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ABSTRACl The SCDAP/REtAPS code has been developed for best-estimate transient simulation of light water reactor coolant systems during a severe accident. The code models the coupled behavior of the reactor coolant system, the core, and the fission products and aerosols In the system during a severe accident transient as well as large and small break loss-of-coolant accidents, operational transients such as anticipated transient without SCRAM, loss of offslte power, loss of feedwater, and loss of flow. A generic modeling approach Is used that permits as much of a particular system to be modeled as necessary. Control system and secondary system components are Included to permit modeling of plant controls, turbines, condensers, and secondary feedwater conditioning systems. The modeling theory and associated numerical schemes are documented In Volumes I and II to acquaint the user with the modeling base and thus aid In effective use of the code. Volume III contains detailed instructions for code application and Input data preparation. In addition. Volume III contains user guidelines that have evolved over the past several years from application of the REtAP5 and SCDAP codes at the Idaho National Engineering taboratory, at other national laboratories, and by users throughout the world. The light water reactor (LWR) severe accident transient analysis code, SC0AP/RELAP5, has been developed at the Idaho National Engineering Laboratory (INEL) for the U. S. Nuclear Regulatory Commission (NRC) to provide an advanced best-estimate predictive capability for use In severe accident applications In support of the regulatory process. Code uses Include analysis required to support rulemaking, licensing audit calculations, evaluation of accident mitigation strategies, and experiment planning and analysis. Specific applications of this capability have Included analytical support for the loss-of-fluld test (LOFT), Power Burst Facility (PBF), ACRR, MISl, ROSA IV, and NRU experimental programs, as well as simulations of transients that lead to severe accidents, such as loss of coolant, anticipated transients without scram (ATWS), and operational transients In LWR systems. SCDAP/RELAP5 Is a highly generic code that. In addition to calculating the behavior of a reactor coolant system (RCS) during a severe accident transient, can be used for simulation of a wide variety of hydraulic and thermal transients In both nuclear and nonnuclear systems Involving steam-water noncondenslble solute fluid mixtures. SC0AP/RELAP5 was developed by Integrating three separate codes, RELAP5/M0D2, SCDAP, and TRAP-MELT. These codes were combined to model the coupled Interactions that occur between the core, the RCS, and the fission products during a severe accident. For example, blockage In the core, caused by fuel rod ballooning and meltdown, can have a significant effect on RCS flows. Fission products released from the core can have a significant effect on the RCS because of the heat produced during decay. These and many other coupled effects can have a sig...

Designs are described for models to calculate the heat transfer and flow losses in porous debris in the lower head of a reactor vessel. The COUPLE model in SCDAP/RELAP5 represents both the porous and nonporous debris that results from core material slumping into the lower head. Previously, the COUPLE model had the capability to model convective and radiative heat transfer from the surfaces of nonporous debris in a detailed manner and to model only in a simplistic manner the heat transfer from porous debris. In order to advance beyond the simplistic modeling for porous debris, designs were developed for detailed calculations of heat transfer and flow losses in porous debris. Correlations were identified for calculating convective heat transfer in porous debris for the following modes of heat transfer; (1) forced convection to liquid, (2) forced convection to gas, (3) nucleate boiling, (4) transition boiling, (5) film boiling, and (6) transition from film boiling to convection to vapor. Interphase heat transfer is modeled in an approximate manner. Correlations were identified for calculating the flow losses and interphase drag of fluid flowing through the interstices of the porous debris. These calculations for flow loss and interphase drag were integrated into the momentum equations in the RELAP5 part of the code. Since the models for heat transfer and flow losses in porous debris in the lower head were designed for general application, a design is also described for implementation of these models into the analysis of porous debris in the core region. An assessment was performed of the capability of the implemented models to calculate the heat transfer and flow losses in porous debris. The assessment showed that SCDAP/RELAP5 is capable of calculating the heat transfer and flow losses occurring in porous debris regions that may develop in a LWR during a severe accident.

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