Question 1

Nuclear basics: pick the single WRONG statement from the following concise definitions and facts about nuclides and radiations.

Accepted Answer

Introduction / Context: This question checks precise nuclear terminology (isotopes, isobars, isotones) and key radiation facts. Many learners mix up these near-sounding terms, so identifying the single incorrect statement requires recalling exact definitions and the basic behavior of α, β, and γ radiation. Given Data / Assumptions: "Nucleons" means protons + neutrons (mass number A). Atomic number Z counts protons; neutron number N = A − Z. Radiation penetration order is a standard property in nuclear physics. Concept / Approach: Definitions to remember: isotopes (same Z, different N ⇒ different A), isobars (same A, different Z), isotones (same N, different Z). Radiation penetration commonly follows: γ > β > α (with appropriate shielding examples: paper blocks α, thin metal blocks β, dense lead/concrete needed for γ). α-emission reduces A by 4 and Z by 2, as a helium-4 nucleus is emitted. Step-by-Step Solution: Check (a): "same A, different Z" → definition of isobars. Correct.Check (b): "same Z, different A" are called isotopes, not "isoters". Hence (b) is wrong.Check (c): γ has highest penetration compared with α and β. Correct.Check (d): α-emission removes 2p+2n ⇒ A decreases by 4. Correct.(e) restates the isotope idea correctly (same element, different N). Verification / Alternative check: Periodic table relationships: isotopes share the same chemical behavior due to identical Z (electron configuration for neutral atoms), while differing A affects nuclear stability and decay modes—not the electron shell directly. Why Other Options Are Wrong: (a) Correct definition of isobars. (c) Correct trend in penetration power. (d) Correct consequence of α-decay. (e) Correct paraphrase of isotopes. Common Pitfalls: Confusing "isotones" (same N) and "isobars" (same A). Also, inventing nonstandard terms like "isoters" can mislead; the accepted term is "isotopes" for same Z. Final Answer: Atoms with the same number of protons (same atomic number Z) but different mass numbers are called isoters.

Question 2

Fuel enrichment check: the U-235 content of enriched uranium commonly used in light-water power reactors (e.g., early Tarapur BWR units) is approximately what percent?

Accepted Answer

Introduction / Context: Light-water reactors (LWRs)—which include boiling water reactors (BWRs) and pressurised water reactors (PWRs)—use uranium fuel enriched in U-235 beyond its natural abundance (~0.71%). A typical commercial enrichment band balances reactivity, fuel cycle economics, and safety margins. Given Data / Assumptions: LWR class reactors (e.g., Tarapur BWRs) historically used low-enriched uranium (LEU). LEU is defined as U-235 enrichment below 20%. Question asks for an approximate, representative value. Concept / Approach: Most LWR cores operate on fuel enriched to roughly 2–5% U-235. A commonly cited nominal value in introductory questions is about 3%. Heavy-water reactors (e.g., CANDU) can use natural uranium; fast reactors use plutonium-bearing fuel; research reactors may use higher enrichments depending on design and HEU-LEU conversion programs. Step-by-Step Solution: Identify reactor type as LWR (BWR at Tarapur).Recall typical U-235 enrichment range (2–5%).Select the closest representative value: ~3%. Verification / Alternative check: Vendor data and nuclear engineering texts align on ~3–5% for many LWR designs. Early Indian BWR units similarly operated with LEU in this range, often quoted as about 3% for pedagogy. Why Other Options Are Wrong: 85% and 97%: Highly enriched uranium (HEU), not used in power LWRs. 50%: Also HEU; not typical for power generation. 20%: Upper bound of LEU definition; not standard for LWR cores. Common Pitfalls: Confusing natural uranium (0.71%) with LEU; assuming higher enrichment always means better performance without considering neutron economy and proliferation controls. Final Answer: 3

Question 3

Pressurised Water Reactor (PWR) primary circuit: the reactor coolant in the primary loop is maintained as high-pressure __________ to prevent bulk boiling in the core.

