Radioactivity

RADIOACTIVITY WHAT IS RADIOACTIVITY

Radioactivity is the process in which unstable atomic nuclei spontaneously decompose to form nuclei with a higher stability by the release of o f energetic sub atomic particles. In order to understand radioactivity the structure o f atoms needs to be understood. ATOMIC STRUCTURE

Atoms are made up o f protons, neutrons and electrons each with their own pro perties. y

Protons have a positive (+) charge

y

Electrons have a negative (-) charge

y

Neutrons

have no charge (Neutral)

The protons and neutrons lie in the nucleus at the centre of the atom and the electrons orbit round the nucleus. The proton and neutron are approximately equal in mass and they are each about 1800 times the mass of an electron. ATOMIC NUMBER AND ATOMIC MASS

The atomic number of an element is the number of protons that lie in the nucleus of o f an atom. The atomic mass or  mass number of an element is the total number of protons and neutrons that lie in the nucleus of an atom. ato m. Example: If we take the example of o f the Lithium atom it contains 3 protons and 4 neutrons in its nucleus. Therefore it's: Atomic number = number of  protons = 3 Mass number = number of  protons + number of n of  neutron = 3 + 4 = 7

Elements are normally represented by their chemica l symbols along with their atomic and mass number, as shown below:

1

ISOTOP ISOTOPES All the atoms of a particular element have the same number of protons. It is the number of protons which determine the chemical properties of an atom. This is because the number of protons determines the number  of electrons which orbit the nucleus. nuc leus. The number of neutrons however can var y. Atoms of t of  the same element that have the same number of  p rotons but diff erent number of n of  neutrons are called isotopes of t of  that element.

Or in other words, atoms of the same element with the same atomic number but diff erent atomi c masse s are called isotopes of that element.

STABLE AND UNSTABLE NUCLEI An atom is electrically neutral. It contains an equal number of positively charged protons and negatively charged electrons and their charges balance. The nucleus however contains only positively charged protons which are closely packed together in a very small volume (remember neutrons have no charge). From the laws of physics (Coulomb¶s Law) one would expect that the protons being of the same charge and so close together  would exert strong repulsive forces on each other. The combined gravitational force from the protons and neutrons in a nucleus is insignificant as an attractive force because their masses are so tiny. This implies there must be an additional attractive force similar in size to the electrostatic repulsion which holds the nucleus together. STRONG NUCLEAR FORCE

A strong force must exist between the protons and neutrons in a nucleus to overcome the electrostatic repulsion between the protons. This additional force is called the strong nuclear force. This force acts is attractive over short distances, but this is not all. If the force was only attractive then it would pull the protons and neutrons together to an ever decreasing small volume of space. We know this not to be the case as each nucleus contains a distinct number of protons and neutrons which retain their identity. The more protons and neutrons, the bigger the nucleus and thus the separation between the protons and neutrons in a nucleus must 2

 be a fixed distance. This means the strong nuclear force must be repulsive at a very small range as well as  being attractive over a slightly larger distance. This is the force that holds the nucleus together and the energy associated with this force is called the binding e nergy. It is the amount of strong nuclear force and the associated binding energy in a nucleus that determines whether it will be stable or unstable. STABLE NUCLEI

A stable atom is an atom that has enough binding energy to hold the nucleus together permanently. The graph below is a plot of neutron number against proton number. It is used as rule to determine which nuclei are stable or unstable.

 Nuclei which lie on the stability line are stable nuclei. From the plot it can be seen that many of the stable nuclei have equal number of protons and neutrons. These are usually the elements in the lighter section of the  periodic table. For the heavier stable nuclei the there are approximately 50% more neutrons to protons. UNSTABLE NUCLEI

In unstable nuclei the strong nuclear forces do not generate enough binding energy to hold the nucleus together permanently. It is unstable nuclei that are radioactive and are referred to as radioactive nuclei and in the case of their isotopes called radioisotopes. The unstable nuclei lie above and below the line of stability in the neutron ± proton plot. This gives information of the type of radioactive decay they will undergo. Nuclei which lie above the line of stability contain too many neutrons to be stable. They are referred to as ³neutron rich´. Those that lie below the line of  stability contain too many protons to be stable and are called ³proton rich´. In summary it is the balance of protons and neutrons in a nucleus which determines whether a nucleus will be stable or unstable. Too many neutrons or protons upset this balance disrupting the binding energy from the 3

strong nuclear forces making the nucleus unstable. An unstable nucleus tries to achieve a balanced state by given off a neutron or proton and this is done via radioactive decay.

