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

Fast fission factor (ε)

Answer

Resonance escape probability (p)

Answer

Thermal utilization factor (f)

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

Neutron leakage increase

Answer

Control rod insertion

Answer

Xenon-135 burnout

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

The shutdown causes a sudden increase in the fission yield of Iodine-135.

Answer

Xenon-135 is produced directly from fission at a higher rate immediately after shutdown.

Answer

The control rods used for shutdown are coated with a material that catalyzes Xenon-135 production.

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

To cool the reactor core during normal operation.

Answer

To provide structural support for the fuel assemblies.

Answer

To moderate (slow down) fast neutrons to thermal energies.

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

Increase exponentially until the reactor becomes critical.

Answer

Decrease to zero regardless of the source.

Answer

Oscillate around an average value determined by k_eff.

Question 6

Which of the following describes the 'thermal non-leakage probability' (P_th or L_th) in the neutron life cycle?

Accepted Answer

The probability that a neutron, once it has reached thermal energy, will be absorbed in the core before it can leak out.

Answer

The probability that a fast neutron will be absorbed by U-238.

Answer

The probability that a fast neutron will slow to thermal energies without leaking from the core.

Answer

The probability that a thermal neutron will be absorbed in the fuel rather than in other materials.