Advanced Physics Topics like quantum mechanics and relativity have revolutionized our understanding of the universe.
How does Nuclear Fusion take place?
The word ‘fusion’ means ‘to join’ or ‘to combine’. The process in which two nuclei of light elements (like that of hydrogen) combine to form a heavy nucleus (like that of helium), is called nuclear fusion. A tremendous amount of energy is produced during the fusion process. We know that the nuclei of atoms are positively charged.
So, when two nuclei are brought together, they repel each other due to their similar charges. Due to this a lot of initial heat energy and high pressure are required to force the lighter nuclei to fuse together to form a bigger nucleus.
So, the conditions needed for carrying out nuclear fusion process are ‘millions of degrees of temperature’ and ‘millions of pascals of pressure’. In other words, nuclear fusion is carried out by heating the lighter atoms’ to extremely high temperatures under extremely high pressure. There is some loss of mass during the fusion process which appears as a tremendous amount of energy. Here is an example of nuclear fusion reaction.
When deuterium atoms (heavy hydrogen atoms of mass number 2) are heated to an extremely high temperature under extremely high pressure, then two deuterium nuclei combine together to form a heavy nucleus of helium, and a neutron is emitted. A tremendous amount of energy is liberated in this fusion reaction. This fusion reaction can be written as :
A fusion process is just the opposite of fission process. The energy produced in nuclear fusion reaction is, however, much more than that produced in a nuclear fission reaction. The energy produced during nuclear fusion has not been controlled so far.
So, nuclear fusion energy could not be used for generating electricity so far. Please note that the nuclear reactions which occur at extremely high temperatures are called thermonuclear reactions. Before we describe a hydrogen bomb, please note that the hydrogen isotope of mass number 3 is called tritium.
Hydrogen Bomb
Thermonuclear reactions (fusion reactions which occur at very high temperatures) are used for producing a weapon of mass destruction called hydrogen bomb. The hydrogen bomb consists of heavy isotopes of hydrogen called deuterium (2H)and tritium (3H) alongwith an element lithium-6 (6Li).
The detonation (or explosion) of hydrogen bomb is done by using an atom bomb (based on the fission of uranium-235 or plutonium-239). When the atom bomb is exploded, then its fission reaction produces a lot of heat. This heat raises the temperature of deuterium and tritium to 107oC in a few microseconds.
At this temperature, fusion reactions of deuterium and tritium take place producing a tremendous amount of energy. This explodes the hydrogen bomb releasing an enormous amount of energy in a very short time (see Figure 39). This energy causes destruction of life and property.
The function of lithium-6 used in hydrogen bomb is to produce more tritium needed for fusion. This is because when lithium is hit by neutrous (produced during fusion reactions), it forms tritium and helium. From the above discussion we conclude that a hydrogen bomb is based on the thermonuclear fusion reactions of heavy hydrogen atoms (like deuterium and tritium) to produce helium atoms.
The hydrogen bomb (based on nuclear fusion) is exploded by using an atom bomb (based on nuclear fission). A hydrogen bomb is actually an uncontrolled nuclear fusion process. Thus, the source of energy of a hydrogen bomb is the same as that of the sun’s energy, the only difference being that sun’s energy supports life on earth, whereas the energy of hydrogen bomb destroys life on earth ! (because it is much more near to us than the sun). Please note that a hydrogen bomb is much more powerful than an atom bomb.
The Source of Sun’s Energy
The sun is a huge mass of hydrogen gas and the temperature in it is extremely high. The sun may be considered a big thermonuclear furnace where hydrogen atoms are continuously being fused into helium atoms. Mass is being lost during these fusion reactions and energy is being produced.
Thus, the sun which gives us heat and light, derives its energy from the fusion of hydrogen nuclei into helium nuclei, which is going on inside it, all the time.
