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PE Nuclear practice questions

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Question 1Nuclear Power Plant Systems and Components

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

  1. Remove moisture from steam entering the turbine
  2. Maintain reactor coolant system pressure Correct answer.
  3. Provide emergency boron injection to the core
  4. Cool the reactor coolant before it enters the core

Explanation

The pressurizer uses a steam bubble, electric heaters, and spray to regulate reactor coolant system pressure. It does not serve as the primary heat sink or as the main boron injection path.

Question 2Nuclear Power Plant Systems and Components

Which feature is characteristic of a boiling water reactor but not a pressurized water reactor?

  1. Steam is produced directly in the reactor vessel Correct answer.
  2. A separate steam generator transfers heat to the secondary loop
  3. Graphite is used to moderate neutrons
  4. Use of heavy water as a moderator

Explanation

In a BWR, the reactor coolant boils in the vessel and the resulting steam flows directly to the turbine. A PWR keeps the primary coolant pressurized and liquid, transferring heat to a secondary loop through steam generators.

Question 3Nuclear Power Plant Systems and Components

What neutron-absorbing material is commonly used in pressurized water reactor control rods?

  1. Uranium dioxide
  2. Hafnium metal
  3. Silver-indium-cadmium alloy Correct answer.
  4. Zirconium alloy

Explanation

PWR control rods commonly use silver-indium-cadmium alloy because it has strong neutron absorption and good mechanical properties. Boron carbide is also used in some designs, but the silver-indium-cadmium alloy is a standard PWR absorber.

Question 4Nuclear Power Plant Systems and Components

What is the main purpose of the main condenser in a steam power plant?

  1. To increase the pressure of exhaust steam
  2. To remove dissolved oxygen from the condensate
  3. To condense turbine exhaust steam and maintain a low backpressure Correct answer.
  4. To heat feedwater before it enters the steam generator

Explanation

The condenser rejects heat from turbine exhaust steam and converts it back to liquid, creating a low-pressure region that improves turbine efficiency. Feedwater heating and oxygen removal are separate functions performed by other equipment.

Question 5Nuclear Power Plant Systems and Components

Under what condition do the emergency core cooling system accumulators begin injecting borated water into the reactor coolant system?

  1. Immediately after a reactor trip signal
  2. When reactor coolant system pressure falls below a predetermined setpoint Correct answer.
  3. When offsite power is lost
  4. When containment pressure rises above atmospheric pressure

Explanation

Accumulators are passive safety injection tanks that discharge through check valves when reactor coolant system pressure drops below the tank pressure setpoint. They do not depend on a reactor trip signal or offsite power.

Question 6Nuclear Power Plant Systems and Components

In a PWR, what is the primary function of the steam generator?

  1. To pressurize the reactor coolant system
  2. To condense steam after it leaves the turbine
  3. To transfer heat from the primary coolant to the secondary water Correct answer.
  4. To remove boron from the reactor coolant

Explanation

The steam generator is a heat exchanger that transfers heat from the radioactive primary coolant to the secondary water, producing steam. The primary and secondary fluids remain separated so that steam entering the turbine is not radioactive under normal conditions.

Question 7Nuclear Power Plant Systems and Components

Which moderator is used in a CANDU reactor to allow the use of natural uranium fuel?

  1. Heavy water Correct answer.
  2. Light water
  3. Beryllium oxide
  4. Graphite

Explanation

CANDU reactors use heavy water as both moderator and coolant because heavy water absorbs fewer neutrons than light water. This low neutron absorption makes it possible to sustain a chain reaction with natural uranium fuel.

Question 8Nuclear Power Plant Systems and Components

What is the primary safety function of a nuclear power plant containment structure?

  1. To confine radioactive material and withstand internal pressure during accidents Correct answer.
  2. To cool the reactor core during normal operation
  3. To support the reactor vessel during an earthquake
  4. To filter radioactive particulates from the turbine building ventilation

Explanation

Containment is the final physical barrier that limits the release of radioactive fission products to the environment. It is designed to withstand accident pressures and temperatures while remaining leaktight.

Question 9Nuclear Power Plant Systems and Components

Which system is used to remove decay heat from the reactor coolant during a normal shutdown?

