SRO Certification Test Study Guide 2025/2026 Accurate
Questions and Correct Detailed Answers || 100%
Guaranteed Pass <Recent Version>
Category 1: Nuclear Theory and Reactor Core Principles (20 Questions)
1. What is the definition of reactivity, and what are its common units?
Answer: Reactivity is a measure of the departure of a nuclear reactor from criticality. It indicates
whether the neutron population (and thus reactor power) is increasing, decreasing, or
remaining constant.
• Units: Delta-k/k (Δk/k), which is dimensionless. It is also commonly expressed
in pcm (percent milli-rho, where 1 pcm = 10⁻⁵ Δk/k) or dollars and cents (relative to the
effective delayed neutron fraction, β_eff).
2. Describe the concept of "Doppler Broadening" and its role in reactor safety.
Answer: Doppler Broadening is the phenomenon where an increase in fuel temperature causes
the resonance absorption peaks of fissile and fertile isotopes (like U-238) to widen or
"broaden." This increases the probability of neutron capture in these resonances, leading to a
negative reactivity feedback. This provides an immediate, inherent shutdown mechanism for
the reactor in response to a power increase.
3. What are delayed neutrons, and why are they critical for reactor control?
Answer: Delayed neutrons are neutrons emitted by fission product precursors (like Br-87, I-137)
seconds to minutes after the fission event. They constitute a small fraction (~0.65% for U-235)
of the total neutrons. They are critical because they increase the average neutron generation
time, allowing reactor power changes to occur on a timescale of seconds/minutes, which is
manageable with control systems. Without them, reactor control would be impossible with
mechanical means.
4. Define "Moderator Temperature Coefficient (MTC)" and explain its typical sign in a PWR.
Answer: The MTC is the change in reactivity per degree change in moderator temperature. In a
PWR, the MTC is typically negative. As moderator temperature increases, its density decreases,
reducing its moderating efficiency and causing more neutrons to leak out or be absorbed, which
introduces negative reactivity.
5. What is the difference between the "Prompt Critical" and "Critical" conditions?
Answer: A reactor is Critical when the chain reaction is self-sustaining using both prompt and
delayed neutrons (k_eff = 1). A reactor is Prompt Critical when the chain reaction is sustained
by prompt neutrons alone (k_eff > 1 + β_eff). This condition is extremely dangerous as power
,can rise many orders of magnitude in milliseconds and is beyond the control of delayed
neutrons.
6. Explain the purpose of "Chemical Shim" (Boric Acid) in a PWR.
Answer: Chemical shim is a soluble neutron absorber (boric acid) dissolved in the reactor
coolant. It provides a means of:
• Burnup Compensation: To offset the long-term reactivity loss from fuel burnup.
• Gross Control: Allows for bulk reactivity changes without moving control rods.
• Shutdown Margin: Provides a supplementary shutdown mechanism.
7. Describe the "Fission Product Poisoning" effects of Xenon-135 and Samarium-149.
Answer:
• Xenon-135: Has an extremely high neutron absorption cross-section. It is produced from
the decay of Iodine-135 (half-life ~6.6 hrs). It causes Xenon Oscillations and can create a
"Xenon Preclude" after shutdown, making restart impossible for a period (typically 24-48
hours).
• Samarium-149: Also a strong neutron absorber. It is a stable poison produced from the
decay of Promethium-149 (half-life ~54 hrs). It builds to a constant equilibrium level
during operation and remains constant after shutdown, representing a permanent
reactivity loss.
8. What is the "Fuel Temperature Coefficient (FTC)" and why is it always negative?
Answer: The FTC (or Doppler Coefficient) is the change in reactivity per degree change in fuel
temperature. It is always negative due to the Doppler Broadening effect. An increase in fuel
temperature immediately increases resonance absorption in U-238, adding negative reactivity
and providing a crucial, fast-acting safety response.
9. Define "Beta-effective (β_eff)" and its significance.
Answer: Beta-effective is the effective delayed neutron fraction. It is the weighted average of
the delayed neutron fractions for all fissile isotopes in the core. It defines the threshold for
prompt criticality (1 β) and is a fundamental parameter for reactor kinetics and safety analyses.
10. What is "Subcritical Multiplication" and how is it used during startup?
Answer: Subcritical multiplication is the phenomenon where a source-driven, subcritical reactor
produces a steady-state neutron population proportional to the source strength. It is used
during startup to calibrate source range and intermediate range detectors, extrapolate to
criticality, and ensure a predictable approach to criticality.
, 11. Explain the term "Burnable Poison." Provide an example.
