RELAP-4, Transient 2 Phase Flow Thermohydraulics, LWR LOCA and Reflood

1 - Description of problem or function: RELAP4 has been 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. RELAP4/MOD6 was developed specifically to add a capability to the earlier RELAP codes for calculating PWR reflood phenomena. REALP-4/6-KFK is a modification of RELAP4/MOD6 update 4 which generates a file for transferring boundary conditions to the fuel rod code SSYST-2 (NEA 0684). RELAP4/MOD7/101: Performs best estimate analyses of nuclear reactors or related systems undergoing a transient. Transient thermal- hydraulic, two-phase phenomena are calculated from formulations of one-dimensional, homogeneous, equilibrium conservation equations for water mass, momentum, and energy. Heat structures are modeled using a transient one-dimensional heat conduction solution that is coupled to the fluid through heat transfer relations. Various explicit models are used to calculate nonhomogeneous, nonequilibrium behavior including a phase separation model, a vertical slip model, and a nonequilibrium model. Other models are used to represent critical flow, reactor kinetics, pressurized water reactor reflood behavior, nuclear fuel rod swelling and blockage, and components such as pumps, valves, and accumulators. 2 - Method of solution: 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. 3 - Restrictions on the complexity of the problem - Maxima of: 9 minor edit variables; 20 time-step cards; 20 trip control cards; 100 junctions (flow paths) between volumes; 5 bubble-parameter sets; 5 time-dependent volume descriptions on cards; 12 pumps; 75 control volumes; 10 check valve types; 5 normalized leak-area-versus-time curves; 20 fill system curves; 50 heat slabs; 20 heat slab geometries; 7 heat slab materials; 50 core sections; 99 data sets for heat exchangers without conduction