inadvertent exposure during complex multi-team operations.

Rationale: Environment Agency: EPR compliance is a condition of operation; continuous monitoring provides the evidence base for the environmental safety case and public dose assessment.

Rationale: Environment Agency: no UK geological disposal facility exists yet for ILW; the plant must demonstrate a credible waste strategy with long-duration interim storage to obtain environmental permits.

Rationale: Local Community and Public: public dose limit under IRR17; community acceptance depends on demonstrating that fusion is radiologically safe under all conditions including accidents.

Rationale: Local Community and Public, S-003/S-004: REPPIR regulations require off-site emergency planning; rapid notification is essential for public protection during tritium release or seismic events.

Rationale: Fusion Research Community (UKAEA, EUROfusion): STEP's dual mission includes advancing fusion science; diagnostic data validates physics models and informs DEMO/commercial reactor design.

Rationale: Fusion Research Community (UKAEA, EUROfusion), S-001: breeding blanket validation is a key STEP mission deliverable; no prior device has operated a breeding blanket in a DT neutron environment at reactor scale.

Rationale: Decommissioning Authority: NDA and ONR require a credible decommissioning strategy at the design stage; 30-year timeline aligns with UK nuclear decommissioning practice and funding models.

Rationale: Decommissioning Authority: minimising ILW reduces long-term storage burden and cost; material choice at design stage is the primary lever for waste classification outcomes.

Rationale: Derives from STK-REQ-009 (net 100 MW to grid) and mission statement. Q≥5 is the minimum gain to produce ~500 MW fusion power from ~100 MW auxiliary heating, yielding ~100 MW net after recirculating power. Pulse duration drives energy yield per campaign.

Rationale: Derives from STK-REQ-009. ~500 MW fusion thermal power requires ~33% gross thermal efficiency to yield ~165 MW gross; after ~65 MW recirculating power for magnets, heating, and auxiliaries, ≥100 MW net is delivered. 25% gross-to-net accounts for all parasitic loads.

Rationale: TBR ≥ 1.1 means 10% excess production over consumption rate. At planned 500 MW fusion power with 50% availability, annual DT consumption is ~4 kg T; 10% surplus yields ~400 g/year reserve accumulation, reaching ≥1 kg within 3 years. The 1 kg reserve provides 2-3 month fuel buffer against breeding blanket underperformance. The original text said 'sufficient tritium' which is non-measurable; this revision specifies both the rate and target inventory. Revised in validation session 519 to address AmbiguityBlacklist finding.

Rationale: Derives from STK-REQ-002, addresses H-001 (SIL 3). 10 ms response time is driven by thermal quench timescale of ~1 ms; mitigation must be initiated during the pre-thermal-quench phase. 0.5 MJ/m² is the tungsten damage threshold for single events.

Rationale: Derives from STK-REQ-001 and STK-REQ-003, addresses H-002 (SIL-3 tritium release). 0.1 g single-event limit derives from UK nuclear site licence conditions and ONR regulatory guides for fusion: at this inventory level, atmospheric dispersion models show ground-level concentration at the site boundary remains below 1% of the occupational exposure limit. Two-barrier requirement derives from nuclear containment principles. 'Nominal operating, anticipated transient, and design basis accident' replaces ambiguous 'normal' to comply with ISO 29148 unambiguous language requirement; updated validation session 520.

Rationale: Derives from STK-REQ-003, addresses H-003 (SIL 2). 50 GJ stored energy in HTS magnets must be extracted before conductor damage occurs; 300 K hot-spot limit prevents irreversible degradation of HTS tape. 30s discharge time balances voltage limits against thermal margin.

Rationale: Passive decay heat removal is safety-critical (SIL-2, hazard H-004 LOCA). VER-REQ-093 specifies physical Test on a full-scale passive decay heat removal test rig. Analysis alone cannot account for as-built thermal resistance, fouling, or water chemistry effects on natural circulation. IEC 61508 SIL-2 requires Test verification for this failure mode. Changed from Analysis to Test in validation session 520 to resolve quality gate blocker.

Rationale: Derives from vacuum constraints and STK-REQ-001 (plasma operation). Impurity ingress from leaks poisons the plasma and triggers disruptions; 1e-6 Pa base pressure is required for plasma breakdown and burn purity.

Rationale: Derives from STK-REQ-005, STK-REQ-006. 4-month campaign duration drives 50% plant availability target. 2 mm positioning accuracy is required for divertor cassette alignment to magnetic field geometry and coolant seal engagement.