ALPHA, BETA AND GAMMA RAYS Radioactivity is the process in which unstable atomic nuclei spontaneously decompose to form nuclei with a higher stability by the release of  energetic sub atomic particles.

The above definition tells us that radioactivity is a random or spontaneous naturally occurring process. The  process cannot be influenced by external factors such as heat, pressure or exposure to a magnetic field. This is not to be confused with the radiation that arises from nuclear fission in nuclear power stations. Here the fission is not spontaneous but is triggered in a nuclear reactor by the bombardment of high energy neutrons. Secondly, it occurs in unstable atoms or more accurately unstable isotopes called radioisotopes. These atoms are unstable because of the unbalanced nuclear forces within their nuclei. The sub atomic particles and their associated energy that are released during the decomposition of the unstable nuclei are referred to as radiation. There are 3 main types of radiations.

y

Alpha particles

y

Beta particles

y

Gamma

rays

Alpha particles are released by high mass, proton rich unstable nuclei. The alpha particle is a helium nucleus;

it consists of two protons and two neutrons. It contains no electrons to balance the two positively charged  protons. Alpha particles are therefore positively charged particles moving at high speeds. Beta particles are emitted by neutron rich unstable nuclei. Beta particles are high energy electrons. These

electrons are not electrons from the electron shells around the nucleus, but are generated when a neutron in the nucleus splits to form a proton and an accompanying electron. Beta particles are negatively charged. Gamma

rays are electromagnetic waves of very short wavelength and high frequency. Gamma rays are

emitted by most radioactive sources along with alpha or beta particles. After alpha or beta emission the remaining nucleus may still be in an excited energy state. By releasing a gamma photon it reduces to a lower  energy state. Gamma rays have no electrical charge associated with them.

PENETRATING PROPERTIES OF RADIATION Radiations from radioactive materials can be dangerous and pose health hazards. By knowing the ability of  the different types of radiation to penetrate matter allows us to gain an understanding on how best to protect ourselves.

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PENETRATION OF ALPHA PARTICLES

Alpha particles can be absorbed by a thin sheet of paper or by a few centimetres of air. As alpha particles travel through air they collide with nitrogen and oxygen molecules. With each collision they lose some of  their energy in ionising the air molecule until eventually they give up all of their energy and are absorbed. In a sheet of paper the molecules are much close together so the penetration of alpha particles is much less than in air. PENETRATION OF BETA PARTICLES

Beta particles travel faster than alpha particles and carry less charge (one electron compared to the 2 protons of an alpha particle) and so interact less readily with the atoms and molecules of the material through which they pass. Beta particles can be stopped by a few millimetres of aluminium. PENETRATION OF GAMMA RAYS

Gamma rays are the most penetrating of the radiations. Gamma rays are highly energetic waves and are poor  at ionising other atoms or molecules. It cannot be said that a particular thickness of a material can absorb all gamma radiation. Many centimetres of lead or many meters of concrete are required to absorb high levels of  gamma rays.

PROPERTIES OF RADIATION ± DEFLECTION IN AN ELECTRIC AND MAGNETIC FIELD

An insight into the properties of radiation can be demonstrated by observing their behaviour in a magnetic and electric field. ALPHA PARTICLES IN AN ELECTRIC FIELD

Alpha particles are attracted to the negatively charged plate. This confirms that they must be positively charged as unlike charges attract. Alpha particles are helium nuclei; they contain 2 protons which gives them their positive charge. BETA PARTICLES IN AN ELECTRIC FIELD

Beta particles are attracted to the positively charged plate. This confirms that they are negatively charged. Beta particles are fast moving electrons with a very low mass and so have a high charge to mass density. They are deflected much more t han the heavier alpha particles. GAMMA

RAYS IN AN ELECTRIC FIELD

Gamma rays are unaffected by an electric field. This shows gamma rays are uncharged. Gamma rays are highly energetic waves with no charge associated with them.

