Considerations concerning the analysis of NPP structures subjected to blast loading

Basic features of the loads induced on structures by air blasts are described in IAEA Design Guide NS-G-1.5 (2003) in terms of a normalized distance that takes into account the amount and type of explosive charge. The guide presents charts that allow the determination of the peak value of the incident pressure, the total impulse of the positive phase and other relevant design parameters, which are generally used for design or verification purposes of sensitive nuclear structures. For the determination of the resulting actions on structures subjected to a specified blast event, the load-time functions induced by the incident pressure wave must next be evaluated, as discussed for example by Kinney and Graham. The authors note that a number of models are available for such purpose, such as Zaslavskii et al., which lead to different predictions and raise the issue of model uncertainty in this problem. In this paper, the uncertainties expected in the various stages of the process are examined in connection with the analysis of the air vent stack of a NPP presently under design. The spatial and temporal pressure distributions induced at the site by the blast were determined by means of a finite element model that simulates the shock wave propagation considering the topographical features of the site and the main structures of the NPP. The numerically determined pressures are compared with the IAEA design guide recommendations. Next, the loads induced in the air vent stack by the shock wave as well as the dynamic response of the stack are determined employing several models. The validity of commonly accepted assumptions in the determination of the response of structures to blast loading is finally discussed.