The Reactor Safety Program is centered on those problems associated with the operation of nuclear reactors which involve the possibility of extreme hazards. The program involves studies of reactor dynamics, chemical reactions, and reactor containment. The earliest large-scale experimental work was the Borax series of tests. The principal objectives of the Borax series were related to the feasibility of boiling reactors but included the study of the self- limiting properties of such systems when subjected to sudden large additions of reactivity. Studies on the maximum step and ramp additions of reactivity that can safely be introduced into a few selected types of reactor cores were completed. Spert-II will be used to examine the influence of prompt neutron lifetime and to study the influence of special factors connected with the use of heawy water. Spert-III wiII be used to assess the importance of pressure and temperature, as well as other special factors, in power plants operating with boilingand pressurized-water reactors. Spert-IV will be used for investigating self-induced oscillations. Extensive changes are proposed in the Spert-I program. (W.L.H.)