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Question 1Nuclear Physics and Reactor Theory

Which particle carries no electric charge and is found in the nucleus of an atom?

  1. Positron
  2. Neutron Correct answer.
  3. Proton
  4. Electron

Explanation

The neutron is electrically neutral and resides in the nucleus alongside protons. Protons are positively charged, electrons are negatively charged and orbit the nucleus, and positrons are antimatter particles not present in stable nuclei.

Question 2Nuclear Physics and Reactor Theory

The atomic number Z of a nuclide represents which quantity?

  1. The number of protons in the nucleus Correct answer.
  2. The number of neutrons in the nucleus
  3. The mass defect of the nucleus
  4. The total number of nucleons

Explanation

Atomic number Z is defined as the number of protons in the nucleus and fixes the chemical element. The neutron number N is separate, and their sum A = Z + N is the mass number. Mass defect is an energy-related quantity, not a count of protons.

Question 3Nuclear Physics and Reactor Theory

Which force binds nucleons together within the nucleus?

  1. Electromagnetic force
  2. Strong nuclear force Correct answer.
  3. Weak nuclear force
  4. Gravitational force

Explanation

The strong nuclear force acts over very short ranges and overcomes proton-proton electrostatic repulsion to bind nucleons. Electromagnetic and gravitational forces are far too weak at nuclear scales, and the weak force mediates certain decays rather than binding.

Question 4Nuclear Physics and Reactor Theory

In the notation for a nuclide, the mass number A equals which of the following?

  1. The number of protons only
  2. The number of protons plus neutrons Correct answer.
  3. The number of neutrons only
  4. The number of electrons plus protons

Explanation

Mass number A counts all nucleons, so A = Z + N, the sum of protons and neutrons. Electrons are not nucleons and are not included. Proton-only and neutron-only counts correspond to Z and N respectively.

Question 5Nuclear Physics and Reactor Theory

Which decay mode decreases the atomic number of a nucleus by one while leaving the mass number unchanged?

  1. Alpha decay
  2. Gamma emission
  3. Beta-plus decay Correct answer.
  4. Beta-minus decay

Explanation

In beta-plus decay a proton converts to a neutron, emitting a positron and a neutrino, so Z decreases by one and A is unchanged. Beta-minus raises Z by one, alpha reduces both A and Z, and gamma emission changes neither.

Question 6Nuclear Physics and Reactor Theory

A radionuclide has a half-life of 10 days. What fraction of the original activity remains after 30 days?

  1. One sixteenth
  2. One eighth Correct answer.
  3. One half
  4. One quarter

Explanation

Thirty days equals three half-lives, and each half-life halves the activity. The remaining fraction is (1/2)^3 = 1/8.

Question 7Nuclear Physics and Reactor Theory

Which quantity is conserved in every nuclear reaction?

  1. Number of neutrons
  2. Total electric charge Correct answer.
  3. Number of protons
  4. Number of alpha particles

Explanation

Total electric charge is strictly conserved in all nuclear reactions. Proton and neutron counts individually can change, for example in beta decay, while the total charge balance is maintained.

Question 8Nuclear Physics and Reactor Theory

The binding energy of a nucleus is best described as which of the following?

  1. The kinetic energy of emitted neutrons
  2. The energy required to separate the nucleus into its individual nucleons Correct answer.
  3. The energy released when the nucleus fissions
  4. The energy of the emitted gamma rays

Explanation

Binding energy is the energy equivalent of the mass defect, the energy needed to disassemble the nucleus into free nucleons. It is a property of the nucleus itself, not of a particular fission or emission event.

Question 9Nuclear Physics and Reactor Theory

Which nuclide has a magic neutron number of 82?

  1. Lead-206
  2. Barium-138 Correct answer.
  3. Uranium-235
  4. Thorium-232

Explanation

Barium-138 has Z = 56 and N = 82, a magic neutron number associated with closed shells and enhanced stability. Uranium-235 has N = 143, lead-206 has N = 124, and thorium-232 has N = 140.

