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The difference is where the water boils. A pressurized-water reactor (PWR) keeps the water that passes through the reactor core under enough pressure to remain liquid. That hot primary water transfers heat through steam-generator tubes to a separate secondary-water loop, where steam is produced for the turbine. A boiling-water reactor (BWR) allows its reactor coolant to boil inside the reactor vessel; separators and dryers remove droplets, and the resulting steam travels directly to the turbine.
In both designs, the turbine converts steam energy into electricity, the exhaust is condensed back into water, and pumps return that water to the heat source. The key question—whether reactor water reaches the turbine—has a different answer for each design: not in a PWR, but yes in a BWR, where the turbine steam is made from reactor coolant.
The difference at a glance
| Feature | Pressurized-water reactor (PWR) | Boiling-water reactor (BWR) |
|---|---|---|
| Where boiling occurs | In a separate secondary loop inside the steam generator | Inside the reactor vessel, in the core flow path |
| Steam sent to the turbine | Steam made from secondary-loop water | Reactor-coolant steam after moisture separation and drying |
| Heat-transfer arrangement | Two water loops coupled through steam-generator tube walls | A direct reactor-vessel-to-turbine steam path |
| Do the water streams mix? | Primary reactor coolant and secondary water remain separate | The reactor coolant is itself the source of turbine steam |
| Moisture control | Steam-generator and turbine-cycle equipment controls steam quality | Moisture separators and dryers are located at the top of the vessel |
| Condensate return | Condensed turbine exhaust is pumped back to the steam generators | Condensed turbine exhaust is pumped back to the reactor vessel |
Both are light-water reactor designs: ordinary water carries heat and, in the core, also serves the reactor’s neutron-moderating function. Their steam cycles look similar after steam reaches the turbine, but the route from fission heat to that steam is fundamentally different.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchHow a pressurized-water reactor produces steam
A PWR has a primary loop that removes heat from the fuel and a secondary loop that receives that heat in a steam generator. The loops are connected thermally, not by a shared water stream.
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1. Fission heats the primary coolant
Fuel inside the reactor core releases heat through fission. Water circulating through the core absorbs that heat and leaves the vessel at a high temperature. This water is called the primary coolant because it is the first fluid to receive heat from the fuel.
2. A pressurizer suppresses boiling in the core
The primary system is kept at high pressure. A pressurizer provides the pressure-control function, allowing the plant to keep the core coolant in the liquid state even though it is very hot. “Pressurized” therefore means that boiling is suppressed in the primary loop; it does not mean that the plant produces no steam anywhere. Steam is made later, on the other side of the steam-generator tubes.
3. Primary water flows through steam-generator tubes
Pumps circulate the heated primary coolant from the reactor vessel to a steam generator. Inside that component, the primary water travels through many tubes. The tube walls form the heat-transfer boundary: heat crosses the metal, while the primary water stays inside its own circuit.
4. Secondary water boils outside the tubes
Feedwater from the secondary cycle surrounds the hot tubes. Heat raises the temperature of this separate water until it boils. The resulting steam belongs to the secondary loop, not the primary reactor-coolant loop.
The U.S. Nuclear Regulatory Commission describes the separation directly: “Water from the reactor and the water in the steam generator that is turned into steam never mix.” That separation keeps most radioactivity associated with the reactor side rather than sending primary coolant through the turbine cycle.
5. Secondary steam drives the turbine-generator
Steam leaves the steam generator through the main steam line and expands through the turbine. The turbine turns a generator, which converts the mechanical rotation into electricity. Since the turbine steam came from secondary water, the turbine is not receiving a direct flow of PWR primary coolant.
6. Exhaust steam is condensed and returned
After expanding through the turbine, the steam enters a condenser. Cooling removes its latent heat and turns it back into liquid water. Feedwater pumps send that condensate back to the steam generators, completing the secondary-side cycle. The primary coolant, meanwhile, remains in its own closed circuit and returns to the reactor vessel after giving up heat.
How a boiling-water reactor produces steam
A BWR removes the separate steam-generator boundary from the basic heat-to-steam path. Reactor coolant boils in the reactor pressure vessel, and the steam produced there is routed to the turbine after its moisture content is reduced.
1. Very pure reactor coolant rises through the core
Fission heats water as it moves upward through the fuel assemblies. The NRC describes this process as follows: “A steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat.” The water is both the reactor coolant and the fluid that will become turbine steam.
2. Boiling occurs inside the reactor vessel
As the heated water rises, part of it changes from liquid to vapor. The vessel therefore contains a two-phase mixture of water and steam. Unlike a PWR primary loop, the BWR core flow path is intentionally designed to permit boiling.
3. Separators remove liquid droplets
The steam-water mixture reaches moisture separators near the top of the vessel. These devices remove entrained liquid droplets and return that water to the vessel. Removing bulk moisture protects the turbine from carrying large amounts of liquid into its steam path.
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Steam then passes through dryers that remove additional fine moisture. The goal is drier steam before it enters the main steam line. The separation and drying equipment is part of the vessel’s internal steam path, rather than a separate steam generator outside the reactor vessel.
5. Reactor steam goes directly to the turbine
The dried steam leaves the vessel through the main steam line and expands through the turbine-generator. In this design, the turbine steam is derived from reactor coolant, so the reactor-water-to-turbine answer is yes for a BWR.
