I85PREP
⌘ K
NTN REACT study materials
NTN REACT study resources

NTN REACT practice questions

One question at a time. Build your NTN REACT understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Reactor Theory and Nuclear Physics

In a thermal reactor, which neutron energy range is most responsible for fission in uranium-235?

  1. Relativistic energies above 100 MeV
  2. Fast energies above 1 MeV
  3. Epithermal energies near 1 keV
  4. Thermal energies near 0.025 eV Correct answer.

Explanation

U-235 has a very large fission cross section at thermal energies, roughly 585 barns for neutrons near 0.025 eV. Most fissions in a moderated thermal reactor therefore occur in this low-energy region. Fast neutrons must first be slowed by moderation before they are efficient at causing fission in U-235.

Question 2Reactor Theory and Nuclear Physics

Which quantity is defined as the average number of fission neutrons produced per neutron absorbed in the fuel?

  1. Sigma (σ)
  2. Kappa (κ)
  3. Lambda (λ)
  4. Eta (η) Correct answer.

Explanation

Eta is the reproduction factor, the number of fission neutrons produced per neutron absorbed in fissile fuel. It differs from nu, which counts neutrons per fission, and from k, the multiplication factor of the whole assembly. Eta captures the effect of non-fission captures in the fuel.

Question 3Reactor Theory and Nuclear Physics

A reactor has a multiplication factor k of 1.008. What is its reactivity in dollars, given a delayed neutron fraction of 0.0065?

  1. About 0.65 dollars
  2. About 8.0 dollars
  3. About 0.008 dollars
  4. About 1.23 dollars Correct answer.

Explanation

Reactivity in dollars equals (k − 1)/k divided by beta. Here (0.008/1.008) equals about 0.00794, and dividing by 0.0065 gives roughly 1.22 dollars. This is just above prompt critical, so the reactor would be supercritical on delayed neutrons alone.

Question 4Reactor Theory and Nuclear Physics

Why is a moderator with a low atomic mass preferred in a thermal reactor?

  1. Heavy nuclei absorb fewer neutrons per collision
  2. Heavy nuclei slow neutrons mainly through inelastic scattering
  3. Low-mass nuclei increase the fission cross section directly
  4. Light nuclei remove more energy per elastic collision Correct answer.

Explanation

In elastic scattering the average fractional energy loss depends on the mass ratio of neutron to nucleus, and it is largest when the target nucleus is light. Hydrogen, deuterium, and carbon therefore slow neutrons efficiently. A good moderator also needs a low absorption cross section, which is why deuterium and carbon are attractive despite being heavier than hydrogen.

Question 5Reactor Theory and Nuclear Physics

What is the physical significance of the effective multiplication factor being exactly one?

  1. The delayed neutron fraction becomes zero
  2. The reactor produces no neutrons at all
  3. All fission neutrons are absorbed in the moderator
  4. The neutron population is steady from one generation to the next Correct answer.

Explanation

When k equals one, each neutron generation produces exactly as many neutrons as the previous generation, so the population and power remain constant. This is the critical condition. Values above or below one give increasing or decreasing populations respectively.

Question 6Reactor Theory and Nuclear Physics

Which statement gives the conventional four-factor expression for k-infinity, before applying the two non-leakage factors in the six-factor formula?

  1. k-infinity equals epsilon times p times f times eta times the fast and thermal non-leakage factors
  2. k-infinity equals the fast non-leakage factor times the thermal non-leakage factor
  3. k-infinity equals eta divided by the resonance escape probability
  4. k-infinity equals epsilon times p times f times eta Correct answer.

Explanation

The four-factor formula multiplies four infinite-medium factors: fast fission factor, resonance escape probability, thermal utilization, and reproduction factor. The two non-leakage factors are applied to convert k-infinity into k-effective. Thus k-infinity is the product of epsilon, p, f, and eta alone.

Question 7Reactor Theory and Nuclear Physics

A bare critical reactor has k-infinity of 1.10. What non-leakage probability is required to make it critical?

