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Question 1Nuclear Science and Technology

In a thermal reactor, the main reason heavy water (D₂O) is a better moderator than light water (H₂O) is that deuterium has a smaller neutron absorption cross-section, which allows a reactor to operate with natural uranium. Which additional property of D₂O supports its role as a moderator?

  1. Deuterium's mass is closer to that of a neutron, giving fewer collisions needed to thermalize. Correct answer.
  2. Deuterium has a lower mass than protium, so energy transfer per collision is larger.
  3. Deuterium has a larger scattering cross-section than protium at thermal energies.
  4. Heavy water has a lower boiling point, improving heat removal.

Explanation

Moderation efficiency depends on how much energy a neutron loses per elastic collision, which is greatest when the moderator nucleus mass is close to the neutron mass. Deuterium (mass 2) is closer to a neutron than protium (mass 1), so each collision removes a larger fraction of neutron energy and fewer collisions are needed to reach thermal energies. Its very low absorption cross-section is what permits natural uranium fuel.

Question 2Nuclear Science and Technology

The effective multiplication factor k_eff of a reactor is defined as the ratio of the number of neutrons in one generation to the number in the preceding generation. A reactor operating at steady-state criticality has which value of k_eff?

  1. k_eff < 1
  2. k_eff > 1
  3. k_eff = 0
  4. k_eff = 1 Correct answer.

Explanation

Criticality means the neutron population is constant from generation to generation, so the production and loss rates balance exactly and k_eff equals 1. If k_eff were less than 1 the chain reaction would die away (subcritical), and if greater than 1 it would grow (supercritical). Zero would mean no neutron multiplication at all.

Question 3Nuclear Science and Technology

In a pressurised heavy water reactor (PHWR) using natural uranium dioxide fuel, the coolant and moderator are kept in separate circuits. What is the principal reason for this separation?

  1. To allow the use of enriched uranium fuel in the coolant channels.
  2. To eliminate the need for control rods in the reactor.
  3. To permit the use of light water as the moderator.
  4. To prevent the moderator from boiling and losing its moderating efficiency while allowing the coolant to be at high pressure. Correct answer.

Explanation

In a PHWR the heavy-water moderator is kept at low temperature and near atmospheric pressure in a calandria, while the heavy-water coolant in pressure tubes is at high pressure and temperature. Separating the circuits prevents moderator boiling and density changes that would alter reactivity. This design also allows on-power refuelling.

Question 4Nuclear Science and Technology

Which of the following best describes the function of a control rod in a nuclear reactor?

  1. It increases the number of fission neutrons by reflecting them back into the core.
  2. It moderates fast neutrons to thermal energies to sustain the chain reaction.
  3. It absorbs neutrons to reduce reactivity and regulate or shut down the chain reaction. Correct answer.
  4. It converts fertile material into fissile material in the blanket region.

Explanation

Control rods are made of strong neutron absorbers such as boron, cadmium, or hafnium. Inserting them removes neutrons from the chain reaction, lowering reactivity, and full insertion provides shutdown capability. They do not moderate neutrons or breed fuel.

Question 5Nuclear Science and Technology

The radioactive decay of a radionuclide follows first-order kinetics with a half-life of 8 days. What fraction of the initial activity remains after 24 days?

  1. 1/16
  2. 1/8 Correct answer.
  3. 1/2
  4. 1/4

Explanation

Twenty-four days equals three half-lives (24 ÷ 8 = 3). After each half-life the activity halves, so after three half-lives the remaining fraction is (1/2)³ = 1/8. This follows directly from the first-order decay law A = A₀ e^(−λt).

Question 6Nuclear Science and Technology

Which type of radiation has the greatest penetrating power in matter and therefore requires the heaviest shielding in a nuclear facility?

  1. Gamma rays Correct answer.
  2. Beta particles
  3. Thermal neutrons
  4. Alpha particles

Explanation

Gamma rays are high-energy photons with no charge or mass, so they interact only probabilistically and can penetrate thick layers of material. They require dense, high-atomic-number shielding such as lead or concrete. Alpha and beta particles are charged and lose energy rapidly, while thermal neutrons are readily captured by suitable absorbers.

Question 7Nuclear Science and Technology

In the nuclear fuel cycle, the term 'enrichment' refers to which process?

  1. Increasing the concentration of the fissile isotope U-235 relative to U-238 in uranium. Correct answer.
  2. Compacting uranium dioxide powder into fuel pellets by sintering.
  3. Removing fission products from irradiated fuel by chemical reprocessing.
  4. Converting uranium ore concentrate into uranium hexafluoride gas.

