Question 1

Radioactive decay arithmetic: after a duration equal to four half-lives, what percentage of the original amount of a radioisotope remains undecayed?

Accepted Answer

Introduction / Context: Half-life problems are common in nuclear chemistry and radiation protection. They test exponential decay understanding and the ability to translate half-life intervals into remaining fractions or percentages of a radionuclide. Given Data / Assumptions: Ideal first-order decay law applies: N = N0 * (1/2)^(t/t_half). No production or replenishment of the isotope during the interval. We measure remaining, not decayed, fraction. Concept / Approach: Each half-life reduces the quantity by half. After n half-lives, remaining fraction is (1/2)^n. Converting to percentage involves multiplying by 100. For four half-lives, n = 4, so remaining fraction is (1/2)^4 = 1/16 = 0.0625 = 6.25% of the original amount. Step-by-Step Solution: Use decay formula: N/N0 = (1/2)^(n).Set n = 4 → N/N0 = (1/2)^4 = 1/16.Convert to percent: (1/16)*100 = 6.25%. Verification / Alternative check: Successive halving: 100% → 50% (1st) → 25% (2nd) → 12.5% (3rd) → 6.25% (4th). Matches the calculation. Why Other Options Are Wrong: 12.50%: This is after three half-lives, not four. 3.125%: This is after five half-lives. 25%: After two half-lives. 50%: After one half-life only. Common Pitfalls: Confusing the amount decayed with amount remaining; stopping one half-life too early; arithmetic errors when converting fractions to percentages. Final Answer: 6.25%

Question 2

Fundamental particle masses: approximately how does the electron’s mass compare to the proton’s mass?

Accepted Answer

Introduction / Context: Mass ratios among subatomic particles are constants used throughout atomic physics, spectroscopy, and accelerator calculations. The electron-to-proton mass ratio is a widely memorized benchmark value. Given Data / Assumptions: Proton rest mass ≈ 1.6726 × 10^-27 kg. Electron rest mass ≈ 9.1094 × 10^-31 kg. We consider rest masses, non-relativistic comparison. Concept / Approach: Divide electron mass by proton mass to obtain the ratio. The accepted approximate ratio is m_e/m_p ≈ 1/1836 (more precisely ≈ 1/1836.15). Hence, the electron is much lighter than the proton by roughly three orders of magnitude. Step-by-Step Solution: Recall numerical values for m_e and m_p.Compute ratio: m_e/m_p ≈ 9.11×10^-31 / 1.67×10^-27 ≈ 1/1836.State the approximate relationship used in problem solving. Verification / Alternative check: Tables of fundamental constants list m_p/m_e ≈ 1836.152673, confirming the common approximation 1/1836 for m_e/m_p. Why Other Options Are Wrong: equal, infinite, about half, 1836 times: All contradict well-established mass ratios from experiment. Common Pitfalls: Mixing up which particle is heavier; misplacing decimal exponents when estimating the ratio. Final Answer: 1/1836 of the proton mass

Question 3

Half-life application: a radioactive element has a half-life of 12 years. How long will it take for the sample to reduce to 50% of its original mass?

Accepted Answer

Introduction / Context: The half-life is the time required for a radioactive sample to decay to half of its initial quantity. This definition is frequently tested to ensure students can directly connect decay fractions to elapsed time without unnecessary computation. Given Data / Assumptions: Half-life t_half = 12 years. Simple first-order decay with no production. We seek the time for exactly 50% remaining. Concept / Approach: By definition, after one half-life, the remaining quantity is 1/2 of the original. Therefore, if the half-life is 12 years, it takes precisely 12 years to reach 50% of the original mass. No further calculations are needed beyond recalling the definition. Step-by-Step Solution: Recognize that 50% remaining corresponds to one half-life.Insert given t_half: 12 years.Hence time to 50% = 12 years. Verification / Alternative check: Using N = N0*(1/2)^(t/t_half): set N/N0 = 0.5 → (1/2)^(t/12) = 1/2 → t/12 = 1 → t = 12 years. Why Other Options Are Wrong: 6 years: Would leave about 70.7%, not 50%. 18 years: Between one and two half-lives; not 50%. 24 or 36 years: Two or three half-lives (25% or 12.5% remaining), not 50%. Common Pitfalls: Overcomplicating with logarithms; confusing the time to 50% with the time to 25% or other fractions. Final Answer: 12 years

Question 4

Uranium mining and milling in India: the Jaduguda (Jadugoda) plant produces which final concentrate as its saleable product (commonly referred to as yellowcake)?