  1. About 0.10
  2. About 0.55
  3. About 0.91 Correct answer.
  4. About 1.10

Explanation

For a finite reactor, k-effective equals k-infinity multiplied by the total non-leakage probability. Setting k-effective to one gives a required non-leakage probability of 1/1.10, about 0.91. About nine percent of neutrons may leak out while the system remains critical.

Question 8Reactor Theory and Nuclear Physics

Which delayed neutron precursor group dominates the delayed neutron fraction in a U-235 fueled reactor?

  1. Bromine-87 Correct answer.
  2. Samarium-149
  3. Xenon-135
  4. Iodine-137

Explanation

Bromine-87 is the most important delayed neutron precursor in U-235 fission, contributing a large share of the total delayed fraction. Its half-life of about 55 seconds places it in the longer-lived groups that govern reactor control. Other precursors such as iodine-137 contribute smaller fractions.

Question 9Reactor Theory and Nuclear Physics

Why are delayed neutrons so important for reactor control even though they are a small fraction of all fission neutrons?

  1. They carry far more energy than prompt neutrons
  2. They eliminate the need for a moderator
  3. They are absorbed only by control rods
  4. They increase the effective neutron lifetime enough for mechanical control systems to respond Correct answer.

Explanation

Delayed neutrons are born seconds to minutes after fission, so their presence lengthens the effective neutron lifetime from milliseconds to tenths of a second. This gives operators and control systems time to respond before power excursions become rapid. Without them, a small excess reactivity would cause a prompt-critical power surge.

Question 10Reactor Theory and Nuclear Physics

A reactor operates at constant power and then the control rods are withdrawn slightly, making it supercritical with a small positive reactivity. Immediately after the change, what happens?

  1. Power remains exactly constant because k is still near one
  2. Power rises instantly to a new steady value
  3. Power rises on the prompt neutron period, then levels to a slower delayed-dominated rise Correct answer.
  4. Power decreases until delayed neutrons are absorbed

Explanation

When positive reactivity is inserted below prompt critical, the prompt neutron population adjusts quickly on the prompt period. The longer-term rise is then governed by delayed neutron precursors, giving a much slower stable period. The power does not jump instantly because the delayed groups build up over time.

Question 11Reactor Theory and Nuclear Physics

What does a negative moderator temperature coefficient imply for a reactor with a negative coefficient?

  1. Rising moderator temperature increases reactivity
  2. Reactivity depends only on control rod position
  3. Moderator temperature has no effect on reactivity
  4. Rising moderator temperature reduces reactivity Correct answer.

Explanation

A negative moderator temperature coefficient means that as the moderator heats up and becomes less dense, reactivity decreases. This provides a self-limiting feedback that opposes power increases. In light-water reactors this behavior is typical and is important for inherent safety.

Question 12Reactor Theory and Nuclear Physics

Which fission product is the dominant neutron poison during the first few hours after reactor shutdown, causing the xenon dead time?

  1. Cesium-137
  2. Xenon-135 Correct answer.
  3. Krypton-85
  4. Strontium-90

Explanation

Xenon-135 has an enormous thermal absorption cross section and builds up after shutdown because iodine-135 continues to decay into it while burnout by neutron absorption stops. This peak creates the xenon dead time during which restart is difficult. The effect can persist for many hours.

Question 13Reactor Theory and Nuclear Physics

In a reactor, the thermal utilization factor f is defined as which ratio?

  1. Thermal neutrons leaking out divided by neutrons produced
  2. Fission neutrons produced divided by neutrons absorbed in fuel
  3. Fast neutrons produced divided by thermal neutrons produced
  4. Thermal neutrons absorbed in fuel divided by thermal neutrons absorbed in all materials Correct answer.

Explanation

Thermal utilization compares useful absorption in fuel to total thermal absorption in fuel plus moderator, structure, and poisons. A value near one means most thermal neutrons are absorbed where they can cause fission. Lower values indicate parasitic absorption.

Question 14Reactor Theory and Nuclear Physics

Which condition defines a reactor as prompt critical?

  1. Reactivity equals zero
  2. The delayed neutron fraction is zero
  3. Reactivity is negative
  4. Reactivity equals or exceeds the delayed neutron fraction Correct answer.

