What is a light-water reactor?

The light-water reactor is nuclear's dependable workhorse: about nine in ten reactors worldwide use ordinary water as both moderator and coolant. In a pressurized-water reactor the water stays liquid under pressure and hands its heat to a clean separate steam loop; in a boiling-water reactor the water boils right in the core.

This is mature, deeply understood technology with one of the best safety and reliability records of any way humans make electricity — which is precisely why it's the trusted baseline that newer designs build on, and why utilities keep extending these plants to 60 and 80 years of service.