NANTeL - National Academy for Nuclear Training e-Learning Reactor Theory and Fundamentals Questions and Answers 1 — 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?
- Fast fission factor (ε)
- Resonance escape probability (p)
- Thermal utilization factor (f)
- Reproduction factor (η) (Correct answer)
Correct answer: Reproduction factor (η)
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?
- Neutron leakage increase
- Control rod insertion
- Doppler broadening (Correct answer)
- Xenon-135 burnout
Correct answer: Doppler broadening
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:
- 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. (Correct answer)
- The shutdown causes a sudden increase in the fission yield of Iodine-135.
- Xenon-135 is produced directly from fission at a higher rate immediately after shutdown.
- The control rods used for shutdown are coated with a material that catalyzes Xenon-135 production.
Correct 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.
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?
- To cool the reactor core during normal operation.
- To provide structural support for the fuel assemblies.
- To moderate (slow down) fast neutrons to thermal energies.
- To absorb neutrons and control the rate of the fission chain reaction. (Correct answer)
Correct answer: To absorb neutrons and control the rate of the fission chain reaction.
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:
- Increase exponentially until the reactor becomes critical.
- Decrease to zero regardless of the source.
- Stabilize at a level proportional to the source strength multiplied by the subcritical multiplication factor. (Correct answer)
- Oscillate around an average value determined by k_eff.
Correct answer: Stabilize at a level proportional to the source strength multiplied by the subcritical multiplication factor.
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.
Question 6: Which of the following describes the 'thermal non-leakage probability' (P_th or L_th) in the neutron life cycle?
- The probability that a fast neutron will be absorbed by U-238.
- The probability that a fast neutron will slow to thermal energies without leaking from the core.
- The probability that a thermal neutron will be absorbed in the fuel rather than in other materials.
- The probability that a neutron, once it has reached thermal energy, will be absorbed in the core before it can leak out. (Correct answer)
Correct answer: The probability that a neutron, once it has reached thermal energy, will be absorbed in the core before it can leak out.
The thermal non-leakage probability is a factor in the six-factor formula that accounts for the finite size of a reactor. It represents the fraction of neutrons that, after successfully slowing down to thermal energies, remain within the reactor core to be absorbed, as opposed to escaping or leaking out.
NANTeL - National Academy for Nuclear Training e-Learning Reactor Theory and Fundamentals Questions and Answers 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?