Question 1

Radioactive decay with half-lives: a sample has a half-life of 100 days. If we start with 1 g, how much remains after 400 days (i.e., after four half-lives)?

Accepted Answer

Introduction / Context: Half-life problems test understanding of exponential decay in nuclear physics. The half-life t1/2 is the fixed interval over which a radioactive sample's quantity reduces to half its current value. This question asks how much of a 1 g sample remains after several half-lives have elapsed. Given Data / Assumptions: Initial mass, m0 = 1 g. Half-life, t1/2 = 100 days. Total time elapsed, t = 400 days. Decay follows N(t) = N0 * (1/2)^(t / t1/2). Concept / Approach: Radioactive decay is a first-order process. Each half-life reduces the remaining amount by a factor of 1/2, regardless of how much remains. The number of half-lives elapsed is n = t / t1/2. The remaining mass is m = m0 * (1/2)^n. No complicated algebra is needed when t is a whole-number multiple of the half-life—simply halve successively. Step-by-Step Solution: Compute n: n = 400 / 100 = 4 half-lives.Apply the decay formula: m = 1 * (1/2)^4.Evaluate the power: (1/2)^4 = 1/16.Hence m = 1/16 g. Verification / Alternative check: Successive halving: 1 g → 1/2 g (100 d) → 1/4 g (200 d) → 1/8 g (300 d) → 1/16 g (400 d). This confirms the calculation without formulas. Why Other Options Are Wrong: 1/2: corresponds to one half-life (100 days), not four.1/4: corresponds to two half-lives (200 days).1/8: corresponds to three half-lives (300 days). Common Pitfalls: Confusing total time with number of half-lives; adding halves linearly instead of multiplying by 1/2 each interval; rounding fractional half-lives when the time matches an exact multiple. Final Answer: 1/16

Question 2

Indian nuclear fuel strategy: fast breeder reactors are especially relevant to India primarily because the country has large deposits of which fertile resource?

Accepted Answer

Introduction / Context: India's three-stage nuclear power programme leverages indigenous resource endowments. The country has relatively modest uranium reserves but very large thorium reserves. Fast breeder and associated fuel-cycle strategies are designed to utilise fertile materials efficiently and expand fissile inventories over time. Given Data / Assumptions: India has abundant thorium-bearing monazite sands along certain coasts. Fast reactors can breed fissile isotopes from fertile nuclides. Programme aims to improve resource utilisation and energy security. Concept / Approach: A fertile isotope becomes fissile after neutron absorption and subsequent nuclear transformations. In breeder strategies, U-238 can breed to Pu-239, and thorium-232 can breed to U-233. India’s strategy exploits initial reactors to generate plutonium for fast breeders, which in turn support the thorium cycle to realise long-term energy independence. Step-by-Step Solution: Recognise resource endowment: thorium is abundant in India compared with uranium.Breeding pathways: Th-232 → U-233 in appropriate reactor/fuel cycles.Therefore, fast-breeder-related development aligns with tapping large thorium reserves. Verification / Alternative check: Policy documents and educational texts consistently describe India’s thorium-centric long-term plan, with fast reactors as a bridge to close fuel cycles and enable Th utilisation. Why Other Options Are Wrong: Plutonium: a produced fissile material, not a native large deposit.Uranium: India’s known uranium reserves are comparatively limited.None of these: contradicts the thorium reality. Common Pitfalls: Confusing breeder technology purpose (creating fissile material) with the inventory of naturally occurring deposits; assuming “plutonium deposits” exist—plutonium is man-made in reactors. Final Answer: thorium

Question 3

Moderator identification: “heavy water” used to slow neutrons in certain reactors is best described as which substance?

Accepted Answer

Introduction / Context: Moderators slow down fast neutrons to thermal energies where fission in certain fuels is more probable. Heavy water is a classic moderator used in specific reactor designs to achieve high neutron economy and enable the use of lower-enrichment or even natural uranium fuel. Given Data / Assumptions: Term: “heavy water.” Application: neutron moderation in reactors. Chemical identity is sought. Concept / Approach: Heavy water is D2O (or primarily HDO in mixtures), where hydrogen is replaced by deuterium (D), an isotope of hydrogen with one proton and one neutron. Replacing protium with deuterium reduces neutron absorption, improving moderation efficiency compared to ordinary light water (H2O). Step-by-Step Solution: Identify heavy water: molecular water with deuterium in place of protium.Note performance: lower neutron absorption cross-section than light water.Therefore, define it as a compound of oxygen and deuterium. Verification / Alternative check: In practice, reactor-grade heavy water has high D2O concentration (> 99%) for optimum neutron economy. Why Other Options Are Wrong: “Highly purified water” describes demineralised H2O, not heavy water.“Water with heavy-metal salts” changes chemistry and does not define heavy water.“None of these” contradicts the correct definition. Common Pitfalls: Assuming “heavy” refers to impurities; confusing isotopic substitution with contamination. Final Answer: A compound of oxygen and deuterium

