Light-water family
Coolant: H2OModerator: H2OCommercial
- PWR: pressurized primary loop, steam generator, dominant global fleet.
- BWR: boils in core, direct steam to turbine, simpler loop geometry.
Quick Reference
The fastest way to classify a reactor is to ask what slows neutrons, what carries heat, and how much pressure the primary loop carries.
| Family | Core clue | Fuel / moderator / coolant | Pressure behavior | Status | Watch out for |
|---|---|---|---|---|---|
| PWR | Heat goes through a steam generator, not straight to the turbine. | Enriched uranium; light-water moderator and coolant. | Primary loop kept at very high pressure so water does not boil. | Commercial | People confuse the secondary steam cycle with primary coolant flow. |
| BWR | Water boils in the core and steam goes to the turbine. | Enriched uranium; light-water moderator and coolant. | Lower primary pressure than a PWR because boiling is intended. | Commercial | Not every reactor with steam in the loop is a BWR. |
| PHWR / CANDU | Heavy water buys neutron economy and usually on-power refueling. | Heavy-water moderator and coolant; often natural uranium fuel. | Pressurized, but the key distinction is moderator chemistry. | Commercial | CANDU is a family label; PHWR is the reactor class. |
| RBMK / LWGR | Graphite moderator with light-water coolant in pressure channels. | Low-enriched uranium oxide; graphite moderator; light-water coolant. | Channel-type, not one big pressure vessel. | Legacy | Do not generalize Chernobyl to all graphite reactors. |
| Gas-cooled | CO2 or helium carries heat; graphite often slows neutrons. | Magnox / AGR / HTGR variants use different fuels and cladding. | Usually lower pressure than light-water reactors. | Legacy / niche | Gas-cooled is a coolant family, not a single reactor. |
| HTGR | Helium coolant, graphite moderator, TRISO fuel, high outlet temperature. | TRISO fuel in pebbles or prismatic blocks. | Low-pressure primary coolant enables different accident behavior. | Limited operating / advanced | High temperature does not mean high power output by itself. |
| SFR | Fast neutrons, sodium coolant, no moderator. | Metal or oxide fuel, often HALEU / plutonium-bearing fuel. | Low-pressure coolant loop, but sodium chemistry is the hard part. | Prototype / demo | Fast spectrum is not the same as "high temperature." |
| MSR | Fuel and/or coolant are molten salts; pressure is low. | Either liquid fuel in salt or solid fuel with salt coolant. | Atmospheric or low pressure compared with water reactors. | Development | Molten salt is not a synonym for thorium. |
| SCWR | Water runs above its critical point so there is no boiling transition. | Usually light-water fuel concepts with supercritical coolant conditions. | About 22.1 MPa critical pressure, so pressure design is demanding. | Concept | Supercritical is a thermodynamic regime, not a safety label. |
| SMR | Modular size and factory assembly; can be PWR, BWR, HTGR, SFR, or MSR. | Depends on the underlying physics family. | Pressure depends on the underlying reactor design. | Deployment class | SMR is not a coolant or moderator type. |
Decision rule: if two designs use the same fuel but different coolant or moderator, they can behave very differently in accidents, refueling, and waste handling. If two designs use the same coolant but different spectrum, they can diverge just as much.
Reactor Family Tree
Signature view: coolant, moderator, fuel, and maturity branch from the same starting point. This is the screenshot-worthy artifact for the page.
Nuclear power reactor families
Heavy-water family
Coolant: D2OModerator: D2OCommercial
- PHWR / CANDU: natural uranium friendly, strong neutron economy, on-power refueling.
Graphite / gas family
Moderator: graphiteCoolant: CO2 / He / waterLegacy + advanced
- RBMK / LWGR: graphite moderator, light-water coolant, pressure channels.
- GCR / AGR / Magnox: legacy gas-cooled power reactors.
- HTGR: helium, graphite, TRISO, high outlet temperature.
Fast spectrum
No moderatorCoolant: sodium / lead / gasPrototype / demo
- SFR: sodium cooled, breeding and actinide management focus.
Molten salt
Fuel or coolant in saltLow pressureDevelopment
- MSR: liquid-fuel and salt-coolant variants share the same headline but not the same engineering.
Supercritical water
Water above 22.1 MPaNo phase changeConcept
- SCWR: efficiency-oriented water reactor concept with a hard materials problem.
SMR class
Deployment / size classFactory modularityCross-cutting
- SMR can be a light-water, fast, gas-cooled, or molten-salt design.
How to Classify Any Reactor
These are the comparison axes worth reading first. Each one changes how the plant behaves, what fails first, and what the regulator cares about.