Explanation

Prompt criticality occurs when the inserted reactivity reaches beta, the delayed neutron fraction, so the chain reaction can be sustained by prompt neutrons alone. Beyond this point power rises on the very short prompt period, which is difficult to control. Reactivity in dollars reaches one at this threshold.

Question 15Reactor Theory and Nuclear Physics

Which statement about the resonance escape probability p is correct?

  1. It is the fraction of thermal neutrons absorbed in fuel
  2. It is the fraction of neutrons that leak from the core
  3. It is the fraction of fissions caused by fast neutrons
  4. It is the fraction of neutrons that avoid capture in resonance energies while slowing down Correct answer.

Explanation

As neutrons slow through the resonance region, some are captured by U-238 resonances. The resonance escape probability is the fraction that survives this slowing-down process without resonance capture. It increases with better moderation and lower fuel-to-moderator ratio in some designs.

Question 16Reactor Theory and Nuclear Physics

A control rod is made of a material with a very high thermal absorption cross section. Which material is commonly used for this purpose?

  1. Zirconium alloy
  2. Graphite
  3. Aluminum
  4. Boron carbide Correct answer.

Explanation

Boron carbide contains boron-10, which has a large thermal neutron absorption cross section and is widely used in control rods. The boron-10 nucleus absorbs a neutron and emits an alpha particle. Materials with low absorption such as zirconium are used for cladding instead.

Question 17Reactor Theory and Nuclear Physics

Which process describes inelastic scattering of a neutron in a reactor?

  1. The neutron transfers energy to a nucleus, which is left in an excited state Correct answer.
  2. The neutron decays into a proton and an electron
  3. The neutron is absorbed and a gamma ray is emitted
  4. The neutron splits a heavy nucleus into two fragments

Explanation

In inelastic scattering the neutron excites the target nucleus and loses kinetic energy, and the nucleus later de-excites by gamma emission. This process has a threshold energy and is important for fast neutrons in heavy nuclei. It differs from elastic scattering, where kinetic energy is conserved.

Question 18Reactor Theory and Nuclear Physics

What is the main reason U-238 is not fissile although it can fission with fast neutrons?

  1. It requires a threshold neutron energy to fission and usually captures thermal neutrons instead Correct answer.
  2. It cannot absorb neutrons at all
  3. It has no fission cross section at any energy
  4. It decays too quickly to be used as fuel

Explanation

U-238 has a fission threshold near 1 MeV, so thermal neutrons cannot cause fission in it. Instead, U-238 captures thermal and resonance neutrons, forming U-239 and eventually plutonium-239. This fertile behavior is important for fuel breeding.

Question 19Reactor Theory and Nuclear Physics

In a homogeneous infinite reactor, k-infinity is 1.25. What is the reactivity in percent, defined as (k−1)/k times 100?

  1. About 25 percent
  2. About 80 percent
  3. About 20 percent Correct answer.
  4. About 1.25 percent

Explanation

Reactivity in percent uses the definition (k−1)/k times 100. With k of 1.25, the numerator is 0.25 and dividing by 1.25 gives 0.20, or about 20 percent. This differs from the simple excess of k, which would be 25 percent.

Question 20Reactor Theory and Nuclear Physics

Which factor in the six-factor formula accounts for fissions caused by fast neutrons before they are moderated?

  1. p
  2. f
  3. Epsilon Correct answer.
  4. eta

Explanation

The fast fission factor epsilon accounts for the extra neutrons produced when fast neutrons cause fission in fertile isotopes such as U-238 before slowing down. It is slightly greater than one. The other factors describe resonance escape, thermal utilization, and reproduction.

Question 21Reactor Theory and Nuclear Physics

A reactor has a neutron generation time of 1e-4 seconds and a small reactivity such that the prompt period is about 0.01 seconds. Which statement best describes the power behavior?

  1. Power oscillates without changing average value
  2. Power stays constant because the period is short
  3. Power decreases slowly over minutes
  4. Power rises rapidly on the prompt period until delayed neutrons and feedback slow the rise Correct answer.