Question 4

India's first underground nuclear test site: identify the location where the first underground test was conducted.

Accepted Answer

Introduction / Context: Understanding milestone events in a nation’s nuclear history is common general knowledge for engineering and GK exams. India’s first underground nuclear test is a frequently asked factoid tied to geography and strategic programmes. Given Data / Assumptions: The question asks for the specific site of the first underground test. We are identifying the location by name. Concept / Approach: India’s inaugural underground nuclear test was conducted at the Pokhran Test Range in Rajasthan. The site is distinct from nearby cities or nuclear power stations and is a purpose-built range in the Thar Desert region. Step-by-Step Solution: Recall the well-known test range name associated with India’s early nuclear tests.Distinguish it from other nuclear locations like Kalpakkam (power/fast-breeder programmes) or Narora (power station).Select Pokhran as the correct site. Verification / Alternative check: Historical references widely identify Pokhran as the test site for the first underground detonation, with subsequent tests also associated with the Pokhran range. Why Other Options Are Wrong: Jaisalmer: a district/city in Rajasthan, not the specific test range.Kalpakkam: a nuclear power and research site in Tamil Nadu, not the test range.Narora: a nuclear power station in Uttar Pradesh, not the test site. Common Pitfalls: Confusing administrative districts with the dedicated range; selecting a nuclear power location instead of the weapons test range. Final Answer: Pokhran

Question 5

Fissile vs fertile recognition: among the following nuclides, which one is fissile under thermal-neutron irradiation?

Accepted Answer

Introduction / Context: Fissile materials can sustain a chain reaction with thermal (slow) neutrons. Fertile materials are not fissile as-is but can be converted to fissile nuclides via neutron capture and decay. Distinguishing between them is fundamental to reactor physics and fuel-cycle engineering. Given Data / Assumptions: Thermal spectrum context. Candidate nuclides: U-238, Th-232, Pu-239. Concept / Approach: Pu-239 is fissile with thermal neutrons. U-238 and Th-232 are fertile: U-238 breeds to Pu-239; Th-232 breeds to U-233. Only Pu-239 in the list undergoes efficient thermal fission to release energy and extra neutrons for chain reaction. Step-by-Step Solution: Identify fissile candidate: Pu-239.Classify others as fertile under thermal conditions.Choose the fissile option accordingly. Verification / Alternative check: Standard fissile set includes U-233, U-235, Pu-239, Pu-241 (thermal). U-238 and Th-232 are excluded as directly fissile in thermal spectra. Why Other Options Are Wrong: U-238: fertile, needs conversion to Pu-239.Th-232: fertile, needs conversion to U-233.“None of these”: incorrect because Pu-239 is indeed fissile. Common Pitfalls: Assuming any actinide is fissile; forgetting the spectrum dependence of fission probabilities. Final Answer: Plutonium-239

Question 6

Identify the incorrect nuclear statement: which of the following is wrong based on basic atomic and nuclear physics?

Accepted Answer

Introduction / Context: This question probes basic facts about nuclear particles and radiation. Spotting the incorrect statement requires recalling what gamma radiation is and how alpha particles and isotopic masses relate. Given Data / Assumptions: Alpha particle identity. Relative masses of deuterium (≈2 u) and helium-4 (≈4 u). Nature of gamma radiation. Order-of-magnitude comparisons involving atomic vs nuclear scales. Concept / Approach: Gamma rays are high-energy photons (electromagnetic radiation), not electrons. High-energy electrons constitute beta-minus radiation. Alpha particles consist of two protons and two neutrons; when stripped of electrons, they are equivalent to He^2+ ions. Deuterium’s mass being roughly half of helium-4 is also consistent with atomic mass units. Step-by-Step Solution: Check (d): “Gamma rays are high-energy electrons” → incorrect; gammas are photons.Check (b): Alpha ≡ He nucleus → as a particle in matter, it corresponds to He^2+ (true).Check (c): Deuterium ~2 u vs helium-4 ~4 u → about half (true).Check (a): The density/scale contrast between nucleus and atom is enormous; 10^12 is an order-of-magnitude statement and acceptable for this context. Verification / Alternative check: Radiation taxonomy: alpha (He nuclei), beta (electrons/positrons), gamma (photons). Mass relations follow standard atomic masses. Why Other Options Are Wrong: (a), (b), (c) are broadly correct in the intended textbook sense.(d) misidentifies the nature of gamma radiation. Common Pitfalls: Confusing beta radiation with gamma; over-focusing on exact numeric ratios instead of order-of-magnitude reasoning for (a). Final Answer: Gamma rays are high-energy electrons.

