Assessment of the hydrodynamic loads due to a LOCA in a 3-loop PWR. Castem-Plexus computations
Résumé
The safety analysis of the PWR plants requires to assess the consequences of a hypothetical Loss Of Coolant Accident in the whole primary circuit and the reactor. This paper is dedicated to the study of the depressurization phase immediately following the break opening and to the evaluation of the mechanical consequences of the transient on the inner structures of a 3-loop PWR.
The primary circuit and the reactor are represented with a pipe-model. A guillotine rupture is applied to one of the cold legs, just downstream the pump. The geometric model and the hydraulic conditions were described in [1]. This paper only deals with the analysis of the results calculated with the CASTEM-PLEXUS code. We suppose that the LOCA occurs in a reactor working at nominal rating.
A first calculation is carried out at nominal rating for 2~s in order to validate the numerical model. The validation is obtained by comparing the computed pressures, volumic flows and pump working conditions with the estimated nominal rating ones. As the computation converges around the initial data (approximating the nominal rating), then the model is correct and can be used for the LOCA computation.
A second calculation is performed during 500~ms, in accidental operation, from the previous initial conditions. The results are globally and locally scrutinized thanks to whole circuit drawings at a given time and local curves showing the evolution of a variable versus time. The analysis deals with the pressures and the void fractions in the complete circuit, and with the local pressure differences in the reactor.
We observe a pressure drop with three phases:
- from 0 to 2~ms: a pressure drop at the break,
- from 2 to 100~ms: a general pressure loss in the whole circuit,
- after 100~ms: a slower diphasic pressure decrease in the primary circuit.
From the break extremities, two depressurization waves propagate, in opposite directions, through the whole circuit. The wave passing is characterized by a pressure fall until the saturation pressure at 8.2~MPa. Both waves superimpose at the higher plenum outlet and in the non broken loop steam generators. In the core area, we note horizontal pressure differences on the core barrel that can reach 1.2~MPa. The vertical gaps don't exceed 0.5~MPa. The wave coming from the reactor side break causes a flow rate increase whereas the wave coming from the pump side break brings about a flow rate decrease including a direction change in some places.
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