Accepted Answer

Introduction / Context: After uranium ore is mined, it is processed (milled) to produce a concentrated product called yellowcake, which is later refined and converted into reactor-grade materials. Indian uranium production historically includes the Jaduguda operation, an important source of domestic uranium concentrate. Given Data / Assumptions: Question refers to the final product leaving the extraction/milling site. Conventional hydrometallurgical processing is assumed. Naming conventions in Indian practice are followed. Concept / Approach: Yellowcake can be produced as different compounds depending on the process, such as ammonium diuranate (ADU), magnesium diuranate (MDU), or U3O8 after calcination. Historical Indian practice at Jaduguda has been associated with magnesium diuranate as the exported/transported concentrate, which is then refined further at other facilities. Step-by-Step Solution: Identify yellowcake forms commonly produced at mills.Associate Jaduguda output with magnesium diuranate concentrate.Select magnesium diuranate as the correct product. Verification / Alternative check: Public domain descriptions of Indian uranium milling note production of diuranate concentrates (not metallic uranium) for transport and further processing. Why Other Options Are Wrong: uranium metal: Produced later in the fuel cycle, not at the mill. U3O8: While some mills calcine to U3O8, the question specifies Jaduguda's product as the diuranate form. uranium carbide: Fuel compound used in specific reactors, not a milling product. ammonium diuranate: Another yellowcake form, but not the one asked here. Common Pitfalls: Assuming all yellowcake is U3O8; ignoring country-specific process chemistry and logistics. Final Answer: magnesium diuranate

Question 5

Moderator selection in reactor physics: ordinary (light) water is often less desirable than heavy water as a moderator because it __________.

Accepted Answer

Introduction / Context: Moderators slow down fast neutrons to thermal energies to increase the likelihood of fission in thermal reactors. The choice of moderator impacts neutron economy, fuel enrichment requirements, and overall reactor design. Heavy water (D2O) and light water (H2O) differ importantly in neutron interaction cross-sections. Given Data / Assumptions: Comparison between ordinary water (H2O) and heavy water (D2O). Focus on absorption and scattering cross-sections. Thermal reactor context. Concept / Approach: A good moderator should have a high scattering cross-section (to slow down neutrons efficiently) and a very low absorption cross-section (to avoid parasitic loss of neutrons). Light water moderates effectively (good scattering) but has higher neutron absorption than heavy water, which hurts neutron economy and necessitates higher fuel enrichment in light-water reactors. Heavy water, with very low absorption, enables operation with natural uranium in certain designs. Step-by-Step Solution: List desired moderator traits: high scattering, low absorption.Assess H2O: good scattering but relatively higher absorption than D2O.Therefore, the drawback is that H2O absorbs neutrons significantly compared to D2O. Verification / Alternative check: Thermal reactor designs (PWR/BWR) require enriched fuel largely because of neutron absorption in light water; heavy-water reactors (e.g., CANDU) can use natural uranium, demonstrating superior neutron economy. Why Other Options Are Wrong: low absorption: Opposite of the drawback. does not absorb at all: Incorrect; H2O does absorb. low scattering: False; H2O scatters well. cannot slow down neutrons: Incorrect; H2O is an effective moderator in practice. Common Pitfalls: Confusing “less desirable than heavy water” with “not used”—light water is widely used but with enriched fuel to compensate for its absorption. Final Answer: absorbs neutrons significantly

Question 6

Boiling Water Reactor (BWR) concept: which statement correctly describes where and how steam is generated in a BWR nuclear power plant?