Accepted Answer

Introduction / Context: In a PWR, the primary circuit removes fission heat from the core while avoiding boiling to maintain stable heat transfer and prevent departure from nucleate boiling margins. The thermal-hydraulic state of the primary coolant is central to safe operation. Given Data / Assumptions: PWR design with pressuriser maintaining high pressure. Boiling is purposely suppressed in the primary loop. Steam is generated only in the secondary side (steam generator). Concept / Approach: By keeping the primary coolant at high pressure (typically ~15–16 MPa), its saturation temperature is raised well above core outlet temperature, ensuring the fluid remains a single-phase liquid (subcooled). This avoids void formation in the core and keeps neutron moderation and heat transfer predictable. Step-by-Step Solution: Identify PWR goal: no bulk boiling in primary.High pressure elevates saturation temperature.Operating temperatures are kept below saturation, hence liquid is subcooled.Steam forms only on the secondary side of steam generators. Verification / Alternative check: Basic PWR schematics show pressuriser and steam generators, with primary water circulating through the reactor vessel and remaining in liquid state throughout normal operation. Why Other Options Are Wrong: Saturated water/steam or two-phase mixture contradicts the PWR primary objective. Superheated steam exists only in the secondary/tertiary cycles, not in the primary circuit. Common Pitfalls: Confusing BWR and PWR designs; in BWRs, boiling occurs in the core and steam goes directly to the turbine. Final Answer: sub cooled water

Question 4

Indian context: the nuclear power reactor at Tarapur (Maharashtra) is of which reactor type?

Accepted Answer

Introduction / Context: Recognising reactor types at specific Indian sites is a frequent exam theme. Tarapur Nuclear Power Station (the earliest commercial nuclear station in India) began with imported boiling water reactor units, distinct from later indigenous heavy-water moderated designs deployed elsewhere. Given Data / Assumptions: Historical identification of Tarapur Unit-1 and Unit-2 technology. Question asks only for the reactor type label. Concept / Approach: BWRs generate steam directly in the reactor core; the steam is dried and sent to the turbine. This contrasts with PWRs, which use a steam generator to isolate the primary loop, and PHWRs (pressurised heavy-water reactors), which use D2O moderator and pressure tubes. Fast breeders are a separate class using liquid-metal coolants. Step-by-Step Solution: Recall Tarapur’s earliest commercial units were BWRs.Match with the options.Select “boiling water.” Verification / Alternative check: Site overviews consistently refer to Tarapur’s original units as BWRs, with later Indian fleets dominated by PHWRs elsewhere. Why Other Options Are Wrong: PWR: different coolant/steam separation concept. Fast breeder: different spectrum and coolant (e.g., sodium). Gas cooled: not applicable to Tarapur’s early units. Heavy-water pressure tubes: characteristic of PHWR/CANDU, not Tarapur BWRs. Common Pitfalls: Mixing Tarapur with PHWR-heavy sites; confusing BWR and PWR characteristics. Final Answer: boiling water

Question 5

Decay identification: in the nuclear transformation 93Np239 → 94Pu239 + ?, identify the missing particle.

Accepted Answer

Introduction / Context: Recognising decay modes from changes in atomic number Z and mass number A is a foundational nuclear skill. The transformation 93Np239 → 94Pu239 keeps A constant and increases Z by 1, pointing to a specific beta process. Given Data / Assumptions: Parent: neptunium-239 (Z = 93, A = 239). Daughter: plutonium-239 (Z = 94, A = 239). Unknown emitted particle to be identified. Concept / Approach: When Z increases by 1 and A remains constant, a neutron in the nucleus converts to a proton. That is β− decay (electron emission): n → p + e− + anti-ν. β+ decay or electron capture would decrease Z by 1; α decay would reduce A by 4 and Z by 2; neutron emission would reduce A by 1. Step-by-Step Solution: Compare Z and A before and after: ΔZ = +1, ΔA = 0.Match to β− decay signature.Therefore the emitted particle is an electron (beta minus), with an antineutrino also emitted. Verification / Alternative check: Decay chains from activation of U-238 (via n,γ to U-239 → β− to Np-239 → β− to Pu-239) illustrate this exact step and particle identity. Why Other Options Are Wrong: Proton or neutron emission would alter A. Positron emission (β+) would reduce Z by 1. Alpha emission changes both Z and A significantly (−2, −4). Common Pitfalls: Forgetting that mass number counts nucleons only; electrons do not change A, so β− keeps A constant while increasing Z. Final Answer: electron

Question 6

Indian uranium mining: the approximate percentage of natural uranium present in uranium ore at Jadugoda (Jharkhand) is closest to which value?