Coolant
What carries heat away from the core
Coolant is the working fluid that removes thermal power from fuel. Example: PWRs and BWRs use light water; SFRs use sodium; HTGRs use helium; MSRs may use salt.
Gotcha: a coolant can also be a moderator, but it does not have to be. Sodium fast reactors deliberately avoid moderation.
Moderator
What slows neutrons down
Moderator choice determines neutron spectrum and fuel economy. Example: light water moderates PWRs and BWRs; heavy water moderates PHWRs; graphite moderates RBMKs, GCRs, and HTGRs.
Gotcha: "graphite reactor" is not enough by itself. Graphite plus water, graphite plus gas, and graphite plus no moderator implications are different designs.
Spectrum
Thermal versus fast
Thermal reactors slow neutrons before fission; fast reactors keep neutrons energetic. Example: PWR, BWR, PHWR, RBMK, HTGR are thermal; SFRs are fast.
Gotcha: "fast" does not mean "hotter" or "more powerful." It means the neutron energy spectrum is different.
Pressure
How hard the coolant loop is being pushed
Pressure changes boiling margin, pipe size, and accident mode. Example: PWR primary loops stay highly pressurized; BWRs let the core boil; SCWRs run above 22.1 MPa; MSRs are low pressure.
Gotcha: low pressure is not a free safety win. Sodium chemistry, salt chemistry, and materials damage still matter.
Fuel form
What the fissile material looks like
Fuel form drives enrichment, burnup, reprocessing, and waste handling. Example: PWR/BWR use UO2 pellets; PHWR often uses natural uranium; HTGRs use TRISO particles; SFRs may use metal fuel.
Gotcha: the same coolant can support multiple fuel forms, and the same fuel form can be used in more than one reactor family.
Refueling mode
How and when fuel is swapped
Some designs refuel during shutdowns; others refuel online. Example: PHWR and RBMK families support on-power refueling, which affects capacity factor, maintenance, and proliferation scrutiny.
Gotcha: online refueling is operationally convenient but does not automatically make the plant simpler or safer.
Containment
What keeps releases inside the plant envelope
Containment is the last barrier around accident releases. Example: PWR and BWR plants rely on robust containments; IAEA notes RBMKs were not designed with a pressure-retaining containment structure capable of limiting severe-core-damage releases.
Gotcha: containment quality matters as much as core physics when you compare accident consequences.
Deployment class
SMR is a size and construction strategy
DOE describes advanced SMRs as reactors ranging from tens to hundreds of megawatts and assembled with factory-made modules. Example: NuScale is a light-water SMR, Natrium is a sodium fast SMR, and Xe-100 is an HTGR SMR.
Gotcha: you cannot infer coolant, moderator, or spectrum from the SMR label alone.
Current Fleet Snapshot
IAEA PRIS last updated its type table on 2026-07-06. These counts are useful as a reality check on what is commercially real versus mostly proposed.
| PRIS type | Descriptive name | Operating units | What it says about the market |
|---|---|---|---|
| PWR | Pressurized Light-Water Moderated and Cooled Reactor | 308 | Dominant commercial family worldwide. |
| BWR | Boiling Light-Water Cooled and Moderated Reactor | 45 | Large commercial fleet, but much smaller than PWR. |
| PHWR | Pressurized Heavy-Water Moderated and Cooled Reactor | 46 | Healthy niche, especially where natural uranium is attractive. |
| GCR | Gas Cooled, Graphite Moderated Reactor | 8 | Legacy gas-cooled power is mostly a UK story. |
| LWGR | Light-Water Cooled, Graphite Moderated Reactor | 7 | Graphite-plus-water channel-type family, historically Soviet. |
| HTGR | High Temperature Gas Cooled Reactor | 1 | Still niche, but real, not just theory. |
| FBR | Fast Breeder Reactor | 2 | Fast-spectrum operation exists, but remains rare. |
Interpretation: the fleet is still overwhelmingly light-water. Advanced designs are real, but commercial scale is concentrated in PWRs, BWRs, and PHWRs.
Safety and Tradeoffs
Safety behavior is not just "good" or "bad." It follows from pressure, chemistry, spectrum, refueling, and whether the plant can shed decay heat without active intervention.
Pressure boundary
High-pressure water reactors store more energy in the coolant system, so rupture control and depressurization matter. Example: PWR primary loops are pressurized to keep water from boiling. Gotcha: that same pressure makes pipe and vessel design more demanding.
