12.142. The RELAP class of codes developed at the Idaho National Engineering Laboratory (INEL) for the U. S. Nuclear Regulatory Com­mission is the nonproprietary workhorse for reactor safety thermal-hydraulic analysis for PWRs only [11]. TRAC codes developed by Los Alamos have a minimum level of modeling simplifications and therefore serve both for accident analysis and “benchmarking” or calibration of simpler modeling systems [12]. Both PWR and BWR versions are available. The Electric Power Research Institute has sponsored the RETRAN series of codes, which evolved from the RELAP effort. These are intended to provide electric utilities with a general-purpose tool for design and safety evaluation [13]. A feature is the continuing use of plant operating data for code calibration.

12.143. RELAP5/MOD2, is the result of a complete rewrite of the earlier RELAP4 to include nonhomogeneous and nonequilibrium features. It is applicable for modeling large and small break LOCAs, operational transients, transients in which the entire secondary system must be mod­eled, and system behavior simulation up to the point of core damage. The code includes a point reactor kinetics model (§5.7), a decay heat model,

and the capability of modeling various functions of the reactor control system.

12.144. A challenge in the development of any system modeling code is to manage the numerical solution techniques so that there is a reasonable balance between accurate representation and computing effort, particularly when one considers the physical complexity of turbulent two-phase flow. In the case of RELAP5/MOD2, modeling with workstation-level com­puters has become possible. Various improvements are being incorporated into a new version, RELAP5/MOD3 [14].

12.145. TRAC-PF1 is intended to model PWR LOCAs and transients while TRAC-BF1 fulfills a similar function for BWRs. RETRAN-02 models all types of LWR transients. They are similar in the general nature of simulation approach to the RELAP codes but differ in modeling details and solution methods. TRAC is a useful design tool. It, like other system codes, includes a number of computing modules that model behavior in individual components. TRAC was extensively used in the analysis of the Three Mile Island accident.