Accepted Answer

Introduction / Context: Uranium ore grades vary widely, but many conventional deposits are of relatively low grade by percentage of uranium content. India’s Jadugoda mines are often cited in exam questions to emphasise that processing plants extract uranium economically even from low-percentage ores. Given Data / Assumptions: The question asks for an order-of-magnitude percentage, not exact geology per vein. We refer to uranium content in ore, typically reported as U or U3O8 equivalent. Concept / Approach: Introductory nuclear engineering and mining overviews generally place Jadugoda ore in the low-grade category around a few tenths of a percent or less. A commonly taught figure for such MCQs is ~0.1%, highlighting the need for large-scale milling and leaching to recover usable uranium. Step-by-Step Solution: Recall that Jadugoda is low-grade relative to very high-grade Canadian deposits.Pick the closest low-percentage choice: 0.1%.Eliminate obviously high values inconsistent with India’s known ore grades at that site. Verification / Alternative check: Educational summaries on Indian uranium resources consistently describe Jadugoda as a low-grade source, aligning with ~0.1% order of magnitude used in standard MCQs. Why Other Options Are Wrong: 1%, 3%, 12%: Too high for the typical values taught for Jadugoda. 0.01%: An order of magnitude lower than the often-quoted figure for exam purposes. Common Pitfalls: Confusing ore grade percentages with U-235 enrichment percentages; mixing site-specific averages with exceptional lenses. Final Answer: 0.1

Question 7

Mineral resources: the principal element obtained from monazite sand (common on certain coastal placer deposits) is which of the following?

Accepted Answer

Introduction / Context: Monazite is a phosphate mineral rich in rare-earth elements and thorium. Coastal placer deposits in several countries, including India, contain monazite-bearing sands that serve as significant thorium resources, important for thorium-based fuel cycle discussions. Given Data / Assumptions: We focus on the main valuable nuclear-related constituent of monazite. Trace or associated elements are not the target of this question. Concept / Approach: Monazite composition commonly includes ThO2 in appreciable amounts along with rare-earth oxides. While uranium can occur in associated minerals, thorium is the hallmark nuclear element linked to monazite, feeding thorium fuel-cycle research and policy plans in thorium-rich countries. Step-by-Step Solution: Identify monazite's notable nuclear element: thorium.Match to options: choose "thorium".Eliminate others that are not characteristic monazite products. Verification / Alternative check: Mineralogy references describe monazite as (Ce,La,Nd,Th)PO4 with variable thorium content, supporting the selection. Why Other Options Are Wrong: Uranium: More associated with uraninite or carnotite, not monazite as the principal source. Polonium, technetium: Artificial/radioactive products, not mined directly from monazite. Hafnium: Typically recovered with zirconium from zircon sands, not monazite. Common Pitfalls: Assuming all radioactive elements are sourced from the same mineral sands; conflating zircon (ZrSiO4) with monazite-rich fractions. Final Answer: thorium

Question 8

Sodium-cooled fast breeder reactors: why is a secondary (intermediate) sodium loop essential before the steam–water system?

Accepted Answer

Introduction / Context: Fast breeder reactors (FBRs) often use liquid sodium as primary coolant to preserve a fast neutron spectrum and enable high heat transfer at low pressure. Integrating this with a water/steam power cycle requires careful interface design to manage chemical reactivity and radiological safety. Given Data / Assumptions: Primary coolant is radioactive sodium after core contact. Water/steam is used to drive turbines on the secondary/tertiary side. Sodium and water react vigorously, generating heat and hydrogen, leading to potential explosions. Concept / Approach: A secondary (intermediate) sodium loop sits between the primary sodium and the steam–water system. This loop transfers heat through a sodium–sodium heat exchanger, so any sodium–water leak in the steam generator does not involve radioactive primary sodium, reducing radiological consequences and protecting the core from water ingress. Step-by-Step Solution: Recognise the chemical hazard: sodium + water → violent reaction + hydrogen.Note the radiological hazard of primary sodium.Insert an intermediate loop to isolate primary sodium from water.Hence, the main reason is to prevent water–primary sodium contact and explosions. Verification / Alternative check: SFR design literature consistently emphasises the intermediate loop as a key safety and availability feature, localising any steam generator leaks to non-radioactive sodium. Why Other Options Are Wrong: Superheat degree depends on steam-side design; not the primary reason. Heat removal rate depends on overall design; the intermediate loop is for isolation, not simply higher flow. Circulation pressure is already low with sodium; isolation is the driver. No steam generator: incorrect—steam generators are required for Rankine cycles. Common Pitfalls: Underestimating sodium–water reactivity; assuming the intermediate loop is optional in power SFRs. Final Answer: To prevent water from contacting radioactive sodium (sodium–water reactions can be explosive)

Question 9

Coolant selection: which gas has a relatively high neutron capture cross-section and is therefore unsuitable as a reactor coolant?