Accepted Answer

Introduction / Context: Commercial light-water reactors are broadly classified into Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs). Understanding their steam cycle configuration is central to power plant engineering and safety systems design. Given Data / Assumptions: Standard BWR arrangement with a single water/steam circuit. Turbine receives steam generated directly in the core region. No steam generators separating primary and secondary loops. Concept / Approach: In a BWR, coolant water enters the reactor core, is heated by fission, and partially boils within the core and upper plenum. The generated steam is separated and dried, then routed to the turbine. This is a single-loop configuration. In contrast, PWRs keep the primary water under high pressure to prevent boiling and transfer heat to a secondary loop via steam generators. Step-by-Step Solution: Identify BWR hallmark: in-core boiling and direct steam to turbine.Recognize contrast with PWR: no boiling in primary, uses steam generators.Discard unrelated reactor types (homogeneous solution, gas-cooled helium systems). Verification / Alternative check: Plant schematics and vendor documentation show moisture separators and steam dryers within the BWR vessel, confirming in-vessel steam generation. Why Other Options Are Wrong: secondary boiler (two-loop): Describes PWR, not BWR. pressurised water prevented from boiling: Again PWR behavior. homogeneous solution reactor: Different concept; not commercial BWR. helium primary: Gas-cooled reactors, not BWRs. Common Pitfalls: Equating all light-water reactors; forgetting that only BWRs intentionally allow boiling in the core. Final Answer: Coolant water is allowed to boil directly within the reactor core to produce steam for the turbine

Question 7

Coolants in nuclear reactors: Which molten metal is most commonly used for heat removal in fast reactors and experimental systems?

Accepted Answer

Introduction / Context: Nuclear reactors require an efficient coolant to transport thermal energy from the core to heat-exchange equipment. Liquid metals are attractive because they have high thermal conductivity and operate at low pressure even at high temperatures. The question asks which molten metal is most commonly used, especially in fast breeder and research reactors. Given Data / Assumptions: We are comparing practical reactor coolants among the listed metals. Focus is on historical and widespread use, not theoretical properties alone. Coolant must offer high heat capacity, good heat transfer, chemical manageability, and acceptable neutron economy. Concept / Approach: Liquid sodium has been the dominant choice for fast reactors (e.g., Phénix, BN series, EBR-II, PFBR) due to excellent heat transfer properties, low neutron moderation, low viscosity, and operation near atmospheric pressure. Alternatives like lead, lead–bismuth eutectic, and sodium–potassium alloy exist, but pure sodium is the most widely implemented molten metal coolant. The other options in the list are either impractical or obsolete as primary coolants. Step-by-Step Solution: Identify required coolant traits: high thermal conductivity, low viscosity, chemical compatibility, neutron characteristics.Compare candidates: sodium vs. calcium, mercury, zinc.Note extensive operational experience with sodium in fast reactors.Conclude that sodium is the most commonly used molten metal coolant. Verification / Alternative check: Many fast reactors worldwide (historical and current) have used sodium primary and secondary loops, validating its status as the common choice. Why Other Options Are Wrong: Calcium: Highly reactive and not used as a mainstream reactor coolant. Mercury: Toxic, poor neutron characteristics, and thermophysical drawbacks. Zinc: Not adopted as a primary reactor coolant in practice. Common Pitfalls: Confusing sodium with lead–bismuth or NaK; while these are used in some designs, sodium has the broadest adoption history. Final Answer: Sodium

Question 8

Radioactive decay of tritium (hydrogen-3): What is the daughter nuclide produced by its beta decay?

Accepted Answer

Introduction / Context: Tritium (symbol T or ³H) is a radioactive isotope of hydrogen with one proton and two neutrons. It undergoes beta minus decay. Understanding its decay product is fundamental in nuclear chemistry, fusion science, environmental tracing, and radiological safety. Given Data / Assumptions: Tritium decays via beta minus (electron) emission. Nucleon (mass) number is conserved in beta decay. Atomic number increases by one in beta minus decay. Concept / Approach: In beta minus decay, a neutron converts to a proton with the emission of an electron and an antineutrino. Therefore, the daughter nucleus has atomic number Z increased by 1, while mass number A remains the same. For tritium (Z = 1, A = 3), the daughter will have Z = 2 and A = 3, which is helium-3 (³He). Options list 'Helium' generally, which correctly captures the element of the daughter nuclide. Step-by-Step Solution: Parent: tritium, Z = 1, A = 3.Beta minus decay: n → p + e⁻ + anti-ν̄.New Z = 1 + 1 = 2; A remains 3.Element with Z = 2 is helium → daughter is helium-3. Verification / Alternative check: Published decay schemes universally show ³H → ³He + e⁻ + ν̄ with a half-life of about 12.3 years. Why Other Options Are Wrong: Lithium: Would require Z = 3, not obtained by a single beta minus step from tritium. Deuterium: Has A = 2; mass number must remain 3. Hydrogen: The product is not another hydrogen isotope because Z increases by 1 in beta minus decay. Common Pitfalls: Confusing beta minus with beta plus or forgetting that A stays constant while Z changes by ±1 depending on decay type. Final Answer: Helium

