NANTeL - National Academy for Nuclear Training e-Learning Reactor Theory and Fundamentals Questions and Answers

Question 1

In the context of the six-factor formula for an infinite reactor (k-infinity), which factor represents the ratio of neutrons produced by thermal fission to the number of thermal neutrons absorbed in the fuel?

Accepted Answer

Reproduction factor (η)

Answer

The reproduction factor (η) is defined as the number of fast neutrons produced from thermal fission divided by the number of thermal neutrons absorbed in the fuel. It quantifies the fuel's efficiency in producing new neutrons from thermal neutron absorption.

Question 2

A reactor operator observes a gradual, unplanned increase in reactor power. Investigation reveals that the fuel temperature has been increasing. Which of the following phenomena is the primary cause of the negative reactivity feedback that will naturally counteract this power increase?

Accepted Answer

Doppler broadening

Answer

Doppler broadening is a key temperature-dependent feedback mechanism. As the fuel temperature rises, the thermal motion of the fuel nuclei (like Uranium-238) increases. This broadens the energy range for neutron resonance absorption, leading to increased neutron capture in the fuel without causing fission. This increased capture removes neutrons from the chain reaction, adding negative reactivity and inherently stabilizing the reactor power.

Question 3

Following a reactor shutdown from extended full-power operation, the concentration of Xenon-135 initially increases before it begins to decrease. This phenomenon, known as the 'iodine pit', occurs primarily because:

Accepted Answer

The decay rate of Iodine-135 is faster than the decay rate of Xenon-135, and Xenon-135 is no longer being burned out by neutron flux.

Answer

After shutdown, the neutron flux drops to near zero, stopping the 'burnout' (removal by neutron absorption) of Xenon-135. However, the inventory of its precursor, Iodine-135, continues to decay into Xenon-135. Since Iodine-135 has a shorter half-life (approx. 6.6 hours) than Xenon-135 (approx. 9.1 hours), the production of Xenon-135 from iodine decay temporarily outpaces its own removal by decay, causing its concentration to peak.

Question 4

Which of the following best describes the primary function of control rods in a nuclear reactor?

Accepted Answer

To absorb neutrons and control the rate of the fission chain reaction.

Answer

Control rods are made of materials with a high neutron absorption cross-section (like boron, cadmium, or hafnium). By inserting them into or withdrawing them from the reactor core, operators can absorb more or fewer neutrons, thereby controlling the neutron population and the rate of the fission reaction to manage the reactor's power level or to shut it down.

Question 5

In a subcritical reactor (k_eff < 1) with a constant external neutron source, the neutron population will:

Accepted Answer

Stabilize at a level proportional to the source strength multiplied by the subcritical multiplication factor.

Answer

In a subcritical assembly, the chain reaction is not self-sustaining. However, the presence of an external source provides an initial number of neutrons. These source neutrons, and the subsequent neutrons from the fissions they induce, create a steady-state neutron population. This effect is known as subcritical multiplication, and the final stable neutron level is higher than the source level alone, determined by the formula N = S / (1 - k_eff), where S is the source strength.

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

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

NANTeL - National Academy for Nuclear Training e-Learning Reactor Theory and Fundamentals Questions and Answers