Questions - Answers
Long Answer Questions (8 Marks)
1. Explain the principle and working of a nuclear reactor with the help of a labelled diagram.
A: Nuclear Reactor: A nuclear reactor is a device in which enormous amount of nuclear energy is produced. The energy it produces can be used for generation of electricity.
* The essential features of a nuclear reactor are
1) Fuel 2) Moderator 3) Control Rods 4) Coolant 5) Shielding.
The block diagram of a nuclear reactor is as shown
Nuclear Reactor
1) Fuel: In a nuclear reactor fissionable material like U235, U238 etc are used as fuel. The fuel is packed in aluminium cylinders. They are called fuel rods. The fuel rods are responsible for fission reaction in a reactor.
2) Moderator: The purpose of a moderator is to slow down the fast moving neutrons and thus promote a chain reaction. Heavy water or graphite are used as moderators.
3) Control rods: The purpose of control rods is to absorb the neutrons. As a result the number of neutrons available for fission reaction decreases and rate of reaction decreases. Boron or Cadmium rods are used as control rods.
4) Coolant: The purpose of coolant is to transfer the heat energy produced in a reactor to an outside agency. Generally cold water is circulated in a reactor through special steel pipes. The water will come out as steam and it is used to generate electrical power.
5) Shielding: To save the workers near the reactor from the harmful effects of radiations emitted from the reactor, thick walls of cement and lead are constructed around the reactor.
Working:
When a few U235 nuclei undergo fission fast neutrons are liberated. These neutrons pass through the surrounding graphite moderator and loose their energy to become thermal neutrons.
These thermal neutrons captured by U235 to carry out chain reaction. By proper arrangement of the control rods (raising and lowering the rods) the fission events are suitably controlled. The heat generated here is used for heating coolant and produce steam. This steam is made to rotate turbine and there by drive a generator for production of electric power.
2. Explain the source of solar and stellar energy. Explain the carbon - nitrogen cycle and proton - proton cycle occurring in stars.
A: The source of solar and stellar energy is nuclear and that the reactions involved are of fusion type, which are likely to happen at very high temperatures. For which hydrogen is the fuel.
The nuclear fusion reaction over the Sun and stars are believed to be of two types.
1) Carbon - Nitrogen cycle
2) Proton - Proton cycle.
Carbon - Nitrogen cycle
Bethe suggested that at high temperature parts of Sun and stars Carbon - Nitrogen cycle may takes place. In this cycle
3. Explain the principle and working of a nuclear reactor with the help of a labelled diagram.
A: Nuclear Reactor: The device in which enormous energy is produced by controlled chain reaction is called nuclear reactor.
Principle: The principle of nuclear reactor is the controlled chain reaction. It is made possible.
* By slowing down the fission neutrons to thermal neutrons using a moderator. Thermal neutrons can initiate fission in U - 235.
* By inserting rods of a material that can absorb neutrons (control rods) and using them to maintain the reaction rate under control such that the value of neutron multiplication factor K = 1.
Working:
1. Fuels: In nuclear reactor U235 can be used like fuel which is filled in the Aluminium cylindrical rod. These rods are main core of the nuclear reactor.
2. Moderator: The materials like D2O heavy water, salt water, graphite block can be used as nuclear reactor. The main purpose of nuclear reactor to slow down the past moving nuclear comes out from the fuel rod.
Control rod: The materials like Berylium and Cadmium rods used as control rods in nuclear reactor which controls chain reaction by absorbing the neutron emitted in the chain reaction.
Coolant: To absorb the heat which may be nuclear reactor, cool water is circulated through the water pipes surround the nuclear reactor. CO2 is also used like coolant.
Protective Shielding: Thick concrete walls near by 10 to 15 m are constructed around the nuclear reactor to prevent the radiations comes out from the nuclear reactor. Lead blocks are also used protective shielding.
Uses:
The main purpose of nuclear reactor is which produces the electricity.
In nuclear reactor radioisotopes are produced which also mainly useful different fields like medical, agricultural & industrial.
4. What is radioactivity? State that the law of radioactive decay. Show that radioactive decay is exponential in nature.
A: Natural Radioactivity: The phenomenon in which an unstable nucleus like uranium undergoes a decay is called radioactivity. Three types of radioactive decay occur in nature.
1. α - decay in which streams of helium nuclei (2He4) are emitted.
2. β - decay in which electrons or positrons are emitted.
3. γ - decay in which high energy photons are emitted.
Law of Radioactive Decay:
The rate of disintegration is directly proportional to number of nuclei present at that instant.
If N is the number of nuclei in the sample and dN undergo decay in time dt, then
Where λ is called decay constant.
ntegrating it on both sides, we get
Here, N0 is the number of radioactive nuclei at time t0 and N is the number of nuclei at time t. Putting t0 = 0 in equation (2), we get
Thus the radioactive decay is exponential in nature. i.e. the number of undecayed nuclei in a radioactive sample decreases exponentially with time.
