NANTeL Reactor Theory and Fundamentals 2 — Questions and Answers
Question 1: What is the primary role of delayed neutrons in reactor control?
- They provide the extra reactivity needed to sustain fission
- They slow the neutron chain reaction enough for mechanical control systems to respond (Correct answer)
- They increase the average neutron energy above the fission threshold
- They are absorbed by control rods to reduce reactor power
Correct answer: They slow the neutron chain reaction enough for mechanical control systems to respond
Delayed neutrons extend the effective neutron lifetime from microseconds to tens of milliseconds, making the reactor slow enough for control rods and other mechanical systems to regulate power.
Question 2: Which quantity represents the average number of neutrons produced per neutron absorbed in the fuel?
- k-effective (k-eff)
- Eta (η) (Correct answer)
- Epsilon (ε)
- Resonance escape probability (p)
Correct answer: Eta (η)
Eta (η) is the reproduction factor, defined as the average number of fission neutrons emitted per neutron absorbed in the fuel.
Question 3: In a thermal reactor, why is a moderator necessary?
- To reflect fast neutrons back into the core
- To slow fast neutrons to thermal energies where fission cross-sections are much higher (Correct answer)
- To absorb excess neutrons and prevent supercriticality
- To shield operators from neutron radiation
Correct answer: To slow fast neutrons to thermal energies where fission cross-sections are much higher
Fission cross-sections for U-235 and Pu-239 are orders of magnitude larger at thermal energies, so the moderator slows neutrons to maximize the probability of causing fission.
Question 4: What does a negative temperature coefficient of reactivity (TCR) mean for reactor safety?
- Reactor power rises as temperature increases, requiring operator action
- An increase in temperature automatically reduces reactivity, creating inherent self-regulation (Correct answer)
- Control rods must be inserted whenever coolant temperature rises
- The reactor will scram automatically if temperature drops below setpoint
Correct answer: An increase in temperature automatically reduces reactivity, creating inherent self-regulation
A negative TCR means the reactor is inherently self-regulating: if power rises and temperature increases, reactivity drops, reducing power back toward equilibrium without operator action.
Question 5: The six-factor formula (k∞) for an infinite reactor is expressed as ηεpf. What does the thermal utilization factor (f) represent?
- Fraction of thermal neutrons that cause fission rather than capture in U-238
- Fraction of thermal neutrons absorbed in fuel compared to all thermal absorptions in the core (Correct answer)
- Fraction of fast neutrons that escape resonance capture while slowing down
- Average number of neutrons produced per thermal neutron absorbed in the moderator
Correct answer: Fraction of thermal neutrons absorbed in fuel compared to all thermal absorptions in the core
Thermal utilization (f) is the fraction of all thermal neutron absorptions in the reactor that occur specifically in the fuel, reflecting competition between fuel and non-fuel absorbers.
Question 6: A reactor is described as 'prompt critical.' What does this mean?
- The reactor is exactly critical using only prompt neutrons, without needing delayed neutrons (Correct answer)
- The reactor has reached its maximum licensed power level
- All delayed neutron precursors have decayed to their equilibrium concentrations
- The reactor control rods are fully withdrawn and at their minimum worth position
Correct answer: The reactor is exactly critical using only prompt neutrons, without needing delayed neutrons
Prompt criticality occurs when the chain reaction is self-sustaining on prompt neutrons alone (k-eff ≥ 1 without delayed neutrons), causing power to rise on the very short prompt neutron lifetime and making control nearly impossible.
Question 7: What is 'reactor period' and how is it used operationally?
- The time in seconds for reactor power to change by a factor of e (~2.718); operators use it to judge rate of power change (Correct answer)
- The calendar interval between required reactor shutdowns for refueling
- The time required for a full control rod insertion during a scram
- The half-life of the most abundant fission product in the core
Correct answer: The time in seconds for reactor power to change by a factor of e (~2.718); operators use it to judge rate of power change
Reactor period (T) is the e-folding time of power change; a short positive period means rapid power rise, while a long or negative period indicates slow rise or power decrease, giving operators a key indicator for controlling ascent rate.
NANTeL Reactor Theory and Fundamentals 2
What is the primary role of delayed neutrons in reactor control?