| Abstract: | The US Nuclear Regulatory Commission (US NRC) has sponsored a research program to investigate the mode and timing of vessel lower head failure. Major objectives of the program were to identify plausible failure mechanisms and to develop a method for determining which failure mode would occur first for different light water reactor designs and accident conditions. Failure mechanisms, such as tube ejection, tube rupture, global vessel failure, and localized vessel creep rupture, were studied. Newly developed models and existing models were applied to predict which failure mechanism would occur first in various severe accident scenarios. So that a broader range of conditions could be considered simultaneously, the calculations relied heavily on models with closed-form or simplified numerical solution techniques. Finite element techniques were employed for analytical model verification and examination of more detailed phenomena. High-temperature creep and tensile data were obtained for predicting the vessel and penetration structural response. This paper summarizes major accomplishments and conclusions from research performed in the NRC sponsored lower head failure project. |
