Reactor Systems Design Diagnostic Exam

What does PWR stand for?

Pressurized Water Reactor.

What is the main purpose of high pressure in a PWR primary system?

To keep the primary coolant from boiling even though its temperature is above 300°C.

What is the typical primary pressure of a PWR?

About 15.5 MPa or 2250 psi.

What is the approximate PWR core inlet temperature?

About 290°C.

What is the approximate PWR core outlet temperature?

About 320–325°C.

What is the hot leg in a PWR?

The pipe carrying heated primary coolant from the reactor vessel to the steam generator.

What is the cold leg in a PWR?

The pipe carrying cooled primary coolant from the steam generator back toward the reactor vessel.

What does the steam generator do in a PWR?

Transfers heat from the radioactive primary coolant to the secondary water, producing steam without mixing the two fluids.

What does the pressurizer do in a PWR?

Controls and maintains primary-system pressure and prevents bulk boiling.

What does the reactor coolant pump do?

Circulates primary coolant through the reactor core and steam generator.

Does primary coolant normally boil in a PWR core?

No. The high primary-system pressure prevents bulk boiling.

What does BWR stand for?

Boiling Water Reactor.

What is the major difference between a PWR and BWR?

A PWR uses a separate secondary loop and steam generator, while a BWR boils water directly in the reactor vessel and sends that steam to the turbine.

Where is steam produced in a PWR?

In the secondary side of the steam generator.

Where is steam produced in a BWR?

Directly inside the reactor vessel.

What is the basic PWR primary-loop path?

Core → hot leg → steam generator → cold leg/reactor coolant pump → core.

What is the basic secondary-loop path in a PWR?

Steam generator → turbine → condenser → feedwater pump → steam generator.

What is the purpose of the condenser?

Condenses turbine exhaust steam back into liquid water while rejecting waste heat.

What is the purpose of the turbine?

Converts the thermal energy of steam into mechanical shaft work.

What is the purpose of the generator?

Converts mechanical shaft work from the turbine into electrical energy.

What is the purpose of the feedwater pump?

Raises the pressure of condensed water and sends it back toward the steam generator.

What is the basic Rankine cycle sequence?

Pump → heat addition/steam generator → turbine → condenser → pump.

What is thermal efficiency?

The fraction of heat input converted into net work: η = W_net/Q_in.

What is the approximate thermal efficiency of a conventional light-water reactor plant?

Roughly 30–35%.

What is the steady-flow energy equation used for?

Relating heat transfer, work, enthalpy, kinetic energy, and potential energy for flowing systems.

What is the simplified heat balance for a flowing liquid?

Qdot = mdot cp ΔT.

If reactor power stays constant but coolant mass flow rate decreases, what happens to coolant temperature rise?

It increases because ΔT = Qdot/(mdot cp).

What happens to coolant temperature rise if mass flow rate increases at constant reactor power?

It decreases.

What is enthalpy?

A thermodynamic property useful for flowing systems, defined as h = u + Pv.

What is specific heat cp?

The energy required to raise the temperature of a unit mass by one degree at constant pressure.

What is Fourier's law of heat conduction?

q = -kA dT/dx.

What does the negative sign in Fourier's law indicate?

Heat flows from higher temperature toward lower temperature.

What is Newton's law of cooling?

q = hA(Ts - T∞).

What is the convection coefficient h?

A measure of how effectively heat is transferred between a surface and a moving fluid.

What is conduction thermal resistance for a plane wall?

Rcond = L/(kA).

What is convection thermal resistance?

Rconv = 1/(hA).

How are thermal resistances in series combined?

Rtotal = R1 + R2 + R3 + …

How is heat transfer related to total thermal resistance?

q = ΔT/Rtotal.

What is the thermal resistance for radial conduction through a cylinder?

R = ln(r2/r1)/(2πkL).

Why is cylindrical conduction important in reactor design?

Fuel rods are approximately cylindrical, so heat travels radially from the fuel toward the coolant.

Where is the maximum temperature usually located in a cylindrical fuel rod with internal heat generation?

At or near the fuel centerline.

What is volumetric heat generation?

Heat produced throughout a material's volume, commonly written q''' in W/m³.

What is heat flux?

Heat-transfer rate per unit area, q'' = Qdot/A.

What is Reynolds number?

Re = ρVD/μ; it compares inertial forces to viscous forces and helps identify the flow regime.

What does a low Reynolds number generally indicate?

Laminar flow.

What does a high Reynolds number generally indicate?

Turbulent flow.

What is the Nusselt number?

Nu = hD/k; it represents convective heat transfer relative to conduction through the fluid.

What is the Prandtl number?

Pr = cp μ/k; it compares momentum diffusivity with thermal diffusivity.

How can the convection coefficient be obtained from Nusselt number?

h = Nu k/D.

Why is turbulent flow often desirable for reactor cooling?

