SIMPLIFIED MODEL OF PWR VESSEL TEARING UNDER SEVEREACCIDENT CONDITIONS.APPLICATION TO LHF TESTS AND REACTOR SITUATIONS

摘要

Between 1999 and 2003, CEA conducted a R&D program on high temperature ductile tearing ofFrench PWR vessel steel 16MND5. The main goal was to develop simplified high temperature tearingmodels, validated on experimental data, and allowing parametrical studies of various severe accidentsituations of vessel failure. These models are to give estimations of the final breach section (andtherefore assess the risks of complete unzipping of the vessel), which are major safety concerns withregard to molten corium ejection and ex-vessel initial conditions.The model presented in this paper concerns circumferential tearing and applies to fast openingsituations, for which breach propagation is coupled to vessel depressurisation. Following anexperimental program of high-temperature tearing tests on Compact Tensile specimens and CentreCrack Panels, a perfect plastic behaviour with a strain rupture criterion have been chosen, and simpleassumptions were made on stress and strain distribution. The equations of the model are based on theenergetic differential balance between the kinetic energy, the external work of the pressure load andthe energy dissipated in the tearing process, coupled to an isothermal depressurisation model. Thissystem is numerically solved to obtain the breach length and section versus time during thedepressurisation process.The application of this model to the LHF tests 3, 5 and 8 shown a very good agreement with theexperimental results concerning the final breach size and the dynamic stability or instability of thestructure. Dimensionless key parameters were identified, allowing simple assessments on the risks ofcatastrophic failure in experimental or reactor situations. Moreover, the importance of pressure historybefore failure was explained : lower and constant pressure values (implying creep failures) lead todynamically stable situations and smaller breach sections, whereas fast plastic failures during pressureramps lead to dynamically unstable situations and catastrophic failures may occur.Finally, a PWR severe accident situation with corium relocation in the vessel bottom head has beenmodelled. The main result is that catastrophic failure (complete unzipping of the vessel bottom head,like in the LHF 5 test) can occur in situations of late vessel reflooding, leading to a re-pressurisation ofthe primary circuit at high vessel temperature. These situations are of low probability, but these resultsmight have important consequences on accident management and safety assessments.