  1. Containment spray system
  2. Emergency diesel generator system
  3. Main turbine bypass system
  4. Residual heat removal system Correct answer.

Explanation

The residual heat removal system provides long-term cooling of the reactor coolant after shutdown, when decay heat is still being produced. Emergency systems and turbine bypass do not serve as the normal shutdown heat removal path.

Question 10Nuclear Power Plant Systems and Components

What is the function of the reactor coolant pumps in a PWR?

  1. To circulate primary coolant through the reactor core Correct answer.
  2. To condense exhaust steam in the main condenser
  3. To supply feedwater to the steam generators
  4. To inject borated water into the core

Explanation

Reactor coolant pumps provide forced circulation of the primary coolant so heat is removed efficiently from the fuel and transported to the steam generators. Feedwater and condensate systems serve the secondary loop.

Question 11Nuclear Power Plant Systems and Components

What is the purpose of the main turbine governor valves?

  1. To vent steam from the containment
  2. To control the flow of feedwater to the steam generators
  3. To isolate the condenser from the turbine
  4. To regulate steam flow to the turbine Correct answer.

Explanation

Governor valves modulate the flow of steam into the turbine to control turbine speed and load. They are part of the turbine control system rather than the feedwater or containment systems.

Question 12Nuclear Power Plant Systems and Components

What is the role of the main feedwater system in a PWR?

  1. To provide cooling water to the main condenser
  2. To supply water to the steam generators to maintain secondary inventory Correct answer.
  3. To lubricate the turbine bearings
  4. To inject boron into the reactor coolant

Explanation

The main feedwater system supplies treated water to the steam generators to replace steam that has been sent to the turbine. It maintains secondary-side inventory and supports heat transfer from the primary system.

Question 13Nuclear Power Plant Systems and Components

What is the function of the pressurizer safety valves in a PWR?

  1. To protect the reactor coolant system from overpressure Correct answer.
  2. To inject boron during a reactivity accident
  3. To vent noncondensable gases from the condenser
  4. To control pressurizer water level during normal operation

Explanation

Pressurizer safety valves open at a set pressure to prevent the reactor coolant system from exceeding its design pressure limit. They are passive overpressure protection devices, not normal level or reactivity control devices.

Question 14Nuclear Power Plant Systems and Components

What is the purpose of the main steam isolation valves in a BWR?

  1. To isolate the steam lines and limit the release of steam from containment Correct answer.
  2. To provide cooling water to the condenser
  3. To control feedwater flow to the reactor vessel
  4. To regulate turbine speed during normal operation

Explanation

Main steam isolation valves close to isolate the reactor vessel and containment from the main steam lines during abnormal conditions. This limits the loss of steam and the potential release of radioactive material.

Question 15Nuclear Power Plant Systems and Components

In a PWR, soluble boron in the reactor coolant is used primarily for what purpose?

  1. To improve heat transfer in the core
  2. To moderate neutrons and increase reactivity
  3. To lubricate the reactor coolant pumps
  4. To absorb neutrons and control reactivity Correct answer.

Explanation

Boron is a strong neutron absorber that is dissolved in the reactor coolant to control long-term reactivity changes. Its concentration is adjusted to compensate for fuel burnup and to provide shutdown margin.

Question 16Nuclear Power Plant Systems and Components

What is the function of the jet pumps in a BWR?

  1. To recirculate coolant through the reactor core Correct answer.
  2. To cool the reactor vessel during a loss-of-coolant accident
  3. To inject borated water into the reactor vessel
  4. To supply feedwater to the reactor vessel

Explanation

BWR jet pumps use recirculation pump flow to drive additional coolant flow through the core. This internal recirculation loop enhances core flow without requiring external recirculation piping.

Question 17Nuclear Power Plant Systems and Components

During a rapid load rejection, reactor coolant system pressure rises. How does the pressurizer spray system help control this pressure?

  1. It injects boron to reduce reactor power
  2. It reduces pressure by condensing steam in the pressurizer Correct answer.
  3. It maintains water level by isolating the surge line
  4. It increases pressure by flashing water into steam

Explanation

Pressurizer spray introduces cooler water into the steam space, condensing some steam and reducing pressure. This helps limit the pressure transient after a load rejection.