Accepted Answer

Introduction / Context: Good reactor coolants should not unduly absorb neutrons, especially in thermal-spectrum systems where maintaining neutron economy is critical. Among candidate gases, differences in thermal neutron capture cross-sections inform suitability for use as coolants or cover gases. Given Data / Assumptions: Comparing common gases: helium, nitrogen, carbon dioxide, hydrogen, argon. Qualitative comparison suffices for MCQ selection. Concept / Approach: Helium has an extremely small capture cross-section and is chemically inert—excellent as a coolant or cover gas. CO2 and Ar also have relatively low captures compared with nitrogen. Nitrogen-14 has a significantly higher thermal capture (~order ~1–2 barns), and captures can produce C-14 via (n,p) or (n,γ)/(p) pathways, undesirable in reactor cores. Hydrogen’s capture is moderate (~0.33 b) but it is mainly used as a moderator in water rather than as a standalone gas coolant; flammability is a separate concern, not the largest capture among the options. Step-by-Step Solution: Rank gases by capture qualitatively: He lowest; CO2 and Ar low; H2 moderate; N2 comparatively high.Select nitrogen (N2) as the gas with comparatively high capture making it unsuitable as a reactor gas coolant. Verification / Alternative check: Gas-cooled reactor designs generally choose helium (or CO2 historically) over nitrogen due to neutron economy considerations and activation products concerns. Why Other Options Are Wrong: He: exceptionally low capture and inert—preferred. CO2: historically used in gas-cooled reactors. H2: capture lower than N2; safety issues aside, it is not the highest absorber here. Ar: relatively low absorption compared with nitrogen; can be used as cover gas. Common Pitfalls: Assuming hydrogen, because it moderates strongly, must also have the highest capture; in fact, nitrogen’s capture and activation issues are more problematic for gas coolant roles. Final Answer: N2

Question 10

Facilities at BARC, Trombay: which of the following best describes the nuclear research reactors at this site?

Accepted Answer

Introduction / Context: The Bhabha Atomic Research Centre (BARC) at Trombay is India’s premier nuclear research complex housing research reactors, critical assemblies, and advanced laboratories. Exam questions often distinguish research facilities from commercial power plants and heavy-water production units located elsewhere. Given Data / Assumptions: Focus is on the type of reactors/facilities historically associated with BARC Trombay. Commercial nuclear power generation units are typically sited away from BARC. Heavy water plants are separate industrial facilities at other locations. Concept / Approach: BARC historically hosted research reactors and critical assemblies such as Apsara (swimming-pool type), CIRUS (heavy-water moderated), ZERLINA (zero energy), Purnima series (critical assemblies), and Dhruva (high-flux research reactor). This portfolio highlights research and isotope/irradiation capabilities rather than commercial power generation or heavy-water production capacity on the same campus. Step-by-Step Solution: Identify the nature of BARC facilities: research/critical reactors.Note that commercial 420 MW class power units are not at BARC.Heavy water production plants of large capacity are located elsewhere.Therefore, the accurate statement is the one listing the research/critical facilities. Verification / Alternative check: Publicly available summaries distinguish Tarapur/Kudankulam/RAPS etc. (power stations) from BARC (research site). Heavy water plants (e.g., Kota, Manuguru, Thal, etc.) are separate industrial sites, not at BARC Trombay. Why Other Options Are Wrong: 420 MW power plant: not a BARC Trombay facility. 100 t/yr heavy water plant: not located at BARC Trombay. "All of the above" cannot be true since (b) and (c) are false. "Only a power reactor": contradicts the research mission of BARC. Common Pitfalls: Conflating research reactors with commercial power stations; assuming heavy-water production is co-located with research reactors. Final Answer: Five research/critical facilities historically including Apsara, Cirus, Zerlina, Purnima, and Dhruva

Question 11

In nuclear reactor engineering, which coolant is typically used in a fast breeder reactor (FBR) to remove heat while avoiding neutron moderation?

Accepted Answer

Introduction / Context: Fast breeder reactors (FBRs) are designed to operate with a fast-neutron spectrum and to convert fertile isotopes (such as U-238) into fissile material (such as Pu-239). The choice of coolant is crucial, because many common coolants moderate (slow) neutrons, which would undermine fast-spectrum operation. This question reinforces why molten sodium is used widely in FBRs. Given Data / Assumptions: Fast breeder reactor requires minimal neutron moderation. Coolant must provide excellent heat removal and good thermal-hydraulic performance. Chemical compatibility and corrosion behavior are manageable with proper materials and cover gas. Concept / Approach: Water (ordinary or heavy) is a strong neutron moderator, which is counterproductive in a fast reactor. Gas coolants like helium have very low moderation but comparatively low volumetric heat capacity, requiring high pressures and large flow rates. Liquid metals—especially molten sodium—offer high thermal conductivity and heat capacity with low neutron moderation, enabling compact cores and efficient heat transfer at near-atmospheric pressure. Step-by-Step Solution: Identify fast-spectrum requirement: minimize moderation.Evaluate coolants: H2O/D2O moderate strongly; helium has low moderation but low heat capacity.Liquid sodium: very high thermal conductivity and heat capacity, low neutron absorption and moderation.Conclusion: molten sodium is the standard FBR coolant. Verification / Alternative check: Historical and contemporary FBR programs (e.g., EBR-II, BN series, PFBR design choices) consistently use sodium for primary cooling due to its superior heat-transfer properties and suitable neutronic behavior in fast spectra. Why Other Options Are Wrong: Heavy water / ordinary water: strong moderators; unsuitable for fast-spectrum cores.Helium: acceptable spectrum-wise but limited by low density and lower heat capacity.Lead–bismuth: used in some concepts but less common than sodium in commercial FBRs. Common Pitfalls: Assuming high thermal efficiency demands water; spectrum requirements dominate.Ignoring sodium's chemical reactivity; while significant, it is managed by design and operating practices. Final Answer: Molten sodium

