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All matter is composed of atoms. Each atom has a central core, or nucleus, consisting of a number of protons and a number of neutrons. Protons have a positive electrical charge; neutrons have no electrical charge. Protons and neutrons together are sometimes called nucleons.


The nucleons inside an atom are held together by a very strong force. If the nucleus is split apart, some of this force is released as heat energy. This process is called nuclear fission and it is the basis of today's nuclear electric power. A very small quantity of matter can release a great deal of energy.


Another possible source of nuclear power is fusion, whereby energy is created by certain atomic nuclei coming together rather than splitting apart. The technology of nuclear fusion for electric power is still at an experimental stage.


Some very small particles, electrons, surround the atomic nucleus. (They are often portrayed similarly to planets revolving about a sun). The electrons have no role in nuclear energy; only the nuclei do.





Substances that are composed of a single type of atom are called elements. Iron, oxygen (O), and carbon (C) are examples of elements. (Most substances are composed of more than one element, for example carbon dioxide, CO2.) Each element is given a number corresponding to the number of protons in its nucleus. This atomic number is unique for each element.


Each element is also identified by a symbol – for example, C for carbon, O for oxygen. The Periodic Table of the Elements gives the atomic number and symbol for each element. You can see this table at a website such as


Although each element has a fixed number of protons, the number of its neutrons may vary. These variants are called isotopes of the element. Isotopes are identified by the number of nucleons (protons and neutrons) in the atomic nucleus.




Some isotopes are unstable: they transform themselves into other isotopes of the same element, or into other elements. They are said to be radioactive. As they transform, they emit various types of radiation. Radioactvity is a natural phenomenon that has existed since the formation of the earth. Some radioactive materials are still found in nature: in rocks, in space, in our food, and even in our own bodies. Depending on its type and intensity, radioactivity can be harmless to humans, beneficial and useful in the analysis and treatment of certain diseases or harmful to our health (we must protect ourselves from it).


The atoms of each radioactive substance decay at a fixed rate, which can be fast or slow. It can be measured very accurately. We measure this rate in terms of the half-life of the substance – the time it takes for half of its unstable atoms to change. As the transformed atoms regain stability, the radioactivity of the substance diminishes.


The half-life may take only seconds or up to billions of years, like potassium-40, which provides half of the radioactivity in our bodies.




Nuclear fission occurs when an atomic nucleus, upon being struck by a neutron, absorbs the neutron and splits apart, releasing two or more other neutrons and a great deal of energy in the form of heat. Material capable of doing this is said to be fissile. If one or more of the released neutrons strike other fissile nuclei and cause them to split and release neutrons, we get a chain reaction – a self-sustaining process. This is what occurs within the core of a nuclear reactor. The chain reaction is carefully controlled to remain at a determined, stable level.


Most nuclear reactors are used in power plants, where the heat is used to generate electricity. The heat can also be used directly, for example to heat buildings or to remove the salt from sea-water.


The only fissile material found in nature is uranium in its isotope 235 (U-235), widely used in today's nuclear reactors. Other fissile materials are created in nuclear reactors in the course of the reaction process. Some can be further used as nuclear “fuel”, notably uranium 233 and plutonium 239.


Nuclear energy is extremely efficient. One pound of U-235 can produce 3 million times more energy than burning one pound of coal!


It's important to understand that a nuclear power plant is very different from a nuclear weapon, in which the fissile materials must be packed together extremely densely within a small fraction of a second. This cannot happen in a nuclear reactor. A nuclear reactor cannot explode like an atomic bomb.




Some materials remaining at the end of the nuclear fuel cycle are radioactive to various degrees and varying durations. Some of these products can be re-used in reactors.


Only highly radioactive materials with a long half-life are truly dangerous. Luckily, these account for a very small part (about 1%) of the waste. They are kept out of harm’s way by being vitrified (melted together with glass) and stored temporarily at ground level and later, permanently, deeply underground.


The Pressurized Water Reactor is the most common type of nuclear reactor in use today. It uses water under high pressure to cool the reactor’s core and carry the heat to where it is needed. The high pressure ensures that the water does not boil, so that it can reach a high temperature (around 300°C) for  more efficient operation.


The water, heated by the nuclear reaction, in turn heats the water in a second, independent circuit in a steam generator. The steam turns a turbine that operates an electricity generator. Electricity feeds into the power grid, which provides electricity for our homes, factories, streetcars, streetlights, and wherever it's needed.


In order to minimize warming of the river, it is cooled by evaporating a small portion of the water taken from the river in high cooling towers before the main flow is re-injected into the river.

The steam in turn is cooled and condensed by a large quantity of cold water, usually pumped from a river or the sea. To minimize warming of the river, a portion of the water taken from the river is then cooled again in high cooling towers before being returned to the river. Don’t worry, the white steam escaping from the towers is not radioactive and it contains no CO2!


The water in the reactor also serves as a moderator that slows the speed of the neutrons. If the reactivity starts to increase too much, the water heats up and expands, and the chain reaction slows down. This is an important automatic safety feature.


The Control rods absorb neutrons. They can be lowered down into the reactor vessel or pulled up, as needed, to slow down or speed up the rate of reaction.


The Containment structure ensures that no radioactivity can escape in the event of a severe accident. This is an essential safety precaution.





Since the first nuclear reactors were built in 1954, there have been three major accidents: at Three Mile Island (USA, 1979), Chernobyl (Ukraine, 1986) and Fukushima (Japan, 2011).