Question 7

Units check in nuclear science: which of the following is a unit of radioactivity (historic non-SI unit)?

Accepted Answer

Introduction / Context: Radioactivity measures the rate of nuclear disintegrations per unit time. While the SI unit is the becquerel (1 Bq = 1 decay per second), older or non-SI units still appear in literature and exams. Recognising which unit corresponds to activity prevents confusion with length or area units used elsewhere in nuclear engineering. Given Data / Assumptions: Focus is on the physical quantity “activity.” Options include various nuclear-related units and constants. Concept / Approach: The curie (Ci) is a traditional unit of radioactivity defined as 3.7 * 10^10 decays per second, historically tied to the activity of 1 gram of radium-226. Other listed units are not activity: barn (nuclear cross-sectional area), fermi (length scale ~ 1 fm = 10^-15 m), angstrom (length scale = 10^-10 m). Step-by-Step Solution: Identify the quantity: activity = disintegrations per second.Recall the unit: curie (historical), SI unit is becquerel.Select “curie.” Verification / Alternative check: Conversion: 1 Ci = 3.7e10 Bq; 1 mCi = 3.7e7 Bq; 1 μCi = 3.7e4 Bq. These are standard in radiological tables. Why Other Options Are Wrong: Barn: unit of cross-section (area).Fermi and angstrom: units of length at nuclear and atomic scales, not activity. Common Pitfalls: Confusing becquerel (SI) with curie (non-SI) or mixing up barn with activity because both appear in reactor calculations. Final Answer: curie

Question 8

CANDU reactor classification: which description correctly characterises a CANDU nuclear reactor?

Accepted Answer

Introduction / Context: CANDU (CANada Deuterium Uranium) is a family of heavy-water-moderated, heavy-water-cooled power reactors. Their design allows the use of natural uranium fuel, providing flexibility in fuel supply and online refuelling capability. Given Data / Assumptions: Design uses D2O as both moderator and coolant (in separate systems). Fuel enrichment: natural uranium (≈0.711% U-235). Pressure tubes and online refuelling are typical features. Concept / Approach: Heavy water’s low neutron absorption enables criticality with natural uranium, avoiding enrichment. The reactor employs pressure tubes carrying coolant and fuel channels, surrounded by a large moderator tank (calandria) filled with heavy water. This configuration is distinct from light-water and fast breeder designs. Step-by-Step Solution: Identify moderator/coolant: both are heavy water systems.Identify fuel: natural uranium.Match to the description that states all three traits. Verification / Alternative check: Technical summaries of CANDU highlight natural-uranium fuel and heavy-water moderation/cooling as core characteristics enabling online refuelling and good neutron economy. Why Other Options Are Wrong: Highly enriched uranium: not characteristic of CANDU baseline.Homogeneous: CANDU is a heterogeneous lattice of fuel channels and moderator.Fast breeder: CANDU is a thermal-spectrum reactor, not a fast breeder. Common Pitfalls: Assuming heavy water automatically implies fast spectrum; confusing “homogeneous” with “uses one fluid” rather than neutronic geometry. Final Answer: Natural uranium fuelled, heavy water cooled and moderated

Question 9

Neutron-to-proton ratio calculation: an element has mass number A = 80 and atomic number Z = 40. What is the neutron-to-proton ratio N/Z?

