How Nuclear Reactors Work — Types and Principles

A nuclear reactor sustains a controlled fission chain reaction to produce heat, which generates steam, which drives turbines for electricity. The physics is straightforward. The engineering is where it gets complicated.

The chain reaction

When U-235 absorbs a neutron, it splits and releases 2-3 new neutrons plus about 200 MeV. Those neutrons trigger more fissions — a chain reaction. The key parameter is k, the multiplication factor. At k = 1 (critical), the reaction is stable. Below 1, it dies. Above 1, it grows exponentially. Reactors operate at k ≈ 1, controlled by absorber rods that soak up excess neutrons.

Why moderators matter

Natural uranium is 99.3% U-238 and only 0.7% U-235. U-238 absorbs fast neutrons without fissioning. So neutrons must be slowed to thermal energies where U-235's fission cross-section is huge. This is done by a moderator — water (PWR, BWR), heavy water (CANDU), or graphite (RBMK).

Reactor types

Pressurized Water Reactors (PWR) are most common — high-pressure primary coolant transfers heat to a secondary steam loop. Boiling Water Reactors (BWR) let coolant boil directly. CANDU reactors use heavy water, allowing natural uranium fuel. Fast breeder reactors skip the moderator, using fast neutrons to convert U-238 into Pu-239 — producing more fuel than they consume. Gen IV designs like molten salt and high-temperature gas reactors aim for improved safety and efficiency.

Use our Q-value calculator for fission reaction energies and the cross-section calculator for neutron interaction probabilities.