NANTeL Reactor Theory and Fundamentals 3 — Questions and Answers
Question 1: Xenon-135 is called a 'reactor poison.' What property makes it so significant?
- It emits high-energy gamma rays that damage fuel cladding
- It has an extremely large thermal neutron absorption cross-section (~2.6 million barns) (Correct answer)
- It builds up rapidly during startup and causes control rod ejection accidents
- It is produced directly from U-235 fission at a yield of over 50%
Correct answer: It has an extremely large thermal neutron absorption cross-section (~2.6 million barns)
Xe-135 has a thermal absorption cross-section of about 2.6 × 10⁶ barns, making it by far the dominant neutron absorber in an operating reactor and a critical factor in power maneuvering.
Question 2: What is 'xenon pit' (xenon precluded startup), and when does it occur?
- A reactor cannot restart after shutdown because xenon-135 has built up to a level exceeding available excess reactivity (Correct answer)
- Control rods become stuck in the xenon pit region of the reactor core
- Xenon gas accumulates in the fuel rods and causes pellet-cladding interaction
- The xenon concentration drops so fast after shutdown that restart is delayed
Correct answer: A reactor cannot restart after shutdown because xenon-135 has built up to a level exceeding available excess reactivity
After a shutdown, Xe-135 initially increases (as I-135 continues decaying into it without neutron burnup) and can exceed the core's available excess reactivity, preventing restart for up to ~30 hours.
Question 3: What is the void coefficient of reactivity, and why does it matter in a boiling water reactor (BWR)?
- Rate of reactivity change per degree of coolant temperature rise; it must be negative for BWR safety
- Change in reactivity per unit change in steam void fraction in the coolant; a negative void coefficient provides automatic power reduction if boiling increases (Correct answer)
- Reactivity worth of removing all coolant from the core in a loss-of-coolant accident
- The increase in reactivity that occurs when the moderator level drops below the active fuel zone
Correct answer: Change in reactivity per unit change in steam void fraction in the coolant; a negative void coefficient provides automatic power reduction if boiling increases
In a BWR, more steam voids mean less moderation, which reduces reactivity — this negative void coefficient gives the reactor important inherent safety characteristics during power transients.
Question 4: Samarium-149 is the second most important fission product poison. How does its behavior differ from xenon-135 after reactor shutdown?
- Samarium-149 decays rapidly after shutdown, causing a large positive reactivity insertion within hours
- Samarium-149 is stable and does not decay; its concentration rises after shutdown as Pm-149 continues to decay into it, then remains constant (Correct answer)
- Samarium-149 causes a xenon-like peak followed by a return to near-zero concentration after about 10 days
- Samarium-149 builds up only during reactor operation and disappears immediately upon shutdown
Correct answer: Samarium-149 is stable and does not decay; its concentration rises after shutdown as Pm-149 continues to decay into it, then remains constant
Unlike Xe-135, Sm-149 is stable (no decay), so after shutdown it accumulates as its precursor Pm-149 decays; the samarium buildup is permanent until the reactor restarts and neutron absorption burns it away.
Question 5: What is the 'Doppler effect' (Doppler broadening) in the context of reactor physics?
- The frequency shift of gamma rays emitted by moving fission fragments
- Broadening of neutron absorption resonance peaks in U-238 as fuel temperature rises, increasing resonance capture and reducing reactivity (Correct answer)
- The increase in neutron speed as reactor temperature increases, shifting the neutron spectrum to higher energies
- Doppler shift in neutron velocity measurements used by in-core detectors
Correct answer: Broadening of neutron absorption resonance peaks in U-238 as fuel temperature rises, increasing resonance capture and reducing reactivity
As fuel temperature rises, thermal motion of U-238 nuclei broadens their resonance absorption peaks, capturing more neutrons during slowing-down and reducing reactivity — this is a prompt, inherently stabilizing feedback.
Question 6: In the context of reactor control, what is 'differential control rod worth'?
- The total reactivity change from fully inserting all control rods simultaneously
- The change in reactivity per unit of control rod withdrawal or insertion at a specific rod position (Correct answer)
- The difference in worth between two different control rod materials (e.g., boron vs. hafnium)
- The reactivity change caused by a single control rod ejection accident
Correct answer: The change in reactivity per unit of control rod withdrawal or insertion at a specific rod position
Differential rod worth (Δρ/Δx) describes how much reactivity changes per inch (or cm) of rod movement at a given position, and is highest near the core midplane where neutron flux peaks.
Question 7: What is meant by the 'moderator-to-fuel ratio' and what happens if a light water reactor is 'undermoderated'?
- Adding more water (moderator) reduces reactivity because water absorbs neutrons; undermoderated cores have excess water
- Adding more water increases reactivity up to an optimum; undermoderated cores gain reactivity if water is added, which is a safer configuration (Correct answer)
- Undermoderated cores are more efficient because fewer neutrons are lost to moderator absorption
- The ratio only matters during initial core loading and has no effect on operating reactor behavior
Correct answer: Adding more water increases reactivity up to an optimum; undermoderated cores gain reactivity if water is added, which is a safer configuration
In an undermoderated light water reactor, adding water increases neutron moderation and raises reactivity, but the loss of water (e.g., from a leak or boiling) reduces moderation and lowers reactivity — an inherently safe negative coolant density feedback.
NANTeL Reactor Theory and Fundamentals 3
Xenon-135 is called a 'reactor poison.' What property makes it so significant?