Accepted Answer

Introduction / Context: Quickly determining neutron and proton counts from A and Z is a core skill in nuclear chemistry and reactor physics. Ratios like N/Z indicate stability trends and help check bookkeeping in reaction equations. Given Data / Assumptions: Mass number A = 80. Atomic number Z = 40. Definitions: Z = protons, N = neutrons = A − Z. Concept / Approach: Compute neutrons directly from A − Z, then form the ratio N/Z. Many medium-mass stable nuclides have N/Z near 1 to slightly above 1; this example lands exactly at 1. Step-by-Step Solution: Compute N: N = A − Z = 80 − 40 = 40.Compute N/Z: 40 / 40 = 1.Select the option equal to 1. Verification / Alternative check: Proton count equals neutron count here, consistent with many nuclides around the middle of the chart where N ≈ Z. Why Other Options Are Wrong: 0.5 implies N = 20 with Z = 40 (not the case).2 implies N = 80 with Z = 40 (exceeds A).4 is impossible since N cannot exceed A. Common Pitfalls: Swapping A and Z; forgetting to subtract Z from A to get N; simplifying the ratio incorrectly. Final Answer: 1

Question 10

Behaviour of activity with time: how does the disintegration rate (activity) of a radioactive sample change as time progresses?

Accepted Answer

Introduction / Context: Activity, denoted A, is the number of nuclear disintegrations per unit time in a sample. It is directly proportional to the number of undecayed nuclei present (N). As nuclei decay, N falls, and so must A. Recognising this monotonic decrease is central to understanding radiometric dating, dosimetry, and inventory calculations. Given Data / Assumptions: First-order decay: dN/dt = −λN. Activity definition: A = λN. Decay constant λ is time-independent for a given nuclide. Concept / Approach: Because A = λN and λ is constant, A follows the same exponential law as N. As time increases, N(t) = N0 * exp(−λt) declines, so A(t) = A0 * exp(−λt) also declines. There is no physical mechanism in spontaneous decay for A to increase with time in a single, isolated nuclide sample. Step-by-Step Solution: Start with A = λN.Use N(t) = N0 * exp(−λt) → A(t) = λN0 * exp(−λt) = A0 * exp(−λt).Exponential term decreases with time, so activity progressively decreases. Verification / Alternative check: Half-life reasoning: each t1/2 reduces both N and A by 1/2. After n half-lives, A = A0 * (1/2)^n, clearly showing a decrease. Why Other Options Are Wrong: “Progressively increases”: contradicts A = λN with λ constant and N decreasing.“Remains constant”: would require λ = 0 or replenishment of N.“May increase or decrease”: spontaneous decay in a closed system does not produce increases. Common Pitfalls: Confusing multi-nuclide chains (where daughter growth can temporarily increase a specific isotope’s activity) with the total sample activity of a single nuclide; mixing up count-rate instrument effects with true activity. Final Answer: Progressively decreases

Question 11

Radioactivity basics: given a radioactive nuclide with half-life T (time for activity to drop to 50%), what is its average life (mean life) expressed in terms of T?

Accepted Answer

Introduction / Context: Understanding the relationship between half-life and average life (also called mean life) is fundamental in nuclear engineering, radiological safety, and decay calculations. Half-life T is the time required for the number of undecayed nuclei (or activity) to fall to one-half its initial value, whereas the average life τ represents the statistical average lifetime of radioactive atoms before disintegration. This question checks whether you can convert between these two commonly used time scales in decay processes using the exponential decay model. Given Data / Assumptions: The decay of the nuclide follows first-order kinetics (radioactive decay law). Half-life is denoted by T. Decay constant is denoted by lambda (λ). Average life (mean life) is denoted by tau (τ). Concept / Approach: Radioactive decay obeys N(t) = N0 * exp(−λ t). Two standard identities connect λ, T, and τ: T = (ln 2) / λ and τ = 1 / λ. Eliminating λ gives τ = T / ln 2. Since ln 2 ≈ 0.693, τ ≈ 1.4427 T. For practical purposes, this is rounded to 1.44 T in hand calculations and MCQs, which is precise enough for most engineering applications. Step-by-Step Solution: 1) Start with T = (ln 2) / λ.2) Mean life is τ = 1 / λ.3) Substitute λ from step 1 into step 2: τ = T / ln 2.4) Evaluate numerically: ln 2 ≈ 0.693 → τ ≈ T / 0.693 ≈ 1.4427 T.5) Round to two decimal places for MCQ reporting: τ ≈ 1.44 T. Verification / Alternative check: Consider two half-lives (t = 2T). The fraction remaining is 0.25. The expected average time to decay must exceed T because many atoms survive beyond one half-life; the factor 1.44 aligns with this intuition and with integral definitions of mean life using the exponential distribution. Why Other Options Are Wrong: 0.144 T: off by a factor of 10; contradicts τ = T / ln 2. 14.4 T: off by a factor of 10 in the opposite direction. 2T: not derived from the decay law; no theoretical basis. T / 10: arbitrary and inconsistent with τ = 1 / λ. Common Pitfalls: Confusing half-life (median of the distribution) with mean life; using log base 10 instead of natural log; rounding ln 2 poorly. Always remember τ = T / ln 2 ≈ 1.44 T for quick estimates. Final Answer: 1.44 T

