PWR vs BWR: Difference and Comparison

Key Takeaways

1. Type of Reactor: PWR (Pressurized Water Reactor) and BWR (Boiling Water Reactor) are two types of light water nuclear reactors, both using water as both coolant and neutron moderator, but with different operational methods.
2. Cooling and Moderation Process: In a PWR, the water in the reactor core is kept under pressure to prevent it from boiling, and the heat generated is transferred to a secondary circuit to produce steam. In a BWR, the water in the reactor core is allowed to boil, and the steam generated is directly used to drive the turbine.
3. Complexity and Efficiency: PWRs are considered more complex due to the need for two separate water circuits, but they benefit from slightly higher thermal efficiency and the added safety provided by the separation of the radioactive primary circuit from the turbine. BWRs have a simpler design with only one circuit, but this also means the steam passing through the turbine is radioactive, requiring additional safety measures.

What is PWR?

The Pressurized Water Reactor, commonly known as PWR, is a prominent and widely used type of nuclear reactor for electricity generation. PWRs employ enriched uranium fuel rods to sustain controlled nuclear fission reactions. The reactor core is immersed in high-pressure water, which serves as both a coolant and a moderator.

As the nuclear reactions occur, they release a significant amount of heat, causing the water to heat up and turn into high-pressure steam. This steam is then directed to a separate turbine system, where it drives turbines connected to electricity generators. 

The PWR’s design includes multiple safety features, such as control rods and emergency cooling systems, to ensure stable and safe operation. Due to its proven track record of safety, efficiency, and power output, the PWR technology remains a crucial player in the global energy landscape, providing a substantial portion of electricity to countless communities worldwide.

What is BWR?

BWR stands for “Boiling Water Reactor.” It is another type of nuclear reactor used for generating electricity through nuclear fission reactions. BWRs are distinct from Pressurized Water Reactors (PWRs) in their design and operation.

In a Boiling Water Reactor (BWR), enriched uranium fuel is placed in the reactor core. The core is immersed in water, which acts as both a coolant and a moderator. The coolant water is allowed to boil directly within the reactor core due to the heat generated from nuclear fission.

The steam produced from the boiling water rises to the top of the reactor vessel, where it is collected and directed to a turbine system.

Difference Between PWR and BWR

1. In a Pressurized Water Reactor, the coolant and moderator are separate. Water is used as the coolant, which circulates through the reactor core and transfers heat to a steam generator. The steam generator, in turn, heats a separate water loop to produce steam for driving turbines and generating electricity. In a Boiling Water Reactor, the coolant and moderator are the same substance: water. The water in the reactor core directly boils due to the heat generated by nuclear fission. The resulting steam rises to the turbine system without the need for a separate steam generator.
2. PWRs operate at high pressure to keep the water coolant in a liquid state, even at elevated temperatures. This pressurized condition is maintained to prevent the coolant from boiling within the reactor core. BWRs operate at lower pressure compared to PWRs since the water coolant is allowed to boil directly in the reactor core. This lower pressure simplifies the design and operation of the reactor.
3. In a PWR, the water in direct contact with the reactor core remains liquid, and only steam is directed to the turbine system. The steam is of high quality (dry steam) with minimal moisture content, contributing to higher turbine efficiency. In a BWR, the steam produced from boiling water is of lower quality (wet steam) since it contains some moisture. The presence of moisture can reduce the efficiency of the turbine system.
4. PWRs have a more complex design with the inclusion of a steam generator, requiring additional components and systems to function. BWRs are relatively simpler in design since they eliminate the need for a separate steam generator, resulting in a more straightforward layout.
5. In a PWR, the water in direct contact with the nuclear fuel remains separate from the water used in the turbine system, reducing the risk of radioactive isotopes reaching the turbines. In a BWR, since the water directly boils in the reactor core, some radioactive isotopes may be present in the steam that drives the turbines. Appropriate safety measures are in place to mitigate any potential risks.

Comparison Between PWR and BWR

Parameters of Comparison	PWR	BWR
Fuel Configuration	Enriched Uranium Fuel Assemblies	Enriched Uranium Fuel Assemblies
Neutron Moderation	Uses Moderator to Slow Down Neutrons	Uses Water as a Moderator
Steam Quality	Dry Steam	Wet Steam
Control Rod Operation	Absorbs Neutrons to Control Reactivity	Absorbs Neutrons to Control Reactivity
Core Pressure	High Pressure Within the Reactor Core	Low Pressure Within the Reactor Core

References

1. https://link.springer.com/article/10.1007/s11661-003-0092-2
2. https://www.sciencedirect.com/science/article/pii/S0168900216307707