Explanation

A short prompt period means the prompt neutron population grows very quickly. However, below prompt critical the sustained growth is moderated by delayed neutrons and by negative feedback such as temperature coefficients. The combination determines the observable power rise.

Question 22Reactor Theory and Nuclear Physics

Why does a large negative void coefficient in a boiling water reactor improve safety?

  1. Voids absorb control rods more effectively
  2. Steam voids reduce moderation, lowering reactivity when voids increase Correct answer.
  3. Voids increase the fuel enrichment automatically
  4. Steam voids increase moderation, raising reactivity

Explanation

In a BWR, steam voids displace water and reduce moderation, which lowers reactivity when power and void fraction rise. This negative feedback tends to stabilize the reactor against power excursions. The effect depends on core design and void fraction distribution.

Question 23Reactor Theory and Nuclear Physics

Which statement about the neutron life cycle in a thermal reactor is correct?

  1. Fast neutrons cause most fissions before moderation
  2. Fast neutrons are moderated, diffuse, and either leak, are captured, or cause fission Correct answer.
  3. Delayed neutrons are produced only in the moderator
  4. Thermal neutrons are produced directly by fission

Explanation

Fission produces fast neutrons that slow through scattering, then diffuse as thermal neutrons until they leak, are absorbed, or cause fission. This sequence is the basis of the four-factor and six-factor descriptions. Delayed neutrons are produced by fission product decay, not by the moderator.

Question 24Reactor Theory and Nuclear Physics

A reactor is subcritical with k of 0.98 and a neutron source present. Which statement describes the neutron population?

  1. It grows without bound
  2. It reaches a steady level determined by the source and the subcritical multiplication Correct answer.
  3. It equals the source strength exactly
  4. It decreases to zero immediately

Explanation

With an external source, a subcritical assembly reaches a steady neutron level where source production balances losses. The multiplication factor 1/(1−k) amplifies the source, here about 50 times. The population is stable rather than decaying to zero.

Question 25Reactor Theory and Nuclear Physics

Which quantity represents the average number of neutrons released per fission of U-235 by thermal neutrons?

  1. About 1.00
  2. About 2.43 Correct answer.
  3. About 6.5
  4. About 0.65

Explanation

Thermal fission of U-235 releases on average about 2.43 neutrons per fission event. This value, denoted nu, exceeds the reproduction factor eta because some neutrons are lost to non-fission capture. The excess over one makes a chain reaction possible.

Question 26Reactor Theory and Nuclear Physics

In a thermal reactor, which quantity is defined as the ratio of the number of thermal neutrons absorbed in the fuel to the total number of thermal neutrons absorbed in all reactor materials?

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

Explanation

The thermal utilization factor, usually written f, measures how effectively thermal neutrons are captured in fuel rather than in moderator, structure, or poisons. A value close to one means most thermal absorption occurs in fuel, improving neutron economy.

Question 27Reactor Theory and Nuclear Physics

Which statement best describes the role of the moderator in a thermal reactor?

  1. It removes fission heat from the core to the steam generator.
  2. It slows neutrons by elastic scattering so they can induce thermal fission. Correct answer.
  3. It multiplies neutrons through fast fission in fertile material.
  4. It absorbs excess neutrons to hold the reactor subcritical.

Explanation

The moderator slows fast fission neutrons through repeated elastic collisions until they reach thermal energies, where fission cross sections for U-235 are much larger. Light nuclei such as hydrogen in water are effective because they remove energy efficiently per collision.

Question 28Reactor Theory and Nuclear Physics

Which particle must be released in a fission event for a self-sustaining chain reaction to be possible?

  1. At least one beta particle per fission on average
  2. At least one gamma ray per fission on average
  3. At least one neutron per fission on average Correct answer.
  4. At least one alpha particle per fission on average

Explanation

A chain reaction requires that fission release neutrons capable of inducing further fissions. On average more than one neutron per fission must be produced and must survive leakage and parasitic absorption for the chain to sustain itself.

Question 29Reactor Theory and Nuclear Physics

Which quantity equals the average number of neutrons produced per neutron absorbed in the fuel, and therefore couples the fission yield to the fuel's absorption behavior?

