Nuclear power – how it works

In a nuclear power plant, energy is derived from splitting atomic nuclei. The process is called fission, and it heats water to form steam. The steam powers a turbine, which in turn powers a generator that generates electricity.

Fission takes place in the reactor. During the process, atomic nuclei are split by bombarding them with neutrons. When an atomic nucleus is split, it emits new neutrons that can split new atomic nuclei, creating a chain reaction. A nuclear power plant typically uses uranium-235, a special isotope of the element uranium, as fuel. In order to control the process, various types of control rod stems are used to absorb the discharged neutrons, reducing the fission rate or stopping it entirely.

There are several different types of nuclear reactors, the most common of which are pressurised water reactors and boiling water reactors.

Pressurised water reactor

Enlarged illustration

The reactor contains water and uranium. When the uranium atoms are split, the water is heated to 325°C. The high pressure inside the reactor is regulated by a pressure vessel, preventing the water from boiling.

The hot water from the reactor is transferred to the steam generator, which is a large heat exchanger. Steam is produced, because the pressure is lower here, and steam is subsequently fed into the turbine. The pressure from the steam causes the turbine blades to rotate. The turbine powers a generator which generates electricity. The steam is then conveyed to a condenser which consists of a large number of small pipes. Sea water is pumped through the pipes and when the steam encounters the cold pipes it condenses and becomes water again. The sea water is pumped back out to the sea again and is then around 10°C warmer than when it entered the condenser.

The water is pumped back from the steam generator into the reactor to then be heated again. The water in the reactor thus circulates in a closed cycle so neither the steam generator's water nor the cooled sea water comes into contact with the water in the reactor.

Boiling water reactor

Enlarged illustration

The reactor contains water and uranium. When the uranium atoms are split, energy is released causing the water in the reactor tank to boil and steam is formed. The steam is transported to the turbine. The steam pressure causes the turbine blades to rotate. The turbine drives an electricity generator that generates electricity. The electricity is transported out through the power lines to the users.

When the steam has supplied its energy to the turbine, it is fed into a condenser consisting of a large number of narrow pipes. The sea water is pumped through the pipes and when the steam encounters the outside of the pipes it cools down and condenses, i.e. it becomes water. The sea water is pumped back out into the sea and is then 10°C warmer than when it was pumped in.

The water from the condenser is pumped back into the reactor to subsequently be heated again and start a new cycle. The water that is in the reactor system forms a closed cycle, and consequently the cooling water from the sea never comes into direct contact with the steam from the reactor.

Multiple barriers and safety systems

Ionising radiation is formed during the fission process in the reactor. To prevent radiation and radioactive substances from affecting the surroundings there are several independent barriers and safety systems.

The fuel itself is a barrier since the ceramic uranium pellets have low solubility in water and air (cf. dissolving a brick in water). It also binds the radioactive substances. The pellets only start to melt at 2,800°C.

The uranium pellets are enclosed in cladding tubes of zirconium alloy, a metal alloy with good properties for use in reactors. The tubes are completely gas-proof.

The third barrier consists of the reactor tank and associated pipe systems. The reactor tank is made of 15-20 cm thick steel and weighs around 400 tonnes.

The reactor is surrounded by the reactor containment, which is made of metre-thick concrete with infused gas-proof steel plates.

The fifth barrier is the building itself, which is designed to withstand strong forces from both inside and outside.

In addition to the barriers, there are also multiple safety systems for cooling the reactor core and preventing radioactive substances from being spread.

Safety barriers

Safety filters provide additional protection

Even if all safety systems were to stop working, radioactivity must not escape to the surroundings. For this reason there are special filters to take care of at least 99.9 per cent of the radioactive substances.

If the pressure in the reactor containment were to become too high, gases and steam may have to be released to the filter. The primary task of the filter is to minimise emissions of radioactive particles and radioactive iodine.

For this reason, the steam and gases are washed in a filter pool, a so-called scrubber. The radioactive particles remain in the scrubber's water while the purified gases are released via a stone filter.

Last updated: 2013-10-01 08:59