Question 12

Hydrogen (H2) versus carbon dioxide (CO2) as reactor coolants: which property primarily makes H2 a better heat-removal medium under similar conditions?

Accepted Answer

Introduction / Context: Gas-cooled systems value coolants that can carry away large amounts of heat with minimal pumping power. Hydrogen has been studied and used in specialized high-temperature applications because of its superior heat-transfer characteristics compared with heavier gases like CO2. This question distinguishes the dominant property that makes H2 an efficient coolant. Given Data / Assumptions: Comparable temperature and pressure ranges. Focus on heat removal capability at similar flow conditions. Materials compatibility and safety engineered via standard practices. Concept / Approach: Hydrogen possesses a very high specific heat and exceptional thermal conductivity for a gas, which together enable better convective heat transfer at a given mass flow. Although density influences pumping and Reynolds number, it is the strong thermophysical properties—chiefly specific heat and conductivity—that drive superior heat removal. Reactivity with structural or fuel materials and neutron absorption are separate, design-managed concerns and are not the primary reason for H2’s cooling advantage over CO2. Step-by-Step Solution: Compare cp: cp,H2 > cp,CO2 → more energy carried per unit mass per degree.Compare k (thermal conductivity): k,H2 >> k,CO2 → higher heat-transfer coefficients.Result: for similar flow, H2 removes heat more effectively.Therefore, the key property is higher specific heat (with high conductivity). Verification / Alternative check: Heat-transfer correlations (e.g., Nu–Re–Pr) highlight the influence of cp and k on convective performance; hydrogen’s properties yield larger Nu and heat flux at comparable conditions than CO2. Why Other Options Are Wrong: Lower density: affects pumping but does not by itself ensure better heat removal.Non-reactivity with uranium: not generally the decisive factor here.Lower neutron capture: hydrogen actually has a higher thermal absorption than C or O.Boiling point: irrelevant for gas-phase coolants at operating conditions. Common Pitfalls: Confusing density effects with core heat-transfer capability.Assuming neutronics dictates coolant choice in all designs; here, thermophysical properties are central. Final Answer: Higher specific heat (and excellent thermal conductivity)

Question 13

Among common minerals, which one is recognized as the principal ore of uranium used for industrial extraction?

Accepted Answer

Introduction / Context: Identifying the principal uranium ore is a fundamental piece of nuclear fuel-cycle knowledge. The mineralogical source affects beneficiation, leaching routes, and downstream refining steps. This question tests recognition of the primary uranium-bearing mineral used globally. Given Data / Assumptions: Industrial-scale uranium extraction context. Typical mineralogy and naming conventions. Comparison with ores of other metals for contrast. Concept / Approach: Pitchblende, also known as uraninite, is a uranium oxide mineral (commonly approximated as UO2 with U3O8 components) and is the most important source of uranium. While monazite may contain trace uranium, it is best known for rare earths and thorium. Cassiterite is tin ore (SnO2), and chalcopyrite is copper–iron sulfide (CuFeS2), neither relevant for uranium production. Step-by-Step Solution: List candidates: pitchblende, monazite, cassiterite, chalcopyrite.Match each to its metal: pitchblende → uranium; monazite → rare earths/thorium; cassiterite → tin; chalcopyrite → copper.Select the principal uranium ore: pitchblende (uraninite). Verification / Alternative check: Historical mining districts and modern operations alike cite uraninite/pitchblende as the dominant uranium ore, supplying feed to milling and hydrometallurgical circuits that produce yellowcake (U3O8). Why Other Options Are Wrong: Monazite: mainly rare earths and thorium, not the principal uranium source.Cassiterite: tin ore, unrelated to uranium.Chalcopyrite: copper ore.Ilmenite: iron–titanium oxide (TiO2 feed), not uranium. Common Pitfalls: Equating trace uranium presence with principal ore status.Confusing ore names across different metals due to similar-sounding minerals. Final Answer: Pitchblende (uraninite)

Question 14

Radiation hazard comparison: among β-rays, X-rays, γ-rays, and ultraviolet rays, which is generally the most harmful to humans for equivalent exposure conditions?

