The moderator used in a nuclear reactor is:
- (a)U2O
- (b)D2O
- (c)Na2O
- (d)Cl2O
Correct — B, D2O.
Fission of uranium-235 throws out fast neutrons, but a U-235 nucleus is far likelier to split when the neutron arriving at it is slow. A reactor therefore needs a substance that drags those neutrons down to thermal speeds without swallowing them.
D2O — heavy water, in which the hydrogen is the isotope deuterium — does exactly that. Its nuclei are light, so each elastic collision strips away a large share of a neutron's energy, and deuterium takes up neutrons only very weakly.
The idea to carry away: a moderator slows neutrons down; it does not soak them up. Soaking them up is the control rod's job, and a material good at one is bad at the other.
Because so few neutrons are lost inside heavy water, a reactor moderated by it can sustain the chain reaction on natural uranium. India's pressurised heavy water reactors are built on that choice.
- (a)U2O — Start with the formula. There is no compound U2O: uranium's oxides are UO2, U3O8 and UO3, and UO2 — uranium dioxide — is what fuel pellets are made of.
Uranium is certainly in the core, but on the other side of the transaction. It is the fuel — the material whose nuclei split and release the neutrons that everything else in the reactor then manages.
A uranium nucleus is also among the heaviest known, and a heavy target barely recoils when a neutron strikes it, so uranium moderates poorly.
- (c)Na2O — Sodium does have a reactor job worth knowing, and it is the coolant of a fast breeder reactor — the prototype fast breeder at Kalpakkam is sodium-cooled. That coolant is liquid sodium metal, not sodium oxide.
Cooling and moderating are different tasks. A fast breeder is designed to keep its neutrons fast, which is precisely why strongly moderating materials are kept out of its core.
- (d)Cl2O — Dichlorine monoxide is a chlorine oxide that reacts with water to give hypochlorous acid, so it belongs to the chemistry of hypochlorites and bleaching agents rather than to a reactor core.
On the physics, chlorine nuclei are heavy beside deuterium and chlorine captures thermal neutrons fairly readily. Removing neutrons from the chain is the opposite of what moderation is meant to achieve.
A fission chain reaction in uranium-235 depends on neutrons striking further U-235 nuclei and splitting them. The neutrons that fission releases come out fast, and a fast neutron is much likelier to pass a U-235 nucleus by than to split it.
The moderator solves this. Neutrons bounce off its nuclei in elastic collisions and shed energy at each bounce, until they reach thermal speeds, where their chance of causing fission rises sharply.
Two properties make a material good at the job. Light nuclei, because a light target recoils and carries off more of the neutron's energy per collision. And weak neutron absorption, so the neutrons being slowed survive to do their work.
This is the hinge of India's civil nuclear programme. Heavy water lets the first stage run on natural uranium — a route that needs no uranium enrichment at all.
That choice shapes the sequence that follows. The heavy water reactors burn natural uranium and breed plutonium, the second stage puts that plutonium into fast reactors, and the third stage aims at thorium.
The question also sits where physics meets general science: isotopes, neutron energy, and the division of labour among reactor materials all meet in the single word 'moderator'.
- Heavy water is deuterium oxide, D2O; deuterium is the hydrogen isotope whose nucleus carries one proton and one neutron.
- A moderator slows fast fission neutrons to thermal energies by elastic collision, raising their chance of causing further fission in uranium-235.
- A good moderator combines light nuclei with weak neutron absorption; deuterium absorbs neutrons far more weakly than ordinary hydrogen does.
- Graphite and ordinary light water are also used as moderators; light water reactors are fuelled with enriched uranium.
- India's pressurised heavy water reactors use heavy water as both moderator and coolant, and run on natural uranium.
- Control rods of cadmium or boron absorb neutrons to regulate or shut down the reaction — a different job from moderation.
- The Heavy Water Board, a unit of the Department of Atomic Energy, produces heavy water in India.
- Harold Urey received the 1934 Nobel Prize in Chemistry for his discovery of heavy hydrogen, deuterium.
The stem asks for the highlighted row. Heavy water appears twice because a pressurised heavy water reactor uses it as moderator and coolant at once.
- Treating 'moderator' as though it meant 'controller'. The moderator only slows neutrons; the control rod absorbs them to regulate or halt the reaction.
- Reading an option as its element and importing the element's other reactor role — sodium's reactor role is as liquid metal coolant in a fast breeder, and it is the metal, not the oxide.
- Reaching for the heaviest-sounding compound. Moderation needs light nuclei, so a heavy nucleus is a weak moderator however impressive its formula looks.
- Mixing D2O up with H2O2 or with hard water. Heavy water is deuterium oxide; neither hydrogen peroxide nor hard water is deuterium oxide, and neither has any role as a reactor moderator.
- Assuming heavy water is the only possible moderator. Graphite and light water also serve; heavy water is the answer that fits among these four options.
- Forgetting that low absorption, not just low mass, is the second condition — which is why a light but neutron-hungry material still fails as a moderator.
The one-line form names a reactor component and asks for the substance, as here. It also runs backwards — the substance is named and you supply the function, the way the UPSC 2011 item asked what heavy water does and keyed slowing the neutrons down.
