The LWR transient analysis code, RELAP5, was developed at the Idaho National Engineering Laboratory (INEL) for the U. S. Nuclear Regulatory Commission (NRC). Code uses include analyses required to support rulemaking, licensing audit calculations, evaluation of accident mitigation strategies, evaluation of operator guidelines, and experiment planning analysis. RELAP5 has also been used as the basis for a nuclear plant analyser. Specific applications have included simulations of transients in LWR systems such as loss of coolant, anticipated transients without scram (ATWS), and operational transients such as loss of feedwater, loss of offsite power, station blackout, and turbine trip. RELAP5 is a highly generic code that, in addition to calculating the behaviour of a reactor coolant system during a transient, can be used for simulation of a wide variety of hydraulic and thermal transients in both nuclear and non-nuclear systems involving mixtures of steam, water, non-condensable, and solute.

The MOD3 version of RELAP5 has been developed jointly by the NRC and a consortium consisting of several countries and domestic organizations that were members of the International Code Assessment and Applications Program (ICAP) and its successor organization, the Code Applications and Maintenance Program (CAMP). Credit also needs to be given to various Department of Energy activities, including the INEL laboratory-directed discretionary funding program. The mission of the RELAP5/MOD3 development program was to develop a code version suitable for the analysis of all transients and postulated accidents in LWR systems, including both large — and small-break loss-of-coolant accidents (LOCAs) as well as the full range of operational transients.

The RELAP5/MOD3 code is based on a non-homogeneous and non-equilibrium model for the two-phase system that is solved by a fast, partially implicit numerical scheme to permit economical calculation of system transients. The objective of the RELAP5 development effort from the outset was to produce a code that included important first-order effects necessary for accurate prediction of system transients but that was sufficiently simple and cost effective so that parametric or sensitivity studies were possible.

The code includes many generic component models from which general systems can be simulated. The component models include pumps, valves, pipes, heat releasing or absorbing structures, reactor point kinetics, electric heaters, jet pumps, turbines, separators, accumulators, and control system components. In addition, special process models are included for effects such as form loss, flow at an abrupt area change, branching, choked flow, boron tracking, and non-condensable gas transport.

The system mathematical models are coupled by an efficient code structure. The code includes extensive input checking capability to help the user discover input errors and inconsistencies. Also included are free-format input, restart, renodalization, and variable output edit features. These user conveniences were developed in the recognition that generally the major cost associated with the use of a system transient code is in the engineering labour and time involved in accumulating system data and developing system models, while the computer cost associated with generation of the final result is usually small.

Main characteristics of the code are as follows:

— One-dimensional system thermal hydraulic code (enabling also pseudo two — or three­dimensional modelling);

— Basic two-phase model with 6 balance equations (mass, momentum and energy equation for each phase) with additional capability to model boron in liquid phase and non­condensable gases in vapour phase;

— Set of closure correlations and models (best-estimate) connected to the basic model by help of system of flow regimes and set of heat transfer modes;

— Point kinetic model;

— Other physical models (dynamic model of fuel-clad gap, radiation heat transfer, metal — water reaction, etc.);

— Special models for NPP components (pumps, various types of turbine, accumulator, pressurizer, etc.);

— Extensive system of trips and control components for modelling control system;

— Very flexible character of the code itself and of the input data.