Accepted Answer

Introduction / Context: Different types of ionizing radiation interact with matter in distinct ways, affecting penetration depth, linear energy transfer, and biological damage. Understanding relative hazards guides shielding, dosimetry, and safety protocols in nuclear and medical settings. This question asks for the most harmful type under comparable exposure conditions. Given Data / Assumptions: Equivalent dose or exposure context, not extreme or unusual geometries. No specialized shielding—conceptual comparison only. Human tissue considered as the target medium. Concept / Approach: Gamma rays are highly penetrating electromagnetic radiation arising from nuclear transitions. They traverse substantial thicknesses of tissue, reaching internal organs and delivering dose throughout the body. X-rays are also penetrating but typically of lower energies in many applications; betas (electrons) are less penetrating and can be stopped by modest shielding or skin to a degree; ultraviolet is mostly non-ionizing at lower energies and is primarily a surface hazard (e.g., skin/eye damage) rather than deep-tissue ionization. Step-by-Step Solution: Assess penetration: γ > X > β > UV for deep-tissue exposure.Consider ionizing potential: γ and X are ionizing; UV mostly non-ionizing except at highest energies.Health impact: systemic/dense ionization from γ poses greater internal hazard per equal unshielded exposure.Therefore, γ-rays are generally most harmful among the listed. Verification / Alternative check: Protection standards (ALARA principles) and shielding practices assign substantial barriers (lead, concrete) for γ-rays; beta and UV primarily require lighter shielding or protective clothing/eyewear. Why Other Options Are Wrong: β-rays: lower penetration; can be shielded with thin materials.X-rays: significant hazard but, for similar conditions, often less penetrating than γ from nuclear sources.Ultraviolet: harmful to skin/eyes but comparatively limited in penetration and ionization.Visible light: non-ionizing; orders of magnitude less hazardous. Common Pitfalls: Confusing dose rate and energy spectrum specifics; here, a general comparison is intended.Assuming α is included; while highly damaging internally, it is not listed in this question. Final Answer: γ-rays (gamma radiation)

Question 15

In fast breeder reactors, why does using a molten metal (such as liquid sodium) as the primary coolant help the core remove heat effectively?

Accepted Answer

Introduction / Context: Fast breeder reactors select coolant materials that do not slow neutrons and that can remove large thermal loads efficiently. Liquid metals, especially sodium, are favored because they combine excellent heat-transfer properties with negligible moderation. This question asks for the principal benefit related to thermal performance. Given Data / Assumptions: Fast-spectrum operation (minimal moderation). High power density in the core. Engineering controls in place for chemical reactivity and leak management. Concept / Approach: Liquid metals possess very high thermal conductivity and substantial heat capacity relative to many alternatives, allowing efficient convective heat removal at modest pressure. This rapid heat transfer lowers temperature gradients, improves thermal margins, and supports compact core designs. The coolant itself does not significantly alter reaction rates other than by removing heat (which impacts temperature feedback), nor does it act as a neutron source or accelerator. Step-by-Step Solution: Identify desired property: high k (thermal conductivity) and good cp.Relate to heat transfer: higher k → higher heat flux for a given ΔT.Assess neutronic neutrality: sodium causes minimal moderation in a fast spectrum.Conclude: liquid metal coolants enable rapid core heat removal. Verification / Alternative check: Design literature shows sodium-cooled loops operating near atmospheric pressure with large heat fluxes, confirming the strong heat-transfer advantage relative to gases at similar pressures. Why Other Options Are Wrong: Accelerating reaction rate: not the coolant’s function; reactivity is governed by core physics.Breeding neutrons: breeding arises from fertile capture (e.g., U-238 → Pu-239), not from coolant.Accelerating neutrons: coolant does not accelerate neutrons.Eliminating secondary loop: many sodium systems still use intermediate loops for safety. Common Pitfalls: Assuming any liquid automatically improves heat transfer; the key is metal properties.Overlooking pressure advantages of liquid metals compared with high-pressure gas coolants. Final Answer: It enables rapid heat transfer from the core due to high thermal conductivity

Question 16

Time dependence of radioactive decay: a radioactive isotope’s activity decreases with time according to which mathematical law?

