The essential difference is where the water boils. A pressurized-water reactor (PWR) keeps its primary coolant under enough pressure that it stays liquid while passing through the reactor core. That hot water transfers heat through steam-generator tubes to a separate, lower-pressure water loop, which boils and sends its steam to the turbine. A boiling-water reactor (BWR) allows reactor coolant to boil inside the reactor vessel; moisture separators and dryers remove liquid droplets, and the resulting steam travels directly to the turbine.
In both designs, the turbine exhaust is condensed back into water and pumped into the reactor’s steam-producing circuit. The key question—whether reactor coolant itself reaches the turbine—therefore has different answers: no for a PWR’s normal steam path, yes for a BWR.
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The two designs 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 | Primary and secondary loops exchange heat through steam-generator tubes | A direct reactor-vessel-to-turbine steam path |
| Do the two water supplies mix? | No. The primary coolant and secondary water remain separated by tube walls. | There is no separate primary-to-secondary steam-generator boundary; reactor coolant becomes the turbine-steam source. |
| Moisture control | The steam generator and turbine-cycle equipment condition the secondary steam | Separators and dryers at the top of the vessel remove droplets before turbine admission |
| 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: water removes heat from the fuel and, ultimately, supplies the steam cycle. “Pressurized” in PWR means that the primary coolant is kept from boiling in the core; it does not mean the plant has no steam.
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Fission in the fuel produces heat. Water carries that heat away from the fuel, and a steam cycle converts the heat into shaft motion in a turbine. The turbine drives a generator. After leaving the turbine, the steam is cooled until it condenses, and pumps return that water to the part of the plant where it will be heated again.
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The shared cycle is easier to understand if it is divided into three functions:
- Heat production: Fission heats water flowing through or above the reactor core.
- Steam production: The design either transfers that heat to a separate water loop (PWR) or permits the reactor coolant to boil in the vessel (BWR).
- Heat rejection and recycle: Turbine exhaust is condensed and returned as feedwater, so the plant continually recirculates water rather than consuming a fresh supply for every revolution of the turbine.
The difference is not whether a turbine exists or whether water is recycled. It is the boundary between the reactor coolant and the steam that enters the turbine.
How a pressurized-water reactor produces steam
A PWR uses two water loops connected by a steam generator. The primary loop passes through the reactor core and carries heat away from the fuel. The secondary loop receives heat through metal tubes in the steam generator and turns into the steam that powers the turbine.
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1. Fission heats the primary coolant
Water circulates through the reactor core, where it absorbs the heat released by fission. This water is called primary coolant because it is part of the system directly associated with the reactor core.
2. Pressure keeps the primary coolant liquid
A pressurizer and high pressure in the primary system keep the coolant from boiling even though it is very hot. The coolant therefore leaves the core as hot liquid rather than as a steam-water mixture. This is the specific meaning of “pressurized” in the PWR name.
Pressure does not eliminate the need to control temperature, flow or pressure. It changes the boiling condition: at the controlled primary-system pressure, the coolant can transport core heat as liquid water.
3. Primary coolant flows through steam-generator tubes
The hot primary water travels through many tubes inside a steam generator. On the other side of each tube is the secondary water. The tube wall separates the two supplies while allowing heat to cross.
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The primary coolant gives up heat as it passes through the tubes and then returns to the reactor vessel to be heated again. It remains in the closed primary circuit; it does not become the steam sent to the turbine.
4. Secondary water boils outside the tubes
Feedwater in the secondary loop surrounds the hot primary-side tubes. Heat crossing the tube walls raises the secondary water to its boiling point and produces steam. Because this water is in a different circuit, the steam is made without mixing the primary coolant into the turbine cycle.
“Water from the reactor and the water in the steam generator that is turned into steam never mix.” — U.S. Nuclear Regulatory Commission, The Pressurized Water Reactor (PWR)
This separation is a central PWR characteristic. The U.S. Nuclear Regulatory Commission explains that it keeps most radioactivity in the reactor area rather than sending primary coolant through the turbine-side water system.
5. Secondary steam drives the turbine-generator
The steam produced in the steam generator enters the main steam line and flows to the turbine. Its expansion turns turbine blades, and the turbine shaft drives the electrical generator. The turbine’s steam is secondary-loop steam, not water that has circulated through the reactor core.
6. Condensed water returns to the steam generators
After expanding through the turbine, the steam is cooled and condensed. Pumps send the resulting water back toward the steam generators, where it can absorb heat from the primary loop and become steam again.
How a boiling-water reactor produces steam
A BWR removes the separate steam-generator boundary from the steam-producing path. Reactor coolant rises through the core, absorbs heat and boils inside the reactor pressure vessel. The steam-water mixture is then dried in the vessel before it leaves for the turbine.
1. Very pure reactor coolant rises through the core
Water circulates upward through the fuel region and absorbs fission heat. In a BWR, this water is both the reactor coolant and the water that will become turbine steam.
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2. Boiling occurs in the reactor vessel
As the heated coolant rises, part of it changes from liquid to steam. The result is a steam-water mixture inside the vessel rather than a single-phase liquid stream like the primary coolant in a PWR.
