Hydrodynamic consequences of a LOCA in a 4-loop PWR
Résumé
This paper presents a numerical simulation with CASTEM-PLEXUS fast dynamics code of the hydrodynamic behaviour of the pressurised primary piping system belonging to a 4-loop nuclear PWR under a sudden pipe break known as a Loss-Of-Coolant-Accident (LOCA). To calculate unsteady pressure and velocity fields a hydrodynamic model of the primary fluid was developed. The fluid behaviour in the whole piping is approximated by Euler equations with a homogeneous equilibrated two-phase flow model. In the broken zone a more precise Moody’s non-equilibrated model is used taking into account the sliding between liquid and steam phases. Under an assumption of plane waves, a finite element discretization of all fluid volumes of the primary circuit is performed using pipe elements of CASTEM-PLEXUS code. An assumption of perfectly rigid structures is used which is justified because anti-whipping devices are used to limit displacements of the primary circuit piping. Using the above hydrodynamic model, the nominal flow rate in the undamaged circuit is first calculated. The stabilisation of nominal rate computation for main variables close to initial conditions proves the coherence of the numerical hydrodynamic model and allows using it for the LOCA computation. The nominal computation was carried out until 1000 ms of physical time. Then, a sudden break of a U-leg of one of the primary-circuit loops is simulated. It generates a strong decompression wave running through the circuit. A LOCA computation is performed for 500 ms more. The curves of evolution of main variables allow understanding the chronology of the blowdown from the rupture time.
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