A nuclear safety inspector is reviewing the physical components of a pressurized water reactor (PWR).

Match each reactor component to its primary mechanical function and material composition.

1. Moderator medium
2. Control rods assembly
3. Biological shield
4. Primary heat transfer fluid (coolant)

Functions and Materials:

• A: Made of high-density concrete and steel; its main purpose is to attenuate gamma rays and absorb stray neutrons, preventing radiation leakage into the environment.
• B: Made of boron-infused steel or cadmium alloys; these are adjusted to absorb excess thermal neutrons, regulating the multiplication factor ($k$) of the chain reaction.
• C: Made of pressurized water or liquid sodium; it circulates through the core to extract thermal energy released by fission and transfer it to a steam generator.
• D: Made of light water, heavy water, or graphite; it collides elastically with fast fission neutrons to reduce their kinetic energy to thermal levels.

Which of the following describes the particle and condition required to initiate a sustained fission chain reaction in a standard thermal reactor utilizing ${}^{235}U$?

• A A high-velocity proton to overcome the electrostatic repulsion of the uranium nucleus.
• B A slow-moving (thermal) neutron, as its longer interaction time increases the probability of capture by the uranium nucleus.
• C A high-energy beta-plus particle (positron) to convert a neutron into a proton within the nucleus.
• D An energetic alpha particle to induce alpha-induced fission via bombardment.

Describe the sequence of physical events that occurs during the induced fission of a uranium-235 nucleus, starting from neutron capture to the generation of a self-sustaining chain reaction.

Detail three key physical or operational differences between the processes of nuclear fission and nuclear fusion.