Explanation

Natural uranium contains about 0.7% U-235; enrichment raises this fraction to the level needed by a given reactor type, typically 3–5% for light-water reactors. Enrichment exploits the small mass difference between U-235 and U-238 isotopes using gaseous diffusion or gas centrifuges.

Question 8Nuclear Science and Technology

Which of the following particles is emitted in beta-minus decay?

  1. A gamma-ray photon
  2. An alpha particle
  3. An electron and an electron antineutrino Correct answer.
  4. A proton

Explanation

In beta-minus decay a neutron in the nucleus converts into a proton, emitting an electron (beta particle) and an electron antineutrino to conserve lepton number and energy. The atomic number increases by one while the mass number stays the same.

Question 9Nuclear Science and Technology

A reactor has a prompt neutron lifetime of about 10⁻⁴ s and a delayed neutron fraction of 0.0065. If the reactor is made supercritical by a reactivity of 0.002 (200 pcm), which statement best describes the resulting behaviour?

  1. The reactor remains subcritical because 200 pcm is less than the delayed fraction.
  2. The reactor is prompt-critical and power rises on the prompt neutron lifetime timescale.
  3. The reactor is exactly critical and the power remains constant.
  4. The reactor is delayed-critical and power rises on a much longer timescale governed by delayed neutrons. Correct answer.

Explanation

The inserted reactivity of 0.002 is less than the delayed neutron fraction β of 0.0065, so the reactor is delayed-critical. Power rises with a period governed largely by the delayed neutron precursors, which is far slower than the 10⁻⁴ s prompt lifetime and therefore controllable. Prompt criticality would require inserting reactivity exceeding β.

Question 10Nuclear Science and Technology

In a thermal reactor, the six-factor formula for the infinite multiplication factor is k∞ = ε p f η. Which factor accounts for resonance neutron capture in U-238 during slowing down?

  1. f (thermal utilization factor)
  2. η (reproduction factor)
  3. p (resonance escape probability) Correct answer.
  4. ε (fast fission factor)

Explanation

The resonance escape probability p is the fraction of neutrons that slow down past the U-238 resonance capture energies without being absorbed. A larger p means fewer neutrons are lost to resonance absorption, increasing k∞. The other factors describe fast fission, thermal utilization, and neutrons produced per thermal absorption.

Question 11Nuclear Science and Technology

Which mechanism is primarily responsible for the negative moderator temperature coefficient in a light-water reactor?

  1. The delayed neutron fraction increases with moderator temperature.
  2. The fuel Doppler broadening of resonance absorption increases as fuel temperature rises.
  3. The moderator density decreases as its temperature rises, reducing moderation and increasing resonance capture. Correct answer.
  4. The control rods automatically insert deeper as the moderator heats up.

Explanation

In a light-water reactor, raising the moderator temperature lowers its density, which reduces moderation and shifts the neutron spectrum toward higher energies. This increases resonance absorption in U-238 and leakage, giving a negative moderator temperature coefficient. Doppler broadening is a separate fuel-temperature feedback, not the moderator coefficient.

Question 12Nuclear Science and Technology

A point source of gamma radiation gives a dose rate of 400 μSv/h at 2 m. Ignoring attenuation in air, what is the approximate dose rate at 8 m?

  1. 50 μSv/h
  2. 12.5 μSv/h
  3. 100 μSv/h
  4. 25 μSv/h Correct answer.

Explanation

For a point source the dose rate falls off with the inverse square of distance: D₂ = D₁ (r₁/r₂)². Here the distance increases by a factor of 4, so the dose rate drops by 4² = 16, giving 400/16 = 25 μSv/h.

Question 13Nuclear Science and Technology

In a boiling water reactor (BWR), the steam produced in the core passes directly to the turbine. Which design consequence follows from this direct cycle?

  1. The turbine hall must be shielded and monitored because the steam is slightly radioactive. Correct answer.
  2. The reactor needs a pressuriser to keep the coolant subcooled.
  3. The reactor must use enriched uranium at more than 20% U-235.
  4. The reactor requires heavy water as moderator to allow direct-cycle operation.

Explanation

In a BWR the steam leaving the core carries short-lived activation products such as N-16, so the turbine and associated piping must be shielded and radiation-monitored. The direct cycle also means the turbine is part of the radiological controlled area. BWRs use enriched uranium and light water and do not need a pressuriser.

Question 14Nuclear Science and Technology

Which quantity is a measure of the biological effectiveness of a given radiation type and is used to convert absorbed dose to equivalent dose?

