Final answer:
To design an efficient fission reactor, a higher ratio of U-235 to U-238 is needed, with typical reactors enriching U-235 to a 3-5% level for a sustainable chain reaction. Enrichment is achieved using methods like gaseous diffusion or centrifuges, taking advantage of the slight mass difference between the isotopes.
Step-by-step explanation:
If you were trying to design the most efficient fission reactor possible, the ideal ratio of U-235 to U-238 would be one that balances the need for a sustainable chain reaction with economical considerations. The enrichment process typically aims to increase the proportion of U-235 because it is the isotope capable of sustaining a nuclear chain reaction. Naturally, uranium consists of approximately 0.72% U-235, with the vast majority being U-238.
To have a self-sustained fission reactor, the U-235 concentration needs to be increased, typically to around 3-5%. Enrichment is accomplished through methods like gaseous diffusion or centrifugation, which capitalize on the slight mass difference between U-235 and U-238. U-235 is more likely to lead to a fission reaction as it more readily absorbs slow-moving (thermal) neutrons compared to U-238, which is better at capturing fast neutrons but does not fission as easily.
Pressurized water reactors are a common design that uses water to slow down neutrons, enhancing the fission capacity of U-235. Therefore, a higher ratio of U-235 would generally be preferable for an efficient fission reactor, while acknowledging that higher enrichment requires more effort and cost, balancing those factors is key to reactor design.