6. Condensate returns to the vessel
Turbine exhaust enters a condenser and becomes liquid water again. Pumps return that water to the reactor vessel, where it re-enters the circulation path and is heated again. There is no PWR-style secondary steam generator between the reactor vessel and the turbine.
Does reactor water go through the turbine?
For a PWR, no. The water that circulates through the core remains in the primary system. It transfers heat through steam-generator tubes to secondary water, and that secondary water becomes the turbine steam. The two waters do not mix during normal heat transfer.
For a BWR, yes in the important process sense: reactor coolant boils in the vessel, and the separated, dried vapor is sent through the main steam line to the turbine. The liquid portion removed by separators returns to the vessel, while the vapor expands through the turbine before being condensed and recycled.
This distinction is more precise than saying that a BWR has “no pressure control.” A BWR still operates its vessel and steam system under controlled pressure. The defining difference is that boiling is allowed in the core flow path, whereas a PWR’s primary pressure is used to keep core coolant liquid.
What happens after the turbine in both designs?
The two reactor types converge on the same broad steam-cycle idea once steam enters the turbine:
- Expansion: steam expands through turbine stages and transfers energy to the turbine shaft.
- Generation: the shaft drives an electrical generator.
- Condensation: low-pressure exhaust steam is cooled in a condenser and becomes liquid water.
- Pumping: pumps raise the condensate pressure and return it to the heat source—steam generators in a PWR or the reactor vessel in a BWR.
The equipment names and detailed water-treatment arrangements can differ by plant design, but the essential purpose is the same: recover the water instead of discarding it, then circulate it through another heating-and-expansion cycle.
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Why the two-loop PWR arrangement matters
Separation of primary and secondary water
The PWR’s steam generator is a heat exchanger. The primary side can be kept within the reactor system while the secondary side supplies the turbine. The NRC identifies this separation as a way to keep most radioactivity in the reactor area. It also gives operators a distinct boundary to monitor between the reactor coolant and the turbine-side water.
Boiling is moved away from the core
Because the primary coolant remains liquid in the core, the phase change that creates steam occurs in the steam generator. The reactor vessel and primary piping therefore carry hot pressurized water rather than a boiling mixture during normal operation.
Steam quality is managed in the secondary cycle
Secondary water boils in the steam generator, and the plant’s steam-cycle equipment delivers suitable steam to the turbine. The PWR still has to manage moisture and water chemistry, but the reactor vessel itself is not the steam-producing component.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the direct BWR arrangement matters
A compact heat path
A BWR sends steam from the reactor vessel to the turbine without a separate steam generator. Heat does not need to cross a second set of tube walls before steam is formed; the reactor coolant changes phase in the vessel itself.
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Boiling naturally creates a mixture of liquid and vapor. Separators and dryers remove droplets before turbine admission. Without that moisture control, liquid carried with the steam could reduce steam quality and impose undesirable conditions on turbine components.
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The turbine-side steam originates in the reactor coolant
Because the steam is made from reactor water, the direct path is the defining BWR feature. The condenser and return pumps then send the recovered water back toward the vessel rather than to a separate steam generator.
Fuel-assembly figures and U.S. fleet context
Fuel-assembly counts illustrate that the two reactor types use different core layouts, but the count alone does not determine how steam is produced. The NRC’s current reactor overview gives these typical ranges:
| Reactor type | Typical number of fuel assemblies | What the figure means |
|---|---|---|
| PWR | 150–200 | A typical core inventory range cited by the NRC; the exact count varies by reactor design. |
| BWR | 370–800 | A typical core inventory range cited by the NRC; the exact count varies by reactor design. |
The U.S. Department of Energy reported in an update published in May 2025 that more than 65% of U.S. commercial reactors are PWRs and roughly one-third are BWRs. Those percentages describe the U.S. fleet at that time, not the global reactor fleet, and they can change as reactors enter or leave service.
Common misconceptions, corrected
“Pressurized” means a PWR has no steam
Incorrect. Pressure suppresses boiling in the PWR primary coolant. Steam is still produced in the secondary side of the steam generator and sent to the turbine.
A BWR has no pressure control
Incorrect. A BWR vessel and steam system operate under controlled pressure. The difference is that the operating design permits coolant to boil in the core flow path.
The PWR steam generator mixes reactor and turbine water
Incorrect. Heat crosses the tube walls, but the primary and secondary waters remain separate. The steam sent to the turbine is made from secondary water.
All steam from a BWR goes straight from fuel to turbine
That description skips critical equipment. The vessel produces a steam-water mixture, and moisture separators and dryers remove liquid before the dried steam enters the main steam line.
The two designs use completely different turbine cycles
Not at the broadest level. Both expand steam through a turbine, condense the exhaust, and recycle the water. Their major difference is the route from reactor heat to the steam entering the turbine.
A practical way to remember the difference
- PWR: reactor water stays liquid, transfers heat through steam-generator tubes, and never mixes with the secondary water that becomes turbine steam.
- BWR: reactor water boils in the vessel, separators and dryers condition the steam, and that reactor-derived steam goes to the turbine.
- Both: turbine exhaust is condensed and pumped back for reuse.
If a diagram shows two separate water loops joined by a steam generator, it is depicting the indirect PWR arrangement. If it shows steam rising from the reactor vessel through separators and then traveling along a main steam line to the turbine, it is depicting the direct BWR arrangement.
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