A 56-kg sphere of U-235 constitutes a critical mass. If the sphere were flattened into a pancake shape, would it still be critical? Explain.
Solution 7E When the sphere of U-235 is flattened to make a pancake shape, the shape will not be able to absorb as many neutrons as the sphere initially. Escape of neutrons will become significant. In this situation, it would no more be critical.