Question 9

Measuring nuclear disintegration rates: Which instrument directly counts ionizing events from radioactive emissions?

Accepted Answer

Introduction / Context: Monitoring radioactivity requires instruments that detect and count ionizing events due to alpha, beta, or gamma radiation. The goal is to measure disintegration rate (activity) or count rate for safety, research, and calibration purposes. Given Data / Assumptions: The device must provide event counts proportional to ionization events. Typical on-site survey and laboratory counting contexts are considered. Concept / Approach: A Geiger–Müller (G–M) counter uses a gas-filled tube biased at high voltage. Ionizing radiation creates electron–ion pairs; Townsend avalanches cause detectable pulses. The instrument counts pulses per unit time, which reflects the rate of nuclear disintegrations reaching the detector (after geometry and efficiency corrections). By contrast, a cyclotron accelerates particles; a cold chamber is not a standard radiation counter; a mass spectrograph separates ions by mass-to-charge and is used for isotopic analysis rather than real-time counting of decay events. Step-by-Step Solution: Define requirement: count ionizing events.Match device principle: G–M tube generates pulses per event.Select G–M counter as the appropriate instrument.Reject accelerator and analytical spectrograph options. Verification / Alternative check: Other counters (proportional counters, scintillation detectors) also measure activity, but the most common general-purpose rate meter is the G–M counter. Why Other Options Are Wrong: Cyclotron: Produces beams; not a rate counter. Cold chamber: Not a standard radiation measurement device for count rates. Mass spectrograph: Identifies isotopes; does not directly count disintegrations in real time. Common Pitfalls: Assuming any radiation-related instrument measures activity; function and operating principle must match the need for counting ionizing events. Final Answer: Geiger–Müller counter

Question 10

Critical mass in fission: What does the term “critical mass” specifically mean in reactor physics and nuclear chain reactions?

Accepted Answer

Introduction / Context: In nuclear fission systems, the concept of critical mass is central to whether a self-sustaining chain reaction can occur. It has precise physical meaning unrelated to economics or plant scale, and depends on material, geometry, density, and presence of reflectors and moderators. Given Data / Assumptions: Fissile material considered (e.g., U-235, Pu-239). Neutron economy determined by production, absorption, leakage. Geometry and reflectors influence effective mass required. Concept / Approach: Critical mass is the minimum mass (under specified conditions) of a fissile assembly at which the effective multiplication factor keff equals 1. At keff = 1, each generation of neutrons produces exactly one subsequent generation on average, so the reaction is self-sustaining (critical). Subcritical (keff < 1) dies away; supercritical (keff > 1) increases in power. This definition is purely physical and does not mandate commercial-scale power production or economic viability. Step-by-Step Solution: Define keff and neutron balance (production vs. losses).Relate geometry/material to neutron leakage and absorption.Identify the minimum mass for keff = 1 as critical mass.Conclude: it is the mass for sustainment of chain reaction. Verification / Alternative check: Standard reactor physics texts define critical mass via keff = 1 and show dependence on reflectors, enrichment, and density. Why Other Options Are Wrong: Commercial scale or economic power: These are engineering/economic outcomes, not the physics definition of critical mass. None of these: Incorrect because option (a) is precisely correct. Common Pitfalls: Confusing critical mass with criticality at operating power; criticality can occur at any power level once control rods set reactivity to zero. Final Answer: Sustainment of chain reaction.

Question 11

Applications of nuclear reactors: Identify which listed application is not feasible for a reactor (practical perspective).

