Answer :
Final Answer:
The critical mass of uranium-235 (U-235) required to sustain a chain reaction is achieved when a single 1-kg ball is present.
- Breaking the ball into smaller chunks disrupts the necessary conditions for a sustained chain reaction due to decreased mass and increased surface area.
Explanation:
A nuclear chain reaction occurs when the fission (splitting) of atomic nuclei releases additional neutrons that go on to cause the fission of other nuclei. To sustain this reaction, a critical mass of fissile material (such as U-235) is required. This critical mass depends on factors like the material's composition, shape, density, and arrangement.
In the case of the 1-kg U-235 ball, its mass is sufficient to achieve criticality, meaning the number of neutrons released by fission is enough to sustain a chain reaction. However, when the ball is broken into smaller chunks, several factors come into play:
Decreased Mass:
The total mass of the smaller chunks becomes less than the critical mass, reducing the chances of achieving a self-sustaining chain reaction.
Increased Surface Area:
Breaking the ball into smaller pieces increases the surface area of the material. This allows more neutrons to escape rather than interact with other U-235 nuclei, making it difficult to maintain a chain reaction.
Reduced Neutron Reflection:
The geometry of the material affects the reflection and moderation of neutrons. Smaller chunks might not reflect neutrons as efficiently as a single larger mass.
All these factors combined lead to a situation where the broken-up chunks are not at critical mass and are unable to sustain a chain reaction. The original 1-kg ball's intact nature provides the ideal conditions for achieving the critical mass required for a chain reaction to occur.
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