Question 12

Nuclear engineering terminology: “critical mass” is defined as the minimum mass of a fissile material required for what key condition in a chain-reacting system?

Accepted Answer

Introduction / Context: Critical mass is a foundational concept in reactor physics and nuclear safety. It specifies the smallest amount of fissile material that, with a given geometry, composition, reflector, and moderator condition, can support a self-sustaining nuclear chain reaction. Recognizing this definition helps distinguish neutronic feasibility from economic or commercial considerations. Given Data / Assumptions: Material is fissile (e.g., U-235, Pu-239). Geometry, density, and presence/absence of reflector/moderator are fixed. We are concerned with neutronic sustainment, not economics. Concept / Approach: The chain reaction is sustained when the effective multiplication factor k_eff equals 1. The critical mass is the minimum mass for which, under specified conditions, neutron production from fission balances neutron losses (absorption in non-fuel and leakage). Below this mass, k_eff < 1 and the reaction dies out (subcritical). Above this mass, k_eff > 1 unless controlled. Step-by-Step Solution: 1) Identify the neutronic criterion: k_eff = 1 for criticality.2) Recognize that mass, geometry, and reflectors affect leakage and absorption.3) The smallest mass at which k_eff reaches 1 is the “critical mass.”4) Therefore, the correct definition focuses on sustainment of the chain reaction, not economic scale. Verification / Alternative check: Textbook relationships (e.g., six-factor formula) and diffusion theory show how size affects neutron leakage; increasing system size reduces surface-to-volume ratio, lowering leakage and enabling k_eff → 1. Why Other Options Are Wrong: Commercial or economic power generation depends on plant engineering and costs, not simply on reaching criticality. “None of these” is false because one option is correct. Prompt-critical excursion is a transient safety condition, not the definition of critical mass. Common Pitfalls: Equating critical mass with power plant sizing; ignoring the roles of reflectors and moderators which substantially reduce the critical mass. Final Answer: Sustainment of chain reaction.

Question 13

Coolants and moderators: heavy water (D2O) is preferred over ordinary water in some reactors primarily because it acts as an efficient moderator while also serving as a coolant. Which statement best reflects this advantage?

Accepted Answer

Introduction / Context: Reactor designers choose coolants and moderators to optimize neutron economy and heat removal. Heavy water (D2O) is prized in certain thermal reactors, such as pressurized heavy water reactors (PHWRs), because it is a superb moderator that slows neutrons with minimal parasitic capture while also functioning as a coolant to carry heat away from the core. Given Data / Assumptions: D2O has a much lower thermal-neutron absorption cross-section than H2O. It remains a liquid under pressure at typical reactor operating temperatures. We compare moderation effectiveness and practical coolant behavior. Concept / Approach: An ideal moderator slows neutrons efficiently via scattering while absorbing very few, thereby enabling a self-sustaining chain reaction even with fuels of low fissile enrichment (including natural uranium). Heavy water meets this goal better than light water. As a coolant, D2O exhibits broadly similar thermophysical properties to H2O, making it usable in conventional heat-removal circuits when appropriately pressurized. Step-by-Step Solution: 1) Compare moderation: D2O has lower neutron absorption → better neutron economy.2) Assess coolant role: D2O removes heat effectively when pressurized, analogous to H2O systems.3) Conclude that the principal advantage is its dual role, especially its superior moderation, enabling natural-uranium fuel cycles. Verification / Alternative check: PHWR designs demonstrate high conversion ratios and the ability to use natural uranium due to D2O moderation. Operating conditions still require pressurization to raise boiling temperature, similar to light-water designs. Why Other Options Are Wrong: (b) Incorrect: D2O does not eliminate the need for pressurization; its normal boiling point is only slightly above H2O. (c) Incorrect: D2O undergoes radiolysis; it is not immune to radiation damage (though rates differ from H2O). (d) Incorrect because (b) and (c) are not fully true as stated. (e) Incorrect: D2O’s key benefit is moderation, not merely corrosion effects. Common Pitfalls: Assuming heavy water allows high-temperature operation without pressure; ignoring radiolysis management common to water coolants. Final Answer: Acts both as an efficient coolant as well as a moderator.