It generally produces a larger convection coefficient and improves heat transfer.

What is pressure drop?

The decrease in fluid pressure caused by friction, fittings, elevation changes, and other flow resistance.

What is the Darcy-Weisbach equation?

ΔP = f(L/D)(ρV²/2) for frictional pressure loss in a pipe.

What are minor losses?

Pressure losses caused by fittings, valves, bends, entrances, exits, and other components.

How are minor losses commonly calculated?

hL = K V²/(2g).

What does a pump supply to a fluid system?

Energy or head needed to overcome pressure losses and maintain flow.

What is pump head?

Mechanical energy added to the fluid per unit weight, often expressed as a height of fluid.

What happens to reactor cooling if pump flow decreases?

Coolant temperature rise generally increases and fuel/cladding temperatures can increase.

What is critical heat flux?

The heat flux at which nucleate boiling becomes unstable and heat-transfer performance deteriorates sharply.

Why is critical heat flux important in reactor safety?

Exceeding it can cause a rapid increase in cladding surface temperature and potentially damage the fuel.

What is nucleate boiling?

Boiling where vapor bubbles form at the heated surface while liquid still effectively wets the surface.

What is departure from nucleate boiling (DNB)?

The transition from efficient nucleate boiling toward a poorer heat-transfer regime in a PWR.

What does DNBR stand for?

Departure from Nucleate Boiling Ratio.

Why is DNBR important?

It provides a safety margin between operating heat flux and the heat flux that would cause departure from nucleate boiling.

What is k-effective?

The ratio of neutron production in one generation to neutron loss in the previous generation.

What does keff = 1 mean?

The reactor is critical and neutron population remains constant.

What does keff < 1 mean?

The reactor is subcritical and neutron population decreases.

What does keff > 1 mean?

The reactor is supercritical and neutron population increases.

What is reactivity?

A measure of departure from criticality: ρ = (k - 1)/k.

What does positive reactivity do?

Increases reactor power or neutron population.

What does negative reactivity do?

Decreases reactor power or neutron population.

What is neutron flux?

The rate at which neutrons pass through a unit area, commonly represented by φ.

What is neutron current?

The net movement of neutrons through a surface.

What is Fick's law for neutron diffusion?

J = -D∇φ.

What is the diffusion coefficient D?

A parameter describing how readily neutrons diffuse through a material.

What is macroscopic cross section?

The probability of a neutron interaction per unit path length; Σ = Nσ.

What is microscopic cross section?

The effective interaction area of a single nucleus, commonly measured in barns.

What is neutron absorption?

A neutron interaction in which the neutron is captured by a nucleus and removed from the neutron population.

What is neutron scattering?

A neutron interaction in which the neutron changes direction and/or energy but remains a neutron.

What is neutron moderation?

Slowing fast neutrons to lower energies through scattering collisions.

Why is water an effective moderator?

Hydrogen nuclei have a mass close to that of a neutron, allowing efficient energy transfer during collisions.

What is geometric buckling?

A measure of neutron leakage determined by reactor geometry and size.

What is material buckling?

A measure of the neutron-producing ability of the reactor material relative to neutron losses.

What condition relates geometric and material buckling at criticality?

Bg² = Bm².

What are prompt neutrons?

Neutrons emitted essentially immediately after fission.

What are delayed neutrons?

Neutrons emitted later by radioactive fission products called delayed-neutron precursors.

Why are delayed neutrons important?

They slow reactor response enough to make practical reactor control possible.

What is prompt criticality?

A condition where the reactor can sustain a chain reaction using prompt neutrons alone.

What is a delayed-neutron precursor?

A fission product that later decays and emits a delayed neutron.

What is reactor period?

The time required for reactor power to change by a factor of e during exponential power change.

What is Xe-135?

A fission product with an extremely large thermal-neutron absorption cross section that acts as a neutron poison.

Why is Xe-135 called a neutron poison?

It strongly absorbs neutrons without producing useful fission.

What isotope is an important precursor to Xe-135?

Iodine-135.

What happens to xenon shortly after reactor shutdown?

Xe-135 concentration can initially increase because existing I-135 continues decaying into xenon while neutron absorption of xenon stops.

What is Doppler feedback?

Increased fuel temperature broadens resonance absorption peaks, increasing neutron absorption and usually adding negative reactivity.

Why is a negative temperature coefficient desirable?

An increase in temperature naturally introduces negative reactivity and tends to reduce reactor power.

What are control rods used for?

To absorb neutrons and control reactor reactivity and power.

What happens when control rods are inserted farther into the core?

Neutron absorption increases and negative reactivity is added.

What happens when control rods are withdrawn?

Neutron absorption decreases and positive reactivity is added.

What is reactor scram?

A rapid emergency shutdown of the reactor, usually involving rapid insertion of control rods.

What is decay heat?

Heat produced by radioactive decay of fission products after the fission chain reaction has been stopped.