What is a significant characteristic of PWRs regarding neutron moderation?

In Pressurized Water Reactors (PWRs), water is the primary medium used for neutron moderation. Water serves to slow down the fast neutrons produced during fission events, increasing the likelihood of these neutrons being captured by fissile fuel, such as uranium-235. This process of slowing down neutrons is essential for maintaining a sustained nuclear reaction, as thermal neutrons (which are slower) are more effective at inducing fission in the fuel compared to fast neutrons.

The use of water for moderation provides not only effective slowing of neutrons but also serves as the primary coolant in the reactor, transferring heat away from the core. Additionally, water’s properties, such as its availability and relatively high heat capacity, make it an ideal moderating material for PWRs.

The effectiveness of water as a neutron moderator is due to its light atomic mass, which allows it to effectively scatter neutrons, reducing their energy and aiding in maintaining the criticality of the reactor. This dual role of water as both the moderator and coolant is a key aspect of PWR design and operation.