The main nuclear fusion reaction taking place in the sun which releases a tremendous amount of energy is the fusion of 4 hydrogen atom nuclei to form a bigger nucleus of helium atom. That is :
The total energy produced by the fusion of hydrogen into helium is tremendous. All this energy is released in the form of heat and light. It is this energy which makes the sun shine and give us heat and light. Thus, nuclear fusion reactions of hydrogen are the source of sun’s energy. Please note that just like the sun, other stars also obtain their energy from the nuclear fusion reactions of hydrogen.
An advantage of nuclear fusion reactions over nuclear fission for producing electricity is that the amount of energy released in a fusion reaction is much more than that liberated in a fission reaction. Moreover, the products of a fusion reaction are not radioactive.
So, they are harmless and can be disposed of easily without causing any contamination in the environment. The biggest disadvantage of-a nuclear fusion reaction is that it has not been possible to have a controlled fusion reaction so far, and to safely use the enormous heat produced during this reaction for the production of electricity.
Advantages of Nuclear Energy
The advantages of nuclear energy are that :
- it produces a large amount of useful energy from a very small amount of a nuclear fuel (like uranium- 235).
- once the nuclear fuel (like uranium-235) is loaded into the reactor, the nuclear power plant can go on producing electricity for two to three years at a stretch. There is no need for putting in nuclear fuel again and again.
- it does not produce gases like carbon dioxide which contribute to greenhouse effect or sulphur dioxide which causes acid rain.
Disadvantages of Nuclear Energy
The disadvantages of nuclear energy are that :
(i) the waste products of nuclear fission reactions (produced at nuclear power plants) are radioactive which keep on emitting harmful nuclear radiations for thousands of years. So, it is very difficult to store or dispose of nuclear wastes safely. Improper nuclear waste storage or disposal can pollute the environment.
(ii) there is the risk of accidents in nuclear reactors (especially the old nuclear reactors). Such accidents lead to the leakage of radioactive materials which can cause serious damage to the plants, animals (including human beings) and the environment.
(iii) the high cost of installation of nuclear power plants and the limited availability of uranium fuel make the large scale use of nuclear energy prohibitive.
Let us solve one problem now.
Example Problem.
The mass numbers of four different elements A, B, C and D are 2, 35, 135 and 239 respectively. Which of them would provide the most suitable fuel for (i) nuclear fission, and (ii) nuclear fusion ?
Solution:
(i) In the process of nuclear fission, a very big atom (having a very big mass number) is used as a fuel. Here, out of the four elements A, B, C and D, the atom of element D is the biggest, having a mass number of 239. So, element D would provide the most suitable fuel for nuclear fission.
(ii) In the process of nuclear fusion, a very small atom (having a very small mass number) is used as a fuel. Here, out of the four elements A, B, C and D,’ the atom of element A is the smallest, having a mass number of 2. So, element A would provide the most suitable fuel for nuclear fusion.
Environmental Consequences
In this Chapter we have studied various sources of energy. The use of each and every source of energy disturbs the environment in one way or the other. For example, the use of fossil fuels causes air pollution and the production of hydroelectricity causes ecological imbalance.
In some cases, the actual operation of the device used for harnessing energy may be pollution-free but the making of the device itself must have caused some environmental damage.
For example, the use of a wind generator, solar cooker and solar cells for obtaining energy is pollution-free but the processes involved in making the materials for these energy devices must have damaged the environment in some way. From this we conclude that, in reality, no source of energy can be said to be pollution-free.
So, when we talk of a ‘clean fuel’, it actually means that it is a cleaner fuel than some other fuel. For example, when we say that CNG is a clean fuel, it means that CNG is a cleaner fuel than, say petrol or diesel. Similarly, we can say that hydrogen is a cleaner fuel than CNG. This will become more clear from the following example.
Example Problem.
Hydrogen has been used as a rocket fuel. Would you consider it a cleaner fuel than CNG ? Why or why not ?
Answer:
Hydrogen is a cleaner fuel than CNG. This is because the burning of hydrogen produces only water, which is totally harmless. On the other hand, burning of CNG produces carbon dioxide gas and water. This carbon dioxide can produce green house effect in the atmosphere and lead to the excessive heating of the environment in the long run.