Question 18Nuclear Power Plant Systems and Components

After a loss-of-coolant accident, when the refueling water storage tank reaches its low-level setpoint, what is the typical long-term alignment for emergency core cooling system pump suction?

  1. The pumps switch to the spent fuel pool
  2. The pumps switch to the service water system
  3. The pumps switch to the main condenser hotwell
  4. The pumps switch to the containment sump Correct answer.

Explanation

Once the refueling water storage tank is nearly empty, ECCS pumps realign to take suction from the containment sump in recirculation mode. This allows continued cooling using water already inside containment.

Question 19Nuclear Power Plant Systems and Components

In a BWR, what dual function do the safety/relief valves perform?

  1. Controlling feedwater and isolating the condenser
  2. Isolating the main steam lines and cooling the containment
  3. Bypassing steam to the condenser and heating feedwater
  4. Providing overpressure protection and automatic depressurization for ECCS Correct answer.

Explanation

BWR safety/relief valves protect the reactor vessel from overpressure and can be opened deliberately to depressurize the vessel. Depressurization allows low-pressure ECCS systems to inject water into the core.

Question 20Nuclear Power Plant Systems and Components

Which indication is most characteristic of a steam generator tube rupture in a PWR?

  1. A rapid increase in feedwater flow
  2. An increase in secondary system radiation and a primary-to-secondary leak Correct answer.
  3. A sudden decrease in primary coolant pressure
  4. A decrease in secondary system radiation

Explanation

A tube rupture allows radioactive primary coolant to leak into the secondary system, raising secondary-side radiation levels. Operators also see indications of a primary-to-secondary leak and may need to isolate the affected steam generator.

Question 21Nuclear Power Plant Systems and Components

What is the primary purpose of the condensate polishing system in a nuclear power plant?

  1. To heat the condensate to improve plant efficiency
  2. To add boron to the condensate
  3. To remove dissolved and suspended impurities from condensate Correct answer.
  4. To cool the condensate before it enters the feedwater system

Explanation

Condensate polishing uses ion exchange and filtration to remove impurities that could cause corrosion or deposits in the steam generators. It helps maintain high water purity in the secondary loop.

Question 22Nuclear Power Plant Systems and Components

Why is hydrogen often used as a cooling medium in large turbine generators?

  1. It has a lower density and higher heat-transfer capability than air Correct answer.
  2. It provides electrical insulation between windings
  3. It seals the generator shaft against steam ingress
  4. It provides lubrication for the generator bearings

Explanation

Hydrogen has a low density, which reduces windage losses, and good heat-transfer properties, which improve cooling of the generator windings. The generator casing is designed to contain the hydrogen safely.

Question 23Nuclear Power Plant Systems and Components

What is the function of the emergency diesel generators in a nuclear power plant?

  1. To supply emergency alternating current power to safety-related loads Correct answer.
  2. To compress air for the containment purge system
  3. To provide steam to the turbine during startup
  4. To supply direct current to the control room batteries

Explanation

Emergency diesel generators start automatically on loss of offsite power and supply AC power to safety-related buses. This powers equipment needed for safe shutdown and accident mitigation.

Question 24Nuclear Power Plant Systems and Components

What are the primary functions of the spent fuel pool cooling and cleanup system?

  1. To moderate neutrons and increase reactivity
  2. To boil off excess water and concentrate boron
  3. To remove decay heat and maintain water clarity and purity Correct answer.
  4. To maintain the pool at high pressure to prevent boiling

Explanation

Spent fuel continues to produce decay heat and must be cooled in the pool. The cooling and cleanup system removes this heat and filters the water to maintain clarity, purity, and radiological cleanliness.

Question 25Nuclear Power Plant Systems and Components

What is the purpose of the moisture separator reheaters located between the high-pressure and low-pressure turbines?

  1. To remove moisture from steam and reheat it before it enters the low-pressure turbine Correct answer.
  2. To remove boron from the secondary steam
  3. To condense extracted steam for feedwater heating
  4. To reduce steam pressure before the condenser

Explanation

Moisture separator reheaters remove water droplets from steam and reheat the steam to improve cycle efficiency. This also reduces droplet erosion of low-pressure turbine blades.