  1. The diffusion coefficient
  2. The reproduction factor eta Correct answer.
  3. k-infinity
  4. The migration length

Explanation

The reproduction factor eta is defined as the fission neutrons produced per neutron absorbed in fuel, equal to nu times the ratio of fission to absorption cross sections. It captures both the neutron yield and competing capture in the fuel.

Question 30Reactor Theory and Nuclear Physics

A reactor has k of 1.02. Using the standard definition of reactivity, what is the reactivity in percent?

  1. 2.00 percent
  2. 0.98 percent
  3. 1.96 percent Correct answer.
  4. 0.02 percent

Explanation

Reactivity is defined as (k minus 1) divided by k, then expressed as a percentage. Here 0.02 divided by 1.02 gives about 0.0196, or roughly 1.96 percent.

Question 31Reactor Theory and Nuclear Physics

A reactor has k of 0.95 and a strong startup neutron source is present. Which statement describes the steady-state neutron population?

  1. The population reaches a constant level set by source strength and subcritical multiplication. Correct answer.
  2. The population grows without bound at a constant period.
  3. The population oscillates indefinitely between two fixed levels.
  4. The population decays to zero and the reactor shuts down.

Explanation

With k below one, each generation produces fewer neutrons than the previous one, but the source continuously adds neutrons. The result is a stable, lower-than-critical population whose size is amplified relative to the source by the subcritical multiplication factor 1/(1 minus k).

Question 32Reactor Theory and Nuclear Physics

A reactor is initially critical at low power. Control rods are withdrawn so that it becomes supercritical with a small positive reactivity, well below prompt critical. Which statement describes the resulting power behavior?

  1. Power rises on a stable period determined mainly by delayed neutrons. Correct answer.
  2. Power remains constant because delayed neutrons compensate exactly.
  3. Power decreases until the rods are reinserted further.
  4. Power rises almost instantaneously on the prompt neutron lifetime.

Explanation

For small positive reactivity below the delayed neutron fraction, the reactor period is governed largely by delayed neutron precursors, giving a slow, controllable rise. Prompt neutrons alone would give an extremely short period, but their contribution is moderated by the delayed fraction.

Question 33Reactor Theory and Nuclear Physics

Which statement about prompt criticality is correct?

  1. It occurs when k-infinity falls below one due to fission product buildup.
  2. It occurs when reactivity equals or exceeds the effective delayed neutron fraction. Correct answer.
  3. It occurs whenever the reactor exceeds its licensed thermal power limit.
  4. It occurs when the moderator temperature coefficient becomes positive.

Explanation

Prompt criticality is reached when the inserted reactivity is at least equal to beta, the effective delayed neutron fraction, so the chain can be sustained by prompt neutrons alone. Power then rises on the prompt neutron lifetime, which is extremely short.

Question 34Reactor Theory and Nuclear Physics

A reactor is operating at power when a large negative reactivity insertion occurs, driving k below one. Which statement describes the neutron population immediately after the insertion?

  1. It rises briefly then settles at a higher constant level.
  2. It increases on a period set by the delayed neutron fraction.
  3. It stays exactly constant because the source term dominates.
  4. It decreases, with the rate of decrease set by the prompt and delayed neutron behavior. Correct answer.

Explanation

A negative reactivity insertion makes k less than one, so the neutron population falls. The decay is initially influenced by prompt neutrons and then dominated by the longer-lived delayed neutron precursors, giving a characteristic shutdown transient.

Question 35Reactor Theory and Nuclear Physics

Which statement correctly describes the effect of a positive void coefficient in a reactor core?

  1. Steam voids increase the fuel absorption cross section and lower reactivity.
  2. Steam voids remove moderator and reduce reactivity, providing inherent shutdown.
  3. Steam voids have no effect on reactivity because they contain no fuel.
  4. Steam voids reduce moderation, which can increase reactivity and challenge safety. Correct answer.

Explanation

In a core where the coolant also moderates, replacing liquid with steam voids reduces moderation, hardening the neutron spectrum. If the core has excess reactivity and resonance absorption is low, this can raise reactivity, so a positive void coefficient is a safety concern.