Statement sets go a layer deeper, testing why a chain reaction sustains itself and which material does the slowing; the UPSC 2001 item put graphite in that role rather than heavy water.
A third form leaves physics behind and asks which Indian organisation produces heavy water or operates fast reactors, so keep the institutions attached to the concept.
UPSC_2011_GS1_Q322011The same fact turned around. UKPSC names the role and asks for the substance; this one names heavy water and asks for its function, keying the option that says it slows the speed of neutrons. What differs is the reasoning demanded — 2011 tests whether you know moderation is about neutron speed rather than cooling or stopping the reaction, while 2025 tests whether you can read heavy water off the formula D2O, set among formulas one of which, U2O, is not a compound at all.
UPSC_2001_GS1_Q1062001Same physics, deeper cut. Its statement III puts graphite in the moderating role — fast neutrons slowed down by graphite — while the rest of the item asks why a chain reaction becomes self-sustaining. What differs is both the material and the format: a multi-statement item on the mechanism of the chain reaction rather than a one-line identification of the moderator, and a reminder that heavy water is one moderator among several.
UPSC_2004_GS1_Q922004Heavy water again, but as an institutional fact rather than a physical one: one of its statements asks whether the Atomic Minerals Directorate for Research and Exploration produces heavy water. What is shared is the substance at the centre of the question; what differs is the knowledge tested — which body inside the Department of Atomic Energy does which job, against UKPSC's test of what the substance does inside the core.
GEO_GS_2023_Q592023The closest content match: it asks what heavy water is used for and keys its use as a moderator in nuclear reactors. What is identical is the fact. What differs is where the difficulty sits — here the wrong options are rival uses of heavy water (fuel, cancer therapy, water softening), so the test is of the substance's role, whereas UKPSC 2025 hides the substance behind a chemical formula and makes you read D2O correctly first.
UKPSC_2016_PRE_PaperI_Q922016Same commission and the same corner of general science. It asks which statement is not associated with nuclear fission, and its options range across neutron-induced reactions, reactors and the atom bomb. What differs is the level: 2016 tests the boundary between fission and fusion, which is where energy generation in stars sits, while 2025 assumes fission and asks about one material inside the reactor that runs on it.
- practice — not a real PYQ
Heavy water is preferred over ordinary water as a moderator mainly because
- (a)deuterium nuclei absorb neutrons far less readily than ordinary hydrogen nuclei
- (b)heavy water boils at a higher temperature, so the core stays cooler
- (c)deuterium nuclei are heavier, so they halt a neutron in a single collision
- (d)heavy water conducts electricity, carrying charge away from the core
Answera — deuterium's very weak neutron absorption is the whole point. Neutrons slowed inside heavy water survive to cause further fission, and that surviving margin is what lets natural uranium sustain a chain reaction.b fails because boiling point is a coolant consideration, and a slightly higher boiling point is not the reason heavy water is chosen to moderate.
c reverses the physics: a heavier nucleus recoils less and takes away less energy per collision, and moderation is a gradual process of many collisions, not one.
d fails because a neutron carries no charge, so electrical conduction has no part in slowing it; pure heavy water is in any case a poor conductor.
- practice — not a real PYQ
In a nuclear reactor, cadmium rods are used to
- (a)slow down fast neutrons to thermal energies
- (b)absorb neutrons and so regulate the rate of the reaction
- (c)carry heat from the core to the steam circuit
- (d)supply the fissile nuclei that undergo fission
Answerb — cadmium is a strong neutron absorber, so pushing the rods further in removes neutrons from the chain and slows or shuts down the reaction.a is the moderator's job, done by heavy water or graphite. c is the coolant's job. d is the fuel's job, done by uranium.
- practice — not a real PYQ
India's pressurised heavy water reactors can run on natural uranium because
- (a)the natural uranium is enriched to about three per cent uranium-235 before loading
- (b)the heavy water moderator absorbs so few neutrons that the chain reaction sustains itself without enrichment
- (c)graphite reflectors raise the uranium-235 content of the fuel during operation
- (d)most of the fissions in such a reactor are caused by fast neutrons
Answerb — heavy water slows neutrons while losing very few of them, so enough survive each generation to keep the reaction going even at the low uranium-235 fraction of natural uranium.a contradicts itself: uranium that has been enriched is no longer natural uranium.
c is wrong because a reflector returns escaping neutrons to the core; it cannot alter the isotopic composition of the fuel.
d is wrong because this is a thermal reactor, in which the fissions are caused by neutrons that the moderator has already slowed.
- practice — not a real PYQ
Which one of the following can serve as a moderator in a nuclear reactor?
- (a)Cadmium
- (b)Graphite
- (c)Boron
- (d)Lead
Answerb — graphite is carbon, a light and weakly absorbing solid, and it has been used as a moderator since the earliest reactors.a and c name cadmium and boron, both strong neutron absorbers used in control rods — the opposite function.
d names lead, a heavy nucleus used for radiation shielding; a heavy nucleus barely recoils, so lead takes little energy from a neutron.