Question 10Nuclear Physics and Reactor Theory

In a nuclear reactor, which process sustains the fission chain reaction?

  1. Alpha particles captured by fuel nuclei
  2. Neutrons released by fission inducing further fissions Correct answer.
  3. Gamma rays ejecting protons from nuclei
  4. Electrons colliding with fuel nuclei

Explanation

Fission releases neutrons that can be absorbed by other fissile nuclei, causing additional fissions and releasing more neutrons. This self-sustaining neutron cycle is what constitutes a chain reaction. Charged particles and gamma rays do not propagate such a chain.

Question 11Nuclear Physics and Reactor Theory

Which neutron energy range is most effective at inducing fission in uranium-235?

  1. Epithermal neutrons near 1 keV
  2. Relativistic neutrons
  3. Thermal neutrons near 0.025 eV Correct answer.
  4. Fast neutrons above 1 MeV

Explanation

The fission cross section of U-235 is largest at thermal energies, around 0.025 eV, which is why reactors use moderators to slow neutrons. Fast and epithermal neutrons have much smaller fission cross sections in U-235.

Question 12Nuclear Physics and Reactor Theory

What is the primary function of a moderator in a thermal reactor?

  1. Reflect neutrons back into the core
  2. Slow fast neutrons to thermal energies Correct answer.
  3. Absorb excess neutrons to control reactivity
  4. Convert fertile material into fissile material

Explanation

A moderator, such as water or graphite, reduces neutron kinetic energy through elastic scattering so neutrons reach thermal energies where fission cross sections are large. Absorption and reflection are performed by control poisons and reflectors, and breeding occurs in fertile fuel.

Question 13Nuclear Physics and Reactor Theory

Which quantity describes the average number of neutrons produced per neutron absorbed in a fissile nuclide?

  1. Multiplication factor k
  2. Fast fission factor epsilon
  3. Resonance escape probability p
  4. Reproduction factor eta Correct answer.

Explanation

Eta is the reproduction factor, the number of fission neutrons produced per neutron absorbed in the fissile material. The multiplication factor k is the overall neutron balance, p is resonance escape probability, and epsilon is the fast fission factor.

Question 14Nuclear Physics and Reactor Theory

A reactor is exactly critical when which condition holds?

  1. k equals 0
  2. k equals 1 Correct answer.
  3. k is greater than 1
  4. k is less than 1

Explanation

Criticality means the neutron population is steady in time, which corresponds to an effective multiplication factor of exactly one. Values below one give a subcritical decaying population and values above one give a supercritical growing population.

Question 15Nuclear Physics and Reactor Theory

Which of the following is a fertile nuclide that can be converted into a fissile nuclide?

  1. Uranium-233
  2. Uranium-235
  3. Uranium-238 Correct answer.
  4. Plutonium-239

Explanation

Uranium-238 is fertile: after neutron capture it becomes U-239, which beta decays to Np-239 and then Pu-239. Uranium-233, plutonium-239, and uranium-235 are already fissile with thermal neutrons.

Question 16Nuclear Physics and Reactor Theory

The delayed neutron fraction in a thermal reactor is approximately which value?

  1. About 0.0065 Correct answer.
  2. About 0.65
  3. About 6.5
  4. About 0.065

Explanation

Delayed neutrons constitute roughly 0.65 percent of fission neutrons in U-235, or a fraction of about 0.0065. This small fraction governs reactor controllability because it sets the prompt critical threshold.

Question 17Nuclear Physics and Reactor Theory

Which term in the six-factor formula accounts for neutrons lost by absorption in non-fuel materials during moderation?

  1. Reproduction factor
  2. Resonance escape probability
  3. Thermal utilization factor Correct answer.
  4. Fast fission factor

Explanation

The thermal utilization factor f represents the fraction of thermal neutrons absorbed in fuel relative to all thermal absorptions, including those in moderator and structure. Resonance escape p concerns slowing-down losses, and the fast fission factor concerns fast neutrons.

Question 18Nuclear Physics and Reactor Theory

Which radiation has the greatest range in air for a given energy?

