circumstances leading to release of material. Each node of the event tree (Figure 4.5-1 of the uSSRA) indicates a point in the sequence of events at which a release mitigation system (including human action) either succeeds or fails. “Success” means that the system functions as expected, not that it is 100% effective. An event tree is developed for each originating location and for each of four possible mechanisms, called pathways, by which pathogens could be released (aerosol, liquid waste, solid waste, and transference). This is accompanied by a table that provides the following for each node of the event tree: failure probabilities and “reduction factors” that are to be applied to the MAR, one reduction factor that is assigned when the mitigation system at each event tree node fails, and another when it is fully functional.

The reliability of the ultimate risk estimates presented in Section 8 depends heavily upon the adequacy of the analyses and results from the accident event modeling of Section 4.


The method applied in Section 4 is a distinct improvement over that applied in the 2010 SSRA. The use of event tree analysis and probabilistic modeling is preferable to the scenario-based, semi-quantitative approach of the earlier assessment, and is consistent with current risk assessment science for facilities like the NBAF. The adoption of International Organization for Standardization (ISO) Standard 31000 terminology is also to be applauded, although the committee notes some concerns about the misuse of terms.

The committee identified a few significant omissions in the conceptual models used to describe containment and in the elucidation of system failures that could lead to a release (the system failures are summarized in the 24 circumstances presented in Table 4.3.1-1 of the uSSRA). The committee finds that the development of the 142 events that could lead to an infectious or non-infectious release is nearly complete and generally takes into account mitigation systems, including human action, identified in the conceptual models. However, critical issues remain that affect other aspects of the risk assessment, and these are discussed below.


The uSSRA generally adheres to ISO 31000 terminology. As previously mentioned in Chapter 3, the committee finds that use of the term Ploss is confusing, and the uSSRA should have adopted a different term. The uSSRA would be less confusing if the “loss” subscript were dropped, inasmuch as Ploss can be easily misread as the probability of loss of FMDv when it carries no such meaning. The term refers to the probability that,

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