  1. The linear attenuation coefficient μ
  2. The absorbed dose expressed in gray
  3. The radiation weighting factor w_R Correct answer.
  4. The mass energy-absorption coefficient

Explanation

Equivalent dose is the product of absorbed dose and the radiation weighting factor w_R, which reflects the relative biological effectiveness of the radiation type (for example, 1 for photons and 20 for alpha particles). The result is expressed in sieverts. The other quantities describe energy deposition and attenuation, not biological effectiveness.

Question 15Nuclear Science and Technology

In a nuclear reactor, xenon-135 is a major fission product poison. Which statement correctly describes the behaviour of xenon-135 after reactor shutdown?

  1. Xenon-135 concentration remains constant because it is a stable isotope.
  2. Xenon-135 concentration decreases immediately because its production from fission stops.
  3. Xenon-135 concentration increases indefinitely because it has an infinite half-life.
  4. Xenon-135 concentration first increases because it continues to be produced from iodine-135 decay, then decreases. Correct answer.

Explanation

After shutdown, direct production of Xe-135 from fission stops, but its precursor I-135 (half-life about 6.6 hours) continues to decay into Xe-135. Because Xe-135 is removed mainly by neutron absorption, which also stops at shutdown, its concentration rises for several hours before decaying with its own half-life (about 9.2 hours), producing the post-shutdown xenon peak.

Question 16Nuclear Science and Technology

The thermal neutron flux in a reactor is 5 × 10¹³ neutrons cm⁻² s⁻¹. Given the microscopic absorption cross-section of U-235 for thermal neutrons as 681 barns and its atom density in the fuel as 2 × 10²¹ atoms cm⁻³, what is the approximate macroscopic absorption cross-section of U-235?

  1. 0.136 cm⁻¹
  2. 1.36 cm⁻¹ Correct answer.
  3. 13.6 cm⁻¹
  4. 0.0136 cm⁻¹

Explanation

The macroscopic cross-section is Σ = Nσ, where N is the atom density and σ the microscopic cross-section. Converting 681 barns to 681 × 10⁻²⁴ cm² gives Σ = (2 × 10²¹)(681 × 10⁻²⁴) ≈ 1.36 cm⁻¹. The flux value is not needed for this calculation.

Question 17Nuclear Science and Technology

Which of the following is the correct description of the role of the calandria in a pressurised heavy water reactor?

  1. It is the steam generator that transfers heat from the primary to the secondary circuit.
  2. It is the fuelling machine that loads fuel bundles during operation.
  3. It is the pressure vessel that contains the high-pressure primary coolant.
  4. It houses the heavy-water moderator at low pressure and surrounds the pressure tubes containing the coolant. Correct answer.

Explanation

In a PHWR the calandria is a large low-pressure vessel filled with heavy-water moderator. It surrounds the horizontal pressure tubes through which the high-pressure coolant flows, keeping the two heavy-water systems physically separated. This arrangement allows the moderator to remain cool and near atmospheric pressure.

Question 18Nuclear Science and Technology

During a loss-of-coolant accident in a light-water reactor, emergency core cooling systems are designed to keep the fuel cladding temperature below a specified limit. What is the main physical reason for limiting cladding temperature?

  1. To prevent the cladding from melting and releasing fission products. Correct answer.
  2. To maintain the moderator at its optimum density for reactivity control.
  3. To keep the coolant from freezing in the loop.
  4. To prevent the formation of xenon-135 in the fuel.

Explanation

Zirconium-alloy cladding undergoes rapid oxidation with steam at high temperature, generating heat and hydrogen and weakening the cladding. Limiting cladding temperature preserves the first barrier against fission product release and prevents cladding failure or melting. Xenon production and moderator density are not the safety criteria involved.

Question 19Nuclear Science and Technology

Which detector is most suitable for measuring thermal neutron flux in a reactor because of the high capture cross-section of the nuclide used?

  1. A Geiger–Müller counter filled with argon
  2. A boron trifluoride (BF₃) proportional counter Correct answer.
  3. An ionisation chamber filled with air at atmospheric pressure
  4. A scintillation detector using sodium iodide

Explanation

BF₃ proportional counters use the high thermal-neutron capture cross-section of B-10, which produces an alpha particle and a lithium ion that deposit charge in the gas. This gives a strong, easily distinguished signal proportional to neutron flux. The other detectors respond mainly to gamma radiation or have low neutron sensitivity.

