Answer :
The critical mass of a fissile material, such as U-235, is the minimum amount required to sustain a self-sustaining chain reaction. It depends on various factors, including the shape, density, and enrichment of the material.
In the case of a solid sphere of U-235 with a mass of 56 kg, it is critical because the shape and density of the sphere are carefully designed to ensure a self-sustaining chain reaction. Any change in the shape or density of the material can potentially affect its criticality.
If the sphere were flattened into a pancake shape, the distribution of the material would change. The pancake shape would increase the surface area of the U-235, which could lead to increased neutron leakage and reduced neutron multiplication. This change in geometry can disrupt the criticality of the system.
Moreover, the pancake shape may also alter the density of the U-235 material. The critical mass depends on the density of the material because a higher density allows for a more efficient neutron capture and fission process. Flattening the sphere could potentially decrease the density, further affecting the criticality.
In summary, changing the shape of the U-235 sphere from a solid sphere to a pancake shape can disrupt the criticality of the system. The specific critical mass and shape requirements for a self-sustaining chain reaction depend on the detailed design and calculations for a particular nuclear reactor or weapon.
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