Answer: Burnable poisons are neutron-absorbing materials (e.g., Gadolinium or Boron)
incorporated directly into the fuel pellets or rods. They are designed to "burn out" at a rate
similar to fuel burnup, helping to flatten the core power distribution and control initial core
excess reactivity. Example: Gadolinia (Gd₂O₃) mixed with UO₂ fuel.
12. What is the difference between "Fast Fission Factor" and "Resonance Escape Probability"?
Answer: These are two of the four factors in the six-factor formula.
• Fast Fission Factor (ε): The ratio of the total number of neutrons produced from both
fast and thermal fissions to the number produced by thermal fissions alone.
• Resonance Escape Probability (p): The probability that a fast neutron will slow down to
thermal energies without being captured in a resonance.
13. Describe the conditions for "Criticality."
Answer: A nuclear system is critical when the neutron production rate from fission equals the
neutron loss rate from both absorption and leakage. Mathematically, the effective multiplication
factor (k_eff) is exactly equal to 1.0.
14. What is meant by "Excess Reactivity"?
Answer: Excess reactivity is the amount of positive reactivity available in the core at any given
time, beyond what is needed to achieve criticality. It is required to compensate for fuel burnup,
fission product poisoning, and temperature changes. It is offset by control systems (rods,
chemical shim).
15. How does a "Control Rod Worth" change as it is inserted into the core?
Answer: Control rod worth is not linear. Due to spatial flux effects, the worth is typically greatest
at the periphery of the core and follows an "S-shaped" curve as it is inserted. The highest
reactivity change per unit of insertion often occurs when the rod tip is near the core mid-plane.
16. What is the "Decay Heat" source after reactor shutdown?
Answer: Decay heat is the thermal energy released from the radioactive decay of fission
products and transuranic actinides produced during operation. It is a significant source of heat
that requires continuous cooling for days to years after shutdown.
17. Define "Linear Heat Generation Rate (LHGR)" and its safety importance.
Answer: LHGR is the power generated per unit length of fuel rod (typically kW/ft). It is a key
parameter for ensuring fuel integrity, as exceeding design limits can lead to fuel centerline
melting, cladding failure, and fission product release.
18. Explain the term "Power Defect" of reactivity.
Answer: The power defect is the total negative reactivity change from zero power to full power.
Questions and Correct Detailed Answers || 100%
Guaranteed Pass <Recent Version>
Category 1: Nuclear Theory and Reactor Core Principles (20 Questions)
1. What is the definition of reactivity, and what are its common units?
Answer: Reactivity is a measure of the departure of a nuclear reactor from criticality. It indicates
whether the neutron population (and thus reactor power) is increasing, decreasing, or
remaining constant.
• Units: Delta-k/k (Δk/k), which is dimensionless. It is also commonly expressed
in pcm (percent milli-rho, where 1 pcm = 10⁻⁵ Δk/k) or dollars and cents (relative to the
effective delayed neutron fraction, β_eff).
2. Describe the concept of "Doppler Broadening" and its role in reactor safety.
Answer: Doppler Broadening is the phenomenon where an increase in fuel temperature causes
the resonance absorption peaks of fissile and fertile isotopes (like U-238) to widen or
"broaden." This increases the probability of neutron capture in these resonances, leading to a
negative reactivity feedback. This provides an immediate, inherent shutdown mechanism for
the reactor in response to a power increase.
3. What are delayed neutrons, and why are they critical for reactor control?
Answer: Delayed neutrons are neutrons emitted by fission product precursors (like Br-87, I-137)
seconds to minutes after the fission event. They constitute a small fraction (~0.65% for U-235)
of the total neutrons. They are critical because they increase the average neutron generation
time, allowing reactor power changes to occur on a timescale of seconds/minutes, which is
manageable with control systems. Without them, reactor control would be impossible with
mechanical means.
4. Define "Moderator Temperature Coefficient (MTC)" and explain its typical sign in a PWR.
Answer: The MTC is the change in reactivity per degree change in moderator temperature. In a
PWR, the MTC is typically negative. As moderator temperature increases, its density decreases,
reducing its moderating efficiency and causing more neutrons to leak out or be absorbed, which
introduces negative reactivity.
5. What is the difference between the "Prompt Critical" and "Critical" conditions?
Answer: A reactor is Critical when the chain reaction is self-sustaining using both prompt and
delayed neutrons (k_eff = 1). A reactor is Prompt Critical when the chain reaction is sustained
by prompt neutrons alone (k_eff > 1 + β_eff). This condition is extremely dangerous as power
,can rise many orders of magnitude in milliseconds and is beyond the control of delayed
neutrons.