Question 14

Subatomic masses: a free neutron has a rest mass approximately equal to the mass of which neutral atom?

Accepted Answer

Introduction / Context: Comparing subatomic particle masses with atomic masses offers useful intuition in nuclear physics. The neutron is electrically neutral and slightly heavier than the proton. Matching its mass to a familiar neutral atom highlights scale and aids in back-of-the-envelope estimates. Given Data / Assumptions: Neutron rest mass ≈ 1.008665 atomic mass units. Hydrogen atom (protium) mass ≈ 1.007825 atomic mass units. Deuterium atom mass ≈ 2.0141 atomic mass units; helium atom ≈ 4.0026 atomic mass units. Concept / Approach: The hydrogen atom consists of one proton and one electron. Despite the electron’s small mass, the hydrogen atom’s total mass is very close to 1 u and is near the neutron’s mass. Deuterium and helium atoms are about two and four atomic mass units, respectively, and therefore not “approximately equal” to the neutron mass. Step-by-Step Solution: 1) List representative masses: m_n ≈ 1.008665 u; m_H(atom) ≈ 1.007825 u.2) Compare differences: |m_n − m_H| ≈ 0.00084 u, a small relative deviation.3) Conclude the closest neutral atom mass to a neutron is hydrogen. Verification / Alternative check: Mass-energy equivalence (E = m*c^2) tracks the small mass differences; binding energies account for fine differences between free nucleons and bound atomic systems. Why Other Options Are Wrong: Deuterium: roughly twice the neutron mass. Helium: roughly four times the neutron mass. None of these/Lithium: much larger deviations from neutron mass. Common Pitfalls: Confusing “hydrogen nucleus” (a proton) with the “hydrogen atom” (proton + electron) when comparing masses; overlooking orders of magnitude. Final Answer: Hydrogen

Question 15

Minerals and ores: identify the principal ore source of thorium that is widely associated with beach sand placer deposits.

Accepted Answer

Introduction / Context: Thorium is a fertile material of strategic interest for advanced nuclear fuel cycles. Knowing its geological sources is essential for resource planning and fuel-cycle education. In many coastal regions, thorium is recovered from monazite-bearing beach sands. Given Data / Assumptions: Monazite is a rare-earth phosphate mineral that often contains thorium and sometimes uranium. Other listed minerals are principal ores of different metals (e.g., galena for lead, pitchblende for uranium). Concept / Approach: Monazite concentrates recovered from heavy-mineral beach sands are a traditional source of thorium compounds. After beneficiation, chemical processing extracts thorium (commonly as ThO2 or related compounds) for industrial or research uses. Step-by-Step Solution: 1) Map minerals to metals: pitchblende → uranium; galena → lead; limonite → iron; cassiterite → tin.2) Identify monazite as the thorium-bearing mineral common in placers.3) Therefore, monazite sand is the principal ore of thorium in many regions. Verification / Alternative check: Resource literature and historical production records consistently cite monazite as a primary thorium source, especially in India and Brazil. Why Other Options Are Wrong: Pitchblende is primarily uranium ore, not thorium. Limonite is an iron ore; galena is lead sulfide; cassiterite is tin oxide. Common Pitfalls: Confusing uranium and thorium ores due to both being actinides; overlooking that “monazite sand” refers to heavy-mineral concentrates within beach sands. Final Answer: Monazite sand

Question 16

Reactor control: what is the primary function of control rods inserted into a nuclear reactor core?

