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NASA-Systems Engineering

# To attach probabilities to outcomes event trees and

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Unformatted text preview: ty estimates and outcome dollar values. NASA Systems Engineering Handbook Management Issues in Systems Engineering Probabilistic Risk Assessment (PRA). A PRA seeks to measure the risk inherent in a system's design and operation by quantifying both the likelihood of various possible accident sequences and their consequences. A typical PRA application is to determine the risk associated with a specific nuclear power plant. Within NASA, PRAs are used to demonstrate, for example, the relative safety of launching spacecraft containing RTGs (Radioisotope Thermoelectric Generators). The search for accident sequences is facilitated by event trees, which depict initiating events and combinations of system successes and failures, and fault trees, which depict ways in which the system failures represented in an event tree can occur. When integrated, an event tree and its associated fault tree(s) can be used to calculate the probability of each accident sequence. The structure and Probabilistic Risk Assessment Pitfalls Risk is generally defined in a probabilistic risk assessment (PRA) as the expected value of a consequence function—that is: R = Σ PS CS S where PS is the probability of outcome s, and CS is the consequence of outcome s. To attach probabilities to outcomes, event trees and fault trees are developed. These techniques have been used since 1953, but by the late 1970s, they were under attack by PRA practitioners. The reasons include the following: • • • • • • Fault trees are limiting because a complete set of failures is not definable. Common cause failures could not be captured properly. An example of a common cause failure is one where all the valves in a system have a defect so that their failures are not truly independent. PRA results are sometimes sensitive to simple changes in event tree assumptions Stated criteria for accepting different kinds of risks are often inconsistent, and therefore not appropriate for allocating risk reduction resources. Many risk-related decisions are driven by...
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