The Clean and Environmentally Safe Advanced Reactor (CAESAR) is a nuclear reactor concept created by Claudio Filippone, the Director of the Center for Advanced Energy Concepts at the University of Maryland, College Park and head of the ongoing CAESAR Project.
There seems to be some discrepancy between "media overview" material and technical details: The CAESAR reactor design exploits the fact that the fission products and daughter isotopes produced via nuclear reactions also decay to produce additional delayed neutrons.
, however point out that it is generally believed that a controlled, sustained chain reaction is not possible with 238U.
[1] Based on these calculations even an infinitely large mass of pure U-238 (or even natural Uranium) is incapable of sustaining a chain reaction with only its own neutron production, so coupling the gas-cooled fast-spectrum core with a moderated outer slow-neutron section is required, or alternatively some level of fissile enrichment is required.
[3] The energy of delayed neutrons is so low that contribution to 238U fission is almost 0.0000, requiring some fissile material to keep the reactor safely under prompt criticality: (e.g. 235U in natural uranium and preferably also some moderator, possibly outside the extra-fast core).