Carbon dioxide in several established designs; helium in many advanced high-temperature concepts; other gases are less common.
Fission System • Broad Coolant Family
Gas-Cooled Reactor
Gas-cooled reactors span several generations and physics choices. Established systems have used carbon dioxide with graphite moderation, while many advanced concepts use helium for high-temperature service; fast-spectrum gas-cooled concepts form a separate research branch.
CO₂ or Helium FamiliesThermal + Fast ConceptsOperating HeritageAdvanced Development
Phase 4 • Page-Specific Technical Summary
What this reactor category means
System definition
A family of reactors that removes core heat with a gas coolant, with moderator, fuel form, pressure, temperature, and conversion cycle selected by the design.
A family of reactors that removes core heat with a gas coolant, with moderator, fuel form, pressure, temperature, and conversion cycle selected by the design.
Intended applications
Grid electricity, cogeneration, high-temperature industrial heat, hydrogen-production studies, and modular energy systems.
Grid electricity, cogeneration, high-temperature industrial heat, hydrogen-production studies, and modular energy systems.
Public boundary
Technology classification, system relationships, maturity, and engineering challenges only. No design parameters, calculations, control logic, procedures, procurement data, or build instructions.
Technology classification, system relationships, maturity, and engineering challenges only. No design parameters, calculations, control logic, procedures, procurement data, or build instructions.
Phase 2 • Public-Level System Architecture
Core System Elements
The full-width rendering above provides the visual system map. The callouts below identify the principal public-level functions without repeating the same reactor graphic a second time.
- Core region; moderator design-dependent
- Reactor vessel
- Gas circulators
- Heat exchanger or direct-cycle interface
- Power or process-heat conversion
- Cooling and heat rejection
System roles are family-level and intentionally qualitative. Exact geometry, operating conditions, safety classification, equipment boundaries, and licensing basis remain design-specific.
Phase 4 • Concise Comparison
Engineering comparison
Usually thermal in graphite-moderated systems; fast-spectrum gas-cooled concepts remain an advanced research category.
Long operating heritage for some CO₂-cooled fleets plus active development and limited deployment of advanced helium-cooled systems.
Electricity and high-temperature useful heat, with configuration-specific passive-safety and efficiency goals.
Pressure-boundary design, gas leakage, circulator reliability, graphite and fuel qualification, high-temperature materials, inspection, heat exchangers, decay-heat removal, licensing, and economics.
Phase 4 • Technical Education + Terminology + Accuracy
Learn the System
This learning layer explains the system in public engineering language. Definitions are intentionally conceptual: they teach terminology and relationships without providing operating procedures, design set-points, construction dimensions, or protected implementation details.
Maturity checkpoint: Includes both historical/commercial operating heritage and advanced designs at demonstration or development maturity.
System walkthrough
How it works — at a safe public level
- Fission heats a gas coolant as it passes through or around the reactor core.
- The gas carries heat to a steam generator, intermediate heat exchanger, or direct gas-turbine system depending on design.
- A power-conversion system produces electricity and may also provide high-temperature process heat.
- The coolant returns to the reactor through a blower, circulator, or natural-circulation path depending on architecture.
- Materials, pressure boundary, graphite behavior, and decay-heat removal dominate the engineering trade space.
Key distinction: “Gas-cooled reactor” is a broad family: established carbon-dioxide-cooled designs and advanced helium-cooled HTGR/VHTR concepts should not be treated as one standardized machine.
Technical vocabulary
Terms worth knowing
Helium coolant
Inert gas used as a reactor coolant in many high-temperature gas-cooled concepts because it remains single phase and does not become radioactive through ordinary chemical activation pathways as readily as many alternatives.
High-temperature gas-cooled reactor
A graphite-moderated reactor family using gas coolant, commonly helium, and fuel capable of high-temperature operation.
Graphite moderator
Graphite used to slow neutrons while tolerating high temperatures; common in several gas-cooled and pebble-bed concepts.
TRISO fuel
Tristructural-isotropic coated-particle fuel. Each tiny fuel kernel is surrounded by multiple ceramic and carbon layers designed to retain fission products.
Heat exchanger
Equipment that transfers heat between fluid systems without mixing the fluids.
Natural circulation
Fluid motion driven by density differences rather than by a mechanical pump.
Process heat
Useful thermal energy supplied directly to industrial processes rather than first being converted to electricity.
Thermal efficiency
The fraction of reactor heat converted into useful electrical or mechanical output.
Decay heat
Heat that continues after the fission chain reaction stops because radioactive fission products continue to decay.
Pressure boundary
The set of vessels, piping, and connected components that retain pressurized reactor coolant or working fluid.
MWe
Megawatts electric: the electrical power output delivered by a generating system.
MWth
Megawatts thermal: the rate of heat production before conversion to electricity or useful process heat.
