Risk modeling of variable probability external initiating events

As components engineering has progressively advanced over the past 20 years to encompass a robust element of reliability, a paradigm shift has occurred in how complex systems fail. While failures used to be dominated by `component failures,' failures are now governed by other factors such as environmental factors, integration capability, design quality, system complexity, built in testability, etc. Of these factors, environmental factors are difficult to predict and assess. While test regimes typically encompass environmental factors, significant design changes to the system to mitigate any failures found is not likely to occur based on the cost. The early stages of the engineering design process offer a significant opportunity to evaluate and mitigate risks due to environmental factors. Systems that are expected to operate in a dynamic and changing environment have significant challenges for assessing environmental factors. For example, external failure initiating event probabilities will change with respect to time and new types of external initiating events can be expect with respect to time. While some of the well exercised methods such as Probabilistic Risk Assessment (PRA) [Error! Reference source not found.] and Failure Modes and Effects Analysis (FMEA) [Error! Reference source not found.] can partially address a time-dependent external initiating event probability, current methods of analyzing system failure risk during conceptual system design cannot. As a result, we present our efforts at developing a Time Based Failure Flow Evaluator (TBFFE). This method builds upon the Function Based Engineering Design (FBED) [Error! Reference source not found.] method of functional modeling and the Function Failure Identification and Propagation (FFIP) [Error! Reference source not found.] failure analysis method that is compatible with FBED. Through the development of TBFFE, we have found that it can provide significant insights into a design that is to be used in an environment with variable probability external initiating events and unique external initiating events. We present a case study of the conceptual design of a nuclear power plant's spent fuel pool undergoing a variety of external initiating events that vary in probability based upon the time of year. The case study illustrates the capability of TBFFE by identifying how seasonally variable initiating event occurrences can impact the probability of failure on a month timescale that otherwise would not be seen on a yearly timescale. Changing the design helps to reduce the impact that time-varying initiating events have on the monthly risk of system failure.