Question 26Nuclear Power Plant Systems and Components

In a boiling water reactor, what is the primary function of the reactor water cleanup (RWCU) system?

  1. Filters condensate upstream of the feedwater heaters to remove corrosion products
  2. Removes dissolved and suspended impurities from reactor recirculation water Correct answer.
  3. Supplies makeup water to the vessel to offset level shrink after a scram
  4. Injects borated water to shut down the reactor following a scram

Explanation

The RWCU system continuously takes a side stream from the reactor recirculation loops (and vessel head and bottom drains), passes it through filters and demineralizers, and returns it to the feedwater or vessel. This removes ionic and particulate impurities, including activated corrosion products, keeping reactor water conductivity low and limiting radiation buildup in the primary system. It is a water-chemistry and activity-control system, not a makeup, condensate-treatment, or shutdown system.

Question 27Reactor Physics and Nuclear Criticality

In a nuclear reactor, the effective multiplication factor k_eff is defined as the ratio of

  1. delayed neutrons emitted divided by prompt neutrons emitted per fission
  2. fission neutrons emitted divided by neutrons absorbed in the fuel only
  3. neutrons produced per generation divided by neutrons lost per generation Correct answer.
  4. thermal neutrons present divided by fast neutrons present in the core

Explanation

k_eff compares the neutron population in one generation with that in the immediately preceding generation, accounting for absorption and leakage. A value of 1 means the population is steady, or critical.

Question 28Reactor Physics and Nuclear Criticality

For an infinite homogeneous multiplying medium, the infinite multiplication factor k_inf is given by which expression?

  1. Σ_f / (ν Σ_a)
  2. ν Σ_f / Σ_a Correct answer.
  3. Σ_a / (ν Σ_f)
  4. ν Σ_a / Σ_f

Explanation

With no leakage, k_inf equals the neutron production rate νΣ_f divided by the total absorption rate Σ_a. This ratio directly measures neutron gain per generation.

Question 29Reactor Physics and Nuclear Criticality

For U-235 thermal fission, the effective delayed neutron fraction β_eff is approximately

  1. 0.00065
  2. 0.065
  3. 0.0065 Correct answer.
  4. 0.65

Explanation

The delayed neutron fraction for U-235 is about 0.0065, or 0.65 percent, which defines one dollar of reactivity. β_eff is slightly lower in a real core because delayed neutrons are born at lower energy and can leak or be captured differently.

Question 30Reactor Physics and Nuclear Criticality

A neutron with energy 0.025 eV is commonly described as

  1. a thermal neutron Correct answer.
  2. an epithermal neutron
  3. a fast neutron
  4. a fission-spectrum neutron

Explanation

0.025 eV is the most probable energy of a neutron in thermal equilibrium at about 20 degrees Celsius, corresponding to 2200 m/s. Neutrons near this energy are called thermal neutrons.

Question 31Reactor Physics and Nuclear Criticality

The primary function of a moderator in a thermal reactor is to

  1. absorb excess neutrons to control the chain reaction
  2. increase the number of neutrons released per fission
  3. reflect neutrons back into the core from the vessel
  4. slow neutrons by scattering so fission probability rises Correct answer.

Explanation

Moderation by elastic scattering reduces neutron energy into the thermal range where the U-235 fission cross section is large. Absorbers and reflectors serve different functions.

Question 32Reactor Physics and Nuclear Criticality

For a typical large PWR, the void coefficient of reactivity is

  1. strongly positive
  2. essentially zero
  3. positive only at hot zero power
  4. negative Correct answer.

Explanation

In a PWR, voiding reduces water density, which degrades moderation and hardens the spectrum. This reduces the fission rate and increases leakage, giving a negative void coefficient.

Question 33Reactor Physics and Nuclear Criticality

Which fission product is the most important thermal neutron poison in an operating thermal reactor?