Some of the environmental consequences of the increasing demand for energy are the following :
- The combustion of fossil fuels is producing acid rain and damaging plants (crops), soil and aquatic life.
- The burning of fossil fuels is increasing the amount of greenhouse gas carbon dioxide in the atmosphere.
- The cutting down of trees from the forest (deforestation) for obtaining fire-wood is causing soil erosion and destroying wild life.
- The construction of hydro-power plants is disturbing ecological balance.
- Nuclear power plants are increasing radioactivity in the environment.
The various factors which we should keep in mind while choosing a source of energy are :
- the ease of extracting energy from that source,
- the cost of extracting energy from the source,
- the efficiency of technology available to extract energy from that source, and
- the damage to environment which will be caused by using that source.
How Long Will Energy Resources of Earth Last
Most of the energy that we use today comes mainly from the three non-renewable energy resources of the earth : coal, petroleum and natural gas (which are called fossil fuels). We have been using these energy resources of the earth at a very rapid rate in the past.
So, the amount of coal, petroleum and natural gas left in the earth is limited. It has been estimated that the world’s known coal reserves are expected to last for another 200 years compared to around 40 years for the known petroleum oil reserves and around 60 years for the known reserves of natural gas.
So, it is high time that we start using alternative sources of energy to conserve the depleting reserves of coal, petroleum and natural gas so that they may last longer. We should also reduce energy consumption wherever possible. Some of the steps which can be taken to reduce energy consumption are as follows :
- Switch off lights, fans, TV and other such electrical appliances when not needed, to save electricity.
- Use energy efficient electrical appliances to save electricity. This can be done by using compact fluorescent lamps (CFL) and tube-lights in place of conventional filament-type electric bulbs (see Figure).
- Good quality stoves should be used to burn fuels like kerosene and LPG so as to obtain maximum heat.
- Pressure cookers should be used for cooking food to save fuel.
- Solar cookers should be used to cook food whenever possible and solar water heaters should be used to get hot water.
- The use of biogas as fuel should be encouraged in rural areas.
- Bicycles should be used for short distances to save precious fuel like petrol (which is used in cars, scooters and motorcycles).
Definition: The phenomenon in which two or more light nuclei combine to form a comparatively heavy nucleus is called nuclear fusion.
Fusion is in fact the reverse phenomenon of fission.
Example: Probability of fusion of two hydrogen nuclei is very low. A good example of nuclear fusion is the fusion between two deuterons i.e., two heavy hydrogen nuclei (1H2).
1H2 + 1H2 → 2He3 + 0n1 …… (1)
Probability of fusion of another hydrogen isotope, tritium (1H3) with deuteron is also high.
1H3 + 1H2 → 2He4 + 0n1 ….. (2)
Energy released ki nuclear fusion: Mass lost during nuclear fusion changes to energy as per mass-energy equivalence. In the equation (1):
Initial mass = total mass of 2 deuterons = 2 × 2.015 = 4.030 u
Final mass = total mass of He3 and neutron
= 3.017+1.009 = 4.026u
∴ Mass loss = 2 × 2.015 – (3.017 + 1.009) = 0.004 u
∴ Energy released = 0.004 × 931 MeV = 3.7 MeV (approx.)
Hence, energy released from 1 g of deuterium will be about 9 × 1010 J. For fusion of tritium with deuteron, the energy released per gram will be more and is about 30 × 1010 J.
Thus, energy released from comparatively easily available deuterium or tritium fusion is greater than that obtained from the fission of U-235.
In addition, in fusion reaction a greater percentage of nucleus takes part than the participant nuclei in fission reaction. Hydrogen bomb is made, based on this fusion reaction.
Conditions of nuclear fusion:
1. Light element: For bringing about fusion of two positively charged nuclei, the electrostatic force of repulsion needs to be overcome. Hydrogen-like lighter elements are convenient because of the low positive charge contained in them and thereby there is less force of repulsion.