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

Explanation

Gamma rays are uncharged photons and interact only probabilistically, giving them the longest range in air. Alpha particles are stopped within centimeters, beta particles within meters, and protons have a well-defined short range.

Question 19Nuclear Physics and Reactor Theory

Which interaction process dominates gamma-ray attenuation in high-Z materials at moderate energies?

  1. Pair production
  2. Compton scattering
  3. Coherent scattering
  4. Photoelectric absorption Correct answer.

Explanation

In high-Z absorbers at moderate gamma energies, the photoelectric effect dominates because its cross section scales strongly with atomic number. Compton scattering dominates in low-Z materials, and pair production requires energies above 1.022 MeV.

Question 20Nuclear Physics and Reactor Theory

In a nuclear reaction, the Q value is negative. What does this indicate?

  1. The reaction produces only neutrons
  2. The reaction requires energy input Correct answer.
  3. The reaction releases energy
  4. The reaction is impossible

Explanation

A negative Q value means the total mass of the products exceeds that of the reactants, so energy must be supplied from kinetic energy for the reaction to proceed. Positive Q values indicate exothermic reactions.

Question 21Nuclear Physics and Reactor Theory

Which statement about the mass defect of a nucleus is correct?

  1. It is always negative
  2. It is equal to the atomic number
  3. It is the difference between the sum of free nucleon masses and the nuclear mass Correct answer.
  4. It equals the mass of the electrons in the atom

Explanation

The mass defect is the difference between the summed masses of the separated nucleons and the actual nuclear mass. This difference corresponds to the binding energy via E = mc^2. It is a positive quantity for bound nuclei.

Question 22Nuclear Physics and Reactor Theory

Which of the following best explains why heavy nuclei release energy upon fission?

  1. Neutrons are heavier than protons
  2. The parent nucleus has no binding energy
  3. The fission products have higher binding energy per nucleon than the parent nucleus Correct answer.
  4. The fission products have lower binding energy per nucleon

Explanation

Fission products near the middle of the binding energy curve have higher binding energy per nucleon than heavy actinides, so the mass difference appears as released energy. The binding energy curve peaks near iron-56, making both fission of heavy nuclei and fusion of light nuclei exothermic.

Question 23Nuclear Physics and Reactor Theory

Which process produces energy in the core of the Sun?

  1. Fission of uranium
  2. Annihilation of protons
  3. Decay of carbon-14
  4. Fusion of hydrogen into helium Correct answer.

Explanation

The Sun fuses hydrogen nuclei into helium through the proton-proton chain, releasing energy because helium has a higher binding energy per nucleon. Fission and radioactive decay do not supply the Sun's energy.

Question 24Nuclear Physics and Reactor Theory

A sample contains 8 x 10^20 atoms of a radionuclide with a decay constant of 0.1 per day. What is the approximate activity at that instant?

  1. 8 x 10^19 decays per day Correct answer.
  2. 8 x 10^21 decays per day
  3. 8 x 10^18 decays per day
  4. 8 x 10^20 decays per day

Explanation

Activity equals the decay constant times the number of atoms, A = lambda N. Multiplying 0.1 per day by 8 x 10^20 atoms gives 8 x 10^19 decays per day.

Question 25Nuclear Physics and Reactor Theory

Which statement about neutron scattering by a moderator is correct?

  1. Elastic scattering transfers the most energy to heavy nuclei
  2. Inelastic scattering is the dominant moderation mechanism
  3. Elastic scattering transfers the most energy to light nuclei Correct answer.
  4. Moderation requires neutron absorption

Explanation

In elastic scattering, the energy transferred to a target nucleus is greatest when the target mass is closest to the neutron mass, so light nuclei such as hydrogen are efficient moderators. Heavy nuclei absorb little energy per collision, and moderation relies on scattering rather than absorption.

Question 26Nuclear Physics and Reactor Theory

In a thermal reactor, the value of the reproduction factor (eta) for uranium-235 is approximately which of the following?

