Nuclear reaction

The energy production of a nuclear power plant is based on uranium fuel fission and a controlled chain reaction.

In nuclear fission, a neutron collides with the nucleus of isotope U-235 and splits it. This fission reaction releases two or three new neutrons and fission products. Some of the released neutrons continue the chain reaction. Every fission reaction releases a lot of energy. This means that very small amounts of uranium fuel can be used to produce large quantities of heat. For example, one gram of fissile material corresponds to 24,000 kWh of energy. In a nuclear power plant, the thermal energy created through fission is converted into electricity via a turbine and an electric generator.

Fission products created in the fission reaction are radioactive, i.e. they decay into other elements. In radioactive decay, the nuclei emit radiation. The radiation of radioactive fission products causes residual heat in the fuel. The removal of residual heat must be ensured at all stages of fuel handling.

Neutrons in the reactor

The operation of a nuclear reactor is controlled by influencing the neutron balance of the reactor. Adding the amount of neutron-absorbing substance in the reactor core reduces the number of neutrons, thus decreasing the power, and vice versa. The water coolant in the reactor also acts as a moderator of the neutrons generated in the fission process. The neutrons released in the fission process are so quick that the likelihood of them causing a uranium nucleus to split is very small. When they collide with the hydrogen atoms in the coolant, the neutrons lose their speed, and, having slowed down sufficiently, they can cause new fission reactions in the fuel. Reactor power control takes place with the control rods and the water flow maintained by the reactor coolant pumps. The control rods contain a substance which effectively absorbs neutrons, such as cadmium, boron, or hafnium.


The German chemists Otto Hahn and Fritz Strassman discovered fission in 1939. They bombarded a uranium solution with neutrons and observed the creation of substances, such as barium, that were not present in the original solution. This mystery was later solved by the physicists Lise Meitner and Otto Frisch who concluded that a uranium nucleus can split into two parts of roughly equal size when a neutron hits it. The neutron had been discovered in 1932 by the English physicist James Chadwick, and it was soon observed that neutrons held a special position among fundamental particles. Electrically neutral, they can be used as missiles that can penetrate an atomic nucleus to induce various reactions.