What is the difference between a PWR and a BWR?

Both are light-water reactors. In a Pressurised Water Reactor the primary coolant stays liquid under high pressure and heats a separate steam generator, so the turbine sees clean steam. In a Boiling Water Reactor the water boils directly in the core and that steam drives the turbine, making it simpler but mildly radioactive on the turbine side.

What does the moderator do in a reactor?

The moderator slows fast neutrons released by fission to lower speeds, where they are far more likely to cause further fission in uranium-235. Common moderators are ordinary light water, heavy water and graphite. Fast reactors omit the moderator and run on fast neutrons instead.

Why can CANDU reactors use natural uranium?

CANDU reactors use heavy water (deuterium oxide) as moderator and coolant. Heavy water absorbs very few neutrons, so the reactor sustains a chain reaction with unenriched natural uranium, avoiding the costly enrichment that light-water reactors require.

What is a small modular reactor?

A small modular reactor (SMR) is a compact design under roughly 300 MWe, built in factory modules and shipped to site rather than constructed in place. Many SMRs are scaled-down pressurised water reactors, and a few are now entering early deployment.

Are the reactor counts exact?

No. The number of operational units changes constantly as reactors are built, restarted, shut down or retired. The counts here are approximate snapshots meant to show the relative prevalence of each type, not a live census.

Nuclear Reactor Type Reference

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Comparing the world’s reactor designs

This reference compares the major commercial and notable nuclear reactor types, from the dominant light-water PWR and BWR through heavy-water CANDU, graphite designs like the RBMK and AGR, fast breeders, high-temperature gas reactors, and the emerging small modular and molten-salt concepts. Each entry lists the moderator, coolant, fuel and an approximate count of operational units.

How it works

Every reactor is defined by three choices. The moderator slows the fast neutrons from fission so they can sustain a chain reaction — light water, heavy water and graphite are the usual options, while fast reactors use none. The coolant carries heat out of the core to make steam; it may be the same water, a gas like CO₂ or helium, or liquid sodium. The fuel is usually uranium dioxide, enriched to a few per cent U-235, though heavy-water designs run on natural uranium and breeders use plutonium-bearing mixed oxide.

These choices interact. Heavy water’s low neutron absorption lets CANDU skip enrichment. Graphite moderation enabled early gas-cooled and Soviet RBMK designs. Removing the moderator entirely gives a fast spectrum that can breed more fuel than it burns. The table makes these trade-offs visible side by side.

Notes and caveats

Light-water PWRs and BWRs make up the great majority of the global fleet.
The RBMK is the Chernobyl-type design; its positive void coefficient was a key factor in that accident and the design is being phased out.
Magnox reactors are fully retired and shown for historical context.
Molten-salt reactors remain at the research stage with no commercial units; counts are approximate and change over time.