Where does the heat come from?
A nuclear reactor generates vast amounts of energy. On this page you will find out how this energy is handled, what forms it takes on, and ultimately how it is converted to electricity for the grid. Let’s start off with a recap of fission, the ultimate source of nuclear energy.
Nuclear fission is a process in which a neutron is absorbed by a nucleus (e.g., uranium-235), causing it to become unstable. The nucleus then splits into fragments called “daughter nuclei.” During this split, two or three neutrons are released, which can go on to collide with other uranium nuclei, leading to further fission reactions. This continuous process of neutron-induced fission is known as a chain reaction and releases huge amounts of energy.
The neutrons emerging from each reaction are typically ‘fast-moving’ neutrons and carry most of the energy from the reaction (about 99%). Before these neutrons can collide with fresh uranium nuclei, they need to be slowed down. This is done by the moderator. As the neutrons slow down, the energy they carry is transferred to the reactor coolant, causing it to heat up.
How is the heat removed?
The energy created by the fission reaction has now taken on the form of heat in the reactor coolant. What happens next?
Reactor coolant, most commonly water, is confined in a number of Primary Cooling Loops. The number of loops depends on the size and design of the reactor, but typically either three or four. The key role of the primary loops is transferring heat away from the reactor core. If this process stops, the temperature of the entire reactor core will rise uncontrollably.
To prevent the coolant from boiling, most reactors keep it under huge pressures. Huge pressure means huge forces and therefore the primary loop is made out of very thick and strong pipework. The coolant is typically circulated using several Reactor Coolant Pumps (RCPs). These pumps carry the coolant towards the Steam Generators. As the coolant passes through the steam generator, it transfers heat to a secondary loop of water. After the coolant leaves the steam generator, it is returned back the reactor core to continue the endless cycle.
How is steam generated?
Steam generators are a crucial component of nuclear power plants. Let’s have a closer look at them!
As described above, heated coolant arrives from the reactor core and enters the steam generator. However, it is not the reactor coolant that turns into steam. The steam is instead produced from the water in the secondary loop. Steam generators act as the interface between these two entirely separate loops. Inside each steam generator, there are thousands of tubes (ranging from 3,000 to 16,000 tubes), each approximately 2cm in diameter. The hot reactor coolant flows through those tubes, while the cold secondary loop water flows around the tubes. Heat is transferred from the primary loop to the secondary loop across this interface, turning the water in the secondary loop into steam.
In commercial power plants, there are typically two to four steam generators per reactor. Each steam generator can measure up to 21 metres in height and weigh as much as 800 tonnes.
Note – Unlike Pressurised Water Reactors (PWRs), Boiling Water Reactors (BWRs) do not use steam generators. In BWRs, the primary coolant directly boils in the reactor core, and the resulting steam is passed through a steam turbine.
What happens to the steam?
The energy created by the fission reaction has now taken on the form of high temperature steam in the secondary cooling loop. What happens next?
The superheated steam leaves the steam generators via the secondary loop piping. It is carried towards the huge steam turbines. Steam turbines convert the energy stored in high-pressure steam into mechanical / rotational energy.
The steam enters the turbine and flows over a set of blades (also called vanes). These blades are attached to a rotor and are designed to efficiently extract energy from the steam. As the high-pressure steam flows over the blades, it exerts a force which causes the rotor to rotate. The rotational energy of the rotor is then transferred to an electrical generator via a shaft.
How is electricity generated?
We’ve finally reached the last step. The energy created by the fission reaction has now taken on the form of rotational energy of the steam turbine and is being transferred to an electrical generator.
There are different types of generators, but we’ll focus on the most common one. An Alternating Current (AC) generator, also known as an alternator, produces alternating current. The alternator contains a rotor (magnet rotor) and a stator (coil of wire). The shaft connected to the steam turbine provides mechanical (i.e. rotational) power to the rotor. As the rotor spins, it induces a changing magnetic field in the stator coil. This changing magnetic field generates an alternating voltage across the coil. The resulting AC voltage is supplied to the grid and can be used by hundreds of thousands, if not millions of homes.
All the processes described on this page happen continuously, 24 hours a day, for years at a time in nuclear power stations. A truly remarkable feat of engineering!
What about cooling towers?
We’ve generated electricity, but the cycle is not yet complete. What happens to the steam after it passes through the turbine?
Steam leaving the turbine has very little useful energy left. Dealing with large volumes of low energy steam is quite tricky. That’s why the steam is instead condensed back into liquid water which is far easier to handle. This is done using the tertiary cooling loop.
The tertiary cooling loop uses water at a lower temperature to condense the steam back into water. This is done using a heat exchanger between the secondary and tertiary loop. The cooled water in the secondary loop is then returned to the steam generator to be heated once again.
There are two main ways of dealing with the heat that is now in the tertiary cooling water. The most recognisable one is recirculating it using cooling towers, where the heat is emitted to the atmosphere. This is typically done in power stations located close to rivers.
Alternatively, power stations located on the coast will simply release the heated water into the sea and suck in fresh seawater to replace it. It is important to note that the tertiary water is not radioactive at all.
Ove Schoeppner
Fission Reaction Diagram – Pass My Exams
Reactor Background – Oak Ridge National Laboratory – Creative Commons Attribution 2.0 Generic
Primary Loop – Σ64 – Creative Commons Attribution 4.0 International
Steam Generator – Mliu92 – Creative Commons Attribution-Share Alike 4.0 International
Steam Generator Background – Nuclear Regulatory Commission – Public domain
Steam Turbine – US Department of Energy – Public Domain
Steam Turbine Background – Siemens Pressebild – free
Electrical Generator Background – NRC – Public Domain
Cooling Towers Diagram – U.S.NRC. – Public Domain
Cooling Towers Background – Bjoern Schwarz – Creative Commons Attribution 2.0 Generic