Coolant chemistry
Sodium is thermally excellent but reacts violently with water and air; salt systems avoid some pressure risks but introduce corrosion, chemistry, and freeze-point issues. Example: SFRs and MSRs both need chemistry discipline, just in different ways.
Decay heat
Every fission reactor keeps producing heat after shutdown. Example: PWRs, BWRs, PHWRs, and HTGRs all need decay-heat removal. Gotcha: the accident question is never "does it stop fission?" but "how does it remove residual heat?"
Containment and confinement
Robust containment lowers severe-release risk. Example: the IAEA notes RBMKs were not designed with a pressure-retaining containment structure capable of limiting releases during severe core damage. Gotcha: this is a design lesson, not a statement that every graphite reactor behaves the same.
Fuel economy versus proliferation
Heavy water and fast-spectrum designs can improve neutron economy, but that can also affect fuel handling and safeguards scrutiny. Example: PHWRs often use natural uranium, while fast breeders can generate more fissile material than they consume. Gotcha: fuel economy is not a pure safety metric.
Operating mode
On-power refueling supports availability but changes maintenance and safeguards planning. Example: CANDU and RBMK families can refuel online. Gotcha: "online refueling" is an operational advantage, not a reason to ignore mechanical complexity.
Advanced and Proposed Designs
These are the reactor names that create most public confusion. The important split is what is deployed now, what is operating only in small numbers, and what is still being licensed or tested.
| Design | Why it matters | Maturity | Representative example | Primary gotcha |
|---|---|---|---|---|
| HTGR | Very high outlet temperature with graphite moderation and helium coolant. | Limited operating; still a small fleet. | PRIS lists 1 operating HTGR as of 2026-07-06. | Great for process heat, but the high temperature claim is not the whole engineering story. |
| SFR | Fast-spectrum breeding and actinide-management potential. | Prototype / demo with a small operating base. | IAEA notes sodium-cooled fast reactors are being developed internationally; NRC tracks Natrium as a 345 MWe sodium fast design. | Sodium chemistry drives much of the plant design burden. |
| MSR | Low-pressure operation and unusual fuel-cycle flexibility. | Development / licensing / test stage. | IAEA says the coolant and/or fuel can be molten salt; NRC issued a construction permit for ACU's MSRR in 2024. | Do not assume thorium, liquid fuel, or breeding unless the specific design says so. |
| SCWR | Efficiency-oriented water reactor concept above the critical point. | Concept / research. | IAEA describes water above 22.1 MPa with no phase change. | The materials and heat-transfer problem is harder than the thermodynamic pitch sounds. |
| SMR | Factory modularity and smaller site footprint. | Mixed: certified, pre-application, and concept designs coexist. | DOE describes advanced SMRs as tens to hundreds of megawatts; NRC tracks multiple developers. | SMR is a deployment strategy, not a reactor physics family. |
| Microreactor | Even smaller power systems for remote or defense-adjacent use cases. | Pre-application and research. | NRC maintains a dedicated microreactor page and reviews developer engagement. | Microreactor is smaller than SMR, but not automatically simpler to license. |
Common Mistakes
1
Nuclear is not one reactor type
Example: a PWR, BWR, PHWR, and SFR all make electricity from fission, but they differ on coolant, moderator, spectrum, and accident behavior. Gotcha: lumping them together erases the actual engineering tradeoffs.
2
Molten salt does not automatically mean thorium
Example: IAEA defines MSRs by salt in the fuel or coolant, not by thorium specifically. Gotcha: some molten-salt concepts use uranium fuel, some use thorium, and some use solid fuel with salt coolant.
3
SMR is not a physics family
Example: NuScale is a light-water SMR, Natrium is a sodium fast SMR, and Xe-100 is an HTGR SMR. Gotcha: the acronym tells you more about construction and size than about neutrons.
4
Fast does not mean high temperature
Example: SFRs use a fast spectrum because they do not moderate neutrons, not because their coolant is automatically hotter than an HTGR. Gotcha: spectrum and outlet temperature are separate design axes.
5
Decay heat still exists after shutdown
Example: any fission reactor that just tripped still needs heat removal. Gotcha: the most important accident question is how the plant sheds residual heat when pumps, power, or operators are unavailable.
6
Paper concepts are not built-plant economics
Example: a vendor cost estimate for an MSR or SMR should not be compared directly with a completed PWR fleet average unless you control for learning curve, financing, and licensing stage. Gotcha: timeline and financing can dominate the headline reactor cost.
Related Pages
Primary sources used
These are the official sources behind the page's definitions and current-status statements. The counts and status notes are intentionally dated where they can drift.