PWRs, BWRs, and the US Fleet

Every operating power reactor in the United States is a light-water reactor (LWR): ordinary water serves as both coolant and moderator. Within that family there are exactly two species, and the map's reactor-type field tells you which one you're looking at.

A pressurized-water reactor (PWR) — about two-thirds of the US fleet — keeps the water touching the fuel under such high pressure (~155 bar, 2,250 psi) that it cannot boil even at 320°C. That superheated primary water flows through thousands of small tubes inside steam generators, boiling a completely separate secondary loop of water into the steam that drives the turbine. The payoff for this complexity is separation: the water that has been through the core never leaves the containment, and the turbine hall handles only clean secondary steam. PWRs descend from naval propulsion reactors, and three vendors built the US fleet — Westinghouse, Combustion Engineering, and Babcock & Wilcox, names you'll see in the detail panels. The newest US reactors, Vogtle 3 and 4 in Georgia, are Westinghouse AP1000s — PWRs with "passive" safety systems that cool the core by gravity and convection rather than pumps.

A boiling-water reactor (BWR) — the remaining third, all built by General Electric — takes the direct route: water boils right in the core at ~75 bar, and that steam goes straight to the turbine. No steam generators, no separate secondary loop, a simpler and lower-pressure vessel. The trade-off is that the turbine receives mildly radioactive steam (mostly short-lived nitrogen-16, gone minutes after shutdown), so the turbine building is part of the radiological plant. BWRs also control power partly with coolant flow — pump harder, and the bubbles ("voids") that weaken moderation collapse, raising power — and their control rods enter from the bottom, since the steam separators occupy the top of the vessel.

For the curious reader the differences run deeper — PWRs adjust long-term reactivity with boron dissolved in the coolant while BWRs cannot; BWR containments are smaller pressure-suppression designs that vent steam through water pools — but the practical summary is this: both designs are mature, both achieve the same outstanding capacity factors, and the US fleet's character comes less from the PWR/BWR split than from its age and consolidation. The median operating US reactor entered service in the mid-1980s. Nearly all have received NRC license renewals from 40 to 60 years, and a growing set — including Surry, North Anna, and Oconee — hold subsequent renewals to 80 years. Ownership has consolidated into large operators: Constellation runs the biggest fleet, with Duke, Entergy, Southern, Dominion, and TVA among the other majors — names that recur in the offtake-deals layer, because a company that owns many reactors is a company that can sign many PPAs.

Worldwide, light water is dominant but not alone: Canada's CANDU line uses heavy water and natural uranium, Britain ran gas-cooled reactors for decades, and Russia still operates RBMKs (post-Chernobyl, heavily modified). The advanced-reactors page covers the new American designs that deliberately leave water behind — sodium, helium, and molten salt — and why their developers think the switch is worth it.