Rationale: Derives from STK-REQ-009 and operational tempo constraint. 50% availability with 100 MW net output yields ~440 GWh per year, demonstrating economic relevance. Availability is limited by inter-pulse turnaround (30 min–4 hr) and unplanned downtime.

Rationale: SIL-3 seismic trip requirement. 100 ms shutdown window is set by maximum halo current exposure duration before structural damage to in-vessel components; beyond 100 ms, asymmetric disruption loads exceed design basis. 10 second full subsystem safe-state transition covers magnet hold current reduction, tritium confinement isolation, and vacuum system standby mode. Removed the adjective 'fast' (ambiguous) as the 100 ms bound is self-defining; added explicit 10-second subsystem transition bound absent from original. Revised in validation session 519.

Rationale: Neutron streaming dose rate must be measured in-situ at full-power operation; MCNP analysis cannot fully account for as-built penetration tolerances, cable routing gaps, and local shielding variations. VER-REQ-095 specifies calibrated dosimetry measurement at all occupied area boundaries during full-power plasma. Changed from Analysis to Test in validation session 520 to resolve quality gate blocker and meet IEC 61508 SIL-1 requirements for occupied area radiation protection.

Rationale: Derives from STK-REQ-017. 40 diagnostic systems is the minimum for comprehensive plasma characterisation of a burning DT plasma; 1 µs synchronisation enables correlation of fast MHD events across diagnostic channels.

Rationale: Derives from STK-REQ-019, STK-REQ-020. Reduced-activation ferritic-martensitic steels (e.g., EUROFER) and tungsten selection at design stage determines activation products and decay timescales. 80% LLW target minimises ILW storage burden.

Rationale: Derives from STK-REQ-009. Grid Code compliance is mandatory for connection; voltage and frequency tolerances are National Grid ESO standard requirements; THD limit prevents interference with other grid users.

Rationale: A fusion power plant generates neutron activation products and tritium, creating radiobiological hazards to workers and the public. UK law requires compliance with IRR 2017 and ONR site licence conditions. The 1 mSv/year limit (above background) is the HSE-recommended constraint for designated areas at nuclear facilities. This requirement flows from STK-REQ-003 (safety assessment principles), STK-REQ-004 (tritium accountability), and the Biological/Biomimetic ontological trait assigned to the STEP system entity reflecting biological dose considerations.

Rationale: STK-REQ-010 mandates 24-hour advance notification; no SYS requirement currently implements this stakeholder need. UK Grid Code CC.6.3.2 requires mandatory advance notice for scheduled plant outages. The 15-minute unplanned interruption notification is derived from National Grid's Balancing and Settlement Code and Grid Code requirements for frequency response obligations: a delay beyond 15 minutes may trigger Grid Code non-compliance penalties and affect National Grid's ability to dispatch balancing mechanisms.

Rationale: STK-REQ-003 requires demonstration of a deterministic nuclear safety case using Design Basis Accidents and basic safety levels; no SYS requirement currently decomposes this. The four DBA categories represent the STEP-specific risk inventory: tritium release is the primary radiological source term; in-vessel component failure is the primary structural hazard; magnet quench cascade is the primary energy release event; and loss of decay heat cooling is the primary post-shutdown hazard. The 1 mSv DBA dose limit is derived from IAEA SSR-2/1 and ONR technical assessment guide NS-TAST-GD-005 for near-field nuclear installations.

Rationale: STK-REQ-013 mandates compliance with environmental permitting regulations; no SYS requirement currently implements this. Environmental Permitting (England and Wales) Regulations 2016 apply to all nuclear sites discharging radioactive substances to the environment; compliance is a condition of the ONR nuclear site licence. ISO 14001 certification provides an independent, internationally recognised framework for ongoing environmental compliance management. Annual third-party audit is the minimum frequency required by ONR guidance for nuclear site environmental management systems.

Rationale: STK-REQ-017 mandates data sharing within 30 days; no SYS requirement implements this. STEP is a publicly funded programme with open-science obligations under UKRI data management policy. The 30-day window is the STK-stated constraint derived from the UK Research and Innovation open data requirements for nationally significant science infrastructure. FAIR data principles are the international standard for scientific data interoperability, adopted by EUROfusion and mandatory for fusion science facilities receiving EU/UK public funding.