Question 20Nuclear Science and Technology

In a uranium-fuelled thermal reactor, the fuel temperature coefficient of reactivity is negative mainly because of Doppler broadening. Which statement correctly explains this effect?

  1. Heating the fuel decreases the U-235 fission cross-section at thermal energies only.
  2. Heating the fuel increases the moderator density, improving moderation.
  3. Heating the fuel shifts the neutron spectrum to higher energies, increasing fission in U-238.
  4. Heating the fuel broadens U-238 resonance absorption peaks, increasing neutron capture in the resonances. Correct answer.

Explanation

As fuel temperature rises, thermal motion of U-238 nuclei broadens the resonance absorption peaks. Because the resonances are already deep, the increase in absorption in the wings outweighs the decrease at the peak, so more neutrons are captured and reactivity falls. This Doppler effect acts very rapidly and is a key inherent safety feature.

Question 21Nuclear Science and Technology

Which of the following best describes the principle of a nuclear reactor's inherent safety in relation to the void coefficient?

  1. The void coefficient is independent of the moderator and depends only on the fuel enrichment.
  2. A positive void coefficient means steam bubbles increase reactivity, which is desirable for stability.
  3. A negative void coefficient means the formation of voids reduces reactivity, providing a self-limiting response. Correct answer.
  4. A zero void coefficient guarantees the reactor cannot become supercritical.

Explanation

The void coefficient describes how reactivity changes when voids (steam bubbles) form in the coolant or moderator. A negative void coefficient means void formation reduces reactivity, so the reactor tends to self-limit rather than escalate. Light-water reactors are designed with negative void coefficients for this reason.

Question 22Nuclear Science and Technology

In a gas centrifuge enrichment cascade, the separation of U-235 from U-238 relies on which physical principle?

  1. The isotopes have different boiling points, allowing fractional distillation.
  2. The isotopes emit different amounts of radiation, allowing electromagnetic separation.
  3. The small mass difference between the isotopes causes slightly different centrifugal forces in the rotating gas. Correct answer.
  4. The isotopes have different chemical reactivities with the centrifuge wall.

Explanation

In a gas centrifuge, UF₆ gas is spun at high speed so the heavier U-238-bearing molecules concentrate near the wall while the lighter U-235-bearing molecules remain closer to the axis. The small mass difference allows a modest separation per stage, so many stages are connected in a cascade. Chemical reactivity and boiling point differences are not the basis of this process.

Question 23Nuclear Science and Technology

A reactor is operating at a steady power level when a small positive reactivity is inserted and then held constant. Which statement best describes the resulting power history?

  1. Power decreases immediately because the inserted reactivity is positive.
  2. Power rises exponentially at first and then approaches a new steady level as feedback effects balance the inserted reactivity. Correct answer.
  3. Power rises linearly and then remains constant at the new level.
  4. Power remains constant because the reactor was already critical.

Explanation

A step insertion of positive reactivity makes the reactor supercritical, so power rises on a period determined by the reactor kinetics. As power and temperature increase, negative feedback effects such as Doppler and moderator feedback reduce reactivity, and the reactor settles at a new steady power where feedback cancels the inserted reactivity. Without feedback the power would continue to rise exponentially.

Question 24Nuclear Science and Technology

In the design of a spent fuel storage pool, the fuel assemblies are typically arranged in a geometry that ensures subcriticality even if the pool is flooded or drained. Which parameter is most directly controlled by this geometry to maintain subcriticality?

  1. The thermal neutron flux level in the pool
  2. The gamma dose rate at the pool surface
  3. The neutron leakage and absorption relative to fissile mass, expressed through k_eff Correct answer.
  4. The decay heat removal rate by natural circulation

Explanation

Spent fuel storage arrays are designed so that k_eff remains below 1 under all credible moderation and flooding conditions. This is achieved by controlling spacing, using neutron-absorbing materials such as borated racks, and limiting the fissile mass per unit volume so that leakage and absorption dominate. Dose rate and decay heat removal are separate safety considerations.

Question 25Nuclear Science and Technology

In a thermal reactor, the fuel Doppler coefficient and the moderator temperature coefficient both provide negative feedback, but they act on different timescales. Which statement correctly distinguishes their response times?

  1. The Doppler coefficient responds almost instantaneously with fuel temperature, while the moderator coefficient responds more slowly with bulk coolant temperature changes. Correct answer.
  2. Neither coefficient responds to temperature; both depend only on control rod position.
  3. Both coefficients act on the same timescale because both depend on the coolant temperature.
  4. The moderator coefficient responds instantaneously, while the Doppler coefficient requires hours to develop.