2. High temperature: To bring about nuclear fusion, hydrogen isotopes are to be raised to a few crore degree celsius temperature. That is why fusion reaction is actually a thermo-nuclear reaction. To reach such high temperature, the most effective way is to set up an uncontrolled fission reaction. Therefore, to get nuclear energy from fusion, nuclear fission has to take place first.
Energy of the sun and the stars: In sun and other stars, the energy at the centre is produced by thermonuclear reaction. The core of stars being at a very high temperature, favours the process. According to the presently accepted theory, the thermonuclear reaction cycle in sun is completed in steps. In every cycle, primarily due to nuclear fusion of four protons, one helium nucleus and two positrons are formed.
1H1 + 1H1 + 1H1 + 1H1 → 2He4 + +1e0 + +1e0
The mass defect = mass of 4 protons – combined mass of 2He4 and 2 positrons = 4 × 1.008 – (4.003 + 2 × 0.00055) = 0.0279 u
Corresponding energy = 0.0279 × 931 = 26 MeV (approx.)
Sun has huge hydrogen store, but per year only 1 part in 1011 of hydrogen stored in sun is used. Also, the energy released due to thermonuclear reaction is about 4 × 1026 W. It is estimated that sun will continue producing energy for another 5 billion years before the total store of energy fuels is exhausted.
Uses Of Radioactive Isotopes
Medical science:
- Studying blood circulation pattern and in investigation of ailments, radioactive sodium (Na-24) and radioactive phosphorus (P-32) are used.
- Radioactive radium or strontium are used for destroying cancer cells. Presently radioactive cobalt (Co-60) is extensively used for this purpose.
- Radioactive phosphorus (P-32) is very effective in treating blood cancer and brain tumour.
- Radioactive iodine (I-131) is used in the treatment of thyroid gland.
Radioactive tracer or indicator: For various investigation purposes P-32 and Na-24 are used as tracer or indicator.
Examples: Different chemical reactions in plants and animals, the reaction of phosphorus containing manure for agriculture, detecting cracks In dams and reservoirs.
Radioactive pigments: A paint in which traces of radium and a fluorescent ZnS are mixed glows even in the darkness. This pigment is used in watch dials, electrical switches, road-sign, etc.
Radiocarbon dating: Cosmic rays bring about a nuclear reaction with atmospheric nitrogen producing some C-14 of half-life about 5600 years. C-14, in atmosphere changes to CO2 (carbon dioxide) and during the process of photosynthesis enters into plant body. In living plant and animal bodies a definite ratio is maintained between radioactive C-14 and normal C-12 . Assume this ratio is 1 : x. The quantity of radiocarbon C-14 decreases exponentially after the death of a carbon enriched sample, but quantity of C-12 remains constant.
So, at the time T, 2T, 3T, ….., the ratio of C-14 and C-12 will be \(\frac{1}{2}\) : x, \(\frac{1}{4}\) : x, \(\frac{1}{8}\) : x, respectively. Hence, by estimating this ratio in archaeological sample, the age of the sample can be estimated. Thus, radiocarbon C-14, acts as a radioactive clock.
Geological time determination: Half-life of C-14 is only 5600 years while earth and other geological specimen are more ancient. Therefore C-14 clock cannot be used for determining their age. Here uranium clock is used by noting the ratio of lead and uranium (half-life = 450 crores of years) in the sample.
Production of energy: Radioactive uranium or plutonium is used as fuel in nuclear power stations.
Numerical Examples
Example 1.
In a piece of ancient wood C-14 and C-12 are present. The ratio of C-14 and C-12 in this wood at present is \(\frac{1}{8}\) part of their ratio hi the ancient wood. Half-life of C14 is 5570 y. What is the age of the wood?
Solution:
Half-life of C14, T = 5570 y.
∴ From table, in time 3T, ratio of C14 and C12 is \(\frac{r}{8}\) i.e, \(\frac{1}{8}\)th of that ratio when T = 0
∴ Age of the piece of wood
= 3T = 3 × 5570 = 16710 y