How does a nuclear power plant work — and how dangerous is it really?

SVET ZA 5 MINÚT · In depth · 16 October 2026 · approximately 7–8 minutes to read

Water at 300 °C that remains liquid. Tiny fuel pellets containing an enormous amount of usable energy. And a power plant that must keep cooling its reactor even after shutting it down. Nuclear energy involves processes that can seem counterintuitive. Understand a few basic principles, however, and the pieces start to fit together.

A nuclear power plant uses energy released by splitting atomic nuclei to produce heat. That heat produces steam, the steam spins a turbine, and a generator converts the motion into electricity. The fundamental difference from a coal-fired plant comes at the beginning: heat is released from atomic nuclei rather than from burning fuel. [1]

Our reel and this article mainly follow a pressurised water reactor, or PWR. The VVER reactors used in Czechia and Slovakia belong to this family. Other designs can work differently, particularly in how they produce steam.

1. What actually happens inside the fuel rods?

Ordinary PWR fuel does not look like the glowing liquid of science-fiction films. It consists of small ceramic uranium dioxide pellets stacked inside long metal tubes. These fuel rods are grouped into fuel assemblies. The metal surrounding the pellets is known as the cladding. [2]

When a suitable neutron strikes a uranium-235 nucleus, it can cause the nucleus to split. Smaller nuclei form, releasing energy and additional neutrons. Those neutrons can split further nuclei, creating a chain reaction. As the moving fission products collide with surrounding material, their kinetic energy becomes heat.

The reactor must keep this reaction under control. Control rods are different from fuel rods: they contain materials that absorb neutrons. Inserting them helps reduce the reaction or shut it down. Water also does more than carry heat away. It acts as a moderator, slowing neutrons so that they are more likely to cause further fission in this type of reactor. [3]

What about plutonium? Some forms inside uranium fuel during operation, and some of it also undergoes fission. Mixed uranium–plutonium fuel, known as MOX, exists too. None of this makes “plutonium rods” an accurate general name for ordinary uranium fuel rods. [2, 4]

2. How can water reach 300 °C without boiling?

Water boils at about 100 °C only at roughly normal atmospheric pressure. Lower the pressure and it boils at a lower temperature; increase the pressure and the boiling temperature rises. A kitchen pressure cooker uses the same basic principle, although a reactor operates under very different conditions.

Primary cooling circuits in pressurised water plants can operate at approximately 120–160 bar, equivalent to 12–16 MPa. At the Temelín plant in Czechia, the operating pressure is about 157 bar, with coolant temperatures of approximately 290–320 °C. The exact temperature depends on the design and where it is measured. [5, 6]

The reactor heats the water. High pressure allows it to remain liquid at that temperature. Pressure itself is not continually heating it to 300 °C. Nor does this mean evaporation has somehow become impossible: the important point is that the main coolant flow does not undergo bulk boiling during normal operation.

3. How does heat reach the turbine without the water mixing?

Hot water from the reactor flows into a steam generator, a large heat exchanger. It travels through tubes, transferring energy through their metal walls to water in a separate circuit. This second circuit operates at a lower pressure, allowing its water to turn into steam. Under normal operating conditions, water from the two circuits does not mix. [7]

Part of the plant	What happens there
Primary circuit	Water removes heat from the fuel and transfers it in the steam generator.
Secondary circuit	Steam forms, drives the turbine, condenses, and returns as water.
Cooling system	Heat is removed from the condenser and released to the surroundings.

The separation matters because primary water passes through the reactor and is handled as a radioactive fluid. Steam for the turbine is produced elsewhere. Operators monitor the integrity of the exchanger; a leaking tube is a fault requiring protective action.

Not every nuclear reactor has this arrangement. In a boiling water reactor, steam forms directly inside the reactor pressure vessel and travels to the turbine. One animation cannot therefore represent every nuclear power plant without qualification. [1]

4. The turbine produces motion. The generator produces electricity

Steam transfers energy to the turbine blades, rotating a shaft connected to the generator's rotor. A magnetic field moving relative to electrical windings induces a voltage. When a load is connected, current flows. The generator does not create energy from nothing: it converts mechanical energy into electrical energy. [8]

After leaving the turbine, steam is cooled in a condenser and becomes liquid water again. Pumps return it to the circuit. The plant does not need to replace all of this water every time it passes through the system.