Accepted Answer

Introduction / Context: Nuclear reactors serve multiple roles beyond electricity generation. They are sources of neutrons, heat, and specific radionuclides. The question asks which of the listed uses is not possible; recognizing real-world reactor applications is key. Given Data / Assumptions: Research reactors routinely produce radioisotopes and neutron beams. Naval propulsion reactors power submarines and surface ships. We assume standard reactor types (research, power, and naval designs). Concept / Approach: Reactors can irradiate targets to produce medical and industrial radioisotopes (e.g., Mo-99, I-131). Research reactors provide high neutron fluxes for beam lines, material studies, and neutron imaging. Naval reactors have been used for decades for marine propulsion in submarines and aircraft carriers. Therefore all listed applications are in fact feasible, so there is no item that a reactor cannot do among the options given. Step-by-Step Solution: Match each application to real-world implementations: isotopes (research reactors), neutron beams (beam tubes/instruments), propulsion (naval reactors).Confirm feasibility and historical precedent for each.Therefore, select 'None of these' as correct. Verification / Alternative check: Examples include isotope production at OPAL, BR2, and HFIR; neutron science at ILL and NIST; propulsion in U.S. Navy and others. Why Other Options Are Wrong: Each of (a), (b), (c) accurately describes a real use of reactors. Common Pitfalls: Assuming only power generation is a valid use; research and naval reactors are large, long-standing applications. Final Answer: None of these.

Question 12

Homogeneous nuclear reactor: Choose the statement that best defines a homogeneous reactor configuration.

Accepted Answer

Introduction / Context: Nuclear reactors are categorized by how fuel and moderator are spatially arranged. Homogeneous designs differ fundamentally from heterogeneous designs in which solid fuel elements are separated from moderator regions. Understanding this definition is important in reactor physics and engineering. Given Data / Assumptions: The term “homogeneous” refers to fuel–moderator mixture at the microscopic scale. Typical examples: uranyl sulfate solutions in water (aqueous homogeneous reactors). Coolant identity is not, by itself, the defining criterion. Concept / Approach: A homogeneous reactor mixes fuel and moderator into a single phase or uniform suspension, enabling uniform neutron moderation throughout the medium. By contrast, using the same substance as moderator and coolant (option b) can be true in some heterogeneous designs (e.g., D2O-moderated, D2O-cooled CANDU), so it is not definitive of homogeneity. Statement (a) is imprecise and refers to distribution within the “reactor mass” rather than the specific fuel–moderator mixture concept. Step-by-Step Solution: Identify key definition: fuel + moderator intimately mixed as one medium.Map that definition to option (c).Reject options not capturing the required mixture concept.Finalize selection of (c) as correct. Verification / Alternative check: Classic aqueous homogeneous reactors (AHRs) used uranyl salts dissolved in water, confirming that mixing defines homogeneity. Why Other Options Are Wrong: (a) Vague; does not necessarily imply fuel–moderator homogeneity. (b) Same substance for cooling and moderation is possible in heterogeneous systems. (d) Cannot be correct because (a) and (b) are not true definitions of homogeneous reactors. Common Pitfalls: Equating coolant identity with reactor classification; the spatial relationship between fuel and moderator is the key differentiator. Final Answer: The fuel and the moderator are mixed to form a single homogeneous material (solution or slurry).

Question 13

Thermonuclear (hydrogen) bomb: Which fusion fuel combination is predominantly used in modern designs?