“A steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core, absorbing heat.” — U.S. Nuclear Regulatory Commission, Boiling Water Reactors
The vessel and steam system still operate under controlled pressure. Calling the design “boiling-water” means boiling is permitted in the core flow path; it does not mean the plant operates without pressure control.
3. Separators remove bulk liquid droplets
The steam-water mixture rises to the upper part of the reactor vessel. Moisture separators remove entrained liquid water from the mixture. This prevents large amounts of liquid from being carried onward with the steam.
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Steam dryers remove additional moisture after separation. The dried steam then leaves the vessel through the main steam line. Moisture control matters because the turbine is intended to receive steam rather than a wet mixture containing substantial liquid droplets.
5. Reactor-coolant steam drives the turbine
The dried steam travels directly from the reactor vessel to the turbine-generator. There is no separate secondary loop making an intermediate batch of steam. The turbine therefore receives steam that originated as reactor coolant and boiled above the core.
6. Condensate returns to the reactor vessel
Steam leaving the turbine is condensed back into water. Pumps return the condensate toward the reactor vessel, where it re-enters the reactor coolant flow, is heated by the core and can boil again.
Does reactor water go through the turbine?
In a PWR, not in the normal steam path. Primary coolant circulates through the reactor and the steam-generator tubes, but it remains on the primary side. The water that boils and enters the turbine is secondary-loop water. The two supplies do not mix.
In a BWR, yes in the design sense. Reactor coolant boils in the vessel, and the resulting steam—after separators and dryers remove moisture—flows through the main steam line to the turbine. The turbine-side steam is therefore produced directly from reactor coolant.
“Goes through the turbine” should be understood as a statement about the steam cycle, not a claim that liquid water from the core is sent unchanged through the turbine. In a BWR, the fluid admitted to the turbine is dried steam; in a PWR, it is steam generated from a physically separate water inventory.
Why the PWR’s two loops matter
The PWR arrangement places a heat-transfer boundary between the reactor coolant and the turbine cycle. Heat crosses the steam-generator tube walls, but the primary and secondary waters stay separate during normal operation. That is why the NRC describes the arrangement as keeping most radioactivity in the reactor area.
The separation also gives the PWR a useful mental model: the reactor loop is a hot-water heat source, while the secondary loop is a conventional steam-producing circuit heated by that source. A reader should not describe the PWR’s turbine steam as “reactor water” without qualification; it is secondary water that received heat from reactor coolant.
Why the BWR needs separators and dryers
Boiling in the vessel creates a two-phase mixture by design. Before that mixture can enter the main steam line, the plant must remove liquid droplets. Moisture separators handle the bulk separation, and dryers remove remaining moisture from the steam headed to the turbine.
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This equipment is not evidence that boiling has been stopped in the core. It is evidence that the plant has conditioned the steam after boiling has occurred. The reactor vessel performs both the heat-generation and initial steam-production roles that are split between the reactor vessel and steam generator in a PWR.
What happens after the turbine in each design
The downstream sequence is conceptually similar:
- Steam expands through the turbine and transfers energy to the generator shaft.
- The turbine exhaust is cooled until it condenses into liquid water.
- Pumps return the condensate to the steam-producing part of the plant.
In a PWR, the returned water goes back to the steam generators as secondary-loop feedwater. In a BWR, it is pumped back toward the reactor vessel as feedwater for the reactor coolant circuit. Thus both designs reuse the water, but they return it to different steam-producing boundaries.
Fuel and fleet context
Fuel-assembly counts illustrate that the two reactor families are not identical in core layout, although the count alone does not determine how steam is produced. The U.S. Nuclear Regulatory Commission’s current reactor overview states that typical PWRs contain about 150–200 fuel assemblies, while typical BWRs contain about 370–800.
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Common misconceptions, corrected
“A PWR does not make steam because it is pressurized.”
Incorrect. The primary coolant is pressurized so it does not boil in the core. Steam is still made—inside the steam generator, in the separate secondary loop.
“A BWR has no pressure control.”
Incorrect. A BWR allows boiling in the core flow path, but its reactor vessel and steam system still operate under controlled pressure.
“Every reactor sends radioactive primary water through the turbine.”
Incorrect. A PWR keeps primary coolant separate from the secondary steam that reaches the turbine. A BWR uses reactor coolant as the source of turbine steam, which is why the two designs have different fluid boundaries.
“The turbine exhaust is discarded.”
Incorrect. In both designs, turbine exhaust is condensed and the water is pumped back into the steam-producing circuit.
“The steam generator is just another name for the reactor vessel.”
Incorrect for a PWR. The reactor vessel contains the core and primary coolant; the steam generator is a separate heat exchanger where secondary water boils. A BWR produces steam in the reactor vessel instead of using a separate steam generator for that function.
A practical way to remember the difference
- PWR: hot pressurized reactor water heats another water loop. The other loop boils, and its steam drives the turbine.
- BWR: reactor water boils in the vessel. Separators and dryers condition that steam, which then drives the turbine.
- Both: turbine exhaust is condensed and recycled.
If a diagram shows a steam generator between the reactor and the turbine, it is depicting the PWR’s indirect steam path. If the main steam line runs from the reactor vessel after internal moisture separation and drying, it is depicting the BWR’s direct steam path.
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