UPSC Prelims Practice Questions — How do tokamaks work to make nuclear fusion possible?

Q1. Consider the following statements: Which of the following is correct in respect of the above statements?

  1. A magnetic field can be used to confine a plasma, but it cannot confine an ordinary gas made up of neutral atoms in the same way.
  2. In a plasma, electrons have been separated from their atoms, so its particles carry electric charge and a magnetic field forces them to gyrate around the field lines.
  • A. Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
  • B. Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
  • C. Statement-I is correct, but Statement-II is incorrect
  • D. Statement-I is incorrect, but Statement-II is correct

Q2. Consider the following statements: Which of the following is correct in respect of the above statements?

  1. Energy is released both when a uranium-235 nucleus splits and when a deuterium nucleus fuses with a tritium nucleus.
  2. In both these reactions, the total mass of the products exceeds the total mass of the reacting nuclei, and this extra mass is the source of the energy released.
  • A. Both Statement-I and Statement-II are correct and Statement-II explains Statement-I
  • B. Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I
  • C. Statement-I is correct, but Statement-II is incorrect
  • D. Statement-I is incorrect, but Statement-II is correct

Q3. With reference to the working of a tokamak, consider the following statements: Which of the statements given above is/are correct?

  1. The confining magnetic field is helical, formed by combining a field running the long way around the ring with one running the short way around it.
  2. Once the reaction starts, the confining field is produced entirely by the plasma itself, so no external magnetic coils are needed.
  3. The chamber is kept under vacuum because stray gas atoms entering the plasma act as impurities that cool it.
  • A. 1, 2 and 3
  • B. 2 and 3 only
  • C. 1 and 3 only
  • D. 1 and 2 only

Q4. In a tokamak running on deuterium-tritium fuel, consider the following statements about the neutrons produced by the fusion reaction: Which of the statements given above is/are correct?

  1. Most of the energy released in each reaction is carried away by the neutron rather than by the helium nucleus.
  2. Because the neutron has no electric charge, the magnetic field does not hold it, and it passes out of the plasma into the surrounding structure.
  3. Neutrons absorbed by lithium placed around the plasma can produce tritium, so a reactor can breed part of its own fuel.
  • A. 1, 2 and 3
  • B. 1 and 2 only
  • C. 2 and 3 only
  • D. 1 and 3 only

Q5. With reference to achieving net energy gain from a fusion device, consider the following statements: Which of the statements given above is/are correct?

  1. Raising the plasma temperature high enough is by itself sufficient to reach net energy gain, irrespective of plasma density or how long the energy is confined.
  2. The gain factor of a fusion plasma compares the fusion power produced with the external heating power supplied; a value above 1 means more fusion power comes out than heating power goes in.
  3. A device whose plasma gain exceeds 1 may still consume more electricity than it could generate, because of losses in heating systems, magnets and the conversion of heat to electricity.
  • A. 1 and 2 only
  • B. 2 and 3 only
  • C. 1 and 3 only
  • D. 1, 2 and 3

Q6. Consider the following facts: I. Nuclear power generated today comes almost entirely from the fission of uranium. II. Decades of research have not produced a tokamak that generates net energy for commercial power. III. Several countries are jointly building a large experimental tokamak, ITER, in France. Which of the following inferences can be drawn from the above facts?

  1. Fusion does not at present make a meaningful contribution to commercial electricity supply.
  2. The tokamak design has been shown to be incapable, in principle, of achieving net energy gain.
  3. Magnetic confinement has been abandoned by the research community in favour of other routes to fusion.
  • A. 1 and 2 only
  • B. 2 and 3 only
  • C. 1 and 3 only
  • D. 1 only

Q7. Consider the following statements comparing nuclear fission and nuclear fusion: Which of the statements given above is/are NOT correct?

  1. Like fission, fusion in a tokamak is sustained by a chain reaction in which neutrons released by one reaction trigger the next.
  2. The uranium fuel used in most fission reactors is enriched because the fissile isotope uranium-235 forms only a small fraction of natural uranium.
  3. Fusion releases less energy per unit mass of fuel than fission does, which is why fusion needs far higher temperatures.
  • A. 1 and 2 only
  • B. 1 and 3 only
  • C. 2 and 3 only
  • D. 1, 2 and 3

Q8. A sealed glass tube containing neon gas at low pressure is kept near a strong magnet. At first the gas shows no response to the magnet. When a high voltage is applied across the tube, the gas begins to glow and the glowing column is visibly deflected by the magnet. Which one of the following best explains this change?

  • A. The atoms have been raised to higher energy levels while staying neutral, and such excited atoms are pulled by magnets
  • B. The voltage has fused some neon nuclei into heavier nuclei whose larger mass makes them respond to the magnet
  • C. The voltage has heated the gas and raised its density, so its neutral atoms now experience a stronger magnetic pull
  • D. Some atoms have lost electrons, producing free electrons and positive ions on which the magnet exerts a force