Q20. Which element among these is not used as nuclear fuel?
[UPPCS Main Examination (2014)]
(A) Uranium
(B) Thorium
(C) Calcium
(D) Plutonium
Answer: (C) Calcium
Explanation: Uranium and plutonium have important fission-related isotopes, while thorium can be converted into fissile uranium-233. Calcium lacks a practical role as a nuclear fuel and is not used for sustaining power-producing fission reactions.
Q19. Which element is the stable end product of major natural uranium radioactive decay chains?
[Chhattisgarh PCS Preliminary Examination (2013)]
(A) Lead
(B) Radium
(C) Thorium
(D) Plutonium
Answer: (A) Lead
Explanation: Uranium-238 undergoes a sequence of alpha and beta decays leading ultimately to stable lead-206. Uranium-235 similarly decays through a series ending in lead-207. Intermediates such as radium remain radioactive rather than being final stable products.
Q18. Concerning thorium fuel:
(1) It is more abundant than uranium.
(2) It may deliver more useful energy per mined mass through appropriate cycles.
(3) It can reduce some long-lived waste.
Which are correct?
[UPSC Civil Services Preliminary Examination (2012)]
(A) 1 only
(B) 2 and 3 only
(C) 1 and 3 only
(D) 1, 2 and 3
Answer: (D) 1, 2 and 3
Explanation: Thorium is relatively abundant and can be bred into uranium-233 in suitable reactors. The intended UPSC answer accepted all three potential advantages. Actual performance and waste characteristics depend strongly on reactor design and fuel-cycle technology.
Q17. Which uranium isotope is conventionally used as fissile fuel for nuclear-electricity production?
[Uttarakhand PCS Preliminary Examination (2012)]
(A) Uranium-233
(B) Uranium-234
(C) Uranium-235
(D) Uranium-239
Answer: (C) Uranium-235
Explanation: Uranium-235 is a naturally occurring fissile isotope capable of sustaining thermal-neutron-induced fission. It constitutes less than one percent of natural uranium. Certain reactor types enrich its proportion, while pressurised heavy-water reactors can use natural uranium.
Q16. Heavy water used within many nuclear reactors principally has what function?
[UPSC Civil Services Preliminary Examination (2011)]
(A) To reduce neutron speeds
(B) To speed up neutrons
(C) Only to cool the turbine
(D) To stop fission completely
Answer: (A) To reduce neutron speeds
Explanation: Heavy water, D₂O, can act as a neutron moderator. By slowing fast neutrons without absorbing many, it makes thermal fission more likely in appropriate fuels. It may also serve as coolant in certain reactor designs.
Q15. Which material listed is used to moderate neutrons inside a nuclear reactor?
[UPPCS Main Examination (2011)]
(A) Thorium
(B) Heavy water
(C) Radium
(D) Ordinary water
Answer: (B) Heavy water
Explanation: Heavy water is a widely used neutron moderator because deuterium transfers kinetic energy from fast neutrons while capturing relatively few. Ordinary water also moderates neutrons in many other reactor types, so the original question’s options can be ambiguous.
Q14. A hydrogen bomb releases its main energy through which reaction?
[UPPCS Preliminary Examination (2010)]
(A) Controlled fusion
(B) Uncontrolled fusion
(C) Controlled fission only
(D) Uncontrolled fission only
Answer: (B) Uncontrolled fusion
Explanation: Thermonuclear explosive energy arises from rapid fusion of light hydrogen isotopes, commonly deuterium and tritium or related fuel systems. Because the release is deliberately explosive, it is an uncontrolled reaction unlike experimental magnetic-confinement fusion.
Q13. What fluid was identified as coolant in the Fast Breeder Test Reactor at Kalpakkam?
[UPPCS Main Examination (2010)]
(A) Carbon dioxide
(B) Heavy water
(C) Seawater
(D) Liquid sodium
Answer: (D) Liquid sodium
Explanation: Liquid sodium is used as coolant in many fast breeder reactors because it transfers heat effectively without significantly slowing neutrons. The Kalpakkam Fast Breeder Test Reactor operates with a fast-neutron spectrum rather than heavy-water moderation.
Q12. What does the term “enriched uranium” mean?
[UPPCS Preliminary Examination (2009)]
(A) Uranium shaped into rods
(B) Uranium with an artificially increased proportion of uranium-235
(C) A mixture of uranium and thorium
(D) Chromium-coated uranium
Answer: (B) Uranium with an artificially increased proportion of uranium-235
Explanation: Natural uranium has only about 0.7% uranium-235. Enrichment increases the fraction of this fissile isotope by separating isotopes, permitting its use in many kinds of nuclear reactors. Enrichment is not simply coating fuel rods.
Q11. A breeder reactor is distinguished by its ability to do what?
[UPPCS Main Examination (2009)]
(A) Operate without nuclear fuel
(B) Use only heavy water
(C) Produce more fissile material than it consumes
(D) Operate entirely from solar cells
Answer: (C) Produce more fissile material than it consumes
Explanation: A breeder reactor converts fertile isotopes, such as uranium-238, into fissile products while generating energy. With suitable neutron economy, it can create more fissile material than it burns, extending available nuclear-fuel resources.
