Radioactive Decay And Radiation Engineering Essay


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In 1899, English scientist Ernest Rutherford discovered alpha particles. If the number of neutrons in the nucleus of an atom is too low in relation to the number of protons, it results in an unstable energy state. In order to restore the balance between protons and neutrons and create a stable atom, alpha particles are released. This process is called alpha decay. These alpha particles consist of two protons and two neutrons and are identical to the nucleus of the helium-4 atom. As the number of protons determine which element an atom would belong to, this loss of protons causes the atom to become an atom of a different element. The protons give the alpha particle a positive charge of +2. The alpha particle is a high-energy particle, and can travel through air at a speed of about one twentieth (1/20) of the speed of light.1 The discovery of alpha particles led to the concept of the atom's structure having negative particles (electrons) orbiting a positive nucleus (containing protons and neutrons).

Figure 1. An Alpha Particle [Accessed 05-04-10]

Henri Becquerel, a French physicist, discovered beta particles in the late 19th century and in 1900 proved these beta particles are identical to electrons. If there are excess neutrons in the nucleus of an atom, a neutron will degenerate and form a proton and an electron.3 This is called a beta particle. While the proton remains in the nucleus, the electron is expelled from the nucleus at a high speed.3 This formation of beta particles decreases the number of neutrons and increases the number of protons, therefore increasing the atomic number and stability3 and changing the atom to a different element.4 As the atom now has an extra electron, its electrical charge is negative. The speed at which the particle can travel depends on the amount of energy it contains, and it is the excess energy which can cause damage to living cells by breaking chemical bonds and forming ions.4

Figure 2. A Beta Particle [Accessed 05-04-10]

Gamma rays were discovered by French physicist Paul Villard. The emission of gamma rays occurs when too much energy is present in the nucleus of a radioactive atom. This can happen due to nuclear explosions, supernova explosions, and radioactive material decaying in space. The emission of gamma rays often occurs after beta particles have been released.7 Beta particles do not get rid of excess energy, so this energy is released from the nucleus in the form of gamma photons.6 Gamma photons do not have a mass or electrical charge, as they are rays of pure electromagnetic energy.7 Gamma rays have short wavelengths, high energy, and are able to travel at the speed of light, allowing them to penetrate many different materials. Only very dense substances, such as lead, are able to slow down or stop these gamma rays.7 Gamma rays are extremely dangerous and have the ability to kill living cells. For this reason, gamma rays have been used in the medical field to kill cancerous cells.7

Figure 3. A Gamma Particle [Accessed 05-04-10]

Why do some isotopes of elements spontaneously undergo radioactive decay?

Some isotopes of elements spontaneously undergo radioactive decay because the nuclei of its atoms are unstable. This instability causes nuclear change within the element, and it will react to form a substance that is more stable and less reactive

than the original element.

All substances which naturally undergo radioactive decay continue to decay by a sequence of alpha and beta decays until they finally become stable isotopes of a particular element. What element is this?

All substances which naturally undergo radioactive decay continue to decay until they become Pb-206, which is a stable isotope of lead.

The rate at which radioactive decay occurs is given in half-life. Explain the term and give the half-life of a few substances.

The term half-life refers to the amount of time it takes for half of the atoms in a sample of a particular substance to decay. This means that radioactive decay will occur quickly when there is a large amount of atoms, but as they decrease, the rate of decay will slow down.11 The rate at which radioactive decay takes place will, therefore, get closer and closer to zero, until it becomes immeasurable, but will never reach it. The half-lives of elements can range from seconds to billions of years. Elements with long half-lives are said to have low specific activities, as their decay into other elements does not happen often. Examples of substances with low specific activities would be uranium-238 (U-238), which has a half-life of 4,5 billion years, potassium-40 (K-40), which has a half-life of 1,3 billion years, and plutonium-239 (Pu-239), which has a half-life of 240 000 years. Other substances, such as nitrogen-16 (N-16), whose half-life is 7 seconds, have extremely short half-lives, and therefore have high specific activities. Technetium-99 (Tc-99) has a half-life of 6 hours, and iodine-131 (I-131) has a half-life of 8 days. These substances are therefore used in nuclear medicine procedures.13 There is an inverse relationship between radioactivity and half-life, as the more radioactive a substance is, the shorter its half-life.13

Carbon dating is used in archaeology to date materials. Explain how carbon dating works.