  1. About 1.0
  2. About 2.07 Correct answer.
  3. About 0.65
  4. About 4.5

Explanation

Eta is the number of fission neutrons produced per neutron absorbed in the fissile nuclide. For U-235 absorbing a thermal neutron, roughly 2.42 neutrons are released per fission while the absorption-to-fission ratio is near 0.85, giving eta close to 2.07.

Question 27Nuclear Physics and Reactor Theory

A point source emits neutrons isotropically. If a detector at 2 m from the source records a certain count rate, what happens to the uncollided flux at a distance of 4 m in the absence of absorption?

  1. It doubles
  2. It remains unchanged
  3. It falls to one quarter of its original value Correct answer.
  4. It falls to one half of its original value

Explanation

For an isotropic point source the uncollided flux follows the inverse-square law, decreasing as the square of the distance. Doubling the distance from 2 m to 4 m therefore reduces the flux by a factor of four.

Question 28Nuclear Physics and Reactor Theory

A reactor operates with a prompt neutron lifetime of 1 x 10^-4 s and a delayed neutron fraction of 0.0065. Using the inhour relation in the limit of small reactivity, which reactivity insertion (in dollars) gives a stable reactor period of about 80 s dominated by delayed neutrons?

  1. About 0.1 dollars Correct answer.
  2. About 10 dollars
  3. About 1.0 dollars
  4. About 0.001 dollars

Explanation

For small positive reactivity the stable period is governed by the delayed precursor decay, and the reactivity in dollars is approximately beta_eff divided by (lambda_eff times period) plus the prompt term. With typical precursor decay constants near 0.08 per second and a period of 80 s, the reactivity is on the order of 0.1 dollars.

Question 29Nuclear Physics and Reactor Theory

Which statement correctly describes the behavior of the effective multiplication factor when a reactor is subcritical with an external neutron source present?

  1. The neutron population oscillates around zero
  2. The neutron population grows without bound
  3. The neutron population approaches a steady level set by the source and the subcritical multiplication Correct answer.
  4. The neutron population decays to zero regardless of the source strength

Explanation

In a subcritical assembly with a source, each generation adds source neutrons plus k times the previous generation, summing to a finite steady population proportional to 1/(1-k). The multiplication factor M = 1/(1-k) describes this subcritical multiplication.

Question 30Nuclear Physics and Reactor Theory

Two nuclides have the same mass number A but differ in proton number by one. Which relationship between their nuclear masses is required for single beta-minus decay to be energetically allowed?

  1. The parent mass must exceed the daughter mass by at least two electron masses
  2. The parent and daughter masses must be exactly equal
  3. The parent mass must exceed the daughter mass plus the electron mass equivalent Correct answer.
  4. The parent mass must be less than the daughter mass

Explanation

In beta-minus decay a neutron converts to a proton with emission of an electron and antineutrino, so the atomic mass of the parent must exceed that of the daughter by more than the electron rest-mass equivalent. The Q value equals the atomic mass difference converted to energy.

Question 31Reactor Operations and Control

In a pressurized water reactor, what is the primary function of the pressurizer?

  1. Control the boron concentration during power changes
  2. Provide emergency makeup water to the steam generators
  3. Maintain reactor coolant system pressure and limit boiling in the core Correct answer.
  4. Filter dissolved corrosion products from the coolant

Explanation

The pressurizer uses electric heaters and spray to hold reactor coolant system pressure above saturation so the core coolant does not bulk-boil. It acts as a pressure and inventory buffer, not a purification or boron-control device.

Question 32Reactor Operations and Control

A reactor is critical at low power with a constant neutron population. Which statement describes the core multiplication factor?

  1. It equals one exactly Correct answer.
  2. It is greater than one and slowly rising
  3. It is less than one but positive
  4. It is zero because power is not changing

Explanation

A constant neutron population defines criticality, which means the effective multiplication factor is exactly one. Power level does not determine criticality; the balance of neutron production and loss does.

Question 33Reactor Operations and Control

Which quantity is the reactor period?