Accepted Answer

Introduction / Context: Thermonuclear weapons (hydrogen bombs) use nuclear fusion to release immense energy. The practicality of a fusion reaction depends on reaction cross-section at achievable temperatures and pressures produced by a fission primary. The common fusion fuel choice in such devices is crucial foundational knowledge in nuclear engineering and defense studies. Given Data / Assumptions: We are considering the fusion stage that follows a fission-driven implosion and radiation-driven compression. Fuel must have a high fusion cross-section at relatively “low” thermonuclear temperatures. Engineering constraints favor materials that can be stored or bred on demand (e.g., lithium deuteride generating tritium in situ). Concept / Approach: The deuterium–tritium (D–T) reaction has the highest fusion cross-section at the lowest temperatures among light isotope combinations, making it the practical choice. In many devices, lithium deuteride serves as a solid store; under neutron bombardment, lithium produces tritium which then fuses with deuterium. Pure deuterium or pure tritium alone are not typical as standalone fuels because D–D requires higher temperature and tritium is scarce and decays, although both participate within the D–T pathway. Helium isotopes are not primary fuels in such weapons. Step-by-Step Solution: Identify fusion pairs: D–T has favorable cross-section and energy yield.Relate to engineering realization: breeding tritium from lithium in situ.Eliminate single-isotope answers as incomplete or impractical.Select D–T mixture as the predominant fuel. Verification / Alternative check: Standard fusion cross-section plots show D–T peak at temperatures markedly lower than D–D or D–He-3, supporting its adoption. Why Other Options Are Wrong: Deuterium only: Requires higher temperatures and yields lower rate. Tritium only: Not a practical sole fusion path; needs deuterium partner. Helium-3 with helium-4: Not a standard weapons fuel pair. Common Pitfalls: Assuming “hydrogen bomb” implies ordinary hydrogen (protium). The effective fuel is a D–T combination (often bred from lithium deuteride). Final Answer: Deuterium–tritium (D–T) mixture

Question 14

Isotopes of hydrogen: How many isotopes of hydrogen are commonly recognized in nuclear science?

Accepted Answer

Introduction / Context: Hydrogen is unique for having well-known isotopes with important roles in chemistry, nuclear energy, and astrophysics. Recognizing how many isotopes are commonly referenced is fundamental background knowledge for students of physical science. Given Data / Assumptions: We refer to commonly recognized isotopes: protium, deuterium, tritium. Protium (¹H) is stable; deuterium (²H) is stable; tritium (³H) is radioactive. Concept / Approach: An isotope is defined by the same atomic number (Z = 1 for hydrogen) and different neutron numbers. Protium has zero neutrons, deuterium has one, and tritium has two. While additional exotic hydrogen isotopes can be created (e.g., ⁴H), they are extremely short-lived and not commonly counted in basic curricula. Therefore, the accepted number for general education is three. Step-by-Step Solution: List known, commonly referenced isotopes: ¹H, ²H (D), ³H (T).Check stability: ¹H and ²H stable; ³H radioactive with half-life ~12.3 years.Conclude that hydrogen has three commonly recognized isotopes. Verification / Alternative check: Standard chemistry and physics texts enumerate the three principal isotopes, with deuterium used in heavy water and tritium in fusion research. Why Other Options Are Wrong: No/one/two: Understate the accepted set of isotopes used in science and technology. Common Pitfalls: Confusing “stable isotopes” (two) with “commonly recognized isotopes” (three). The question is not restricted to stability. Final Answer: Three

Question 15

Indian nuclear facilities: Where is the Fast Breeder Test Reactor (FBTR) located?

Accepted Answer

Introduction / Context: India’s nuclear programme includes research and power reactors across multiple sites. The Fast Breeder Test Reactor (FBTR) is a key facility for demonstrating fast reactor technology and breeding capabilities, important for closing the fuel cycle and utilizing plutonium effectively. Given Data / Assumptions: FBTR refers specifically to India’s sodium-cooled fast test reactor. We identify the physical location/site and associated center. Concept / Approach: FBTR is situated at the Indira Gandhi Centre for Atomic Research (IGCAR) campus at Kalpakkam, on India’s southeast coast near Chennai (formerly Madras). Other nuclear sites mentioned are significant but house different facilities: Kota is near Rajasthan Atomic Power Station (PHWRs); Trombay (BARC) hosts research reactors and labs; Tarapur hosts early BWRs and PHWRs, not FBTR. Step-by-Step Solution: Identify FBTR as a sodium-cooled fast reactor.Recall that IGCAR at Kalpakkam is India’s fast reactor hub.Match location to option: Kalpakkam (near Madras).Eliminate other nuclear sites hosting different reactor types. Verification / Alternative check: Published information on India’s nuclear programme consistently lists FBTR at Kalpakkam, alongside PFBR (Prototype Fast Breeder Reactor) development. Why Other Options Are Wrong: Kota (Rajasthan): PHWR units at RAPS, not FBTR. BARC (Trombay): Research reactors like Apsara and Dhruva; not FBTR. Tarapur: Early BWRs and later PHWRs; not FBTR. Common Pitfalls: Confusing FBTR with PFBR or with research reactors at Trombay; each has distinct technology and location. Final Answer: Kalpakkam (near Madras)

Question 16

In basic materials science and chemical engineering data tables, what is the melting point of elemental sodium at 1 atm (atmospheric pressure), expressed in °C?