Carbon-14 (C-14) is produced when cosmic rays collide with an atom in the atmosphere creating an energetic neutron which in turn collides with nitrogen-14 (N-14) atoms. A proton gets displaced by a neutron and the nitrogen atom turns into a radioactive isotope of carbon. Carbon-14 combines with oxygen to form carbon dioxide. Plants absorb this carbon dioxide during photosynthesis and animals take in C-14 by eating these plants as well as breathing.14 This radiocarbon is unstable and it will undergo spontaneous decay into N-14. When an organism dies, it will no longer consume radiocarbon, but the C-14 which is in its body already, will decay into nitrogen,15 while the level of C-12, the most common, stable isotope of carbon will remain constant.14 As we assume the ratio of C-14 to C-12 atoms is one to one trillion, and that C-14's half-life is 5730 years,15 it is possible to date a specimen by measuring the ratio of C-14 to C-12. For example, if the ratio of C-14 to C-12 is one to two trillion, we know that the organism would have died 5730 years ago, as half of the C-14 has decayed. However, carbon dating can only be used to date specimens up to 60 000 years old, as after 10 half-lives, the amount of C-14 becomes too small to measure.16 There is much controversy about the accuracy of carbon dating, as we assume that the rate of decay or half-life of C-14 and the ratio of C-12 to C-14 in the atmosphere were the same at the time of the specimen's death as it is now.16 Nevertheless, carbon-14 dating has proven to be very accurate over the many years of its use. [Accessed 05-04-10]

Figure 4. Carbon-14 dating

What is a Geiger counter?

A Geiger counter is an instrument used to measure radioactivity. It is made up of a metal or glass cylinder filled with a gas such as helium, neon or argon. A metal wire is then run through the centre and the tube is sealed.17 The wire is then charged with about 1000 volts.17 When ionised particles such as radiation penetrate the tube they collide with the gas particles,18 ionising the atoms and producing electrons.17 Due to the positive voltage of the wire, the electrons are attracted to it,17 colliding with more atoms and ionising them.18 Each particle that is created causes an electrical pulse and by counting these particles, the amount of radiation present can be counted.17

Nuclear fission and nuclear fusion

Questions 7 - 10

Nuclear fission and nuclear fusion

Questions 7 - 10

It is possible to cause artificial radioactivity by bombarding a nucleus with high velocity neutrons. This process is called nuclear fission. This reaction is the basis of nuclear power stations as well as the nuclear bomb. With the aid of a diagram, briefly explain this fission reaction.

Nuclear fission is the splitting of an atom by adding neutrons to its nucleus. The nucleus must be bombarded with neutrons as they are the only particles with a definite mass and that are electrically neutral, therefore they will not be repelled from the nucleus which is positively charged. An element which commonly undergoes nuclear fission is uranium-235, and this is used to make the nuclear bomb as well as in nuclear reactors in power stations. When a stray neutron collides with the nucleus of the uranium atom, it is absorbed, creating a uranium-236 atom. This U-236 is unstable and causes the atom to fission or split.19 There are many elements that the uranium can fission to, but the mass of the fragments will be slightly less than the mass of the original atom, as some of the mass has been converted into and released as energy. When an atom fissions, one or two extra neutrons are released, thus creating a chain reaction.19 Nuclear fission can continue until all the uranium-235 fuel has been used. When it is not controlled it is called a runaway nuclear reaction.19

Figure 5. A Nuclear Fission Reaction [Accessed 30-04-10]

What is meant by the term "critical mass"?

