Citation

Heising CD. Reliab. Eng. 1983; 6(4): 229-254.

Copyright

(Copyright © 1983, Elsevier Publishing)

DOI

10.1016/0143-8174(83)90006-9

PMID

unavailable

Abstract

Analytical expressions have been derived for the calculation of one- and two-component system unavailability under test conditions for unrepaired and undetectable failures. Time-dependent unavailability models have been combined with Markov models, including common-cause failures using the beta-factor formulation of Fleming. The resulting expressions represent a new approach to calculating system unavailability that can be compared to the more commonly used approximate expressions that include random failures only. Results of the comparison show that, for identical components arranged in parallel, the approximate expressions can result in gross underestimation of system unavailability of up to several orders of magnitude, depending upon the relative magnitude of the component beta-factor. However, for systems of identical components arranged in series, the inclusion of common-cause failures actually reduces the calculated system unavailability. Recommendations are made for the implementation of these expressions in large-scale nuclear power plant probabilistic risk assessments (PRAs).