The multiplication factor: k = 1
Every nuclear reactor on Earth operates on one simple principle: maintain the multiplication factor k at exactly 1.000. This means that for every fission event, exactly one of the neutrons released goes on to cause another fission. The other neutrons are either absorbed by control rods or escape the core entirely.
When k = 1, the reactor is critical — power output is steady and predictable. When k drops below 1, the reactor is subcritical and the chain reaction dies out. When k rises above 1, the reactor is supercritical and power increases exponentially. The difference between a power plant and a bomb is simply how fast and how far k moves above 1.
In a bomb, k is pushed far above 1 in microseconds — there is no time for control systems to respond. In a reactor, k is kept within a narrow band around 1 using control rods — neutron-absorbing rods made of boron, cadmium, or hafnium that can be inserted or withdrawn from the core to fine-tune the reaction rate.
Drag control rods up and down in the Reactor Core Simulator. Hold k between 0.98 and 1.02 for 10 seconds to stabilise the core. Experience subcritical, critical, and supercritical states firsthand.
From heat to electricity: the turbine loop
A fission reactor does not produce electricity directly. It produces heat — enormous amounts of it. The heat boils water into steam, the steam spins a turbine, and the turbine drives a generator. This is the same process used in coal and gas plants. The only difference is the heat source.
In a typical pressurised water reactor (PWR), the core heats water to around 315°C under high pressure so it does not boil. This hot water flows through a heat exchanger called a steam generator, where it transfers its heat to a secondary water loop. The secondary loop boils and produces steam at lower pressure, which drives the turbine. This two-loop design means the radioactive primary water never touches the turbine.
One uranium fuel pellet — about the size of a fingertip — contains the same energy as 17,000 litres of oil or 1,780 tonnes of coal. A typical reactor contains hundreds of thousands of these pellets, arranged in fuel rods that are bundled into fuel assemblies. A single assembly might produce 200 megawatts of thermal power.
E = mc²: where the energy actually comes from
When a Uranium-235 nucleus splits, the fragments weigh slightly less than the original nucleus. That missing mass — about 0.1% of the total — did not disappear. It became energy via E = mc². Multiply that tiny mass by the speed of light squared (a very large number) and you get 200 million electron-volts per split.
To put that in perspective: burning one atom of coal releases about 4 electron-volts. Splitting one uranium atom releases 50 million times more energy. This is not because uranium is "more flammable" — it is because nuclear reactions convert mass directly into energy, while chemical reactions only rearrange electrons. The mass defect in chemical reactions is so small it is undetectable.
A single kilogram of uranium-235, if fully fissioned, would release about 83 terajoules of energy — equivalent to roughly 14,000 tonnes of TNT. In practice, reactors only burn a few percent of their fuel before the fuel rods must be replaced, but even that fraction produces staggering amounts of power from a tiny volume of fuel.
Pick a reaction — U-235 fission, D-T fusion, chemical combustion — and see exactly how many kilograms of mass disappear. Watch the energy bar fill and compare nuclear to chemical reactions side by side.
Safety systems: multiple lines of defence
Modern nuclear reactors are designed with defence in depth — multiple independent safety systems, each capable of preventing an accident if the others fail. The philosophy is simple: no single failure should ever lead to a release of radiation.
First line: Control rods. If anything goes wrong, the rods drop into the core by gravity within seconds, absorbing neutrons and shutting down the chain reaction. This is called a scram or reactor trip. Every reactor has multiple independent scram systems.
Second line: The containment vessel. A thick steel-and-concrete shell surrounds the reactor core, designed to withstand the pressure and temperature of a worst-case accident. The containment at Three Mile Island held — despite a partial meltdown, almost no radiation escaped.
Third line: Emergency cooling. Even after the chain reaction stops, the fuel continues to produce decay heat — about 7% of full power immediately after shutdown, declining over days. If cooling is lost, this residual heat can melt the fuel. Emergency core cooling systems (ECCS) inject water automatically if normal cooling fails.
Fourth line: Passive safety. Newer reactor designs — like the AP1000 and small modular reactors (SMRs) — use gravity, natural convection, and pressurised tanks rather than pumps and valves that can fail. If power is lost, these systems activate automatically without any human intervention or electricity.
Chernobyl (1986): A safety test was performed with the reactor in an unstable configuration. The operators withdrew too many control rods, making the reactor highly supercritical. When the test triggered a power surge, the reactor exploded — not a nuclear explosion, but a steam explosion from superheated water flash-boiling. The containment vessel was absent by design.
