What must be true of the enrichment of U-235 for maximum possible nuclear reaction efficiency?

The enrichment of U-235 must be sufficient to sustain the fission chain reaction for maximum nuclear reaction efficiency. In a nuclear reactor, U-235 acts as fuel, and for a sustained chain reaction to occur, there needs to be a proper concentration of U-235 relative to U-238, which is its more abundant isotope.

When U-235 is enriched to a higher level, it enhances the probability of fission occurring when a neutron interacts with the nucleus. This is critical because a chain reaction relies on the ability of the process to continually produce more neutrons through the fission of U-235. If the enrichment is too low, there may not be enough U-235 present to absorb neutrons efficiently, leading to a failure to sustain the chain reaction.

In summary, the correct answer emphasizes the necessity of having adequate enrichment levels to ensure that there is a sufficient number of fission events occurring, which allows for a self-sustaining reaction that maximizes the energy output from the nuclear fuel.