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ALPHA PARTICLES IN A MAGNETIC FIELD

When a charged particle cuts through a magnetic field it experiences a force referred to as the motor effect. Alpha particles are deflected by a magnetic field confirming that they must carry a charge. The direction of  deflection which can be determined by Fleming¶s left hand rule demonstrates that they must be positively charged. Remember for Fleming¶s left hand rule the second finger is aligned with the direction of the current which is from positive to negative. BETA PARTICLES IN A MAGNETIC FIELD

Beta particles are deflected by a magnetic field in an opposite direction to alpha particles confirming they must hold a charge opposite to alpha particles. Beta particles are fast moving electrons and are thus negatively charged. GAMMA

RAYS IN A MAGNETIC FIELD

Gamma rays are unaffected by a magnetic field. This shows gamma rays are uncharged as they do not experience a force when passing through the lines of a magnetic field. Gamma rays are highly energetic waves with no charge associated with them.

DETECTING RADIOACTIVITY ± THE GEIGER MULLER TUBE

Alpha, Gamma and Beta radiations are invisible to humans and exposure to these radiations can be hazardous to the health of living organisms. It is therefore extremely important that suitably designed detectors are available in order to gain information on the type and amount of radiation present. The Geiger-Muller tube or Geiger counter

Alpha, Beta and Gamma radiations are all ionising radiations. This means that all three forms of radiations have enough energy to pull electrons from atoms turning them into ions. The Geiger-Muller tube makes use of  this fact. A Geiger-Muller tube consists of a sealed metallic tube filled with argon or another noble gas mixed with a small amount of alcohol vapour or bromine gas. The argon gas is called the detecting gas whereas the bromine gas or alcohol vapours are referred to as the quenching gas. The gas mixture inside the tube is at a pressure  below atmospheric pressure. A thin metal wire runs through the centre of the tube. An electric potential of up to 1 kilovolt is maintained between the metal wire (the anode) and the cylinder (the cathode). In the absence of any radiation no current flows between the wire and the cylinder. When a radioactive particle enters the tube it ionises an argon atom. The resulting electron is accelerated towards the metal wire or anode. As the electron approaches the metal wire it experiences an increasing electric field strength which in turn applies a greater accelerating force on the electron. The accelerating force becomes so strong that on collision with other argon atoms the electron can ionise them. The electrons from these ionisations can go onto to generate a cascade of further electrons, an effect called the avalanche effect. The ionisation by one particle can result in millions of electrons striking the metal wire. 6

This migration of electrons inside the tube results in an electric discharge. This gives a measurable voltage   pulse in the external circuit of the Geiger-Muller tube. The counter registers the number of pulses and converts them into sound signals or displays them as a measure on the screen. QUENCHING GAS

The purpose of the quenching gas is to absorb the positive argon ions as they accelerate to the cathode. Without the quenching gas these positive ions will be neutralised at the cathode in an exited state or could even also dislodge electrons from the cathode. These dislodged electrons or excited atoms could trigger  further ionisation creating a further voltage discharge giving inaccuracies in the measure from the device. When the quenching gas migrates to the cathode it recombines at ground state and so does not present the  potential to cause any further ionisation.

RADIOACTIVE HALF LIFE

The release of radiation by unstable nuclei is called radioactive decay. This process occurs naturally and cannot be influenced by chemical or physical processes. The release of radiation is also a random event and overtime the activity of the radioactive material decreases. It is not possible to predict when an individual nucleus in a radioactive material will decay. But it is possible to measure the time taken for half of the nuclei in a radioactive material to decay. This is called the half life of radioactive material or radioisotope.  Half life can be defined as the time taken for the number of nuclei in a radioactive material to halve. It can also be defined as the time taken for the count rate of a sample of radioactive material to fall to half of its  starting level. The count rate is measured by using an instrument called a Geiger-Muller tube over a period of 

time. A Geiger-Muller tube detects radiations by absorbing the radiation and converting it into an electrical  pulse which triggers a counter and is displayed as a count rate. At the start of the measurement the radioisotope has 10,000 unstable nuclei. Over 2 days 5,000 of these unstable nuclei undergo radioactive decay to stable nuclei. Therefore half of the original radioactive nuclei have decayed in 2 days so the half life for this radioisotope is 2 days. Over the next 2 days 2500 of the remaining radioactive nuclei undergo decay. This is half of the remaining 5000 radioactive nuclei. The isotope undergoes further decay and it takes 2 days for the number of radioactive nuclei to halve in number. The half life for this radioisotope is 2 days. Therefore the number of radioactive nuclei is always half the number there was 2 days ago. The curve produced is called a radioactive decay curve.