Application Context
Where the family fits
Commercial Operating Heritage
Carbon-dioxide-cooled systems provide decades of operating experience, though fleet futures and designs vary.
High-Temperature Industry
Helium-cooled concepts target useful heat for hydrogen, chemicals, and other industrial processes.
Advanced Modular Systems
Prismatic and pebble-bed variants pursue modular deployment with different fuel-handling and maintenance strategies.
10-Phase Reactor Program • Phase 2
Standardized fission technical specification
Standardized public-safe fields support comparison across every fission and fusion family. Values are intentionally qualitative; licensing data, dimensions, operating windows, calculations, control logic, and build instructions remain offline.
Carbon dioxide or helium, depending generation and design
Usually graphite-moderated thermal spectrum; variants exist
Metal-clad, coated-particle, prismatic, or other design-specific fuel
Legacy large plants and advanced modular high-temperature concepts
Steam or gas-cycle electricity and industrial heat
Commercial heritage in selected countries; advanced variants remain development
Waste depends on fuel and moderator. Graphite management, activated structures, spent fuel, and decommissioning remain material lifecycle issues.
Specification boundary: Family-level technology classification and lifecycle context—not a vendor datasheet, safety analysis, procurement specification, or construction package.
Ten-Phase Reactor Program · Phase 5
Safety, licensing, lifecycle & public trust
Phase 5 treats safety as a full lifecycle obligation: site and license basis, fuel/material supply, operation, environmental controls, safeguards, emergency preparedness, waste, decommissioning, and disciplined public claims.
Pressure-boundary integrity, depressurization, coolant inventory, graphite oxidation where applicable, high-temperature materials, decay heat, confinement, and power-conversion interfaces.
In the United States, Parts 50 and 52 remain available, and Part 53 became available in 2026 as a risk-informed, performance-based, technology-inclusive pathway for eligible class 103 commercial plants. For microreactors, the NRC has also proposed Part 57; it is not yet an available licensing route. Exact applicability depends on design, mission, fuel, and site.
Gas cooling can reduce some water dependence but does not eliminate heat rejection. Evaluate the selected gas, high-temperature materials, graphite inventories where used, blower/compressor power, process-heat interfaces, cooling system, construction, and site-specific emissions or discharge permits.
Fuel form and enrichment are design-specific. Many advanced-reactor concepts depend on fuels or supply chains that differ from today's light-water fleet, including some designs that require HALEU. Fuel qualification, fabrication capacity, transport packages, and secure material availability can be deployment gates.
Advanced fuel forms can change waste streams but do not eliminate them. Account for spent or used fuel, activated structures, coolant or salt cleanup where applicable, contaminated maintenance hardware, storage, transportation, decommissioning, and disposal pathways.
Apply graded material control and accounting, physical protection, cybersecurity, transport controls, insider-risk controls, and international safeguards as applicable to the material, facility, mission, and jurisdiction. Public pages must not reproduce Safeguards Information, sensitive security layouts, response tactics, or adversary-useful detail.
Eligible SMRs, non-light-water reactors, and non-power facilities may use the performance-based 10 CFR 50.160 emergency-preparedness framework. EP scope and any plume-exposure planning zone are consequence- and site-informed; neither reactor size nor passive features automatically reduce the requirement to zero.
State the maturity actually achieved, name the applicable authority, separate family-level evidence from project approval, and distinguish demonstrated performance from projections or marketing.
May describe a design family as research, demonstration, early deployment, or commercially operating only where the cited design actually supports that maturity. May say U.S. licensing pathways exist without claiming that a particular design is licensed.
Do not convert passive features into 'walk-away safe,' fuel robustness into 'meltdown-proof,' smaller size into 'no emergency planning,' or advanced fuel into 'waste-free' or 'proliferation-proof.' Do not treat concept maturity as project approval.
Phase 5 evidence:NRC — Advanced Reactor Licensing ↗NRC — Part 53 / Advanced Reactor Rulemaking ↗NRC — Performance-Based Emergency Preparedness ↗DOE — HALEU Availability Program ↗
Public classification only: This is not a safety finding, licensing opinion, environmental permit, emergency plan, security plan, safeguards determination, waste-acceptance decision, or legal advice. Exact obligations belong to the applicable regulator and project authority.
Phase 7 • Release Certification & Production Integration
Release certification
This page has passed the Reactor Programs Phase 7 branch-level release review for internal routing, asset references, canonical metadata, maturity and terminology controls, source-authority links, accessibility essentials, responsive markup, and release traceability.
Production boundary: this certification covers the Reactor Programs branch package. Parent-site shared CSS, navigation, search, logo, and global assets remain controlled by the Aurora website root and must already exist in production. Live deployment is not implied until the package is uploaded and the production URLs are rechecked.