Final answer:
The U-235 isotope constitutes approximately 0.711% of natural uranium found in Earth's crust and seawater, and this consistency is due to the predictable rate of radioactive decay. This ratio is expected to be consistent throughout our solar system based on common stellar processes. However, in a distant solar system, the U-235 abundance may differ depending on the system's age and formation history.
Step-by-step explanation:
The percentage of Uranium-235 (U-235) is roughly 0.711% in natural uranium found on Earth, including in seawater and uranium ores. This uniformity is due to the process of radioactive decay which happens at a predictable rate. U-235 has a half-life of approximately 700 million years, meaning over a geologic time scale, the amount of U-235 has decreased from a higher initial percentage when the Earth formed to the current state of 0.711%. The percentage of U-235 would likely be the same throughout our solar system, including in an asteroid near Jupiter, assuming that all the material originated from the same stellar processes. This ratio can differ in a distant solar system based on the age of the system and its unique formation history.
For example, if we calculate the percentages of U-235 and U-238 when Earth formed 4.5 billion years ago, knowing the half-lives of both isotopes, we can deduce that there was a higher percentage of U-235 at Earth's formation than there is now. We know that the useful fissile isotope U-235 is needed in higher concentrations for nuclear reactors, typically about 3-5%, as opposed to its natural occurrence which is much lower. The enrichment process required to achieve this concentrates U-235 from its natural percentage by separating it from the more abundant U-238.