Nuclear Power Plants Interview Questions and Answers:

1. Why are nuclear power stations becoming popular ?

Ans. The need of nuclear power plant lies in the fact that the hunger for electricity is virtually unending and after each decade the world demand for electricity is doubled owing to booming increase in the population and industrial growth. Moreover, the reserves of fossil fuels i.e., coal, oil and gas are fast depleting. Again it would be better to utilise these minerals, particularly oil, for chemical industry as raw materials. Thus there is tendency to seek alternative source of energy and the nuclear power is the only alternative source which can meet the future energy demands of the world. One of its main attractions is the huge amount of energy that can be released from a small quantity of active material. The energy obtainable by completely burning of 1 kg of uranium would give energy equivalent 3,000 tonnes of high grade coal i.e., uranium has 3 million times the energy of coal. The possible energy reserves in the form of uranium and thorium is many times greater than that of fossil fuels.

The factors which are in favour of nuclear energy are: it is practically independent of geographical factors, it has no combustion products and it is clean source of power which does not contribute to air pollution. Further it does not require fuel transportation mechanism and large storage facilities.

2. Why nuclear power plants are used as base load plants ?

Ans. The operating cost of nuclear power plants is quite low and once the installation is completed, the loading of the power plant will have no effect on the generation cost. Therefore, a nuclear power plant is always operated as a base load plant. The nuclear power plants are usually not operated at a load factor less than 0.8.

3. Name three important factors in choosing a site for atomic power plant.

Ans. Three important factors in choosing a site for atomic power plants are :

(i) availability of water supply (ii) distance from populated area and (iii) transportation facilities

4. Why the site near a sea or river or lake and away from thickly populated area is considered an ideal site for nuclear power plants ?

Ans. Keeping in view the huge requirement of cooling water and facility for disposal of nuclear waste and effluents and safety from danger of presence of radioactivity in the atmosphere near the plant the site near a sea or river or lake and away from thickly populated area is considered an ideal site for nuclear power plants.

5. Name three different types of fuels used in nuclear reactors.

Ans. Three different types of fuels used in nuclear reactor are : (i) natural uranium U-235 (ii) plutonium (iii) enriched uranium.

6. How much power is generated by a fissioning of 1 gm of U235 per day ?

Ans. 2.57/2.68 MW

7. What is meant by radioactivity ?

Ans. The disintegration or decay of unstable atoms accompanied by emission of radiation is called the radioactivity.

8. What is decay constant ?

Ans. The decay or disintegration constant may be defined as the proportion of atoms of an isotope decaying per second.

9. What is half life ?

Ans. The half life or half life period of a radioactive isotope is the time required for one half of the isotope to decay.

10. What is meant by mass defect ?

Ans. The difference between the experimental mass and calcu­lated mass of the nucleus is called the mass defect or mass deficit.

11. What is binding energy ?

Ans. The energy that binds the nucleus together in the nucleus is called the nuclear binding energy. Binding energy is the energy equivalent of the mass defect.

12. What is mass energy equivalence ?

Ans. 1 amu = 931.5 MeV; amu stands for atomic mass unit.

13. What is nuclear fission ?

Ans. The splitting of a heavy nucleus into two or more smaller nuclei is termed nuclear fission.

14. What is critical mass ?

Ans. The minimum mass of fissionable material required to sustain a chain reaction is called the critical mass.

15. Distinguish between fusion and fission.

Ans. When two or more lighter nuclei (H2 or H3) with lower binding energy per nucleon are combined or fused together into a heavier and more stable nucleus, there is a release of energy. This process of releasing energy is known as fusion.

The splitting of a heavy nucleus into two or more smaller nuclei is termed nuclear fission.

16. Distinguish between fissile and fertile materials.

Ans. The materials which are known to undergo neutron fission are U-235, U-233 and Pu-239. These are fissile materials. There are materials (238U92 and 232Th90) which occur in nature, are called fertile materials. Fissionable materials 238U92 and 233Th92 are formed in the, nuclear reactors during fission process from 238U92 and 232Th90. Th respectively due to absorption of neutrons without fission.