Question 36Reactor Theory and Nuclear Physics

A neutron scatters off a heavy nucleus and transfers some kinetic energy while leaving the nucleus in an excited state that later emits a gamma ray. Which process is described?

  1. Elastic scattering
  2. Radiative capture
  3. Fission
  4. Inelastic scattering Correct answer.

Explanation

In inelastic scattering the neutron loses kinetic energy that excites the target nucleus, which then de-excites by gamma emission. It differs from elastic scattering, where kinetic energy is conserved among the collision partners.

Question 37Reactor Theory and Nuclear Physics

A reactor is critical with a neutron generation time of 1e-4 seconds and a small positive reactivity of 0.001 in absolute terms. Using the simple prompt approximation, what is the approximate e-folding time of the neutron population?

  1. 1 second
  2. 10 seconds
  3. 100 seconds
  4. 0.1 seconds Correct answer.

Explanation

For small reactivity the prompt approximation gives the e-folding time as the generation time divided by reactivity, about 1e-4 divided by 1e-3, or 0.1 seconds. In a real reactor delayed neutrons lengthen the period well beyond this prompt estimate.

Question 38Reactor Theory and Nuclear Physics

A reactor has k-infinity of 1.08 and a total non-leakage probability of 0.96. What is the effective multiplication factor k-eff?

  1. 1.037 Correct answer.
  2. 0.96
  3. 1.04
  4. 1.08

Explanation

The effective multiplication factor is k-infinity multiplied by the non-leakage probabilities, so 1.08 times 0.96 equals about 1.037. Leakage reduces the infinite-medium multiplication to the finite-core value.

Question 39Reactor Theory and Nuclear Physics

A thermal reactor's fuel is changed from low-enriched uranium to a fuel with a higher U-235 enrichment. Which combination of effects on the six-factor formula is expected, all else equal?

  1. Eta increases and the thermal utilization factor f tends to decrease. Correct answer.
  2. Eta decreases and the resonance escape probability p increases sharply.
  3. Eta increases and the fast fission factor epsilon decreases to below one.
  4. Eta decreases and thermal utilization f increases strongly.

Explanation

Higher enrichment raises the ratio of fissile to fertile fuel, increasing eta because fission absorption competes less with U-238 capture. However, the fuel's macroscopic absorption per unit volume also rises, which tends to lower the thermal utilization factor f.

Question 40Reactor Theory and Nuclear Physics

A reactor is exactly critical at k-eff of 1.000. Which statement about the neutron balance is correct?

  1. Neutron production equals fission absorption only, ignoring leakage.
  2. Neutron production is less than losses but the source maintains power.
  3. Neutron production exceeds losses, so power rises steadily.
  4. Neutron production equals losses from absorption plus leakage. Correct answer.

Explanation

Criticality means the effective multiplication factor is one, so each neutron generation replaces itself exactly. Production therefore balances all losses, including parasitic absorption and leakage from the core.

Question 41Reactor Operations and Control

In a nuclear reactor, the rate of fission reactions is controlled primarily by adjusting which parameter?

  1. The coolant flow rate through the steam generator
  2. The condenser vacuum pressure
  3. The turbine generator load setpoint
  4. The neutron population within the core Correct answer.

Explanation

Reactor power is proportional to the neutron population, and control rods and other reactivity devices act by changing the neutron balance in the core. Coolant flow, turbine load, and condenser vacuum are secondary-side parameters that follow, rather than directly control, fission rate.

Question 42Reactor Operations and Control

Which term describes the condition in which a reactor's neutron population remains constant over time?

  1. Critical Correct answer.
  2. Prompt critical
  3. Supercritical
  4. Subcritical

Explanation

A critical reactor has an effective multiplication factor of exactly one, so each generation of neutrons replaces the previous one and power stays steady. Subcritical and supercritical describe decreasing and increasing populations respectively.

Question 43Reactor Operations and Control

Control rods in a thermal reactor are most commonly made from which material because of its high neutron absorption cross-section?

