Coupled 3D thermo-mechanical analysis of nordic BWR lower head failure in case of core melt severe accident

Abstract

Consulta en la Biblioteca ETSI Industriales (Riunet)

[ES] In the present Master Thesis a hypothetical core melt severe accident in a Nordic BWR is considered. The molten core material is assumed to be relocated and quenched in the lower head of the reactor vessel forming an internally heated debris bed and eventually a melt pool of corium, which will inflict thermal and mechanical loads to the vessel wall and penetrations leading to its failure. The mode and timing of the vessel failure determine melt ejection characteristics and the success of ex-vessel melt retention strategy proposed in Nordic BWR as a means of terminate the severe accident progression. A coupled thermo-mechanical approach using plant-scale 3D models of the lower head geometry with penetrations is followed in the present work in order to reduce uncertainties in the mode and timing of vessel failure. The calculations are performed coupling the Phase change Effective Convectivity Model (PECM), which simulates the debris bed heat transient and thermal load to the lower head, with ANSYS finite element structural models of the vessel wall and penetrations. Furthermore, several scenarios are considered in terms of (i) implementation of control rod guide tube (CRGTs) cooling as a severe accident management strategy, and (ii) different amounts of the core relocated in the lower plenum, with the aim to investigate the influence of these factors on the mode and timing of (a) failure through the vessel penetrations, and (b) failure through the vessel wall by creep. A total of four possible modes of vessel failure at different times have been identified depending on the scenario. (i) The failure through instrumentation guide tube (IGT) ejection is the earliest mode of failure for both cooling scenarios. In addition, two different modes of vessel wall failure depending on the amount of core relocated in the lower head were found; (ii) vessel wall creep failure by ¿localized creep¿ at the vicinity top periphery of the wall, and (iii) vessel wall creep failure by ¿ballooning¿ of the vessel bottom. These results are consistent with previously obtained results from analytical calculations using 3D-slice models of the reactor lower head, although slight differences in the timing of failure as a consequence of considering a 3D quadrant model are obtained. Finally, a new possible mode of failure that was not previously being identified was found, that is, (iv) Failure by accelerated creep at the CRGTs penetrations, which can take place earlier than the vessel wall failure for the scenarios without CRGTs cooling.