Nuclear Technology MCQs for UPSC Prelims

Correct answer: A - Slow down the speed of neutrons

Heavy water (deuterium oxide) acts as a moderator in a nuclear reactor: it slows down fast neutrons to thermal speeds so they can efficiently cause further fission of uranium, which is option (a). Option (b) is the opposite of its role; it slows neutrons rather than speeding them up. Option (c) confuses the moderator with a coolant; although water can carry heat, the defining function of heavy water here is moderation. Option (d) describes the role of control rods (which absorb neutrons), not heavy water.

Correct answer: D - It can build fusion reactors for power generation

ITER, being built at Cadarache in France with India as one of the seven partners, is a tokamak project to demonstrate that nuclear fusion of hydrogen isotopes can be sustained as a net energy source. If it succeeds, member countries including India gain the technology base to build fusion reactors for electricity generation. Thorium utilisation relates to India's three-stage fission programme, not fusion; satellite navigation is the domain of ISRO's NavIC project; and ITER deals with fusion, so it cannot improve the efficiency of existing fission reactors.

Correct answer: C - Statement-I is correct but Statement-II is incorrect

Statement-I is correct: coal is the mainstay of India's power sector, generating roughly 70% or more of its electricity, even though India holds modest uranium reserves and a large thorium endowment. Statement-II is incorrect: nuclear power reactors run on low-enriched uranium, typically enriched to only about 3-5% U-235; enrichment to 60% or higher is associated with research reactors and, beyond 90%, with weapons-grade material, not routine electricity generation. Since Statement-I is correct but Statement-II is factually wrong, option (c) is the answer. Options (a) and (b) wrongly accept Statement-II as correct, and (d) wrongly rejects the true Statement-I.

Correct answer: B - 2 and 3 only

RTGs convert heat from the natural radioactive decay of an isotope into electricity, so they are not fission reactors, which makes statement 1 incorrect. They are widely used to power onboard systems of deep-space probes where sunlight is too weak for solar panels, so statement 2 is correct. They commonly use Plutonium-238, which is produced in connection with nuclear weapons and reactor programmes, so statement 3 is correct. Hence statements 2 and 3 are right, giving option (b).

Correct answer: B - Some use imported uranium and others use domestic supplies

India keeps reactors that use imported uranium under IAEA safeguards because supplier countries require assurance that the fuel is used only for civilian purposes, while reactors running on domestically sourced fuel stay outside safeguards under the 2008 civil-military separation plan. The distinction is not about uranium versus thorium, nor about foreign versus domestic operators, nor about public versus private ownership. Hence the answer is imported versus domestic uranium supply.

Correct answer: D - 1, 2 and 3

Thorium is several times more abundant in the Earth's crust than uranium, and India in particular holds among the world's largest thorium reserves in its monazite sands, so statement 1 is correct. Because almost all of mined thorium (Th-232) can be bred into fissile U-233, whereas natural uranium contains only about 0.7% fissile U-235, thorium yields far more energy per unit mass of mined mineral, so statement 2 is correct. Thorium-based fuel cycles also produce significantly smaller quantities of long-lived transuranic and other harmful radioactive waste, so statement 3 is correct. As all three advantages hold, the answer is 1, 2 and 3. (A few coaching keys dispute statement 2, but the official UPSC answer and the majority of keys treat all three as correct.)

Correct answer: A - 1 and 3 only

Statements 1 and 3 are correct. Isotopes of an element have the same atomic number (same number of protons) but different mass numbers because they differ in neutron count. Some isotopes are radioactive (radioisotopes) and are used in medical imaging (e.g., technetium-99m) and in radiotherapy for cancer (e.g., cobalt-60). Statement 2 is wrong: since isotopes have the same atomic number, they occupy the SAME position in the periodic table (the word 'isotope' literally means 'same place'). Hence only 1 and 3 are correct.

Correct answer: D - 1, 2 and 3

All three statements are correct. A breeder reactor is designed so that neutron capture in fertile material produces new fissile material faster than fissile fuel is consumed, giving a breeding ratio above one. Fertile Thorium-232 captures a neutron and, after beta decays, becomes fissile Uranium-233, the basis of the thorium fuel cycle. India's three-stage programme uses Pressurised Heavy Water Reactors in stage one, Fast Breeder Reactors using plutonium and depleted uranium (producing more plutonium and breeding U-233 from thorium) in stage two, and thorium-U233 based reactors in stage three. Hence all three statements are correct.