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8

CALCULATING HALF LIFE

We know that after one half life, one half of the original radioactive nuclei remain. After two half lives, one half of this half remains or one quart er of the original radioactive nuclei remain. We can therefore say: 1 half life = ½ of the original radioactive nuclei remain 2 half lives = ¼ of the original radioactive nuclei remain This can be written as an equat ion as:

Example:

A radioactive isotope has a radioactivity measured on a Geiger-Muller tube at 10,000 Becquerel¶s or 10,000 counts per second. What is the activity of the radioisotope after 3 half lives?

The amount remaining =

x Original amount

The amount remaining =

x 10,000

The amount remaining =

x 10,000

The amount remaining = 1250 Bq For this question we did not need to know the half life of the isotope, we simply needed to remember the equation and the fact that the activity is halved for every half life.

USES OF RADIOACTIVITY RADIATION IN THE TREATMENT OF CANCER 

The three types of radiation are highly energetic particles. Each type has its own distinct ionising and  penetrating property. Radiation can ionise the molecules in living cells, in particular the DNA molecule. This can affect the ability of the cell to grow and divide. This affect of radiation on living cells is made use of in the treatment of cancer. The process of using radiation in the treatment of cancer is called radiation therapy or radiotherapy. During radiotherapy both cancer cells and normal cells are damaged, however normal cells can recover from the effects of radiations.

9

There are three types of radiation therapy;

y

External radiation therapy

y

Internal radiation therapy

y

Systemic radiation therapy

EXTER NAL RADIATION THERAPY

In this type of treatment a machine is used to aim high energy radiation to the cancerous cells from outside the   body. A machine called a linear accelerator (LINAC) is commonly used for this type of treatment. In this device a highly energetic electron beam is used to generate x-rays or gamma rays through the collision with a suitable target material. The x-rays/gamma rays are then focused into a beam which targets the region requiring treatment. External radiation is used to treat most type of cancers for e.g. cancer of the breasts,  brain, bladder, lungs etc. It is also used in certain cases when the cancer spreads to other parts of the body from the primary site. INTER NAL RADIATION THERAPY

In this type of therapy the radioactive material is sealed in needles, wires, seeds or flexible tubes and placed directly into or near the cancer cells. The radioactive isotopes used in internal radiation therapy are Iodine 125 or Iodine 131, Strontium 89, Phosphorous, Palladium, Cesium, Iridium or Cobalt. As the isotopes decay naturally, they emit radiations which target the nearby cancer cells. SYSTEMIC RADIATION THERAPY

In this type of therapy a radioactive substance is swallowed or injected into the body and travels through the  blood/circulatory system, locating and destroying the cancer cells. Systemic radiation therapy is used in the treatment of some types of thyroid cancer. The radioactive isotope, Iodine 131 is used. This is because thyroid cells naturally take up this radioactive isotope. For some types of cancer treatment the radioactive substance is  joined to an antibody. The antibody travels through the blood and recognises the target cells; it binds to the  proteins of these cells exposing them to the radioactive substance. ALPHA PARTICLES IN SMOKE DETECTORS

Smoke detectors make use of the ionising properties of alpha particles. They contain an ionisation chamber  which consists of a positive and negative electrode along with a very small amount of the radioisotope Americium-241. Amercium-241 has a half life of 432 years and is a good source of alpha particles. The long half life is useful as it ensures a continuous source of alpha particles meaning the detector is very reliable and will not stop functioning. It also means the alarm does not require regular replacement; however the battery does require regular replacing and this is indicated by a warning beep or light on the alarm. The ionisation chamber consists of open channels allowing the air form the room to flow through it. The alpha  particles emitted from the Americium-241 collide with the oxygen and nitrogen molecules in the air causing them to ionise. To ionise means to knock off electron/electrons from an atom. Thus the alpha particles knock  off an electron from the oxygen and nitrogen molecules resulting in negatively charged electrons and   positively charged atoms. The negatively charge electrons are attracted to the positive electrode and the  positively charged atoms to the negative electrode. A very small current is generated which is detected by the electrical circuit in the smoke detector. 10