Three Mile Island (Pennsylvania, USA)


On March 28, 1979, as a result of a combination of mechanical failures and misunderstanding of information by the operators, the core of one of the two reactors in this plant overheated and melted. Nevertheless, the reactor vessel, containing the damaged fuel and its radioactive isotopes, remained intact. No radioactive material reached the outside. Nobody died from the accident. After this accident, safety requirements became more stringent.


Chernobyl (Ukraine, under the former Soviet Union)


On April 26, 1986, the management of the Chernobyl plant decided to proceed with a planned experiment. However, as the conditions for the test were not met, the operators deliberately violated the safety instructions by order of their superiors. The reactor overheated. The fuels inside the reactor began to melt, causing the cooling water to turn into steam and explode, tipping the 1000-ton concrete slab that covered the reactor. The graphite tubes that served as moderator caught fire. Large quantities of radioactive matter was dispersed into the atmosphere.


About 30 firemen and rescue workers died from overexposure to radioactivity, several hundred other people suffered radiation sickness, and several thousand cases of thyroid cancer were diagnosed or foreseen - about half of one percent of the population. Apart from the health consequences of the accident, the most stressful aspect for the inhabitants of the region was the evacuation of some 300,000 of them, a very harsh experience, especially as the country was in difficulty after the dissolution of the Soviet Union.


This reactor was modelled on a design already known to be unstable under certain conditions. In addition, the reactor was not surrounded by a reinforced concrete containment, a safety standard that was proven necessary at Three Mile Island and now recognized as indispensable. Nuclear engineers have learned to avoid these mistakes.


Ironically, three decades after the disaster, wildlife thrive in the Chernobyl exclusion zone. Moose, deer, boars, beavers, bears, owls, ravens, songbirds, lynxes, wolves, badgers and even wild Przewalski's horses abound in the absence of human competition.


Fukushima (Japan)


On March 11, 2011, an earthquake of a rare intensity - magnitude 9 on the Richter scale - gave rise to a huge tsunami. A tital wave 45 feet high washed over the Fukushima nuclear plant, overwhelming its 33-foot protective sea wall.


The earthquake cut off the power supply, and the tsunami flooded the emergency diesels, preventing the cooling of the reactors and spent fuel pools. The plant resisted for 2 days under an emergency cooling system powered by batteries, but eventually the batteries lost their charge. The reactors began to overheat. The heat caused a series of hydrogen explosions, releasing radioactive materials into the atmosphere and discharging contaminated cooling water into the sea.


The tsunami claimed nearly 20,000 lives. There were no casualties from radiation at the plant. The radioacitvity to which the inhabitants of the surrounding area were exposed is likely to cause a very limited number of cancers in the long term, but the evacuation of the inhabitants was a heartbreaking experience, and their resettlement in the abandoned areas will take time, causing serious psychological disorders.


Nine years after the accident, radioactivity in the coastal waters has returned to normal. The average radioactivity of a fish affected by the accident is now equivalent to that of a banana. (Bananas contain some of the element potassium, which has a tiny amount of a radioactive isotope. Don't worry, bananas are perfectly safe to eat.)


Lessons we have learned


These accidents have provided important lessons for nuclear safety. We know that the reaction must stop automatically in the event of a problematic event or human error. The reactor vessel must be protected by a robust containment structure to prevent the release of radioactive material into the environment. Reactor cooling must be ensured even in the event of a disaster (earthquake, flood, etc.), and off-site response teams must be ready within 24 hours.


These precautions have already been applied to most reactors. Third-generation reactors confine molten fuel, letting no radioactivity escape; the reactors of the future will be even safer so as not to require any evacuation of the population, even in the event of a severe accident. No human activity carries a zero risk - not even crossing the street - but the production of electricity by nuclear power has a high safety record compared to almost all alternatives, even before taking greenhouse gas emissions into account.





Generation IV, an initiative launched by the USA in 2000, is a forum for international cooperation to develop a new generation of nuclear systems. Among its objectives: to improve nuclear safety, reduce long-life radioactive waste, use uranium up to 100 times more efficiently, and make nuclear power more affordable. The members recognize nuclear technologies’ role in satisfying the world’s increasing energy needs and preventinig climate change.


Many countries are pursuing research and development of nuclear technologies that meet these criteria. One example is the use of liquid rather than solid fuels, including thorium. This type of reactor is very different from those in use today. Its ability to operate at normal atmospheric pressure will make it much safer and cheaper...but much remains to be done.

New reactors, including fast breeder reactors (which produce more fuel than they consume), are a very promising avenue for the long-term production of the electricity the world needs, and will allow for the reuse of a lot of nuclear waste by recycling it as fuel. Automatic safety devices will make serious nuclear accidents almost impossible. More diverse and abundant fuels will make nuclear energy sustainable over time.




Most of the energy used in the world today comes from fossil fuels: coal, oil and natural gas. Their use produces greenhouse gases, leading to climate change. It is very unlikely that renewable energies (hydro, solar, wind, geothermal, biomass, wave power, etc.) can replace fossil fuels unless the human population is much smaller and we return to a more frugal way of life as it was before the industrial revolution.


There are excellent reasons for including nuclear energy in the energy solution: it is non-carbon emitting, non-polluting, abundant (uranium is available in large quantities on Earth for thousands of years) and causes the least health consequences per unit of energy produced.


Plentiful electric power for our many needs including transportation, manufacturing and heating, can be a huge help in fighting climate change.

You can learn more about nuclear energy here.

Pressurized Water Reactor.jpg

A Chain Reaction

A Pressurized Water Reactor






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