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A Layman’s Guide to Nuclear Terminology


What is nuclear energy?

Nuclear energy first emerged in the early twentieth century and is the newest form of energy to be used on a large scale. Today the dominant source of energy in the industrialized world and in developing countries is fossil fuels (coal, oil, natural gas) but nuclear energy may become more widespread in the future.

Matter consists of tiny particles called atoms. Each atom has a center, or nucleus, which consists of particles called protons and neutrons. In certain types of atoms the nucleus is unstable and it breaks up, releasing various particles such as protons, neutrons and also energy. This is known as fission and it can happen naturally or can be artificially induced.

Sometimes the products of a fission reaction cause similar reactions in other atoms resulting in a chain reaction.

When the atoms split, the fission produces energy which is released in the form of heat. This is nuclear energy.

Nuclear power plants use this heat to create electricity.

What materials can be used to create nuclear energy?

Not all materials will fission. Readily fissile materials are uranium-235 and plutonium-239. The number that follows the name of the element designates the number of protons and neutrons in its nucleus. This can vary, even within the same type of element, and the variants are called isotopes. For example, uranium-235 and uranium-238 are two different isotopes of the same element – uranium.

Uranium-235 and plutonium-239 are often used in power plants to create nuclear energy. However, these materials are hard to come by naturally and a large part of the nuclear energy process is creating these materials or gathering large quantities of them.

What is uranium?

Uranium is a heavy material and is one of the elements that can be used to create nuclear energy. It occurs naturally and is extracted from opencast pits or by underground mining.

Its chemical symbol is U and it was discovered in 1789.

There are 16 isotopes of uranium. All of them will split spontaneously but not all are fissile, which means they cannot all be used to cause a chain reaction and create nuclear energy.

One efficient type of fissile uranium is called uranium-235 (or U-235) which means it has 235 particles (143 neutrons and 92 protons) in its nucleus. This is the isotope usually used to create nuclear energy.

However, most of the uranium found naturally is a different isotope called uranium-238 (or U-238).

In every 1,000 atoms of uranium, only 7 atoms will be U-235, the type that can be used to create nuclear energy. The rest of the atoms will mostly be U-238. If you do not have a high concentration of fissile material (in this case U-235), you will not have a sufficient amount to sustain a chain reaction and create nuclear energy. This is why uranium has to be enriched.

What is uranium enrichment?

Uranium enrichment is the process of “weeding out” those unwanted atoms of uranium and providing more U-235 atoms.

The enrichment does not mean you will end up with pure U-235. It just means that after the enrichment process, in every 1,000 atoms of uranium, instead of having just seven atoms of U-235, you will have between 35 and 40 of these atoms.

There are currently four methods of enriching uranium. One of them is by using centrifuges.

What are centrifuges?

One method of enriching uranium is using centrifuges. This is a device that holds uranium and spins it very fast. Since the uranium-238 is heavier, the spin forces the U-238 to the outer side of the centrifuge and separates it from the desirable U-235.

Uranium that is made to contain a lot of U-238 is called depleted uranium.

What is yellowcake?

When uranium is mined, it is “stuck” in another material. It is sent to a mill that crushes this material and then it is covered with a substance that will separate the uranium from the waste rock. When the uranium is recovered it appears as uranium oxide. Uranium oxide is also referred to as yellowcake. This is the form in which uranium is sold. It still needs to go through several processes in order to be able to produce nuclear energy.

Countries with large deposits of uranium include Australia, Brazil, Canada, Kazakhstan, Namibia, Russia, South Africa and the United States.

Sizewell nuclear plant in the United Kingdom (Uranium Information Center)

What is plutonium?

Another material that is readily fissile (the fission process can be artificially induced in this material and it can also cause a chain reaction) is plutonium-239.

Plutonium is less effective in creating nuclear energy because it needs to be hit faster by moving neutrons than U-235 in order for it to fission.

Unlike uranium, plutonium is an artificial element. Scientists believe it once existed naturally but its quantity decreased considerably over the years and if any exists naturally, it is very rare. Therefore, the plutonium used in nuclear reactors is man-made.

Plutonium was discovered in 1940 and its chemical symbol is Pu.

Do these materials change when used for making nuclear bombs?

Yes. Bomb-grade uranium is highly enriched. Nearly 90 percent of its atoms will be U-235 (as opposed to 3-4% in a normal nuclear reactor.)

There is also a difference between fuel-grade plutonium and weapons-grade plutonium.

Virtually any combination of plutonium isotopes can be used to make bombs but the most effective for this purpose is Pu-239. Weapons-grade plutonium contains about 93% Pu-239. In contrast, plutonium used in civilian nuclear reactors will contain more of the higher isotopes.

Apart from uranium and plutonium, can any other materials be used to make nuclear energy?