Accepted Answer

Introduction / Context: Radioactive decay is a stochastic process characterized by a constant probability of decay per nucleus per unit time. Despite the randomness at the microscopic level, the macroscopic behavior of large populations of nuclei follows a predictable mathematical form. Recognizing the exponential law underpins concepts such as half-life and decay constant. Given Data / Assumptions: Nuclei decay independently. Decay constant (lambda) is time-invariant. No production or removal of the parent isotope during the interval considered. Concept / Approach: The rate equation for radioactive decay is dN/dt = −lambda * N. Solving this first-order differential equation yields N(t) = N0 * exp(−lambda * t). Activity A(t) is proportional to N(t), so it also decays exponentially. This elegant law explains why the fraction remaining after equal time intervals is constant (leading to the concept of half-life), unlike linear or inverse-square processes. Step-by-Step Solution: Write decay rate: dN/dt = −lambda * N.Separate variables: dN/N = −lambda dt.Integrate to obtain: N(t) = N0 * exp(−lambda * t).Relate activity: A(t) ∝ N(t) → exponential as well. Verification / Alternative check: Plotting ln(N) versus time produces a straight line of slope −lambda. This diagnostic linearization is a standard verification in radiochemistry and nuclear physics labs. Why Other Options Are Wrong: Logarithmic: implies very slow variation, not physically consistent here.Linear: would yield constant-rate decrease; not observed for decay.Inverse-square: pertains to intensity versus distance for point sources.Sinusoidal: oscillatory behavior, not applicable to decay. Common Pitfalls: Confusing exponential decay with half-life that depends on the starting amount; half-life is constant for first-order decay.Mixing activity decay (counts per second) with detector efficiency changes. Final Answer: Exponential law

Question 17

Why is the unit power-generation cost in a nuclear power plant typically lower than in a coal-based thermal plant?

Accepted Answer

Introduction / Context: Levelized cost of electricity (LCOE) reflects multiple contributors: capital, fuel, operations and maintenance (O&M), and decommissioning. For nuclear plants, fuel contributes a relatively small fraction of total generation cost compared with fossil plants, even though nuclear has high capital and stringent O&M requirements. This question focuses on the principal recurring cost advantage. Given Data / Assumptions: Comparing steady-state generation costs, not total overnight capital cost. Typical modern coal plant efficiencies around the mid-30% range; nuclear thermal efficiencies often slightly lower. Fuel-cycle costs include mining, conversion, enrichment, and fabrication for nuclear. Concept / Approach: The energy density of nuclear fuel is orders of magnitude higher than that of coal. Consequently, the quantity of fuel required per kWh is minuscule, making the fuel cost per unit energy comparatively low even after accounting for the entire fuel cycle. By contrast, coal plants consume large tonnages daily, making fuel a dominant operating expense. While nuclear plants do not have higher thermal efficiency than coal, their fuel cost component remains smaller per kWh generated. Step-by-Step Solution: Identify cost components: capital, fuel, O&M.Recognize nuclear fuel’s extremely high energy density.Translate to operating costs: much lower fuel cost per kWh for nuclear.Conclude: fuel cost advantage lowers unit generation cost. Verification / Alternative check: Utility cost breakdowns typically show fuel as a small slice for nuclear generation compared with coal and gas, despite nuclear’s higher fixed costs. Why Other Options Are Wrong: Higher thermal efficiency: nuclear is usually lower, not higher, than coal on a percentage basis.Maintenance cost inherently lower: nuclear O&M is rigorous and not necessarily cheaper.No fuel handling costs: incorrect; nuclear fuel cycle handling is specialized and nonzero. Common Pitfalls: Confusing LCOE drivers; capital cost dominates nuclear but the question targets fuel cost per kWh.Assuming efficiency equals cost—fuel price and energy density are decisive. Final Answer: Fuel cost per unit power generated is lower for nuclear plants

Question 18

To initiate nuclear fission, the critical (excitation) energy of the compound nucleus should be how large relative to the neutron binding energy of the target nucleus?

Accepted Answer

Introduction / Context: Nuclear fission by neutron absorption forms a compound nucleus in an excited state. Whether fission occurs depends on whether the excitation energy exceeds the fission barrier. The relation between this critical (threshold) energy and neutron binding energy determines the need for fast neutrons or whether thermal neutrons suffice for certain nuclides. Given Data / Assumptions: Compound nucleus model (neutron capture followed by de-excitation paths). Fission barrier exists and must be overcome. Binding energy refers to the energy required to remove the captured neutron. Concept / Approach: When a neutron is captured, the released binding energy plus the neutron’s kinetic energy sets the excitation energy of the compound nucleus. For fission to be energetically possible, this excitation must exceed the fission barrier (critical energy). In nuclides that fission with thermal neutrons, the binding energy alone is typically sufficient; in others, additional kinetic energy from a faster neutron is required, implying that the critical energy must be above the binding energy level. Step-by-Step Solution: Capture neutron: target + n → compound nucleus + excitation energy.Excitation energy = binding energy + kinetic energy of neutron.Fission condition: excitation > fission barrier (critical energy).Therefore, critical energy must exceed binding energy to ensure threshold behavior. Verification / Alternative check: Fast-fission thresholds observed in U-238 illustrate cases where thermal neutrons lack sufficient energy, requiring higher kinetic energy to push excitation above the barrier, consistent with “more than binding energy.” Why Other Options Are Wrong: Equal / less than: would imply no threshold for nuclides that clearly have one.Either more or less: contradicts the known threshold behavior in non-thermal-fissioning isotopes.Exactly half: arbitrary and unphysical. Common Pitfalls: Confusing binding energy with fission barrier; they are distinct energies.Assuming all actinides fission with thermal neutrons; many require fast neutrons. Final Answer: More than the neutron binding energy