Accepted Answer

Introduction: The melting point of an element at 1 atm is a fundamental property used in process design, safety assessment, and materials selection. Sodium is a common alkali metal with distinctive thermal behavior that affects storage and handling in reactors and heat-transfer systems. Given Data / Assumptions: Standard pressure is 1 atm. We seek the bulk melting point of pure sodium (no alloying, no significant impurities). Temperature is to be reported in degrees Celsius. Concept / Approach: Sodium is an alkali metal with low melting point compared to most structural metals. Reliable handbooks list its melting point very close to 98 °C (more precisely around 97.8 °C). In engineering calculations, it is typically rounded to 98 °C for convenience, which is sufficiently accurate for most design estimations and operational guidelines. Step-by-Step Solution: Identify the standard melting point of sodium at 1 atm from reference data: approximately 97.8 °C.Round appropriately for an MCQ context: 98 °C.Select the option that matches: 98. Verification / Alternative check: Multiple materials databases and physical-chemistry handbooks converge on the value near 98 °C for sodium’s melting point under atmospheric pressure, confirming the selection. Why Other Options Are Wrong: 58 °C: Too low, inconsistent with standard data. 348 °C and 588 °C: Far above sodium’s known melting range; these are more typical of higher-melting metals or salts. 92 °C: Closer but still not the accepted standard value. Common Pitfalls: Confusing °C with °F, or mixing sodium’s melting point with those of potassium (lower) or other metals. Another error is quoting values under nonstandard pressures or for sodium alloys rather than pure sodium. Final Answer: 98

Question 17

In nuclear reactor physics, what key property should an effective moderator (used in a thermal reactor) possess regarding neutron absorption?

Accepted Answer

Introduction: Moderators in thermal nuclear reactors slow down fast neutrons to thermal energies, greatly increasing the probability of fission in isotopes like U-235. Understanding the required properties of a moderator is essential for safe, efficient reactor design and operation. Given Data / Assumptions: We are referring to thermal reactors, not fast reactors. Moderator function is to reduce neutron energy primarily via elastic scattering. Question focuses on absorption characteristics, not phase or mechanical form. Concept / Approach: An ideal moderator has a high scattering cross section and a very low absorption cross section. Low absorption prevents loss of neutrons that are needed to sustain the chain reaction, while scattering on light nuclei (e.g., hydrogen in water, carbon in graphite) efficiently reduces neutron energy. Step-by-Step Solution: Identify moderator role: slow fast neutrons to thermal energies.Desirable property: low neutron absorption (i.e., a poor absorber) to minimize neutron losses.Select the option that states “poor absorber of neutrons.” Verification / Alternative check: Common moderators (light water, heavy water, graphite) are chosen precisely because they scatter well and absorb little compared to strong absorbers like cadmium or boron compounds. Why Other Options Are Wrong: Good absorber: This would poison the reactor, reducing reactivity. “Must be a solid” or “must be a liquid”: Phase is not the defining criterion; both graphite (solid) and water (liquid) are used successfully. High atomic number: Heavy nuclei are less efficient at moderating; light nuclei work best. Common Pitfalls: Confusing moderators with control materials (e.g., cadmium, boron) whose purpose is to absorb neutrons and control reactivity. Final Answer: Poor absorber of neutrons

Question 18

Thermal neutrons used to induce fission in U-235 have energies below about 0.025 eV. What is the typical speed of such a neutron at room temperature, expressed in m/s?

