PRA as a design tool

PRA (Probabilistic Risk Assessment) has been used in nuclear and other industries for a number of years. Generally, the PRA model is based on a completed or as-built design and is updated as the plant or system is modified. This provides the benefit of ensuring that the new modification or design makes the system more reliable or at least that the new “risk” is understood and acceptable. Using PRA as a design tool has been discussed for years, but the disadvantages have often outweighed the advantages of such an endeavor. Given the high risk environment and the emphasis on a “safe” and “reliable” replacement for the Space Shuttle, NASA chose to incorporate the use of PRA methodology into the design requirements for the Constellation Program in order to allow management to make better risk informed decisions. A variety of “lessons” and planning considerations can be learned from NASA´s experience. The most obvious advantage of using PRA methodology as a design tool is that PRA helps identify potential risks early in the design process, allowing designers to mitigate or eliminate unacceptable risks. Areas for improvement that can occur when using PRA models early in the design process, include: understanding how to address the problems of limited design information and data, planning for the delay between the current PRA model and the “latest” design changes, how to use and document the series of assumptions needed to supplement the lack of detail in the design, how to use the model for “trade” studies, and how to interpret the numbers and “cut-sets” to make appropriate “risk” based decisions. Understanding and planning for these circumstances would improve the effectiveness, cost and schedule impacts of using this methodology to mitigate or eliminate a variety of safety risks in the design stage instead of re-work after the system has been built.