NANTeL Reactor Theory and Fundamentals 3

Question 1

Xenon-135 is called a 'reactor poison.' What property makes it so significant?

Accepted Answer

It has an extremely large thermal neutron absorption cross-section (~2.6 million barns)

Answer

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?

Accepted Answer

A reactor cannot restart after shutdown because xenon-135 has built up to a level exceeding available excess reactivity

Answer

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)?

Accepted 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

Answer

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?

Accepted 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

Answer

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?

Accepted Answer

Broadening of neutron absorption resonance peaks in U-238 as fuel temperature rises, increasing resonance capture and reducing reactivity

Answer

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.

(NANTeL) National Academy for Nuclear Training e-Learning Practice Test

(NANTeL) National Academy for Nuclear Training e-Learning Practice Test

NANTeL Reactor Theory and Fundamentals 3