Yes. There is an element called thorium which can also be applied but since its efficiency is much lower than that of uranium and plutonium, its application remains largely on the drawing board.

How does a nuclear reactor work?

If the reactor uses uranium, this element will be assembled in such a way that a controlled fission chain reaction can take place.

The chain begins when a uranium atom is hit by a neutron causing it to split. Products of this split include three neutrons which in turn make three other atoms split, so the chain is continued and expanded.

Heat released in the chain reaction is used to create steam.

The steam spins a turbine which drives a generator and that makes the electricity.

In the United States there are more than 100 reactors operating, supplying more than 20% of the country’s electricity.

Illustration of the nuclear reactor cycle. (Uranium Information Center)

What are fuel rods?

After uranium is enriched, the uranium goes through other chemical processes and is then formed into fuel pellets. These pellets are placed in thin metal tubes which are put in bundles. The bundles of rods contain the fuel for the core of the nuclear reactor. The core is made up of several hundred of these clusters.

What are control rods?

The speed of the chain reaction that takes place in the core of the nuclear reactor is regulated with control rods. These contain a material that can absorb neutrons which would otherwise cause other atoms to split.

Control rods slow down the rate of energy production and they can be inserted or removed to regulate the required power level.

What is spent fuel?

After uranium has been used in a reactor to create electricity for three or four years, the uranium is no longer efficient because the U-235 isotopes have been depleted. The uranium is then removed from the reactor. This is called spent fuel and it is highly radioactive. Spent fuel can either be reprocessed to recover the usable uranium in it, or it can be stored and later disposed of as waste.

What is heavy water?

Heavy water is chemically the same as regular water, but it is made with hydrogen atoms containing an additional neutron in their center which makes this hydrogen “heavier” than a normal atom of hydrogen.

Heavy water is used in certain types of nuclear reactors as a neutron moderator. The neutrons taking part in the fission process cannot be too “energetic,” otherwise the chain reaction will not take place. Moderators slow down the neutrons emitted during the fission process so they are slow enough to carry on the chain reaction.

Regular water can also act as a moderator in some nuclear reactors. When using regular water as a moderator, there is less chance that neutrons will provoke the next stage in the chain reaction and in this case the uranium used will need to be more enriched. However, when using heavy water as a moderator, the uranium does not have to be enriched as much.

Heavy water can also be used to assist in creating plutonium from uranium. Because of this its production is monitored and its export controlled.

Another common moderator is graphite.

What is a nuclear bomb?

There are two types of nuclear bombs – an atom bomb, also called an A-bomb, and a hydrogen bomb, also called an H-bomb or a thermonuclear bomb.

Hydrogen bombs are much more powerful than atom bombs and have never been applied in warfare. The nuclear bombs dropped on Hiroshima and Nagasaki at the end of the Second World War were both atom bombs.

Although both types of bomb utilize nuclear energy, they differ in the method used to create this energy.

Atom bombs are based on the fission process, where nuclei containing many neutrons and protons, such as uranium or plutonium, are forced to release neutrons and energy is released in the process. Civilian nuclear reactors are based on this method.

(In a nuclear bomb, the chain reaction is not controlled as it is in the reactor. Because of this, and because the uranium is more enriched, the chain reaction will take place
in a fraction of a second, releasing an enormous amount of energy in a short time.)

A hydrogen bomb is based on a different process called fusion, where atoms of light elements such as hydrogen (the most abundant element in the world) or helium, are bundled together. In this case, it is the union of these atoms that creates energy.

But to set off a hydrogen bomb, you need very unique conditions of extreme density, high temperature and high pressure. To detonate a hydrogen bomb, an atom bomb will be detonated next to it to create these conditions.

So, the fission process is used in the atom bomb but it is also applied as a means to detonate a hydrogen bomb.

There are no nuclear reactors based on fusion (civilian or otherwise) because due to the extreme conditions necessary for fusion reaction, such installations are not feasible at this time. However, this is an area of extensive research because of the great potential of energy production it could yield.

How can you tell whether a nuclear program is for civilian purposes or for making weapons?

There are several indications. The most obvious would be traces of highly enriched uranium found in samples near a nuclear site. The only reason a country would need to produce weapons-grade uranium is if they wish to manufacture materials for a military program.

The United Nations International Atomic Energy Agency (IAEA) takes samples of earth near nuclear sites and seeks traces of uranium. The type of uranium bears a signature which can indicate the purpose of the nuclear activity or even which country the uranium came from.

If the international community finds a hidden enrichment facility, this may also indicate that a party is hiding military activity. If the enriched uranium in this case is not weapons-grade, some countries would not consider this a “smoking gun” but a concealed facility would raise questions about the purpose of the nuclear activity.

Other indications would be bomb design information or technologies and materials which are only useful in producing nuclear bomb materials, such as beryllium or polonium.

The Media Line would like to thank physicist Elad Zinger for his assistance.