  1. The time required to reach full licensed power
  2. The time for neutron population to change by a factor of e Correct answer.
  3. The average neutron lifetime in the core
  4. The time for reactivity to change by one dollar

Explanation

Reactor period is the time required for the neutron population (and thus power) to change by a factor of e, about 2.718. It is related to reactivity through the inhour equation.

Question 34Reactor Operations and Control

Why do delayed neutrons matter so much for reactor control?

  1. They prevent the formation of xenon after shutdown
  2. They lengthen the effective neutron lifetime and slow power changes Correct answer.
  3. They increase the number of neutrons produced per fission
  4. They supply most of the total fission heat

Explanation

Delayed neutrons are a small fraction of the total but their longer emission times dominate the effective lifetime, making power changes controllable. Without them, a small reactivity insertion would cause an extremely fast power rise.

Question 35Reactor Operations and Control

What is the effect of a positive reactivity insertion on a critical reactor?

  1. Power increases only if the coolant temperature rises
  2. Power stays constant while period shortens
  3. Power increases until a new steady level is reached Correct answer.
  4. Power decreases until a new steady level is reached

Explanation

Positive reactivity makes the core supercritical, so the neutron population rises. The rise continues until feedback effects or operator action return the core to criticality at a new power level.

Question 36Reactor Operations and Control

Which signal is used by the reactor protection system to trip the reactor on high neutron flux?

  1. Out-of-core ion chamber background
  2. Intermediate range log power level
  3. Power range linear power level Correct answer.
  4. Source range detector count rate

Explanation

High-flux trips are derived from the power range detectors, which measure linear power near operating levels. Source and intermediate range channels cover startup and are used for other protections.

Question 37Reactor Operations and Control

What is the purpose of the control rod drive mechanism scram function?

  1. Withdraw rods slowly for power ascension
  2. Insert control rods rapidly to shut down the reactor Correct answer.
  3. Rotate the rods to equalize burnup
  4. Hold rods at a fixed position during steady operation

Explanation

A scram releases the rod drives so the rods fall or are driven rapidly into the core, adding negative reactivity to shut down the chain reaction. Slow withdrawal and position holding are normal control functions, not scram functions.

Question 38Reactor Operations and Control

Which condition is a typical reactor trip on loss of reactor coolant flow?

  1. High containment temperature only
  2. High pressurizer level only
  3. Low coolant flow coincident with power above a setpoint Correct answer.
  4. Low steam generator level only

Explanation

Loss-of-flow trips are designed to actuate when flow falls below a setpoint while the reactor is at power, protecting against inadequate heat removal. Other parameters alone do not directly indicate loss of core flow.

Question 39Reactor Operations and Control

A reactor is operating at power when a small positive reactivity is inserted and the core returns to a stable higher power without operator action. Which feedback explains this?

  1. Positive void coefficient
  2. Negative moderator temperature coefficient Correct answer.
  3. Constant moderator density
  4. Positive fuel temperature coefficient

Explanation

A negative moderator temperature coefficient reduces reactivity as coolant temperature rises, offsetting the inserted reactivity and stabilizing power. This self-limiting behavior is a key inherent safety feature.

Question 40Reactor Operations and Control

After a reactor trip from full power, why does the power level briefly rise again after an initial drop?

  1. Xenon is burned out immediately
  2. Decay heat from fission products continues to produce heat Correct answer.
  3. Control rods are withdrawn automatically
  4. Delayed neutron precursors decay and add neutrons

Explanation

Following a trip, fission stops but decay heat from fission products continues, producing a heat source that decays slowly. This decay heat must be removed by the residual heat removal systems.

Question 41Reactor Operations and Control

Which parameter is controlled by the steam generator water level control system in a PWR?

  1. Secondary side inventory and level Correct answer.
  2. Control rod position
  3. Reactor coolant system pressure
  4. Primary coolant boron concentration

Explanation

Steam generator level control maintains secondary inventory so that heat transfer and turbine steam supply remain stable. Primary pressure and boron are controlled by other systems.

Question 42Reactor Operations and Control

What is the effect of withdrawing control rods in a critical reactor at power?