Critical mass is the quantity of a certain substance needed to ensure that the chain reaction can become self-sustaining. This means that neutrons are not lost at a quicker rate than they are created by nuclear fission. The critical mass of a substance can be reduced by limiting the amount of neutrons which are lost. This can be done by forming the substance into a sphere, which allows minimum surface area for the loss of neutrons, or by using a neutron reflector, which also reduces the loss of neutrons from a substance.

The stars emit radiations due to nuclear fusion reactions. What is the difference between nuclear fusion and nuclear fission?

While nuclear fission is the splitting of atoms due to the addition of neutrons, nuclear fusion is the joining of two elements with low atomic numbers to form a different element.22 Nuclear fusion releases nuclear energy as well as a neutron. Nuclear fission can only occur until the fuel is used up, but there is no limit to the amount of nuclear fusion that can occur. The biggest example of a nuclear fusion reaction is the sun.

Nuclear fusion reactions produce no nuclear waste and would thus be an ideal source of energy. What are the main reasons why fusion reactions have not been used in nuclear reactors?

Nuclear fusion reactions have not been used in nuclear reactors due to the difficulty of containing an environment similar to that of the sun. As fusion reactions can reach temperatures of up to 1 000 000°C, strong magnetic fields would be necessary to ensure that the elements remained in the solid phase. Also due to the extreme temperature, gravitational pressures would have to be produced to force the nuclei of the atoms to fuse together and release their bond energy. The amount of energy needed to maintain these conditions is higher than that which would be extracted.

Nuclear power stations

Questions 11 - 15

Nuclear power stations

Questions 11 - 15

Using a diagram, briefly explain the layout and functioning of a nuclear power station. In your answer refer to the three separate water circuits.

Figure 6. Layout of a Nuclear Power Station

Chaplin [Date unknown], 6

A nuclear power station consists or a reactor vessel in the centre of the reactor building, surrounded by three steam generators. This forms the primary circuit. The secondary circuit consists of the turbine generator, found in the turbine hall, with the condenser directly beneath it. The generator is driven by the turbine to produce electricity.

The nuclear fuel (usually uranium) is contained in the reactor pressure vessel. The fuel undergoes nuclear fission and heats up to about 2 500°C. This heat is transferred to the water which runs through the centre of the reactor vessel. This reactor coolant then moves to the steam generators. The water is at a temperature of 323°C, but is still in the liquid phase due to the high pressure. The hot water in the steam generators creates steam which is received by the turbine. As the steam passes through the high pressure cylinder, the moisture separator-reheaters and the low pressure cylinders, the steam loses pressure and temperature, thus providing energy to turn the turbine and the generator. The steam then passes through the condenser and the condensate is returned to the steam generator.

The feed water system extracts water from the condenser through the condensate extraction pumps. As it has cooled down, the temperature is raised again by passing it through the drain coolers and the low and high pressure feed heaters. The water is then returned to the steam generators for the cycle to be repeated.

The circulating water system takes water from the condenser to the cooling tower, where it is cooled by falling down and giving off heat to the air as the air rises. It is then returned to the condenser. At Koeberg, however, no cooling tower is present as the cooling water is drawn from the Atlantic Ocean. It is then passed through the condenser and returned to the ocean.

The generator water cooling system is intended to cool the stator windings. As the stator windings can conduct electricity, demineralised water is passed through these hollow conductors. The pressure of this water has to be kept slightly below that of hydrogen to ensure that the water does not leak into the generator.

Figure 7. Koeberg Nuclear Power Station [Accessed 30-04-10]

How is the rate of the fission reaction regulated in a nuclear reactor?

Nuclear fission is used in nuclear reactors, but has to be controlled in order to avoid an explosion. Only one of the released neutrons is allowed to strike another nucleus and cause a fission reaction. The amount of free neutrons is controlled using control rods made of materials which are neutron-absorbing such as boron or cadmium. The fast moving neutrons are slowed down by using a moderator such as water.26

Uranium fuel pellets used in nuclear power stations produce far more energy per kilogram than the energy produced by burning coal. How much coal has to be burnt to produce the same amount of energy as is generated by a kilogram of uranium fuel pellets?

