How Nuclear Reactors Work

Nuclear power plants use uranium to generate heat and boil water intosteam. Uranium has the largest atomsof the 92 naturally occurring elements onearth, making them more likely thanother atoms to split.

When subatomic particles called neutrons come into contact with uranium atoms, the atoms split, releasing heat energy. This occurs all the time in nature, but at a very slow rate. Nuclear reactors are able to greatly speed up this process by slowing down the neutrons and increasing the likelihood that they will hit and split the uranium atoms. When uranium atoms split they also release more neutrons which can then go on and split additional atoms ensuring a chain reaction of atom splitting. This is called nuclear fission.

At the heart of every nuclear reactor are fuel pellets no bigger than the tip of your finger. Despite their small size, these fuel pellets hold the potential to produce tremendous amounts of energy.

Canada’s nuclear reactors use fuel pellets that are made from naturally occurring uranium that is mined in Canada. The pellets are inserted into tubes about half-a-metre in length made from zirconium alloy, a special type of metal that has a high resistance to corrosion. The tubes are welded shut and several are then assembled together into what is called a fuel bundle. One of these half-metre fuel bundles can provide enough electricity to power 100 homes for a year.

Hundreds of fuel bundles are inserted into the core of a nuclear reactor where the uranium atoms split giving off vast amounts of heat. This heat is used to boil water to create steam, which then spins a turbine and generator producing electricity.

Nuclear power stations are able to produce tremendous amounts of electricity from a very small amount of fuel. A single 1.65 cm nuclear fuel pellet can produce the same amount of energy as 400 kilograms of coal, 410 litres of oil, or 350 cubic metres of natural gas.

As well, because nuclear power plants do not burn any fuels, they produce virtually no smog or greenhouse gas emissions (GHGs). They do however produce nuclear waste which needs to be handled and stored very carefully.

Basic Components of Nuclear Reactors

The basic parts of a nuclear reactor are the core, a moderator, control rods, a coolant, and shielding.

The core of a reactor contains the uranium fuel. CANDU heavy water reactors use natural uranium, of which 0.7% is U-235, while light water reactors use uranium that has been enriched so that U-235 makes up about 3 – 5% of the total.

The moderator is a light material, such as water, that allows the neutrons to slow down without being captured. By slowing down the fast neutrons created during fission, it can increase their efficiency of causing further fission.

Control rods are made of materials that absorb neutrons, such as boron, silver, indium, cadmium, or hafnium. They are introduced into the reactor to reduce the number of neutrons and thus stop the fission process when required. They are also used to control the level and distribution of power in the reactor.

A coolant is a fluid circulating through the reactor core that is used to absorb and transfer the heat produced by nuclear fission. At the same time, it maintains the temperature of the fuel within acceptable limits.

Shielding is a structure around the reactor and its steam generators, designed to protect it from intrusion and to protect those outside from the effects of radiation in the event of any serious malfunction inside. It is typically a metre-thick concrete and steel structure.

Nuclear Fission and the Moderator

In order to control the generation of electricity from a nuclear reactor, it is necessary to have a critical, or self-sustained, fission reaction within the reactor. When the fission process occurs, fast moving neutrons (about 10% of the speed of light) are one of the products. These neutrons can be absorbed or slowed by other materials or absorbed by a fuel atom (e.g., 235U) which in turn may cause that atom to fission. Since the fission process releases several neutrons (either two or three for 235U), it is possible for the fission process to occur at an increasing rate with each new “generation” of neutrons. This is known as a super-critical reaction. If too many neutrons are absorbed or if there is insufficient fissile material to fission, a sub-critical state occurs, in which case the chain reaction is not sustainable. A critical reaction, then, is one in which the chain reaction is stable and there is no increase in the population of free neutrons within the reactor.

As a “quirk” of physics, some fissile material tends to better absorb slow or thermal neutrons rather than the fast ones produced in the fission process. In fact with 235U a fast neutron has about 1000 times smaller fission probability than one travelling at 1/100,000,000 the speed. The moderator’s function is to slow down the fast neutrons to increase the efficiency of the fission reaction. It turns out that the best moderators are graphite (a form of carbon) and heavy water (D2O – the hydrogen atoms each have a proton and a neutron, and are known as deuterium, D). These have the advantage of not absorbing many neutrons. Ordinary or light water, H2O is also a good moderator, however, it has the tendency to absorb neutrons thereby dampening the reaction. Reactors that use light water as a moderator require enriched fuel to overcome this loss of neutrons. Enrichment concentrates the 235U isotope in the fuel; increasing it from the natural value of 0.711% to 2–5% (the other 99.289% in natural uranium is the 238U isotope which is more difficult to fission).

The energy from this controlled reaction is absorbed into a material (a coolant, which is often the moderator) and its temperature increases. In some plants, the steam is produced directly: in others, the heated material is pumped to a steam generator, and heat is transferred to water, converting it to steam. This steam is then used to turn a steam turbine creating electricity.

Sources:

NEA Nuclear Energy Today, 2012, pp. 18–19, http://www.oecd-nea.org/pub/nuclearenergytoday/6885-nuclear-energy-today.pdf
www.cleansafeenergy.org/
Ontario Power Generation: www.opg.com/education/program.asp