Nuclear reactors differ mainly in three respects: the fuel they use, what moderates neutrons to sustain the chain reaction, and what carries heat out of the core.
Pressurised Water Reactor (PWR): About seven in ten reactors worldwide are PWRs. Ordinary water serves as both coolant and moderator. A key safety feature: if the water overheats and forms steam bubbles, the chain reaction slows automatically, since steam moderates neutrons far less effectively than liquid water.
Boiling Water Reactor (BWR): Roughly 14% of the global fleet. Also uses ordinary water as coolant and moderator, but the water boils directly inside the reactor vessel, and the resulting steam drives the turbine directly, without a separate steam generator loop.
This makes BWRs simpler and cheaper to build than PWRs, though it also makes the turbine mildly radioactive during operation, mainly from short-lived nitrogen isotopes (radioactivity that fades within minutes of shutdown).
Pressurised Heavy-Water Reactor (PHWR/CANDU): Uses natural, unenriched uranium fuel, made possible by heavy water (deuterium-based) as a moderator, which slows neutrons while absorbing fewer of them than ordinary water.
Fuel sits in hundreds of horizontal pressure tubes rather than one large vessel… [Download the Nuclear Energy Volume 2026 for the glossary on the reactor types at our disposal and more stories]
