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. The concept's key element is the use of steam as a moderator, making it a type of reduced moderation water reactor. Because the density of steam may be controlled very precisely, Filippone claims it can be used to fine-tune neutron fluxes to ensure that neutrons are moving with an optimal energy profile to split 238
92U
nuclei – in other words, cause fission.

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. Filippone claims that unlike conventional water-cooled fission reactors, where fission occurring in enriched 235
U
fuel rods moderated by liquid-water coolant ultimately creates a Maxwellian thermal neutron flux profile, the neutron energy profile from delayed neutrons varies widely. In a conventional reactor, he theorizes, the moderator slows these neutrons down so that they cannot contribute to the 238
U
reaction; 238
U
has a comparatively large cross-section for neutrons at high energies.

Filippone maintains that when steam is used as the moderator, the average neutron energy is increased from that of a liquid water-moderated reactor such that the delayed neutrons persist until they hit another nucleus. The resulting extremely high neutron economy, he claims, will make it possible to maintain a self-sustaining reaction in fuel rods of pure 238
U
, once the reactor has been started by enriched fuel.

Skeptics, however point out that it is generally believed that a controlled, sustained chain reaction is not possible with 238
U
. It can undergo fission when impacted by an energetic neutron with over 1 MeV of kinetic energy. But the high-energy neutrons produced by 238
U
fission (after quickly losing energy by inelastic scattering), are not, themselves, sufficient to induce enough successive fissions in 238
Ureference