RELAP4 is a digital computer program, written in FORTRAN IV, developed to describe the behavior of water-cooled nuclear reactors subjected to postulated transients such as those resulting from loss of coolant, pump failure, or power excursions. The program calculates fluid conditions such as flow, pressure, mass inventory, and quality; thermal conditions such as surface temperatures, temperature profiles, and energy distributions; and heat fluxes in power generating and dissipating elements. The program also calculates reactor power, decay heat, and reactivity. In addition to describing transients in boiling- water and pressurized-water reactors, the program is sufficiently versatile to describe transients in experimental thermal-hydraulic systems. The RELAP4 user must define the geometric features of the system to be analyzed as well as an appropriate set of initial conditions. RELAP4 then solves an integral form of the fluid conservation and state equations applied to each user-defined control volume. Currently, the number of these control volumes is limited to 75 and the number of junctions (flow paths) between volumes is limited to 100. By simply increasing the array sizes, these limits could be raised, if necessary, until the capacity of the particular computer is reached. RELAP4 source decks are available in double precision (EBCDIC) versions.more » Special timing and input-output routines are available and used by the RELAP4 program for the IBM 360 systems. Some of these routines are necessary and, if other computers are used must be rewritten for those computers. Sample problems have been run on the IBM 360/75 computer. (37 references) (auth)« less