5. Explain the source of stellar energy. Explain the carbon - nitrogen cycle, proton - proton cycle occuring in stars.
A: Source of stellar energy: The energy from the sun and other stars is called stellar energy. The source of stellar energy is nuclear fusion. Nuclear fusion may take place in two cycles.
1. Carbon - Nitrogen cycle
2. Proton - Proton cycle
1. Carbon - Nitrogen Cycle: In stars like sun which are at high temperature of the order of 106 K, carbon-nitrogen cycle takes place. In this, hydrogen is converted into helium and carbon, nitrogen act as catalysts.
It's nuclear reactions are as follows.
Proton-Proton Cycle: In stars of less temperature, proton-proton cycle takes place. The nuclear reactions of proton-proton cycle are as follows.
Thus 4 hydrogen atoms combine to from a helium atom with a release 26.7 MeV energy.
6. Define mass defect and binding energy. How does binding energy per nucleon vary with mass number? What is its significance?
A: Mass Defect: The difference between the total mass of all the nucleons of a nucleus and the actual mass of the nucleus is called mass defect (ΔM)
ΔM = [Zmp + (A - Z) mn] - M
Where Zmp = total mass of protons
(A- Z)mn = total mass of neutrons
M = Mass of nucleus.
Binding Energy: The mass defect that is occuring in the formation of nucleus is converted into energy called binding energy (ΔE) of the nucleus.
It can also be defined as the energy required to break the nucleus into its constituent protons and neutrons.
Einstein's mass-energy equivalence relation is E = mc2
Binding Energy Eb = ΔMC2
But 1 amu = 931.5 MeV
Eb = ΔM × 931.5 MeV
Binding Energy per Nucleon:
The binding energy of a nucleus (Eb) divided by its mass number (A) is called binding energy per nucleon (Ebn)
Variation of binding energy per nucleon with mass number:
The graph shown in the figure explains how the binding energy per nucleon (Ebn) varies with mass number (A).
The graph shows the following facts:
1. From A = 30 to A = 170, the binding energy per nucleon (Ebn) is constant.
Ebn: 8.75 MeV is maximum for Fe-56 and
Ebn: 7.6 MeV is very low for U-238.
2. Ebn is low for both light nuclei (A < 30) and heavy water (A > 170).
Significance:
Greater binding energy per nucleon means greater stability to the nucleus. The light nuclei of A < 30 and heavy nuclei of A > 170 are unstable because their Ebn is low. That is why properties like radioactivity, fission, fusion exist in the region only.
The mid nuclei are stable because their Ebn is high.
Short Answer Questions
1. Write a short note on the discovery of neutron.
A: Neutron: The electrically neutral particle with mass approximately equal to that of proton and present in all nuclei except that of hydrogen is called neutron.
Discovery of Neutron: when Beryllium nucleus was bombarded with α - particle, a powerful and neutral radiation came out. Bothe and Becker wrongly predicted this radiation as γ - rays. The nuclear reaction given by them is
Chadwick proved experimentally that when beryllium sheet is bombarded with alpha particles, the powerful, neutral radiation produced is of a new kind of particles called neutrons and not of γ - rays.
The nuclear reaction given by Chadwick is,
2. What are the properties of neutron?
A: Properties of Neutron:
1. Neutron is an uncharged particle.
2. Neutron has high penetrating power and low ionising power.
3. Inside the nucleus, neutrons are stable. But out side the nucleus, neutron is unstable, its average life is 1000 s.
3. What are nuclear forces? Write their properties?
A: Nuclear Forces:
The forces that hold the nucleons together wtihin the nucleus are called nuclear forces.
Properties of Nuclear Forces:
1. The nuclear force is attractive and the strongest in nature.
2. Nuclear force is a short range force.
3. The nuclear force is independent of charge.
4. Nuclear force is spin dependent.
5. Nuclear force is an exchange force.
4. Define half life period and decay constant for a radioactive substance. Deduce relation beween them?
A: Half life (T1/2) of a Radioactive Substance:
It is defined as the time required for the number of radioactive nuclei of the substance to disintegrate to half of its original number. It is denoted by T.
Decay constant: It is the ratio of the rate of radioactive decay to the number of nuclei present at that instant. It is proportional constant. It is denoted by λ.
The number of nuclei present at t = 0 is N0 and after time T and 
are left and after time 2T only remain.
T depends upon the disintegration constant λ of the radioactive substance.
5. Define average life of a radioactive substance. Obtain the relation between decay constant and average life?
A: Average life time (T):
The average life time is defined as the ratio of the total life time of all, the N0 nuclei to the number of original nuclei N0.
(or)
The total life time of all the nuclei present in a substance divided by the total number of nuclei is called average life of the radioactive substance (τ).
Relation between Average Life and decay cosntant:
Activity of a radioactive sample
This gives the number of nuclei that decay in time dt. These dN atoms had a life time between t and (t + dt). Since dt is very small interval of time. These atoms has lived for time t, the total life of all the nuclei would be
dN = tλN0 eλt dt
... The total life time of all the nuclei present in the sample is given by
This is the relation between the average life and decay constant.
