Partial Correlation and Dependence Between Seismic Fragilities of Multiple Adjacent Structures with Significant Soil-Structure-Interaction Effects

September 13, 2019
Publication: 25th Conference on Structural Mechanics in Reactor Technology
Author(s): Mohamed Talaat Robert P. Kennedy

Seismic Probabilistic Risk Assessment (SPRA) of nuclear power plants (NPPs) is typically performed using three major analysis components: probabilistic seismic hazard, seismic fragility, and plant response. The fragility analysis develops conditional probabilities of failure for plant structures, systems, and components (SSCs) given ground motion intensity. Fragility combines the probability distributions of the strength of the SSC and the seismic demands given the ground motion intensity. The plant response analysis combines the seismic fragilities into a single plant-level fragility. In the plant logic model, the SSC failure events are typically modeled as either fully correlated or uncorrelated, but partial correlation is not explicitly modeled. Correlation is assigned based on judgment between components that experience similar seismic demands, e.g., co-located and oriented in the same direction, and have similar construction and anchorage (see Bohn and Lambright, 1990). Partial correlation between structure fragilities is often not considered, except maybe for co-located identical structures. This treatment is often conservatively biased, since modeling structure failures as independent typically increases seismic risk estimates. However, this conservative bias is often minor, since (1) partial correlation between structure fragilities is typically weak and (2) structure fragility contribution to NPP seismic risk is typically minor compared to other SSCs fragilities.
Recent SPRAs have shown an increasing contribution of structure fragilities to NPP risk, especially to that of large early release frequency (LERF). Modifications and upgrades to NPP equipment over time have made them more seismically robust while the buildings that house them did not change. For building structures founded on sites with significant-soil-structure interaction (SSI) effects, the SSI response is largely determined by the soil properties and the ground motions. Variability in these properties can have a significantly higher influence on the variability in seismic fragility than do structure properties and material strengths. Since the former factors are common to all the structures, their seismic fragilities can have strong partial correlation. Ignoring this correlation can result in overly conservative risk estimates.

Markets: Nuclear