When smoke enters the ionisation chamber the alpha particles collide with the smoke particles instead of the air particles. The collisions with smoke particles does not result in ionisation so the current drops. The electric circuit registers the drop in electric current and t riggers the alarm to beep. BETA RADIATION IN THICK NESS CONTROL

In industries involving the production of materials with a specific thickness i.e. paper, plastic films, aluminium foil, steel etc beta radiations are used to measure and control the thickness. Strontium 90 is an example of a radioisotope used for this type of application. It is important to know the properties of the radioisotope when selecting its use. Although there are other radioisotopes which emit beta radiation their    properties may not be best suited for use in a manufacturing environment. Strontium 90 has a half life of  about 29 years; this means the radioactive source for the thickness controller does not need regular  replacement. This is beneficial in a manufacturing environment as it means the processing line does not need to be interrupted regularly to replace the source. It also means that the user does not have to constantly worry about disposing of a radioactive material. An additional property of Strontium 90 is that the beta particles resulting from its decay are moderate in energy and this reduces the risk of exposure to any workers in its  proximity.

It is important to k now the properties of  a radioisotope when determining its use. The main properties to consider are: The half  lif e ± A long half life in equipment which uses the radioisotope as a radiation source i.e. smoke

detectors and thickness control. This reduces the frequency of replacement and waste disposal. Short half life radioisotopes for tracer use and radiotherapy. The penetrating properties  ± In the use of thickness control, alpha rays would not be suitable as they are

stopped by a thin sheet of paper. Gamma rays are the most penetrating of the radiations and would not be suitable in applications where thickness control is up to a few millimeters as they would all pass through. Beta rays are ideally suited as they have enough energy to pass through thin sheets of metal. The energy of  the beta emission ± Radioisotopes can emit beta radiation at different energies varying from

low, medium to high energies. It is important to know the energy of the emission so that people exposed to the source can take the correct level of precautions and protection. BETA RADIATION IN LEAK DETECTION

Leaks in underground pipelines can be detected by the injection of a radioactive isotope. This is achieved by adding a small amount of a radioisotope which is a source of beta radiation to the fluid. The area above ground where a high intensity of beta radiation is detected will pin point the leak source in the pipeline. This saves time as the correct area is dug up. The use of a radioisotope that emits beta radiation is important as beta radiations have the correct penetrating   power to travel through the soil to give a measureable reading above ground. At the source of the leak the intensity of the beta radiations will be high as no energy is lost to the pipe wall. An alpha source would be of  no use as the alpha radiations would be absorbed by the soil. Gamma rays on the other hand have too much energy and would pass through the walls of the pipe and therefore be no more intense at the source of the leak. 11

It is important to use a radioisotope with a half life of a few hours or days. This is so it remains long enough for the leak to   be detected but not too long that it may  pose a safety or health risk. Sodium 24 is an example of a radioisotope used in leak  detection. It has a half life of about 15 hours and emits beta radiation along with gamma radiation.

GAMMA

RAYS IN STERILISATION

The process by which an object is exposed to radiation is called irradiation. Gamma irradiation can be used to  preserve foods. The radioisotope Colbalt-60 is used as the source for gamma rays. The gamma rays from this source kill bacteria, mould, parasites and insects. They also inhibit germination and premature ripening in fruits and vegetables. This helps to prolong the shelf life of the food. The food itself does not become radioactive. Medical equipment is also sterilised using gamma radiation. Sterilisation by this method is more advantageous than heating in the case o f plastic equipment such as syringes as it avo ids heat damage. Radiocarbon dating

