Final answer:
Both the assertion that it is desirable to slow down fast moving neutrons to sustain controlled chain reactions and the reason that slow moving neutrons efficiently collide with U²³⁵ are correct.option A is correct answer.
Step-by-step explanation:
The question pertains to the reasons why it is desirable to slow down fast moving neutrons to sustain controlled chain reactions. The assertion made is that it is desirable to slow down fast moving neutrons to sustain controlled chain reactions. The reason given is that slow moving neutrons efficiently collide with U²³⁵. Both the assertion and reason are correct. Slow moving neutrons, often referred to as thermal neutrons, are indeed more effective in causing fission in U²³⁵ because their kinetic energy allows them to be absorbed by the nucleus, which leads to instability and subsequent fission. This process releases a significant amount of energy and additional neutrons that can continue the chain reaction.
Neutrons generated from fission are high in energy ('hot') and thus more likely to bypass other uranium nuclei without being absorbed. This is why a moderator, such as water, is used in nuclear reactors—to slow down the neutrons, thereby increasing the probability of their absorption by U²³⁵ nuclei and sustaining the controlled chain reaction. The control rods in a reactor then help to maintain the chain reaction by absorbing excess neutrons, thereby preventing a runaway reaction.
Therefore, the correct option would be A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.