QUALIFICATION OF THE CFD CODE TRIO _ U FOR FULL SCALE NUCLEAR REACTOR APPLICATIONS

Numerical and experimental research on nuclear safety is in the end dedicated to understand, on a plant scale , the fundamental physical phenomena which are associated to specific accident scenarios. Hence, the results derived from single effect experiments or reduced scale analysis have to be extrapolated to plant scale whereas plant scale experiments should be evaluated with respect to their applicability to the physics of the specific scenario. For several years, IRSN and CEA have used Computational Fluid Dynamics (CFD) codes for detailed nuclear safety analyses on plant scale. The paper presents a procedure which has been used to qualify the Trio_U code for the prediction of the boron concentration at the core inlet of a French Pressurized Water Reactor (PWR) in accidental conditions (inherent dilution problem) 1 . A ROCOM experiment as well as an UPTF Tram-C3 experiment has been used for this purpose. Introduction In PWR, boron acid is used as a neutron absorber for reactivity control. If the boron concentration in the core region is reduced (boron dilution), it might result in a power excursion with possible fuel damage. Within nuclear reactor safety analyses, one of the events that could potentially lead to such a criticality accident is a small break loss of coolant accident (LOCA) inducing a dilution in the steam generators tubes by reflux condensation phenomena then followed by the restart of the natural circulation /1/. This situation can only be mitigated by the mixing at the core inle t. The increase of computer power allows nowadays the use of CFD for the detailed analysis of the three dimensional thermal hydraulic phenomena which occur under such accident conditions. Nevertheless, the application to full scale PWR thermal hydraulics safety issues is a challenge for CFD due to the complexity of the underlying physical phenomena and the limited field of application of the physical models which are implemented in the codes. Best Practice Guidelines (BPG) /2/, /3/ can provide CFD users with simple and practical advices on the correct application of computational methods. However, BPG can not help in the definition of the modelling hypothesis, this means to define the problem, to identify the important physical phenomena and to select an adequate solution strategy. 1 This work has been partly financed by IRSN