ISPH Nuclear Energy Practice Test

PWR fuel uses low-enriched uranium so that a thermal reactor with water as a moderator can sustain a controlled fission chain reaction. Natural uranium has only a tiny fraction of U-235, which isn’t enough to maintain a steady reaction in a light-water environment. By increasing the U-235 content to a modest level, the fuel becomes capable of sustaining the reaction over the fuel burnup while still allowing for safe reactor operation, fuel management, and licensing. That enrichment level is chosen to balance several factors: it provides enough fissionable material to achieve and maintain criticality with the reactor’s neutron spectrum, it supports efficient fuel burnup over the fuel cycle, and it avoids the higher proliferation and safety considerations associated with much higher enrichments. In short, PWRs use a relatively small, practical enrichment—more than natural uranium but well below what’s used for highly enriched fuel.