How a fission nuclear power plant works

Part 1. How a PWR nuclear power plant works in a nutshell

This section describes the operation of pressurized water reactors (PWRs), the world’s main nuclear power plant. There are three stages in the operation of a nuclear power plant: the primary circuit, the secondary circuit and cooling.

  1. Primary circuit. Plant activity begins with the heat produced by the nuclear reactor. Fuel rods are immersed in water. Their fission produces a great deal of heat, which heats the water to several hundred degrees. The water is nevertheless pressurized to prevent it turning to steam. This is a closed circuit.
  2. Secondary circuit. A pipe containing pressurized, superheated water from the primary circuit passes through a heat generator. The water in the generator will itself overheat and be pressurized. This water (67.5 bars and 283°C) is then divided in two. One part is used to turn a high-pressure turbine. A portion of this steam still has energy left after that. It joins the other part of the pressurized water, and together they drive the low-pressure turbines. The turbines drive an alternator, which produces an alternating electric current. Its voltage is then increased for transmission over high-voltage lines.
  3. Cooling circuit. Water from the low-pressure turbines is transferred to a space where it is cooled by water taken from the environment. At around 9 bar and 180°C, it then flows into a reservoir before being returned, under pressure (71.5b), to the primary circuit. The cooling water is sent to the cooling towers. There, it evaporates, is cooled by a cold air intake system, falls back and is returned to the environment. The rest is the water vapour that escapes from the cooling towers.

Part 2. Parts of a fission power plant

Broadly speaking, a nuclear power plant is divided into two buildings: the nuclear island, which contains the installations specific to nuclear energy, and the conventional island, whose parts are similar to those of thermal power plants in general. However, I won’t go into that level of detail. We’ll look at the main parts on which the operation of a fission nuclear power plant is based:

  • The heart of the plant is the nuclear reactor. This is where the nuclear reaction takes place: fissile fuel is agitated to produce nuclear fission. Immersed in water, the heat generated is transferred to the reactor, which is then harnessed by a turbine – the center of technological innovation in this field. The major advances are the 3rd generation reactors, the 4th generation reactors and the SMR (Small and Medium Reactors).
  • Turbines, driven by the steam produced by the nuclear reactor. This is a crucial component that determines the efficiency of the system: how much of the energy produced by the steam can be captured?
  • As with all power plants, the direct current produced must be transformed into alternating current. This is done by means of an alternator. Once this is done, the voltage needs to be raised to send the current to the high-voltage lines. This is the role of the transformer.
  • The cooling tower. This is the culmination of the cooling system. Pipes spray cooling water. This water evaporates rapidly. This creates a very hot zone at the base of the tower. The air rises and creates a draft at the base of the tower. Two air inlets allow cold outside air to rush in. This is the “chimney effect”. This cold air cools the water, causing it to fall back in droplets. In the end, only a small proportion of the water escapes: ~2%. However, towers are not systematic: it is also possible to exchange water with the sea or a nearby river.

To recognize a nuclear cooling tower:

Part 3. Nuclear fuel and its life cycle

Nuclear power plants obviously need nuclear fuel to operate. There are many different types. The most important is enriched uranium: a combination of uranium-238, with a small proportion of uranium-235. It is also possible to use MOX (an acronym for “mixed uranium and plutonium oxide”), produced from used uranium. Research is also underway into the use of thorium. The 4th generation fast neutron reactors could use natural uranium (containing almost only uranium 238).

The question is, once the fuel has been used, what should be done with it? Some of the fuel will have been degraded, however, and will have to be stored as nuclear waste. Another part, the uranium 238, will be stored. In France, fuel can be recycled once. This is the question of the nuclear fuel life cycle.

Part 4. The life cycle of nuclear power plants

  • How long does it take to build a nuclear power plant?
  • How long can a nuclear power plant operate?
  • How do you dismantle a nuclear power plant?

Part 5. Heat production: the nuclear cogeneration route

As we have seen, only a fraction of the heat produced by nuclear fuel is converted into electricity: around 30-40%, like all thermal power plants. One way of making better use of the energy produced could be to use the heat generated directly by “cogeneration”. There are several ways of doing this, the most interesting of which are district heating networks and high-temperature electrolysis.

Heat networks:


EHT electrolysis


To find out more:

  • INA, Presentation of a nuclear reactor on 13h, 1979
  • Very well explained thread with many illustrations:
  • Visit the Czech Témelin power plant: