Nuclear & Radioactivity — Set 5

Correct Answer: B. Sievert (Sv)

• **Sievert (Sv)** = The sievert is the SI unit of equivalent dose and effective dose; it accounts for both the absorbed dose (in gray) and the biological effectiveness of the radiation type via the radiation weighting factor. • **H = D × w_R** — For gamma and beta, w_R = 1 (so 1 Gy = 1 Sv); for alpha, w_R = 20 (so 1 Gy alpha = 20 Sv); background radiation gives ~2–3 mSv/year globally. • 💡 Wrong-option analysis: Becquerel (Bq): the becquerel measures activity (disintegrations per second) — it tells how radioactive a source is, not how much dose the body receives; Watt (W): the watt measures power (joules per second) in electrical or mechanical systems — it is not a radiation dose unit; Volt (V): the volt measures electric potential difference — it has no connection to biological radiation dose.

Correct Answer: A. Monitor radiation exposure of a person

• **Monitor radiation exposure of a person** = A film badge is a personal dosimeter worn by radiation workers; photographic film inside the badge is darkened by ionising radiation, and the degree of darkening is measured to determine cumulative radiation dose. • **Replaced by TLDs and OSLDs** — Modern dosimeters often use thermoluminescent dosimeters (TLD) or optically stimulated luminescence dosimeters (OSLD); regulatory dose limits in India are 20 mSv/year for occupational workers. • 💡 Wrong-option analysis: Produce alpha particles: a film badge is a passive measurement device — it produces no radiation of any kind; Increase nuclear reaction rate: this is the function of reducing control rod insertion in a reactor — a film badge has no effect on reaction rates; Measure electric current: current measurement is done with an ammeter or galvanometer — a film badge uses photographic darkening, not electrical measurement.

Correct Answer: D. A radioactive noble gas

• **A radioactive noble gas** = Radon (element 86, Rn) is a noble gas (Group 18) and is naturally radioactive; it is produced by the radioactive decay of radium-226 and is the densest naturally occurring gas. • **Rn-222, T₁/₂ = 3.82 days** — Radon seeps from soil and rocks containing uranium/radium and can accumulate in poorly ventilated buildings; it is the second leading cause of lung cancer after smoking in some countries. • 💡 Wrong-option analysis: A liquid fuel: radon is a gas at room temperature, not a liquid fuel; fuels like petrol or diesel are completely different; A stable metal: radon is neither a metal nor stable — it is a radioactive gas; noble gases have no metallic properties; A non-radioactive salt: radon does not form stable chemical compounds in ordinary conditions (noble gas), and it is definitely radioactive — not a non-radioactive salt.

Correct Answer: B. Undergo fission with slow (thermal) neutrons

• **Undergo fission with slow (thermal) neutrons** = U-235 is fissile because it has a very high fission cross-section for thermal neutrons (~585 barns); slow neutrons efficiently induce fission, sustaining a chain reaction in reactors. • **Natural abundance 0.72%** — Natural uranium contains only 0.72% U-235 and 99.27% U-238; enrichment increases the U-235 fraction to 3–5% for power reactors or >90% for weapons. • 💡 Wrong-option analysis: Only emit alpha particles: U-235 does decay by alpha emission naturally, but that is not why it is called fissile — fissility refers to the ability to fission with thermal neutrons; Never absorb neutrons: U-235 actively absorbs neutrons — that is the very mechanism that initiates fission; Only emit gamma rays: gamma emission alone would not constitute fissility; gamma emission accompanies but does not define the fission process.

Correct Answer: A. Uranium-238

• **Uranium-238** = U-238 captures a neutron to form U-239, which undergoes two successive beta-minus decays (via Np-239) to produce Pu-239: U-238 + n → U-239 → Np-239 → Pu-239. • **U-238 abundance 99.27%** — U-238 is the dominant isotope in natural uranium; breeder reactors exploit this to convert fertile U-238 into fissile Pu-239, effectively creating more fuel than they consume. • 💡 Wrong-option analysis: Carbon-12: carbon-12 is used as a moderator (in graphite form) but its neutron capture does not produce plutonium — it converts to C-13; Carbon-12 is far too light; Uranium-235: U-235 is the fissile fuel that undergoes fission; neutron capture in U-235 produces U-236, not Pu-239; Helium-4: helium-4 (alpha particle) is an extremely light noble gas nuclide — neutron capture in He-4 cannot produce a heavy element like plutonium.

Correct Answer: A. D2O

• **D₂O** = Heavy water (deuterium oxide) has the formula D₂O, where each hydrogen atom is replaced by deuterium (²H), which has an extra neutron compared to ordinary hydrogen. • **D₂O density = 1.105 g/cm³** — Heavy water moderates neutrons effectively while absorbing fewer neutrons than ordinary water, making it efficient enough to use natural (unenriched) uranium as fuel, as in CANDU reactors. • 💡 Wrong-option analysis: CO₂: carbon dioxide is used as a coolant in gas-cooled reactors (Magnox type), not as a heavy water substitute for moderation; NaCl: sodium chloride is common table salt — it is not used in reactor moderation and would be chemically reactive with reactor materials; H₂O₂: hydrogen peroxide is a common disinfectant — it is not heavy water and is not used as a reactor moderator.

Correct Answer: B. Produce more fissile fuel than it consumes

• **Produce more fissile fuel than it consumes** = A breeder reactor converts fertile material (U-238 or Th-232) into fissile material (Pu-239 or U-233) at a rate faster than the fissile fuel is consumed, yielding a breeding ratio greater than 1. • **Breeding ratio > 1** — The fast breeder reactor (FBR) uses fast neutrons without a moderator; India's PFBR at Kalpakkam is designed to breed Pu-239 from U-238 as part of its three-stage nuclear programme. • 💡 Wrong-option analysis: Use only gamma rays: reactors use neutron-induced fission, not gamma rays, as the primary driver of the chain reaction; Convert coal into uranium: coal is a fossil fuel with no nuclear conversion pathway to uranium — this is physically impossible; Stop all nuclear reactions: stopping all nuclear reactions describes a reactor shut-down, not the design purpose of a breeder reactor.

Correct Answer: C. Sustain a self-propagating chain reaction

• **Sustain a self-propagating chain reaction** = The critical mass is the minimum mass of a fissile material in a given geometry and purity at which neutron losses to the surface are just balanced by neutron production from fission, giving k = 1. • **Critical mass of pure U-235 ≈ 52 kg (bare sphere)** — With a neutron reflector the critical mass is much reduced; the critical mass depends on geometry, enrichment, density, and surrounding materials. • 💡 Wrong-option analysis: Produce chemical energy: chemical reactions involve electron interactions, not neutron-induced fission — the concept of critical mass is nuclear, not chemical; Reduce temperature to zero: reducing temperature to absolute zero is a thermodynamic concept (Third Law) with no connection to nuclear chain reactions; Emit only alpha particles: emitting alpha particles is natural radioactive decay; critical mass refers specifically to sustaining a neutron chain reaction, not alpha emission.

Correct Answer: A. Net energy released or absorbed in the reaction

• **Net energy released or absorbed in the reaction** = The Q-value is defined as Q = (sum of reactant masses − sum of product masses) × c²; positive Q means energy is released (exothermic); negative Q means energy must be supplied (endothermic). • **Q = (M_reactants − M_products) c²** — For fission of U-235, Q ≈ +200 MeV; for D-T fusion, Q ≈ +17.6 MeV; these are millions of times larger than chemical reaction Q-values (~eV range). • 💡 Wrong-option analysis: Color of the radioactive sample: sample color depends on chemistry and crystal structure — it has no relation to nuclear energy balance; Number of protons only: the number of protons is the atomic number Z, not a measure of reaction energy; Time period of a pendulum: the pendulum period depends on length and gravity — a completely unrelated classical mechanics concept.

Correct Answer: D. Beta plus (β+) decay

• **Beta plus (β+) decay** = In beta-plus decay, a proton inside the nucleus converts to a neutron, emitting a positron (e⁺, the antiparticle of the electron) and a neutrino; the daughter nucleus has Z−1 and the same mass number A. • **p → n + e⁺ + νₑ** — The emitted positron quickly annihilates with a nearby electron to produce two 0.511 MeV gamma photons — the basis of PET (positron emission tomography) scanning. • 💡 Wrong-option analysis: Alpha decay: alpha decay involves emission of a helium-4 nucleus (charge +2), not a positron; Alpha decay reduces both Z and A; Neutron emission: neutron emission is a separate nuclear process (e.g., from excited nuclei) — it is not the same as positron emission; Gamma decay: gamma decay (isomeric transition) involves emission of a photon with no change in Z or A — not the same as positron emission.