An analysis of the annual probability of failure of the waste hoist brake system at the Waste Isolation Pilot Plant (WIPP)

The Environmental Evaluation Group (EEG) previously analyzed the probability of a catastrophic accident in the waste hoist of the Waste Isolation Pilot Plant (WIPP) and published the results in Greenfield (1990; EEG-44) and Greenfield and Sargent (1993; EEG-53). The most significant safety element in the waste hoist is the hydraulic brake system, whose possible failure was identified in these studies as the most important contributor in accident scenarios. Westinghouse Electric Corporation, Waste Isolation Division has calculated the probability of an accident involving the brake system based on studies utilizing extensive fault tree analyses. This analysis conducted for the U.S. Department of Energy (DOE) used point estimates to describe the probability of failure and includes failure rates for the various components comprising the brake system. An additional controlling factor in the DOE calculations is the mode of operation of the brake system. This factor enters for the following reason. The basic failure rate per annum of any individual element is called the Event Probability (EP), and is expressed as the probability of failure per annum. The EP in turn is the product of two factors. One is the open-quotes reportedclose quotes failure rate, usually expressed as the probability of failure per hour and the other is the expected number of hours that the element is in use, called the open-quotes mission timeclose quotes. In many instances the open-quotes mission timeclose quotes will be the number of operating hours of the brake system per annum. However since the operation of the waste hoist system includes regular open-quotes reoperational checkclose quotes tests, the open-quotes mission timeclose quotes for standby components is reduced in accordance with the specifics of the operational time table