Capabilities and limitations of thermalhydraulic codes

Advanced thermalhydraulic codes such as CATHARE, RELAP 5 or TRAC are now part of the everyday tools used for Pressurized Water Reactor safety studies. In the last few years methodologies for code uncertainty evaluation have been developed. The relevance of these methods is still under discussion and it may be argued that they do not fully take into account all the inherent limitations of such codes. In relation to this problem, it is useful to get a clear view of the capabilities and limitations of a thermalhydraulic code. As a contribution to this work some conclusions from the analyses of the CATHARE code assessment calculations are presented. The first part of this paper considers aspects of the code development process in order to keep in mind the successive simplifying assumptions required to get the final model. This process includes the derivation of averaged balance equations from exact local instantaneous equations, the choice or development of the constitutive relations, the discretization and the solution methods. Each of these steps is associated with a degradation of the physical description. Next a review of code prediction deficiencies during calculations of analytical tests is presented leading to conclusions on the accuracy and range of validity of some constitutive relations. Analysing the mispredictions of integral tests, various kinds of difficulties have been pointed out. At first, problems may be related to a questionable choice of schematization. Limitations are also faced when a constitutive relation is used out of its validity range. A more critical case in encountered when a specific physical situation has not been yet studied in analytical tests and is not described by any of the models. Finally some transients may themselves be highly sensitive to certain physical processes. This paper focusses on problems encountered in predicting core swell level, flooding limit and CCFL in complex geometries, loop seal clearing, condensation at EEC injection and gravity driven reflooding. (author). 12 refs