  1. It has no effect on reactivity at power
  2. It adds negative reactivity and lowers power
  3. It reduces the delayed neutron fraction
  4. It adds positive reactivity and raises power Correct answer.

Explanation

Control rods absorb neutrons, so withdrawing them reduces absorption and adds positive reactivity. The reactor becomes supercritical and power rises until feedback restores criticality.

Question 43Reactor Operations and Control

A reactor operator observes a decreasing reactor period (shorter period) while power is rising. What does this indicate?

  1. The reactor is becoming more subcritical
  2. The reactor is becoming more supercritical Correct answer.
  3. The reactor has reached criticality
  4. The reactor is shutting down

Explanation

A shorter period means a faster rate of power increase, which corresponds to larger positive reactivity and a more supercritical state. Criticality corresponds to an infinite period.

Question 44Reactor Operations and Control

Which system provides borated water to the reactor coolant system to compensate for reactivity changes during power operation?

  1. The chemical and volume control system Correct answer.
  2. The emergency diesel generator system
  3. The component cooling water system
  4. The main steam bypass system

Explanation

The chemical and volume control system adjusts boron concentration in the primary coolant, providing long-term reactivity control. It also purifies and maintains inventory.

Question 45Reactor Operations and Control

Why is a reactor trip required on low pressurizer pressure?

  1. To prevent excessive coolant boiling and inadequate core cooling Correct answer.
  2. To increase coolant flow through the core
  3. To reduce boron concentration
  4. To raise secondary steam pressure

Explanation

Low primary pressure reduces the margin to saturation, risking bulk boiling and inadequate heat removal from the core. The trip protects against departure from nucleate boiling and loss of cooling.

Question 46Reactor Operations and Control

In a PWR, what is the purpose of the safety injection system during a small-break loss-of-coolant accident?

  1. Maintain core inventory and cooling with borated water Correct answer.
  2. Supply cooling water to the turbine condenser
  3. Reduce containment pressure by venting steam
  4. Increase reactor power to offset heat loss

Explanation

Safety injection adds borated water to the primary system to restore inventory and remove decay heat after a loss of coolant. It does not intentionally change power or serve the secondary plant.

Question 47Reactor Operations and Control

What happens to reactivity when the moderator temperature increases in a core with a negative moderator temperature coefficient?

  1. Reactivity becomes positive
  2. Reactivity increases
  3. Reactivity decreases Correct answer.
  4. Reactivity remains unchanged

Explanation

A negative moderator temperature coefficient means rising moderator temperature reduces reactivity, typically through reduced moderator density. This provides a self-limiting response to power increases.

Question 48Reactor Operations and Control

Which action would an operator take to reduce reactor power slowly during normal operation?

  1. Insert control rods in small steps Correct answer.
  2. Open the pressurizer spray valve fully
  3. Start the emergency diesel generator
  4. Bypass the reactor protection system

Explanation

Small, controlled rod insertions add negative reactivity and lower power gradually while feedback stabilizes the core. Other listed actions do not provide controlled reactivity reduction.

Question 49Reactor Operations and Control

Why does xenon-135 buildup after a reactor trip require careful power ascension planning?

  1. Xenon increases the fission cross section of uranium
  2. Xenon is a strong neutron absorber that reduces reactivity Correct answer.
  3. Xenon provides additional delayed neutrons
  4. Xenon raises the moderator temperature coefficient

Explanation

Xenon-135 has a very large neutron absorption cross section, so its buildup after shutdown adds negative reactivity. Operators must plan restart power ascension to avoid being unable to overcome the xenon load.

Question 50Reactor Operations and Control

A reactor is at 100% power when a turbine trip occurs. Which automatic action is expected?

  1. Reactor trip on high steam pressure or low steam flow Correct answer.
  2. Main feedwater increases to maximum
  3. Control rods withdraw to maintain power
  4. Pressurizer heaters energize to raise pressure

Explanation

A turbine trip removes the heat sink, raising secondary pressure and reducing steam flow, which actuates a reactor trip to protect the primary system. The trip prevents an overpower condition from the sudden loss of load.