  1. Xe-133
  2. I-135
  3. Kr-85
  4. Xe-135 Correct answer.

Explanation

Xe-135 has an enormous thermal absorption cross section and is produced directly and from I-135 decay. Its buildup and burnout drive post-shutdown reactivity changes.

Question 34Reactor Physics and Nuclear Criticality

A reactor becomes prompt critical when reactivity equals

  1. two dollars
  2. zero
  3. one dollar, β_eff Correct answer.
  4. the delayed neutron precursor decay constant

Explanation

Prompt criticality occurs when reactivity reaches β_eff, or one dollar. At that point the chain reaction is sustained by prompt neutrons alone and power can rise very rapidly.

Question 35Reactor Physics and Nuclear Criticality

A reactor has k_eff equal to 0.990. Its reactivity in pcm is closest to

  1. -100 pcm
  2. +1010 pcm
  3. -1010 pcm Correct answer.
  4. -990 pcm

Explanation

Reactivity is rho = (k - 1)/k = (0.990 - 1)/0.990 = -0.0101, or -1010 pcm, since 1 pcm equals 10^-5.

Question 36Reactor Physics and Nuclear Criticality

A subcritical system with k_eff equal to 0.95 is driven by a constant neutron source. The source multiplication factor is

  1. 10
  2. 5
  3. 20 Correct answer.
  4. 1.05

Explanation

A steady source in a subcritical system produces a neutron population proportional to 1/(1 - k_eff). Here 1/(1 - 0.95) equals 20.

Question 37Reactor Physics and Nuclear Criticality

The negative Doppler coefficient in a U-235/U-238 fueled reactor arises primarily because increasing fuel temperature

  1. increases the fission cross section of U-235 sharply
  2. reduces the moderator density and thus moderation
  3. broadens U-238 resonances, raising capture Correct answer.
  4. increases leakage from the core by expansion

Explanation

As fuel temperature rises, Doppler broadening of U-238 resonances increases neutron capture in the resonance region before neutrons thermalize. The resulting reduction in k is the negative Doppler feedback.

Question 38Reactor Physics and Nuclear Criticality

Which expression represents the four-factor formula for k_inf of a thermal reactor?

  1. k_inf = η p / (ε f)
  2. k_inf = η ε / (p f)
  3. k_inf = ε + p + f + η
  4. k_inf = ε p f η Correct answer.

Explanation

The four-factor formula is k_inf = ε p f η, where ε is the fast fission factor, p the resonance escape probability, f the thermal utilization, and η the reproduction factor. The factors multiply because each acts on the neutron population in sequence.

Question 39Reactor Physics and Nuclear Criticality

For a bare spherical core, the geometric buckling B_g^2 is given by

  1. (π/H)^2
  2. (2.405/R)^2
  3. (π/R)^2 Correct answer.
  4. (2.405/H)^2 + (π/R)^2

Explanation

For a bare sphere, the geometric buckling is B_g^2 = (π/R)^2, obtained from the lowest eigenvalue of the diffusion equation with zero flux at the extrapolated boundary.

Question 40Reactor Physics and Nuclear Criticality

The differential worth of a control rod inserted from the top of a core typically

  1. decreases monotonically as the rod enters the core
  2. peaks near the core midplane and is small at the ends Correct answer.
  3. is constant along the full inserted length
  4. peaks at the bottom of the core and is zero at the top

Explanation

Differential rod worth is proportional to the neutron flux squared at the rod tip. It is small at the core ends and largest near the midplane where flux peaks, producing an S-shaped integral worth curve.

Question 41Reactor Physics and Nuclear Criticality

For a bare spherical assembly, if the density of fissile material is doubled, the critical mass becomes approximately

  1. unchanged from the original
  2. double the original
  3. one quarter of the original Correct answer.
  4. half the original

Explanation

For a bare sphere the critical radius varies as 1/rho and the critical mass as 1/rho^2. Doubling density therefore reduces the critical mass to about one quarter.

Question 42Reactor Physics and Nuclear Criticality

The conversion ratio of a reactor is defined as the ratio of

  1. fissile material consumed to fertile material consumed
  2. thermal fissions to fast fissions
  3. fission neutrons produced to neutrons absorbed in control rods
  4. fissile atoms produced to fissile atoms consumed Correct answer.