Accepted Answer

Introduction / Context: Maintaining reactor criticality and power levels requires precise control of the neutron population. Control rods are essential safety and control devices in thermal and many fast reactors. They enable operators and automated systems to adjust reactivity quickly and reliably. Given Data / Assumptions: Control rods are typically made from materials with very high neutron-absorption cross-sections (e.g., boron carbide, silver-indium-cadmium alloys, cadmium). Reactivity adjustment is achieved by varying insertion depth. Concept / Approach: By absorbing neutrons, control rods reduce the number of neutrons available to cause fission events. Inserting rods increases overall absorption, decreasing k_eff; withdrawing rods has the opposite effect. Temperature and pressure are affected secondarily via power changes, but rods do not directly regulate them. Step-by-Step Solution: 1) Identify desired control variable: reactivity (neutron population growth/decay).2) Recognize material choice: strong neutron poisons maximize absorption.3) Mechanism: insert to reduce reactivity; withdraw to increase.4) Conclude the primary function is absorption of neutrons to control chain reaction. Verification / Alternative check: Start-up, shutdown, and scram procedures rely on rapid rod insertion to drive the reactor subcritical, demonstrating their neutron-absorption role. Why Other Options Are Wrong: Temperature/pressure are plant parameters influenced indirectly; they are not directly controlled by rods. Fuel consumption schedules are operational logistics, not a rod function. Converting thermal to fast neutrons is not a rod function; moderators and spectra define neutron energies. Common Pitfalls: Confusing coolant-system controls (pressurizers, valves) with neutronic controls; assuming rods directly regulate thermodynamic variables. Final Answer: Absorption of neutrons

Question 17

Isotope counting: how many neutrons are present in the nucleus of Uranium-233 (denoted 92U233)?

Accepted Answer

Introduction / Context: Interpreting nuclide notation is a basic skill in nuclear science. The superscript denotes the mass number A (total protons + neutrons), and the subscript is the atomic number Z (protons). The difference A − Z yields the number of neutrons N in the nucleus. Given Data / Assumptions: Nuclide: Uranium-233 with A = 233 and Z = 92 (uranium). We want N, the neutron count. Concept / Approach: The relationship N = A − Z is universal for nuclides. Using Uranium-233 values gives a straightforward arithmetic result. Step-by-Step Solution: 1) Write the known values: A = 233, Z = 92.2) Compute neutrons: N = A − Z = 233 − 92.3) Evaluate: N = 141. Verification / Alternative check: Compare with Uranium-235, which has N = 235 − 92 = 143, confirming that U-233 has two fewer neutrons than U-235, consistent with isotope definitions. Why Other Options Are Wrong: 92: equals proton number (Z), not neutrons. 233: total nucleons, not neutrons alone. 325: not physically related to A or Z here. 115: incorrect subtraction. Common Pitfalls: Mixing up A and Z; forgetting that A includes both protons and neutrons. Always verify with N = A − Z. Final Answer: 141

Question 18

Fusion feasibility: why is commercial-scale power generation from controlled fusion reactors not yet a reality?

Accepted Answer

Introduction / Context: Fusion promises abundant, low-carbon energy by combining light nuclei (e.g., D + T) to release large amounts of energy. However, translating this promise into an operating power plant requires meeting stringent physics and engineering conditions, notably ignition criteria and steady-state operation with net energy gain. Given Data / Assumptions: Practical fuels: deuterium (abundant in seawater) and tritium (bred from lithium). Initiation requires extremely high temperatures and adequate particle density with sufficient confinement time. Lawson criterion provides the product n * T * τ_E needed for net energy. Concept / Approach: The central obstacle is simultaneously achieving and confining the required plasma temperature, density, and energy confinement time. Reaching these conditions without intolerable losses (radiation, turbulence, disruptions) is challenging. While control is also complex, the primary barrier to date is establishing and maintaining the initiation/ignition conditions with a positive energy balance on a commercial scale. Step-by-Step Solution: 1) Identify the physics requirement: satisfy Lawson criterion for D–T fusion.2) Recognize two major pathways: magnetic confinement (tokamaks, stellarators) and inertial confinement (laser-driven).3) Both approaches struggle with achieving/maintaining the necessary n, T, and τ_E while minimizing losses.4) Therefore, the principal reason is the difficulty in initiating and confining a plasma at fusion conditions with net gain. Verification / Alternative check: Experimental devices have achieved significant fusion yields for short periods, but continuous, economically viable operation with robust tritium breeding and power conversion remains unrealized. Why Other Options Are Wrong: (a) “Control” is challenging, but the chief barrier has been achieving ignition-level conditions long enough for net energy. (b) Deuterium is abundant; tritium can be bred from lithium—scarcity is not the primary blocker. (d) Fusion requires tiny fuel masses; the issue is conditions, not quantity. (e) Neutron effects matter for materials, not immediate plasma “poisoning.” Common Pitfalls: Assuming fuel supply is the main limitation; conflating plasma control issues with the more fundamental challenge of reaching and sustaining ignition parameters. Final Answer: It is difficult to initiate fusion by achieving and confining the required temperature and density.

