THE ROLE OF STRUCTURAL MATERIALS IN THE VULNERABILITY OF NUCLEAR POWER PLANTS

Abstract

The nuclear power plants (NPPs) world-wide are generally very robustly designed and constructed, capable to stand very extreme conditions. Small design differences from this point of view can be found among the various reactor types of the same generation; PWR, WWER, etc. The NPP structures are thus designed to accommodate all originally thinkable unwanted conditions, to cope with various extreme scenarios and respond safely to the various considered initiating events. In addition to the robust design, a series of complex redundant and diverse safety barriers, following a defence in depth concept, have been developed to avoid negative consequences, or at least mitigate the consequences of the events. Recently, questions and debates are appearing with regard to the vulnerability of the NPPs and their possible exposure to external threats; like for example terrorist attacks involving few individuals able to by-pass security and introducing small charges of explosive inside or near-by such containments. The role of the structural materials is in these situations very important for the safety of the NPP. The worst consequences of an event can contemplate of course huge environmental damage, like release of radio-activity combined with possible human losses and considerable direct costs, and financial and logistic indirect consequences. Such negative consequences are especially impacting the nuclear industry; in fact, it can be foreseen that a single accident or serious incident may put in danger the complete NPP fleet operation simply due to public opinion justified pressure. The response of the structures subjected to non-design impacts is discussed and reviewed in this paper. Although the main focus is on structural integrity, the paper also discusses the overall risk assessment of terrorist attacks presenting the link between structural analyses and plant risk analysis.