Radiocarbon or Carbon-14 dating is a technique used by scientist to date bones, wood, paper and cloth. Carbon-14 is a radioisotope of Carbon. It is produced in the Earth¶s upper atmosphere when Nitrogen-14 is   broken down to form the unstable Carbon-14 by the action of cosmic rays. The unstable Carbon-14 is transported down to the lower atmosphere by atmospheric activity such as storms. Carbon-14 reacts identically to Carbon-12 and is rapidly oxidised to form (Carbon-14)Dioxide. Since all living organisms on Earth are made up of organic molecules that contain Carbon atoms derived from the atmosphere, they therefore contain Carbon-14 atoms. The Carbon-14 within a living organism is continually decaying, but as the organism is continuously absorbing Carbon-14 throughout its life the ratio of Carbon-14 to Carbon-12 atoms in the organism is the same as the ratio in the atmosphere. Once an organism dies it stops taking in Carbon in any form. The unstable Carbon-14 within the organism begins to decay to form Nitrogen14 by emitting a beta particle. Over time there is a gradual decrease in the amount of Carbon-14 and the ratio of Carbon-14 atoms to other Carbon atoms declines.

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The half life for  Carbon-14 is 5730 years.

Therefore

half of the Carbon14

has

decayed

after 5730 years. Half

of

the

remaining Carbon14

then

decays

over the next 5730 years leaving one fourth original

of

the

amount.

By measuring the ratio of Carbon-14 in a sample and comparing it to the amount recently

in

a

deceased

sample its date can  be determined.

WHAT IS NUCLEAR FISSION? Nuclear

Fission is the process in which the nucleus of a heavy atom, such as Uranium-235 or  Plotonium-

236, is split into two smaller or daughter nuclei and three neutrons along with the release of energy when

 bombarded by a slow moving neutron.  Nuclear fission releases a tremendous amount of energy. The mass of products from the fission reaction are slightly less than the mass of the parent nucleus. The mass that is lost in the reaction has been converted to energy. The amount of energy released from this mass can be ca lculated using Einstein's equation below: 2

Energy Released = mass x [velocity of light] (Joules)

(kilograms)

(meters per second)

or  2

E = mc

The energy resulting from the mass loss is in the form of kinetic energy of the daughter nuclei and neutron and electromagnetic radiation in the form of gamma rays.

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A slow moving neutron bombards a heavy Uranium-235 nucleus. The neutron is absorbed by the nucleus to  produce a Uranium-236 nucleus. This neutron rich nucleus is unstable and beg ins to oscillate. The unstable energetic nucleus d istorts and the repulsion between the protons begins to separate the nucleus. The repulsion between the protons becomes strong enough to separate the nucleus into two separate nuclei along with the release of a large amount of energy. Two daughter nuclei are formed from the fission and possess a large amount of the energy from the nuclear  reaction in the form of kinetic energy. These are accompanied with three neutrons.

CHAIN REACTION A heavy nucleus undergoing fission splits into two smaller nuclei and 2 or 3 neutrons with the release of  energy. The neutrons released in the fission can go on to produce fission in three other Uranium-235 nuclei. These Uranium nuclei split to produce further neutrons which in turn trigger fission in further Uranium-235 nuclei and so on. This is referred to as a chain reaction.

Chain reactions are made use of in nuclear reactors and atomic bombs. In an atomic bomb Uranium is used above a critical size to get an uncontrolled chain reaction by ensuring all the available fissile material undergoes fission in the minimum time possible. In nuclear reactors the concentration of Uranium-235 is much less and the chain reaction is controlled in order to reduce it and more importantly stop it if required.

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 A

NUCLE AR RE ACTOR

The energy released from nuclear fission is much greater than the chemical energy released when burning fuels. 1 kg of Uranium-235 gives the same amount of energy through nuclear fission as is released by burning about 3 million tonnes of coal. The energy from nuclear fission can be used to generate electricity in a nuclear   power plant. In order to safely harness the energy from nuclear fission the chain reaction needs to be controlled. This is achieved in a nuclear reactor. The nuclear fuel used in the reactor contains a small concentration of fissionable atoms, typically 3 to 4%. This low concentration reduces the risk of a runway reaction. However, in order to establish a chain reaction with this low concentration one neutron from each fission event must go on to cause fission in another atom. This is achieved in the reactor with the use of a moderator. A moderator is a material used in a nuclear reactor to slow down the neutrons produced from fission. By slowing the neutrons down the probability of a neutron interacting with Uranium-235 nuclei is greatly increased thereby maintaining the chain reaction. Moderators are made from materials with light nuclei which do not absorb the neutrons but rather slow them down by a series of collisions. Carbon in the form of graphite is a material used for moderators as is heavy water which is Deuterium an isotope of Hydrogen with an atomic mass of 2  bonded to Oxygen. The moderator only slows neutrons down in order to increase the interaction with Uranium nuclei. They do not give any protection if the reaction goes out of control. If a chain reaction is heading out of control the reactors needs to be able to reduce the concentration of neutrons. For this the reactor uses control rods. Control rods are made from material with the ability to absorb neutrons; Cadmium and Boron are examples of  suitable materials. By inserting control rods between the fuel rods the chain reaction can be slowed down or  shut down. Withdrawing the control rods can restart or speed up the react ion. A large mass of moderator encases the nuclear fuel rods slowing down the neutrons to increase their  interactions with the fissionable atoms. If the chain reaction shows signs of being out of control the control rods are inserted to absorb neutrons thereby slowing down the reaction. They can then be withdrawn to allow the reaction to proceed in a controlled manner.

