Differences Between BWRs and PWRs

Kofi Owusu Agyeman
March 20, 2018

Submitted as coursework for PH241, Stanford University, Winter 2018


Fig. 1: Pressurized Nuclear Reactor. (Source: Wikimedia Commons)

Pressurized water reactors (PWRs) and boiling water reactors (BWRs) are classified as light water reactors (LWRs). In the United States LWRs are used in the production of electric power. As of 2016, 69 out of 104 commercial nuclear power plants licensed by the U.S Nuclear Regulatory Commission are PWR's. [1,2]


Both PWRs and BWRs use light water or normal water (H2O) as the coolant and the neutron moderator. Furthermore, they both use enriched Uranium as fuel with cylindrical vessel types. The general structure of both reactors are also very similar, as they both consist of the main components of a nuclear reactor: a containment vessel, a reactor vessel, which houses the reactor core, and a steam generating turbine.


Fig. 2: Boiling Water Reactor. (Source: Wikimedia Commons)

The main difference between the PWR and BWR lies in the process of steam generation. A PWR generates steam indirectly by using two water circuits, a primary one and a secondary one. On the other hand, a BWR produces steam directly using a single water circuit. [3,4]

In a PWR (Fig 2), heat from the reactor core is used to heat the primary reactor coolant at temperatures over 300°C. This water is kept liquid under high pressure. The heat from the primary water circuit is then transferred to the secondary circuit by way of the pressurized liquid. The secondary circuit then uses this heat to convert liquid water into steam for the turbine. The steam is later condensed and recycled.

In a BWR (Fig 2), steam is directly produced by the boiling the water coolant. The steam is separated from the remaining water in steam separators positioned above the core and passed to the turbines. The steam is later condensed and recycled.


One of the major concerns of electricity production with nuclear energy has to do with safety. As with BWRs, the most severe operating condition affecting a PWR is the loss of coolant accident (LOCA). [3,4] With increased research and development in the issues surrounding LOCA, the safety of LWRs can be improved as they are used widely throughout the world.

© Kofi Owusu Agyeman. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] B. Zarubin, "Introduction to Light Water Reactors," Physics, Stanford University, Winter 2016.

[2] S. Shaw, "Advantages of Pressurized Water Reactors (PWR)," Physics 241, Stanford University, Winter 2017.

[3] A. Andrews and P. Folger, "Nuclear Power Plant Design and Seismic Safety Considerations," Congressional Research Service, R41805, Jan 2012.

[4] R. Kraus, "Modern Pressurized Water Reactor Safety Systems," Physics 241, Stanford University, Winter 2017.