Explanation

Doppler broadening depends on the fuel temperature itself and responds within milliseconds, making it a very fast inherent feedback. The moderator temperature coefficient depends on changes in bulk coolant or moderator temperature, which are governed by heat transfer and flow and therefore develop over seconds to minutes. This difference in timescale is important in transient analysis.

Question 26Nuclear Science and Technology

In a heavy-water-moderated reactor such as a PHWR, heavy water is preferred over light water primarily because of which nuclear property?

  1. Its high thermal neutron absorption cross-section, which reduces the critical size of the core
  2. Its very low thermal neutron absorption cross-section combined with good neutron slowing-down power Correct answer.
  3. Its high boiling point, which allows the moderator to operate as a single-phase coolant at high pressure
  4. Its ability to moderate neutrons without any inelastic scattering events

Explanation

Deuterium has a much smaller thermal neutron capture cross-section than protium, so heavy water moderates neutrons with very little parasitic loss, giving a high moderating ratio. This permits natural uranium to be used as fuel. The slowing-down ability comes from the light mass of deuterium relative to the neutron.

Question 27Nuclear Science and Technology

Which statement correctly describes the function of control rods in a thermal reactor during normal power operation?

  1. They remove heat from the core during a loss-of-coolant accident
  2. They convert fast neutrons into thermal neutrons to sustain the chain reaction
  3. They adjust reactivity by changing neutron absorption, compensating for burnup and regulating power Correct answer.
  4. They provide the primary long-term heat sink after reactor shutdown

Explanation

Control rods contain strong neutron absorbers such as boron, cadmium, or hafnium, and their insertion depth changes the neutron balance and hence reactivity. They are used for power regulation, shutdown, and compensating reactivity changes from burnup and xenon. They are not a cooling or moderation system.

Question 28Nuclear Science and Technology

In a nuclear reactor, the term 'reactivity' is conventionally defined in terms of the multiplication factor k. Which expression gives reactivity ρ in dollars when k is close to unity?

  1. ρ = k·β, expressed in dollars by multiplying by the delayed neutron fraction β
  2. ρ = (1 − k)/k, expressed in dollars by dividing by the prompt neutron lifetime
  3. ρ = ln(k), expressed in dollars by dividing by the effective delayed neutron fraction β
  4. ρ = (k − 1)/k, expressed in dollars by dividing by the delayed neutron fraction β Correct answer.

Explanation

Reactivity is defined as ρ = (k − 1)/k, which is positive when the reactor is supercritical and negative when subcritical. Expressing it in dollars means dividing this value by the effective delayed neutron fraction β, so one dollar corresponds to ρ = β. This normalisation is useful because it measures how far the reactor is from prompt criticality.

Question 29Nuclear Science and Technology

Which of the following best explains why a reactor cannot be made prompt critical during normal operation?

  1. Because the moderator temperature coefficient is always positive
  2. Because reactivity insertions are kept well below the delayed neutron fraction β Correct answer.
  3. Because control rod drives are mechanically limited to slow insertion speeds
  4. Because the delayed neutron fraction is large enough to keep k below unity

Explanation

Prompt criticality occurs when reactivity reaches or exceeds β, at which point the chain reaction can be sustained by prompt neutrons alone and the power rises on the prompt neutron lifetime. Operational limits, interlocks, and design keep inserted reactivity far below β so that power changes remain governed by delayed neutrons. This provides the time constants needed for control and protection systems to act.

Question 30Nuclear Science and Technology

A radionuclide has a physical half-life of 8 days and is also removed from the reactor coolant by a purification system with a constant removal half-time of 8 days. What is the effective half-life of the activity in the coolant?

  1. 16 days
  2. 8 days
  3. 5.7 days
  4. 4 days Correct answer.

Explanation

For two independent first-order removal processes acting in parallel, the decay constants add: λ_eff = λ_physical + λ_removal. Equal half-lives mean equal decay constants, so λ_eff = 2λ, giving T_eff = T/2 = 4 days. Effective half-life is always shorter than the shortest individual half-life.

Question 31Nuclear Science and Technology

Which of the following is the principal reason that a nuclear reactor core is arranged in a lattice with discrete fuel assemblies separated by moderator?