Fukushima (2011): A tsunami flooded the backup diesel generators, cutting power to the cooling pumps. The reactors scrammed successfully, but decay heat could not be removed. Fuel melted, hydrogen built up, and explosions damaged the containment buildings. The reactors themselves were not designed for a tsunami of that height — a failure of site assessment, not reactor physics.
Types of nuclear reactors
Not all reactors are the same. The design choices — coolant, moderator, fuel enrichment — determine how safe, efficient, and expensive a reactor is. Here are the main types in operation today.
Pressurised Water Reactor (PWR): The most common design worldwide. Water under high pressure carries heat from the core to a steam generator. The water never boils in the primary loop. Used in the US, France, Japan, and most of the world.
Boiling Water Reactor (BWR): Water boils directly in the core, and the steam goes straight to the turbine. Simpler design but the turbine becomes slightly radioactive. Used mainly in the US, Japan, and Sweden.
Heavy Water Reactor (CANDU): Uses heavy water (deuterium oxide) as both coolant and moderator. Can run on natural uranium without enrichment — Canada exports this technology widely.
Fast Breeder Reactor: Uses fast neutrons (no moderator) and "breeds" more fuel than it consumes by converting Uranium-238 into Plutonium-239. Potentially solves fuel supply concerns but raises proliferation risks.
Small Modular Reactor (SMR): Factory-built reactors under 300 MW that can be deployed in remote areas or added incrementally to grids. Many designs use passive safety and can be buried underground.
Each scenario shows live reactor readouts — temperature, neutron flux, control rod position. Identify what's wrong and choose the correct intervention. Based on real accidents: Chernobyl, Fukushima, Three Mile Island, and more.
Nuclear waste: the unsolved problem
After three to five years in the reactor, fuel rods are removed. They are still highly radioactive — not from the uranium, but from the fission products created during operation. These include isotopes like Caesium-137 (30-year half-life) and Strontium-90 (28-year half-life) that emit intense beta and gamma radiation.
Spent fuel is first stored in cooling pools at the reactor site for several years. The water blocks radiation and carries away decay heat. After cooling, it can be moved to dry cask storage — thick concrete and steel containers that sit on concrete pads. Some countries reprocess spent fuel to extract remaining uranium and plutonium for reuse.
The long-term solution is deep geological disposal — burying waste hundreds of metres underground in stable rock formations. Finland is building the first such facility, Onkalo, designed to isolate waste for 100,000 years. No country has yet completed a permanent repository, meaning most waste remains in temporary storage.
The total volume of high-level waste from all nuclear power ever generated worldwide would fit in a single football field to a depth of about 10 metres. The challenge is not volume — it is making sure it stays contained for longer than human civilisation has existed.
Why nuclear energy matters
Nuclear power provides about 10% of the world's electricity and roughly 25% of low-carbon electricity. A single 1-gigawatt reactor prevents about 4 million tonnes of CO₂ emissions per year — equivalent to taking 1 million cars off the road. Unlike solar and wind, nuclear runs 24/7 regardless of weather, making it ideal for baseload power.
The trade-offs are real: high upfront construction costs, long build times, proliferation risks, and the unresolved waste question. But as climate targets tighten, many countries are reconsidering nuclear. France gets 70% of its electricity from nuclear. Sweden gets 30%. Even Japan, after Fukushima, is restarting reactors to meet emissions goals.
The physics is settled: fission works, it is safe when properly engineered, and it produces vast amounts of energy with minimal carbon emissions. The remaining questions are economic, political, and social — not scientific.
A nuclear reactor maintains a self-sustaining chain reaction at k=1 using control rods to absorb excess neutrons. Water heated by fission boils into steam, spins a turbine, and generates electricity. E=mc² converts tiny mass defects into enormous energy — roughly 83,000 GJ per kilogram of uranium fuel. Modern reactors use multiple redundant safety systems including automatic scrams, containment vessels, emergency cooling, and passive safety designs. Spent fuel remains radioactive for millennia and requires secure long-term storage. Nuclear provides 10% of global electricity and 25% of low-carbon power, with trade-offs between energy security, waste, and cost.
Stabilise a live reactor core, calculate E=mc² for real reactions, and diagnose five control room emergencies. Everything in one 12-minute interactive lesson with instant feedback and worked examples.
The Nuclear section covers fission, chain reactions, isotopes, radioactive decay, and nuclear reactors through interactive simulations and challenges.