Why must uranium-235 be enriched for use in most light-water reactors?

Uranium-235 must be enriched for use in most light-water reactors (LWRs) because naturally occurring uranium contains only about 0.72 percent of the fissile uranium-235 isotope, with the remainder being primarily uranium-238.1

In a light-water reactor, ordinary water serves as both the coolant and the moderator to slow down neutrons, which is necessary to sustain a chain reaction. However, because ordinary water also tends to absorb more neutrons than other types of moderators, a higher concentration of fissile material is required to maintain criticality. To compensate for this absorption and ensure efficient operation, the fuel used in LWRs is enriched to contain 3 to 5 percent uranium-235. This enrichment process, typically achieved through methods like gas centrifugation, increases the probability of fission occurring within the reactor core.2341