How Nuclear Fission Powers Reactors: Chain Reactions and Reactor Design

One Kilogram of Uranium Holds the Energy of 3,000 Tonnes of Coal

A single kilogram of uranium-235 fuel, fully fissioned, releases approximately 80 terajoules of energy — equivalent to burning 3,000 tonnes of coal or consuming 14,000 barrels of oil. This extraordinary energy density makes nuclear fission the most energy-concentrated fuel source humanity has ever harnessed for electricity generation. As of 2024, 413 operating nuclear reactors in 32 countries supply approximately 10% of global electricity, with France generating nearly 70% of its national electricity from nuclear power.

The Physics of Fission

Fission occurs when a heavy atomic nucleus — most commonly uranium-235 or plutonium-239 — absorbs a neutron and becomes unstable. The nucleus splits into two lighter nuclei (fission products), releases two to three additional neutrons, and emits a burst of thermal energy. The mass of the fission products is slightly less than the original nucleus — that missing mass converts to energy following Einstein's E=mc², where even tiny mass losses produce enormous energy.

The critical breakthrough is the chain reaction. Each fission event releases neutrons that can trigger additional fissions in neighboring atoms. If exactly one neutron from each fission event causes one more fission, the reaction sustains itself at a constant rate — called a critical state. Below criticality, the reaction dies out. Above criticality (supercritical), the reaction accelerates exponentially. Controlled nuclear power requires maintaining precise criticality.

Components of a Thermal Reactor

PWR vs. BWR: The Two Dominant Designs

Two light-water reactor designs account for approximately 85% of operating commercial reactors worldwide.

Pressurized Water Reactors (PWRs) keep the primary coolant water under approximately 155 atmospheres of pressure, preventing it from boiling despite temperatures reaching 315°C (600°F). This hot pressurized water passes through a steam generator, transferring heat to a secondary water loop that does boil and drives turbines. The separation of primary and secondary circuits means the turbine side does not contact radioactive coolant. PWRs represent about 70% of operating reactors globally.

Boiling Water Reactors (BWRs) operate at lower pressure (about 75 atmospheres), allowing coolant to boil directly inside the reactor vessel. Steam generated in the reactor drives turbines directly, eliminating the need for a steam generator. BWR designs are mechanically simpler but require turbine maintenance in a slightly radioactive environment. BWRs make up about 18% of global reactors. The Fukushima Daiichi plant used GE-designed Mark I BWRs.

Control Rods: The Brake Pedal of a Nuclear Reactor

Control rods are neutron-absorbing materials inserted into the reactor core to regulate the chain reaction. Inserting rods absorbs more neutrons, slowing fissions and reducing heat output. Withdrawing rods allows more neutrons to cause fission, increasing power output. In an emergency shutdown (SCRAM), all control rods insert simultaneously — reducing the fission rate to near zero within seconds.

• Boron-10 captures thermal neutrons with exceptional efficiency
• Control rods can fine-tune power output from 20% to 100% of rated capacity
• Modern designs include passive safety features that insert control rods by gravity if power fails

Lessons from Chernobyl and Fukushima

Both disasters reshaped global reactor design standards and safety culture.

Chernobyl (April 26, 1986) involved a Soviet RBMK reactor — a graphite-moderated design with a positive void coefficient, meaning steam bubbles forming during overheating actually accelerated the chain reaction rather than slowing it. A safety test was conducted improperly, triggering a power surge. The chain reaction accelerated uncontrollably, causing a steam explosion and graphite fire that released approximately 5,200 petabecquerels of radioactive material. Western light-water reactors do not have a positive void coefficient; the RBMK design was never exported.

Fukushima Daiichi (March 11, 2011) demonstrated the vulnerability of backup power systems. The earthquake and tsunami knocked out both grid power and diesel generators, eliminating coolant circulation for three BWR reactors. Fuel melted and produced hydrogen gas that exploded, breaching containment in Units 1 and 3. The disaster led to global mandates for passive cooling systems that function without electrical power.

Energy Output and Carbon Footprint

Nuclear's lifecycle emissions are among the lowest of any energy source, comparable to wind. Its 90%–93% capacity factor — the proportion of time a plant generates near-maximum output — far exceeds intermittent renewables, making nuclear a reliable baseload power source regardless of weather conditions.