Initiating event frequency analysis, risk

Risk Analysis—The development of a quantitative estimate of risk based on engineering evaluation and mathematical techniques for combining estimates of initiating event frequency and independent protection layers and consequences. 

Hora, S.C. Iman, R.L. 1990. Bayesian modeling of initiating event frequencies at nuclear power plants. Risk Analysis, 10(1) 103-109. 

The chance of an incident is generally a function of the distance traveled. Thus, the frequency of an accident is often expressed as an accident rate per mile. Contributions from non-accident-initiated events are typically expressed on a frequency-per-hour or per-year basis. Thus, the duration of the hazardous materials movement is a key parameter. Figure 5.3 illustrates the basic calculation sequence for one trip or movement. If multiple trips are made, the total risk is equal to the number of trips times the risk per trip. The basic calculation sequence will have minor variations for each mode of transport and can be broken down into greater detail as needed. Increased detail might include different accident rates and lengths for each segment of a route or might explicitly address the accident rates and release probabilities for different accident causes. Inputs to the analysis that may be altered or may influence the calculation include ... 

If control system equipment failure can cause an initiating event, then quantitative analysis must be done for all components where failure might initiate a hazard. For those failures with no other protection layer, the frequency of failure will result directly in an incident. The detailed quantitative analysis must show that these failures will not increase risk beyond tolerable levels. 

Having identified a number of initiating events, the demand tree can be used as an input to other analysis techniques to carry out a more detailed risk assessment. This further stage would typically use either a fault-tree analysis or a layer of protection analysis (so long as the LOPA methodology used has sufficient flexibility to treat each cause separately and then combine them when assessing the frequency of the hazardous event). 

The reviews of the event trees indicate that the trees accurately model the responses to the postulated initiating events, given the simplifying assumptions used to reduce the complexities of the analysis. The principal scoping assumption, i.e., that the reactor is operating,at full power, is typical of most reactor PRAs. Some concern was expressed that shutdown operations and operation at less than full power were not modelled. Commercial reactor experience has shown that analyses of these operational phases can provide important insights. These concerns need not be resolved before restart, because it is expected they will have only small contributions to core damage frequency and plant risk. 

SESSION II Risk Theory and Risk Analysis for Landslides. [1] Landslide Risk Management concepts and framework and examples (2.5 h) [2] Deterministic and Probabilistic models for slope stability evaluation (2 h) [3] Introduction to modelling of catastrophic landslide events (2 h) [4] Empirical models for travel distance (1.5 h) [5] Application examples of probabilistic methods and semi quantitative methods for landslide hazard zonation (2h) [6] Landslide Frequency Assessment (1.5 h) [7] Different components of vulnerability to landslides. Prevention and long term management of landslides (3.5 h) [8] Case Studies coal waste dump risk assessment, example from motorway in La Reunion Island, Aknes Rock slope in Norway (2 h) [9] Application of QRA to other geotechnical problems Internal erosion of dams, crater lake hazard (1.5 h) [10] Advanced numerical models initiation of landslides, propagation of sediments/climate change effects (3.5 h). 

Does the hazard and risk analysis clearly identify the Initiating causes and associated frequency of occurrence for potentially hazardous events ... 

The principal tool supporting this activity is the full-scope Probabilistic Risk Assessment (PRA) that WSRC is developing for the SRS reactors. The PRA, when completed, will include Level 1, 2, and 3 analyses. The Level 1 analysis identifies core damage sequences and their frequencies. The Level 2 analysis studies each accident sequence from core melt progression, through confinement system response, to the release of radionuclides to the environment. The Level 3 analysis takes the release from each accident sequence and estimates the consequences. Accident initiators Include Internal events, which are associated with component failures in the reactor systems, and external events, which are outside the reactor systems, such as earthquakes, fire, or flooding. 

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