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A NUCLEAR POWER STATION

The Uranium fuel rods are inserted into a large mass of moderator and are arrange in the reactor core to form the fuel assembly.

The moderator slows the neutrons down so they interact more strongly with the Uranium-235 nuclei. The fission reaction is regulated using cadmium control rods which absorb neut rons. The reactor core is housed in a reactor vessel constructed from steel 20 ± 25 cm thick.

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Coolant (either water or carbon dioxide at high pressures to prevent boiling) flow through the reacto r core. Heat generated by the fission reaction is transferred to the coolant and removed from the co re. The coolant then flows through a boiler where it transfers the heat from the reactor to water thereby ge nerating steam.

The high pressure steam produced at the boiler is forced through the steam turbine. The steam turns the turbine blade which drives the turbine shaft. The generator is housed around t he turbine shaft. A powerful electromagnet mounted on the t urbine shaft produces electricity in the generator windings as it rotates.

The generated electricity goes to a transformer where the voltage is increased and transmitted along pylons.

The steam from the turbines is cooled in a condenser. Here it passes over a maze of pipes containing cold water sourced from a nearby water supply such as a river. The steam condenses to water and recycled back to 17

the steam generator. The water in pipes sourced from the river is no much warmer after gaining heat from the steam and is cooled in a cooling tower. Radioactive Waste

Radioactive wastes are generated by different kinds of facilities. The major contributor is the nuclear power  industry but other users such as the defence industry, hospitals, manufacturing industries and educational facilities produce radioactive wastes in a variety of p hysical and chemical forms. With the associated health and environmental risks associated with radioactivity, it is imperative that the radioactive waste is disposed off safely and responsibly. The methods employed in disposal of radioactive waste depend on several factors some of which are listed below: y

The activity and concentration

y

The half life

y

The chemical properties i.e. associated chemical h azards, reactivity and combustibility.

y

The physical properties i.e. state (solid, liquid or gaseous), size and weight and solubility.

For example Plutonium-239 undergoes alpha decay. Alpha particles are relatively simple to provide   protection against as their high ionisation means they cannot penetrate more than a few centimetres of air. However, Plutonium-239 has a half life of 24000 years and it is extremely toxic and corrosive therefore any disposal system for this radioactive isotope needs to be very secure for many thousands of years. Radioactive waste has been categorised by a classification system. The categories are as follows: High Level Waste (HLW)

This is radioactive waste produced by the chemical reprocessing of nuclear fuel. It contains mainly fission  products and other heavy nuclei that are generated in the reactor core. Besides being radioactive it is thermally hot. Disposal methods for high level waste include solidifying the waste in a glass matrix and sealing it in a corrosion resistant steel lined drum. The drums are stored in specially engineered cooling pools or storage vaults. Intermediate Level Waste (ILW)

This is radioactive waste with a radioactive content that requires shielding but requires no cooling as it is not thermally hot. Intermediate level waste includes the old components from a nuclear reactor, chemical residues and other support structures form a nuclear reactor core. Intermediate level wastes are mixed with cement and solidified in stainless steel drums which are stored in special facilities above ground or in shaft or trenches underground. Low Level Waste (LLW)

This is radioactive waste with a low level of radioactivity requiring no shielding. Examples of low level waste include discarded protective clothing, packaging material and medical equipment such as syringes and needles. Other than nuclear power stations this waste is generated from hospitals and other industries. This

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type of waste is short lived and is stored on sites with special licences until it has decayed and is then disposed off as ordinary waste.