  1. Boron carbide Correct answer.
  2. Stainless steel
  3. Zirconium alloy
  4. Graphite

Explanation

Boron carbide is a strong neutron absorber and is widely used as a control rod material because boron-10 captures thermal neutrons effectively. Zirconium and graphite are chosen for low absorption, and steel is a structural material.

Question 44Reactor Operations and Control

A reactor operator notices that reactor power has increased slightly and the coolant outlet temperature is rising accordingly. Which control action is consistent with restoring the reactor to its original power level?

  1. Increase feedwater flow to the steam generator
  2. Insert control rods to add negative reactivity Correct answer.
  3. Withdraw control rods to add positive reactivity
  4. Reduce the turbine load setpoint

Explanation

Inserting control rods adds negative reactivity, reducing the neutron population and returning power toward its prior value. Withdrawing rods would worsen the power increase, while secondary-side actions do not directly reduce fission rate.

Question 45Reactor Operations and Control

In reactor kinetics, what does the term 'delayed neutrons' refer to?

  1. Neutrons emitted from fission products after a measurable delay Correct answer.
  2. Neutrons produced directly at the instant of fission
  3. Neutrons scattered elastically by the moderator
  4. Neutrons that leak from the core before causing fission

Explanation

A small fraction of neutrons are released by the decay of certain fission products, minutes to seconds after fission, and their presence gives operators time to control the reactor. Prompt neutrons are released essentially instantly at fission.

Question 46Reactor Operations and Control

The reactor protection system initiates a scram (trip) primarily to accomplish which objective?

  1. Raise the pressurizer level setpoint
  2. Rapidly insert negative reactivity to shut down the fission chain reaction Correct answer.
  3. Reduce the turbine speed setpoint
  4. Increase coolant flow to the steam generators

Explanation

A scram rapidly inserts control rods or otherwise adds large negative reactivity to terminate the fission chain reaction. Secondary-side actions such as turbine trip occur as a consequence but are not the purpose of the scram.

Question 47Reactor Operations and Control

Which parameter is the most direct indicator of the instantaneous fission rate in a reactor core?

  1. Neutron flux measured by in-core or ex-core detectors Correct answer.
  2. Steam generator water level
  3. Condenser circulating water temperature
  4. Turbine bearing vibration

Explanation

Neutron flux is proportional to the fission rate, so neutron detectors provide the most direct measurement of reactor power. The other listed parameters are secondary-side or mechanical indicators that respond with delay.

Question 48Reactor Operations and Control

Why is the reactor coolant system maintained at high pressure in a pressurized water reactor?

  1. To prevent bulk boiling of the primary coolant in the core Correct answer.
  2. To increase the neutron absorption by the moderator
  3. To lower the melting point of the fuel pellets
  4. To reduce the thermal power produced by fission

Explanation

High primary pressure raises the coolant saturation temperature, keeping the coolant in the liquid phase and preventing bulk boiling in the core. Pressure does not set fission power and does not change the fuel melting point.

Question 49Reactor Operations and Control

A reactor is operating at steady full power when the operator observes that the power defect from xenon buildup is being offset by gradual control rod withdrawal. What underlying phenomenon explains this behavior?

  1. Xenon-135 accumulates after power changes and adds negative reactivity over time Correct answer.
  2. Xenon-135 decays rapidly and adds positive reactivity within minutes
  3. Samarium-149 is produced only during shutdown
  4. Iodine-131 directly absorbs neutrons and drives the power defect

Explanation

Xenon-135 is a strong neutron absorber that builds up as iodine-131 decays after power changes, gradually adding negative reactivity that must be compensated. This reactivity change is the xenon power defect, and operators withdraw rods to offset it.

Question 50Reactor Operations and Control

During a reactor startup, the operator observes that neutron count rate has doubled while the source range instrument remains on scale. What does this observation indicate about the reactor period?

  1. The reactor is shutdown with no source neutrons
  2. The reactor is supercritical with a positive period Correct answer.
  3. The reactor is exactly critical with an infinite period
  4. The reactor is subcritical with a negative period

Explanation

A steadily doubling count rate shows an increasing neutron population, which corresponds to a supercritical state and a finite positive period. Criticality would produce a constant count rate, and subcriticality would produce a decreasing rate.