Index09 SEPT 20265 min
How Does a Water-Cooled SMR Work? An Interactive Diagram
Follow the heat from nuclear fuel to electricity in a water-cooled SMR. Explore the primary loop, steam generator, turbine and condenser in an interactive diagram.
- Nuclear
- Engineering
A water-cooled small modular reactor uses nuclear fission to heat water. In the pressurised-water example here, the primary water stays liquid and transfers heat to a separate water-and-steam circuit. Steam drives a turbine; the turbine drives an electrical generator. A condenser and a heat sink complete the cycle.
The most useful way to read a small modular reactor diagram is to follow energy and fluid separately. Heat can cross a metal wall without the water on one side crossing to the other.
Follow a stage · the other circuits keep running →
Start with heat in the fuel.
Fission energy becomes heat. It crosses the fuel and cladding into pressurised primary water. That water remains liquid in this example.
Illustrative net efficiency fixed at 30%. These are energy accounts at steady state, not a load-following simulation or a rating for a specific SMR. Rejected heat includes all non-electric output, not only the condenser.
What the diagram leaves out
This is a conceptual integral pressurised-water SMR with natural primary circulation. The picture separates the circuits for readability. It omits the containment, pressuriser details, valves, protection systems and cooling-system pumps. Moving tracers show fluid direction; the amber pulses across the exchanger show heat, not mixing. Animation and shaft speeds are illustrative and do not predict flow or rotational speed. Selecting a stage does not stop the other physical processes.
In an intact steam generator, heat crosses the tube wall while primary and secondary fluids remain separate. Tube-side arrangements differ between designs. The coil symbol does not specify which side of a real tube carries each fluid.
What makes this reactor small and modular?
The IAEA describes SMRs as reactors with an electrical capacity of up to 300 MW per unit, with modular construction intended to allow factory fabrication and transport of components or modules. “Small” describes the unit's scale; it does not specify a coolant or a thermodynamic cycle. IAEA: What are SMRs?
This article follows an integral pressurised-water reactor, or integral PWR: the core and steam generators share a reactor vessel. It is one water-cooled SMR arrangement. Boiling-water SMRs use a different steam path; compare the BWR guide in the Reactor Atlas wiki. Other SMRs use different coolants. The illustration is deliberately one coherent example rather than a composite of incompatible designs.
Where does the heat start?
Fissions deposit energy in the fuel. Heat conducts through a fuel pellet and its surrounding cladding, then transfers to the flowing coolant. The cladding is a material barrier and part of the thermal path; it is not an empty gap that energy jumps across.
Water also affects the neutron population. In this light-water reactor it slows neutrons through collisions, while fuel temperature and water density influence neutron interactions. The neutron calculation determines heating, and the evolving material state feeds back into the neutron calculation.
That conversation is the subject of the worked neutronics–thermal-hydraulics coupling example. Here we zoom out to see where the heat goes next.
Does the water in the core become the turbine steam?
Not in this PWR example. Pressure keeps the primary coolant liquid at its operating temperature. At the steam generator, heat passes through metal tubes into a separate secondary stream, where boiling produces steam. In normal operation with intact heat-transfer surfaces, the fluids remain separate. The NRC describes this distinction between PWRs and boiling-water reactors in its steam-generator explainer.
Pressurising water and pumping water do different jobs. Pressure helps set the boiling condition. A pump supplies a pressure difference that can drive circulation around a loop. A reactor may operate at high pressure while its primary circulation is driven by buoyancy.
| Path | What circulates? | What does it do? |
|---|---|---|
| Primary | Pressurised liquid water | Carries heat from the core to the steam generator |
| Secondary | Water and steam | Receives heat, produces shaft work and returns as feedwater |
| Cooling system | A separate cooling medium | Transfers rejected heat toward the environment |
The interactive picture uses a water cooling circuit for the last path. Real sites can use different heat-rejection arrangements. A cooling tower, if present, belongs to that cooling system; it is not the reactor vessel.
Why is a condenser essential?
After expanding through the turbine, steam still carries energy. The condenser transfers heat to a colder cooling medium and returns the secondary fluid to liquid. A feedwater pump can then send that liquid back toward the steam generator. The closed water-and-steam path is the working cycle of the heat engine. NRC: steam and condensate path
The electrical output is only part of the thermal input. At steady state, for a plant exporting electricity and no useful process heat, the overall energy account is
Here “rejected” collects all non-electric energy leaving the plant boundary. The condenser handles a major part of that heat; the equation does not assign every loss to the condenser alone. Net electricity is what remains after the plant's own electrical consumption.
For the interactive example, choose a fixed, illustrative net efficiency of 30%:
At 100 MW thermal, the account is 30 MW net electric plus 70 MW rejected heat. At 50 MW thermal it becomes 15 MW plus 35 MW. These are constructed examples, not performance figures for a named design. The slider changes a steady energy balance; it does not calculate changing pressures, turbine efficiency or control-system response.
Do water-cooled SMRs need primary pumps?
Some designs use pumps and some use natural circulation. In the latter, the density difference between a hotter rising path and a cooler returning path provides a driving pressure difference. The layout must allow that driving head to overcome the circuit's resistance.
One documented example is the NuScale US600 design: the NRC describes its natural-circulation primary system, with the core and helical-coil steam generators in a common vessel. That identifies a specific design, rather than claiming that all SMRs share the same arrangement. NRC: NuScale US600 overview
The primary loop in our conceptual diagram follows that general buoyancy-driven arrangement. Its feedwater pump belongs to the secondary circuit. “No primary circulation pump” does not mean “no pumps anywhere in the plant.”
What changes when the reactor shuts down?
Stopping the self-sustaining chain reaction does not instantly remove all heat generation: radioactive decay continues to release energy. A shutdown therefore still requires a heat-removal path. The normal power-conversion diagram above does not specify an emergency cooling system or establish its performance.
Natural circulation can participate in a heat-removal strategy, but the relevant circuit, heat sink and available water inventory must be identified for that design. The IAEA report on natural circulation in water-cooled plants treats both the physics and the system conditions involved.
A better way to read the next SMR diagram
Trace one fluid circuit until it returns to its starting point. Then locate every boundary across which energy leaves it. Finally, identify what drives the flow and where the plant ultimately sends its rejected heat.
Those three checks connect a drawing to conservation laws. They also expose what a picture leaves out: pressure control, decay-heat removal, protection systems and operating limits all require more detail than an energy-flow sketch can provide.
For a broader overview of SMR families, passive safety and current designs, continue with How an SMR reactor works in the Reactor Atlas wiki. To explore the core feedback behind this heat source, continue with the interactive coupling experiment. For a broader numerical model, see what is inside Criticality.
Working on a reactor explanation or simulation? Let’s discuss the model and the questions it needs to answer.