The table below summarises the categories of radioactive waste.

Radioactive waste category

Examples

High

Fission products and other heavy Solidifying the waste in a glass matrix and sealing

Level

Disposal method

Waste

nuclei that are generated in the it in a corrosion resistant steel lined drum. Storing

(HLW)

reactor core.

the drums in specially engineered cooling pools or  storage vaults.

Intermediate

Old

Level

reactors, chemical residues and other  drums which are stored in special facilities above

(ILW)

Waste

components

from

nuclear  Mixed with cement and solidified in stainless steel

support structures form a nuclear  ground or in shaft or trenches underground. reactor core.

Low

Level Discarded

protective material

clothing, and

Stored on sites with special licences until it

Waste

  packaging

medical radioactive decay complete and then disposed off as

(LLW)

equipment such as syringes and ordinary waste usually in landfills. needles.

WHAT IS NUCLEAR FUSION?

Nuclear

f usion is the process in which the nuclei of light elements combine, or fuse together, to give heavier 

nuclei. An example of a fusion reaction is that of two deuterium nuclei fusing together to give a helium nucleus. Deuterium is an isotope of hydrogen. The reaction is as follows:

Fusion reactions are accompanied by a much greater mass to energy conversion than in fission reactions.  Nuclear fusion requires extremely high temperatures. This is because the small nuclei require enough kinetic energy to overcome their electrostatic repulsion. The energy we receive from the sun is from nuclear fusion.

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The sun is made up mainly of hydrogen and helium. Within the sun the temperature is millions of degrees Celsius, there is the constant fusion of small nuclei into larger nuclei. Both nuclei contain 1 proton and 1 neutron. They carry a positive charge and repel each other. An

increase in temperature increases the k inetic energy of  the nuclei and they move around at increasing speeds. At very high temperatures (millions of  degrees Celsius) the k inetic energies of the nuclei overcome their

forces of  repulsion and collide. The collision results in the two Deuterium nuclei f using to form a Helium nucleus along with a tremendous amount of  energy.

Back ground Radiation

Humans are exposed to radiation all the time. This radiation is called background radiation. Most of the   background radiation the body is exposed to is from natural sources and is called natural back ground radiation. A small amount of the background radiation is from artificial sources and is called artificial back ground radiation. Natural

back ground radiation

The table below lists the main sources o f natural background radiation. Cosmic radiation Cosmic rays originating from the sun, stars and other major events in outer space are continuously striking the Earth. The majority of these cosmic rays are absorbed by the Earth atmosphere but the more energetic radiations interact with the atoms in the atmosphere creating energetic neutrons. Other energetic rays such as gamma rays can make it through the atmosphere to the Earth's surface.

Radioactive rocks The Earth contains radioactive rocks. Since the Earth was formed 4   billion years ago many of the short-lived radioactive rocks have decayed. However radioisotopes with long half lives such as Uranium235, Uranium-238 and Thorium-232 still remain along with the radioisotopes formed from the decay of these long lived isotopes. During the radioactive decay of Uranium Radon gas is produced which seeps through rocks underground and introduced into the atmosphere. Radon gas is radioactive and accounts for a large proportion of the natural background radiation. 20

Human body and food Small traces of radioactive materials are present in the human body. These come from natural radioactive sources such as Carbon-14 in the air we breathe. Fruits and crops take in radioactive materials from the soil as well as Carbon-14 from the air. These are introduced into the  body when they are eaten or via the food chain when meat or milk from animals grazing on the crops are consumed.

Artificial back ground radiation

This is background radiation resulting from human activity through the creation and use of artificial sources of radiation. The table below lists the main sources o f artificial background radiation. Medical sources Humans are exposed to radiations by medical procedures such as x-rays and radiotherapy.

 Nuclear power stations Major incidents from nuclear power stations have released radiations into the environment. Nuclear waste from power station also accounts for a small proportion of artificial background radiation.

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 Nuclear weapons  Nuclear weapon testing in the 1950s and 1960s resulted in an increase of  radiation in the environment.

The pie chart below shows the average contribution from the various sources to background radiation.

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