Question 19

Fast breeder reactor coolants: why is molten sodium commonly selected as the primary coolant in fast breeder reactors?

Accepted Answer

Introduction / Context: Fast breeder reactors operate with a fast neutron spectrum, requiring coolants that do not moderate neutrons while efficiently transporting heat from the core to the heat exchangers. Liquid metals, particularly molten sodium, satisfy these requirements and have been used extensively in experimental and prototype breeder systems. Given Data / Assumptions: Sodium has high thermal conductivity and a high boiling point at near-atmospheric pressure. Fast reactors avoid moderators to maintain a fast spectrum for breeding (e.g., from U-238 to Pu-239). Coolant selection should minimize neutron absorption and moderation while maximizing heat removal. Concept / Approach: Molten sodium efficiently removes heat because of its excellent thermal properties and allows operation at high temperatures without high system pressure. Its low neutron moderation maintains fast-spectrum conditions, which is critical for breeding performance. Additionally, sodium's low vapor pressure contributes to plant safety margins relative to pressure excursions. Step-by-Step Solution: 1) Identify coolant requirements for fast spectrum: minimal moderation, strong heat transfer.2) Evaluate sodium: high thermal conductivity, high boiling point → strong heat removal at low pressure.3) Confirm neutronic compatibility: low moderation keeps spectrum fast.4) Therefore, sodium is chosen primarily for rapid heat removal under suitable neutronic conditions. Verification / Alternative check: Operational experience from sodium-cooled fast reactors demonstrates effective heat transport, with design attention to chemical reactivity of sodium with water/air and the need for intermediate loops to isolate sodium from steam generators. Why Other Options Are Wrong: (a) Incorrect: sodium is a poor moderator, which is actually desirable for fast reactors. (b) Incorrect: breeding occurs in fertile material (e.g., U-238), not in the coolant. (d) Incorrect: coolant does not directly increase reaction rate; it affects temperature and feedbacks but not by design to raise fission rate. (e) Incorrect and irrelevant to reactor physics. Common Pitfalls: Confusing moderation needs of thermal reactors with fast reactors; ignoring sodium's chemical reactivity and the design measures taken to mitigate it. Final Answer: Because it enables rapid heat removal due to high thermal conductivity and high boiling point.

Question 20

Uranium extraction: the recovery of uranium from its ores (e.g., pitchblende) is primarily accomplished using which type of processing method?

Accepted Answer

Introduction / Context: Uranium production involves mining, beneficiation, and extraction of uranium values into a concentrated product (commonly yellowcake). The core step is converting uranium-bearing minerals into soluble forms so that uranium can be separated and purified for the nuclear fuel cycle. Given Data / Assumptions: Typical uranium ores include pitchblende (uraninite) and other minerals. Industrial practice favors hydrometallurgical routes for extraction. Beneficiation (crushing, grinding, sizing) precedes chemical leaching. Concept / Approach: Hydrometallurgy employs chemical leaching to dissolve uranium from ore. Common leaching systems are sulfuric-acid-based oxidizing leaches, carbonate/bicarbonate leaches in the presence of oxidants, and in-situ leach (ISL) operations in permeable deposits. Post-leach solutions are processed by ion exchange or solvent extraction, followed by precipitation/calcination to produce U3O8. Step-by-Step Solution: 1) Prepare ore by comminution and, if applicable, physical upgrading.2) Dissolve uranium using chemical leaching (acidic or alkaline with oxidant).3) Recover uranium from solution using ion exchange or solvent extraction.4) Precipitate and calcine to yellowcake specification. Verification / Alternative check: Industry flowsheets across major uranium-producing countries consistently rely on hydrometallurgy; pyrometallurgy is uncommon for primary uranium extraction from ores. Why Other Options Are Wrong: Electrometallurgical and electrolytic methods are not the primary route for ore extraction. Pyrometallurgical methods are rare for initial uranium recovery from ores. Physical beneficiation alone cannot produce concentrated uranium product without leaching. Common Pitfalls: Assuming uranium extraction mirrors base-metal smelting; confusing beneficiation with hydrometallurgical extraction. Final Answer: Chemical (hydrometallurgical) methods

Nuclear Power Engineering Questions