OVERVIEW OF NRC SPONSORED LWR LOWER HEAD FAILURE RESEARCH-PROGRAM

The US Nuclear Regulatory Commission (US NRC) has sponsored a research program to investigate the mode and timing of vessel lower head failu re. 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 accid ent conditions. Failure mechanisms, such as tube ejection, tube ruptur e, global vessel failure, and localized vessel creep rupture, were stu died. Newly developed models and existing models were applied to predi ct which failure mechanism would occur first in various severe acciden t 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 tech niques were employed for analytical model verification and examination of more detailed phenomena. High-temperature creep and tensile data w ere obtained for predicting the vessel and penetration structural resp onse. This paper summarizes major accomplishments and conclusions from research performed in the NRC sponsored lower head failure project.