Yes, natural uranium can go critical under certain conditions. Criticality refers to a self-sustaining chain reaction in which the rate of neutron production and absorption is balanced. In the case of natural uranium, it contains a mixture of uranium-238 (U-238) and a small amount of uranium-235 (U-235). U-235 is a fissile isotope, meaning it can sustain a chain reaction, while U-238 is a fertile isotope that can be converted into fissile material through neutron capture and subsequent radioactive decay.
Natural uranium typically has a low concentration of U-235, around 0.7% of the total uranium content. In order for natural uranium to go critical, it requires an increased concentration of U-235. This can be achieved through a process called enrichment, where the U-235 content is increased to a level where a sustained chain reaction can occur. Enriched uranium is used as fuel in nuclear reactors or as a component in nuclear weapons.
Without enrichment, natural uranium is not capable of self-sustaining chain reactions. However, it can still undergo subcritical reactions and release energy, such as in the case of conventional nuclear power reactors that utilize enriched uranium or other fuels.