  1. To allow neutrons born in fission to slow down in the moderator before being absorbed in neighbouring fuel Correct answer.
  2. To reduce the total mass of uranium required for criticality by minimising the moderator volume
  3. To make the fuel elements easier to manufacture and inspect
  4. To ensure that all fission neutrons are absorbed within the same fuel pin where they were born

Explanation

A lattice separates fuel and moderator so that fast fission neutrons can lose energy through scattering in the moderator before encountering fuel again. This reduces resonance capture in U-238 and improves the thermal utilisation factor. The spacing is optimised between excessive leakage and excessive parasitic absorption in the moderator.

Question 32Nuclear Science and Technology

Which of the following describes the role of boron in the primary coolant of a pressurised water reactor?

  1. It acts as a soluble neutron poison to compensate for reactivity changes during the fuel cycle Correct answer.
  2. It acts as a burnable poison only in the form of solid pellets in the fuel
  3. It provides additional moderation by slowing down fast neutrons
  4. It increases the moderator temperature coefficient to a positive value

Explanation

Boric acid dissolved in the primary coolant provides a distributed, adjustable neutron absorber. Its concentration is reduced as the fuel burns and fission product poisons build up, maintaining criticality. This is chemical shim control, distinct from solid burnable poisons.

Question 33Nuclear Science and Technology

In a nuclear reactor, which of the following is the most significant contributor to the decay heat immediately after shutdown?

  1. Beta and gamma decay of fission products and actinides Correct answer.
  2. Heat stored in the moderator and structural materials
  3. The residual fission chain reaction sustained by delayed neutrons
  4. Delayed neutron emission from fission products

Explanation

After shutdown, the fission chain reaction stops almost immediately, but radioactive decay of fission products and actinides continues to release energy. This decay heat is initially about 6-7% of full power and must be removed to prevent fuel damage. Stored sensible heat is much smaller and decays quickly.

Question 34Nuclear Science and Technology

Which of the following best explains why spent fuel assemblies are stored in a pool with neutron-absorbing material such as borated stainless steel or boron-containing racks?

  1. To reduce the probability of an inadvertent criticality when assemblies are closely spaced Correct answer.
  2. To prevent corrosion of the cladding during long-term storage
  3. To increase the heat transfer coefficient from the fuel to the water
  4. To reduce the radiation dose rate at the pool surface

Explanation

Closely spaced spent fuel can, under certain conditions, approach criticality if moderation increases, for example on flooding or rearrangement. Neutron absorbers in the racks ensure the array remains subcritical with a margin. Heat removal and shielding are provided separately by the pool water.

Question 35Nuclear Science and Technology

Which of the following is the correct order of the nuclear fuel cycle stages for an open, once-through cycle using enriched uranium?

  1. Mining, conversion, enrichment, reactor irradiation, fuel fabrication, interim storage, final disposal
  2. Mining, conversion, fuel fabrication, enrichment, reactor irradiation, interim storage, final disposal
  3. Mining, enrichment, conversion, fuel fabrication, reactor irradiation, reprocessing, final disposal
  4. Mining, conversion, enrichment, fuel fabrication, reactor irradiation, interim storage, final disposal Correct answer.

Explanation

Uranium ore is mined and converted to uranium hexafluoride, which is suitable for enrichment. The enriched uranium is then converted to uranium dioxide and fabricated into fuel assemblies before irradiation. In an open cycle, the spent fuel is stored and eventually disposed of without reprocessing.

Question 36Nuclear Science and Technology

In a pressurised water reactor, the coolant is maintained at high pressure primarily to do which of the following?

  1. Prevent bulk boiling of the coolant in the core Correct answer.
  2. Increase the solubility of boron in the coolant
  3. Increase the neutron moderation by the coolant
  4. Reduce the corrosion rate of the fuel cladding

Explanation

PWR primary pressure, typically around 15 MPa, keeps the coolant subcooled so that it does not bulk-boil in the core. This allows high heat removal with liquid-phase heat transfer and avoids the complications of two-phase flow in the primary loop. The steam generator on the secondary side produces steam at lower pressure.

Question 37Nuclear Science and Technology

Which of the following statements about the conversion ratio of a reactor is correct?

  1. It is the ratio of thermal neutrons to fast neutrons in the core
  2. It is the ratio of moderator atoms to fuel atoms in the lattice
  3. It is the ratio of fissile material produced to fissile material consumed Correct answer.
  4. It is the ratio of delayed neutrons to prompt neutrons produced in fission

Explanation

The conversion ratio measures how much new fissile material, for example Pu-239 from U-238, is produced per fissile atom consumed. A value greater than one defines a breeder reactor. It is a fuel-cycle metric and is distinct from neutron balance or lattice ratios.