Accepted Answer

Introduction: In reactor physics, the term “thermal neutron” refers to neutrons in approximate equilibrium with the moderator at room temperature. Their speed determines reaction rates and resonance escape probabilities, influencing core design and control. Given Data / Assumptions: Thermal energy reference is about 0.025 eV at roughly 293 K. We assume Maxwell-Boltzmann distribution; the quoted speed is a representative mean value. Units required: metres per second (m/s). Concept / Approach: Thermal neutron speed v is commonly cited around 2.2 × 10^3 m/s at room temperature. This order-of-magnitude is standard in nuclear engineering references and is used for quick estimates of diffusion length and reaction rates. Step-by-Step Solution: Recognize “thermal” implies ~0.025 eV at room temperature.Corresponding typical speed ≈ 2,200 m/s.Choose the option that matches 2,200. Verification / Alternative check: Using kinetic energy relation (E ≈ 1/2 m v^2) with neutron mass gives the same order of magnitude, validating the standard value. Why Other Options Are Wrong: 1 or 220 m/s: Far too low; would correspond to much lower energies. 3 × 10^11 or 9 × 10^21 m/s: Physically impossible or nonsensical; they exceed realistic particle speeds and even the speed of light. Common Pitfalls: Confusing instantaneous particle speeds with macroscopic flow velocities, or mixing up eV (energy) with m/s (speed). Final Answer: 2,200

Question 19

Radioactivity and mass number: for which emissions does the mass number of a nucleus remain unchanged?

Accepted Answer

Introduction: Different radioactive emissions alter the composition of a nucleus in different ways. Knowing which emissions change the mass number is vital in balancing nuclear equations and predicting daughter nuclides. Given Data / Assumptions: Mass number A counts total nucleons (protons + neutrons). We consider α, β, and γ emissions in their common forms. Concept / Approach: Alpha emission ejects a helium nucleus (2 protons + 2 neutrons), decreasing mass number by 4. Beta emission converts a neutron to a proton (β−) or vice versa (β+), changing Z but not the total number of nucleons; thus, A remains unchanged. Gamma emission is the release of a photon from an excited nucleus and does not change Z or A. Step-by-Step Solution: α: A decreases by 4 → mass number changes.β: neutron↔proton transformation; A unchanged.γ: energy-only de-excitation; A unchanged.Therefore, emissions that do not change A: β and γ. Verification / Alternative check: Writing balanced nuclear equations for typical decays confirms that only α alters mass number by removing nucleons. Why Other Options Are Wrong: α and β or γ and α: include α, which changes A. All three: incorrect because α changes A. Only γ: incomplete; β also leaves A unchanged. Common Pitfalls: Assuming β emission removes or adds particles to the nucleus. It changes identity (Z) but not the nucleon count. Final Answer: β and γ emissions

Question 20

Control and safety in reactor types: which reactor type, if any, may operate without control rods for reactivity management under normal practice?

Accepted Answer

Introduction: Reactivity control is essential in all power and research reactors to ensure safe startup, power maneuvering, and shutdown. Control rods or functionally equivalent systems (e.g., soluble boron, burnable absorbers) are standard. Given Data / Assumptions: Normal reactor operation, not emergency-only systems. “Control rod” includes equivalent negative reactivity devices even if named differently. Reactor types listed: fast breeder, CANDU, liquid-metal-cooled, and others. Concept / Approach: Regardless of neutron spectrum or coolant, each reactor design provides controllable negative reactivity insertion to track fuel burnup, temperature feedbacks, and load changes. Fast reactors lack moderators but still rely on control rod banks and sometimes burnable poisons. CANDU reactors use adjustable control/shutoff rods and sometimes liquid zone control. Liquid-metal-cooled systems similarly use control rod mechanisms compatible with the coolant environment. Step-by-Step Solution: Identify requirement: all reactor types must manage reactivity over time.Check each listed type: all employ control devices for startup, regulation, and scram.Conclusion: none operate without reactivity control systems. Verification / Alternative check: Safety standards and design documents universally include a rapid shutdown (scram) system and routine control mechanisms irrespective of reactor class. Why Other Options Are Wrong: Fast breeder/CANDU/liquid-metal-cooled: each uses control rods or equivalent; saying they may not need them is unsafe and incorrect. Gas-cooled graphite-moderated reactors also require control systems. Common Pitfalls: Confusing the presence or absence of a moderator with the need for control rods. Moderator choice affects spectrum, not the necessity of reactivity control. Final Answer: None of these (all require control rods or equivalent reactivity control)

Nuclear Power Engineering Questions