How many isotopes are there of uranium?

Uranium has a number of isotopes, but uranium-238 and uranium-235 are the most important naturally occurring ones. Uranium-238 makes up about 99.3% of natural uranium, while uranium-235 makes up about 0.7%. U-235 is fissile, meaning it can sustain a nuclear chain reaction under the right conditions, which is why it is crucial in nuclear power and weapons. U-238 is not fissile in the same way, but it can be converted into plutonium-239 in reactors. Uranium’s isotope mix is one reason enrichment is such an important part of nuclear fuel production.