Question 38Nuclear Science and Technology

A sample contains a mixture of two gamma-emitting radionuclides. Which of the following techniques is most suitable for identifying the radionuclides present?

  1. Measuring the neutron emission rate with a BF3 counter
  2. Gamma-ray spectrometry with a high-purity germanium detector Correct answer.
  3. Measuring the gross alpha activity with a ZnS scintillation detector
  4. Measuring the total beta activity with a gas-flow proportional counter

Explanation

High-purity germanium detectors have excellent energy resolution, allowing characteristic gamma-ray photopeaks to be identified and quantified. Each radionuclide has a distinctive gamma spectrum, so spectrometry can identify components of a mixture. Gross counting methods do not provide the energy information needed for identification.

Question 39Nuclear Science and Technology

Which of the following is the correct definition of the absorbed dose quantity and its SI unit?

  1. Energy fluence per unit time, expressed in watt per square metre
  2. Number of ionisations per unit path length, expressed in keV per micrometre
  3. Charge produced per unit mass of air, expressed in coulomb per kilogram
  4. Energy imparted per unit mass, expressed in gray Correct answer.

Explanation

Absorbed dose D is the mean energy imparted by ionising radiation to matter per unit mass, with SI unit the gray, equal to one joule per kilogram. Exposure in coulomb per kilogram and stopping power in keV per micrometre are different quantities. Absorbed dose is a fundamental quantity for radiation protection and dosimetry.

Question 40Nuclear Science and Technology

A worker receives an absorbed dose of 2 mGy from alpha particles with a radiation weighting factor of 20. What is the equivalent dose?

  1. 2 mSv
  2. 40 mSv Correct answer.
  3. 0.1 mSv
  4. 10 mSv

Explanation

Equivalent dose is the product of absorbed dose and the radiation weighting factor: 2 mGy × 20 = 40 mSv. The gray and sievert are numerically equivalent for this conversion. Alpha particles are densely ionising, which is why their weighting factor is high.

Question 41Nuclear Science and Technology

Which of the following is the main reason for using a boron carbide or hafnium absorber in a reactor shutdown system rather than relying only on the control rods used for power regulation?

  1. To reduce the delayed neutron fraction so that power changes more slowly
  2. To increase the thermal neutron flux in the core during normal operation
  3. To allow the reactor to operate at higher power without exceeding fuel temperature limits
  4. To provide a diverse and independent means of rapidly inserting large negative reactivity Correct answer.

Explanation

Shutdown systems are designed to be independent of the regulating system and to insert sufficient negative reactivity quickly and reliably, often using diverse absorber materials and drive mechanisms. This defence-in-depth ensures safe shutdown even if the normal control system fails. The absorber itself does not change the delayed neutron fraction or increase flux.

Question 42Nuclear Science and Technology

In a gas-cooled reactor using carbon dioxide as coolant, which property of the gas is most important in determining the heat transfer capability at a given mass flow rate?

  1. Its specific heat capacity Correct answer.
  2. Its dielectric strength
  3. Its density at standard temperature and pressure
  4. Its neutron absorption cross-section

Explanation

For a given mass flow rate, the heat carried away is proportional to the specific heat capacity multiplied by the temperature rise. A gas with higher specific heat capacity removes more heat for the same flow and temperature change. Neutron absorption is a nuclear property, and dielectric strength is relevant to insulation, not heat transfer.

Question 43Nuclear Science and Technology

Which of the following best describes the phenomenon of xenon-135 'post-shutdown peaking'?

  1. Xenon concentration remains constant after shutdown because production and removal are balanced
  2. Xenon concentration builds up after shutdown because iodine-135 decay continues while xenon burnout by neutron capture stops Correct answer.
  3. Xenon concentration increases only if the reactor is restarted within one hour of shutdown
  4. Xenon concentration decreases immediately after shutdown because iodine production stops

Explanation

After shutdown, the neutron flux collapses, so xenon burnout by neutron capture ceases. Iodine-135 already present continues to decay into xenon-135 with a half-life of about 6.6 hours, so the xenon inventory grows before decaying away. This can prevent restart for many hours unless sufficient reactivity margin exists.

Question 44Nuclear Science and Technology

In a thermal reactor, which of the following explains why the resonance escape probability p is less than one?

  1. Some neutrons leak from the core before reaching thermal energies
  2. Some neutrons are captured in U-238 resonances while slowing down Correct answer.
  3. Some fast neutrons cause fission in U-238 before moderation
  4. Some neutrons are absorbed by control rods at thermal energies

Explanation

Resonance escape probability is the fraction of neutrons that survive slowing down past the resonance region without being captured, principally in U-238 resonances. Leakage is accounted for separately by the non-leakage probabilities. Control rod absorption occurs mostly at thermal energies and is not part of p.

