The CATHARE code strategy and assessment

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The French thermal-hydraulic code CATHARE is being developed in Grenoble by EDF, FRAMATOME and CEA. The best-estimate objectives of CATHARE require a very rigorous methodology for development and assessment. In particular, the validation process is performed in two steps: qualification of the models against separate effect tests and verification of the code on integral loop tests. The validation reports and the resulting evaluation document and user guidelines manual are delivered to the users. This paper illustrates the methodology for the case of CATHARE 1 version 1.3. The qualification process was conducted with reference to a matrix of 300 tests selected from experiments on critical flow, flow pattern determination, blowdown of heated or adiabatic test sections in various geometries, reflooding, boil-off, steam generator and pump behaviour, fuel thermomechanics. The results show the adequacy and the consistency of the models but indicate some items to be further studied: droplet diameter correlation, incipience of boiling model, critical flow in diaphragms, top-down rewet or reflood, interphase friction in rod bundle geometry. The verification process was performed on 21 tests coming from LOBI, LOFT, LSTF, PKL facilities. The natural circulation post-test calculations show good results for single phase or two-phase mass flowrate evaluation and a satisfactory prediction of the reflux condensor phase. The various phases of the small break or large break transients are generally well described by CATHARE. However, some quantitative discrepancies underline limited deficiencies in the domain of condensation at safety injection, CCFL at steam generator inlet or core upper tie plate, multidimensional effects in annular downcomer and in core region, modelling of steam generator secondary side. The consideration of these items led to the introduction of significant improvements and new capabilities in the CATHARE 2 version which consequently has to be validated following the same two-step process and with an extended test matrix. The efficiency of such a rigorous methodology has induced the French nuclear industry (EDF, FRAMATOME) to use CATHARE for safety research and development studies.