As uranium-235 produces 80 million MJ/kg, and coal only produces 24 MJ/kg, 3 333 333,333 kg of coal has to be burnt to generate the same amount of energy as 1 kilogram of uranium fuel pellets.

Nuclear power stations cause far less air pollution then coal burning power stations. Briefly discuss the different types of air pollution produced by coal burning power stations.

There are many different types of air pollution which are produced by the burning of fossil fuels such as coal. Particulate matter, or black carbon pollution, consists of small particles of burnt fossil fuels (ash) floating in the air. It is not only the visible particles, but the fine particles which can cause health problems. Smog is a hazy substance consisting of a mixture of gases and particles of pollutants. Sulphur dioxide and nitrogen oxide, both contributors to acid rain, are produced by the burning of fossil fuels in car engines and power plants. Nitrogen oxide can also form toxic chemicals and harm the quality of water as well as that of air. Carbon monoxide is a gas formed by the burning of fossil fuels such as coal, kerosene and gas. It is toxic to humans as it combines with the iron found in haemoglobin in blood, and prevents the blood from absorbing enough oxygen to sustain life.

Unfortunately, the nuclear waste which is generated by nuclear power stations and the disposal of such waste is an enormous problem. What are the three levels of waste produced at Koeberg and how is this waste disposed of?

The three levels of waste produced at Koeberg are categorised according to their levels of radioactivity and each level must be disposed of differently.

Low level waste consists mostly of clothing, plastics, insulation material or paper, which could have been contaminated with small amounts of radioactive material from the controlled radiological areas of the power station. This waste is stored in metal drums before it is taken by truck to Vaalputs National Nuclear Waste Repository, which is found about 500km north of Koeberg.

Intermediate level waste is more radioactive than low level waste and it consists of evaporator concentrate, spent resins, filter cartridges and contaminated scrap metal.31 This type of waste has to be mixed with concrete before it is sealed into concrete drums to ensure that it cannot harm the environment or people. It is also shipped by truck to Vaalputs.30

High level waste is dangerous material derived from fuel which has been used in the fission process as well as reprocessed to extract uranium to be reused. High level waste is divided into three categories. Fission products are extremely radioactive when they leave the reactor vessel30 but decay to low radioactivity in 1 000 years. Actonides are isotopes of uranium and take 10 000 years to decay.31 Structural materials have only a small amount of radioactivity through the irradiation by neutrons and only take 500 years to decay.31 These need to be stored separately. This waste is stored in fuel pools31 for 10 years after which it has lost enough radioactivity to be moved into thick-walled casks and stored for another 40 years. [Accessed 30-04-10]

Figure 8. Low Level Waste [Accessed 30-04-10]

Figure 9. Fuel Pool


Cosmic rays: Radiations from space etc. that reach the earth from all directions, usually with high energy and penetrative power.

Electromagnetic: Having both an electrical and a magnetic character or properties.32

Electron: A stable elementary particle with a charge of negative electricity.32

Ion: An atom or group of atoms that has lost one or more electrons, or gained one or more electrons.32

Ionise: Convert or be converted into an ion or ions.32

Isotope: One of two or more forms of an element differing from each other in relative atomic mass, and in nuclear but not chemical properties.32

Neutron: An elementary particle of about the same mass as a proton but without en electric charge.32

Nucleus: The positively charged central core of an atom that contains most of its mass.32

Proton: A stable elementary particle with a positive electric charge, equal in magnitude to that of an electron, and occurring in all atomic nuclei.32

Radioactivity: The spontaneous disintegration of atomic nuclei, with the emission of usually penetrating radiation or particles.32

Supernova: A star that suddenly increases very greatly in brightness because of an explosion ejecting most of its mass.32

Voltage: Electromotive force or potential difference expressed in volts.32

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