Computation of total risk in an external hazard PRA is dependent on hazard assessment, fragility assessment, and systems analysis. A systems analysis for the propagation of component fragilities is conducted using event and fault trees. The event and fault trees for an actual power plant can be fairly large in size, which imposes computational challenges. Hence, certain assumptions are employed for computational efficiency. The assumptions are valid for the case of design basis conditions, but may lead to excessively conservative risk estimates in the case of beyond design basis conditions. We provide state-of-the-art tools, including Bayesian networks, to estimate the system-level risk accurately and computationally efficiently compared to traditional PRA tools.