Industrial application of the advanced thermal hydraulics code F-COBRA-TF

Full text of publication follows: Framatome-ANP is currently involved in developing the COBRA-TF computer code, now called FCOBRA- TF, for sub-channel analysis of PWRs and BWRs. F-COBRA-TF is a transient thermal hydraulic sub-channel code which is based on an heterogeneous two-fluid representation of the two-phase flow in the reactor core. It solves the conservation equations for the three fields - continuous liquid (film), entrained liquid (droplets), and vapor - separately. A great variety of heat transfer regimes are taken into account to calculate the complete boiling curve including phenomena like transition boiling, film boiling, dryout, and post dryout. Co-current two-phase flow and countercurrent flow can be handled as well as cross flow situations. The F-COBRA-TF code is applicable for both Pressurized Water Reactors (PWR) and Boiling Water Reactors (BWR). It is able to handle normal operating states as well as faulted conditions like the loss of coolant accident (LOCA), the main steam line break accident (MSLB), etc. At Framatome ANP, the code is mostly used to calculate PWR transients as well as to support BWR fuel assembly design studies. The aim for latter is to reduce the extent of the complex and very expensive full-scale tests usually carried out to determine thermal-hydraulic characteristics of different geometries. Recently, significant improvements in modeling entrainment and deposition for annular film flow have been done to obtain more reliable predictions of dryout in BWR fuel assemblies. In addition the influence of grids on mass, momentum, and energy exchange between adjacent sub-channels is being modeled in more detail. Improvements have also been made in the modeling of the single and two-phase mixing as well as the void drift effects. F-COBRA-TF has been validated over a wide range of experiments. Among them are dryout tests in tubes, 3 x 3 bundles, 4 x 4 bundles, as well as realistic state-of-the-art Framatome- ANP BWR fuel assemblies. (authors)