6. Distinguish between nuclear fission and nuclear fusion.
A: Nuclear fission:
1. It is a process in which a heavy nucleus is splits up into two lighter nuclei of nearly equal mass.
2. Neutrons are required for it.
3. It takes places at ordinary temperature.
4. Atom bomb is constructed on this principle.
5. Energy of 200 MeV is released for each fission.
Nuclear Fusion:
1. It is a process in which two lighter nuclei combined together to form a heavy nucleus.
2. Protons are required for it.
3. It require very high temperature of the order of 107 K to 108 K.
4. Hydrogen bomb is constructed on this principle.
5. Energy of about 24.68 MeV is released per Fusion.
7. What is nucleur fission? give an exmaple to illustrate it.
A: Nucleur fission: The process of breaking a heavy nucleus like uranium-235 in two or more roughly equal fragments with the release of enormous amount of energy is called nuclear fission.
Nuclear fission was discovered by Otto Haun and Strassman.
Example:
8. What is Nuclear fusion? Write the conditions for nuclear fusion to occur.
A: Nuclear Fusion: The process in which two or more light nuclei fuge together to from a single nucleus with the release of enormous energy is called nuclear fusion.
Nuclear fusion takes place only at high temperature of the order of 106 K
e.g.: In Sun 4 hydrogen nuclei fuge together to form a helium nucleus
Very Short Answer Questions
1. What are isotopes and isobars?
A: Isotopes: The nuclei having same atomic number (Z), but different mass number (A) are called isotopes.
Isobars: The nuclei having same mass number (A), but different atomic numbers (Z) are called isobars.
2. What are isotones and isomers?
A: Isotones: The nuclei having the same neutron number (N), but different atomic numbers (Z) are called isotones.
Isomers: The nuclei having same atomic number (Z) and same mass number (A) but different nuclear properties such as radioactive decay and magnetic moments are called isomers.
Here m = meta stable and g = ground state.
3. What is amu? What is its equivalent energy?
A: Atomic mass unit:
Half of the mass of Carbon - 12 atom is called atomic mass unit (U).
1 U = 1.66 × 10-27 kg
The equivalent energy of atomic mass unit is 1U = 931.5 MeV
4. What will be the ratio of the radii of two nuclei of mass numbers A1 and A2?
A: Nuclear radius R = R1 A1/3
The ratio of nuclear radii
R1 : R2 = R0 A11/3 : R0 A21/3
(or)
R1 : R2 = A11/3 : A21/3
5. Neutrons are the best projectiles to produce nuclear reactions. Why?
A: Neutrons are electrically neutral. In the case of protons and α - particles it is difficult to collide the nuclei due to force of repulsion. But neutrons do not have that repulsion. Therefore, they collide the nuclei easily and produce nuclear reactions.
6. Neutrons cannot produce ionization. Why?
A: Ioinsation of particles in a medium, usually, takes place either by losing electrons or by gaining electrons. This is possible with charged particles only.
As neutrons are neutral, they cannot produce ionisation. Because they can neither attract nor repel electrons.
7. What are thermal neutrons? What is their improtance?
A: Thermal Neutrons: The slow neutrons which can initiate fission are called thermal neutrons.
Importance: Fast neutrons cannot initiate to controlled chain reaction of fission. Only thermal neutrons can do it. That is why, in nuclear reactor, the speed of the neutrons is reduced by the moderator to make them thermal neutrons.
8. What is the value of neutron multiplication factor in a controlled chain reaction and in an un controlled chain reaction?
A: In controlled chain reaction, the value of multiplication factor K = 1, In uncontrolled chain reaction, K > 1.
9. What is the role of controlling rods in a nuclear reactor?
A: Controlling rods: The rods used in the nuclear reactor to absorb neutrons are called controlling rods. The substances used for controlling rods are cadmium, beryllium, boron etc.
10. Define Becqueral and Curie?
A: Becqueral: If a radioactive sample disintegrates at the rate of 1 disintegration per second, its activity (or decay rate R) is called 1 becqueral (Bq).
1 Bq = 1 disintegration/second
Bq is the SI unit of activity.
Curie: If a radioactive sample disintegrates at the rate of 3.7 × 1010 disintegration per second, its activity (or decay rate R) is called 1 curie (Ci).
1 Ci = 3.7 × 1010 Bq.
11. What is chain reaction?
A: A self sustained nuclear fission reaction producing tremendous amount of energy is called chain reaction.
In chain reaction, the number of neutrons increases in geometrical progression and the nuclei undergoing fission also increase in geometric progression. Hence, enormous amount of energy is released within a very short duration.
12. What is the function of moderator in a nuclear reactor?
A: Moderator: The function of moderator is to slow down the fisson neutrons (2 MeV) to thermal neutrons (0.025 MeV).
Heavy Water, Graphite are the moderators.
13. What is the energy released in the fusion of 4 protons to form a helium nucleus?
A: 26.7 MeV