Question 19

Identify the mineral that is not an ore of uranium among the following commonly cited names.

Accepted Answer

Introduction / Context: Ore identification is essential in the nuclear fuel cycle. While several minerals carry uranium in significant quantities (e.g., pitchblende/uraninite, carnotite, uranophane), many common minerals are unrelated to uranium. This question checks the ability to differentiate uranium ores from unrelated minerals. Given Data / Assumptions: Typical uranium-bearing minerals vs. non-uranium minerals. Common textbook examples used in competitive exams. Names may be similar sounding but indicate different chemistries. Concept / Approach: Pitchblende (uraninite) and carnotite are well-known uranium ores; uranophane is a hydrated uranium silicate also found in secondary deposits. Kyanite, by contrast, is an aluminum silicate (Al2SiO5) used as a refractory material and contains no uranium. The task is to exclude the non-uranium mineral from the list. Step-by-Step Solution: Match minerals to uranium content: pitchblende ✓, carnotite ✓, uranophane ✓.Identify kyanite: Al2SiO5, refractory, not a uranium ore.Select the outlier: kyanite. Verification / Alternative check: Mineralogy references catalog kyanite among aluminosilicate polymorphs (with andalusite, sillimanite), not uranium-bearing species. Uranium ores are dominantly oxides, vanadates, phosphates, or silicates of uranium. Why Other Options Are Wrong: Pitchblende: primary uranium oxide ore.Carnotite: potassium uranium vanadate; an important uranium–vanadium ore.Rescolite: sometimes cited as a synonym/misspelling in legacy lists; not as common as others but historically associated with uranium occurrences in some texts.Uranophane: hydrated uranium silicate found in oxidized zones. Common Pitfalls: Confusing refractory aluminosilicates with uranium minerals due to unfamiliar names.Assuming every rare-sounding mineral is uranium-bearing. Final Answer: Kyanite

Question 20

In reactor physics terminology, what does a moderator do to neutrons within the core?

Accepted Answer

Introduction / Context: Moderators are central to thermal reactor design. Many fissile isotopes (such as U-235) have substantially higher fission cross sections for thermal (slow) neutrons than for fast neutrons. Therefore, thermal reactors employ a moderator to reduce neutron kinetic energy efficiently while minimizing neutron absorption. Given Data / Assumptions: Typical thermal reactors (PWR, BWR, HWR) are considered. Moderator choices include light water, heavy water, and graphite. Goal is to enhance the probability of fission in the fuel with minimal losses. Concept / Approach: A moderator slows neutrons primarily through elastic scattering, transferring kinetic energy from neutrons to moderator nuclei. Effective moderators have low atomic mass (to maximize energy loss per collision) and low neutron-absorption cross sections (to preserve the neutron population). Heavy water and graphite excel in low absorption; light water moderates well but absorbs more than heavy water. Step-by-Step Solution: State function: reduce neutron energy from MeV to eV scale.Mechanism: multiple elastic collisions with moderator nuclei.Selection criteria: high scattering / low absorption, suitable thermal and mechanical properties.Outcome: higher thermal fission probability and sustained chain reaction. Verification / Alternative check: Cross-section data show the fission cross section of U-235 increases markedly at thermal energies; moderator presence shifts the neutron spectrum accordingly, improving reactivity in thermal designs. Why Other Options Are Wrong: Absorbs strongly: describes poisons or control materials, not moderators.Accelerates: not physically meaningful for neutrons in cores.Reflects: reflection is a separate function handled by reflectors; some moderators also reflect, but moderation specifically means slowing.Converts neutrons to protons: not a moderator function. Common Pitfalls: Confusing moderator with reflector or absorber.Assuming all moderators are equal; absorption penalties vary significantly (e.g., H2O vs D2O). Final Answer: Slows down the neutrons

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