However, converting heat into electricity is not perfectly efficient. Some energy must be rejected to the environment. This is why many plants have cooling towers or use river or seawater. The visible white cloud above a cooling tower consists mainly of tiny droplets of condensed water. It is not smoke from burning fuel, nor is it steam coming directly from the reactor. [9]

5. Why can't we simply switch the reactor off?

Stopping the chain reaction dramatically reduces power, but the heat does not immediately disappear. Radioactive fission products continue to decay, releasing decay heat. This heat output declines over time, but remains significant shortly after shutdown. That is why a shut-down reactor still needs cooling. [10]

Safety systems must perform three essential tasks: control the reaction, remove heat, and keep radioactive substances contained. Plants use several layers of protection, backup equipment, and procedures for abnormal conditions. It is also important to prevent one event, such as flooding, from disabling all the backup systems together. [11]

Safety is therefore much more than a thick wall around the reactor. It depends on design, maintenance, staff training, operational oversight, and independent regulation. Nobody can honestly promise zero risk, but both the probability of an accident and its consequences can be systematically reduced.

6. Can a power plant explode like an atomic bomb?

A conventional power reactor cannot explode like a nuclear weapon. Its fuel and physical arrangement are fundamentally different. That does not mean nothing explosive can happen at a power plant. A severe accident can involve a sudden pressure increase or a hydrogen explosion. [12]

At Chernobyl in 1986, dangerous characteristics of the RBMK design at the time combined with an unsuitable operating state and serious safety failures. A rapid power increase damaged the reactor. Its design was different from the pressurised water reactors used in Czechia and Slovakia. [13]

At Fukushima in 2011, the operating reactors shut down after the earthquake, but the tsunami disabled crucial power supplies and cooling. Fuel overheated, and hydrogen generated during the damage subsequently exploded, damaging buildings. [14]

Both accidents show why saying “a nuclear explosion is impossible” is not a complete safety explanation. Preventing overheating, fuel damage, and releases of radioactive material remains essential.

7. What remains after the fuel has been used?

Spent fuel is still radioactive and still generates heat. It is initially placed in pools, where water provides both cooling and radiation shielding. Once sufficiently cooled, it may be transferred to protective dry-storage systems, depending on the chosen management approach. Storage is not the same as final disposal. [15]

Deep geological repositories are being developed and implemented to isolate high-level radioactive waste from the environment over long periods. Protection combines the waste form itself, containers, engineered barriers, and suitable surrounding rock. Assessments must consider groundwater movement and the site's long-term evolution. [16]

Not all radioactive waste poses the same hazard. Used protective equipment, contaminated plant components, and spent fuel require different management. Reprocessing does not make waste disappear either. It can recover some material for further use, but radioactive waste still remains and must be managed. [4, 17]

8. Is nuclear energy environmentally friendly — and why is it expensive?

A reactor does not burn coal or gas to produce heat. But an emissions assessment must also count fuel mining and processing, plant construction, and eventual decommissioning. Nuclear power remains a low-emissions source when its full life cycle is considered. Calling it “completely emissions-free”, however, would be inaccurate. [18]

Low emissions do not mean zero environmental impact. Resource extraction, waste management, water withdrawals and consumption, and heat released to the surroundings all matter. Depending on the location and cooling system, heatwaves and drought can constrain plant operation. [19]

A major strength of nuclear power is its ability to provide large amounts of electricity without depending directly on whether the sun is shining or the wind is blowing at that moment. Nevertheless, nuclear units need scheduled outages and can experience faults. Electricity systems therefore need multiple resources, networks, and reserves.

A new plant also requires a substantial upfront investment. Construction time, interest rates, project management, and access to skilled workers strongly influence the final cost. Low emissions, technical safety, and an attractive price are three separate questions. Success in one does not automatically guarantee success in the others. [20]

9. Does radioactivity automatically mean danger?

Knowing that something emits radiation is not enough to assess its risk. The type of radiation, dose, duration, and route of exposure all matter. Radiation protection therefore involves measuring doses, shielding, distance, and limiting time near a source. The mere presence of radioactivity does not tell us how hazardous a particular situation is. [21]

Normal operation must also be distinguished from an accident. Controlled, monitored discharges are not equivalent to an uncontrolled release after fuel damage. Regulatory oversight involves checking measurable quantities and compliance with limits. A useful public discussion should therefore go beyond the word “radiation” and ask: what was measured, at what dose, where, and over what period? [22]

Nuclear energy deserves discussion without treating catastrophe as inevitable or promising a miraculous solution. Understanding it means recognising both its capabilities and its obligations: substantial usable energy, reliable cooling, long-term waste management, and responsibility for decisions whose effects last for decades.

Sources and further reading

Numbers in square brackets refer to the sources below. The technical explanation is simplified; operating parameters and safety systems differ between plants. Sources checked on 16 October 2026. Reading time excludes the reference list.