17. How does a nuclear power plant differ from a conventional thermal power plant ?

Ans. The concepts of nuclear power generation are much similar to that of conventional steam power generation. The difference lies only in the steam generation part i.e., coal or oil burning furnace and the boiler are replaced by nuclear reactor and heat exchanger.

18. Name the main components of nuclear power plants.

Ans. A nuclear power plant consists of a nuclear reactor (for heat generation), heat exchanger (for converting water into steam by using heat generated in the nuclear reactor), steam turbine, alternator, condenser etc.

19. What is nuclear reactor and what are its basic parts ?

Ans. Reactor is that part of nuclear power plant where nuclear fuel is subjected to nuclear fission and the energy released in the process is utilised to heat the coolant which may in turn generate steam or be used in a gas turbine.

A nuclear reactor consists of reactor core, moderator, control rods, coolant, reflector, thermal shielding, reactor vessel and biological shield.

20. In a nuclear reactor, multiplication factor is kept almost equal to one. Why ?

Ans. As mentioned earlier, all the neutrons released in the fission reaction are not used up in propagating the chain reaction but some of these are lost to the surroundings. For maintaining chain reaction, it is therefore, essential that the number of neutrons after the fission should be slightly more than the number before it to allow for the escape or leak of neutrons from the reactor core. The ratio is known as multiplication factor. The multiplication factor k for any reactor is defined as

multiplication factor

Unity value of k indicates that the chain reaction will continue at a steady rate (critical). If k is less than unity, the chain reaction will stop and the system is called subcritical. While for k exceeding unity, the chain reaction will build up and the system is called super critical.

Thus, for keeping chain reaction continuous multiplication factor in a nuclear reactor is kept almost equal to one.

21. How are reactor classified ?

Ans. Nuclear reactors may be classified in several ways i.e., on basis of their applications, type of fission, fuel used, state of fuel, fuel cycle, arrangement of fissile and fertile material, arrangement of fuel and moderator, moderator material, cooling system employed, coolant used etc.

22. How are reactors classified according to type of coolant used ?

Ans. On the basis of coolant used the reactors may be classified as (i) gas (ii) water (iii) heavy water and (iv) liquid metal reactors.

23. What is the purpose of a moderator in a nuclear power reactor ?

Ans. The purpose of a moderator in a nuclear power reactor is to slowdown the neutrons.

24. Name three moderators commonly used.

Ans. Three commonly used moderators are: (i) graphite (ii) ordinary or natural water and (iii) heavy water.

25. What is the purpose of a control rod in a nuclear power reactor ?

Ans. Control rods are meant for controlling the rate of fission of U-235.

26. Name any two materials used for control rods.

Ans. The materials used for control rods must have very high absorption capacity for neutrons. The commonly used materials for control rod are cadmium, boron or hafnium.

27. Distinguish between fast and thermal reactors.

Ans. In fast reactors, the fission is affected by fast neutrons without use of moderator but in thermal reactors, the fast neutrons are slowed with the use of moderators.

28. What is CANDU type reactor ?

Ans. CANDU reactor was first developed by Canada and is, therefore, known as CANDU type reactor. The word CANDU stands for Canadian Deuterium Uranium. These reactors make use of heavy water, composed of the heavy hydrogen isotope, 1H2, as moderator to have maximum neutron economy and as coolant also. Such reactors are meant for those countries which do not have uranium enrichment facilities. Enrichment of uranium is costly affair and such reactors use natural uranium as fuel.

29. Enumerate the main disadvantages of sodium graphite reactor.

Ans. The main disadvantages of sodium graphite reactor are (i) relatively complex core (ii) requirement of enriched fuel

(iii) requirement of triple cycle cooling system with dual heat exchangers to minimize hazards etc.

30. How is nuclear waste disposed of ?

Ans. The nuclear waste is disposed of as follows:

  1. Solid waste is stored in suitable containers, which are sunk to the bottom of seas and oceans.
  2. Liquid waste is diluted and disposed in river and sea after treatment.
  3. Gaseous waste is discharged to atmosphere at high level after filteration and treatment.