Estimation likelihood

Estimate likelihood of attack (combination of vulnerability, threat, and attractiveness) or conditional risk. 

Whilst it is quickly possible to get bogged down in mathematical detail, the takeaway message is to ensure that in the estimation of likelihood one looks at the probability of actual harm occurring. Stopping elsewhere along the hazard-to-harm path is fraught with difficulty as the estimated likelihood will depend on exactly which event constitutes harm in the eyes of the assessor. Similarly, by looking at the end-to-end chain of events the presence or absence of controls can be factored in to the likelihood estimation. 

If the source term for the SCB fire in a DBE were increased from that associated with two SCBs to four SCBs, the dose consequences for DBEs 4, 5, and 6 would approximately double. Also, if the SCB fire were different than that assumed in this analysis (0.01) in the event of a DBE, the likelihood of these three DBEs would increase or decrease in direct proportion to the estimated likelihood of the fire. However, in all cases, both the overall likelihood and the potential consequences result in minimal risk. 

Level 2 is a streamlined version of Level 3, intended to be less resource intensive, and suitable for inspector, rather than expert, use. It uses estimated likelihoods of failure obtained from generic databases, and of consequences based on estimated inventories - essentially a first order quantification of the factors addressed in Level 1 on a cost or square-metre-affected basis. The risk classes of Table 2.1 can now be replaced with order-of-magnitude numerical ranges. Since it is commonly observed that around 20% of the components carry 80% of the risk, it is often appropriate to restrict Level 3 analysis to the most critical items, use a Level 2 approach for the intermediate, and not proceed beyond Level 1 for the remainder. 

In safety-critical environments it can be agreed that items that handle safety aspects are always placed in quadrant I, regardless of their estimated likelihood and impact numbers. Note that this is not needed by default, as other precautions are already taken for safety issues (results from FMEA or safety hazard analsysis). 

Estimate likelihood of occurrence (requires practical knowledge, good judgment power, inter-discipline interaction, technical literature)... 

The estimated likelihood of gross failure needs to be very low or the safety case claims gross failure can be discounted. 

Likelihood is one parameter in the risk equation. Risk is the safety measure of a potential future event, stated in terms of event likelihood and event severity. Hazard likelihood is the expected likelihood that the identified hazard will be activated and becomes an actual mishap. Hazard likelihood is the estimated likelihood of a hazard transitioning from a conditional state to an actual mishap event state, resulting in an actual mishap with undesired outcome. 

In a deterministic approach, we obtain a single value of the two parameters and can obtain the factor of safety against failure. However, the advantages of assessing using probabilistic approach are (i) the outcome is expressed as a (estimated) likelihood of failure by taking into account various sources of uncertainty involved in the assessment (ii) sensitivity analysis can be conducted to identify key factors that affect the outcome (the results are useful to improve the... 

Figures 5.1-7 and 5.1-8 show the uncertainty in the probability of early containment failure conditional on the occurrence of three different classes of accident sequences for the plants analyzed in NUREG-1150. Containment bypass scenarios are not included in these figures, and the results are for internally initiated accidents only. The plant-specific mean frequency of the accident class is listed to the right of each uncertainty interval. For some of the plants (e.g., Zion and Surry) the best estimate of the conditional probability of early containment failure is quite small (about 1%) however, for all plants the uncertainty in the estimated likelihood of early containment failure is quite large. This uncertainty arises as a result of corresponding uncertainties in both the pressures and temperatures that would exist within the containments and the ability of the containments to withstand these pressures and temperatures. In addition, for several of the containments there is uncertainty regarding the mode (structural mechanism, location, size of opening, etc.) by which containment would fail.

Hazardous Event or Situation Caused by/ sequences of events Consequences, immediate/ ultimate Estimated likelihood. Suggested measures to reduce likelihood Emergency measures (to reduce consequences) Action required... 

Estimate likelihood of these factors occurring, in the right column for each choice. (Use a scale of 1 to 10.)... 

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