6. Explain the purpose of "Chemical Shim" (Boric Acid) in a PWR.
Answer: Chemical shim is a soluble neutron absorber (boric acid) dissolved in the reactor
coolant. It provides a means of:
• Burnup Compensation: To offset the long-term reactivity loss from fuel burnup.
• Gross Control: Allows for bulk reactivity changes without moving control rods.
• Shutdown Margin: Provides a supplementary shutdown mechanism.
7. Describe the "Fission Product Poisoning" effects of Xenon-135 and Samarium-149.
Answer:
• Xenon-135: Has an extremely high neutron absorption cross-section. It is produced from
the decay of Iodine-135 (half-life ~6.6 hrs). It causes Xenon Oscillations and can create a
"Xenon Preclude" after shutdown, making restart impossible for a period (typically 24-48
hours).
• Samarium-149: Also a strong neutron absorber. It is a stable poison produced from the
decay of Promethium-149 (half-life ~54 hrs). It builds to a constant equilibrium level
during operation and remains constant after shutdown, representing a permanent
reactivity loss.
8. What is the "Fuel Temperature Coefficient (FTC)" and why is it always negative?
Answer: The FTC (or Doppler Coefficient) is the change in reactivity per degree change in fuel
temperature. It is always negative due to the Doppler Broadening effect. An increase in fuel
temperature immediately increases resonance absorption in U-238, adding negative reactivity
and providing a crucial, fast-acting safety response.
9. Define "Beta-effective (β_eff)" and its significance.
Answer: Beta-effective is the effective delayed neutron fraction. It is the weighted average of
the delayed neutron fractions for all fissile isotopes in the core. It defines the threshold for
prompt criticality (1 β) and is a fundamental parameter for reactor kinetics and safety analyses.
10. What is "Subcritical Multiplication" and how is it used during startup?
Answer: Subcritical multiplication is the phenomenon where a source-driven, subcritical reactor
produces a steady-state neutron population proportional to the source strength. It is used
during startup to calibrate source range and intermediate range detectors, extrapolate to
criticality, and ensure a predictable approach to criticality.
, 11. Explain the term "Burnable Poison." Provide an example.
Answer: Burnable poisons are neutron-absorbing materials (e.g., Gadolinium or Boron)
incorporated directly into the fuel pellets or rods. They are designed to "burn out" at a rate
similar to fuel burnup, helping to flatten the core power distribution and control initial core
excess reactivity. Example: Gadolinia (Gd₂O₃) mixed with UO₂ fuel.
12. What is the difference between "Fast Fission Factor" and "Resonance Escape Probability"?
Answer: These are two of the four factors in the six-factor formula.
• Fast Fission Factor (ε): The ratio of the total number of neutrons produced from both
fast and thermal fissions to the number produced by thermal fissions alone.
• Resonance Escape Probability (p): The probability that a fast neutron will slow down to
thermal energies without being captured in a resonance.
13. Describe the conditions for "Criticality."
Answer: A nuclear system is critical when the neutron production rate from fission equals the
neutron loss rate from both absorption and leakage. Mathematically, the effective multiplication
factor (k_eff) is exactly equal to 1.0.
14. What is meant by "Excess Reactivity"?
Answer: Excess reactivity is the amount of positive reactivity available in the core at any given
time, beyond what is needed to achieve criticality. It is required to compensate for fuel burnup,
fission product poisoning, and temperature changes. It is offset by control systems (rods,
chemical shim).
15. How does a "Control Rod Worth" change as it is inserted into the core?
Answer: Control rod worth is not linear. Due to spatial flux effects, the worth is typically greatest
at the periphery of the core and follows an "S-shaped" curve as it is inserted. The highest
reactivity change per unit of insertion often occurs when the rod tip is near the core mid-plane.
16. What is the "Decay Heat" source after reactor shutdown?
Answer: Decay heat is the thermal energy released from the radioactive decay of fission
products and transuranic actinides produced during operation. It is a significant source of heat
that requires continuous cooling for days to years after shutdown.
17. Define "Linear Heat Generation Rate (LHGR)" and its safety importance.
Answer: LHGR is the power generated per unit length of fuel rod (typically kW/ft). It is a key
parameter for ensuring fuel integrity, as exceeding design limits can lead to fuel centerline
melting, cladding failure, and fission product release.
18. Explain the term "Power Defect" of reactivity.
Answer: The power defect is the total negative reactivity change from zero power to full power.
