Inside a pressurized water reactor

In a pressurized water reactor, fission in uranium fuel heats water that is kept under such high pressure it cannot boil. That hot primary water flows through a steam generator, where it boils water in a separate secondary loop. The steam spins a turbine and generator, then is condensed and pumped back, while control rods that absorb neutrons set how fast the chain reaction runs.

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Control rod insertion

Plant thermal efficiency

Step through it

The coreFuel rods of uranium oxide sit in a grid. Each fission releases heat and neutrons that can split more uranium.
Control rodsRods of neutron absorbing material slide between fuel assemblies. In this simplified model, inserting them lowers the power.
Primary loopWater at about 155 bar and over 300 degrees C carries heat to the steam generator without boiling, held there by the pressurizer.
Turbine and generatorSteam from the secondary loop spins the turbine, which turns the generator. It never touches the reactor water.
Condenser and coolingSpent steam is turned back to water in the condenser. About two thirds of the heat leaves here, often through cooling towers.

From heat to electricity

Electric output is thermal power times plant efficiency. For a steam cycle limited to the temperatures a water reactor can reach, efficiency is about 33 percent, so a reactor making roughly 3,400 MW of heat delivers about 1,100 MW of electricity.

The rest of the heat must go somewhere. That is why nuclear plants sit by rivers, lakes or the sea, or use cooling towers.

Try it: drive the control rods from 0 to 100 percent. Thermal power falls, the core glow dims and the turbine slows. Then raise efficiency to 37 percent and see how much more electricity the same heat yields.

A deliberately simple model

Real reactor power is controlled by rods, boron dissolved in the coolant and strong temperature feedback, and it does not fall in a straight line with rod depth. This model maps insertion to power linearly so the energy bookkeeping stays visible. Even fully shut down, fuel keeps making decay heat that must be cooled.

Sources

Pressurized Water ReactorsU.S. Nuclear Regulatory Commission
PWR Description (22.06 Engineering of Nuclear Systems)MIT OpenCourseWare
Shippingport Atomic Power Station (HAER PA-81)Library of Congress, Historic American Engineering Record

How to cite this model

Avid Solutions International. (2026, November 11). Inside a pressurized water reactor [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/pwr-reactor

Last updated November 11, 2026

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