Explanation

The conversion ratio is the number of fissile atoms produced from fertile capture divided by the number of fissile atoms consumed. A value above one indicates breeding.

Question 43Reactor Physics and Nuclear Criticality

A reactor with β_eff equal to 0.0065 has a reactivity of +0.00325. Expressed in dollars, this is

  1. 2.00 dollars
  2. 0.25 dollars
  3. 0.50 dollars Correct answer.
  4. 1.00 dollars

Explanation

Reactivity in dollars is rho divided by β_eff. With 0.00325 divided by 0.0065, the value is 0.50 dollars.

Question 44Reactor Physics and Nuclear Criticality

As the moderator-to-fuel ratio of a thermal reactor is increased from a very low value, k_inf first rises and then falls because

  1. fission cross sections increase at high moderation
  2. resonance escape improves while moderator absorption grows Correct answer.
  3. delayed neutron fraction decreases with moderation
  4. fertile capture increases without limit

Explanation

At low moderator-to-fuel ratio resonance escape is poor; as moderation improves, p rises. Beyond the optimum, thermal utilization f falls because the moderator absorbs more neutrons, so the product peaks.

Question 45Reactor Physics and Nuclear Criticality

Two bare cores contain the same mass of fissile material, one a sphere and one a thin slab. The slab has

  1. identical leakage because leakage depends only on mass
  2. lower leakage because of its larger surface area
  3. higher leakage per unit volume and a larger critical mass Correct answer.
  4. higher leakage but a smaller critical mass

Explanation

Leakage depends on surface area relative to volume. For the same mass, a thin slab has more surface area per unit volume than a sphere, so it leaks more and requires a larger critical mass.

Question 46Reactor Physics and Nuclear Criticality

Following a small step increase in reactivity well below β_eff, reactor power

  1. jumps promptly, then rises on the stable period Correct answer.
  2. rises immediately with no delayed contribution
  3. decreases before rising on the stable period
  4. remains constant until delayed precursors decay

Explanation

Below prompt critical, power first jumps by the prompt jump as prompt neutrons adjust, then rises with the stable reactor period governed by delayed neutron precursors.

Question 47Reactor Physics and Nuclear Criticality

A bare spherical reactor has material buckling B_m^2 equal to 0.010 cm^-2. Its critical radius is closest to

  1. 14.0 cm
  2. 10.0 cm
  3. 31.4 cm Correct answer.
  4. 22.0 cm

Explanation

At criticality, geometric buckling equals material buckling: (π/R)^2 = 0.010 cm^-2. Thus R = π divided by the square root of 0.010, which is 31.4 cm.

Question 48Reactor Physics and Nuclear Criticality

In the one-group delayed neutron model, the stable reactor period T for small positive reactivity satisfies which relation when the prompt lifetime is neglected?

  1. rho = β λ T
  2. rho = β/(1 + λT) Correct answer.
  3. rho = β/(λT)
  4. rho = β(1 + λT)

Explanation

With one delayed group and the prompt lifetime neglected, the inhour relation reduces to rho = β/(1 + λT), where λ is the precursor decay constant. This shows that long periods correspond to small positive reactivity.

Question 49Reactor Physics and Nuclear Criticality

Which statement about the moderator temperature coefficient in a PWR is correct?

  1. It is typically negative and more negative as boron decreases Correct answer.
  2. It is positive because water expansion increases moderation
  3. It is positive at beginning of cycle and negative at end
  4. It is independent of soluble boron concentration

Explanation

The PWR moderator temperature coefficient is normally negative because heating expands water and reduces moderation. It becomes more negative at low soluble boron concentration, near end of cycle.

Question 50Reactor Physics and Nuclear Criticality

Given diffusion coefficient D equal to 0.80 cm and macroscopic absorption cross section Σ_a equal to 0.020 cm^-1, the diffusion length L is closest to

  1. 2.5 cm
  2. 40 cm
  3. 6.3 cm Correct answer.
  4. 0.16 cm

Explanation

The diffusion length is L = sqrt(D/Σ_a). Substituting gives sqrt(0.80/0.020) = sqrt(40) = 6.3 cm.