CDS (I) 2023 General Knowledge · Question 46 of 120

Correct — A, nuclear fusion and fission. Read the pairing in the order the stem sets it: the hydrogen bomb first, the uranium bomb second. A hydrogen bomb releases energy by fusing light nuclei — isotopes of hydrogen, deuterium and tritium — into helium, and because that reaction needs temperatures of millions of degrees to begin, it is called thermonuclear and is set off by a fission device acting as its trigger. A uranium bomb works the other way. A heavy nucleus, uranium-235, is split by a neutron into lighter fragments, releasing more neutrons that split further nuclei in a chain reaction. Both processes release energy for the same underlying reason: the products are more tightly bound than the reactants, and the missing mass appears as energy through the mass-energy relation. Binding energy per nucleon peaks around iron, so the way to release energy is to move towards iron from either side — fusing nuclei lighter than iron, or splitting nuclei heavier than it. The two remaining options can be dismissed on their wording alone, since 'geothermal' describes heat from inside the Earth and has no place in nuclear physics.

• (b)fission and thermonuclear fusion — The right two processes in the wrong order — this is the option the paper is really testing against. It would make the hydrogen bomb a fission device and the uranium bomb a fusion device, which is the reverse of both.
• (c)geothermal fission and fusion — Geothermal energy is heat drawn from the Earth's interior and has nothing to do with either bomb. The order of fission and fusion is wrong as well.
• (d)geothermal fusion and fission — The order of the two nuclear processes is right, but the word 'geothermal' makes the description false. Fusion in a hydrogen bomb is thermonuclear, not geothermal.

Nuclear fission splits a heavy nucleus such as uranium-235 or plutonium-239 into lighter fragments, releasing neutrons that can sustain a chain reaction; it is the process used in nuclear reactors and in the first atomic bombs. Nuclear fusion joins light nuclei, typically isotopes of hydrogen, into a heavier one, and requires enormous temperature and pressure to overcome the electrical repulsion between the nuclei. Fusion powers the Sun and the stars, and it is the principal energy source of a hydrogen bomb.

The device the question uses is a respectively pairing, which means the answer can be lost purely by reading the options in the wrong order. Fix the anchor first — a hydrogen bomb fuses hydrogen, which is in its name — and then check that the option puts fusion first. Beyond the exam, the contrast between the two processes is worth holding. Fission is controllable and already generates electricity, but leaves long-lived radioactive waste. Fusion produces far more energy per unit mass and much less long-lived waste, but no one has yet built a reactor that sustains it for net power, which is the point of the international ITER project that India is a member of.

• A hydrogen bomb releases energy by fusing isotopes of hydrogen into helium, and is triggered by a fission device.
• A uranium bomb releases energy by fission of uranium-235 in a neutron-driven chain reaction.
• Both processes release energy because the products have a higher binding energy per nucleon than the reactants; the curve peaks near iron.
• Fusion is the energy source of the Sun and the stars.
• India is a member of the International Thermonuclear Experimental Reactor project, which aims to show that controlled fusion can produce power.

• Reversing the order in a 'respectively' question, which is the single most common way to lose this mark.
• Assuming fusion must be easier than fission because the nuclei are lighter; fusion needs extreme temperature to overcome electrical repulsion.
• Treating geothermal energy as a nuclear process; it is heat conducted from the Earth's interior.

As a respectively pairing, as a which-process-powers-the-Sun item, or as a statements item on the differences between fission and fusion.

• practice — not a real PYQ

  The energy radiated by the Sun is produced by which one of the following processes?

  • (a)Nuclear fission of uranium
  • (b)Nuclear fusion of hydrogen into helium
  • (c)Chemical combustion of hydrogen
  • (d)Radioactive decay of heavy elements

  Answer(b) Nuclear fusion of hydrogen into helium — the same process a hydrogen bomb uses, sustained in the Sun by its enormous core temperature and pressure.

• practice — not a real PYQ

  In a nuclear reactor, control rods serve which one of the following purposes?

  • (a)They slow down the neutrons
  • (b)They absorb excess neutrons to regulate the chain reaction
  • (c)They supply additional fuel
  • (d)They carry heat away from the core

  Answer(b) They absorb excess neutrons to regulate the chain reaction — slowing neutrons is the moderator's job and carrying heat away is the coolant's.