Probability assessment failure sequences

It is fundamental for assessing human error in systems analyses to identify and describe the human acts with importance for the event sequence under analysis (qualitative assessment). This corresponds to the task analyses, which are characteristic of ergonomic studies. Firstly, the important actions, the moment in time at which they are required and the time period available for their execution have to be determined. Furthermore, the requirements for the action, the information necessary, respectively available, the possibilities of correction in case of omission or faulty execution must be estabhshed. Additionally, other factors of important influence on human reliabihty such as the state of knowledge on the process in question, ergonomically favourable or disadvantageous layout of the workplace, the tools or the environment are identihed. On the basis of this task analysis reliability data (normally failure probabilities on demand) are assigned to the tasks identified. They stem from existing data collections (cf. Table 9.21). 

The sequences of events that may lead to vessel failure and their frequencies are determined from probabilistic risk assessment (PRA) analyses. The pressure, temperature and heat transfer coefficient time histories at the vessel inner surface are determined from thermal hydraulic analyses for the events identified by the PRA analyses. These time histories are used together with probabilistic fracture mechanics (PFM) analysis to calculate the conditional probability of RPV failure. Discussion of the methodology used to perform the PRA analyses and define the transient events and associated frequencies, and the thermal hydraulic analyses used to define the event pressure and temperature time histories are outside the scope of this chapter. Consequently, the remainder of this chapter focuses on the PFM evaluation assumptions and procedures. 

In this example, failure of BI contributes 80% of the risk of being unable to evacuate a platform in the teeth of a predicted storm. Beyond this loss of control point, event tree analysis in employed to assess the probability of the worst case outcome. There are two event sequences, each with a balance of probability, firstly as to whether the storm severity exceeds platform design parameters and lastly whether or not the platform is destroyed. The inclusion of an event tree analysis following a fault tree top event is termed a cause-consequence model. 

The structure corresponds to the logical sequence of the analyses required for the definition of the design basis flood from the site survey stage up to the definition of the design basis and the periodic safety review on the basis of monitoring results. In particular. Section 3 deals with the preliminary investigation for site selection and Section 4 with final data collection for the site assessment up to the definition of the parameters (and probabilities, if required) for the cause of initiation of the flood (precipitation, tornadoes, earthquake and dam failures). 

Quantitative risk assessment is a complex and hotly debated subject. Practitioners use techniques such as Event Tree Analysis or Fault Tree Analysis to give estimated failure rates to key actions in the sequence of events. An example from everyday life might be the probability of stopping... 

The Reactor Safety Study attempted to make a realistic estimate of the potential effects of LWR accidents on the public health and safety. One BWR, Peach Bottom Unit 2, and one PWR, Surry Unit 1, were analyzed in detail. The Reactor Safety Study team used previous information from Department of Defense and NASA to predict the effect of failures of small components in large, complex systems. Events that could potentially initiate core melt accidents were first identified. Event trees were then used to delineate possible sequences of successes or failures of systems provided to prevent core meltdown and/or the release of radionuclides. Fault trees were used to estimate the probabilities of system failures from available data on the reliability of system components. Using these techniques, thousands of possible core melt accident sequences were assessed for their occurrence probabilities. The consequences of such accident sequences were then estimated to complete the risk assessment. 

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