Q30. A reactor and a nuclear bomb both involve a chain reaction. Which statement correctly distinguishes them?
[UPSC CDS-II General Knowledge (2017)]
(A) Reactors have no chain reaction
(B) Reactor reactions are controlled, weapon reactions are uncontrolled
(C) Only bombs involve any criticality
(D) Reactors always use bomb-type moderators
Answer: (B) Reactor reactions are controlled, weapon reactions are uncontrolled
Explanation: A functioning fission reactor controls neutron multiplication to maintain useful steady heat output. A nuclear fission weapon produces an extremely rapid, uncontrolled energy release. Both rely on chain reactions, but their reaction rates and engineering purposes differ.
Q29. Consider:
(1) Nuclear Security Summits were convened directly under the UN.
(2) The International Panel on Fissile Materials is part of IAEA.
Which is right?
[UPSC Civil Services Preliminary Examination (2017)]
(A) 1 only
(B) 2 only
(C) Both
(D) Neither
Answer: (D) Neither
Explanation: Nuclear Security Summits were intergovernmental initiatives rather than meetings organised under the UN framework. The International Panel on Fissile Materials is an independent expert group, not an organ of the International Atomic Energy Agency.
Q28. What would be India’s principal prospective benefit if the ITER fusion experiment proves successful?
[UPSC Civil Services Preliminary Examination (2016)]
(A) Direct conversion of thorium into reactor electricity
(B) Satellite navigation capability
(C) Improved efficiency of existing fission plants alone
(D) Knowledge enabling future fusion power plants
Answer: (D) Knowledge enabling future fusion power plants
Explanation: ITER investigates magnetic confinement of very hot plasma to demonstrate large-scale fusion technology. Its success could provide knowledge and experience essential for future fusion electricity generation, rather than directly modifying the operation of conventional fission reactors.
Q27. Which claim is NOT characteristic of nuclear fission?
[Uttarakhand PCS Preliminary Examination (2016)]
(A) It can be induced by neutrons
(B) It is the main process producing energy in stars
(C) It is used in ordinary nuclear reactors
(D) It can release vast energy in fission weapons
Answer: (B) It is the main process producing energy in stars
Explanation: Stars such as the Sun produce energy mainly through fusion of light nuclei, not fission of heavy nuclei. Fission involves splitting a heavy nucleus and is harnessed in commercial reactors and fission weapons.
Q26. About proposed fusion reactors:
(i) They split heavy nuclei.
(ii) They commonly employ tokamak designs.
(iii) They require very high temperatures.
Select the correct statements.
[UPPSC RO/ARO Preliminary Examination (2016)]
(A) i and iii only
(B) i and ii only
(C) ii and iii only
(D) All three
Answer: (C) ii and iii only
Explanation: Fusion involves combining light nuclei, so statement one incorrectly describes fission. Tokamaks use magnetic fields to confine plasma, and the reacting fuel must be heated to extreme temperatures to overcome the electrostatic repulsion between nuclei.
Q25. During gamma decay of a nucleus, which nuclear quantities change?
[UPSC CDS-II General Knowledge (2016)]
(A) Mass number only
(B) Atomic number only
(C) Both change
(D) Neither atomic nor mass number changes
Answer: (D) Neither atomic nor mass number changes
Explanation: Gamma emission releases excess nuclear excitation energy as a high-energy photon. Because no proton or neutron is added or removed, both atomic number and mass number remain unchanged. The nucleus merely drops to a lower energy state.
Q24. Among China, France, India, Israel and Pakistan, which are nuclear-weapon states recognised under the NPT?
[UPSC Civil Services Preliminary Examination (2015)]
(A) China and France only
(B) China, India, Israel and Pakistan
(C) France, Israel and Pakistan
(D) All five
Answer: (A) China and France only
Explanation: The Nuclear Non-Proliferation Treaty recognises as nuclear-weapon states countries that detonated a nuclear explosive before 1 January 1967: China, France, Russia, the United Kingdom and the United States. India, Israel and Pakistan are not recognised in that category.
Q23. Graphite serves which purpose inside certain nuclear reactors?
[UPPSC Main Examination (2015)]
(A) Fissile fuel
(B) Mechanical lubricant
(C) Neutron moderator
(D) Radiation detector
Answer: (C) Neutron moderator
Explanation: Graphite can slow fast neutrons through elastic collisions without absorbing too many of them. In thermal reactors, this moderation can improve the probability that fissile nuclei capture neutrons and undergo fission, helping sustain the chain reaction.
Q22. Which substance is NOT normally treated as a nuclear fuel?
[Chhattisgarh PCS Preliminary Examination (2014)]
(A) Uranium
(B) Thorium
(C) Plutonium
(D) Lead
Answer: (D) Lead
Explanation: Uranium and plutonium contain isotopes used directly as fissile reactor fuels; thorium is fertile and can breed uranium-233. Ordinary lead is not used as a fissile nuclear fuel, although lead-based materials may serve as coolant or shielding.
Q21. Which metal was the correct choice in a question about material used in nuclear-bomb manufacture?
[MPPSC Preliminary Examination (2014)]
(A) Zirconium
(B) Uranium
(C) Molybdenum
(D) Vanadium
Answer: (B) Uranium
Explanation: The examination identified uranium, whose isotope uranium-235 is capable of sustaining a rapid fission chain reaction under suitable conditions. Zirconium is important in reactor cladding, but is not the fissile material indicated by the question.