Question 45Nuclear Science and Technology

Which of the following is the primary function of the pressuriser in a pressurised water reactor?

  1. To separate steam from water before it enters the turbine
  2. To reduce the boron concentration in the primary coolant
  3. To remove decay heat from the core after shutdown
  4. To maintain primary system pressure and accommodate coolant volume changes Correct answer.

Explanation

The pressuriser contains a steam bubble that buffers volumetric expansion and contraction of the primary coolant and controls system pressure through heaters and spray. It keeps the coolant subcooled during power changes and transients. It is not a heat sink and does not perform steam separation.

Question 46Nuclear Science and Technology

A reactor is critical with a neutron source present. If the effective multiplication factor is then slightly reduced below one, which of the following best describes the resulting behaviour of the neutron population?

  1. It oscillates with a period determined by the delayed neutron fraction
  2. It remains constant because the source compensates for the subcriticality
  3. It falls exponentially towards a level determined by the source strength Correct answer.
  4. It rises exponentially with a stable positive period

Explanation

When k is less than one, the chain reaction cannot sustain itself and the neutron population decays exponentially with a period set by the subcritical reactivity. The external source continues to produce neutrons, so the population approaches a steady subcritical level rather than going to zero. The decay constant is related to the subcritical multiplication.

Question 47Nuclear Science and Technology

In a nuclear reactor, which of the following is the principal reason that the prompt neutron lifetime has only a weak influence on the response of the reactor to a small reactivity insertion?

  1. Because the effective delayed neutron fraction dominates the kinetics when reactivity is well below β Correct answer.
  2. Because the prompt neutron lifetime is independent of the core size and composition
  3. Because the delayed neutron fraction is small compared with one, so prompt neutrons dominate the neutron balance
  4. Because the prompt neutron lifetime is always longer than the delayed neutron precursor half-lives

Explanation

For reactivity insertions well below β, the reactor period is governed mainly by the delayed neutron precursors, whose decay constants are of the order of 0.01 to 0.1 per second. The prompt neutron lifetime is many orders of magnitude smaller and only becomes dominant as reactivity approaches prompt criticality. This is why delayed neutrons make the reactor controllable.

Question 48Nuclear Science and Technology

A fast reactor uses a mixture of plutonium and uranium oxide fuel with a high fissile content and no moderator. Which of the following best explains why the neutron spectrum is hard compared with a thermal reactor?

  1. The fuel is enriched in U-238, which emits fast neutrons on capture
  2. The coolant absorbs the low-energy neutrons preferentially
  3. Fission neutrons are not slowed appreciably because there is little light-nucleus scattering material in the core Correct answer.
  4. The high fissile content causes fission neutrons to be emitted with much higher average energy

Explanation

In a fast reactor, the core contains little or no light moderator, so elastic scattering by heavy nuclei removes only a small fraction of neutron energy per collision. Most fissions therefore occur at high energy. The average energy of fission neutrons themselves is essentially independent of fuel composition.

Question 49Nuclear Science and Technology

In a reactor core, the power distribution is often flattened by using graded fuel enrichment or burnable poisons. Which of the following is the main reason for flattening the radial power distribution?

  1. To increase the average thermal neutron flux in the core
  2. To eliminate the need for control rods during normal operation
  3. To reduce the peak fuel temperature and increase the margin to thermal limits Correct answer.
  4. To reduce the total fissile inventory required for criticality

Explanation

A flattened radial power profile lowers the peak-to-average power ratio, which reduces the maximum fuel centreline temperature and the margin to critical heat flux or melting. This improves safety margins and allows higher total core power for the same limits. It does not remove the need for control rods.

Question 50Nuclear Science and Technology

Which of the following best describes the physical process by which a nuclear reactor's containment building limits the release of radioactive material during a severe accident?

  1. It chemically neutralises fission products before they can escape
  2. It converts radioactive gases into stable isotopes by neutron irradiation
  3. It ensures that no fission products can leave the reactor coolant system
  4. It provides a low-leakage physical barrier and allows pressure suppression through sprays or suppression pools Correct answer.

Explanation

Containment is a leak-tight structure designed to withstand accident pressures and temperatures and to limit the release of radioactive material. Systems such as sprays, suppression pools, and filtered venting reduce pressure and retain activity. It is a physical barrier, not a chemical or nuclear conversion system.